Validation of qrs-polarity algorithm with special emphasis in parahisian pathways / Validación del algoritmo de la polaridad del qrs con especial énfasis en vías parahisianas

Abstract Background: In 1996 Iturralde et al. published an algorithm based on the QRS polarity to determine the location of the accessory pathways (AP), this algorithm was developed before the massive practice of invasive electrophysiology. Purpose: To validate the QRS-Polarity algorithm in a modern cohort of subjects submitted to radiofrequency catheter ablation (RFCA). Our objective was to determinate its global accuracy and its accuracy for parahisian AP. Methods: We conducted a retrospective analysis of patients with Wolff-Parkinson-White (WPW) syndrome who underwent an electrophysiological study (EPS) and RFCA. We employed the QRS-Polarity algorithm to predict the AP anatomical location and we compared this result with the real anatomic location determined in the EPS. To determine accuracy, the Cohen's kappa coefficient (k) and the Pearson correlation coefficient were used. Results: A total of 364 patients were included (mean age 30 years, 57% male). The global k score was 0.78 and the Pearson's coefficient was 0.90. The accuracy for each zone was also evaluated, the best correlation was for the left lateral AP (k of 0.97). There were 26 patients with a parahisian AP, who showed a great variability in the ECG features. Employing the QRS-Polarity algorithm, 34.6% patients had a correct anatomical location, 42.3% had an adjacent location and only 23% an incorrect location. Conclusion: The QRS-Polarity algorithm has a good global accuracy; its precision is high, especially for left lateral AP. This algorithm is also useful for the parahisian AP.

Resumen Antecedentes: En 1996 Iturralde y colaboradores publicaron un algoritmo basado en la polaridad del QRS para determinar la ubicación de las vías accesorias (VA), este algoritmo fue desarrollado antes de la práctica masiva de la electrofisiología invasiva. Objetivo: Validar el algoritmo de la polaridad del QRS en una cohorte moderna de sujetos sometidos a ablación con catéter por radiofrecuencia (ACRF). Nuestro objetivo fue determinar su precisión global y su precisión para las VA parahisianas. Métodos: Realizamos un análisis retrospectivo de pacientes con síndrome de Wolff-Parkinson-White (WPW) a los que se les realizó estudio electrofisiológico (EEF) y ACRF. Empleamos el algoritmo de la polaridad del QRS para predecir la ubicación anatómica de la VA y comparamos este resultado con la ubicación anatómica real determinada en el EEF. Para determinar la precisión se utilizaron el coeficiente kappa de Cohen (k) y el coeficiente de correlación de Pearson. Resultados: Se incluyeron un total de 364 pacientes (edad media 30 años, 57 % varones). La puntuación k global fue de 0,78 y el coeficiente de Pearson de 0,90. También se evaluó la precisión para cada zona, la mejor correlación fue para las VA laterales izquierdas (k de 0.97). Hubo 26 pacientes con VA parahisianas, que mostraron una gran variabilidad en las características del ECG. Empleando el algoritmo de la polaridad del QRS, el 34,6 % de los pacientes tenía una ubicación anatómica correcta, el 42,3 % tenía una ubicación adyacente y solo el 23 % una ubicación incorrecta. Conclusión: El algoritmo de la polaridad del QRS tiene una buena precisión global; su precisión es alta, especialmente para VA lateral izquierdo. Este algoritmo también es útil para la VA parahisiana.

Deadly ECG Sign: An Aspect to Recognize in the Settings of STEMI

Learning Objectives: Learn how to identify and differentiate the shark fin EKG pattern from other EKG patterns and abnormalities. Explore the potential diagnostic and prognostic implications of the shark fin EKG pattern, including its association with increased risk of arrhythmias and sudden cardiac death. Background: The Lambda-wave ECG, also known as the Shark fin or Thombostone pattern, has been linked to ST-elevation myocardial infarction's (STEMI) hazardous implications. It is characterized by merged QRS, ST, and T waves, forming a 'triangular QRS-ST-T waveform' or 'giant R waves.' This treacherous signature poses an alarming risk, with chances of cardiogenic shock and ventricular fibrillation leading to fatality. To tackle this peril, implementing swift thrombolysis or percutaneous intervention for reperfusion is critical for successful treatment. For optimal outcomes, the preferred setting to utilize ventricular assist devices is the ICU. Misdiagnosis can happen and be seen as either wide complex tachycardia or ECG changes induced by hyperkalemia. Imperative to prevent severe complications, early detection and treatment are crucial. We are reporting three cases of this Ekg pattern.

Fetal electrocardiogram signal extraction and analysis method combining fast independent component analysis algorithm and convolutional neural network / 生物医学工程学杂志

Fetal electrocardiogram (ECG) signals provide important clinical information for early diagnosis and intervention of fetal abnormalities. In this paper, we propose a new method for fetal ECG signal extraction and analysis. Firstly, an improved fast independent component analysis method and singular value decomposition algorithm are combined to extract high-quality fetal ECG signals and solve the waveform missing problem. Secondly, a novel convolutional neural network model is applied to identify the QRS complex waves of fetal ECG signals and effectively solve the waveform overlap problem. Finally, high quality extraction of fetal ECG signals and intelligent recognition of fetal QRS complex waves are achieved. The method proposed in this paper was validated with the data from the PhysioNet computing in cardiology challenge 2013 database of the Complex Physiological Signals Research Resource Network. The results show that the average sensitivity and positive prediction values of the extraction algorithm are 98.21% and 99.52%, respectively, and the average sensitivity and positive prediction values of the QRS complex waves recognition algorithm are 94.14% and 95.80%, respectively, which are better than those of other research results. In conclusion, the algorithm and model proposed in this paper have some practical significance and may provide a theoretical basis for clinical medical decision making in the future.

O Escore de Selvester QRS é um Preditor de Mortalidade na Insuficiência Cardíaca com Fração de Ejeção Preservada / Selvester QRS Score is a Predictor of Mortality in Heart Failure with Preserved Ejection Fraction

Resumo Fundamento O escore Selvester QRS (S-QRS) em um eletrocardiograma (ECG) de 12 derivações está associado tanto à quantidade de cicatriz miocárdica quanto ao mau prognóstico em pacientes com infarto do miocárdio. Entretanto, seu valor prognóstico na insuficiência cardíaca (IC) com fração de ejeção preservada (ICFEp) é desconhecido. Objetivo Este estudo tem como objetivo investigar o valor preditivo do escore S-QRS para mortalidade na ICFEp. Métodos 359 pacientes foram incluídos retrospectivamente neste estudo. As características eletrocardiográficas, ecocardiográficas e laboratoriais dos pacientes foram registradas. O escore S-QRS simplificado foi medido e registrado. O tempo médio de seguimento dos pacientes foi de 38,1±9,5 meses. A significância estatística foi estabelecida em p < 0,05. Resultados Dos 359 pacientes, 270 estavam no grupo sobrevivente e 89 no grupo falecido. Idade, PCR-us, troponina, pro-BNP, diâmetro do átrio esquerdo (AE), índice de volume do AE, duração do QRS, Tpe e escore do S-QRS foram estatisticamente altos no grupo falecido. Na análise de regressão logística multivariada, idade, PCR-us, NT-proBNP, diâmetro do AE, índice de volume do AE, Tpe e escore S-QRS mostraram-se fatores de risco independentes para mortalidade. Na análise da característica operacional do receptor (ROC), o valor de corte do escore S-QRS foi de 5,5, a sensibilidade foi de 80,8% e a especificidade foi de 77,2% (AUC: 0,880, p:0,00). Na análise de Kaplan-Meier, verificou-se que a mortalidade foi maior no grupo com escore S-QRS ≥ 5,5 do que no grupo com escore S-QRS < 5,5. (Long-rank, p:0,00) Conclusão Acreditamos que o escore S-QRS pode ser usado como um indicador prognóstico de mortalidade a longo prazo em pacientes com ICFEp.

Abstract Background The Selvester QRS (S-QRS) score on a 12-lead electrocardiogram (ECG) is associated with both the amount of myocardial scar and poor prognosis in myocardial infarction patients. However, its prognostic value in heart failure (HF) with preserved ejection fraction (HFpEF) is unknown. Objective This study aims to investigate the predictive value of the S-QRS score for mortality in HFpEF. Methods 359 patients were retrospectively enrolled in this study. Electrocardiographic, echocardiographic, and laboratory features of the patients were recorded. The simplified S-QRS score was measured and recorded. The mean follow-up time of the patients was 38.1±9.5 months. Statistical significance was set at p < 0.05. Results Of 359 patients, 270 were in the survivor group, and 89 were in the deceased group. Age, Hs-CRP, troponin, pro-BNP, left atrial (LA) diameter, LA volume index, QRS duration, Tpe, and S-QRS score were statistically high in the deceased group. In multivariate logistic regression analysis, age, Hs-CRP, NT-proBNP, LA diameter, LA volume index, Tpe, and S-QRS score were shown to be independent risk factors for mortality. In the receiver-operating characteristic (ROC) analysis, the cut-off value of the S-QRS score was 5.5, the sensitivity was 80.8%, and the specificity was 77.2% (AUC:0.880, p:0.00). In Kaplan-Meier analysis, it was found that mortality was higher in the group with S-QRS score ≥ 5.5 than in the group with S-QRS score < 5.5. (Long-rank, p:0.00) Conclusions We think that the S-QRS score can be used as a prognostic indicator of long-term mortality in patients with HFpEF.

Wolff-Parkinson-White Apresentado como QRS Alternante e Outros Diagnósticos Diferenciais em uma Grande Coorte de Triagem de ECG Pré-Participação / Wolff-Parkinson-White Presenting as QRS Alternans and Other Differential Diagnoses in a Large Pre-Participation ECG Screening Cohort

Resumo Fundamento A síndrome de Wolff-Parkinson-White (WPW) é uma condição pró-arrítmica que pode exigir restrição de atividades extenuantes e é caracterizada por sinais de ECG, incluindo ondas delta. Observamos casos de padrões intermitentes de WPW apresentando-se como QRS alternante ('WPW alternante') em uma grande coorte de triagem de ECG pré-participação de homens jovens que se candidataram ao recrutamento militar. Objetivos Nosso objetivo foi determinar o padrão de WPW alternante, as características do caso e a prevalência de outros diagnósticos diferenciais relevantes apresentando-se como alternância de QRS em um ambiente de pré-participação. Métodos Cento e vinte e cinco mil cento e cinquenta e oito recrutas militares do sexo masculino prospectivos foram revisados de janeiro de 2016 a dezembro de 2019. Uma revisão de prontuários médicos eletrônicos identificou casos de WPW alternante e padrões ou síndrome de WPW. A revisão de prontuários médicos eletrônicos identificou casos de diagnósticos diferenciais relevantes que podem causar alternância de QRS. Resultados Quatro indivíduos (2,2%) apresentaram WPW alternante em 184 indivíduos com diagnóstico final de padrão ou síndrome de WPW. Dois desses indivíduos manifestaram sintomas ou achados eletrocardiográficos compatíveis com taquicardia supraventricular. A prevalência geral de WPW alternante foi de 0,003%, e a prevalência de WPW foi de 0,147%. As WPW alternantes representaram 8,7% dos indivíduos com QRS alternantes, e QRS alternantes tiveram prevalência de 0,037% em toda a população. Conclusões A WPW alternante é uma variante da WPW intermitente, que compreendeu 2,2% dos casos de WPW em nossa coorte de triagem pré-participação. Não indica necessariamente um baixo risco de taquicardia supraventricular. Deve ser reconhecido na triagem de ECG e distinguido de outras patologias que também apresentam QRS alternantes.

Abstract Background Wolff-Parkinson-White (WPW) syndrome is a proarrhythmic condition that may require restriction from strenuous activities and is characterized by ECG signs, including delta waves. We observed cases of intermittent WPW patterns presenting as QRS alternans ('WPW alternans') in a large pre-participation ECG screening cohort of young men reporting for military conscription. Objectives We aimed to determine the WPW alternans pattern, case characteristics, and the prevalence of other relevant differential diagnoses presenting as QRS alternans in a pre-participation setting. Methods One hundred twenty-five thousand one hundred fifty-eight prospective male military recruits were reviewed from January 2016 to December 2019. A review of electronic medical records identified cases of WPW alternans and WPW patterns or syndrome. Reviewing electronic medical records identified cases of relevant differential diagnoses that might cause QRS alternans. Results Four individuals (2.2%) had WPW alternans out of 184 individuals with a final diagnosis of WPW pattern or syndrome. Two of these individuals manifested symptoms or ECG findings consistent with supraventricular tachycardia. The overall prevalence of WPW alternans was 0.003%, and the prevalence of WPW was 0.147%. WPW alternans represented 8.7% of individuals presenting with QRS alternans, and QRS alternans had a prevalence of 0.037% in the entire population. Conclusions WPW alternans is a variant of intermittent WPW, which comprised 2.2% of WPW cases in our pre-participation screening cohort. It does not necessarily indicate a low risk for supraventricular tachycardia. It must be recognized at ECG screening and distinguished from other pathologies that also present with QRS alternans.

Relationship between fragmented QRS wave with ventricular arrhythmia's incidence situation and heart rate variability in patients with old myocardial infarction / 中国综合临床

Objective:To explore the relationship between fragmented QRS complex and heart rate variability (HRV) and ventricular arrhythmia in patients with old myocardial infarction.Methods:From August 2018 to October 2019, 200 patients with old myocardial infarction were first treated in the Department of cardiac function examination of the First Affiliated Hospital of Hebei North University. The patients were divided into 99 cases of old myocardial infarction with fragmented QRS wave group and 101 cases of old myocardial infarction without fragmented QRS wave group according to the case bank data and conventional 12 lead ECG diagnosis in our hospital for the first time. Then, the 24-h ambulatory ECG reexamined within 1 year after discharge was retrospectively analyzed. The incidence of ventricular arrhythmia was compared between the two groups by χ 2 test. The difference of heart rate variability between the two groups was compared by rank sum test. Multiple logistic regression was used to analyze the value of different indexes of heart rate variability in the evaluation of fragmented QRS complex in old myocardial infarction. Drawing the receiver operating characteristic (ROC), and the area under the curve (AUC) was used to analyze the diagnostic accuracy of different indexes of heart rate variability in the broken QRS complex of old myocardial infarction. Results:According to the Lown classification of ventricular premature contraction, the number of positive ventricular arrhythmias in patients with Grade Ⅰ of ventricular premature contraction and Grade Ⅲ-Ⅴ of ventricular premature contraction in the old myocardial infarction fragmented QRS group was higher than that in the old myocardial infarction non fragmented QRS group (Grade Ⅰ of ventricular premature contraction: 54.5% (54/99)and 39.6%(40/101); χ 2=4.484, P<0.05;Grade Ⅲ-Ⅴ of ventricular premature contraction: 34.3% (34/99) and 9.9%(10/101); χ 2=17.406, P<0.05)). Ventricular premature contraction Grade 0 old myocardial infarction fragmented QRS group was lower than old myocardial infarction non fragmented QRS group (8.1% (8/99) and 48.5% (49/101); χ 2=37.995, P<0.05). The total number of positive cases of ventricular arrhythmia in the old myocardial infarction group with fragmented QRS wave was higher than that in the old myocardial infarction group without fragmented QRS wave (91.9% (91/99) and 51.5%(52/101); χ 2=57.146, P<0.05)). There was no significant difference in the number of positive ventricular arrhythmias between the old myocardial infarction fragmentation QRS group and the old myocardial infarction non fragmentation QRS group ( P>0.05). The standard deviation of NN intervals (SDNN) and the standard deviation of average NN intervals (SDANN) of HRV time domain indexes in the old myocardial infarction fragmented QRS group were higher than those in the old myocardial infarction non fragmented QRS Group (SDNN:143.00(122.00,166.00) vs. 110.00(95.00,130.50), Z=5.780, P<0.05; SDANN:112.00(100.00,136.00) vs. 96.00(76.00,118.50), Z=4.013, P<0.05). Multiple Logistics regression analysis results of HRV domain shows that HRV time domain SDNN and SDANN have diagnositic value in diagnosis fQRS after OMI(SDNN: OR=0.949, 95% CI:0.922-0.977, P<0.001; SDANN: OR=1.036, 95% CI:1.005-1.068, P=0.022). Area under ROC curve of HRV time domain SDNN and SDANN have particular diagnositic accuracy in diagnosis fQRS after OMI(SDNN: AUC 0.737, 95% CI 0.666-0.807, Sensitivity 0.818, Specificity 0.634; SDANN: AUC 0.664, 95% CI 0.587-0.741, Sensitivity 0.737, Specificity 0.673. 0.5<AUC<1). Conclusion:Fragmented QRS complex was positively correlated with the incidence and severity of ventricular arrhythmia in patients with old myocardial infarction, and positively correlated with time-domain indexes SDNN and SDANN of heart rate variability in patients with old myocardial infarction.

Relationship between electrocardiogram QRS wave duration and pathological parameters of patients with acute myocardial infarction and prognosis analysis / 中国医师杂志

Objective:To explore the relationship between electrocardiogram QRS wave duration and pathological parameters of acute myocardial infarction and its prognosis.Methods:The patients with acute myocardial infarction treated in Hunan Brain Hospital from January 2018 to June 2019 were divided into normal group (≤110 ms, NG group) and extended group (>110 ms, EG group) according to the time limit of QRS wave group of electrocardiogram (ECG). The clinicopathological data of the two groups were collected, and the differences of relevant pathological parameters between the two groups were compared. The correlation between ECG QRS complex time limit and pathological parameters of acute myocardial infarction and the independent risk factors affecting the occurrence of malignant cardiovascular events after acute myocardial infarction were analyzed.Results:A total of 134 patients were enrolled, 73 in the EG group, and 61 in the NG group. The plasma levels of N-terminal pro-brain natriuretic peptide (N-proBNP) in EG group were significantly higher than those in NG group ( P<0.05), and left ventricular ejection fraction (LVEF) was significantly lower than that in NG group ( P<0.05). The time limit of QRS wave group in EG group was positively correlated with the plasma levels of N-proBNP ( r=0.981, P<0.001) and negatively correlated with LVEF ( r=-0.979, P<0.001). The ratio of Killip grade Ⅰ patients in EG group were lower than those in NG group ( P<0.05), while the ratio of grade Ⅱ and Ⅲ patients were significantly higher than those in NG group ( P<0.05), and there was no significant difference between the two groups in grade Ⅳ patients ( P>0.05). The incidence of malignant cardiovascular events (cardiogenic shock, malignant arrhythmia, acute pulmonary edema, cardiogenic death and re-infarction) in EG group was significantly higher than that in NG group ( P<0.05). Multivariate logistic regression analysis showed that elevated serum N-proBNP, decreased LVEF, prolonged QRS wave duration and high Killip cardiac function were independent risk factors for malignant cardiovascular events after acute myocardial infarction. Conclusions:After acute myocardial infarction, EG patients have worse cardiac function, higher incidence of malignant cardiovascular events and worse prognosis than NG patients.

Research on the detection algorithm of electrocardiogram characteristic wave based on energy segmentation and stationary wavelet transform / 生物医学工程学杂志

The detection of electrocardiogram (ECG) characteristic wave is the basis of cardiovascular disease analysis and heart rate variability analysis. In order to solve the problems of low detection accuracy and poor real-time performance of ECG signal in the state of motion, this paper proposes a detection algorithm based on segmentation energy and stationary wavelet transform (SWT). Firstly, the energy of ECG signal is calculated by segmenting, and the energy candidate peak is obtained after moving average to detect QRS complex. Secondly, the QRS amplitude is set to zero and the fifth component of SWT is used to locate P wave and T wave. The experimental results show that compared with other algorithms, the algorithm in this paper has high accuracy in detecting QRS complex in different motion states. It only takes 0.22 s to detect QSR complex of a 30-minute ECG record, and the real-time performance is improved obviously. On the basis of QRS complex detection, the accuracy of P wave and T wave detection is higher than 95%. The results show that this method can improve the efficiency of ECG signal detection, and provide a new method for real-time ECG signal classification and cardiovascular disease diagnosis.

Acciones cardiovasculares directas del paracetamol / Direct cardiovascular actions of paracetamol

Introducción: El paracetamol es uno de los antiinflamatorios no esteroideos con efecto analgésico y antipirético más utilizados a nivel mundial. Pocos estudios se enfocan en esclarecer los mecanismos de acción a nivel cardiovascular. Objetivos: Evaluar la acción del paracetamol sobre la fuerza de contracción de anillos de aorta torácica y sobre la actividad eléctrica y contráctil de corazones aislados y perfundidos de ratas Wistar. Métodos: Se midieron los efectos del paracetamol sobre anillos de aorta de rata denudados de su endotelio vascular. Se estudiaron las acciones del fármaco sobre los corazones aislados y perfundidos de las ratas por el método de Langendorff. Se evaluó la amplitud de la fuerza de contracción cardiaca y los intervalos QT, QTc, QRS y RR del electrocardiograma. Las condiciones (control y presencia de paracetamol) fueron comparadas con una prueba t de Student para muestras pareadas (p < 0,05), previa comprobación de la normalidad de los datos. Resultados: El paracetamol no tuvo efectos sobre el músculo liso vascular de los anillos aórticos ni sobre los intervalos QT, QTc, QRS y RR del electrocardiograma bajo ninguna de las concentraciones empleadas. Por otra parte, mostró efecto inotrópico negativo estadísticamente significativo en los corazones aislados, de forma dependiente de la concentración del fármaco. La IC50 estimada para la inhibición de la fuerza de contracción cardiaca fue de 17,15 ± 5,33 µmol/L. Conclusiones: Las acciones cardiovasculares directas del paracetamol son modestas, lo cual contribuye al buen margen de seguridad para su uso en clínica, en pacientes sin enfermedad cardiovascular(AU)

Introduction: Paracetamol is among the non-steroidal anti-inflammatory, analgesic and antipyretic drugs most commonly used worldwide. Few studies have focused on clarifying its mechanisms of action on a cardiovascular level. Objectives: Evaluate the action of paracetamol on the force of contraction of thoracic aortic rings and on the electrical and contractile activity of isolated perfused Wistar rat hearts. Methods: Measurements were taken of the effects of paracetamol on rat aortic rings denuded of their vascular endothelium. Analysis was performed of the actions of the drug on the isolated perfused rat hearts using the Langendorff method. Evaluation was conducted of the amplitude of the force of cardiac contraction and of intervals QT, QTc, QRS and RR of the electrocardiogram. The conditions (control and presence of paracetamol) were compared with a paired samples Student's t-test (p < 0.05) upon verification of the normality of the data. Results: Paracetamol had no effects on the vascular smooth muscle of aortic rings or on intervals QT, QTc, QRS and RR of the electrocardiogram at none of the concentrations used. On the other hand, it displayed a statistically significant negative inotropic effect on the isolated hearts dependent on drug concentration. The IC50 estimated for inhibition of the force of cardiac contraction was 17.15 ± 5.33 µmol/L. Conclusions: The direct cardiovascular actions of paracetamol are modest, which contributes to a good safety margin for its clinical use in patients without cardiovascular disease(AU)

Acuerdo inter- e intraobservador de la cuantificación del ángulo QRS-T espacial calculado por el método de transformación visual. Estudio piloto / Inter- and intra-observer agreement in the quantification of the spatial QRS-T angle calculated by visual transformation. A pilot study

Resumen Introducción: El ángulo QRS-T espacial es la diferencia entre el vector del QRS y la T. Se conoce el valor diagnóstico y pronóstico de este marcador en enfermedades cardíacas. Es prioritario determinar si estas mediciones son reproducibles con confiabilidad en nuestro medio. Métodos: En 30 adultos se obtuvieron electrocardiogramas, independientemente del diagnóstico. Las mediciones las realizaron dos estudiantes de Medicina de quinto año, un médico interno y un cardiólogo, posterior a un reentrenamiento sobre la medición del ángulo por el método de transformación visual. Con identificación cegada, ingresaron en un aplicativo web las mediciones del QRS y la T. El ángulo fue estimado a partir de la plantilla de Cortez y colaboradores. Sobre el 20% de las lecturas se estimó un acuerdo intraobservador y en el 100% de las lecturas un acuerdo interobservador. Los puntos de corte para estimar el acuerdo Kappa fueron < 105s (normal), 105° a 135° (limítrofe) y > 135° (anormal). Resultados: Se excluyó un ECG por calidad deficiente. El rango del ángulo estuvo entre 72.9° y 176.2°. El acuerdo interobservador entre los 4 evaluadores fue kappa = 0.786 (IC95% 0.728-0.823, p < 0.0001) y el intraobservador kappa = 1.0 (p 0.007). Conclusión: Es el primer estudio sobre el ángulo QRS-T espacial realizado en Colombia. Se logró un acuerdo inter- e intraobservador adecuado en la medición del ángulo por el método de transformación visual, en observadores con diferente experticia, con un kappa mayor de 0,75. Este resultado apoya la reproducibilidad de esta medición en estudios posteriores en Colombia.

Abstract Introduction: The spatial QRS-T angle is the difference between the QRS vector and T vector. The diagnostic and prognostic value of this marker in heart disease is well-known. It is essential to determine whether these measurements are reproducible and reliable in Colombia. Methods: Electrocardiograms (ECG) were obtained from 30 adults, regardless of the diagnosis. The measurements were made by two fifth-year medical students, a houseman/ intern, and a cardiologist. They were all re-trained on the measurement of the angle using the visual transformation method. With blinded identification, they entered the QRS and T measurements into a web application. The angle was estimated from the template of Cortez et al. About 20% of the readers showed an intra-observer agreement and an inter-observer agreement in 100%. The cut-off points to estimate the agreement Kappa were < 105° (normal), 105° a 135° (borderline), and >135° (abnormal). Results: One ECG was excluded due to poor quality. The range of the angle was between 72.9° and 176.2°. The inter-observer agreement between the 4 evaluators gave a kappa = 0.786 (95% CI; 0.728-0.823, P < .0001), and the intra-observer agreement a kappa = 1.0 (P = .007). Conclusion: This the first study on the spatial QRS-T angle performed in Colombia. A good inter- and intra-observer agreement was achieved in the measurement of the angle by the visual transformation method by observers with different levels of expertise, with a Kappa greater than 0.75. These results support the reproducibility of this measurement in subsequent studies in Colombia.

Fibrilación auricular por vía accesoria con patrón de preexcitación ventricular: Informe de un caso / Atrial fibrillation by accessory pathway with pattern of ventricular pre-excitation: A case report

RESUMEN La fibrilación auricular por vía accesoria es una peligrosa enfermedad que puede provocar muerte súbita; es poco frecuente y necesita una conducta exacta, mucho más si aparece un patrón de preexcitación ventricular. Se presenta el caso de un adolescente masculino de 17 años de edad y piel blanca, con antecedentes de palpitaciones cortas a esfuerzos moderados y pérdida ocasional del conocimiento de breve duración, sin otra sintomatología asociada. Al momento del ingreso presenta un cuadro de palpitaciones, fatiga, palidez e hipotensión arterial, y en el electrocardiograma se observa una taquicardia con QRS ancho, irregular, con onda delta y frecuencia ventricular rápida, que fue interpretada como fibrilación auricular preexcitada. Se realizó cardioversión eléctrica sincronizada al QRS con 100 julios y se logró restaurar el ritmo sinusal con patrón electrocardiográfico de preexitación ventricular (síndrome de Wolff-Parkinson-White).

ABSTRACT Atrial fibrillation by accessory pathway is a dangerous disease that can cause sudden death, it is rare and it needs a careful management, even more, if a pattern of ventricular pre-excitation appears. The case of a white skin 17-year-old teenager with a history of short palpitations at moderate effort, and occasional loss of consciousness of short duration, without other associated symptoms, is presented. At the moment of admission, he presents palpitations, fatigue, pallor and low blood pressure; in the electrocardiogram a tachycardia with wide, irregular QRS, with delta wave and rapid ventricular rate is observed, which was interpreted as pre-excited atrial fibrillation. Synchronized electrical cardioversion to QRS with 100 joules was performed, and sinus rhythm was restored with electrocardiographic pattern of ventricular pre-excitation (Wolff-Parkinson-White syndrome).

Diagnóstico diferencial de las taquicardias de complejo ancho: un desafío permanente / Wide QRS tachycardia: a diagnostic challenge

El diagnóstico electrocardiográfico correcto de la causa de una taquicardia de complejo QRS ancho (TCA) es fundamental, ya que tanto el manejo, como el pronóstico del paciente, es diferente según su etiología, y define el estudio que debemos realizar. Numerosos criterios y algoritmos han sido descritos para diferenciar el origen de estas taquicardias. Sin embargo, muchos de estos son complejos y difíciles de aplicar para el médico menos experimentado. Esto es particularmente importante en los servicios de emergencia, donde se necesita una definición rápida que permita un manejo agudo apropiado. En la presente revisión analizamos los diferentes mecanismos de las TCA y los principales criterios diagnósticos en el ECG, reforzando, especialmente, aquellos de aplicación rápida y de alto rendimiento diagnóstico.

The correct electrocardiographic diagnosis of the cause of a wide QRS complex tachycardia (WCT) is essential since both management and prognosis of the patient. The correct electrocardiographic diagnosis of the cause of a wide QRS complex tachycardia (WCT) is essential since both management and prognosis is different according to its etiology and defines the study that we should perform. Numerous criteria and algorithms have been described to differentiate the origin of these tachycardias. However, many of these are complex and difficult to apply to the less experienced doctor. This is particularly important in emergency rooms, where a rapid definition is needed to allow proper therapy. In this review we analyze the different mechanisms of WCT and the main EKG diagnostic criteria, emphasizing those which can be applied rapidly and have high diagnostic value.

Índice de varianza versus duración del complejo QRS para el diagnóstico de dispersión mecánica del ventrículo izquierdo

Introducción. En el complejo universo de la falla cardíaca, el pronóstico y el tratamiento de la misma sumaron un aspecto de valor durante la última década del siglo pasado, a partir del reconocimiento de la disincronía mecánica durante la sístole y las consiguientes propuestas de resincronización cardíaca. Desde los inicios la predicción de disincronía mecánica se ha fundamentado en la duración del complejo QRS, las imágenes y más recientemente el análisis del índice de varianza del QRS. Objetivo. Tomando como estándar la dispersión mecánica ventricular izquierda medida por ecocardiografía, valorar la concordancia entre duración promedio del QRS del electrocardiograma (ECG) versus el índice de varianza, en cuanto a sensibilidad, especificidad y valor predictivo de dispersión anormal. Métodos. Población de pacientes consecutivos citados para ecocardiografía strain, sin escara ventricular, para medir dispersión mecánica, duración promedio del QRS del ECG e índice de varianza espacial. Resultados. Se evaluaron 54 pacientes y 57 escenarios distintos. Edad promedio: 66,3±14,2 años; duración promedio del QRS fue de 119,7±31,5 mseg. El índice de varianza promedio fue 0,52±0,35. Un índice de varianza >0,4 demostró sensibilidad y especificidad de 82,6% y 64,7% respectivamente, para diagnóstico de dispersión mecánica anormal (e"56 mseg); valor predictivo positivo: 61,3%, negativo: 84,6%. La duración promedio de QRS e"130 mseg logró sensibilidad y especificidad de 52,6% y 71%, con valores predictivos positivo y negativo de 54,5% y 68,6%, respectivamente. La duración promedio del QRS no guarda dependencia con la disincronía mecánica (p=0,1458); la disincronía mecánica (>56 mseg) y la electrocardiografía de varianza, por otra parte, sí guardan dependencia muy significativa (p=0,0012). Los métodos no son comparables para diagnosticar disincronía (kappa=0,376). Conclusión. El ECG de varianza tiene mejor sensibilidad y valor predictivo negativo para el diagnóstico de dispersión mecánica ventricular izquierda valorada por ecocardiografía.

Introduction. The prediction of mechanical dyssynchrony has generally been based on the duration of the QRS complex, although other resources are available, such as the QRS variance index; these methods have not been compared with each other. Objective. Taking as a standard the left ventricular mechanical dispersion measured by speckle-tracking strain echocardiography, to assess the concordance between the average QRS duration of the standard electrocardiogram versus the spatial variance index in terms of sensitivity, specificity and predictive value of abnormal mechanical dispersion. Methods. Population of consecutive patients cited for strain echocardiography and without ventricular scar tissue, to measure mechanical dispersion, average QRS duration of the standard electrocardiogram and the variance index. Results. Fifty-four patients and 57 different scenarios were evaluated; the average age was 66.3±14.2 years, the QRS average duration on standard electrocardiogram was 119.7±31.5 ms. Average ID was 0.52±0.35. A spatial variation index >0.4 showed sensitivity and specificity of 82.6% and 64.7% respectively, for the diagnosis of abnormal mechanical dispersion (e"56 ms); positive predictive value: 61.3%, negative: 84.6%. The average duration of QRS e"130 ms achieved sensitivity and specificity of 52.6% and 71%, and the positive and negative predictive values were 54.5 and 68.6%, respectively. The average QRS duration are not interdependent with mechanical dyssynchrony (p=0.1458); mechanical dyssynchrony (>56 ms) and spatial variance electrocardiography are, nevertheless, clearly interdependent (p=0.0012). The methods are not comparable between them (kappa=0.376). Conclusion. The QRS variation electrocardiogram has better sensitivity and negative predictive value respect to the average QRS duration for the diagnosis of left ventricular mechanical dispersion prevalence, measured by echocardiography.

Introdução. No complexo universo da insuficiência cardíaca, seu prognóstico e tratamento agregaram valor durante a última década do século passado, com base no reconhecimento da dissincronia mecânica durante a sístole e nas propostas subsequentes de ressincronização cardíaca. Desde o início, a previsão da dissincronia mecânica tem sido baseada na duração do complexo QRS, nas imagens e, mais recentemente, na análise do índice de variância QRS. Objetivo. Tomando como padrão a dispersão mecânica do ventrículo esquerdo medida pelo ecocardiograma, avalie a concordância entre a duração média do QRS do eletrocardiograma (ECG) versus o índice de variância, em termos de sensibilidade, especificidade e valor preditivo de dispersão anormal. Métodos População de pacientes consecutivos citados para ecocardiograma de deformação, sem escara ventricular, para medir dispersão mecânica, duração média do QRS do ECG e índice de variância espacial. Resultados. Foram avaliados 54 pacientes e 57 cenários diferentes. Idade média: 66,3±14,2 anos; a duração média do QRS foi de 119,7±31,5 mseg. O índice de variância médio foi de 0,52±0,35. Um índice de variância >0,4 mostrou sensibilidade e especificidade de 82,6% e 64,7%, respectivamente, para diagnóstico de dispersão mecânica anormal (e"56 mseg); valor preditivo positivo: 61,3%, negativo: 84,6%. A duração média do QRS e"130 mseg alcançou sensibilidade e especificidade de 52,6% e 71%, com valores preditivos positivos e negativos de 54,5% e 68,6%, respectivamente. A duração média do QRS não depende de dissincronia mecânica (p=0,1458); dissincronia mecânica (>56 mseg) e eletrocardiografia de variância, por outro lado, têm dependência muito significativa (p=0,0012). Os métodos não são comparáveis para diagnosticar dissincronia (kappa=0,376). Conclusão. A variância do ECG apresenta melhor sensibilidade e valor preditivo negativo para o diagnóstico de dispersão mecânica do ventrículo esquerdo avaliada pelo ecocardiograma.

A case of ventricular tachycardia precipitated by caffeine: a rare occurrence

The effects of caffeine vary according to the dose and patient. Thus, there is no bright line defining thresholds. Here, we present an unusual case of ventricular tachycardia precipitated by a dose of 450-500 mg of caffeine in a healthy 33-year-old man.

Terapia de resincronización cardíaca: Índice del QRS como predictor de respuesta / Cardiac resynchronization therapy: QRS index as a response predictor

RESUMEN Introducción: La terapia de resincronización cardíaca (TRC) es indicación en la insuficiencia cardíaca con QRS ancho y disminución grave de la fracción de eyección del ventrículo izquierdo. Objetivos: Definir los predictores de respuesta favorable a la TRC. Método: Se realizó un estudio observacional, descriptivo, retrospectivo, para evaluar el índice del QRS (diferencia entre anchura del QRS antes y después del implante, dividido entre su valor antes del implante, multiplicado por 100) como predictor de respuesta favorable a la TRC. Se realizaron electrocardiogramas antes del procedimiento, a los 6 y a los 12 meses del implante. Las mediciones se hicieron por dos observadores independientes, la primera digital en el monitor del salón de operaciones y el resto manual. Resultados: Se incluyeron 91 pacientes (edad media 61,2 años, 76% hombres), QRS mayor de 120 ms y fracción de eyección menor de 35%. Se obtuvo respuesta favorable en un 59%. No hubo diferencias significativas pre-implante en la duración del QRS entre respondedores y no respondedores (151,3 ms vs 151,34 ms, p=0,98), pero sí post-implante (100 vs 115 ms, p<0,0001), así como en el porcentaje de disminución del QRS (33,2% vs 24,3%, p<0,0001). La curva ROC mostró que un valor de corte del índice de QRS del 30% fue sensible (62%) y específico (75%), para predecir respuesta favorable. Conclusiones: La disminución de la anchura del QRS luego del implante de la TRC se relaciona con una respuesta favorable a la misma.

ABSTRACT Introduction: Cardiac resynchronization therapy (CRT) is an indication in heart failure with wide QRS and severely reduced left ventricular ejection fraction. Objectives: To define the response predictors favorable to CRT. Method: An observational, descriptive, retrospective study was conducted to evaluate the QRS index (difference between the QRS width before and after implantation, divided by its value before implantation, multiplied by 100) as a predictor of favorable response to CRT. Electrocardiograms were performed before the procedure, at 6 and 12 months after implantation. The measurements were made by two independent observers, the first digital on the operating room monitor and the rest manual. Results: A total of 91 patients (mean age 61.2 years, 76% men) were included, with QRS wider than 120 ms and ejection fraction less than 35%. A favorable response was obtained in 59%. There were no significant pre-implant differences in the QRS duration between responders and non-responders (151.3 ms vs. 151.34 ms, p=0.98), but there were differences post-implant (100 vs. 115 ms, p<0.0001), as well as in the QRS percentage of decrease (33.2% vs. 24.3%, p<0.0001). The ROC curve showed that a cut-off value of the QRS index of 30% was sensitive (62%) and specific (75%) in order to predict a favorable response. Conclusions: The decrease in the QRS width after the CRT implant is related to a favorable response to it.

Arritmias provocadas por ondansetrón: Presentación de un caso / Arrhythmias caused by ondansetron: Case report

RESUMEN El ondansetrón se usa para prevenir las náuseas y los vómitos causados por la quimioterapia, radioterapia y cirugías, pertenece a los antagonistas de receptores de serotonina 5-HT3, una sustancia natural que puede causar náuseas y vómitos, y bloquea su acción. El ondansetrón viene envasado en forma de tabletas de desintegración rápida, como una solución para tomar por vía oral y en ámpulas, para su uso parenteral. Se presenta el caso de una paciente de 67 años de edad con diagnóstico de carcinoma de mama, a la cual se realizó mastectomía radical con vaciamiento axilar, y recibió quimioterapia con adriamicina, ciclofosfamida y paclitaxel; así como ondansetrón para tratar las náuseas y vómitos. La paciente presentó una taquicardia con QRS ancho después de utilizar el fármaco.

ABSTRACT The ondansetron is used to prevent nausea and vomiting caused by chemotherapy, radiotherapy and surgery, belonging to the serotonin 5-HT3 receptor antagonists, a natural substance that can cause nausea and vomiting, and it blocks its action. The ondansetron is packaged in the form of rapid disintegration tablets, as a solution to be taken orally and in ampules, for parenteral use. The case of a 67-year-old female patient is presented, with a diagnosis of breast carcinoma, who underwent radical mastectomy with axillary dissection was performed, and who received chemotherapy with adriamycin, cyclophosphamide and paclitaxel; as well as ondansetron to treat nausea and vomiting. The patient presented a wide QRS complex tachycardia after taking the drug.

Fragmentación del QRS y muerte súbita en paciente con displasia arritmogénica del ventrículo derecho / Fragmented QRS and sudden death in a patient with arrhythmogenic right ventricular dysplasia/cardiomyopathy

RESUMEN La miocardiopatía o displasia arritmogénica del ventrículo derecho es una cardiopatía de origen genético cuyo diagnóstico supone, a menudo, un reto para el clínico. Es una de las causas más comunes de muerte súbita cardíaca en la adolescencia y en los adultos jóvenes. Se presenta el caso de un paciente con historia de arritmias ventriculares malignas y de muerte súbita cardíaca recuperada, por displasia arritmogénica del ventrículo derecho, con fragmentación del QRS en las derivaciones precordiales derechas, como marcador de la presencia de un sustrato propicio para el surgimiento de la fibrilación ventricular espontánea. Se comenta la patogenia, el diagnóstico y el tratamiento de esta enfermedad.

ABSTRACT The arrhythmogenic right ventricular dysplasia or cardiomyopathy is a genetic heart disease whose diagnosis is often a challenge for the clinician. It is one of the most common causes of sudden cardiac death in adolescence and in young adults. We present the case of a patient with a history of malignant ventricular arrhythmias and recovered sudden cardiac death due to arrhythmogenic right ventricular dysplasia, with QRS fragmentation in the right precordial leads, as a marker of the presence of a suitable substrate for the emergence of spontaneous ventricular fibrillation. The pathogenesis, diagnosis and treatment of this disease are discussed.

Association Between Non-Dipping and Fragmented QRS Complexes in Prehypertensive Patients / Associação entre Ausência de Descenso da Pressão Arterial Durante o Sono e a Fragmentação do Complexo QRS em Pacientes Pré-Hipertensos

Abstract Background: Fragmented QRS (fQRS) is a sign of adverse cardiovascular events in various cardiovascular diseases. It is also associated with increased blood pressure and non-dipping in hypertensive patients. However, no study has investigated the importance of fQRS in prehypertensive patients. Objectives: The aim of our study is to investigate the relationship between fQRS and non-dipper status in prehypertensive patients. Methods: Two hundred and sixteen eligible, newly diagnosed prehypertensive patients who underwent 24-hour ambulatory blood pressure monitoring (ABPM) for further evaluation of blood pressure between June 2015 and July 2016 were included into the study. Patients were divided into three groups according to ABPM results: normotensives, dipper prehypertensives, and non-dipper prehypertensives. Groups were compared regarding presence of fQRS on electrocardiography. Additionally, multinomial logistic regression analysis was used to determine the relationship between fQRS and blood pressure pattern in prehypertensive patients. Results: According to ABPM recordings, 61 patients had normotensive blood pressure pattern (systolic blood pressure < 120 mmHg and diastolic blood pressure < 80 mmHg). Of the remaining 155 prehypertensive patients, 83 were dippers and 72 were non-dippers. Non-dipper prehypertensives had a significantly higher frequency of fQRS compared to normotensives (p = 0.048). Furthermore, multinomial logistic regression analysis revealed that fQRS is an independent predictor of non-dipping blood pressure pattern in prehypertensive patients (p = 0.017, OR: 4.071, 95% CI: 1.281-12.936). Conclusions: We found that fQRS is a predictor of non-dipping in prehypertensives. As a marker of fibrosis and higher fibrotic burden within myocardium, fQRS may be useful in identifying high-risk prehypertensive patients before the development of hypertension.

Resumo Fundamento: A fragmentação do QRS (fQRS) é um sinal de eventos cardiovasculares adversos em várias doenças cardiovasculares. É associado também à pressão arterial elevada e à ausência de descenso da pressão arterial durante o sono (non-dipping) em pacientes pré-hipertensos. Objetivos: O objetivo do estudo presente é investigar a relação entre fQRS e ausência de descenso da pressão arterial durante o sono em pacientes pré-hipertensos. Métodos: Duzentos e dezesseis pacientes elegíveis, recém-diagnosticados com pré-hipertensão, que foram submetidos a monitorização ambulatorial da pressão arterial (MAPA) durante 24 horas para avaliação mais aprofundada da pressão arterial entre junho de 2015 e julho de 2016, foram incluídos no estudo. De acordo com os resultados da MAPA, os pacientes foram divididos em três grupos: normotensos, pré-hipertensos com descenso da pressão arterial durante o sono (padrão dipping) e pré-hipertensos com ausência de descenso da pressão arterial durante o sono (padrão non-dipping). Os grupos foram comparados quanto à presença de fQRS no eletrocardiograma. Adicionalmente, utilizou-se a análise de regressão logística multinomial para determinar a relação entre a fQRS e o padrão de pressão arterial em pacientes pré-hipertensos. Resultados: De acordo com os registos da MAPA, 61 pacientes apresentavam padrão de pressão arterial normotenso (pressão arterial sistólica < 120 mmHg e pressão arterial diastólica < 80 mmHg). Dos 155 pacientes pré-hipertensos, 83 tinham padrão dipping e 72 tinham padrão non-dipping. Os pacientes pré-hipertensos com padrão non-dipping tinham uma frequência significativamente mais alta de fQRS em comparação com os pacientes normotensos (p = 0,048). Além disso, a análise de regressão logística multinomial revelou que fQRS é um preditor independente do padrão non-dipping de pressão arterial em pacientes pré-hipertensos (p = 0,017, OR: 4,071, 95 % CI: 1,281-12,936). Conclusões: Verificamos que a fQRS é um preditor do padrão non-dipping em pacientes pré-hipertensos. Como marcador de fibrose e aumento na carga fibrótica do miocárdio, a fQRS pode ser útil na identificação de pacientes pré-hipertensos de alto risco antes do desenvolvimento da hipertensão.

The role of frontal plane cardiac axis shift in the diagnosis of wide QRS tachycardia / 实用医学杂志

Objective To explore the value of axis shift between the baseline normal sinus rhythm (NSR) and WCT in diagnosis of wide QRS-complex tachycardia (WCT). Methods 390 surface ECGs of 186 patients with WCT were obtained from April 2012 to April 2018 at Ningbo Medical Center Lihuili Hospital at which the arrhythmia diagnosis was proven by intracardiac electrophysiological study. The axis shift between the baseline NSR and WCT was calculated by table lookup method. Then we analyzed the role of axis shift in diagnosis of WCT. Results Among the 186 patients with WCT, 147 (79.03％) were ventricular tachycardia (VT) , and 39 (20.97％) were supraventricual tachycardia (SVT) with conduction abnormalities. In the 95％ confidence interval, the axis shift showed an outstanding discrimination performance. The area under the ROC curve is 0.708 (0.579-0.817, P =0.007). Compared with left axis deviation, right axis deviation, the right axis deviation of LBBB morphology, the axis shift> 68 degree is more sensitive (53.06％) , and the specificity (91.43％) is also more desirable. Moreover, if the axis shift set> 130 degree, the specificity can reach 100％, and the sensitivity (12.24％) is equivalent to northwestern axis. Conclusion A significant axis shift between the baseline NRS and WCT can distinguish WCT accurately. Given the ease of grasping, it can probably be feasible to popularize as a routine diagnosis method for WCT in primary hospitals.

Distribution characteristics of fragmented QRS in patients with coronary atherosclerotic heart disease and its relationship with left ventricular remodeling / 上海交通大学学报（医学版）

Objective: To explore the diagnostic value of fragmented QRS (fQRS) for coronary atherosclerotic heart disease (CHD), and to analyze it's relationship with left ventricular remodeling. Methods: From Nov. 2016 to Oct. 2018, 498 hospitalized patients in the Department of Cardiovascular Medicine of Shanghai Ninth People's Hospital, Shanghai Jiao Tong University School of Medicine were selected consecutively. During the hospitalization, all the patients underwent coronary angiography. According to the angiographic results, the patients were divided into the control group (203 patients with negative or coronary stenosis < 30%), the mild to moderate stenosis group (155 patients with coronary stenosis 30% to 75%), and the severe stenosis group (140 patients with coronary stenosis≥75%). The incidences of fQRS(+) in the normal electrocardiogram among the three groups were compared by chi-square test of R×C contingency table. Two hundred and thirty patients with single-vessel stenosis≥30% were divided into the anterior descending branch group (128 cases), the right coronary branch group (59 cases), and the circumflex branch group (43 cases), and the relationship between fQRS(+) leads and diseased vessels was analyzed by nonparametric test. Finally, all the patients were divided into fQRS(+) group (86 cases) and fQRS(-) group (412 cases). The correlation between fQRS and left ventricular ejection fraction (LVEF), left ventricular end-diastolic volume (LVEDV), left ventricular end-systolic volume (LVESV), interventricular septum thickness (IVST) and left ventricular posterior wall thickness (LVPWT), respectively, were analyzed by binary Logistic regression model. Results: The chi-square test of R×C contingency table showed that the incidences of fQRS(+) in the three groups were 8.89%, 16.13% and 30.71%, respectively, with statistically significant differences (all P < 0.05). The nonparametric test showed that the fQRS(+) leads reflecting the anterior wall (V3, V4) were more common in the anterior descending branch group, and the fQRS(+) leads reflecting the interior wall and right ventricular (Ⅱ, III, AVF, V1, V2) were more common in the right coronary branch group, the fQRS(+) leads reflecting upper lateral wall (, AVL) were more common in the circumflex branch group, with statistically significant differences (all P<0.05). Binary Logistic regression analysis showed that fQRS was negatively correlated with LVEF (r=-0.030, OR=0.971, 95% CI 0.945-0.997, P=0.029), and positively correlated with LVESV (r=0.042, OR=1.043, 95% CI 1.005-1.082, P=0.026). Conclusion: fQRS has certain reference value in the clinical diagnosis of CHD, and left ventricular remodeling may be one of the mechanisms of fQRS.