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 extremely helpful for the controlled adjustment of mechanical ventilation in sick neonates and infants. However, these patients have small lung volumes and usually have been intubated by uncuffed endotracheal tubes (ETT). Therefore, the open-circuit nitrogen washout technique (N2wo) may give false FRC values if the inspired oxygen concentration (F) is high and leakage around the ETT is present. We evaluated the N2wo as supplied by the Pediatric Pulmonary System 2600 (Sensor-Medics) in a small-sized lung model by 570 measurements using five different ventilator settings, an F increasing up to 0.9, different bypass flows between 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, volumes as small as 14 mL could be measured using an F as high as 0.9 with only slight loss of accuracy (error, 4%; CV, 2,8%). During leakage, FRC had been underestimated with a very strong correlation to the total amount of leakage over the measurement period, which was irrespective of the ventilatory parameters (r = 0.9, P < 0.001). The regression equation could, therefore, be used for FRC correction in the lung model. However, most of the miscalculation was due to N, loss during expiratory leakage, which quite simply and reliably can be excluded by an end-inspiratory occlusion test. In conclusion, we evaluated an N2wo system with a lung model and found it suitable to measure small lung volumes as found in preterm babies, even if they were ventilated with high F. In the presence of leakage, exclusion of leakage by an end-inspiratory occlusion was found t i be the most practical way of obtaining reliable FRC-values. Pediatr Pulmonol. 1994; 17:106–112. © 1994 Wiley-Liss, Inc.