Fluid Responsiveness: A Validation of Stroke Volume Variations Estimated With Pulse Wave Transit Time in Mechanically Ventilated and Spontaneously Breathing Patients After Off-pump Coronary Artery Bypass Grafting

Postoperative period after off-pump coronary artery bypass grafting (OPCAB) can be a challenging area for emerging methods for less-invasive continuous hemodynamic monitoring.It is well known that OPCAB results in rapid changes in cardiac performance and vascular tone affecting many hemodynamic variables. The patients demonstrating any pattern of hemodynamic instability during and after major cardiac surgery can benefit from assessment of fluid responsiveness both invasively and non-invasively. In addition to continuous monitoring of cardiac output (CO), functional hemodynamic parameters as well as multiple dynamic tests are of value when elaborating the goal-directed treatment plan and clinical decision-making. Fluid responsiveness can be assessed using stroke volume variation (SVV) and pulse pressure variation (PPV), as well as by non-invasive heart-lung interaction index (HLI, G5-Hamilton, Switzerland) and pulse variability index (PVI, Masimo, USA). However, these parameters vary in predictive value in different categories of patients. Thus, passive leg raising (PLR), transient increase in positive end-expiratory pressure (PEEP) and mini-fluid load test (mFL) can be also used to assess the fluid status, both perioperatively and in critically ill patients. The main advantages of these tests are their reversibility and simplicity. Both PLR and mFL test can be readily performed in spontaneously breathing patients. The restoration of the spontaneous breathing has been supposed to be a major limitation for the predictive value of the functional hemodynamic parameters based on the cardiopulmonary interactions, namely, SVV and PPV. However, recent studies have shown that spontaneous breathing activity does not completely eliminate the accuracy of these parameters only expanding the "grey zone" of indefinite values.

Among emerging techniques for continuous measurement of SVV and cardiac output, the assessment of pulse wave transit time (PWTT) has recently become available in the clinical practice. The latter method is based on the relationship between R-wave on ECG, SpO2, and blood pressure, allowing assessment of estimated continuous stroke volume, SVV and CO (esSVV and esCCO, Nihon Kohden, Japan). The potential advantage of this technique is relative non-invasiveness. However, possible limitations include the dependence on signal quality, calibration and intrinsic hemodynamic characteristics. The results of our pilot study of esCCO in OPCAB demonstrate that during OPCAB, esCCO values overestimate CO and demonstrate poor accuracy, precision, and trending ability compared to transpulmonary thermodilution (TPTD). However, postoperatively esCCO showed better agreement with TPTD. The determination of esSVV when CO2 is measured on the bedside monitor also requires further validation in changing clinical settings both during mechanical ventilation and spontaneous breathing.

Goals of the study The primary goal of this study is to assess the accuracy of estimated stroke volume variation (esSVV, Nihon Kohden, Japan) compared with SVV determined by conventional pulse contour analysis (SVVPCA), respectively.

The secondary goals of this study are to assess the predictive value of esSVV in the early postoperative period during complex dynamic tests including series of dynamic tests (passive leg raising, increased PEEP, and mini-fluid load) followed by the standard fluid load in both mechanically ventilated and spontaneously breathing patients.

Methods Before OPCAB, after receiving the informed consent, all the patients will be subjected to standard premedication. After transfer to the operation room, the catheterization of radial artery using standard 20 G radial arterial catheter (Arteriofix, B/Braun, Germany) and peripheral vein will be performed before induction of anesthesia. The monitoring will also include ECG and SpO2.