Standard intraoperative monitoring

What it is

Standard monitoring is the minimum watched in every anaesthetic, whether general, regional or monitored sedation. The first and most important monitor is a qualified anaesthesia professional present throughout, because machines only report and alarm. Oxygenation is checked with an inspired oxygen analyser with a low-concentration alarm and with continuous pulse oximetry, whose variable pitch tone lets the whole team hear a falling saturation. Ventilation is checked by watching the chest and reservoir bag and by continuous capnography, which confirms that a tracheal tube is in the trachea and shows apnoea, disconnection, obstruction and changes in carbon dioxide production or lung perfusion; when a ventilator is used, a circuit disconnection alarm must be active. Circulation is checked with a continuous electrocardiogram, blood pressure measured non-invasively at least every 5 minutes (or invasively through an arterial line when needed), and assessment of perfusion. Temperature is measured when changes are intended, expected or suspected. When neuromuscular blockers are used, a nerve stimulator, ideally quantitative, guides dosing and confirms recovery; processed electroencephalography is used in selected cases to guide depth. Monitors are attached before induction, alarms are kept on with limits set for the patient, and values are recorded at regular intervals.

Key points

• Monitors are attached before induction to record baseline values and catch the first changes.
• Alarms must be audible and set to sensible limits; silencing them removes the safety net.
• Pulse oximetry measures oxygen saturation, not ventilation: a patient on oxygen can be hypoventilating with normal saturation.
• Capnography detects apnoea, disconnection, oesophageal intubation, rebreathing and a sudden fall in cardiac output.
• Invasive monitoring (arterial line, central venous pressure) is added for major surgery, unstable patients or frequent blood sampling.

Components

• Oxygenation: inspired oxygen analyser, pulse oximetry, skin colour and the lighting to see it.
• Ventilation: chest movement, reservoir bag, capnography, airway pressure and expired volume, disconnection alarm.
• Circulation: continuous ECG, non-invasive or invasive blood pressure, pulse, capillary refill, urine output in long cases.
• Temperature: nasopharyngeal, oesophageal, bladder or skin probe.
• Extra: quantitative neuromuscular monitoring, processed EEG for depth, anaesthetic agent analysis.

How it works

• Pulse oximetry compares red and infrared light absorbed by pulsatile arterial blood to estimate haemoglobin oxygen saturation.
• Capnography measures carbon dioxide in each breath with infrared absorption; the end-tidal value reflects arterial carbon dioxide when ventilation and perfusion are matched.
• The oscillometric cuff estimates mean pressure from the size of arterial pulsations and derives systolic and diastolic values.
• A nerve stimulator applies a train of four impulses to a peripheral nerve; fade between the twitches shows residual block.

Step by step

1. Check the monitors and alarms work before the patient arrives — a faulty monitor found mid-crisis is no monitor.
2. Attach pulse oximetry, ECG and blood pressure before induction and record baseline values — changes are judged against them.
3. Connect capnography to the circuit before the airway is secured — the first breaths confirm tube position.
4. Set alarm limits for this patient and keep them audible — the team hears trouble before seeing it.
5. Add temperature and neuromuscular monitoring as the case requires — hypothermia and residual block are common and silent.
6. Record values regularly and treat the patient, not the number — check an alarming value against the patient before acting.

What each standard monitor measures and catches

Next step

• If an alarm sounds, look at the patient first, then the airway, breathing and circulation, then the monitor.
• Call a senior anaesthetist at once for falling saturation, loss of the capnography trace or a new arrhythmia with hypotension.
• Learn the capnography trace well: its role in confirming intubation and in cardiac arrest is covered on the capnography page.
• Continue the same core monitoring during transfer and in the recovery unit until the discharge criteria are met.

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Elsewhere in DurgiPedia

• Pulse oximetry at the bedside: placement and pitfalls
• Cardiac monitoring at the bedside: leads, alarms and rhythm changes to report
• CO₂ transport and the Haldane effect
• Pulse oximetry

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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: Capnography (StatPearls (NCBI Bookshelf), 2026); General Anesthesia for Surgeons (StatPearls (NCBI Bookshelf), 2023).

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