Postoperative Hypotension

Postoperative hypotension is systolic pressure under 90 mmHg or mean arterial pressure under 65 mmHg, or a fall of over 20% to 30% from baseline, after surgery.

Epidemiology. Occurs in about 10% to 30% in the recovery room. Continuous monitoring shows episodes in 30% to 50% of patients on wards in the first 3 days. It is linked to acute kidney injury, myocardial injury, stroke and death.

Causes by frequency.

• Hypovolaemia: bleeding, third space loss, dehydration, vomiting, ileus.
• Vasodilation: residual anaesthetic, neuraxial block, sepsis, anaphylaxis, opioids, antihypertensives.
• Cardiac: infarction, arrhythmia, heart failure, valve disease.
• Obstructive: tension pneumothorax, pulmonary embolism, tamponade.
• Endocrine: adrenal insufficiency, hypothyroidism.
• Vagal reactions and hypothermia.

Pathophysiology.

Blood pressure reflects cardiac output and systemic vascular resistance. A fall in preload, contractility or vascular tone reduces perfusion pressure.

Organ autoregulation fails under its lower limit, which is higher in chronic hypertension, so hypoperfusion injures kidneys, heart and brain.

Low blood pressure, tachycardia (absent with beta blockade), cool and clammy or warm and vasodilated skin, oliguria under 0.5 mL/kg/h, confusion, delayed capillary refill, chest pain and dizziness.

Red flags.

• Shock index (heart rate over systolic pressure) over 1.0.
• Lactate over 2 mmol/L.
• Distended abdomen, bloody drains or wound bleeding.
• Raised jugular venous pressure with muffled heart sounds.
• Tracheal deviation or unilateral absent breath sounds.
• Urticaria, wheeze or angioedema.
• Chest pain with ECG change.

• Repeat blood pressure manually and check the arterial line transducer.
• Examine for bleeding, volume status, abdomen and chest.
• ECG, point-of-care ultrasound (ventricular function, inferior vena cava, effusion, right heart), haemoglobin, lactate, blood gas, urea, electrolytes, troponin, cultures and cross-match.
• Cortisol if adrenal insufficiency is possible.
• Imaging (chest radiograph, focused abdominal ultrasound, CT) if bleeding or collection is suspected.

Differential diagnosis.

• Haemorrhage.
• Hypovolaemia.
• Sepsis.
• Anaphylaxis.
• Neuraxial vasodilation.
• Myocardial infarction.
• Pulmonary embolism.
• Tension pneumothorax.
• Tamponade.
• Adrenal insufficiency.
• Drug effect.

Target. Mean arterial pressure at least 65 mmHg, higher if chronic hypertension, and adequate urine output and lactate clearance.

Fluid. Give a balanced crystalloid bolus of 250 to 500 mL over 5 to 15 minutes, then reassess. Stop if there is no response or signs of pulmonary oedema. Transfuse blood for haemorrhage at haemoglobin under 70 g/L (under 80 g/L with cardiovascular disease), or earlier with active bleeding, using balanced massive transfusion for major bleeding.

Vasopressors.

• Noradrenaline: 0.01 to 0.5 microgram/kg/min IV infusion, started at 0.05 microgram/kg/min; titrated to mean arterial pressure; reassess the diagnosis above 0.5 microgram/kg/min; continuous arterial pressure and peripheral site checks (central line preferred); reduce if arrhythmia or ischaemia.
• Phenylephrine (vasodilation or neuraxial block): 50 to 100 microgram IV bolus, repeated every 2 to 3 minutes; infusion 0.5 to 1.4 microgram/kg/min; monitor heart rate and blood pressure; avoid with bradycardia or low cardiac output.
• Ephedrine (vasodilation with bradycardia): 3 to 6 mg IV every 3 to 4 minutes; monitor heart rate; tachyphylaxis limits repeated doses.

Cause directed.

• Anaphylaxis: adrenaline 0.5 mg (500 microgram) IM into the anterolateral thigh, repeated every 5 minutes if needed; monitor ECG and blood pressure; use IV titration in arrest or refractory shock.
• Adrenal insufficiency: hydrocortisone 100 mg IV, then 50 mg every 6 hours or 200 mg over 24 hours; stop when stable; monitor glucose and sodium.
• Tension pneumothorax: decompress and place a drain.
• Tamponade: pericardiocentesis or surgical drainage.
• Haemorrhage: surgical or radiological control.
• Neuraxial vasodilation: raise the legs, give fluid and a vasopressor.

Withhold antihypertensives until stable.

Monitoring. Arterial line if persistent or severe, lactate, urine output, haemoglobin and ECG.

Referral and escalation. Critical care if a noradrenaline infusion is needed or hypotension is refractory. Surgical review for bleeding or collections. Cardiology for infarction or tamponade.

Special groups. In chronic hypertension, aim for a higher mean arterial pressure. In heart failure give fluid in smaller aliquots.