What anaesthesia does to the heart, lungs, brain and kidneys

What it is

General anaesthesia is a reversible, drug-induced loss of consciousness with amnesia, analgesia and immobility, but the drugs that produce it act on almost every organ. Volatile agents and propofol relax vascular smooth muscle and depress the myocardium, and all hypnotics reduce sympathetic tone and blunt the baroreceptor reflex, so blood pressure falls, most at induction and most in patients who are hypovolaemic, old or already on vasodilators. Positive-pressure ventilation adds to this by reducing venous return. In the lungs, anaesthesia depresses the ventilatory response to carbon dioxide and to hypoxia, relaxes the upper airway and abolishes protective reflexes, and lowers functional residual capacity, so dependent lung regions collapse (atelectasis) within minutes and ventilation–perfusion matching worsens. In the brain, intravenous hypnotics such as propofol lower metabolic rate and cerebral blood flow together, whereas volatile agents lower metabolism but dilate cerebral vessels, which can raise intracranial pressure at higher concentrations; ketamine tends to raise sympathetic tone. The kidneys receive less blood because of lower arterial pressure and the surgical stress response (antidiuretic hormone, renin–angiotensin–aldosterone, catecholamines), so urine output falls and salt and water are retained. Anaesthesia also impairs temperature regulation, causing early heat redistribution from core to periphery. Knowing these effects lets the anaesthetist anticipate hypotension, hypoxaemia and low urine output instead of being surprised by them.

Key points

• All general anaesthetics depress the brain and, to differing degrees, the heart, vessels and breathing.
• The effects are dose-related and add up when drugs are combined (hypnotic plus opioid plus volatile agent).
• Neuraxial blocks cause sympathetic block: vasodilation below the level of the block and, if high, bradycardia.
• The surgical stress response raises cortisol, catecholamines and glucose and retains salt and water.
• Hypothermia begins in the first hour from redistribution of heat to the periphery.

Components

• Heart and vessels: lower systemic vascular resistance, reduced contractility, blunted baroreflex; ketamine and etomidate keep blood pressure better.
• Lungs and airway: reduced respiratory drive, loss of upper-airway tone and reflexes, fall in functional residual capacity, atelectasis, impaired mucociliary clearance.
• Brain: loss of consciousness and memory; changes in cerebral blood flow and intracranial pressure that depend on the agent.
• Kidneys: lower renal blood flow and glomerular filtration rate, hormonal water and salt retention.
• Liver and temperature: lower hepatic blood flow; loss of shivering and vasoconstriction thresholds, so the patient cools.

How it works

• Hypnotics enhance inhibitory and reduce excitatory transmission in the brain, which removes consciousness and also central sympathetic outflow.
• Volatile agents relax vascular and airway smooth muscle; some, given rapidly, can briefly stimulate the sympathetic system and raise heart rate.
• Nitrous oxide mildly stimulates the sympathetic system and can raise pulmonary vascular resistance.
• Supine position, loss of muscle tone and a cephalad diaphragm shrink lung volume; high inspired oxygen speeds absorption atelectasis.
• Positive-pressure ventilation raises intrathoracic pressure and lowers venous return, which matters most in the hypovolaemic patient.

Main effects of general anaesthesia by organ (in principle; they vary by drug and depth)

In practice

• Anticipate hypotension at induction: titrate the hypnotic, choose a more cardiostable agent when the reserve is poor, and have a vasopressor ready.
• Limit atelectasis with good preoxygenation, lung recruitment and positive end-expiratory pressure set by the anaesthetist, not by a fixed rule.
• In raised intracranial pressure, intravenous anaesthesia and attention to carbon dioxide and blood pressure protect cerebral perfusion.
• Low urine output under anaesthesia: check the catheter, blood pressure and volume status before assuming kidney injury.
• Older adults and patients with heart failure, sepsis or bleeding have less reserve and show these effects sooner and more strongly.

Next step

• Before induction, judge each patient's reserve (heart, lungs, volume status, age) and choose drugs and monitoring to match.
• Treat a sustained fall in blood pressure early: reduce anaesthetic depth if possible, give fluid if hypovolaemic and a vasopressor as needed.
• Call a senior anaesthetist for hypotension or hypoxaemia that does not respond to first measures.
• Monitor core temperature in longer cases and warm actively from the start.
• After surgery, follow breathing, oxygen saturation, blood pressure and urine output in recovery until they return towards baseline.

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Elsewhere in DurgiPedia

• Cardiac output and what sets it
• Lung and chest wall compliance
• Ventilation–perfusion matching and West zones
• Cerebral blood flow, brain metabolism and intracranial pressure
• Renal blood flow, autoregulation and tubuloglomerular feedback
• Body temperature regulation

Connections

← All of 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: Anesthesia Inhalation Agents and Their Cardiovascular Effects (StatPearls (NCBI Bookshelf), 2025); General Anesthesia for Surgeons (StatPearls (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.