Hypotensive Anaesthesia

Definition

Indications (Procedures Benefitting Most)

• Cerebral aneurysm repair (reduces transmural tension, prevents rupture/rebleeding, facilitates clipping)
• Brain tumour resection
• Total hip arthroplasty
• Radical neck dissection
• Radical cystectomy
• Major spinal surgery (scoliosis correction, etc.)
• Ear surgery (middle ear, stapes surgery) - reduces ooze in the operative field
• Endoscopic sinus surgery (FESS) - target MAP ~90 mmHg (systolic) or HR ~60 bpm
• Operations with expected major haemorrhage

Target Blood Pressure

• Healthy young adults: MAP can be lowered to 50-60 mmHg safely
• Chronically hypertensive patients: should not drop more than 20-30% below baseline
• Patients with a history of TIAs may not tolerate any reduction in cerebral perfusion pressure
• Recent evidence suggests the lower limit of cerebral autoregulation may be higher than previously assumed; hence use of this technique is declining

Methods of Achieving Controlled Hypotension

Non-Pharmacological

• Neuraxial anaesthesia (spinal/epidural) - reduces SVR and sympathetic tone
• Elevation of the surgical site - selectively reduces BP at the wound
• Positive-pressure ventilation (PPV) - increases intrathoracic pressure, impedes venous return, lowers cardiac output and MAP

Pharmacological Agents

Contraindications (Relative)

• Severe anaemia (already reduced O2-carrying capacity)
• Hypovolaemia
• Atherosclerotic cardiovascular disease
• Renal or hepatic insufficiency
• Cerebrovascular disease
• Uncontrolled glaucoma (raised IOP worsens with hypotension)

Complications

• Cerebral thrombosis / stroke
• Hemiplegia (decreased spinal cord perfusion)
• Acute tubular necrosis (renal ischaemia)
• Massive hepatic necrosis
• Myocardial infarction
• Cardiac arrest
• Blindness - retinal artery thrombosis or ischaemic optic neuropathy (especially in prone spine surgery)
• Risk is amplified in patients with coexisting anaemia
• Patients in beach chair / sitting position (shoulder surgery) are at particular risk for cerebral hypoperfusion and perioperative infarction

Monitoring

• Intra-arterial BP monitoring - transducer zeroed at the level of the external auditory meatus (level of the circle of Willis) to accurately measure cerebral perfusion pressure
• Cerebral oximetry (NIRS) - recommended when hypotensive anaesthesia is used
• ECG, pulse oximetry, urine output, temperature monitoring as standard

Key Points (Summary)

1. Hypotensive anaesthesia = elective lowering of BP to minimise blood loss and optimise the surgical field
2. Safe MAP target: 50-60 mmHg in healthy patients; no more than 20-30% reduction from baseline in hypertensives
3. Achieved by neuraxial block, patient positioning, volatile agents, vasodilators (SNP), beta-blockers, or remifentanil-based TIVA
4. Relative contraindications include cerebrovascular, cardiovascular, renal, and hepatic disease
5. Complications range from cerebral ischaemia and MI to renal failure and blindness; incidence declining as safer alternatives are available