Valor preditivo da razão entre espaço morto e volume corrente fisiológicos no processo de desmame da ventilação mecânica em crianças / Predictive value of the physiological deadspace/tidal volume ratio in the weaning process of mechanical ventilation in children

OBJETIVO: Avaliar a razão entre espaço morto e volume corrente fisiológicos (V D/V T) como preditor do fracasso na extubação em 42 crianças ventiladas (idade média: 4,75 anos). MÉTODO: Prontidão para extubação foi determinada usando os critérios propostos pela 6ª Conferência Internacional de Consenso em Medicina Intensiva adaptados a crianças. RESULTADOS: A ventilação não invasiva (VNI) foi usada em quatro pacientes que desenvolveram insuficiência respiratória após a extubação; nenhum foi reintubado. Crianças que precisaram de VNI para evitar a reintubação tiveram razão V D/V T significativamente maior do que as que foram extubadas sem VNI (p < 0,001). O valor de corte da razão V D/V T foi 0,55, e a área sob a curva ROC foi 0,86. CONCLUSÃO: Nossos achados confirmam o bom valor preditivo do sucesso/fracasso do desmame pela razão V D/V T e sugere seu papel como preditor da necessidade de VNI após extubação.

OBJECTIVE: To evaluate the physiological deadspace/tidal volume ratio (V D/V T) as a predictor of extubation failure in 42 ventilated children (median age: 4.75 years). METHOD: Extubation readiness was determined using the criteria proposed by the 6th International Consensus Conference on Intensive Care Medicine adapted to children. RESULTS: Non-invasive ventilation (NIV) was used in four patients who developed respiratory failure after extubation; none was reintubated. Children who needed NIV to avoid reintubation had a significantly higher V D/V T ratio than those who were extubated without NIV (p < 0.001). The cut-off value of V D/V T ratio was 0.55 and the area under the receiver operating characteristic curve was 0.86. CONCLUSION: Our findings confirm the good predictive value of weaning success/failure of the V D/V T ratio and suggest its role for predicting the need for NIV after extubation.

Predictive value of the physiological deadspace/tidal volume ratio in the weaning process of mechanical ventilation in children.

OBJECTIVE: To evaluate the physiological deadspace/tidal volume ratio (VD/VT) as a predictor of extubation failure in 42 ventilated children (median age: 4.75 years). METHOD: Extubation readiness was determined using the criteria proposed by the 6th International Consensus Conference on Intensive Care Medicine adapted to children. RESULTS: Non-invasive ventilation (NIV) was used in four patients who developed respiratory failure after extubation; none was reintubated. Children who needed NIV to avoid reintubation had a significantly higher VD/VT ratio than those who were extubated without NIV (p < 0.001). The cut-off value of VD/VT ratio was 0.55 and the area under the receiver operating characteristic curve was 0.86. CONCLUSION: Our findings confirm the good predictive value of weaning success/failure of the VD/VT ratio and suggest its role for predicting the need for NIV after extubation.

Evaluation of inhaled .NO in a model of rat neonate brain injury caused by hypoxia-ischaemia.

OBJECTIVES: Inhaled NO (INO), at 5-40 parts per million (ppm) in the air, is indicated for treating neonatal hypoxic respiratory failure. Whether these doses of INO are protective or toxic towards brain was here evaluated in laboratory animals. METHODS: In rat neonates (postnatal day 7), a brain injury based on permanent right carotid artery occlusion plus transient (90 min) respiratory hypoxia (8% O(2)) was challenged by two NO dosages (10 and 40 ppm) given either before, during or after transient hypoxia. Three weeks later, animal brains were studied for the loss of cerebral matter (infarct or atrophy). RESULTS: In right hemispheres, significant increases (26-39%) in lesion sizes were induced by 40 and not 10 ppm INO, whatever the inhalation period. The two doses reduced significantly the left hemisphere volume only when NO was inhaled at the re-oxygenation period. DISCUSSION: Our results suggest that high doses of INO, brain damaging events and inhalation at re-oxygenation might affect brain integrity when these conditions are cumulated. However, the clinical relevance of this (infarct or atrophy) and previously described (haematomas) brain toxicity associated with INO remains to be clarified in the human neonates, for instance through non-invasive cerebral imagery follow-up of patients given INO.

Effects of inertance on respiratory mechanics measurements in mechanically ventilated children.

OBJECTIVE: The use of the first-order linear single compartment model when studying respiratory mechanics classically neglects inertance (Irs). We hypothesized that Irs would affect compliance (Crs) and resistance (Rrs) estimates in mechanically ventilated young children. DESIGN: Prospective study; single-center evaluation. SETTING: University-affiliated tertiary pediatric intensive care unit. PATIENTS: Forty-four patients with and without respiratory disease. INTERVENTIONS: Patients were studied during volume-controlled constant inspiratory flow ventilation. MEASUREMENTS AND MAIN RESULTS: Pressure (PaO) and flow (V') were analyzed according to two different models: a one-compartment first-order linear model according to PaO = (1/Crs) x V + Rrs x V' and a one-compartment second-order linear model according to PaO = (1/Crs) x V + Rrs x V' + Irs x V''. Irs was higher in children with vs. those without respiratory disease (median 0.00224 cm H2O/L/sec2, Q1-Q3 0.00180-0.00321 vs. median 0.00133 cm H2O/L/sec2, Q1-Q3 0.00072-0.00210; p < .001)). A positive correlation between Irs and the difference of Crs estimates between the first- and the second-order model was found in both groups (r = .84, p < .05 and r = .67, p < .05). Rrs estimates were similar in both groups. CONCLUSIONS: This study showed that the linear single-compartment model may not adequately estimate the respiratory mechanical properties in mechanically ventilated children, particularly in the presence of respiratory disease. Including an Irs term significantly diminished Crs estimates. A one-compartment second-order linear model might be a useful clinical tool in more adequately measuring respiratory mechanics and optimizing ventilatory settings in children with respiratory disease.

Evaluation of a noninvasive cardiac output monitor in mechanically ventilated children.

OBJECTIVE: To compare measurements of cardiac output (CO) and cardiac index (CI) obtained by a recently developed noninvasive continuous cardiac output system, NICO (CONICO), and transthoracic Doppler echocardiography (COTTE) in mechanically ventilated children. DESIGN AND SETTING: Prospective study in a university-affiliated tertiary pediatric intensive care unit. PATIENTS: A total of 21 mechanically ventilated children, weighing >15 kg, in stable respiratory and hemodynamic condition. MEASUREMENTS: Sets of three successive measurements of CO with the NICO system and transthoracic Doppler echocardiography were obtained. Bland-Altman analysis was used to compare the agreement between the two methods. RESULTS: The mean +/- sd CO values were 4.06 +/- 1.43 L/min for CONICO and 4.67 +/- 1.78 L/min for COTTE. Bias +/- sd between the two methods was -0.61 +/- 0.94 L/min. The variability of the difference between the two methods increased as the magnitude of the CO measurement increased. Similar results were obtained for cardiac index: 4.01 +/- 1.40 L.min.m for CINICO and 4.59 +/- 1.48 L.min.m for CITTE. Bland-Altman analysis revealed a nonuniform relationship between CI difference and the magnitude (y = -0.299 - 0.0655 x mean). The variability of the differences did not increase as the magnitude of the CO measurement increased (sd of estimate was 0.827 L.min.m). With both CONICO and CINICO, each measurement was highly repeatable, with coefficient of variation of only 2.88% +/- 2.31%. Repeatability with Doppler echocardiography was 7.02% +/- 4.33%. CONCLUSIONS: The NICO system is a new device that measures CO easily and automatically in mechanically ventilated children weighing >15 kg. CO values obtained with this technique were in agreement with those obtained with Doppler echocardiography in children in respiratory and hemodynamic stable condition. The NICO system needs further investigation in children in unstable respiratory and hemodynamic condition.

Does taking endurance into account improve the prediction of weaning outcome in mechanically ventilated children?

INTRODUCTION: We conducted the present study to determine whether a combination of the mechanical ventilation weaning predictors proposed by the collective Task Force of the American College of Chest Physicians (TF) and weaning endurance indices enhance prediction of weaning success. METHOD: Conducted in a tertiary paediatric intensive care unit at a university hospital, this prospective study included 54 children receiving mechanical ventilation (> or = 6 hours) who underwent 57 episodes of weaning. We calculated the indices proposed by the TF (spontaneous respiratory rate, paediatric rapid shallow breathing, rapid shallow breathing occlusion pressure [ROP] and maximal inspiratory pressure during an occlusion test [Pimax]) and weaning endurance indices (pressure-time index, tension-time index obtained from P(0.1) [TTI1] and from airway pressure [TTI2]) during spontaneous breathing. Performances of each TF index and combinations of them were calculated, and the best single index and combination were identified. Weaning endurance parameters (TTI1 and TTI2) were calculated and the best index was determined using a logistic regression model. Regression coefficients were estimated using the maximum likelihood ratio (LR) method. Hosmer-Lemeshow test was used to estimate goodness-of-fit of the model. An equation was constructed to predict weaning success. Finally, we calculated the performances of combinations of best TF indices and best endurance index. RESULTS: The best single TF index was ROP, the best TF combination was represented by the expression (0.66 x ROP) + (0.34 x Pimax), and the best endurance index was the TTI2, although their performance was poor. The best model resulting from the combination of these indices was defined by the following expression: (0.6 x ROP) - (0.1 x Pimax) + (0.5 x TTI2). This integrated index was a good weaning predictor (P < 0.01), with a LR+ of 6.4 and LR+/LR- ratio of 12.5. However, at a threshold value < 1.3 it was only predictive of weaning success (LR- = 0.5). CONCLUSION: The proposed combined index, incorporating endurance, was of modest value in predicting weaning outcome. This is the first report of the value of endurance parameters in predicting weaning success in children. Currently, clinical judgement associated with spontaneous breathing trials apparently remain superior.

Development of a new model to investigate the fetal nociceptive pathways.

OBJECTIVE: The aim of the study was to develop an experimental model to investigate the fetal nociceptive pathways and fetal analgesia. METHODS: We tested the electromyographic (EMG) response from the biceps femoris to electrical stimulation of the sural nerve in chronically-prepared fetal lambs with and without sufentanil. RESULTS: An EMG response could be recorded 140 ms after the electrical stimulation above a threshold of current's intensity. The response presents the characteristics of a nociceptive flexion reflex. The reflex magnitude increased with the stimulus intensity. Sufentanil decreased the response. Bradycardia was noted 10 s after the stimulation and was not observed after sufentanilinfusion. Catecholamine concentrations were not altered by the stimulation. CONCLUSION: Our study shows that a nociceptive flexion reflex can be recorded in the ovine fetus. We suggest that this reflex can be used as a new tool to study the ontogenesis of the nociceptive pathways and the effects of analgesic drugs during fetal life.

Influence of respiratory system impedance on volume and pressure delivered at the Y piece in ventilated infants.

OBJECTIVES: Tidal volume (VT) delivered to infants' airways are overestimated and pressure underestimated when measured in the ventilator and not at the Y piece. This study aimed at evaluating the influence of respiratory system impedance on expiratory VT (VTE) and pressure measurement difference. DESIGN: Prospective observational study. SETTING: Pediatric intensive care unit at a university hospital. PATIENTS: Data were collected between February 2000 and October 2001 for 30 infants (range, 1-23 months) ventilated in the pressure-controlled or volume-controlled mode. INTERVENTIONS: Measurements of VTE, pressure obtained at the same time at the Y piece and on the ventilator Servo 300, were collected in ventilated infants. Respiratory system impedance was calculated from data obtained at the Y piece. Circuit compliance was measured in vitro. VTEs were corrected for compressible volume. MEASUREMENTS AND RESULTS: VTEs were overestimated by the Servo 300 in the pressure-controlled and volume-controlled modes (from 5% to 62% of the value displayed on Servo 300). Maximal inspiratory pressures were underestimated by the Servo 300 in the pressure-controlled mode (difference from -2 to +19 cm H(2)O). Measurement difference increased with increasing respiratory system impedance. Ventilator VTE corrected for circuit compliance did not offer a sufficiently accurate estimation of VTE at the Y piece. CONCLUSIONS: VT and pressure measurements must be performed at the Y piece, especially in infants with increased respiratory system impedance (i.e., decreased respiratory system compliance or increased resistance). Correcting VTE for circuit compliance cannot replace measurement of VT at the Y piece.

Role of the alpha2-adrenoceptors on the pulmonary circulation in the ovine fetus.

Recent in vitro studies reported that nitric oxide release and pulmonary vasorelaxation can be mediated by endothelial alpha2-adrenoceptor activation. As norepinephrine (alpha1-,alpha2-, and beta1-adrenoceptor agonist) was found to induce pulmonary vasodilation in the ovine fetus, we hypothesized that alpha2-adrenoceptors may modulate basal pulmonary vascular tone and mediate the vascular effect of norepinephrine during fetal life. To determine the role of alpha2-adrenoceptors and the mechanisms of norepinephrine-mediated vasodilation in the fetal pulmonary circulation, we tested, in chronically prepared late-gestation fetal lambs, the hemodynamic response to 1). yohimbine (alpha2 antagonist); 2). UK 14304 (alpha2 agonist) with and without l-nitro-arginine (nitric oxide synthase inhibitor); and 3). norepinephrine infusion with and without yohimbine. We found that yohimbine increased mean pulmonary artery pressure by 15% (p < 0.05), decreased pulmonary flow by 22% (p < 0.01), and increased pulmonary vascular resistance by 51% (p < 0.01). UK 14304 increased pulmonary flow by 145% (p < 0.01) and decreased pulmonary vascular resistance by 58% (p < 0.01). l-Nitro-arginine abolished the UK 14304-mediated pulmonary vasodilation. Norepinephrine (0.5 microg x kg(-1)x min(-1) increased both pulmonary flow by 61% (p < 0.01) and pulmonary arterial pressure by 13% (p < 0.01) and decreased pulmonary vascular resistance by 33% (p < 0.01). Yohimbine abolished the norepinephrine-induced pulmonary vasodilation. This study suggests that 1). a basal alpha2-adrenoceptor activation-induced pulmonary vasodilation exists during fetal life; 2). the pulmonary vascular effects of alpha2-adrenoceptor activation are related at least in part to nitric oxide production; and 3). the norepinephrine-mediated pulmonary vasodilation involves alpha2-adrenoceptor activation. As a surge of norepinephrine exists at birth, we speculate that norepinephrine and endothelial alpha2-adrenoceptor activation may play a significant role in pulmonary vasodilation at birth.

Alveolar capillary dysplasia: a cause of persistent pulmonary hypertension of the newborn.

UNLABELLED: The term alveolar capillary dysplasia refers to complex vascular abnormalities which have recently been identified in some infants with persistent pulmonary hypertension. We report four cases admitted to our institution for severe pulmonary hypertension unresponsive to maximal cardiorespiratory support, including high-frequency ventilation, inhaled nitric oxide and extracorporeal membrane oxygenation. The four infants died of refractory hypoxaemia. The diagnosis of alveolar capillary dysplasia was established by necropsy. We have used these cases as an opportunity for a thorough review of the literature containing comments regarding aetiology, pathophysiology, clinical presentation, associated malformations and treatment trials. CONCLUSION: alveolar capillary dysplasia should be ruled out in all newborn infants presenting severe idiopathic pulmonary hypertension associated with malformations. Open lung biopsy may prevent from using costly, invasive and probably ineffective procedures such as extracorporeal membrane oxygenation.

Effect of increasing inspiratory time on respiratory mechanics in mechanically ventilated neonates.

OBJECTIVE: To investigate the effect of inspiratory time and inspiratory flow on the respiratory mechanics of intubated and ventilated neonates. DESIGN: Physiology study. SETTING: Tertiary university neonatal intensive care unit. PATIENTS: Neonates requiring mechanical ventilation with (group 1, n = 9) and without lung disease (group 2, n = 6). INTERVENTIONS: All infants were ventilated with a Servo 900C Siemens ventilator in the volume-controlled constant-flow mode. Flow and pressure were measured at the Y-piece, while different inspiratory times (25%, 33%, 50%, and 67% of the respiratory cycle) were applied randomly without changing tidal volume. MEASUREMENTS: The constant flow end-inspiratory airway occlusion technique allowed partitioning of the total respiratory system resistance (R(tot,rs)) into a standard intrinsic flow resistance (R(int,rs)) and a lung/thorax tissue viscoelastic component (DeltaR(rs)), and it allowed partitioning of the dynamic respiratory system elastance (E(dyn,rs)) into a static (E(st,rs)) and a lung/thorax tissue viscoelastic component (DeltaE(rs)). A two-compartment model of the respiratory system was applied to the experimental data. MAIN RESULTS: All respiratory mechanics components were significantly higher in group 1 compared with group 2. Both groups showed increasing R(int,rs) with increasing flow and increasing DeltaR(rs) with increasing inspiratory time. DeltaR(rs) represented 40% to 75% of R(tot,rs) whatever the group. E(dyn,rs) and E(st,rs) changed with inspiratory time in the very low (<0.4 secs) and the very long inspiratory time range (>1.0 secs). No change was found when clinically, commonly used inspiratory times were applied (0.4-1.0 secs). DeltaE(rs) represented 17% to 19% of E(dyn,rs). The relationship between DeltaR(rs) and increasing inspiratory time fitted the exponential two-compartment model (r =.99, p <.001). CONCLUSIONS: Total respiratory mechanics and its components in ventilated newborns with and without lung disease showed inspiratory time dependence. DeltaR(rs) increased with increasing inspiratory time as predicted by the two-compartment lung model, whereas standard R(int,rs) and E(dyn,rs) decreased.