[Relation between oxygen consumption and cardiac output during inhalation anesthesia under the influence of...

The metabolic regulation of tissue blood flow manifests itself in a linear relation between blood flow and oxygen consumption (VO2). It is unknown, however, if this fundamental physiological principle operates also during inhalation anaesthesia and catecholamine therapy, both known to be associated with changes of cardiac output (Q) and VO2 in opposite directions.

On different days, 17 trained, healthy dogs (26-33 kg) with chronically implanted flow probes around the pulmonary artery were either anaesthetized with halothane, enflurane, isoflurane, sevoflurane, and desflurane at increasing minimum alveolar concentrations (1-3 MAC) or treated with one of the endogenous catecholamines epinephrine and norepinephrine or the synthetic ones dobutamine and dopexamine (a total of 186 experiments). Cardiac Output (ultrasound transit-time flowmeter) and VO2 (indirect calorimetry) were measured continuously and the relations between both variables (Q/VO2 relations) analyzed.

In awake dogs under basal metabolic conditions, VO2 was 4.6 +/- 0.1 ml x kg-1 x min-1 and Q 105 +/- 3 ml x kg-1 x min-1 (mean +/- SEM). During inhalation anaesthesia, VO2 and Q decreased in parallel, yielding a uniform Q/VO2 relation, which was nearly linear. Above 2 MAC, O2 extraction increased by 50%, indicating compromised oxygen delivery (DO2). Imposed increases in Q and thus DO2 during anaesthesia to rates comparable to that in the awake state did not restore VO2 to baseline. Catecholamines increased both VO2 and Q in a dose-dependent manner, albeit to a different extent. The resulting Q/VO2 relations were linear up to the maximum effects, but their slopes increased about threefold in the order norepinephrine (34), epinephrine (54), dobutamine (86), and dopexamine (105). Despite these differences, VO2 and Q correlated linearly over the whole range studied, which covered a doubling of VO2 and an up to fourfold increase in Q.

The metabolic regulation of blood flow apparently also operates during inhalation anaesthesia, regardless of the anaesthetic. Catecholamines (except norepinephrine) preferentially stimulated Q rather than VO2 (excess perfusion), so that agents like dopexamine might be preferred in the treatment of low cardiac output states because of its low metabolic costs. Our observations imply that cardiac output alone provides little information on the function of the circulation during inhalation anaesthesia and catecholamine therapy unless related to metabolic demands, i.e. to VO2.