CRITICAL HEMOGLOBIN CONCENTRATION IN ANESTHETIZED DOGS - COMPARISON OF 2 PLASMA SUBSTITUTES

We have explored systemic and regional tolerance to haemodilution duri ng anaesthesia with two different synthetic colloids. Eighteen dogs un dergoing mechanical ventilation during anaesthesia with ketamine were submitted to progressive normovolaemic haemodilution with either gelat in (GEL; n=9) or hydroxyethylstarch (HES; n=9) administered on a 1:1 r atio. Systemic oxygen delivery was calculated From measurement of ther modilution cardiac output and arterial oxygen content, while systemic oxygen consumption was determined from expired gas analysis. Mesenteri c oxygen delivery and consumption were determined using ultrasonic flo w measurements, and arterial and mesenteric venous oxygen contents. Th e critical haemoglobin concentration (i.e. the haemoglobin value below which oxygen consumption becomes oxygen delivery dependent) was mean 3.6 (SD 0.8) g dl(-1) in the GEL and 3.5 (1.5) g dl(-1) in the HES gro up. The mesenteric critical oxygen extraction ratio (O2ER) (GEL 50.1 ( 12.1) %; HES 48.5 (13.4) %) was significantly lower than the systemic critical O2ER (GEL 66.1 (8.4) %; HES 67.7 (7.1) %). There were no sign ificant differences between the GEL and HES groups for any of these va riables, or in the amount of colloid administered. During the study, o xygen delivery decreased almost linearly with reduction in haemoglobin , indicating a lack of cardiac output response to anaemia during ketam ine anaesthesia.