VENTILATORY RESPONSE TO CONSECUTIVE SHORT HYPERCAPNIC CHALLENGES IN CHILDREN WITH OBSTRUCTIVE SLEEP-APNEA

In healthy adults, a ventilatory pattern characterized by progressivel y increased tidal volume (VT), and decreased respiratory rate (RR) acc ompany repeated short hypercapnic ventilatory challenges, while minute ventilation (VE) remains constant. We hypothesized that the peculiar ventilatory pattern seen in adults would be blunted in children with o bstructive sleep apnea syndrome (OSAS) who undergo comparable intermit tent or chronic alveolar Pco(2) elevation. We measured ventilatory res ponses to five challenges of 2-min duration (C1-C5) with 5% CO2 in O-2 , separated by 5-min room-air breathing intervals (R1-R4), in nine chi ldren with OSAS and in eight age-, sex-, and body mass index-matched c ontrols. In all children, CO2 significantly increased VE when compared with baseline conditions (22.3 +/- 2.2 vs. 9.5 +/- 0.9 (SE) 1/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), whe reas 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 VE increases across CO2 cha llenges (22.3 +/- 4.9 1/min in C1 vs. 25.1 +/- 5.0 1/min in C5; P < 0. 005). The RR decrease was primarily related to progressive prolongatio n 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 consisten t fashion in OSAS patients with repeated CO2 challenges (OSAS vs. cont rol: P < 0.0001). Furthermore, in OSAS, VE was similar with repeated c hallenges (22.4 +/- 2.2 1/min in C1 vs. 23.9 +/- 1.9 1/min; P = not si gnificant), such that changes in VE over time significantly differed i n OSAS and controls (P < 0.001). We conclude that healthy children mod ify their ventilatory strategy to repeated hypercapnia. We speculate t hat 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 respons e.