[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anaesthesia and ICU](https://durgical.com/en/anaesthesia/) › [Intraoperative crises](https://durgical.com/en/anaesthesia/areas/crises/) › Intraoperative hypotension

# Intraoperative hypotension

- Crisis

Low blood pressure during an operation; it is common, usually treatable, and sometimes the first sign of a serious problem.

Updated Nov 13, 2026

## Key points

- Common thresholds: mean arterial pressure below about 65 mmHg or more than a fifth below baseline; even brief episodes harm the kidneys and heart.
- Mean arterial pressure = cardiac output × systemic vascular resistance; sort causes into vasodilation, low preload, pump failure and obstruction.
- The commonest cause after induction is the anaesthetic itself; treat with lighter anaesthesia, fluids and a vasopressor.
- Phenylephrine suits hypotension with a normal or fast heart rate; ephedrine suits hypotension with bradycardia.
- Hypotension that does not respond means bleeding, anaphylaxis, pneumothorax, tamponade, embolism or ischaemia until proved otherwise.

On this page

- The basics Key points
- What it is
- How to recognise it
- Why it happens
- What to do at once
- The causes of intraoperative hypotension sorted by mechanism
- Costly mistakes
- Next step
- Memory aid
- Elsewhere in DurgiPedia

- Study Test yourself

- Connections Connections

## What it is

Intraoperative hypotension is a fall in blood pressure during anaesthesia below a level that keeps organs well perfused. Definitions vary; common thresholds are a mean arterial pressure below about 65 mmHg or a fall of more than a fifth from the patient's baseline, and even short periods below these levels are associated with acute kidney injury, myocardial injury and stroke. Mean arterial pressure is the product of cardiac output and systemic vascular resistance, and cardiac output depends on heart rate and stroke volume, so every cause fits into a few groups: vasodilation from anaesthetic drugs, neuraxial block or sepsis and anaphylaxis; low preload from bleeding, dehydration, positive-pressure ventilation, pneumoperitoneum or compression of the vena cava; a failing pump from myocardial depression, ischaemia or arrhythmia; and obstruction from tension pneumothorax, tamponade or embolism. Most episodes after induction are drug-induced and respond to reducing anaesthetic depth, fluids and a vasopressor, but the anaesthetist must always confirm the reading, look at the whole picture, talk to the surgeon and consider the dangerous causes when the pressure does not respond. Patients on angiotensin-converting enzyme inhibitors or angiotensin receptor blockers, the elderly and those with heart disease are more vulnerable.

## How to recognise it

- Blood pressure below the threshold set for the patient, on two readings or a continuous arterial trace.
- Check the heart rate and rhythm: bradycardia, tachycardia or a new arrhythmia point to different causes.
- A falling end-tidal CO2 suggests low cardiac output, embolism or arrest.
- High airway pressures with hypotension suggest anaphylaxis, tension pneumothorax or gas trapping.
- Look at the surgical field and suction canisters for bleeding, and at the timing: induction, neuraxial block, positioning, pneumoperitoneum.

## Why it happens

- Most anaesthetic agents dilate vessels, depress the heart and blunt the baroreceptor reflex.
- Neuraxial block causes sympathetic blockade, with vasodilation and, when high, bradycardia.
- Positive-pressure ventilation, raised abdominal pressure and compression of the vena cava reduce venous return.
- Drugs such as angiotensin-converting enzyme inhibitors leave the patient dependent on the renin-angiotensin system, which anaesthesia suppresses.

## What to do at once

- Confirm the reading: cuff size and position, transducer level and damping, and a palpable pulse — artefacts are common.
- Look at the whole monitor: heart rate, rhythm, end-tidal CO2, airway pressure, saturation — the pattern points to the cause.
- Tell the surgeon and ask about bleeding, traction or vessel compression — surgical causes need surgical help.
- Reduce anaesthetic depth if it is excessive, while keeping the patient asleep — deep anaesthesia dilates vessels and depresses the heart.
- Give a fluid bolus if preload may be low, and reassess — fluids help only if the heart responds to volume.
- Give a vasopressor: phenylephrine if heart rate is normal or fast, ephedrine if slow — they restore vascular tone while the cause is found.
- Treat bradycardia with atropine and arrhythmias by their own algorithms — a slow or chaotic heart lowers cardiac output.
- Correct mechanics: tilt pregnant patients left, release pneumoperitoneum, reduce high airway pressure — this restores venous return.
- If it persists, start a noradrenaline infusion, place an arterial line, send a blood gas with haemoglobin, and call a senior.
- Think of the dangerous causes: anaphylaxis, bleeding, tension pneumothorax, tamponade, embolism, ischaemia — each has a specific treatment.

## The causes of intraoperative hypotension sorted by mechanism

| Mechanism | Examples | First treatment |
| --- | --- | --- |
| Vasodilation | Anaesthetic agents, neuraxial block, anaphylaxis, sepsis | Lighter anaesthesia, vasopressor; adrenaline for anaphylaxis |
| Low preload | Bleeding, dehydration, positive-pressure ventilation, pneumoperitoneum, caval compression | Fluids or blood, relieve compression |
| Pump failure | Myocardial depression, ischaemia, arrhythmia, bradycardia | Treat rate and rhythm, inotrope, treat ischaemia |
| Obstruction | Tension pneumothorax, tamponade, pulmonary or gas embolism | Decompress, drain or support while the cause is relieved |

## Costly mistakes

- Repeating vasopressor boluses without looking for the cause.
- Ignoring a falling end-tidal CO2 or rising airway pressure, the clues to embolism, anaphylaxis or pneumothorax.
- Giving fluid after fluid to a patient whose heart no longer responds to volume.
- Accepting long periods of low pressure in elderly or hypertensive patients, whose organs need a higher pressure.
- Forgetting that angiotensin-converting enzyme inhibitors and angiotensin receptor blockers taken that morning cause resistant hypotension.

## Next step

- Call a senior if hypotension does not respond to the first measures, or if a dangerous cause is suspected.
- Continue close monitoring in recovery and set a target blood pressure for the ward, based on the patient's baseline.
- Check for organ injury after a long or severe episode: urine output, creatinine, and troponin with an ECG if ischaemia is possible.
- Review the medicines: plan when to restart antihypertensives, and note any agent that caused a marked fall.
- Record the lowest pressure, its duration, the cause and the treatment in the anaesthetic record.

## Memory aid

Is it real, is it the drugs, is it the tank, the pump or the pipes, or is it blocked? Check the reading, then volume, heart, vessels and obstruction.

## Test yourself

Think of the answer, then tap the question to see it.

## Which thresholds are commonly used to define intraoperative hypotension?

A mean arterial pressure below about 65 mmHg or a fall of more than a fifth from the patient's baseline; even short periods are linked to organ injury.

## Which vasopressor suits hypotension with bradycardia, and which suits it with tachycardia?

Ephedrine, which also raises heart rate, suits bradycardia; phenylephrine, a pure alpha agonist that may slow the heart, suits a normal or fast rate.

## What is the first step when the monitor shows a very low blood pressure?

Confirm the reading: check the cuff, the transducer level and damping, and feel for a pulse, while looking at the other monitors.

## Which causes must be considered when hypotension does not respond to fluids and vasopressors?

Bleeding, anaphylaxis, tension pneumothorax, cardiac tamponade, pulmonary or gas embolism and myocardial ischaemia.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anaesthesia/intraoperative-hypotension/)

## Elsewhere in DurgiPedia

- [Cardiac output and what sets it](https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/)
- [Preload, afterload and contractility](https://durgical.com/en/physiology/preload-afterload-and-contractility/)
- [The baroreceptor reflex](https://durgical.com/en/physiology/the-baroreceptor-reflex/)
- [Arterial line insertion](https://durgical.com/en/clinical-skills/arterial-line-insertion/)
- [Shock: the clinical signs and the types](https://durgical.com/en/clinical-exam/shock-the-clinical-signs-and-the-types/)

## Connections

## Related 8

- [Shock: a bedside approach by type](https://durgical.com/en/anaesthesia/shock-a-bedside-approach-by-type/)
- [Vasopressors and inotropes by receptor and mechanism](https://durgical.com/en/anaesthesia/vasopressors-and-inotropes-by-receptor-and-mechanism/)
- [Fluid responsiveness: when more fluid helps](https://durgical.com/en/anaesthesia/fluid-responsiveness-when-more-fluid-helps/)
- [Haemodynamic monitoring: arterial line, CVP, cardiac output and lactate](https://durgical.com/en/anaesthesia/haemodynamic-monitoring-arterial-line-cvp-cardiac-output-and-lactate/)
- [High or total spinal block](https://durgical.com/en/anaesthesia/high-or-total-spinal-block/)
- [Massive haemorrhage in theatre](https://durgical.com/en/anaesthesia/massive-haemorrhage-in-theatre/)
- [Anaphylaxis under anaesthesia](https://durgical.com/en/anaesthesia/anaphylaxis-under-anaesthesia/)
- [Which medicines to continue or stop before surgery](https://durgical.com/en/anaesthesia/which-medicines-to-continue-or-stop-before-surgery/)

## Related conditions 8

- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)
- [Cardiogenic shock](https://durgical.com/en/conditions/cardiogenic-shock/)
- [Anaphylaxis](https://durgical.com/en/conditions/anaphylaxis/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [Pneumothorax](https://durgical.com/en/conditions/pneumothorax/)
- [Cardiac tamponade](https://durgical.com/en/conditions/cardiac-tamponade/)
- [Pulmonary embolism](https://durgical.com/en/conditions/pulmonary-embolism/)
- [Myocardial Infarction (Heart Attack)](https://durgical.com/en/conditions/myocardial-infarction/)

## In Pharmacology 9

- [Phenylephrine](https://durgical.com/en/pharmacology/phenylephrine/)
- [Ephedrine](https://durgical.com/en/pharmacology/ephedrine/)
- [Norepinephrine](https://durgical.com/en/pharmacology/norepinephrine/)
- [Epinephrine](https://durgical.com/en/pharmacology/epinephrine/)
- [Atropine](https://durgical.com/en/pharmacology/atropine/)
- [Vasopressors and inotropes](https://durgical.com/en/pharmacology/classes/vasopressors-and-inotropes/)
- [Vasopressin](https://durgical.com/en/pharmacology/vasopressin/)
- [ACE inhibitors](https://durgical.com/en/pharmacology/classes/ace-inhibitors/)
- [Angiotensin II receptor blockers (ARBs)](https://durgical.com/en/pharmacology/classes/angiotensin-ii-receptor-blockers/)

[← All of anaesthesia](https://durgical.com/en/anaesthesia/)

Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: [Hypotension](https://www.ncbi.nlm.nih.gov/books/NBK499961/) (StatPearls (NCBI Bookshelf), 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/anaesthesia/intraoperative-hypotension/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anaesthesia/intraoperative-hypotension/