Common causes

How to use this: a safe approach

• Treat the patient first, then the monitor: if something looks wrong, check the patient (colour, chest movement, pulse) and call for help early.
• Use an ABCDE approach: Airway, Breathing, Circulation, Disability (depth/brain), Exposure (temperature/bleeding).
• Assume equipment problems are possible: check connections, oxygen supply, circuit, and monitoring leads before escalating interventions.
• If in doubt: give 100% oxygen, hand-ventilate, and reassess.

Common causes of low oxygen saturation (desaturation)

• Airway obstruction: soft tissue collapse, biting, laryngospasm (often after airway irritation), kinked tube.
• Displaced airway device: endotracheal tube (ETT) too shallow/deep, LMA not seated, accidental extubation.
• Bronchospasm: wheeze, prolonged expiration, rising airway pressures (often asthma, anaphylaxis, light anaesthesia).
• Atelectasis (collapse): common after induction, obesity, laparoscopic surgery; improves with recruitment/PEEP and good positioning.
• Aspiration/regurgitation: sudden desaturation, wheeze/crackles, soiling; higher risk with full stomach, bowel obstruction, pregnancy.
• Pneumothorax (rare but important): sudden desaturation + hypotension + high airway pressures (especially with positive pressure ventilation).
• Pulmonary oedema: pink froth, crackles, difficult ventilation (e.g., fluid overload, negative pressure pulmonary oedema).
• Monitor artefact: poor probe position, cold peripheries, movement, nail varnish—confirm with waveform and clinical check.

Common causes of high airway pressure / difficult ventilation

• Circuit problems: closed APL valve, kinked/occluded tubing, water in filter/HME, blocked catheter mount.
• ETT/LMA issues: kinked tube, biting, cuff herniation, endobronchial intubation (ETT too deep), secretions/mucus plug.
• Bronchospasm: wheeze, ‘shark-fin’ capnography, high pressures with low tidal volumes.
• Reduced compliance: obesity, pneumoperitoneum, Trendelenburg, tight drapes, abdominal distension.
• Pneumothorax: rising pressures with falling saturations and possible hypotension—consider especially after central line insertion or barotrauma.

Common causes of low end-tidal CO2 (EtCO2) or sudden loss of capnography trace

• Disconnection: most common—check circuit, sampling line, filter, and ETT/LMA connections.
• Apnoea or very low ventilation: check chest movement, ventilator settings, neuromuscular blockade.
• Oesophageal intubation: no sustained CO2 trace, poor chest rise—treat as emergency and re-establish airway.
• Low cardiac output / cardiac arrest: EtCO2 drops with reduced pulmonary blood flow—check pulse and start ALS if needed.
• Sampling line issues: kinked/blocked line, water in line, loose connection to monitor.

Common causes of high EtCO2

• Hypoventilation: low minute ventilation, increased dead space, exhausted CO2 absorber (if using circle system).
• Increased CO2 production: shivering, sepsis, thyrotoxicosis, malignant hyperthermia (MH).
• Rebreathing: faulty unidirectional valves, exhausted soda lime, inadequate fresh gas flow (especially in circle).
• CO2 insufflation (laparoscopy): absorption increases EtCO2—often responds to increased ventilation.

Common causes of hypotension (low blood pressure)

• Anaesthetic drugs: induction agents, volatile agents, neuraxial block causing vasodilation (common and expected).
• Hypovolaemia: bleeding, dehydration, third-space losses; look for tachycardia, low urine output, surgical field loss.
• Reduced venous return: high airway pressures, pneumoperitoneum, aortocaval compression (pregnancy), positioning.
• Cardiac causes: arrhythmia, myocardial ischaemia, poor ventricular function.
• Anaphylaxis: hypotension often with bronchospasm, rash, swelling—may be the first sign under drapes.
• Sepsis or vasoplegia: warm peripheries, low SVR picture (post-induction in unwell patients).
• Measurement error: wrong cuff size/position, arterial line damping/flush issues.

Common causes of hypertension and tachycardia

• Light anaesthesia/analgesia: surgical stimulation, inadequate opioid, awareness risk—check MAC, infusion rates, and patient signs.
• Pain on emergence: common in PACU—ensure multimodal analgesia and regional blocks where appropriate.
• Hypoxia/hypercapnia: can drive sympathetic response—check airway and ventilation.
• Full bladder, tourniquet pain, shivering, anxiety (awake/neuraxial cases).
• Drug-related: vasopressors, anticholinergics; withdrawal states (e.g., alcohol, beta-blocker omission).

Common causes of bradycardia

• Vagal stimulus: laryngoscopy, peritoneal traction, ocular surgery (oculocardiac reflex), high spinal block.
• Drug effects: opioids, propofol, beta-blockers; reversal drugs can also affect rate depending on agent used.
• Hypoxia: always consider and treat early.
• Conduction disease or myocardial ischaemia (especially inferior MI).

Common causes of arrhythmias (peri-operative)

• Electrolyte and acid–base problems: potassium, magnesium, calcium abnormalities; acidosis.
• Hypoxia, hypercapnia, pain, light anaesthesia.
• Myocardial ischaemia or structural heart disease.
• Drugs: volatile agents, sympathomimetics, local anaesthetic toxicity (LAST).
• Artefact: loose ECG leads, diathermy interference—check the pulse and waveform.

Common causes of delayed wake-up (slow emergence)

• Residual anaesthetic drugs: volatile agent, propofol infusion, opioids, benzodiazepines (most common).
• Residual neuromuscular blockade: inadequate reversal or monitoring—use quantitative neuromuscular monitoring where available.
• Hypothermia: slows drug metabolism and causes drowsiness; check core temperature.
• Metabolic causes: hypoglycaemia, hypercarbia, electrolyte disturbance, renal/hepatic impairment.
• Neurological event (rare): stroke, seizure, intracranial event—consider if focal signs or unexpected course.

Common causes of low urine output (oliguria)

• Low circulating volume: dehydration, bleeding, third-space losses; check trends and surgical losses.
• Low blood pressure/poor renal perfusion: prolonged hypotension, high vasopressor requirement.
• Obstruction/kinked catheter: check tubing, bag position, and bladder scan if unsure.
• Pre-existing renal impairment or nephrotoxic exposure (contrast, NSAIDs in susceptible patients).

Common causes of hypothermia (and why it matters)

• Heat loss: cold theatre, exposed patient, large open surgery, cold IV fluids and irrigation.
• Anaesthesia effects: impaired thermoregulation and vasodilation after induction.
• Consequences: coagulopathy, wound infection risk, delayed drug clearance, shivering and discomfort.
• Prevention: forced-air warming, warmed fluids, cover exposed areas, monitor temperature in longer cases.