Myocardial oxygen supply during hypocapnia and hypercapnia in the dog

Abstract

It has been postulated that a coronary vasoconstriction during hypocapnia might be opposed by a compensating coronary vasodilatation due to impaired myocardial oxygen supply. The present study was performed first to examine whether a maximal decline in coronary sinus (CS) oxygen content was reached during hypocapnia. During hypercapnia a myocardial "over perfusion" has been demonstrated. The second purpose of the present study was to examine whether a myocardial "over perfusion" is essential to maintain a sufficient myocardial tissue oxygen supply during hypercapnia. Closed-chest dogs were anesthetized with pentobarbital and hypocapnia was induced by hyperventilation. Nitrogen gas and carbon dioxide could both be added to the inspiratory gas to create arterial hypoxemia (arterial SO2 65%) and hypercapnia, respectively. Arterial hypoxemia during hypocapnia increased myocardial blood flow (MBF) by 50%, while CS SO2 decreased significantly. The decrease in CS SO2 demonstrates a reserve capacity of myocardial oxygen extraction during hypocapnia, thereby ruling out any major coronary vasoconstriction during hypocapnia. Hypercapnia during normoxemia increased MBF, myocardial oxygen delivery, and CS SO2 substantially, but this was not observed when hypercapnia was created during arterial hypoxemia. From the present results we conclude that hypocapnia does not cause any major coronary vasoconstriction, while hypercapnia results in a myocardial "over perfusion," which is a luxury perfusion not essential to maintain sufficient myocardial oxygen supply during hypercapnia.

Respiratory alkalosis and primary hypocapnia in Labrador Retrievers participating in field trials in high–ambient-temperature conditions

“…8 The mechanisms underlying these clinical signs include cerebral and myocardial hypoperfusion caused by arteriolar constriction, minor alterations in electrolyte balances such as decreased ionized calcium and potassium concentrations, and decreased release of oxygen to tissues as a result of a left shift in the oxyhemoglobin dissociation curve. 13 In anesthetized dogs, Paco 2 of approximately 20 mm Hg (which was probably within the range recorded in the dogs of the present study) reduced mean cerebral blood flow by 36%. 13 In anesthetized dogs, Paco 2 of approximately 20 mm Hg (which was probably within the range recorded in the dogs of the present study) reduced mean cerebral blood flow by 36%.…”

Physiologic Responses in Healthy Labrador Retrievers during Field Trial Training and Competition

“…14 Studies of dogs during acute hypocapnia have described reductions in both cerebral blood flow and myocardial blood flow, 21 but not all studies have found reduced myocardial blood flow. [22][23][24] The retrieving drill has been compared to dogs exercised on a treadmill for similar durations. 1 However, the speed and gradient of the treadmill and the V O 2max of the individual dogs are additional factors to consider.…”