Resistance differences between blood and crystalloid cardioplegic solutions with myocardial cooling

Abstract

The effectiveness of a cardioplegic solution is dependent on maintaining adequate myocardial perfusion as the heart is cooled. Due to dissimilar rheological properties of blood (BC) and crystalloid (CC) cardioplegic solutions, differences in coronary vascular resistance (CVR) would be expected to develop during postarrest, hypothermic perfusion. To characterize rheologic differences between hypothermic BC and CC, we measured the viscosity of both solutions at 5°C intervals from 30 to 10°C with a Brookfield concentric cylinder viscometer. To evaluate hypothermic changes in CVR, we measured myocardial temperature, perfusion pressure, and perfusate flow during the injection of BC at 10°C in five dogs and CC at 10°C in five dogs. CVR was calculated from simultaneous, postarrest pressure and flow measurements taken at 5°C intervals from 30 to 15°C. The viscosity of BC increased from 1.72 ± 0.25 cP at 30°C to 2.67 cP at 10°C, while the viscosity of CC increased from 0.94 ± 0.02 to 1.44 ± 0.02 cP over the same temperature range. For BC, CVR increased from 0.134 ± 0.035 mm Hg·ml −1·min at 30°C to 0.199 ± 0.069 mm Hg·ml −1·min at 15°C, while CVR for CC increased from 0.0972 ± 0.024 to 0.111 ± 0.020 Hg·ml −1·min at 15°C. Two-way analysis of variance demonstrated that viscosity ( F = 48.5, P = 0.0001) and CVR ( F = 5.82, P = 0.042) were greater for BC than CC. Decreasing temperature resulted in significant differences in viscosity ( F = 347.3, P = 0.0001) and CVR ( F = 11.45, P = 0.0001) for the cardioplegic solutions. We have demonstrated that myocardial cooling results in a more pronounced increase in CVR for BC than for CC which can, in part, be attributed to differences in viscosity. As a consequence, BC will require a perfusion pressure higher than that of CC to maintain coronary flow with decreasing myocardial temperature.