An alternative method to measure the diffusing capacity of the lung for carbon monoxide in infants

Lung diffusion assessed by the uptake of carbon monoxide (DL_CO ) and alveolar volume (V_A ) by inert gas dilution are readily assessed in cooperative older subjects; however, obtaining these measurements in infants has been much more difficult. Our laboratory has measured DL_CO and V_A in sleeping infants using a mass spectrometer, which continuously measures gas concentrations, and demonstrated that infants with bronchopulmonary dysplasia (BPD) have lower DL_CO , but no difference in V_A compared to full-term controls. The mass spectrometer is expensive and lacks portability; therefore, we evaluated whether measurement of end-expiratory alveolar gas concentrations using a gas chromatograph would provide an alternative approach.

(1) Using our previously digitized data for infants with BPD and full-term controls, DL_CO and V_A were calculated at end-expiration rather than between 60 and 80% of expired volume, as previously reported. (2) In a new group of infants, DL_CO and V_A were measured using gas concentrations obtained at end-expiration with a mass spectrometer and a gas chromatograph.

(1) Using end-expiratory concentrations, infants with BPD (n = 49) had significantly lower DL_CO , but similar V_A compared to healthy controls (n = 34) (DL_CO : 4.2 vs 4.6 mL/min/mmHg, P = 0.047; V_A : 614 vs 608 mL, P = 0.772). (2) Among newly evaluated infants (n = 28), DL_CO and V_A obtained with a mass spectrometer and a gas chromatograph were highly correlated (R²  = 0.94 and 0.99, respectively), and were not significantly different for the two analyzers.

Measuring DL_CO and V_A at end-expiration using a gas chromatograph can provide an effective assessment of gas exchange in sleeping infants.