Effect of aging on metabolic respiratory control in sleeping dogs.

We examined the effects of aging on the metabolic respiratory control system by measuring changes with time in steady-state minute volume of ventilation (VI), alveolar carbon dioxide pressure (PACO2), and ventilatory and arousal responses to hypercapnia and hypoxia during slow-wave sleep (SWS). Studies were performed longitudinally in six healthy dogs over a span of 3 to 7 yr, corresponding biologically to 12 to 24 human yr. In each of the dogs aging was associated with a decrease in steady state VI during SWS, from 6.53 +/- 1.08 (mean +/- SEM) to 5.56 +/- 0.90 L/min (p < 0.01), and with an increase in PACO2 from 36.2 +/- 1.0 to 38.5 +/- 1.1 mm Hg (p < 0.01). However, ventilatory and arousal responses to hyperoxic hypercapnia (four dogs) remained unchanged. In contrast there was a decrement in the response of VI to isocapnic hypoxia during SWS (five dogs), from 1.22 +/- 0.12 to 0.70 +/- 0.07 L/min/% fall in arterial O2 saturation (SaO2) (p < 0.02), and a decrease in arousal SaO2, from 83.3 +/- 3.2 to 73.5 +/- 2.3 percent (p < 0.001). The findings indicate that aging is accompanied by impairment of ventilatory and arousal responses to hypoxia during SWS, and point to a specific effect of aging on the carotid-body chemoreceptors, as opposed to the brainstem respiratory controller or the ventilatory pump.