Circulatory responses of the dog to acute isovolumic anaemia in the presence of high-grade adrenergic beta-receptor blockade.

The haemodynamic responses to isovolumic anaemia (dextran-for-blood exchange) were studied in seven dogs pretreated with propranolol 20 mg/kg body weight for 3 weeks. Adrenergic beta-receptor blockade did not prevent significant increases in heart rate (up to 19%). Cardiac output increased by about 20% while systemic vascular resistance decreased by the same proportion. Oxygen extraction increased from 24% to 35% and this was reflected by a decrease in mixed venous PO2 by 1.2 kPa. The modest hyperdynamic response of the circulation to haemodilution in the presence of high-grade adrenergic beta-receptor blockade is in sharp contrast with the large hyperdynamic response that has been reported in its absence and indicates that normal compensatory mechanisms are considerably impaired.