The hyperoxic test in infants reinvestigated

The hyperoxic test (HT) examines peripheral chemoreceptor function (PCF) by measuring the decrease in ventilation (V E) after 100% O(2) inhalation. A 30-s HT has been previously used in infants with calculation of the ventilatory response (VR) as the mean percentage change in V E during HT as compared with normoxia. However, it has been shown that during hyperoxia V E rises secondarily after the initial drop because of loss of PCF. We hypothesized that the mean V E change over a 30-s HT may underestimate the strength of PCF and may be poorly reproducible. We performed breath-by-breath analysis during 30-s HTs, calculating VR at the response time (RT) defined as the time from HT onset to the first significant HT-related change in V E. Eighteen infants (postnatal age, 21 +/- 4 d) underwent two HTs (quiet sleep, face mask attached to a pneumotachograph, and inspired and expired O(2) and CO(2) fractions measured using mass spectrometry). V E, VT, and VT/TI decreases at the RT were significantly greater than the corresponding means (-21 +/- 7 versus -15 +/- 7%, -21 +/- 8 versus -13 +/- 8%, and -22 +/- 11 versus -17 +/- 11%, respectively). Intra-individual coefficients of variation of V E, VT and VT/TI were significantly smaller when RT values were considered rather than means. We conclude that calculation of the VR to HT at RT improves assessment of PCF and enhances HT reproducibility in infants.