Different cardiorespiratory responses to hemorrhage and hyperoxia in normotensive (WKY) and spontaneously hypertensive (SHR) rats

Objective: To compare the cardiorespiratory responses underlying the beneficial effects of hyperoxia during blood loss between normotensive (WKY) and hypertensive (SHR) rats. Methods: Experiments were carried out in anesthetized animals with both carotid bifurcations either innervated or denervated. The effects of breathing 60% O2 in N2 were studied either in combination with non-hypotensive hemorrhage or during hemorrhagic hypotension. Results: In normocia arterial pressure fell more in SHR than in WKY for a given blood loss. During hyperoxia, nerve-intact rats showed initial suppression of ventilation, but bifurcation-denervated rats a powerful enhancement. In all groups, hyperoxia increased the overall tone of venous capacitance vessels. Conclusions: The greater blood in SHR than in WKY when bleeding down to a given arterial pressure results from a stronger construction of venous capacitance vessels. Hyperoxia improves the ability of the cardiorespiratory system to resist the effect of hemorrhage by increasing the overall venous tone, thus supporting cardiac filling, and in some cases also by increasing alveolar ventilation, probably secondary to improved cerebral oxygenation. The beneficial effects of hyperoxia were: not prevented by carotid denervation, and thus were presumably direct tissue effects of oxygen, strikingly weaker in SHR than in normotensive (WKY) rats