Ventilatory response to consecutive short hypercapnic challenges in children with obstructive sleep apnea

In healthy adults, a ventilatory pattern characterized by progressively increased tidal volume (VT), and decreased respiratory rate (RR) accompany repeated short hypercapnic ventilatory challenges, while minute ventilation (V̇E) remains constant. We hypothesized that the peculiar ventilatory pattern seen in adults would be blunted in children with obstructive sleep apnea syndrome (OSAS) who undergo comparable intermittent or chronic alveolar PCO₂ elevation. We measured ventilatory responses to five challenges of 2- min duration (CL-C5) with 5% CO₂ in O₂, separated by 5-min room-air breathing intervals (R1-R4), in nine children with OSAS and in eight age-, sex-, and body mass index-matched controls. In all children, CO₂ significantly increased V̇E when compared with baseline conditions (22.3 ± 2.2 vs. 9.5 ± 0.9 (SE) l/min; P < 0.001). In control subjects, progressive VT increases from 0.67 ± 0.10 liter in C1 to 0.92 ± 0.13 liter in C5 occurred (P < 0.01), whereas RR decreased from 33.9 ± 5.1 breaths/min in C1 to 27.8 ± 3.7 breaths/min in C5 (P < 0.02), resulting in V̇E increases across CO₂ challenges (22.3 ± 4.9 l/min in C1 vs. 25.1 ± 5.0 l/min in C5; P < 0.005). The RR decrease was primarily related to progressive prolongation of expiratory time (TE) (1.1 ± 0.1 s in C1 to 1.5 ± 0.2 s in C5; P < 0.002). In contrast, VT, RR, and TE did not change in a consistent fashion in OSAS patients with repeated CO₂ challenges (OSAS vs. control: P < 0.0001). Furthermore, in OSAS, V̇E was similar with repeated challenges (22.4 ± 2.2 l/min in C1 vs. 23.9 ± 1.91/min; P = not significant), such that changes in V̇E over time significantly differed in OSAS and controls (P < 0.001). We conclude that healthy children modify their ventilatory strategy to repeated hypercapnia. We speculate that in OSAS these mechanisms are already fully implemented because of recurrent alveolar hypoventilation accompanying increased upper airway resistance, leading to blunted temporal trends of ventilatory response.