Number 29, 2006
Bypass surgery for coronary artery disease: a vanishing treatment?

Managing contractile dysfunction after cardiac surgery

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Armand R.J. Girbes, A.B. Johan Groeneveld
Department of Intensive Care Medicine, VU University Medical Center, Amsterdam, The Netherlands
Correspondence: Professor A.R.J. Girbes, Department of Intensive Care, VU University Medical Center, PO Box 7057, 1007 MB Amsterdam, The Netherlands.
Tel: +31 20 4443924; fax: +31 20 4443901; e-mail: arj.girbes@vumc.nl

Introduction
Patients after cardiac surgery are in general treated in the intensive care unit (ICU). Although recovery from the operation may be rapid, the initial treatment is of utmost importance to prevent additional complications. In the immediate postoperative phase, several potential risks and dangers have to be faced. For prompt and appropriate diagnosis and response, an ICU organization that has sufficient medical and nurse staffing is, in our view, the best environment in which to deal with these problems [1].

Complications after cardiac surgery
Potential complications depend on the consequences of conditions present both before and during the operative procedure. Re-operation, emergency procedures, preoperative use of intra-aortic balloon pump pulsation, congestive heart failure, combined coronary artery bypass graft and valve surgery, older age and pre-existing comorbidity such as renal function loss, chronic obstructive pulmonary disease, diabetes mellitus, and cerebrovascular disease are predictors of postoperative morbidity [2]. In addition, the duration of cardiopulmonary bypass, amount of blood loss, and hemodynamic instability during operation together will determine the clinical status of the patient who arrives in the ICU after cardiac surgery. Several more or less severe derangements are present in the initial phase in the ICU: hypothermia, inadequate intravascular volume, systemic inflammatory response syndrome with capillary leakage, continuing mediastinal blood loss (although this can be minimal), and incomplete emergence from anesthesia. Apart from all this, myocardial dysfunction is present.
Myocardial dysfunction can be related to the preoperative situation, such as diastolic dysfunction with low left ventricular compliance as a result of myocardial hypertrophy, as in the case of chronic hypertension or aortic valve stenosis. Repair of mitral valve incompetence may also unmask left ventricular systolic failure [3]. Furthermore, right ventricular dysfunction may occur, especially in patients with previous (unknown) pulmonary hypertension. However, apart from all these “common causes”, myocardial dysfunction will occur in all patients, in relation to global myocardial ischemia and restoration of (coronary) blood flow after cardiopulmonary bypass. The postischemic but viable myocardium requires a period of hours to days before function is fully restored. Heyndrickx et al [4] were the first to report that reversibly injured myocardium failed to contract. This prolonged, postischemic dysfunction of viable tissue salvaged by reperfusion is called “stunning”. The duration of stunning is dependent on the duration and extent of ischemia, in addition to the adequacy of the return of arterial flow [5].

Clinical management
The heart is designed to deliver oxygen to peripheral tissues. This delivery must also be adequate directly after cardiac surgery. As outlined above, several factors may jeopardize the delivery of sufficient oxygen by the heart, therefore the clinician should make a diagnosis and determine which factor – or, most often, which several factors – may cause problems in a specific patient. Furthermore, failure of the operation, such as occlusion of bypass grafts or insufficiently corrected valvular lesions, should be considered. A group of clinical parameters are the indicators of insufficient oxygen delivery [6]. These include blood pressure, heart rate, cardiac output/stroke volume, oxygen delivery, central venous or mixed venous saturation, filling pressures, diuresis if not affected by diuretics, serum lactate concentration, and blood gas analysis. In our view, there is no single indicator that possesses sufficient specificity or sensitivity to be reliable in clinical practice.
It is important to realize that, in general, more than one problem is present to a varying degree in the early postoperative course of the cardiac surgical patient. Apart from vasoconstriction caused by hypothermia or related to inadequate filling of the vascular bed, vasodilatation may occur as a result of a systemic inflammatory response, associated with cardiopulmonary bypass [7,8]. The vasodilatation is accompanied by capillary leakage, which underscores the need for superior surveillance of fluid load and adequate fluid resuscitation in the early phase. Adequate fluid resuscitation establishing “optimal preload” of the ventricles, as we described previously [6], will ensure maximum cardiac performance for the lowest expenditure of energy.

Treatment of stunning
In patients after cardiopulmonary bypass, stunning of the myocardium is, generally speaking, always present. An important feature of the biology of the stunned myocardium is that the tissue responds well to inotropic stimulation. Previous concern that inotropic stimulation of the myocardium worsens long-term recovery of the heart has been shown to be ill-founded: recovery to normal of the stunned myocardium has been found to occur at the same rate as that in myocardium that had not received inotropic stimulation [9,10].
Dobutamine, dopamine, epinephrine, norepinephrine, and phosphodiesterase inhibitors such as enoximone or milrinone can all be used as inotropic drugs to treat myocardial stunning. There are no randomized controlled studies of sufficient power that have compared the efficacy or safety of the different inotropic drugs, therefore making a correct hemodynamic diagnosis and knowledge of the pharmacological properties of these vasoactive drugs are the basis for appropriate prescription. Their properties are listed in Table I.

Table I. Effects of naturally occurring and synthetic catecholamines on adrenergic receptors and dopamine (DA) receptors.

Table II. Suggested hemodynamic treatment (combined with “optimal” preload) in patients exhibiting signs of insufficient oxygen delivery as indicated by, for example, low mixed venous saturation, lactic acidosis, or poor organ perfusion.

Conclusion
Hemodynamic problems in patients' post cardiac surgery are, in general, complex. After cardiopulmonary bypass surgery, myocardial stunning is one of the problems encountered, but other contributing factors should also be sought, and treatment should be based on the diagnosis. Myocardial stunning can safely be treated pharmacologically with inotropes, and the choice and combination of the inotropic drugs to be used depends on whether additional effects are desired from treatment with vasoactive drugs, if necessary complemented by a mechanical assist device.

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