6
Physiological evaluation of work of breathing during acute respiratory failure in children and effect of noninvasive positive ventilation
S Essouri 1 MD, Ph Durand1 MD, L Chevret1 MD, V Haas1 MD, C Perot1 MD, A Clement2 MD, PhD, D Devictor1 MD, and B Fauroux 2 MD, PhD.
Online supplement
NPPV equipment
Oesophageal (Pes) and gastric pressures (Pga) (1-4) were For infants, an adult nasal mask (Respironics Inc., Murrysville, PA) was used as a face mask. For older children, the most comfortable facial mask (type S or M, Respironics Inc., Murrysville, PA) was chosen. The mask was connected to the ventilator by an infant ventilator circuit dual heated with MR 290 autofeed chamber (Fisher Paykel, New Zealand) for patients weighting < 15 kg and by an adult ventilator circuit dual heated (Fisher Paykel, New Zealand) for patients weighting ≥ 15 kg.
Measurements
measured using a 2.1 mm external diameter catheter with two integrated pressure transducers, mounted 5 and 35 cm from the distal tip (Gaeltec, Dunvegan, Isle of Skye, UK) inserted per nasally after administration of local anaesthesia (Lidocaïne 5% Nebuliseur, Astra Zeneca, Rueil-Malmaison, France). Placement of the Pga transducer was checked by gentle manual pressure on the patient’s abdomen to observe fluctuations in Pga, which should be absent on the Pes trace. Placement of the Pes transducer was checked by the presence of a negative deflection during inspiration and an occlusion test as recommended by Baydur et al. (5).
Experimental protocol
After a minimum of 2 continuous hours of NPPV, and within the first 12 hours of NPPV, the patients were studied. In practice, all patients were recorded between the 2nd and 4th NPPV session, in semi-recumbent position. After insertion of the oeso-gastric catheter, the first period was a SB period with additional oxygen delivered by nasal prongs to achieve a SaO2 ³ 94%. All data, breathing pattern and respiratory muscle output, were recorded during a 2-min period following a 3-min period of stabilisation.
This SB period was followed by a period of NPPV with positive end expiratory pressure (PEEP) without a back-up rate. The initial NPPV settings were a PS at 6 cm H2O (defined as the inspiratory pressure delivered above the PEEP level), a PEEP at 4 cm H2O, an inspiratory trigger set at its most sensitive value (0.3-1.5 l/min), and an inspiratory flow set at the highest ramp. In case of auto-triggering, sensitivity of the inspiratory trigger was decreased by steps of 0.2 L/min. These settings were then adapted by the physician on clinical parameters such as the disappearance of retractions and the decrease of RR. Once this clinical setting was obtained, a first clinical NPPV period named NPPVClin was recorded for 2 min after a 10-min stabilisation period.
Then the NPPV settings were adjusted to obtain first, the optimal unloading of the respiratory muscles, as reflected by the normalisation (i-e 5-8 cm H2O) or the maximal decrease in Pes and transdiaphramatic pressure (Pdi) swings, and second, the best patient-ventilator synchronisation 15-17 (see online supplement) . The inspiratory trigger was set at its most sensitive value without auto-triggering or ineffective triggering (6). Auto-triggering was defined as the delivery of a positive-pressure waveform by the ventilator not preceded by a negative deflection in Pes. In case of auto-triggering, the sensitivity of the inspiratory trigger was decreased. Ineffective triggering was defined as a decrease in Pes > 1 cm H2O not followed by positive-pressure waveform (7, 8). In case of ineffective triggering, level of PS and PEEP were changed by steps of 1 cm H2O. In both situations, asynchrony events were expressed as a percentage defined by the number of auto-triggering or ineffective efforts divided by the total respiratory efforts X 100 (9).
Data analysis
Total inspiratory work of breathing (WOBtot) during spontaneous ventilation was calculated (2) and expressed per minute (J.min -1). WOBtot was partitioned into the elastic (WOBel) and resistive work of breathing (WOBres) components by a line drawn between Pes values at points of zero flow. The slope of this line indicated dynamic lung compliance (CLdyn). Total pulmonary resistance (RL) was calculated by dividing mean resistive pressure by mean inspiratory flow. Intrinsic positive end-expiratory pressure (PEEPi) was taken as the difference in Pes between the starting Pes value and the value at the point of zero flow.
The diaphragmatic (PTPdi/breath) and oesophageal pressure-time products per breath (PTPes/breath) were obtained by measuring the area under the Pdi and Pes signal between the onset of the inspiration, defined as the point at which occurred the deflection on the Pes trace, and the end of the inspiration defined as the peak of Pdi. Both PTPdi and PTPes were also expressed per minute by multiplying the pressure-time products per breath by the RR (PTPdi/min and PTPes/min) (3, 10, 11).
References
1. Fauroux, B., F. Nicot, S. Essouri, N. Hart, A. Clement, M. I. Polkey, and F. Lofaso (2004) Setting of noninvasive pressure support in young patients with cystic fibrosis. Eur Respir J 24:624-30.
2. Fauroux, B., J. Pigeot, M. I. Polkey, D. Isabey, A. Clement, and F. Lofaso (2001) In vivo physiologic comparison of two ventilators used for domiciliary ventilation in children with cystic fibrosis. Crit Care Med 29:2097-105.
3. Sassoon, C. S., R. W. Light, R. Lodia, G. C. Sieck, and C. K. Mahutte (1991) Pressure-time product during continuous positive airway pressure, pressure support ventilation, and T-piece during weaning from mechanical ventilation. Am Rev Respir Dis 143:469-75.
4. Stell, I. M., S. Tompkins, A. T. Lovell, J. C. Goldstone, and J. Moxham (1999) An in vivo comparison of a catheter mounted pressure transducer system with conventional balloon catheters. Eur Respir J 13:1158-63.
5. Baydur, A., P. K. Behrakis, W. A. Zin, M. Jaeger, and J. Milic-Emili (1982) A simple method for assessing the validity of the esophageal balloon technique. Am Rev Respir Dis 126:788-91.
6. Goulet, R., D. Hess, and R. M. Kacmarek (1997) Pressure vs flow triggering during pressure support ventilation. Chest 111:1649-53.
7. Parthasarathy, S., A. Jubran, and M. J. Tobin (1998) Cycling of inspiratory and expiratory muscle groups with the ventilator in airflow limitation. Am J Respir Crit Care Med 158:1471-8.
8. Vitacca, M., S. Nava, M. Confalonieri, L. Bianchi, R. Porta, E. Clini, and N. Ambrosino (2000) The appropriate setting of noninvasive pressure support ventilation in stable COPD patients. Chest 118:1286-93.
9. Thille, A. W., P. Rodriguez, B. Cabello, F. Lellouche, and L. Brochard (2006) Patient-ventilator asynchrony during assisted mechanical ventilation. Intensive Care Med 32:1515-22.
10. Barnard, P. A., and S. Levine (1986) Critique on application of diaphragmatic time-tension index to spontaneously breathing humans. J Appl Physiol 60:1067-72.
11. Field, S., S. Sanci, and A. Grassino (1984) Respiratory muscle oxygen consumption estimated by the diaphragm pressure-time index. J Appl Physiol 57:44-51.