BACKGROUNDThe functional residual capacity (FRC), the only lung volume to be routinely measured in infants, is an unreliable volume landmark. In addition to FRC, the residual volume (RV) was measured by nitrogen washout using rapid thoracoabdominal compression (RTC) in nine infants with cystic fibrosis aged 5-31 months.
METHODSA commercial system for nitrogen washout to measure lung volumes and a custom made system to perform RTC were used. Lung volume was raised to an airway opening pressure of 30 cm H₂O (V₃₀). RTC was performed from V₃₀. The jacket pressure (Pj; 65-92 cm H₂O) which generated the highest forced expiratory volume (mean 40.2 ml/kg; 95% confidence interval (CI) 33.03 to 47.33) was used during the RV manoeuvre. The infants were manually hyperventilated to inhibit the respiratory drive briefly. RTC was initiated during the last passive expiration. RV was estimated by measuring the volume of nitrogen expired after end forced expiratory switching of the inspired gas from room air to 100% oxygen while jacket inflation was maintained at the time of switching into oxygen during the post-expiratory pause.
RESULTSIn each infant RV and FRC measurements were reproducible and did not overlap; the difference between mean values, which is the expiratory reserve volume, was statistically significant (p<0.05). Mean RV was 21.3 (95% CI 18.7 to 24.0), FRC was 25.5 (95% CI 22.8 to 28.1), and TLC₃₀ (total lung capacity at V₃₀) was 61.5 (95% CI 54.4 to 68.7) ml/kg. These values were dependent on body length, weight and age. When measuring RV the period between switching to oxygen and the end of the Pj plateau was 0.301 (95% CI 0.211 to 0.391) s. The washout duration was longer for RV than for FRC measurement (80.9 s (95% CI 71.3 to 90.4) versus 72.4 s (95% CI 64.9 to 79.8)) (p<0.001).
CONCLUSIONSA new non-invasive and reliable technique for routine measurement of RV in infants is presented.


Keywords: lung function tests; cystic fibrosis; lung volume; infants; nitrogen washout