Spinal Anaesthesia-Induced Hypotension

Spinal anaesthesia induced hypotension is a fall in systolic blood pressure (usually under 80% of baseline, or under 90 mmHg) after subarachnoid block, caused by sympathetic blockade.

Epidemiology. It is the commonest complication of spinal anaesthesia, affecting 15% to 33% of general surgical patients and up to 60% to 70% of women having caesarean section without prophylaxis.

Risk factors. Block higher than T5, older age (over 40 years), obesity, hypovolaemia, pregnancy, chronic hypertension, emergency surgery, baseline systolic blood pressure under 120 mmHg, combined sedatives, and heart rate under 60/min.

Pathophysiology.

Block of preganglionic sympathetic fibres (T1 to L2) causes venous pooling (reduced preload) and arterial dilation. High block removes cardiac accelerator tone (T1 to T4), adding bradycardia (Bezold-Jarisch reflex). In pregnancy aortocaval compression adds to the effect.

Nausea, vomiting, dizziness, pallor, sweating, restlessness, yawning, chest discomfort, bradycardia or tachycardia, and in severe cases loss of consciousness. Maternal hypotension can cause fetal acidosis.

Red flags.

• Systolic blood pressure under 80 mmHg or fall over 30% from baseline.
• Bradycardia under 40/min.
• Chest pain or ECG ischaemia.
• Loss of consciousness.

Clinical and by blood pressure measurement every 1 to 2 minutes (every minute during caesarean). Check block height, bleeding, anaphylaxis and cardiac causes if the response is unusual.

Differential diagnosis.

• High spinal.
• Haemorrhage.
• Anaphylaxis.
• Local anaesthetic toxicity.
• Pulmonary embolism.
• Myocardial infarction.
• Aortocaval compression.

Lay the patient flat with left uterine displacement in pregnancy. Give oxygen. Treat promptly, because the trigger is vasodilation.

• Phenylephrine (first line in pregnancy, normal or high heart rate): 50 to 100 microgram IV bolus, or infusion 25 to 50 microgram/min started with block; titrate to systolic blood pressure of 100% of baseline; maximum 200 microgram/min; monitor heart rate and blood pressure; avoid in bradycardia under 60/min.
• Noradrenaline: 4 to 8 microgram IV bolus, or infusion 0.05 to 0.1 microgram/kg/min; titrate; maximum 0.5 microgram/kg/min; monitor blood pressure and heart rate; extravasation risk with peripheral infusion.
• Ephedrine: 6 to 12 mg IV every 2 to 3 minutes (when bradycardia is present); maximum 50 mg; monitor heart rate and blood pressure; tachyphylaxis and fetal acidosis with repeated doses.
• Atropine: 500 microgram IV for bradycardia; maximum 3 mg.
• Fluids: coload with 500 to 1,000 mL crystalloid at the time of block (preloading is less effective than coloading); avoid excess in heart failure and pre-eclampsia.

Prevention. Prophylactic vasopressor infusion started with the block, reduced spinal dose in older adults and obese patients, left uterine displacement and correction of hypovolaemia.

Monitoring. Blood pressure every minute for the first 10 minutes, then every 2 to 5 minutes, ECG, heart rate and nausea.

Referral and escalation. Senior help for refractory hypotension, severe bradycardia or arrest.

Special groups. In pre-eclampsia use lower vasopressor doses. Older adults need slow titration.