Continuous mixed venous oxygen saturation, not mean blood pressure, is associated with early bupivacaine cardiotoxicity in dogs

Purpose: To investigate changes of continuous mixed venous oxygen saturation (cSvO₂) and mean arterial blood pressure (MBP) in dogs with bupivacaine-induced cardiac depression. Methods: Bupivacaine was infused into pentobarbital-anesthetized mongrel dogs (n = 8) at a rate of 0.5 mg·kg^-1·min^-1 until the MBP was 40 mmHg or less (end of bupivacaine infusion; BIE). The infusion time was divided into the early period, first 30 min of bupivacaine infusion and the late period, which was from 30 min of bupivacaine infusion until BIE. cSvO₂ was monitored using a fibreoptic pulmonary artery catheter, and MBP and cardiac output (CO) were measured every ten minutes after the initiation of bupivacaine infusion. Arterial blood gas, serum electrolyte and bupivacaine concentration were measured simultaneously. The relationships between CO and cSvO₂, and of CO vs MBP were compared by regression analysis in the early and late periods. Results: The Pearson's correlation coefficients between CO and cSVO₂ were 0.782 (P = 2.1 × 10^-7) in the early period and 0.824 (P = 1.3 × 10^-6) in the late period. The correlation coefficients between CO and MBP were 0.019 (P = 0.921) in the early period and 0.799 (P = 4.8 × 10^-6) in the late period. Conclusions: cSvO₂, but not MBP, is associated with CO changes in bupivacaine-induced cardiac depression during the early period of bupivacaine intoxication. Decrease of MBP with low cSvO₂ observed during the late period might imply severe cardiac depression induced by bupivacaine infusion.