Instrumental monitoring

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