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

Background: Measurements of intrathoracic blood volume (ITBV) provide volum etric information about cardiac preload and are used to investigate the cau se 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 mathem atical coupling of data. The aim of this prospective, clinical study was to investigate whether a variation in CO induced by high-dose beta-blockade i nfluences thermodilution measurements of ITBV in the absence of changes in intravascular volume in patients undergoing minimally invasive coronary art ery bypass grafting. Methods: Sixteen patients undergoing elective minimally invasive direct cor onay artery bypass (MIDCAB) surgery were studied. Transpulmonary thermodilu tion measurements of ITBV and CO were simultaneously performed before bypas s grafting, during beta-blockade induced by high-dose esmolol and at the en d 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 administratio n). After the end of esmolol administration, CO significantly increased by 79%. Again, ITBV remained virtually unchanged (860 +/- 61 ml m(-2) during e smolol administration versus 911 +/- 38 ml m(-2) after esmolol administrati on). Conclusions: The results of the present study demonstrate that substantial alterations in CO as a consequence of high-dose esmolol infusion are not as sociated with changes in ITBV. Because haemodynamic changes were induced by factors other than variation of preload, these findings suggest that chang es in cardiac output do not influence thermodilution measurements of ITBV i n this setting.