Ventilation in conscious dogs during acute and chronic hypercapnia

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Abstract
Minute ventilation was measured in conscious dogs, at rest and during exercise (1 mph), over 60 min immediately following the acute inhalation of 5% CO2 in air and at 2, 4, 7 and 14 days while breathing the same gas mixture in a chamber. The dogs were also studied in the immediate period of air recovery from chronic hypercapnia and 1 day later. Control studies were carried out with the dogs breathing air in the chamber under comparable conditions. A triphasic ventilation change was observed in dogs at rest over the 14 days of hypercapnia. After an initial marked increase in ventilation during acute hypercapnia, ventilation returned to control levels by 2 days and then appeared to be elevated above control studies from 4 to 14 days at a time when blood acid-base balance became compensated. When the same dogs were studied during exercise, ventilation was also not different from air control at 2 days of hypercapnia; during exercise, unlike the resting studies, there was only a tendency for a secondary increase in ventilation at 7 and 14 days of hypercapnia. During the immediate recovery from chronic hypercapnia when the dogs breathed air there was no evidence of hypoventilation either at rest or exercise despite arterial alkalosis. At 24 h of recovery it appeared that dogs while at rest had a slightly reduced ventilatory response to 5% CO2 relative to control studies. The findings provide suggestive evidence that other factors, in addition to acid-base balance, might contribute to the regulation of ventilation during chronic hypercapnia and the recovery from chronic hypercapnia.