In vitro estimation of pressure drop across tracheal tubes during high-frequency percussive ventilation.

Tracheal tubes (TT) are used in clinical practice to connect an artificial ventilator to the patient's airways. It is important to know the pressure used to overcome tube impedance to avoid lung injury. Although high-frequency percussive ventilation (HFPV) has been increasingly used, the mechanical behavior of TT under HFPV has not yet been described. Thus, we aimed at characterizing in vitro the pressure drop across TT (ΔPTT) by identifying the model that best fits the measured pressure-flow (P-V̇) relationships during HFPV under different working pressures (PWork), percussive frequencies and mechanical loads. Three simple models relating ΔPTT and flow (V̇) were tested. Model 1 is characterized by linear resistive [Rtube ⋅ V̇(t)] and inertial [I · V̈(t)] terms. Model 2 takes into consideration Rohrer's approach [K1· V̇(t) + K2 ⋅V̇(t)] and inertance [I ·V̈(t)]. In model 3 the pressure drop caused by friction is represented by the non-linear Blasius component [Kb· V̇(1.75)(t)] and the inertial term [I· V̈(t)]. Model 1 presented a significantly higher root mean square error of approximation than models 2 and 3, which were similar. Thus, model 1 was not as accurate as the latter, possibly due to turbulence. Model 3 presented the most robust resistance-related coefficient. Estimated inertances did not vary among the models using the same tube. In conclusion, in HFPV ΔPTT can be easily calculated by the physician using model 3.

Interpretación de las curvas del respirador en pacientes con insuficiencia respiratoria aguda / Interpretation of ventilator curves in patients with acute respiratory failure

La ventilación mecánica es una intervención terapéutica de sustitución temporal de la función ventilatoria enfocada a mejorar los síntomas en los pacientes que sufren insuficiencia respiratoria aguda. Los avances tecnológicos han facilitado el desarrollo de ventiladores sofisticados que permiten visualizar y registrar las ondas respiratorias, lo que constituye una fuente de información muy valiosa para el clínico. La correcta interpretación de los trazados es de vital importancia tanto para el correcto diagnóstico como para la detección precoz de anomalías y para comprender aspectos de la fisiología relacionados con la ventilación mecánica y con la interacción paciente-ventilador. El presente trabajo da una orientación de cómo interpretar las curvas del ventilador mediante el análisis de trazados de presión en la vía aérea, flujo aéreo y volumen en distintas situaciones clínicas (AU)

Mechanical ventilation is a therapeutic intervention involving the temporary replacement of ventilatory function with the purpose of improving symptoms inpatients with acute respiratory failure. Technological advances have facilitated the development of sophisticated ventilators for viewing and recording the respiratory waveforms, which are a valuable source of information for the clinician. The correct interpretation of these curves is crucial for the correct diagnosis and early detection of anomalies, and for understanding physiological aspects related to mechanical ventilation and patient-ventilator interaction. The present study offers a guide for the interpretation of the airway pressure and flow and volume curves of the ventilator, through the analysis of different clinical scenarios (AU)

[Interpretation of ventilator curves in patients with acute respiratory failure]. / Interpretación de las curvas del respirador en pacientes con insuficiencia respiratoria aguda.

Mechanical ventilation is a therapeutic intervention involving the temporary replacement of ventilatory function with the purpose of improving symptoms in patients with acute respiratory failure. Technological advances have facilitated the development of sophisticated ventilators for viewing and recording the respiratory waveforms, which are a valuable source of information for the clinician. The correct interpretation of these curves is crucial for the correct diagnosis and early detection of anomalies, and for understanding physiological aspects related to mechanical ventilation and patient-ventilator interaction. The present study offers a guide for the interpretation of the airway pressure and flow and volume curves of the ventilator, through the analysis of different clinical scenarios.

In vitro measurements of respiratory mechanics during HFPV using a mechanical lung model.

High-frequency percussive ventilation (HFPV) may be defined as flow-regulated time-cycled ventilation that creates controlled pressure and delivers a series of high-frequency subtidal volumes in combination with low-frequency breathing cycles. In recent years, the usefulness of HFPV has been clinically assessed as an alternative to conventional mechanical ventilation. In the clinical practice, HFPV is not an intuitive ventilatory modality and the absence of real-time delivered volume monitoring produces disaffection among the physicians. For this purpose, it would be useful to develop a monitor able to realize a complete online characterization of high-frequency percussive ventilators and to identify the best combination of their parameters according to the specific pathological situation. This paper describes an innovative acquisition and elaboration system based on the use of new generation pressure transducers presenting high sensitivity and fast response. Such a system is compact and inexpensive, and it allows the user to carry out a more correct online characterization of high-frequency percussive ventilators. This output allowed best real-time ventilatory setting, minimizing the potential baro-volutrauma hazard.

The role of admission surveillance cultures in patients requiring prolonged mechanical ventilation in the intensive care unit.

We undertook a prospective observational cohort study in intensive care unit (ICU) patients requiring mechanical ventilation for four days or more to evaluate normal and abnormal bacterial carriage on admission detected by surveillance cultures of throat and rectum. We assessed the importance of surveillance and diagnostic cultures for the early detection of resistance to third generation cephalosporins employed as the parenteral component of the selective decontamination of the digestive tract. Finally, we sought the risk factors of abnormal carriage on admission to the ICU. During the 58-month study 621 patients were included: 186 patients (30%) carried abnormal flora including methicillin-resistant Staphylococcus aureus (MRSA) and aerobic Gram negative bacilli (AGNB) on admission to the ICU Both MRSA and AGNB carriers were more commonly present in the hospital group of patients than in patients referred from the community (P < 0.001), although overgrowth was equally present both in community and in hospital patients. The incidence of infections during ICU stay was higher in abnormal (n=120, 64.5%) than in normal carriers (n=185, 42.5%) (P < 0.0001), with an odds ratio of 2.46 (95% confidence interval 1.72 to 3.51). Third generation cephalosporins covered ICU admission flora in 482 (78%) of the studied population. AGNB resistant to cephalosporins and MRSA were detected in surveillance cultures of 139 patients (22%), while the same resistant micro-organisms were identified only in 49 diagnostic samples (7.9%). Parenteral cephalosporins were modified in patients with abnormal flora (P < 0.0001). One hundred and ninety-six patients received antibiotics before admission to the ICU and 42% carried AGNB resistant to cephalosporins. Previous antibiotic use was the only risk factor for abnormal carriage in the multivariate analysis (OR 3.5; 95% confidence interval 2.1 to 5.8). The knowledge of carriage on admission using surveillance cultures may help intensivists to identify patients with abnormal carriage on admission and resistant bacterial strains at an early stage even when diagnostic samples are negative. Third generation cephalosporins covered admission flora in about 80% of the enrolled population and were modified in patients with abnormal flora who received antibiotic therapy before ICU admission. Our finding of overgrowth present on admission may justify the immediate administration of enteral antimicrobials.

Prediction of successful defibrillation in human victims of out-of-hospital cardiac arrest: a retrospective electrocardiographic analysis.

In the present study we sought to examine the efficacy of an electrocardiographic parameter, 'amplitude spectrum area' (AMSA), to predict the likelihood that any one electrical shock would restore a perfusing rhythm during cardiopulmonary resuscitation in human victims of out-of-hospital cardiac arrest. AMSA analysis is not invalidated by artefacts produced by chest compression and thus it can be performed during CPR, avoiding detrimental interruptions of chest compression and ventilation. We hypothesised that a threshold value of AMSA could be identified as an indicator of successful defibrillation in human victims of cardiac arrest. Analysis was performed on a database of electrocardiographic records, representing lead 2 equivalent recordings from automated external defibrillators including 210 defibrillation attempts from 90 victims of out-of-hospital cardiac arrest. A 4.1 second interval of ventricular fibrillation or ventricular tachycardia, recorded immediately preceding the delivery of the shock, was analysed using the AMSA algorithm. AMSA represents a numerical value based on the sum of the magnitude of the weighted frequency spectrum between two and 48 Hz. AMSA values were significantly greater in successful defibrillation (restoration of a perfusing rhythm), compared to unsuccessful defibrillation (P < 0.0001). An AMSA value of 12 mV-Hz was able to predict the success of each defibrillation attempt with a sensitivity of 0.91 and a specificity of 0.97. In conclusion, AMSA analysis represents a clinically applicable method, which provides a real-time prediction of the success of defibrillation attempts. AMSA may minimise the delivery of futile and detrimental electrical shocks, reducing thereby post-resuscitation myocardial injury.

Volumetric capnography in the mechanically ventilated patient.

Expiratory capnogram provides qualitative information on the waveform patterns associated with mechanical ventilation and quantitative estimation of expired CO2. Volumetric capnography simultaneously measures expired CO2 and tidal volume and allows identification of CO2 from 3 sequential lung compartments: apparatus and anatomic dead space, from progressive emptying of alveoli and alveolar gas. Lung heterogeneity creates regional differences in CO2 concentration and sequential emptying contributes to the rise of the alveolar plateau and to the steeper the expired CO2 slope. The concept of dead space accounts for those lung areas that are ventilated but not perfused. In patients with sudden pulmonary vascular occlusion due to pulmonary embolism, the resultant high V/Q mismatch produces an increase in alveolar dead space. Calculations derived from volumetric capnography are useful to suspect pulmonary embolism at the bedside. Alveolar dead space is large in acute lung injury and when the effect of positive end-expiratory pressure (PEEP) is to recruit collapsed lung units resulting in an improvement of oxygenation, alveolar dead space may decrease, whereas PEEP-induced overdistension tends to increase alveolar dead space. Finally, measurement of physiologic dead space and alveolar ejection volume at admission or the trend during the first 48 hours of mechanical ventilation might provide useful information on outcome of critically ill patients with acute lung injury or acute respiratory distress syndrome.

High-frequency percussive ventilation during surgical bronchial repair in a patient with one lung.

We report the case of a patient that had undergone a left pneumonectomy during which a double-lumen tube was used and an undetected right bronchial laceration occurred. After diagnosis the patient underwent a second operation to repair the tear. The role of high-frequency percussive ventilation in enabling adequate gas exchange during the bronchial repair is described and discussed.

Mechanical loads modulate tidal volume and lung washout during high-frequency percussive ventilation.

High-frequency percussive ventilation (HFPV) has been proved useful in patients with acute respiratory distress syndrome. However, its physiological mechanisms are still poorly understood. The aim of this work is to evaluate the effects of mechanical loading on the tidal volume and lung washout during HFPV. For this purpose a single-compartment mechanical lung simulator, which allows the combination of three elastic and four resistive loads (E and R, respectively), underwent HFPV with constant ventilator settings. With increasing E and decreasing R the tidal volume/cumulative oscillated gas volume ratio fell, while the duration of end-inspiratory plateau/inspiratory time increased. Indeed, an inverse linear relationship was found between these two ratios. Peak and mean pressure in the model decreased linearly with increasing pulsatile volume, the latter to a lesser extent. In conclusion, elastic or resistive loading modulates the mechanical characteristics of the HFPV device but in such a way that washout volume and time allowed for diffusive ventilation vary agonistically.

Sepsis and organ dysfunction: an ongoing challenge.

In recent years the problem of infection has become increasingly significant, especially in intensive care hospital wards such as Intensive Care Units (ICU), emergency medicine, surgery and critically ill patient care departments. Sepsis is a complex, multifactorial syndrome that can develop into conditions of different severity, described as severe sepsis or septic shock. In these conditions the triggering event may coincide with the functional impairment of one or more vital organs or systems, thus leading to poorer prognosis in patients with overt signs of sepsis or systemic inflammation syndromes. The available data are quite alarming, as most prevention and treatment is performed empirically and requires considerable human and technological resources. Clinical signs are often misleading and, in some circumstances, it may be difficult or even impossible to identify the source of the infection which might otherwise be removed relatively simply, using proper antimicrobial treatment or a less invasive surgical removal of the area from which the infection originates based on needle-guided radiology. In addition, the complex pathophysiological mechanisms involved can be an obstacle to gaining a full understanding of the various biohumoral interactions or mediators action mechanisms. It may not be easy to enroll patients belonging to homogeneous groups in terms of age, underlining disease, immune profile or genetic predisposition, although the use of specific severity indexes has proved helpful also to establish the prognosis. Although the interpretation of generalised inflammation as a warning sign also in the absence of clear signs of infection or a state of overt inflammation has to rely largely on simple intuition, it has helped to drive experimental and clinical research work towards the investigation of interaction between different factors such as infection and sepsis, or inflammation and coagulation. An additional useful tool is the possibility of modulating the endothelial response which may support the process of disseminated thrombosis typical of sepsis evolution. In this context the improvement of standards of care can shed light on the efficacy of different treatments.

Systemic and organ dysfunction response during infusion of recombinant human activated protein C (rhAPC) in severe sepsis and septic shock.

AIM: The aim of this study was the assessment of the efficacy of recombinant human activated protein C (rhAPC) in septic patients. METHODS: A continuous observational prospective study on ICU patients with severe sepsis and septic shock was carried out. Applying the inclusion criteria of a national trial on the use of rhAPC, 15 patients (12 males and 3 females) were enrolled, mean age was 65.9 (SD 9.6), APACHE II score was > or =25. The following variables were assessed on 7 time-points (T1-T7): overall SOFA score; organ-specific SOFA score; APACHE II score; PCR, APTT, INR, fibrinogen, platelet count. Wilcoxon's statistical test and Spearman's correlation test (rho coefficient) between the SOFA and APACHE II scores were used. Test results with a P value below 0.05 were deemed significant. RESULTS: A significant correlation was identified between the APACHE II and SOFA scores. No significant change was found in Friedman's test and the respiratory, haematological and hepatic SOFA score, whereas cardiovascular, renal and neurological SOFA scores showed a significant trend between the ranks at the 7 time-points (chi2=14; df=6; P=0.029). During rhAPC treatment Friedman's test showed significant changes of PCR values over the 7 time-points (chi2=19.2; df=6; P=0.02). Wilcoxon's test indicated a significant decrease in the values recorded during the T2-T6 period. On day 28, 12 of the 15 patients originally enrolled were still alive. Mortality rate was therefore 20% (CI 95%). CONCLUSIONS: RhAPC is the first biological agent approved for the treatment of severe sepsis and septic shock. Our experience is confined to patients with severe sepsis and septic shock, and some severity indexes showed a modulation of the inflammatory processes and haemostatic balance, 2 factors which play a key role in the evolution of sepsis and organ dysfunction.

Total intravenous anaesthesia in endoscopic sinus-nasal surgery.

Aim of this randomized study (64 patients) was to improve the control of bleeding during functional endoscopic sinusal surgery by means of controlled hypotension achieved through either total intravenous anaesthesia using remifentanyl and propofol (27 patients), or inhaled using isoflurane and fentanyl (37 patients). The following parameters were monitored before administration of anaesthesia (T0), then after 15 (T1), and 30 minutes (T2): systolic, diastolic, and mean arterial pressure; heart rate; concentration of tele-exhaled carbon dioxide (PetCO2) and percentage of peripheral saturation of haemoglobin (SPO2); bleeding according to the Fromme-Boezaart scale at T2. Mean arterial pressure values were maintained between 60-70 mmHg throughout surgery. At T0, systolic arterial pressure, diastolic arterial pressure and mean arterial pressure values were seen to overlap in the two groups. Both types of anaesthesia were effective in reducing the pressure values of T0-T1 and T1-T2 trends (p<0.0001). Systolic arterial pressure at T1 is lower with total intravenous anaesthesia compared to isoflurane and fentanyl (p=0.02). PetCO2 and heart rate show a decreasing trend independently of the type of anaesthesia employed. In conclusion, the hypotensive effect of total intravenous anaesthesia and of isoflurane and fentanyl is equivalent, but only total intravenous anaesthesia is effective in reducing bleeding during functional endoscopic sinusal surgery.

Effects of mechanical load on flow, volume and pressure delivered by high-frequency percussive ventilation.

High-frequency percussive ventilation (HFPV) has proved its unique efficacy in the treatment of acute respiratory distress, when conventional mechanical ventilation (CMV) has demonstrated a limited response. We analysed flow (V(dot)), volume (V) and airway pressure (Paw) during ventilation of a single-compartment mechanical lung simulator, in which resistance (R) and elastance (E) values were modified, while maintaining the selected ventilatory settings of the HFPV device. These signals reveal the physical effect of the imposed loads on the output of the ventilatory device, secondary to constant (millisecond by millisecond) alterations in pulmonary dynamics. V(dot), V and Paw values depended fundamentally on the value of R, but their shapes were modified by R and E. Although peak Paw increased 70.3% in relation to control value, mean Paw augmented solely 36.5% under the same circumstances (maximum of 9.4 cm H2O). Finally, a mechanism for washing gas out of the lung was suggested.

High frequency percussive ventilation (HFPV). Principles and technique.

In recent years, the usefulness of high frequency ventilation (HFV) has been clinically reassessed as an alternative to conventional mechanical ventilation (CMV). HFV has often been combined with or in some cases even completely replaced CMV in the attempt to reduce iatrogenic injury. High frequency percussive ventilation (HFPV) is a specific mode of HFV that has been successfully applied in the treatment of acute respiratory failure after smoke inhalation; it has also been more widely used in pediatric than in adult patients. This article gives an introduction to and a description of the basic principles of HFPV, a mode of ventilation which we found particularly versatile and reliable in our preliminary clinical experience with the maneuver.

High frequency percussive ventilation (HFPV). Case reports.

Treatment of acute respiratory failure is still a hot issue in intensive care everyday practice: in the last few years high frequency ventilation techniques have been employed as a therapy for adult respiratory distress syndrome (ARDS) and acute respiratory failure (ARF). We applied high frequency percussive ventilation (HFPV) to 3 patients affected by ARDS or ARF, who did not improve after 24 hours of conventional mechanical ventilation (CMV). All our patient underwent 12 hours of HFPV, and showed an improvement of both respiratory exchange and radiological imaging. Even if the pathogenesis of ARF was quite different, in all patient we registered a good response and no complications.

Noninvasive positive pressure ventilation in trauma patients with acute respiratory failure.

The effectiveness of noninvasive pressure support ventilation (NIPSV) in treating trauma patients with acute respiratory failure (ARF) was evaluated in a retrospective clinical study. Forty-six conscious patients with ARF admitted to the general intensive care units (ICUs) of three hospitals between July 1988 and July 1991 were surveyed. Patients received NIPSV after a period of spontaneous breathing with supplemental oxygen. Blood gas levels and respiratory parameters were measured before the application of the mask and after 1, 6 and 12 h of NIPSV. Thirty-three (72%) patients were successfully weaned to spontaneous breathing (success group). Nine patients with hypercapnia and four with hypoxaemic respiratory failure failed to respond to prolonged mask ventilation and were intubated (failure group). Of the 13 patients who failed NIPSV, nine died after switching to invasive ventilation after a mean time of 10 +/- 3 days. No deaths occurred during NIPSV. A mean pressure support ventilation (PSV) of 11.7 +/- 4.2 cmH2O and positive end-expiratory pressure (PEEP) of 4.5 +/- 2.7 cmH2O were required to significantly increase arterial oxygen tension (Pa,O2)/inspiratory oxygen fraction (Fi,O2) from 152.4 +/- 41.7 (spontaneous breathing) to 277.9 +/- 108.7 (NIPSV) (p < 0.01) within the first hour. The expiratory tidal volume (VT) increased from 356.1 +/- 103.7 (spontaneous breathing) to 648.1 +/- 77.1 mL (NIPSV) (p < 0.01) with a concomitant reduction in the respiratory frequency (fR) from 31.4 +/- 5.2 (spontaneous breathing) to 20.4 +/- 4.3 (NIPSV) without significant differences between the success and failure group. In the 22 patients who were hypercapnic at the point of entering the study, the arterial carbon dioxide tension (Pa,CO2) decreased from 73.0 +/- 1.0 kPa (52.5 +/- 7.8 mmHg) (spontaneous breathing) to 5.5 +/- 1.0 kPa (41.5 +/- 7.5 mmHg) (NIPSV) (p < 0.01) and pH increased from 7.29 +/- 0.05 to 7.33 +/- 0.04 (p < 0.05). The median length of time of use of NIPSV was 55.5 h (range 6-144). In conclusion, noninvasive pressure support ventilation might effectively be used in a selected group of trauma patients as a means of treating respiratory failure.

Volumetric capnography in patients with acute lung injury: effects of positive end-expiratory pressure.

The aim of the study was to analyse the effects of positive end-expiratory pressure (PEEP) on volumetric capnography and respiratory system mechanics in mechanically ventilated patients. Eight normal subjects (control group), nine patients with moderate acute lung injury (ALI group) and eight patients with acute respiratory distress syndrome (ARDS group) were studied. Respiratory system mechanics, alveolar ejection volume as a fraction of tidal volume (VAE/VT), phase III slopes of expired CO2 beyond VAE and Bohr's dead space (VD/VT(Bohr)) at different levels of PEEP were measured. No differences in respiratory system resistances were found between the ALI and ARDS groups. VD/VT(Bohr) and expired CO2 slope beyond VAE were higher in ALI patients (0.52+/-0.01 and 13.9+/-0.7 mmHg x L(-1), respectively) compared with control patients (0.46+/-0.01 and 7.7+/-0.4 mmHg x L(-1), p<0.01, respectively) and in ARDS patients (0.61+/-0.02 and 24.9+/-1.6 mmHg x L(-1), p<0.01, respectively) compared with ALI patients. VAE/VT differed similarly (0.6+/-0.01 in control group, 0.43+/-0.01 in ALI group and 0.31+/-0.01 in ARDS group, p<0.01). PEEP had no effect on VAE/VT, expired CO2 slope beyond VAE and VD/VT(Bohr) in any group. A significant correlation (p<0.01) was found between VAE/VT and expired CO2 slope beyond VAE and lung injury score at zero PEEP. Indices of volumetric capnography are affected by the severity of the lung injury, but are unmodified by the application of positive end-expiratory pressure.

Single-breath method for assessing the viscoelastic properties of the respiratory system.

In order to explain the time dependency of resistance and elastance of the respiratory system, a linear viscoelastic model (Maxwell body) has been proposed. In this model the maximal viscoelastic pressure (Pvisc.max) developed within the tissues of the lung and chest wall at the end of a constant-flow (V') inflation of a given time (tI) is given by: Pvisc,max = R2V'(1-e(-tI/tau2), where R2 and tau2 are, respectively, the resistance and time constant of the Maxwell body. After rapid airway occlusion at t1, tracheal pressure (Ptr) decays according to the following function: Ptr(t) = Pvisc(t) + Prs,st = Pvisc,max(etocc/tau2)+ Prs,st, where tocc/is time after occlusion and Prs,st is static re-coil pressure of the respiratory system. By fitting Ptr after occlusion to this equation, tau2 and Pvisc,max are obtained. Using these values, together with the V' and tI pertaining to the constant-flow inflation preceding the occlusion, R2 can be calculated from the former equation. Thus, from a single breath, the constants tau2, R2 and E2 (R2/tau2) can be obtained. This method was used in 10 normal anaesthetized, paralysed, mechanically ventilated subjects and six patients with acute lung injury. The results were reproducible in repeated tests and similar to those obtained from the same subjects and patients with the time-consuming isoflow, multiple-breath method described previously.