The Purpose of Monitoring
"A monitor does not treat the patient; the anaesthetist does. Monitoring buys time — it converts a slowly evolving catastrophe into an early warning that a trained clinician can act on. The most important monitor in the room remains the vigilant anaesthetist watching the patient and the surgical field; the electronic monitors extend, but do not replace, that vigilance."
Synthesised from Miller's Anesthesia, 9th Ed — Monitoring.- Oxygenation — inspired O₂ (analyser) and arterial saturation (pulse oximetry).
- Ventilation — capnography (the gold standard), airway pressures, chest movement.
- Circulation — ECG, blood pressure (non-invasive or invasive), perfusion.
- Temperature, neuromuscular function, and depth of anaesthesia — for the relevant cases.
- Monitoring must be continuous and begin before induction and continue into recovery, including during transfer.
Pulse Oximetry & Capnography
"Capnography is the single most useful monitor in anaesthesia. A normal, continuous end-tidal CO₂ waveform confirms that the lungs are being ventilated, that the tracheal tube is in the trachea, and — by its very presence — that there is a cardiac output delivering CO₂ to the lungs. Its sudden loss is one of the earliest signs of disconnection, obstruction, or circulatory arrest."
Synthesised from Barash — Clinical Anesthesia; Morgan & Mikhail's Clinical Anesthesiology.- Measures arterial oxygen saturation from the different absorption of red (660 nm) and infrared (940 nm) light by oxy- vs deoxy-haemoglobin, gated to the pulsatile (arterial) signal.
- It is a late warning of hypoventilation in a preoxygenated patient (the SpO₂ stays high while apnoeic) — capnography detects the problem sooner.
- Pitfalls: poor perfusion/cold/vasoconstriction, motion, nail polish, ambient light; falsely normal in carbon-monoxide poisoning (COHb read as O₂Hb); methaemoglobinaemia pulls the reading toward ~85%.
- The normal waveform has a flat baseline (I), rapid upstroke (II), alveolar plateau (III, end of which = EtCO₂ ~4.5–6.0 kPa / 35–45 mmHg) and inspiratory downstroke.
- Sudden loss of trace → disconnection, oesophageal intubation, total obstruction, or cardiac arrest — "no trace, wrong place."
- Sudden fall (not to zero) → pulmonary embolism (air/thrombus), fall in cardiac output, hypotension, hyperventilation.
- Rising EtCO₂ → hypoventilation, exhausted soda-lime (raised baseline = rebreathing), rising CO₂ production (sepsis, laparoscopy insufflation, malignant hyperthermia).
- Sloped/"shark-fin" plateau → bronchospasm or airway obstruction (COPD/asthma, kinked tube).
ECG, Blood Pressure, Temperature, TOF & Depth
- ECG — rhythm, rate, ischaemia (lead II for rhythm/inferior; V5 for anterolateral ischaemia; "II + V5" catches most).
- Non-invasive BP (NIBP) — oscillometric; the correct cuff width is ~40% of arm circumference (too small over-reads, too large under-reads).
- Invasive arterial line — beat-to-beat pressure, waveform (and dynamic indices), repeated ABGs; for major surgery, haemodynamic instability, or when tight BP control matters. Zero at the level of the right atrium (mid-axillary line).
- Central venous / cardiac output monitors — CVP is a poor guide to fluid responsiveness; prefer dynamic measures and, for major cases, oesophageal Doppler or arterial-waveform cardiac-output monitoring (see the critical-care haemodynamic monitoring page).
- Temperature — monitor core temperature for any anaesthetic >30 min; warm actively (avoid inadvertent hypothermia, and detect the rare malignant hyperthermia early).
- Neuromuscular monitoring (TOF) — mandatory whenever a non-depolarising relaxant is used. Train-of-four ratio should be >0.9 before extubation to exclude residual block; post-tetanic count guides deep block.
- Depth of anaesthesia (processed EEG — BIS etc.) — target ~40–60 for general anaesthesia; particularly useful with TIVA + neuromuscular blockade, where end-tidal agent cannot be used to guard against awareness.
Pulse oximetry and capnography are the two monitors that save the most lives and are now expected in every anaesthetising location — the ISA and global "Lifebox" initiatives have driven capnograph and oximeter availability into district and rural theatres. Where invasive or depth monitors are scarce, disciplined use of end-tidal agent monitoring, TOF and vigilant clinical observation covers most needs. Always confirm monitors are working before induction, and carry battery-backed monitoring for transfers where wall power is unreliable.
The Minimum Standards (AAGBI 2021)
• Pulse oximetry (SpO₂)
• Capnography (EtCO₂) — for all anaesthetics, including sedation and during transfer of intubated patients
• Electrocardiography (ECG)
• Non-invasive blood pressure
• Inspired oxygen concentration (O₂ analyser) and, for GA, inspired/expired volatile agent and airway pressures
• Temperature (for anaesthetics >30 min) and a means of neuromuscular monitoring whenever relaxants are used.
Pattern Recognition
Capnograph trace disappears
- Check the patient/airway first: disconnection, oesophageal tube, total obstruction, or cardiac arrest → confirm a pulse, call for help, follow ALS
Sudden fall in EtCO₂ (not to zero)
- Think fall in cardiac output / hypotension, pulmonary or air/venous gas embolism, hyperventilation — look at BP, rhythm, surgical field
Rising EtCO₂ + rising temperature + tachycardia
- Consider malignant hyperthermia — stop trigger agents, call for help & dantrolene, hyperventilate with 100% O₂
Falling SpO₂
- Increase FiO₂, confirm ventilation/capnograph, check the airway & circuit, auscultate (bronchospasm, intubation of a bronchus, pneumothorax); is it a true fall or a probe artefact?
"Shark-fin" capnograph
- Sloping upstroke = expiratory obstruction — bronchospasm, kinked/obstructed tube, or severe COPD; treat the cause
Attaching & Checking Monitors
Before induction
- Attach SpO₂, ECG, NIBP and connect capnography before giving any drug; record a baseline
- Confirm alarms on and set to sensible limits; O₂ analyser calibrated
At induction/airway
- Confirm tracheal placement with sustained EtCO₂; check bilateral ventilation & airway pressures
Maintenance
- Add temperature & TOF (if relaxant used); depth monitor if TIVA + paralysis; invasive lines as indicated
- Scan the monitors in a regular sequence; watch the surgical field and the patient
Emergence & transfer
- Confirm TOF >0.9 before extubation; continue monitoring during transfer (portable capnography for intubated patients)
Common Mistakes in Monitoring
In a preoxygenated patient, saturation stays high for minutes after ventilation stops. Capnography detects apnoea/obstruction immediately — it is the earlier, better warning.
Permanently silenced or badly-set alarms defeat the purpose of monitoring. Set sensible limits and keep them audible — many critical incidents began with a muted alarm.
Head-lift and grip are unreliable for residual block. Use quantitative TOF and confirm a ratio >0.9 before extubation to prevent post-operative respiratory failure.
A cuff too small over-reads (spurious hypertension); too large under-reads. Use a cuff ~40% of arm circumference and cross-check a suspicious reading.
Standard pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin and read falsely high. Use co-oximetry/ABG when CO exposure is possible (burns, smoke).
Deaths occur moving patients between theatre, recovery and ICU. Continue full monitoring — including capnography for intubated patients — throughout transfer.
An un-zeroed or wrongly-levelled transducer gives false pressures and bad decisions. Zero to atmosphere and level at the right atrium (mid-axillary line).
Exam Pearls
Q: List the AAGBI minimum monitoring standards.
Pulse oximetry, capnography, ECG, non-invasive BP, inspired O₂ (and inspired/expired agent + airway pressure for GA), temperature (>30 min), and neuromuscular monitoring when relaxants are used — continuously, from before induction to recovery, with the anaesthetist present throughout.
Q: Why is capnography the most important monitor?
A sustained EtCO₂ waveform confirms ventilation, tracheal tube position and the presence of a cardiac output. Its loss is the earliest sign of disconnection, oesophageal intubation, obstruction or arrest.
Q: What causes a sudden fall in EtCO₂?
A drop in cardiac output/hypotension, pulmonary or venous air embolism, circuit disconnection or a sampling problem — a sudden fall to zero implies disconnection/oesophageal tube/arrest.
Q: How does a pulse oximeter work, and when does it lie?
It compares absorption of red (660 nm) and infrared (940 nm) light gated to the pulsatile signal. It is unreliable with poor perfusion, motion, nail polish and dyes, reads falsely high in CO poisoning, and trends to ~85% in methaemoglobinaemia.
Q: What TOF ratio confirms adequate reversal?
A train-of-four ratio >0.9, measured quantitatively — clinical signs (head-lift, grip) are insufficient to exclude residual block.
Q: Target processed-EEG (BIS) value for GA, and when is it most useful?
~40–60. Most valuable with TIVA plus neuromuscular blockade, where end-tidal agent cannot be used to guard against awareness.
Q: Which capnograph pattern suggests bronchospasm?
A sloping, "shark-fin" upstroke with loss of the normal square plateau, indicating expiratory airflow obstruction.
Q: How do you set up an invasive arterial line correctly?
Zero the transducer to atmosphere and level it at the right atrium (mid-axillary line); an un-levelled or un-zeroed line gives false pressures.
References
- Klein AA, Meek T, Allcock E, et al. (AAGBI). Recommendations for standards of monitoring during anaesthesia and recovery 2021. Anaesthesia. 2021;76:1212–1223.
- Gropper MA, et al. (eds). Miller's Anesthesia, 9th Edition. Monitoring; Neuromuscular Monitoring. Elsevier; 2020.
- Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail's Clinical Anesthesiology, 6th Edition. Patient Monitors. McGraw-Hill; 2018.
- Barash PG, et al. (eds). Clinical Anesthesia, 9th Edition. Commonly Used Monitoring Techniques. Wolters Kluwer; 2023.
- Checketts MR, Alladi R, Ferguson K, et al. (AAGBI). Recommendations for standards of monitoring during anaesthesia and recovery 2015. Anaesthesia. 2016;71:85–93.