Cardiovascular changes during acute episodic repetitive hypoxic and hypercapnic breathing in rats.

It has been shown that chronic repetitive ambient hypoxia, simulating pulmonary gas disturbances observed in apnoea, leads to systemic hypertension in rats. However, the relative roles of hypoxia (HO), hypercapnia (HC), gas stress and vigilance on the cardiovascular changes have not yet been demonstrated. The aim of this study was to investigate the acute haemodynamic changes observed during the repetitive inhalation of various gas mixtures in rats for HO alone and HO+HC, and to analyse the effects of vigilance and of the stress of gas administration. We studied 6 unanaesthetized Wistar rats chronically instrumented with an aortic catheter. Nitrogen, nitrogen+CO2 mixtures and compressed air were randomly administered in a Plexiglass chamber for 10 s and then flushed by compressed air for 20 s. Two cycles were repeated every min for 10 to 12 min. The inhaled gas fractions (FI,O2,FI,CO2) were monitored by O2 and CO2 analysers. Blood pressure (BP) was measured by a P23XL transducer. The blood gases were analysed by a 1306 IL meter. In control experiments, with compressed air alone, there were no significant acute changes in heart rate (HR) and BP. During HO there were no changes in HR or BP at FI,O2 values from 0.05-0.14, whilst at FI,O2 values from 0-0.05 systolic blood pressure (SBP) rose significantly (+25.3 +/- 25.7 (SD) mmHg) and HR decreased (-93.8 +/- 124.1 bpm). During HOHC, SBP rose (+35.1 +/- 26.4 mmHg) and HR decreased (-139.3 +/- 75.7 bpm), significantly more than in HO alone. SBP was linearly correlated with Pa,O2 during HO (r = 0.53) and also during HOHC (r = 0.44) and was not directly related to Pa,CO2 which has, nevertheless, an additive effect to HO. SBP rose with each challenge significantly more when the rats were awake than when asleep (behavioural sleep). We conclude that in this acute repetitive inhalation model, the rise in SBP is not related to gas stress or to Pa,CO2 but to a decrease in Pa,O2 and is enhanced by wakefulness.