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Instrumental monitoring

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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Technology advances at a rapid rate, but there is no substitute for observation of basic clinical signs. Nevertheless, instrumental monitoring of general anaesthesia may be with a selection of:

  • the inhaled concentration of volatile agent, thought to closely represent the minimum alveolar concentration (MAC) of the agent:
    • the minimum concentration of anaesthetic agent within the alveoli which abolishes movement in response to a standard skin incision in 50% of the population
    • deduced from the concentration of anaesthetic gas in the inspired / expired stream measured by infrared analyser
    • the MAC gives a scale of the magnitude of the appropriate dose, but this is titrated according to the patient's needs
    • depends on the attainment of a state of equilibrium
  • ventilatory volume, compared to the direct observation of chest movements
  • pulse oximetry: can calculate arterial oxygen saturation and pulse rate
  • digital plethysmography
  • electrocardiogram: rate, rhythm and ischaemic changes
  • central venous pressure line: used to distinguish reduced cardiac output from hypovolaemia
  • electromyography: stimulation of motor nerve and detection of muscular contraction can indicate the depth of neuromuscular blockade
  • oesophageal contractility: depth of anaesthesia is proportional to the spontaneous contraction of the smooth muscle of the lower third of the oesophagus
  • EEG:
    • poor correlation of basic output with awareness
    • processed signal total amplitude and frequency are more useful
  • evoked potentials: electrical reponse of the cerebral cortex or midbrain to external stimuli, e.g. somatosensory, auditory and visual

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