Anesthetic Complications & Monitoring

Monitoring is often taught as a list of devices, which conceals what makes one monitor more valuable than another.

A monitor is worth what its warning time is worth.

Anaesthesia removes the patient's own protective responses, so the clinician substitutes for them. The question for every monitor is therefore not what it measures but how quickly it detects a problem relative to when harm begins.

Capnography is the clearest example. It detects a circuit disconnection, an oesophageal intubation or a fall in cardiac output within a breath or two, while pulse oximetry will not fall until the oxygen reservoir is exhausted, which may be minutes later. That interval is where patients are lost.

A second principle organises the complications. Most intraoperative crises present as one of a small number of patterns: hypotension, hypoxia, rising or falling carbon dioxide, or rising temperature. Working from the pattern rather than from a list of diagnoses is what allows a structured response under pressure.

1. The Minimum Monitoring Standard

Certain monitors are required from before induction until the patient has recovered, and their presence is not discretionary.

Pulse oximetry measures saturation and gives a continuous audible signal, which is why the tone is deliberately audible in theatre.

Capnography confirms ventilation and tube position and detects circuit problems.

Electrocardiography detects arrhythmia and ischaemia.

Non-invasive blood pressure at intervals appropriate to the case.

Temperature for any case beyond short duration, since hypothermia is common and consequential.

Inspired oxygen concentration and airway pressures guard against hypoxic mixtures and disconnection.

Neuromuscular monitoring wherever a blocker is used, because clinical assessment detects residual paralysis poorly.

The anaesthetist's continuous presence is itself the monitor that matters most, and no device substitutes for it.

2. Reading the Capnograph

The capnograph carries more information per glance than any other trace.

A sudden fall to zero means a disconnection, an obstructed or displaced tube, or oesophageal placement.

A sudden fall to a low but non-zero value means a fall in cardiac output or pulmonary embolism, including air or amniotic fluid, because carbon dioxide delivery to the lung has dropped.

A gradual rise means hypoventilation, absorbed carbon dioxide during laparoscopy, or increased production as in malignant hyperthermia.

A sloping plateau indicates obstructive airway disease or a partially obstructed tube.

A raised baseline means rebreathing, from exhausted soda lime or a faulty valve.

The value most examined is the sudden drop with a normal ventilator, because it distinguishes an equipment problem from a circulatory catastrophe within seconds.

3. Hypotension

The commonest intraoperative event, and it is best approached by asking which of three variables has changed.

Anaesthetic agents cause hypotension by vasodilatation and myocardial depression, and the effect is exaggerated in the hypovolaemic patient because sympathetic tone was maintaining pressure before induction.

Aortocaval compression in pregnancy deserves separate mention. Beyond about 20 weeks the gravid uterus compresses the inferior vena cava when the woman lies supine, reducing venous return and causing hypotension. Left lateral tilt or manual uterine displacement is the first response, before fluids or vasopressors.

4. Hypoxia During Anaesthesia

Approach by location, from the machine to the tissues.

Delivery: hypoxic gas mixture, disconnection, empty cylinder.

Airway: oesophageal intubation, endobronchial intubation, obstructed tube, laryngospasm, bronchospasm.

Lung: atelectasis, aspiration, pneumothorax, pulmonary oedema.

Circulation: low cardiac output, shunt.

Endobronchial intubation is a common and easily corrected cause, produced when the tube advances into the right main bronchus with neck flexion or patient repositioning, leaving the left lung unventilated.

The first response to unexplained hypoxia is 100 per cent oxygen and hand ventilation, because manual ventilation tells you about compliance and resistance directly, which no monitor conveys as quickly.

5. Malignant Hyperthermia

A pharmacogenetic disorder of skeletal muscle, inherited in an autosomal dominant pattern, involving the ryanodine receptor.

Triggers are the volatile anaesthetic agents and suxamethonium. Nitrous oxide, propofol, opioids and local anaesthetics are safe.

The mechanism is uncontrolled release of calcium from the sarcoplasmic reticulum, producing sustained muscle contraction with massive metabolic activity.

The earliest and most sensitive sign is an unexplained rise in end-tidal carbon dioxide, which precedes the temperature rise. Waiting for hyperthermia to appear is waiting too long, and the name is therefore misleading.

Other features are masseter spasm after suxamethonium, generalised rigidity, tachycardia, arrhythmia, acidosis, hyperkalaemia and rhabdomyolysis with myoglobinuria.

Management is immediate. Stop the trigger, call for help, hyperventilate with 100 per cent oxygen through a clean circuit, give dantrolene, cool actively, and treat hyperkalaemia, acidosis and arrhythmia.

Dantrolene acts by blocking calcium release from the ryanodine receptor, and its availability is what converted a condition with very high mortality into a survivable one.

6. Anaphylaxis Under Anaesthesia

Distinctive because the usual warning symptoms are unavailable in an anaesthetised patient.

The presenting features are cardiovascular rather than cutaneous. Sudden severe hypotension, tachycardia, rising airway pressure from bronchospasm and a fall in end-tidal carbon dioxide are typical, while rash may be absent or hidden under drapes.

Neuromuscular blocking agents are the commonest trigger in the anaesthetic setting, followed by antibiotics, chlorhexidine and latex.

Adrenaline is the treatment, given intravenously in titrated doses in this monitored setting, alongside stopping the suspected agent, fluid resuscitation and airway support.

Serum tryptase samples taken at intervals after the event support later confirmation, since tryptase released from mast cells peaks within an hour or two and returns toward baseline, so a sample at presentation, at one to two hours and a later baseline sample together establish the rise and fall. Referral for allergy testing is mandatory because the patient must know what to avoid for life, and the suspected agents must be documented prominently in the record.

7. Other Recurring Complications

Postoperative nausea and vomiting is the complication patients report as most distressing. Risk rises with female sex, non-smoking status, previous postoperative nausea or motion sickness, and postoperative opioid use. Prophylaxis is given according to risk score rather than universally, and multiple agents with different mechanisms outperform higher doses of one.

Hypothermia develops in most patients through anaesthetic-induced vasodilatation redistributing heat from core to periphery, and it increases wound infection, bleeding, cardiac events and recovery time. Active warming is treatment, not comfort, and the largest single fall occurs in the first hour from that redistribution, which is why prewarming before induction is effective.

Awareness under anaesthesia is rare but devastating, and risk is higher in cardiac and obstetric surgery, in trauma where anaesthetic doses are reduced for haemodynamic reasons, and where neuromuscular blockade prevents the patient from signalling.

End-tidal anaesthetic agent monitoring guards against it during inhalational anaesthesia, and processed electroencephalographic monitoring is used where total intravenous technique removes that safeguard. Post-traumatic stress disorder follows a substantial proportion of episodes, so structured follow-up after a suspected event is part of management rather than an optional courtesy.

Nerve injury from positioning is preventable. The ulnar nerve at the elbow and the common peroneal nerve at the fibular head are the classic sites, and the brachial plexus is injured by arm abduction beyond 90 degrees.

Perioperative visual loss is rare and catastrophic, associated with prolonged prone spinal surgery, hypotension, anaemia and long duration, and ischaemic optic neuropathy is the usual mechanism.

Venous thromboembolism risk is assessed for every surgical patient, since anaesthesia, immobility and surgical inflammation together satisfy all three components of Virchow triad.

8. Preoperative Assessment and Risk

The purpose of preoperative assessment is not to clear a patient but to identify and modify risk, and to plan for what cannot be modified.

The ASA physical status classification grades systemic disease from I, a normal healthy patient, through II with mild systemic disease, III with severe systemic disease that limits activity, IV with severe disease that is a constant threat to life, V a moribund patient not expected to survive without the operation, to VI a declared brain-dead organ donor. The letter E is appended for emergency surgery, which independently raises risk.

It describes physical status rather than predicting outcome directly, though it correlates with perioperative mortality well enough to be used in every risk discussion.

Functional capacity is the single most useful bedside assessment. A patient who can climb two flights of stairs without stopping has a metabolic equivalent above four and generally tolerates surgery, and the enquiry outperforms most investigations for this purpose.

What actually reduces risk

Smoking cessation improves outcomes, with benefit to secretions and ciliary function over weeks and to carboxyhaemoglobin within a day.

Optimising anaemia before elective surgery reduces transfusion, and iron deficiency is investigated rather than transfused around.

Continuing rather than stopping most cardiac medication is the current position, since beta blockers and statins are continued while angiotensin blockade is often withheld on the morning of surgery because of induction hypotension.

Routine investigations in a healthy patient do not improve outcome and generate false positives, so tests are directed by history, examination and the magnitude of surgery rather than ordered as a panel.

9. Pulse Oximetry and Its Limits

Pulse oximetry is the most widely deployed monitor in medicine, and knowing where it fails matters as much as knowing what it shows.

How it works

Two wavelengths of light, red at around 660 nm and infrared at around 940 nm, are passed through tissue. Oxyhaemoglobin and deoxyhaemoglobin absorb these two wavelengths differently, and the ratio of absorbances gives saturation.

The device isolates the pulsatile arterial component from the constant venous and tissue background, which is why it needs a pulse to work at all.

Where it misleads

Carboxyhaemoglobin absorbs light almost identically to oxyhaemoglobin at 660 nm, so a patient with carbon monoxide poisoning reads falsely normal or high, sometimes at 100 per cent while profoundly hypoxic. Co-oximetry is required instead.

Methaemoglobin absorbs both wavelengths similarly, driving the ratio toward unity, so the reading trends toward about 85 per cent regardless of true saturation.

Poor peripheral perfusion, hypothermia, vasoconstriction, motion and severe anaemia all degrade the signal.

Nail polish, particularly dark colours, and dyes such as methylene blue interfere with light transmission.

The lag that matters

The oxyhaemoglobin dissociation curve explains why saturation falls late. Its flat upper portion means that a substantial fall in partial pressure of oxygen produces almost no change in saturation until the shoulder is reached, at around 90 per cent, after which saturation falls steeply.

A patient whose saturation has just begun to fall has already lost most of their reserve, which is precisely why capnography and clinical vigilance detect problems earlier.

10. Worked Examples

Example 1. Thirty minutes into an operation the end-tidal carbon dioxide rises steadily, the patient is tachycardic and the temperature has risen by 1 degree. What is the diagnosis and the first action?

Malignant hyperthermia. The rising end-tidal carbon dioxide is the earliest and most sensitive sign, and it appears before temperature rises, which is why the name is misleading.

The mechanism is uncontrolled calcium release through a defective ryanodine receptor, producing sustained muscle contraction with a massive increase in metabolic rate, which generates the carbon dioxide, the heat, the acidosis and eventually rhabdomyolysis.

The first action is to stop the trigger, meaning the volatile agent, and to call for help while switching to a clean circuit with 100 per cent oxygen at high flows and hyperventilating.

Dantrolene is given without waiting for confirmation, since it blocks calcium release at the ryanodine receptor and is the only specific treatment. Active cooling follows, alongside treatment of hyperkalaemia, acidosis and arrhythmia, and monitoring for myoglobinuria and renal failure.

Example 2. During laparoscopy the capnograph falls abruptly from 38 to 8 mmHg while the ventilator continues normally. What are the possibilities and how do you distinguish them?

An abrupt fall in end-tidal carbon dioxide with unchanged ventilation means either that gas is no longer reaching the sampling line or that carbon dioxide is no longer reaching the lungs.

The first group is equipment: circuit disconnection, tube displacement or obstruction. These are excluded within seconds by looking at the circuit, checking the tube and hand-ventilating to assess compliance.

The second group is circulatory, and in laparoscopy specifically it raises venous gas embolism, in which insufflated carbon dioxide enters an open vein and obstructs the pulmonary outflow, producing a sudden fall in delivery to the alveoli.

Other circulatory causes are massive pulmonary embolism, sudden severe hypotension and cardiac arrest.

The response is to check the circuit and tube immediately, stop insufflation and release the pneumoperitoneum, give 100 per cent oxygen, position head-down with left lateral tilt if embolism is suspected, and support the circulation. The value of capnography here is that it announces the event within a breath, whereas saturation would fall considerably later.

Example 3. A woman at 34 weeks becomes hypotensive shortly after being positioned supine for surgery. What is the mechanism and the first intervention?

Aortocaval compression. Beyond about 20 weeks of gestation the gravid uterus compresses the inferior vena cava against the vertebral column when the woman lies supine, obstructing venous return and reducing cardiac output.

The first intervention is mechanical rather than pharmacological: left lateral tilt of at least 15 degrees, or manual displacement of the uterus to the left, which relieves the obstruction directly.

Fluids and vasopressors are given alongside but do not address the cause, since the problem is obstructed venous return rather than absolute hypovolaemia or vasodilatation.

The same reasoning applies in resuscitation of a pregnant woman, where manual uterine displacement is performed during chest compressions, because compressions are ineffective if venous return is obstructed.

Example 4. Shortly after induction with a neuromuscular blocker and antibiotic, the patient becomes profoundly hypotensive with high airway pressures. There is no rash. What is happening?

Anaphylaxis until proved otherwise, and the absence of rash does not exclude it.

Under anaesthesia the presentation is cardiovascular rather than cutaneous. The patient cannot report itch, flushing or breathlessness, and the skin is largely covered by drapes, so severe hypotension, tachycardia, bronchospasm with rising airway pressures and a falling end-tidal carbon dioxide are what announce it.

Neuromuscular blocking agents are the commonest trigger in this setting, followed by antibiotics, chlorhexidine and latex, and both suspects are present here.

Management is to stop administration of all likely agents, call for help, give adrenaline intravenously in titrated doses since the patient is monitored and cannulated, give large-volume fluid resuscitation, and maintain the airway with 100 per cent oxygen.

Afterwards, serial tryptase samples support the diagnosis, and referral for allergy testing is mandatory so the patient knows what to avoid for life.

Example 5. A patient is extubated and appears awake but has weak grip, cannot lift their head and desaturates in recovery. What is the likely cause and how should it have been prevented?

Residual neuromuscular blockade.

Recovery of consciousness and recovery of neuromuscular function are separate processes, so a patient can appear awake while pharyngeal and respiratory muscles remain weak. The pharyngeal muscles are among the most sensitive to residual blockade, which is why airway obstruction and aspiration risk persist even when the patient can open their eyes.

Clinical tests perform poorly. Head lift for five seconds and grip strength detect only substantial residual weakness and miss the degree that still impairs airway protection.

Prevention is quantitative neuromuscular monitoring, using train-of-four ratio measured objectively rather than assessed by eye, with reversal given until an adequate ratio is documented before extubation. Sugammadex reverses aminosteroid blockers rapidly and completely, and neostigmine with an antimuscarinic remains an option for partially recovered blockade.

Immediate management is oxygen, airway support and, if the patient is not protecting their airway, reintubation with further reversal.

Summary

A monitor is worth what its warning time is worth.

Capnography detects disconnection within a breath; saturation lags by minutes.

Continuous presence of the anaesthetist is the monitor that matters most.

Neuromuscular monitoring is required wherever a blocker is used.

A sudden capnograph fall to zero means disconnection, displacement or obstruction.

A sudden fall to a low non-zero value means reduced cardiac output or embolism.

A sloping plateau indicates obstructive airway disease.

A raised baseline indicates rebreathing.

Hypotension is approached as preload, afterload or contractility.

Anaesthetic agents cause hypotension by vasodilatation and myocardial depression.

Aortocaval compression is relieved by tilt or manual displacement first.

Unexplained hypoxia is met with 100 per cent oxygen and hand ventilation.

Endobronchial intubation is a common and easily corrected cause.

Malignant hyperthermia is triggered by volatiles and suxamethonium.

Its earliest sign is rising end-tidal carbon dioxide, before temperature.

Dantrolene blocks calcium release at the ryanodine receptor.

Anaphylaxis under anaesthesia presents cardiovascularly, often without rash.

Neuromuscular blockers are the commonest anaesthetic trigger.

Hypothermia increases infection, bleeding and cardiac events, so warming is treatment.

Residual blockade is detected by quantitative monitoring, not by clinical tests.