Intraoperative hypotension: differential and management
Why examiners ask this
Hypotension is the commonest intraoperative event in every stem, so it is where the examiner tests whether I think in a structure or reach for phenylephrine by reflex. A good answer is organized by physiology, ranks the likely causes for this patient and this operation, and treats while diagnosing. It stresses organization and adaptability, because the first treatment usually fails in the stem.
The 60-second answer
I confirm the number first: check the cuff size or flush and level the arterial transducer, and feel a pulse. Then I treat and think at the same time. I reduce the anesthetic depth if it is excessive, give a vasopressor to restore perfusion pressure, and ask the surgeon what is happening in the field. I work through the physiology, which maps onto the four shock mechanisms of hypovolemic, distributive, cardiogenic and obstructive:1 preload (bleeding, venous pooling, caval compression, pneumoperitoneum, positive pressure or auto-PEEP), afterload (anesthetic vasodilation, neuraxial block, anaphylaxis, sepsis), contractility (ischemia, drugs, a failing heart) and rate and rhythm. I look for the time-critical causes: tension pneumothorax, tamponade, embolism and anaphylaxis. I keep the mean arterial pressure at or above 60–65 mmHg (or systolic at least 90 mmHg),2 and within 20% of baseline in a chronically hypertensive patient, because sustained values below that are linked to kidney and myocardial injury.
Decision points and the "why"
- Confirm before treating — a damped line or a cuff on the wrong arm creates hypotension that does not exist; it takes five seconds.
- Phenylephrine or ephedrine first — phenylephrine 50–100 mcg for vasodilation with a normal or fast heart rate; ephedrine 5–10 mg when the rate is slow, because it adds chronotropy. Persistent need moves me to a norepinephrine infusion, which I run peripherally at low dose.
- Fluid only when responsive — I give a 250–500 mL bolus when pulse-pressure variation exceeds about 13% in a ventilated patient in sinus rhythm, or when there is clear loss; unguided fluid drowns a failing ventricle and swells the bowel.
- Vasopressin for ACE inhibitor or ARB vasoplegia — 1–2 units IV; the renin-angiotensin system is blocked, so catecholamines work poorly. Methylene blue 1–2 mg/kg is my next step for refractory vasoplegia. Prevention is holding these drugs for 24 hours before surgery in patients with controlled hypertension.2
- Epinephrine early for anaphylaxis — hypotension with bronchospasm, rash or a rising peak pressure after an antibiotic or relaxant is anaphylaxis; I give epinephrine 10–100 mcg IV boluses escalating, because phenylephrine does not stabilize mast cells.
- Echo when the answer is unclear — TEE or transthoracic echo tells me in a minute whether the ventricle is empty, failing, compressed or obstructed, and I ask for it rather than escalate blindly.
- Communicate — the surgeon controls the commonest cause: bleeding, retraction on the vena cava, insufflation pressure and vagal traction.
The complication they will inject
The pressure falls to 60/30 despite phenylephrine, the peak airway pressure has climbed from 22 to 40 cmH2O, and the SpO2 is falling after a subclavian line placed in the case. I recognize tension pneumothorax: hypotension plus rising airway pressure plus hypoxemia after a procedure near the pleura; positive-pressure ventilation makes tension more likely, and central line placement is a classic iatrogenic cause.3 I call for help, go to 100% oxygen, check breath sounds and lung sliding with ultrasound if it is immediately available, and decompress without waiting for a radiograph,3 with a 14- or 16-gauge catheter in the second intercostal space at the midclavicular line, followed immediately by a chest tube.3 In adults, current trauma teaching prefers the fourth or fifth space just anterior to the midaxillary line, where the chest wall is thinner. I reassess pressure and compliance. If the airway pressure is high but breath sounds are equal, I think of bronchospasm from anaphylaxis and give epinephrine.
Pitfalls that cost points
- Listing a differential without treating the pressure while I search.
- Giving fluid to every hypotensive patient regardless of the heart.
- Repeating phenylephrine five times without changing strategy.
- Forgetting surgical causes and never talking to the surgeon.
- Missing the patterns that cannot wait: tension pneumothorax, tamponade, anaphylaxis, embolism and local anesthetic toxicity; the first two are obstructive shock that no vasopressor fixes.1
Rapid-why
Q: Why is phenylephrine a poor choice in severe aortic stenosis with bradycardia? A: The reflex slowing lowers cardiac output in a fixed-output lesion; I want norepinephrine, which supports pressure with less reflex bradycardia.
Q: Why does pulse-pressure variation fail in a laparoscopy? A: High abdominal pressure, low tidal volumes, arrhythmias or an open chest change the heart-lung interaction, so the threshold no longer predicts fluid response.
Q: Why does hypotension follow intubation? A: Induction agents remove sympathetic tone and positive pressure lowers venous return, worst in a patient who is volume depleted or chronically on renin-angiotensin blockade.
Q: Why is a MAP of 65 a target? A: Large cohort data link time below about 65 mmHg with acute kidney injury and myocardial injury; it is an association, so I individualize to baseline.
Numbers to know
| Item | Value |
|---|---|
| MAP floor | ≥60–65 mmHg (SBP ≥90),2 or within 20% of baseline |
| Phenylephrine bolus | 50–100 mcg IV |
| Ephedrine bolus | 5–10 mg IV |
| Norepinephrine infusion | 0.02–0.2 mcg/kg/min |
| Vasopressin bolus | 1–2 units IV |
| Pulse-pressure variation | >13% predicts fluid response |
Links
- Vasopressors and inotropes: choosing and titrating, anaphylaxis, Cardiac tamponade, including post-cardiac surgery, Pneumothorax and tension pneumothorax, Fluid management: crystalloid, colloid, goal-directed
- Atlas: Monitor atlas: perioperative cardiac events; Volume status, IVC and fluid responsiveness
- Perioperative Quality Initiative (POQI) consensus statement on intraoperative blood pressure
Sources
- 1.Shock · Merck Manual Professional Edition · merckmanuals.com
- 2.2024 ACC/AHA guideline on perioperative cardiovascular management before noncardiac surgery: What's new? · Cleveland Clinic Journal of Medicine, 2025 · ccjm.orgCohn SL, CCJM 92(4), April 2025. Reports the 2024 AHA/ACC intraoperative target of MAP at least 60-65 mmHg or SBP at least 90 mmHg.
- 3.Pneumothorax · Merck Manual Professional Edition · merckmanuals.com
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.