Variability of ventilatory responses to hypoxia and hypercapnia

Abstract

Hypoxic ventilatory responses (HVR) and hypercapnic ventilatory responses (HCVR) were studied in 8 normals repeatedly on the same day and on different days over 7 mo. No significant mean change occurred in HVR or HCVR measured every 30 min over 2 h. The mean coefficient of variation over 2 h for HVR within each individual was 19.4% (range 7.6-63.8%) and for HCVR 17.9% (range 8.3-26.3%). Variability between days was greater than within days for HVR and HCVR in all but 1 subject. In order to determine if variation in metabolism, body temperature, or acid-base status contributed to between-day variability, 3 normal subjects had these variables measured on 6 different occasions. A significant inverse correlation was observed between HVR and blood pH (P < 0.01), and the pH variation was due primarily to fluctuations in CO2 pressure (PCO2) (P < 0.05). Ventilatory responses were not correlated with changes in metabolism or body temperature. No significant systematic change occurs in HVR and HCVR over a 2 h period mitigating against a training effect or interaction between measurements. Ventilatory response between days is more variable than within-day ventilatory response requiring cautious interpretation of the effects of experimental interventions on between-day ventilatory response. Changes in pH partially explain the between-day variance in HVR.