Changes in cardiac output and intrathoracic blood volume: a mathematical coupling of data?

Background: Measurements of intrathoracic blood volume (ITBV) provide volumetric information about cardiac preload and are used to investigate the cause of alterations in cardiac output (CO). On the other hand, CO is required to calculate ITBV Thus, concerns have been raised with respect to a mathematical coupling of data. The aim of this prospective, clinical study was to investigate whether a variation in CO induced by high-dose beta-blockade influences thermodilution measurements of ITBV in the absence of changes in intravascular volume in patients undergoing minimally invasive coronary artery bypass grafting. Methods: Sixteen patients undergoing elective minimally invasive direct coronay artery bypass (MIDCAB) surgery were studied. Transpulmonary thermodilution measurements of ITBV and CO were simultaneously performed before bypass grafting, during beta-blockade induced by high-dose esmolol and at the end of surgery. Results: During esmolol administration, CO significantly decreased by 33%, whereas ITBV remained unchanged compared to control values (876 +/- 46 ml m(-2) during control versus 860 +/- 61 ml m(-2) during esmolol administration). After the end of esmolol administration, CO significantly increased by 79%. Again, ITBV remained virtually unchanged (860 +/- 61 ml m(-2) during esmolol administration versus 911 +/- 38 ml m(-2) after esmolol administration). Conclusions: The results of the present study demonstrate that substantial alterations in CO as a consequence of high-dose esmolol infusion are not associated with changes in ITBV. Because haemodynamic changes were induced by factors other than variation of preload, these findings suggest that changes in cardiac output do not influence thermodilution measurements of ITBV in this setting.