Hemodilution on microvascular oxygen delivery potential of the blood during coronary bypass surgery

The hematocrit-to-whole blood viscosity ratio (Hct/WBV) reflects the blood O₂ delivery potential (O₂-DP). WBV is variable to the dynamic vascular shear rate (SR), 1-5/s at microcirculation and 300/s at larger vessels. To estimate the impact of hemodilution on the blood O₂-DP to the myocardium, we analyzed the hemodilution-induced change of Hct/WBV at SR 5/s (Hct/WBV₅) during off-pump coronary bypass (OPCAB) surgery.

During OPCAB surgery (n = 21), 10% acute normovolemic hemodilution (HD 10%) was applied. Arterial blood samples were taken: one before and two after HD 10%. One of which after HD 10% underwent an additional 33% in vitro hemodilution (reaching 40% hemodilution in total, HD 40%). WBV of all blood samples was determined using a scan-capillary tube viscometer (Hemovister^™). The changes of Hct/WBV₅ were analyzed as a primary measure of the study and compared with those of Hct/WBV at SR 300/s (Hct/WBV₃₀₀).

Median[IQR] of Hct/WBV₅ [3.5 (2.8-4.2)%/cPoise] was significantly increased by HD 10 and HD 40% [3.6 (3.2-4.6)%/cPoise and 4.2 (3.3-5.2)%/cPoise, respectively, all P < 0.001], but the degrees of changes after HD 10 and HD 40% were not different. Median[IQR] of Hct/WBV₃₀₀ [10.3(8.6‒10.8)%/cPoise] was not changed by HD 10% [10.3(9.1-11.1)%/cPoise], but it was significantly decreased by HD 40% [8.4(7.4‒9.2)%/cPoise, P < 0.001].

The increased Hct/WBV₅ suggests that 10-40% hemodilution improves the blood O₂-DP to the myocardium during OPCAB surgery. The SR-specific discrepancy in Hct/WBV changes advocates using microvascular WBV and Hct/WBV to evaluate the blood O₂-DP changes to the myocardium. Further study is warranted to assess the actual changes in myocardial O₂ delivery.