50. PERIOPERATIVE MYOCARDIAL ISCHAEMIA

In a conscious patient, myocardial ischaemia usually causes chest pain and/or shortness of breath (dyspnoea). In an anaesthetised patient, myocardial ischaemia is usually recognised by changes in the ECG. Myocardial ischaemia occurs when myocardial oxygen demand exceeds myocardial oxygen supply. Myocardial oxygen demand depends mainly on ventricular wall tension, heart rate and contractility. Myocardial oxygen supply depends mainly on coronary blood flow and arterial oxygen content.

Prevention

The anaesthetist must identify which patients are at risk of myocardial ischaemia and avoid and treat perioperative events that worsen the balance between myocardial oxygen supply and demand.

Patients with a high risk of myocardial ischaemia include those with unstable coronary artery disease, recent myocardial infarction, untreated congestive cardiac failure, severe valvular disease and symptomatic ventricular arrhythmias or supraventricular arrhythmias with a rapid ventricular rate.

Patients with intermediate risk include mild angina, previous myocardial infarction, treated heart failure and diabetes.

Patients at low risk include old age, abnormal ECG and uncontrolled hypertension.

High and moderate risk surgery includes vascular, thoracic, carotid, abdominal, major orthopaedic and emergency surgery.

Myocardial oxygen supply must be maintained to meet demand. Myocardial oxygen supply will be reduced by reducing coronary blood flow (tachycardia, hypotension) and by reducing arterial oxygen content (anaemia, hypoxaemia). Myocardial oxygen demand will be increased by increased wall tension (hypertension, hypervolaemia), tachycardia and increased contractility.

Treatment

The anaesthetist must check that the patient is oxygenated and ventilating. Give 100% oxygen.

The blood pressure and heart rate must be assessed. Treat any precipitating event.

Myocardial oxygen demand must be reduced. Tachycardia is the most important determinant of increased myocardial oxygen demand. Deepen the anaesthesia if appropriate. Reduce the heart rate with a beta-blocker. (Intravenous repeated doses of esmolol 0.25 to 0.5 mg/kg, labetolol 5 to 10 mg or propranolol 0.25 to 1 mg). Aim for a heart rate of 50 to 60/minute.

Treat hypertension. Nitrates will reduce preload (wall tension) by venodilation, thus reducing myocardial oxygen demand. (Sublingual nitroglycerine 0.3 mg. Intravenous nitroglycerine 10 micrograms per minute infusion, increasing by 10 micrograms every 3 to 5 minutes, until there is a reduction in symptoms or hypotension).

 Ensure adequate coronary perfusion by treating bradycardia and hypotension. Use inotropic drugs with care as they may increase myocardial oxygen demand.

Aspirin 160 to 325 mg should be given unless there is a contraindication.

Inform the surgeon and discuss completing the surgery as soon as possible. If possible, transfer the patient to a high dependency ward for postoperative management.

If the myocardial ischaemia fails to respond to treatment it is important to re-evaluate the patient. They may have an acute coronary syndrome. Patients with reversible ST segment changes or T wave inversion should be treated as angina. Those with non-reversible ST segment elevation should be investigated for possible myocardial infarction and evaluated for reperfusion by thombolysis as soon as possible (if available).

Postoperatively patients should be monitored for ischaemia. The risk of ischaemia may be reduced by postoperative oxygen, maintaining the blood volume, avoiding anaemia, continuing beta blockade, aspirin and excellent pain management.

Untreated myocardial ischaemia can cause myocardial infarction, arrhythmias and cardiac arrest.