Respiratory and cardiovascular responses to hypoxemia and the effects of anesthesia.

The normoxic ventilatory drive contributes to the normal level of ventilation, and the hypoxic ventilatory drive contributes to the maintenance of adequate gas exchange in the presence of ventilation/blood flow maldistribution and increased mechanical load to breathing. This respiratory drive arises principally from stimuli at the carotid chemoreceptors. The reflex cardiovascular responses to hypoxia also contribute to the delivery of O2 to vital organs, and their efficacy depends on the integrity of the respiratory response and the autonomic nervous system as well as the function of the vascular system. Prolonged exposure to hypoxemia from altitude, cyanotic congenital heart disease, and chronic pulmonary disease impair the ventilatory response to hypoxia. In addition, the respiratory and cardiovascular responses to hypoxemia are impaired by familial or acquired abnormalities of the autonomic effector system. There is growing evidence that impaired respiratory response to hypoxemia is a major factor in recurrent respiratory failure in obesity, obstructive pulmonary disease, idiopathic or familial "hypoventilation," and contributes to disturbances in oxygenation during sleep [152, 189, 192, 202]. Although the ventilatory response to hypoxemia was traditionally thought to be resistant to the effects of inhalational anesthetics, barbiturates, and narcotics, there is abundant evidence that in fact the ventilatory response to hypoxia is more sensitive to depression by drugs than the ventilatory response to CO2. In addition, the hemodynamic responses to hypoxia are modified by anesthesia and anesthetic techniques. The clinical implications of these observations are wide. The ventilatory and cardiovascular response to hypoxemia will be altered, and usually depressed by age, disease processes, premedicant and anesthetic drugs, and autonomic blocking drugs. The cardiovascular responses will be modified indirectly by altered ventilatory control due to neuromuscular blocking drugs and controlled ventilation. Thus, not only will the responses to hypoxemia be depressed by anesthesia but the early clinical hemodynamic signs will be modified or absent, or indeed the cardiovascular response will further impair oxygen delivery. Furthermore, it is not only anesthetic doses that impair the reflex respiratory responses, but also subanesthetic doses of inhalational anesthetics and premedicant doses of barbiturates and narcotics. Hence the patient in the perioperative period continues to have impaired respiratory response to hypoxemia. As anesthetic and surgical care extends to older patients, patients with systemic disease, and recipients of cardiovascular peripheral and central drugs, the clinical implications of the impairment of ventilatory and cardiovascular responses to hypoxia, and the maintenance of organ and system function, escalate. Only a few hesitant steps have been taken into this vast arena of clinical and experimental research.