Surface-induced hypothermia: Effects on coronary blood flow autoregulation and vascular reserve

Abstract

Surface-induced hypothermia has been shown to exert a protective effect in canine models of myocardial infarction. However, its effects on coronary blood flow (CBF) autoregulation and coronary vascular reserve (CVR) have not been investigated. The effects of mild (32°C) and moderate (27°C) hypothermia on CBF autoregulation and CVR (at 60 mm Hg diastolic pressure) were studied using a chronically instrumented canine preparation. Coronary artery pressure-flow relations were obtained over a wide range of coronary diastolic pressures (10 to 106 mm Hg) with autoregulation intact and during adenosine-induced maximal coronary conductance (MCC) at 37, 32, 27°C ( n = 7 dogs), and after rewarming ( n = 5 dogs). Halothane (1 MAC end-tidal concentration, temperature adjusted) was the anesthetic. Autoregulation remained intact during hypothermia. CBF remained relatively constant between diastolic pressures of 43.1 ± 9.0 and 84.0 ± 14.4 mm Hg (mean ± SD). No significant differences were observed between temperatures in the autoregulated pressure range. CBF correlated well with myocardial oxygen consumption (MVO 2) ( r 2 = 0.81, P < 0.0001). There were no significant changes in MVO 2, CBF, MCC, or CVR at 32°C. At 27°C, MVO 2(3.65 ± 1.3 at 37°C vs 2.35 ± 1.4 ml O 2 · min −1 at 27°C), autoregulated CBF (34.9 ± 15.1 vs 19.5 ± 10.8 ml · min −1), the slope of the line of MCC (4.31 ± 0.7 vs 2.7 ± 0.4 ml · mm −1 · min −1), and CVR (147.1 ± 24.6 vs 90.1 ± 27.3 ml · min −1) were all less than control ( P < 0.05). After rewarming to 37°C, no significant changes from control were noted. The authors conclude that coronary autoregulation remains intact at both 32 and 27°C, although MCC and CVR are significantly decreased at 27°C.