Hypotensive anaesthesia 5 mark answer
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Hypotensive Anaesthesia
Definition
Hypotensive (controlled) anaesthesia is the deliberate, elective lowering of arterial blood pressure during surgery to a safe but haemodynamically controlled level in order to reduce intraoperative blood loss and improve the surgical field. - Morgan and Mikhail's Clinical Anesthesiology, 7e, p. 475
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
| Agent | Mechanism |
|---|---|
| Volatile anaesthetics (isoflurane, sevoflurane) | Reduce SVR; note - may paradoxically worsen nasal field in FESS by vasodilating nasal microvasculature |
| Sodium nitroprusside (SNP) | Peripheral vasodilator; rapid onset/offset; risk of cyanide toxicity with prolonged use |
| Nitroglycerin | Predominantly venodilator; reduces preload |
| Beta-blockers (esmolol, labetalol) | Reduce HR and cardiac output; preferred in FESS - HR ~60 bpm is the target |
| Calcium channel blockers | Reduce SVR |
| Remifentanil (short-acting opioid) | Used in TIVA protocols; reduces HR and MAP |
| Sympathetic antagonists (ganglionic blockers) | Reduce sympathetic outflow |
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)
- Hypotensive anaesthesia = elective lowering of BP to minimise blood loss and optimise the surgical field
- Safe MAP target: 50-60 mmHg in healthy patients; no more than 20-30% reduction from baseline in hypertensives
- Achieved by neuraxial block, patient positioning, volatile agents, vasodilators (SNP), beta-blockers, or remifentanil-based TIVA
- Relative contraindications include cerebrovascular, cardiovascular, renal, and hepatic disease
- Complications range from cerebral ischaemia and MI to renal failure and blindness; incidence declining as safer alternatives are available
Source: Morgan and Mikhail's Clinical Anesthesiology, 7e, pp. 475-476; Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol 1 & 2
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