EFFECT OF ENDOTRACHEAL-TUBE LEAKAGE ON FUNCTIONAL RESIDUAL CAPACITY DETERMINATION BY NITROGEN WASHOUT METHOD IN A SMALL-SIZED LUNG MODEL

The determination of functional residual capacity (FRC) would be extre mely helpful for the controlled adjustment of mechanical ventilation i n sick neonates and infants. However, these patients have small lung v olumes and usually have been intubated by uncuffed endotracheal tubes (ETT). Therefore, the open-circuit nitrogen washout technique (N-2wo m ay give false FRC values if the inspired oxygen concentration (F-IO2 i s high and leakage around the ETT is present. We evaluated the N-2wo a s supplied by the pediatric pulmonary System 2600 (SensorMedics) in a small-sized lung model by 570 measurements using five different ventil ator settings, an F-iO2 increasing up to 0.9, different bypass flows b etween 0 and 12 L/min, and various patterns of leakage, either during inspiration or exhalation, or both. We found the most reliable results (error, 0.6%; CV, 0.7%) with a bypass flow of 6 L/min. Absolute N-2 v olumes as small as 14 mt could be measured using an F-IO2 as high as 0 .9 with only slight loss of accuracy (error, 4%; CV, 2,8%). During lea kage, FRC had been underestimated with a very strong correlation to th e total amount of leakage over the measurement period, which was irres pective of the ventilatory parameters (r = 0.9, P < 0.001). The regres sion equation could, therefore, be used for FRC correction in the lung model. However, most of the miscalculation was due to N-2 loss during expiratory leakage, which quite simply and reliably can be excluded b y an end-inspiratory occlusion test. In conclusion, we evaluated an N- 2wo system with a lung model and found it suitable to measure small lu ng volumes as found in preterm babies, even if they were ventilated wi th high F-IO2. In the presence of leakage, exclusion of leakage by an end-inspiratory occlusion was found be the most practical way of obtai ning reliable FRC-values. (C) 1994 Wiley-Liss, Inc.