Heart and Metabolism

Number 29, 2006
Bypass surgery for coronary artery disease: a vanishing treatment?
Hibernating myocardium and coronary artery bypass surgery

Ian Hunt, Tunde Ogunyiluka
Department of Cardiology, St Thomas’ Hospital, London, UK
Correspondence: Mr Ian Hunt, Department of Cardiothoracic Surgery, St Thomas’ Hospital, Lambeth Palace Road, London, UK.
Tel: +44 207 1887188; fax: +44 207 1881006; e-mail: ian.hunt@gstt.nhs.uk

Abstract

Left ventricular dysfunction occurs commonly as a result of coronary artery disease and may be associated with hibernating myocardium, a state of resting myocardial dysfunction resulting from reduced coronary blood flow that can be partially or completely reversed after myocardial revascularization. Assessment of myocardial viability can be achieved by several different methods, none of which is currently regarded as the gold standard. A readily available noninvasive method is dobutamine stress echocardiography. Surgical revascularization in patients with left ventricular dysfunction and suspected hibernating myocardium is associated with significant improvement in symptoms and outcome compared with medical treatment. However, selection of patients is of paramount importance.

Keywords: Coronary artery disease, hibernating myocardium, myocardial stunning, surgical revascularization

Case report
An 83-year-old man was referred to the chest pain clinic with a 1-year history of progressive exertional angina that had recently become more frequent (Canadian Cardiac Society [CCS] grade III) and breathlessness on exertion (New York Heart Association [NYHA] grade III). The patient had a past history of hypercholesterolemia, hypothyroidism, and had been an ex-smoker for 15 years. He performed an exercise stress test (Bruce protocol) as part of the initial assessment and completed 2 min 37 s. The test was stopped because of chest pain associated with significant ST-segment depression (3.5 mm) in precordial leads V4 and V5 (Figure 1). He was started on a medication regimen comprising bisoprolol and a glyceryl trinitrate spray in addition to aspirin and atorvastatin, and was referred for a coronary angiogram.

Figure 1. Exercise stress test showing significant ST-segment depression in the anterior territory. Baseline trace on the left, maximum (MAX.) ST-segment changes on the right. METs, metabolic equivalent of task units.

Coronary arteriography revealed an occluded right coronary artery that filled retrogradely, a normal left main stem, severe diffuse disease in the left anterior descending artery, and a proximally occluded circumflex artery (Figure 2). Left ventricular injection suggested moderate ventricular function. A subsequent transthoracic echocardiogram revealed hypokinesia of the lateral, mid, and inferior apical left ventricular walls, with an estimated ejection fraction of 50–55%. The patient's case was discussed in a joint Cardiology–Cardiac surgery meeting and he was felt to be a candidate for surgery if evidence of viable myocardium could be demonstrated. A dobutamine stress echocardiogram was requested.

Figure 2. Angiogram showing a diffusely diseased left anterior descending artery and a proximal 90% circumflex stenosis. Retrograde filling of the right coronary artery from the left system suggests that this vessel is occluded.

On stress echocardiography, all myocardial segments demonstrated increased contractility in response to low-dose dobutamine, suggestive of viability (Figure 3), with 2–3 mm anterolateral ST-segment depression in leads V3–V6 on the electrocardiogram.

Figure 3. Stress echocardiograms. (a) Baseline apical four-chamber view during diastole. (b) Baseline apical four-chamber view during systole. Note that, apart from the basal interventricular septum, there is little or no further thickening of the left ventricular wall compared with that in diastole. (c) Apical four-chamber view in diastole during infusion of dobutamine 40 μg/kg per min. A contrast solution was administered to improve the delineation of the left ventricular wall. (d) Systole during infusion of dobutamine 40 μg/kg per min. Compare with (c), and note the marked thickening of all regions of the left ventricular wall.

The patient underwent on-pump double coronary artery bypass surgery with a left internal mammary artery anastomosed to the distal left anterior descending artery and a saphenous vein graft to the first Obtuse Marginal (OM1) branch of the circumflex artery. His postoperative recovery was relatively uneventful, but he developed a superficial sternal infection, which responded to flucloxacillin. At 6-week follow-up, he was symptomatically much better, with an improved exercise tolerance (CCS grade I, NYHA grade II).

Discussion
Coronary artery disease, presenting as myocardial ischemia or infarction, is a leading cause of left ventricular dysfunction, with significant associated morbidity and mortality. Through better understanding of the pathophysiological mechanisms of left ventricular dysfunction and developments in cardiac imaging, it has become apparent that dysfunctional myocardium may remain potentially viable, affording an opportunity to restore blood flow by means of revascularization [1].
“Hibernating myocardium” (Table I) is a term used to describe a chronic condition of resting left ventricular dysfunction, caused by reduced coronary blood flow, that can be partially or completely reversed after myocardial revascularization, by reducing myocardial oxygen demand, or both [2,3].

Table I. Characteristics of myocardial hibernation and methods of detection.

A phenomenon of reversible global left ventricular dysfunction after a brief period of coronary arterial occlusion and reperfusion has also been identified, and is referred to as “myocardial stunning”, a state of postischemic myocardial dysfunction [4]. This state may contribute to reversible cardiac failure after acute infarction (with or without thrombolysis), immediately after coronary artery bypass grafting (CABG) or subsequent to cardiac arrest [2]. It has also been postulated that repetitive, intermittent ischemic episodes leading to a chronic myocardial stunned state could underlie the baseline contractile dysfunction of hibernating myocardial segments [5,6].
Several tests can assist in the evaluation of myocardial viability and contractile reserve and the identification of patients in whom there is the potential for recovery of left ventricular dysfunction with revascularization. These tests include myocardial perfusion imaging by thallium scanning or positron emission tomography (PET) – an increase in inotropy using pharmacological or nonpharmacologic stimulation during left ventriculography, echocardiography, or PET. Computed tomography and magnetic resonance imaging can also detect hibernating myocardium.
Detecting viable myocardium, whether hibernating or stunned, is of paramount clinical importance. The best method to assess viability, and the gold standard to which the techniques should be held, remain unclear. In the patient described here, dobutamine stress echocardiography was utilized. This method of noninvasive assessment of detecting hibernating myocardium has emerged as an important, readily available technique familiar to most cardiac surgeons [7]. Stress echocardiography examines the “inotropic reserve” of dysfunctional but viable myocardium through stimulation by inotropic agents – most commonly dobutamine, but also dopamine [8]. Viable myocardium shows improved global (ejection fraction) or regional contractile function (inotropic reserve), as assessed by simultaneous transthoracic echocardiography, in response to these agents [9].
It has been demonstrated in patients with myocardial viability on dobutamine stress echocardiography that a better outcome occurred with surgical revascularization than with medical treatment [10]. Similarly, the results of stress echocardiography can predict long-term outcome on the basis of the degree of myocardial viability (as measured by the number of segments, out of 12) before and after CABG [11]. However, despite a high predictive value, this method does appear to underestimate the extent of viable myocardium compared with results from thallium perfusion scanning [12]. The potential for recovery is assumed to be great enough to recommend revascularization when the total of hibernating and ischemic, but still functioning, myocardium is more than 60% of the left ventricle. In contrast, when more than 40% of the left ventricle is considered to be scarred or is metabolically inactive, surgical mortality is much greater, and the likelihood of the recovery of left ventricular dysfunction from CABG is much less. Accordingly, this generally means that the anterior wall territory as supplied by the left anterior descending artery must be viable [5,13,14].
In addition to myocardial viability, the degree of left ventricular enlargement is another important factor for the surgeon to take into account before considering the patient for revascularization. Several studies have demonstrated a relationship between left ventricular end-systolic volume as assessed by echocardiography and improvement in left ventricular dysfunction after CABG. When the left ventricular end-diastolic dimension is greater than 7 cm, operative mortality is likely to be high [15,16].

Conclusion
Most [17–20], but by no means all [21,22], studies have shown that revascularization of hibernating myocardium – identified by various techniques, including dobutamine stress echocardiography – can improve left ventricular dysfunction, clinical symptoms, and outcome, compared with what is achieved with medical therapy.
The indications, choice, and sequence of investigations in patients with left ventricular dysfunction and suspected hibernating myocardium remain a topic of debate, but several algorithms have been proposed (Figure 4). Essentially, all algorithms should include clinical assessment of heart failure, imaging to evaluate the presence of myocardial viability, and, if the patient is a candidate for invention, a coronary angiogram to assess their suitability for revascularization.

Figure 4. Indications for intervention and investigative strategy in patients with viable myocardium. CABG, coronary artery bypass grafting; CHF, congestive heart failure; PET, positron emission tomography. (Adapted from Marwick [23], with permission.)

REFERENCES

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Number 29, 2006Bypass surgery for coronary artery disease: a vanishing treatment? Hibernating myocardium and coronary artery bypass surgery

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

Number 29, 2006
Bypass surgery for coronary artery disease: a vanishing treatment?
Hibernating myocardium and coronary artery bypass surgery

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