UNCOMPENSATED BLOOD-LOSS IS NOT TOLERATED DURING ACUTE NORMOVOLEMIC HEMODILUTION IN ANESTHETIZED PIGS

Clinically, hemodilution to a hematocrit of 9% has been studied, but t he effects of hypovolemia during this degree of hemodilution have not been elucidated. We studied the response to blood loss during extreme hemodilution and evaluated indicators of hypovolemia. Systemic and myo cardial hemodynamics, oxygen transport, and blood lactate concentratio ns were measured in 12 anesthetized pigs exposed to a graded blood los s of 10, 20, 30, and 40 mL/kg. Six animals were hemodiluted (hematocri t 10.8% +/- 1.4%, mean +/- SD), and six animals served as controls (he matocrit 34.6% +/- 1.5%). Hemodilution decreased systemic oxygen deliv ery to 9.5 +/- 0.6 mL . kg(-1) . min(-1) (controls 21.7 +/- 3.9 mL . k g(-1) . min(-1)) (P < 0.01) despite a 31% increase in cardiac output. Systemic oxygen uptake was unchanged. Arterial lactate increased to 3. 3 +/- 1.1 mM/L (controls 1.6 +/- 0.6 mM/L) (P < 0.05), and mixed venou s oxygen saturation (Svo(2)) decreased to 38.2% +/- 4.8% (controls 68. 6% +/- 2.9%) (P < 0.01). At a blood loss of 10 mL/kg, cardiac output c ontinued to be greater in the hemodiluted animals (P < 0.01). Arterial blood pressure decreased to 61 +/- 8 mm Hg (controls 84 +/- 18 mm Hg) (P ( 0.05), whereas heart rate was unchanged. Systemic oxygen deliver y decreased to 8.8 +/- 1.2 mL . kg(-1) . min(-1) (controls 14.1 +/- 2. 5 mL . kg(-1) . min(-1)) (P < 0.01). Systemic oxygen uptake was mainta ined by a further increase in oxygen extraction, and Svo(2) decreased to 29.7% +/- 7.3%, compared with 55.3% +/- 9.0% in controls (P < 0.01) . Arterial lactate increased to 4.9 +/- 1.4 mM/L (controls 1.8 +/- 0.8 mM/L) (P < 0.01). Myocardial oxygen delivery and lactate uptake were unchanged. When the blood loss equaled 30 mL/kg, myocardial lactate pr oduction occurred, and two hemodiluted animals died of circulatory fai lure. Central venous and capillary wedge pressures changed minimally d uring the blood loss and did not differ between groups. We conclude th at a decrease in arterial blood pressure and Svo(2) were early signs o f hypovolemia during hemodilution, whereas central venous pressure and pulmonary capillary wedge pressure were insensitive indicators. Impli cations: Anesthetized pigs with extremely low hemoglobin levels tone t hird of normal) showed poor tolerance to blood loss >10 mL/kg. A decre asing arterial blood pressure, a decreasing oxygen saturation in the v enous blood, and an increase in arterial blood lactate concentration w ere useful indicators of blood loss.