Influence of changes in cardiac output on the acid-base status of arterial and mixed venous blood

While maintaining the arterial CO₂ tension constant near the normal level of the dog (4.3 kPa), we studied the influence of decreasing cardiac output on both the arterial and mixed-venous blood acid-base status in anaesthetized, artificially ventilated dogs. Cardiac output was manipulated by applying positive end-expiratory pressure (PEEP), and by -adrenergic blockade to suppress a compensatory heart rate response. The systemic vascular response was attenuated by -adrenergic blockade. Metabolic rate remained virtually unchanged when cardiac output decreased. Under these conditions a fall in cardiac output led to a shift of the arterial acid-base status in the direction of a metabolic acidosis. The changes occurring in the mixed-venous blood resembled those of an in-vivo CO₂ bufferline, with the shift being such as if a respiratory acidosis was developing.