Ventricular Stimulation as Predictor of Hypovolemia After Cardiac Surgery

In the immediate postoperative period of cardiac surgery and in intensive care unit (ICU), a systemic arterial hypotension is frequently observed. This hypotension is often due to hypovolemia but also to pump dysfunction and/or vasoplegia. Early recognition of the hypotensive mechanism is necessary because it requires immediate specific treatment.

The interest of a vascular filling is usually estimated by a preload dependence test (fluid responsiveness), showing an increase in the stroke volume when preload is increased according to the Starling the law of the heart. To date, a preload dependency test is usually performed by passive leg raise (PLR) or by measuring the respiratory pulse variation (ΔPP).

The PLR response varies according to the legs venous volume and capacitance. The derivation of ΔPP has been validated in patients intubated, sedated, and ventilated in standard conditions, in sinus rhythm but the ΔPP directly displayed by the monitors is unreliable and the manual calculation on pressure curve is fastidious. Faced with these difficulties, the trend is to realize a blind filling with physiologic saline solution and to judge his interest on clinical and / or hemodynamic criteria.

After cardiac surgery, ventricular epicardial electrodes are systematically placed allowing a temporary stimulation by an external pacemaker in case of postoperative atrio-ventricular blocks. The ventricular stimulation without atrial pacing (VESAP) leads to a loss of the atrial systole and therefore a decrease in the left ventricular preload. During the functional test of epicardial electrodes at a frequency greater than the patient's, it is possible to observe in certain circumstances a decrease of the arterial blood pressure, but of varying importance.

The investigators suspect that the eventual fall in blood pressure during a ventricular pacing test, could be correlated with preload dependence. Therefore, investigator hypothesize that ventricular pacing can be used to realize a functional testing of preload dependence, easy to achieve. Indeed, if doctors admit that during a brief ventricular stimulation, heart rate and systemic vascular resistance remain unchanged, the drop in blood pressure should reflect the decrease in stroke volume.

The main objective of this study is to demonstrate that the response to VESAP can predict the preload dependence of patients after a cardiac surgery. The primary endpoint will study the predictive value of the change in blood pressure during the VESAP and the change in stroke volume after an infusion of 500 cc of crystalloid solution. Secondary endpoints will 1) compare the predictive values of the VESAP and ΔPP; 2) look for pertinent VESAP threshold in correlation with the expected increase in stroke volume; and 3) collection of the possible complications due to the VESAP.

30 patients will be enrolled in this monocentric, open label, non-randomized study.