45. INTRA-OPERATIVE HYPOTENSION

Hypotension is a fall in blood pressure of more than 20% below the preoperative blood pressure or a mean arterial pressure of less than 60 mmHg. Hypotension is an unintended event that often occurs during anaesthesia. The anaesthetist must treat the hypotension, diagnose the cause and treat the cause at the same time. The most common causes of hypotension include a relative overdose of anaesthetic agents, hypovolaemia and epidural/spinal anaesthesia.

Prevention

All patients must be assessed before anaesthesia. Patients who are hypovolaemic must receive intravenous fluids before induction of anaesthesia. The dose of anaesthetic agents must be adjusted depending on the health, age and weight of the patient.

Management

The anaesthetist should aim to keep the patient’s blood pressure within 20% of the normal preoperative blood pressure. If hypotension occurs they must ensure that the patient is oxygenated and ventilating. Increase the inspired concentration of oxygen if the patient is poorly oxygenated or if the hypotension is severe. Hypoxaemia will cause hypotension.

Check the blood pressure and look at the ECG. Both bradycardia and tachycardia can cause hypotension. Arrhythmias and myocardial ischaemia will cause hypotension.

Decrease the anaesthetic agents. A relative overdose of anaesthetic agent is a common cause of hypotension.

Increase the circulating blood volume by giving intravenous fluids (10 to 20 ml/kg). Check the response to the intravenous fluid. Hypovolaemia is a very common cause of hypotension. If hypovolaemia is the cause, continue giving intravenous fluids until the heart rate and blood pressure return to normal. Assess the blood loss, urine output, capillary return and intravenous fluids given.

Give a vasopressor (e.g. ephedrine, epinephine, metaraminol).

If hypotension is severe notify the surgeon and call for help.

If the initial assessment and treatment does not treat the hypotension, continue to try to restore the blood pressure to normal with intravenous fluids and vasopressors, ensuring that the patient is oxygenated and ventilated.

Consider less common causes of intra-operative hypotension:

1.             mechanical obstruction of venous return by surgical           instruments or aortocaval compression in obstetric patients.

2.             pericardial tamponade.

3.            pulmonary embolus.

4.            valvular heart disease.

5.            increased intrathoracic pressure: tension pneumothorax.

6.            anaphylaxis.

7.            severe sepsis.

8.            cardiac failure.

9.            endocrine abnormalities (for example, addisonian crisis,  hypothyroidism or hypogylcaemia).

10.            myocardial ischaemia/infarction