Monitoring depth of anesthesia

depth of anesthesia

1. MONITORING DEPTH OF ANESTHESIA - Dr. Richa Kumar

2. One of the objectives of modern anaesthesia is to ensure adequate depth of anaesthesia To prevent awareness without inadvertently overloading the patients with potent drugs. Various methods have been described to measure the depth of anaesthesia from time to time.

3. In 1957, Woodbridge defined anaesthesia as having four components; sensory blockade, motor blockade, blockade of autonomic reflexes and loss of consciousness. John Snow in 1847, Described five degrees of narcotism for ether anaesthesia . These were later refined by Guedel into four stages on the basis of somatic muscle tone, respiratory parameters and ocular signs. In 1954, Artusio divided Guedel’s stage I into 3 planes.

4. According to Prys -Roberts common feature of general anaesthesia is suppression of conscious perception of noxious stimuli. He divided the noxious stimuli into somatic and autonomic components, which were further, divided into sensory, motor and respiratory, haemodynamic , pseudomotor and hormonal Analgesia, autonomic stability and muscle relaxation are desirable but not actual components of anaesthesia .

5. General anaesthesia is a state of drug-induced, reversible loss of consciousness. It includes the time of induction of anaesthesia to emergence during which patients will not be conscious of their surgery or their surroundings. Unintended awareness (‘awareness’) occurs when general anaesthesia has failed. Awareness is unpleasant, feared by patients, and is a cause of psychological harm to both the patient and the anaesthetist

6. RECALL. It is the patient’s ability to retrieve stored memories Assessed by a patient’s report of previous events, in particular, events that occurred during general anaesthesia . Recall can be either explicit or implicit. Explicit memory : It refers to intentional or conscious recollection of prior experiences as assessed so called direct memory test. Implicit memory (perception without conscious recall): The patient denies recall, but may remember “something” under hypnosis . DEFINITIONS

7. AWARENESS Generally considered to have occurred when, after completion of anaesthesia , an individual has explicit recall of intraoperative events with or without pain. Detection of awareness is necessarily retrospective and dependent on patient recall; There may be confusion as to when in the perioperative period the event being recalled took place.

8. Only 50% of affected patients report awareness immediately after operation, and recall may be reported up to a month after anaesthesia . Particular difficulties arise in differentiating memories from dreams. Detection can be facilitated by a structured interview, and the one most commonly used in both research and clinical practice is based on that by Brice

9. The problem of awareness first became clinically important during the 1940s after the introduction of curare, and the practice of ‘balanced’ anaesthesia . With administration of neuromuscular blocking agents, it was possible for patients to be conscious but unable to respond to environmental stimuli Despite continued advances in anaesthesia delivery and monitoring over 50 years, neuromuscular blocking (NMB) drugs remain the single most frequently implicated risk factor for awareness

10. AMNESIA . Amnesia is the absence of recall Many anaesthetic drugs produce amnesia at concentrations well below those necessary for suppression of consciousness Anterograde amnesia is intended when a drug with amnestic properties is administered before induction of anaesthesia Retrograde amnesia is intended when a drug such as a benzodiazepine is administered after an event that may have caused or been associated with intraoperative consciousness in the hope that it will suppress memory formation and “rescue” from recall.

11. DEPTH OF ANAESTHESIA Depth of anaesthesia or depth of hypnosis refers to a continuum of progressive central nervous system depression and decreased responsiveness to stimulation.

12. MONITORING DEPTH OF ANESTHESIA METHODS OF MONITORING: Clinical techniques and conventional monitoring Pharamcological principles Brain electrical activity monitoring Spontaneous EEG activity monitoring Evoked brain electrical activity monitoring

13. A. CLINICAL TECHNIQUES AND CONVENTIONAL MONITORING

14. I. Clinical signs: The most commonly used scoring system incorporates the PRST or Evan’s score This assesses autonomic activity related to P (systolic blood Pressure ), R (heart Rate ), S ( Sweating ) and T ( Tears ). Advantage of being simple and not requiring any specialized equipment D/A is parameters are not specific for the effects of anaesthesia and the values can vary widely among individuals. The scores range from 0 to 8 but the midpoint is seldom exceeded, reflecting the inadequacy of this scoring system.

15. Measurement of heart rate and blood pressure & regularly assessing pupil size, and the presence of sweating and lacrimation provides useful information regarding the adequacy of analgesia and depth of anaesthesia Tachycardia secondary to anticholinergic drugs such as atropine make the heart rate uninterpretable Beta -adrenergic blocking drugs , opiates and regional anaesthetic techniques will obtund the sympathetic nervous system response to pain Conventional monitoring systems (e.g., electrocardiogram, blood pressure, HR, end-tidal anaesthetic analyzer, capnography) are valuable and should be used to help assess intraoperative consciousness

16. II. Skin conductance : is the phenomenon that the skin momentarily becomes a better conductor of electricity when either external or internal stimuli occur that are physiologically arousing. It is a quantification of the clinical sign of sweat production. Skin conductance is initially low and is increased as anaesthetic depth increases, reducing again with surgical incision. Other factors affecting sweating (e.g. atropine, autonomic neuropathy) can reduce the accuracy of this monitoring.

17. III. Isolated forearm technique: A tourniquet is applied to the patient’s upper arm, inflated above systolic blood pressure before the administration of muscle relaxants. Movement of the arm either spontaneously or to command indicated wakefulness, although not necessarily explicit awareness. It has been used previously as a means of detecting awareness during caesarean section under general anaesthesia and during clinical trials assessing rates of awareness. Not all patients responding have recall. One limitation of this technique is the limited time available before patients are unable to move their arm due to tourniquet induced ischaemia

18. IV. SPONTANEOUS SURFACE ELECTROMYOGRAM (SEMG) In patients who are not completely paralyzed, spontaneous surface electromyogram (SEMG) can be recorded from various muscle groups, especially facial, abdominal and neck muscles. Frontalis muscle is innervated by a branch of the facial nerve and is less affected by the neuromuscular blockade. A stick on electrode positioned over the frontalis muscle can record the frontalis electromyogram (FEMG). The level of FEMG has been observed to fall during anaesthesia and to rise to pre- anaesthetic levels just before awakening

19. V. Lower oesophageal contractility (LOC) The non-striated muscles in the lower half of oesophagus retain their potential activity even after full skeletal muscle paralysis by neuromuscular blocking agents. SPONTANEOUS LOC PROVOKED LOC It arises spontaneously and can be detected by a pressure transducer.. These result from sudden distension of the oesophagus , as if due to the arrival of a food bolus. It can be induced by emotion and stress in the awake individual. PLOC are induced by the rapid inflation of a balloon catheter in the lower oesophagus . This causes smooth muscle contraction and is detected by a more distally placed pressure transducer. It is believed to be in control of a central oesophageal motility centre The dose-response curve for PLOC is shallower than that of SLOC. the activity of which is influenced by higher centres Reduce in latency and amplitude during general anaesthesia . reduce in latency and amplitude during general anaesthesia .

20. VI. HEART RATE VARIABILITY Anaesthetic agents either directly or indirectly first acts on the brain stem and then (probably) inhibit the cerebral cortex via ascending efferent projections from the midbrain. Therefore, objective measurement of brain stem-mediated autonomic tone that is not affected by any factor other than anaesthetic depth may be a good indicator of depth of anaesthesia . .

21. The special analysis of HRV revealed 3 components: Low frequency fluctuations; believed to be circadian. Medium frequency fluctuations; attributed to baroreceptor reflex. High frequency fluctuations HRV coincides with the frequency of ventilation, in which heart rate increases during inspiration and decreases during expiration, through a predominantly parasympathetic reflex connecting stretch receptors in the lungs and aorta to vagal motor neurons innervating the heart. This is called as respiratory sinus arrhythmia (RSA). It is typically characterized by greater than 10% variation in the ECG P-wave interval over 5 minutes

22. Surgical stimulation during light anaesthesia elicits a greater increase on RSA than seen during lightening anaesthesia alone. 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.

23. B. PHARMACOLOGICAL PRINCIPLES OF MEASURING DEPTH OF ANAESTHESIA

24. Depends primarily on the following factors: The equilibration of the drug’s concentration in plasma with the concentration of the drug at its site of action and with the measured drug effect. The relationship between drug concentration and drug effect The influence of noxious stimuli.

25. INHALATIONAL AGENTS The purposeful movement of any part of the body in response to noxious perioperative stimuli is one of the most useful clinical sign of depth of anaesthesia . Minimum alveolar concentration (MAC) of inhaled anaesthetics as the concentration required to prevent 50% of subjects from responding to painful stimuli. MAC-intubation : that would inhibit movement and coughing during endotracheal intubation. MAC-incision : To prevent movement during initial surgical incision. MAC-BAR : To prevent adrenergic response to skin incision, as measured by the venous concentration of catecholamine. MAC-awake: that would allow opening of the eyes on verbal command during emergence from anaesthesia .

26. The MAC curves (representing the relationship between the concentration of anaesthetic agent and the probability of response) are located from left to right in the order: MAC-awake < MAC-incision < MACintubation < MAC-BAR In contrast to somatic reflexes, haemodynamic responses to noxious stimuli do not correlate well with end-tidal drug concentration. Consequently the relationship between somatic (movement) and autonomic ( haemodynamic ) responses is poor during inhalational anaesthesia

27. C. BRAIN ELECTRICAL ACTIVITY MONITORING

28. Brain electrical activity for the purpose of assessing anaesthetic effect is monitored by EEG activity from electrodes placed on the forehead. Systems can be subdivided into those that process spontaneous EEG and EMG activity and those that acquire evoked responses to auditory stimuli i.e. auditory evoked potential (AEPs). From raw EEG or AEP analog signal is amplified and converted into digital domain  various alogorithms are applied to the frequency, amplitude, latency, and/or phase relationship data and a single number is generated referred to as an “index,” typically scaled between 0 and 100. This index represents the progression of clinical states of consciousness (“awake,” “sedated,” “light anaesthesia ,” “deep anaesthesia ”)

29. Value of 100 is associated with the awake state and values of 0 occurring with an isoelectric EEG (or absent middle latency AEP). EMG activity from scalp muscles can be considered an artifact from the viewpoint of pure EEG analysis, it may be an important source of clinically relevant information. Sudden appearance of frontal (forehead) EMG activity suggests somatic response to noxious stimulation resulting from inadequate analgesia and may give warning of impending arousal. For this reason, some monitors separately provide information on the level of EMG activity.

30. 1. Spontaneous EEG activity monitors:

31. ( i ) EEG: An EEG can be obtained using the standard 19-electrode method Time-consuming and impractical and requires expert interpretation. In its unprocessed form, it is not a practical tool for monitoring depth of anaesthesia . Increasingly sophisticated, automated analysis of various EEG components has generated several potential quantitative descriptors of anaesthetic depth. There are two generic problems with processed EEG technologies: 1. Dissimilar anaesthetic agents generate different EEG patterns or signatures and 2. Various pathophysiological events also affect the EEG (e.g. hypotension, hypoxia, hypercarbia). Such events may modify both the patient’s level of consciousness and the expected EEG signature that any given anaesthetic agent generates, thus confounding interpretation

32. (ii) Compressed spectral analysis: The compressed spectral array (CSA) is obtained by superimposing linear plots of successive epochs of time on each other Generates a three-dimensional ‘hill and valley’ display of the power amplitude vertically (y-axis), frequency horizontally (x-axis) and time (z-axis). Anaesthesia causes a reduction in high-frequency and an increase in low-frequency amplitudes, which is easier to interpret than raw EEG.

33. However the problems of patient- and agent variability and the confounding effects of other pathophysiological processes such as hypoxia, hypotension and hypercarbia remain It is nota reliable monitor of the depth of anaesthesia , but can provide a trend for use in conjunction with clinical observations.

34. (iii) Cerebral function monitor (CFM): This device is modified from the conventional EEG It uses a single biparietal or bitemporal lead (three wires) to obtain an EEG signal. This signal is filtered, semi-logarithmically compressed, and rectified. The output is displayed at a very slow chart speed, 1 mm/minute, giving a trace

35. As a result of this processing, the output is presentation of the overall electrocortical background activity of the brain. A high reading on the chart indicates a high level of activity. A low value indicates low activity. It has been used in cardiac, neuro- and vascular surgery, where trends in activity may reflect changes in cerebral perfusion.

36. (iv) Bispectral Index: BIS is a proprietary algorithm that converts a single channel of frontal EEG into an index of hypnotic level (BIS). It is a statistically based, empirically derived complex parameter Composed of a combination of time domain, frequency domain and high order spectral sub parameters. It is unique as it integrates several disparate descriptors of EEG into a single variable, based on a large volume of clinical data to synthesize a combination that correlates behavioral assessments of sedation

37. Insensitive to the specific anaesthetic or sedative agent chosen. It is a numerical index, ranging from 100 (awake) to 0 (isoelectric EEG). Correlates well with the level of the responsiveness and provide an excellent prediction of the level of consciousness with propofol, midazolam and isoflurane anaesthesia . Also correlates with the haemodynamic response to intubation, patient’s response to skin incision and verbal command during inhalational as well as total intravenous anaesthesia .

38. It reduces the cost by saving anaesthetic use and stay in PACU and provides a useful guide for titration of anaesthetic agents in cardiac surgery, elderly and paediatric patients Senile dementia may be a confounding factor in interpretation of BIS value. In some instances BIS has been observed to increase with the use of N2O and ketamine Routine intraoperative events (e.g., administration of depolarizing muscle relaxants, activation of electromagnetic equipment or devices, patient warming or planned hypothermia) may interfere with BIS functioning and patient experiencing awareness despite monitored values indicating adequate depth of anaesthesia

39. (v)Entropy: Based on acquisition and processing of raw EEG and FEMG signals by using the Entropy algorithm. Entropy describes the irregularity, complexity, or unpredictability characteristics of a signal. Entropy of the signal drops when a patient falls asleep and increase again when the patient wakes up. FEMG quiets down as the deeper parts of the brain are increasingly saturated with anaesthetics.

40. Deeper the level of consciousness the patterns are more regular and ordered Correlates with level of consciousness Paradoxical high readings with N2O or Ketamine

41. Entropy measurement is an objective monitoring and is of two types- Response Entropy (RE) and State Entropy (SE) indicating analgesic and hypnotic levels during general anaesthesia .

42. (vi) Narcotrend ®: The Narcotrend is an EEG monitor designed to measure the depth of anaesthesia. The newest Narcotrend software version includes index from 100 (awake) to 0 (electrical silence). Two commercially available electrodes are placed on the forehead of the patient; a third electrode serves as a reference After artifact analysis a multivariate statistical algorithm transforms the raw EEG data finally resulting in a 6-letter classification of the depth of anaesthesia.

43. A(awake), B(sedated), C(light anaesthesia), D (general anaesthesia),E (general anaesthesia with deep hypnosis),F (general anaesthesia with increasing burst suppression). Sub-classifications resulting in a total 14possible sub-stages :A, B0–2,C0–2, D0– 2, E0–1, and F0–131. In the most recent version

44. (vii) Cerebral State Monitor/Cerebral State Index (CSI): It is a handheld device that analyses a single channel EEG and presents a CSI scaled from 0to100. It also provides EEG suppression percentage and a measure of EMG activity (75–85 Hz). The EEG waveform is derived from the signal recorded between the frontal and mastoid electrodes. The frequency content is 2-35Hz. The performance of the CSI is based on the analysis of the frequency content of the EEG signal. The energy of the EEG is evaluated in specific frequency band called alpha and beta

45. The monitor also on-line evaluates the amount of instantaneous burst suppression(BS) in each thirty-second period of the EEG. The CSI is a unit-less scale from 0 to 100, where 0 indicates flat EEG and 100 indicates EEG activity corresponding to the awake state. The range of adequate anaesthesia is designed to be between 40 and 60. All values in the table are approximate values based on the mean values of the patient behaviour.

46. 2. EVOKED BRAIN ELECTRICAL ACTIVITY MONITORING

47. I. SSEP A supramaximal stimulus is applied to peripheral nerves while a recording scalp electrode is placed over the appropriate sensory area. Most anaesthetic agents increase the latency and decrease the amplitude in dose-dependent manner. Etomidate consistently increases the amplitude.

48. II. Visual evoked potentials (VEP): Light-emitting diodes are incorporated into specialized goggles and the optic nerve is stimulated at 2 Hz. EEG electrodes take recordings from the occiput Most anaesthetic agents increase the latency and decrease the amplitude in dose-dependent manner. VEP are considered less reliable than AEP They have been used to monitor function during surgery for lesions involving the pituitary gland, optic nerve and chiasma

49. III Auditory evoked potential(AEP): Defined as the passage of electrical activity from the cochlea to the cortex, which produces a waveform consisting of 15 waves The waveform can be divided into three parts: Brainstem Auditory Evoked Potential (BAEP) Middle Latency Auditory Evoked Potential (MLAEP) and Long Latency Auditory Evoked Potential (LLAEP). These parts indicate the sites in the brain from which the various waves are thought to originate

50. The brainstem response is relatively insensitive to anaesthetics Early cortical responses (MLAEPs) change predictably with increasing concentrations of both volatile and intravenous anaesthetics. There is increased latency and decreased amplitude of the various waveform components with anesthetic agents in dose dependent manner

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