Current Questions in Patient Safety: Is Monitoring for Hypoxemia More Important than for Hypercarbia?

David J. Cullen, M.D

Question: Is monitoring for hypoxemia more important than for hypercarbia?

Answer: Hypercarbia is, by itself, a serious but not usually life-threatening condition. If, however, hypoventillation leads to hypoxia, respiratory and/ or cardiac arrest often follow.

Hypercarbia may result from increased C02 production relative to C02 elimination (as in hyperthermia, emergence excitement, or excessive caloric intake from hyperalimentation solutions) or from hypoventillation due to incorrect ventilator settings, airway obstruction, depressed central respiratory drive (from anesthetic overdose), or inadequate mechanics of ventilation (residual muscle relaxation). The direct physiologic effect of hypercarbia is peripheral vasodilation. Indirectly, hypercarbia stimulates the sympathetic nervous system via the vasomotor center to increase myocardial contractility, heart rate, stroke volume, and blood pressure. The vigor of these responses is reduced by anesthesia.

Hypoxemia plus hypercarbia is extremely dangerous because acidosis (respiratory) potentiates the depressant effects of hypoxemia on myocardium and brain, and leads to bradycardia and cardiac arrest. Even with successful cardiac resuscitation, brain damage often results. To maintain oxygen transfer, more than oxygen content is required. A high enough PaO2 is needed to get oxygen into cells.

As soon as hypoxemia develops, anaerobic metabolism is utilized by cells to produce energy, a very inefficient method. Depending on the severity and duration of hypoxemia, vital functions fail. Thus, brain function diminishes, manifested by disorientation, obtundation, and coma; myocardial dysfunction reduces contractility, heart rate, blood pressure, and stroke volume and often leads to arrhythmias. Other organ functions deteriorate, though at a slower pace. Unlike in the unanesthetized situation when hypoxemia stimulates the vasomotor center via the carotid body chemoreflex to increase cardiac function, this mechanism is almost never operative when residual anesthesia let alone surgical levels of anesthesia are present. Of equal importance, the vigorous respiratory response to hypoxemia seen in unanesthetized intact preparations is essentially abolished by either narcotics or inhalation anesthetics. This can result in further hypoventillation, hypoxemia, and cardiac and/or respiratory arrest within minutes.

Thus, in answer to the question: Is monitoring for hypoxemia more important than for hypercarbia?, the answer is a qualified yes. Hypercarbia, if due to hypoventillation leads to hypoxemia, which in all cases from whatever cause, must be detected to initiate corrective therapy and prevent anesthesia related disasters.

Response by David J. Cullen, M.D. Harvard Medical School

Editors note: Please address patient saw related questions to John H. Eichhorm, M.D.