Intraoperative Hypotension & MAP

Defining intraoperative hypotension

There's no universal definition, which partly explains why intraoperative hypotension (IOH) has been so hard to study rigorously. The most common approaches:

• Absolute: MAP <65 mmHg for any duration
• Relative: MAP falling more than 20–30% below the pre-induction baseline
• Time-weighted: Duration-weighted average MAP below 65 mmHg during the case

By most definitions, IOH occurs in 30–75% of general anaesthetic cases. For decades it was treated as an incidental side effect of anaesthesia. That changed when large observational cohort studies showed a clear dose-response relationship: the longer and deeper the MAP fell below 65 mmHg, the higher the rate of postoperative AKI and myocardial injury. Even episodes lasting 1–5 minutes below MAP 55 mmHg showed independent harm signal in the Sessler et al. 2018 Anesthesiology study of over 57,000 patients.

Why it matters — the evidence base

The consequences of IOH break down by organ:

• AKI — the most consistently linked outcome. Renal autoregulation is impaired under anaesthesia, making kidneys particularly vulnerable. Postoperative AKI means longer ICU stays, dialysis risk, and CKD progression
• Myocardial Injury after Non-cardiac Surgery (MINS) — troponin rise meeting MI criteria without symptoms. Occurs in about 8% of patients over 45 undergoing non-cardiac surgery. IOH is an independent predictor, particularly in patients with pre-existing CAD
• Postoperative delirium — prolonged cerebral hypoperfusion contributes to cognitive changes, particularly in elderly patients with pre-existing cerebrovascular disease
• 30-day mortality — independently associated with sustained IOH in multiple large cohort studies

POISE-2 and INPRESS — the pivotal trials

POISE-2 (Devereaux et al., NEJM 2014) randomised 10,010 patients undergoing non-cardiac surgery to perioperative clonidine or placebo. Clonidine lowers blood pressure reliably. The result: clonidine significantly increased the rate of MAP below 65 mmHg for more than 5 minutes, and this increase was accompanied by higher 30-day mortality and non-fatal cardiac arrest. Pharmacological lowering of perioperative MAP below 65 mmHg is harmful. The trial made that concrete at population scale.

The INPRESS trial (Futier et al., JAMA 2017) took a different angle: 298 high-risk surgical patients randomised to individualised MAP targets (≥80% of the pre-induction baseline) versus standard care. Individualised targeting significantly reduced the composite of postoperative organ dysfunction. Taken together, POISE-2 and INPRESS argue for the same thing: maintain MAP above a patient-specific threshold, actively, throughout the case.

What causes IOH

Goal-directed haemodynamic therapy

GDHT shifts perioperative haemodynamic management from reactive (treat hypotension when it happens) to proactive (prevent it using real-time monitoring). The additional parameters beyond MAP:

Multiple RCTs support GDHT for high-risk major abdominal surgery — reduced postoperative AKI and GI complications, shorter hospital stay. ERAS guidelines recommend it. For low-risk routine cases, standard MAP monitoring is appropriate.

Vasopressor choice in the OR

Phenylephrine — pure α1 agonist. First-line for spinal-induced hypotension, especially in obstetrics. Fast, predictable, short-acting. Causes reflex bradycardia and reduces CO in low-output states — use with caution in cardiac patients. In obstetrics, preferred over ephedrine because it causes less fetal acidosis.

Ephedrine — α and β effects together. Better when hypotension accompanies bradycardia (common with high spinal anaesthesia). Crosses the placenta, can cause fetal tachycardia — phenylephrine preferred in obstetric spinals.

Norepinephrine infusion — increasingly used for major surgery requiring sustained MAP support, particularly when a distributive component is present (post-induction vasodilation from volatile agents). More controllable than repeated phenylephrine boluses over a long case.

Monitoring methods

• NIBP at 1–5 minute intervals — adequate for most routine low-risk cases. Standard in most ORs. Misses brief MAP excursions between readings.
• Continuous non-invasive BP (CNAP/ClearSight) — finger cuff technology, beat-to-beat MAP without arterial cannulation. Accuracy is lower than direct arterial measurement at very low MAPs but useful for intermediate-risk cases.
• Invasive arterial line — gold standard. Beat-to-beat MAP plus SVV/PPV for fluid responsiveness. Standard for cardiac surgery, major vascular surgery, and high-risk non-cardiac cases.

Key takeaways

• IOH (MAP <65) occurs in 30–75% of GA cases and independently causes AKI, myocardial injury, and increased 30-day mortality
• Even brief episodes (<5 minutes) below MAP 55 mmHg show independent harm signal — there's no truly "safe" floor below 65
• POISE-2: pharmacological lowering of perioperative MAP below 65 mmHg increased mortality and cardiac arrest
• INPRESS: individualised MAP targets (≥80% of baseline) reduced postoperative organ dysfunction vs standard care
• GDHT combines MAP with SVV, CO, and stroke volume — reduces complications in high-risk major surgery
• Phenylephrine for spinal-induced hypotension; norepinephrine infusion for sustained major surgery support
• Withhold ACE-I and ARBs on the morning of major surgery — they cause refractory IOH
• Chronic hypertensives need MAP targets ≥80% of their individual baseline, not just 65 mmHg

Sources & references

• Devereaux PJ et al. Aspirin in Patients Undergoing Noncardiac Surgery. N Engl J Med 2014 (POISE-2)
• Futier E et al. Effect of Individualised vs Standard Blood Pressure Management on Postoperative Organ Dysfunction. JAMA 2017 (INPRESS)
• Sessler DI et al. Perioperative Quality Initiative consensus statement on intraoperative blood pressure. Br J Anaesth 2019
• Feldheiser A et al. Enhanced Recovery After Surgery: consensus statement for anaesthesia practice. Acta Anaesthesiol Scand 2016.