The Hemodynamic and Metabolic Effects of Shivering During Acute Normovolemic Hemodilution

To assess the hemodynamic and metabolic effects of shivering during extreme normovolemic hemodilution, we anesthetized 16 pigs with fentanyl-midazolam-pancuronium. Mild hypothermia (36.5° ± 0.1°C) was induced by surface cooling, and the animals were randomized to either a control group (hemoglobin 118 ± 3 g/L) or a hemodilution group (hemoglobin 52 ± 2 g/L). In the latter group, blood was replaced with an isotonic Ringer's acetate/dextran 70 solution. Shivering was allowed to occur by a controlled decrease in the infusion rate of pancuronium. Shivering increased oxygen consumption (Vo 2 ) in both groups (P < 0.001). Initially, this was predominantly compensated for by an increased oxygen extraction ratio (ER), but when Vo 2 was 2.3 ± 0.2 times baseline, critical levels of mixed venous oxygenation (Svo 2 = 18% ± 2%; PvO 2 = 22.5 ± 1.5 mm Hg) and ER (82% ± 3%) were recorded in anemic animals. Control animals did not reach critical levels until Vo 2 was maximal (3.7 ± 0.3 times baseline). Maximal attained Vo 2 was less (2.9 ± 0.1 times baseline) in the anemic animals (P = 0.01), and at this stage two of these pigs had myocardial lactate production, one of which died in ventricular fibrillation. Coronary perfusion pressure was significantly less (P < 0.001) in the anemic animals. We conclude that in this experimental model, maximal shivering as measured by Vo 2 was limited in hemodiluted animals, and left ventricular oxygen balance was marginal, as evidenced by a decreased lactate uptake and extraction.