Heart rate variability (HRV)
Click the figure below for larger image of heart rate variability trace
Some monitors use HRV at respiratory frequency or respiratory sinus arrhythmia (RSA) as a method of assessing anaesthetic depth. This is useful, but depends on an intact autonomic nervous system and healthy myocardial conducting system. Beta-blockers, conduction abnormalities, autonomic neuropathy and sepsis all cause problems.
The ‘Fathom’ (Amtec Medical Limited) is based on the use of HRV, and does not use cortical activity directly but depends on the influence of respiration on the brain stem and the resulting change in heart rate. At this stage, experience with the Fathom monitor is very limited.
[i] Heart rate variability, BIS and "depth of anaesthesia". Br J Anaesth 1999; 82: 659-62
Frontalis (scalp) electromyogram (FEMG) The frontalis muscle receives both visceral and somatic fibres from the facial nerve. The dual nerve supply means that this muscle can be influenced by autonomic activity. Two surface electrodes record compound action potentials from this muscle. The amplitude of the EMG decreases with increasing depth of anaesthesia, but this cannot be used in the paralysed patient.
FEMG has the advantages of being non-invasive and convenient, and it is easy to apply the electrodes. However, the simultaneous recording of electroencephalography (EEG) signals poses technical problems due to low amplitude and interference. There is a wide inter-individual variability in measured FEMG.
Entropy is related to the amount of disorder, complexity, or unpredictability of the system. It is a property of a physical system or data string consisting of a great number of elements. The concept is used in physical sciences and information theory.
By adding the measurement of the cortical electrical activity, the clinician can assess the effect of anaesthetics more comprehensively. EEG recordings change from irregular to more regular patterns when anaesthesia deepens. Similarly, FEMG quiets down as the deeper parts of the brain are increasingly saturated with anaesthetics. Entropy measures the irregularity of EEG and FEMG signals.
The entropy of the EEG signal within a certain time window can be calculated from the signal itself or its spectrum. Entropy of the signal has been shown to drop when a patient falls asleep and increase again when the patient wakes up.
[i] ASA Abstracts:The role of facial EMG and entropy in evaluating adequacy of anesthesia
[ii] Assessment of anaesthetic adequacy with upper facial and abdominal wall EMG. Eur J Anaesthesiol 1989; 6: 111-9
[iii] Comparison of changes in electroencephalographic measures during induction of general anaesthesia: influence of the gamma frequency band and electromyogram signal. Br J Anaesth 86: 50-8