Influence of Anesthesia on Mechanical Efficiency of Left Ventricle in Patients Undergoing Open Heart Surgery

Influence of Anesthesia on Mechanical Efficiency of Left Ventricle in Patients Undergoing Open Heart Surgery

The aim of the study is to delineate and compare the changes in coupled mechanical properties of left ventricle and arterial vascular bed caused by two popular anesthetic protocols commonly used in cardiac surgery (intravenous, based on the infusion of propofol, versus inhalational, based on isoflurane) on different stages of the surgery.

Background

From mechanical point of view the goal of the cardiovascular system is to distribute the kinetic energy of blood ejection from the left ventricle (LV) to body organs with minimal loss and greatest efficiency. The left ventricle and the arterial circulation work as "coupled" system. The ideal coupling supposes that a maximum of the energy produced by the LV is converted into forward flow to perfuse the body organs. This matching between "the source" (LV) and "the load" (arterial circulation) is governed by the mechanical properties of these parts of cardiovascular system.

Variety of pathological conditions change this coupling in such a way that may adversely affect the organ blood flow in the presence of unchanged or even high cardiac output, or significantly increase the metabolic demand on the LV for the maintenance of adequate systemic perfusion due to decrease of mechanical efficiency of the work produced by LV.

Although effects of anesthetic agents on myocardial contractility and peripheral vascular tone were extensively studied in clinical conditions, little is known about their influence on ventriculo-arterial coupling. Propofol and inhalational agents appear to impair this equilibrium in animal experiments. This is not surprising, since these agents cause complex dose-dependent changes in many physiologic parameters, including myocardial contractility, left ventricular preload and afterload, diastolic properties of myocardium and baroreceptor control of hemodynamics. Although these properties of anesthetic agents have been delineated extensively, very limited data characterizing their influence on ventriculo-arterial coupling in clinical conditions exist. This information is especially meaningful while planning the anesthetic management of patients undergoing cardiac surgery, where maintenance of circulatory homeostasis is of outmost importance. Since the main hemodynamic goal during anesthesia of the patient with cardiovascular disease is to provide optimal tissue perfusion with minimal myocardial oxygen demand, i.e. with maximal mechanical efficiency, knowledge of anesthetic induced changes in ventriculo-arterial coupling is extremely relevant from the clinical point of view.

The most convenient method for the evaluation of ventriculo-arterial coupling is the analysis of the relationship between LV end-systolic elastance, load-independent measure of myocardial contractility, and effective arterial end-systolic elastance, measure mechanical loading conditions8. Physiological data necessary for the calculation of these parameters may be acquired in relatively non-invasive way by combining and analyzing together recording of arterial blood pressure waveform and data of changes of LV volume, which may be obtained by means of echocardiography.

Objectives

General

The aim of the study is to delineate and compare the changes in ventriculo-arterial coupling caused by two popular anesthetic protocols commonly used in cardiac surgery (intravenous, based on the infusion of propofol, versus inhalational, based on isoflurane) on different stages of the surgery.

Specific

1. To compare influence of two anesthetic protocols on myocardial contractility at the end of the surgery.
2. To assess net changes in effective arterial elastance produced by surgery with the use of cardiopulmonary bypass and evaluate possible differences between these two anesthetic protocols in respect to changes of arterial tone.
3. To evaluate the preservation of ventriculo-arterial coupling by two different anesthetic modes.

Inclusion Criteria:

• Patients undergoing elective cardiac surgery

Exclusion Criteria:

• Emergent surgery.
• History of previous cardiac surgery.
• Significant arrhythmias.
• More than trivial valvular disorder.
• Absence of written informed consent.
• Contraindications for transesophageal echocardiography.
• Pregnant women.