Adequacy of perfusion during hypothermia: regional distribution of cardiopulmonary bypass flow, mixed venous and regional venous oxygen saturation - hypothermia and distribution of flow and oxygen

Abstract

Background: Hypothermia during CPB is used to reduce metabolic activity, thus protecting organs and tissues. The aim of this study was to investigate the relationship between regional and mixed venous oxygen saturation and distribution of pump flow with respect to hypothermia.

Methods: Twenty-five patients undergoing a Ross procedure were included in a prospective, controlled study. During standard CPB, temperature was reduced stepwise to 28 degrees C. Blood gases (a-stat regimen) were analysed in samples from the inferior (IVC) and the superior vena cava (SVC), arterial and mixed venous blood. Flow was detected separately in the SVC, IVC, arterial, and collecting venous line. Samples were taken, and flows were measured before CPB, during hypothermia, during rewarming, and 30 min after CPB discontinuation.

Results: Oxygen saturation in the IVC was lower than in the SVC and in mixed venous blood at all times (max. difference - 17.3 +/- 3.0 % during hypothermia, - 23.8 +/- 2.9 % during rewarming, p < 0.01). There was a statistical correlation of mixed and IVC venous oxygen saturation (r = 0.79, p < 0.001) but not of SVC venous blood. Hypothermia had a major influence on pump flow distribution as backflow from the SVC decreased significantly in favour of IVC flow with increasing degree of hypothermia (increase of flow difference from 1.15 +/- 0.23 l/min to 1.49 +/- 0.36 l/min, p < 0.01). Temperature profiles were similar when detected in aorta, pulmonary artery, tympanum and nasopharygeum, but differed significantly from other sites.

Conclusions: During hypothermic CPB, regional deoxygenation occurs in spite of normal mixed venous saturation. The level of hypothermia has a major impact on bypass flow distribution with cerebral perfusion reduction. Methods of regional oxygenation assessment are needed, and altered strategies during hypothermia have to be taken into consideration.