Bedside Assessment of the Microvascular Venous Compartment in Cardiac Surgery Patients With Valvular Diseases Undergoing Cardiopulmonary Bypass

OBJECTIVE: Blood volume reserve for venous return and the effects of cardiopulmonary bypass (CPB) on microvascular bed partitioning and blood flow were examined in patients with valvular diseases. DESIGN: Prospective, consecutive, case-control study. SETTING: Single university hospital. PARTICIPANTS: The study comprised 20 adult cardiac surgery patients and 20 healthy volunteers. INTERVENTIONS: Cardiovascular and microvascular variables were collected soon after the induction of anesthesia, after commencement of CPB, 20 minutes after separation from CPB, and in the intensive care unit. MEASUREMENTS AND MAIN RESULTS: The unstressed and stressed volumes (Vu, Vs) and pressures therein (Pit, Ps) were measured in the brachioradial muscle with near-infrared spectroscopy, applying incremental venous occlusions. At the first time point, Vs and Pit showed lower and higher values, respectively, than those of control patients, but Vs increased with Vu during the study, whereas Pit remained unchanged. Fluid balance correlated with Pit (r = 0.83, p<0.001) and hemoglobin (r = 0.78, p = 0.004). A nonlinear regression was found between fluid balance and ΔVu (r = 0.90, p<0.001) [y = 1.85+37.43(-0.01×x)]. The Vu/Pit and Vs/Ps ratios were lower than those of the control patients. Blood flow correlated to Vs/Ps (r = 0.75, p<0.001). The time constant was lower than reference (p = 0.005) and increased 10 times after CPB. CONCLUSIONS: Cardiac surgery patients have a limited blood volume reserve for venous return due to a reduced microvascular bed capacitance. This study demonstrated that during CPB a positive fluid balance induced an extravascular pressure increase and further reduced blood volume reserve.