Anesthesia Depth Monitoring - Significance, Methods, and Outcome

Introduction:

With the emerging newer anesthetic techniques like intravenous anesthesia, potent opiate analgesics, newer volatile agents, and more complicated regional nerve blocks, measuring the depth of anesthesia becomes an integral part of the procedure. The prevention of awareness begins with an accurate anesthetic technique. This involves checking all equipment, ensuring the uninterrupted delivery of anesthetic to patients through intact circuits and intravenous access, and the use of familiar, appropriate techniques by competent practitioners.

Why Should the Depth of Induced Anesthesia Be Monitored?

Patient and equipment monitoring is used to assess the administration of anesthetic medication, detect physiologic variations, allow intervention before the patient suffers harm or excessive anesthetic depth, improve patients’ outcomes, and detect and correct equipment malfunction.

What Are the Important Signs Checked to Monitor the Depth of the Induced Anesthesia?

The main fundamental aspects of anesthetic monitoring are:

• Oxygenation (circulatory and respiratory function).

• Ventilation (respiratory function).

• Circulation (circulatory function with an emphasis on cardiac output).

What Are the Signs of Inadequate Anesthetic Depth?

Important signs of inadequate anesthetic depth that occur due to stress or painful stimuli are:

• Movement.

• Increased breathing or heart rate.

• Increased blood pressure.

Can Intraoperative Depth of Anesthesia Monitoring Predict Outcome from Surgery?

Awareness remains of great concern to anesthetists and their patients undergoing surgery. The generous approach of giving larger anesthetic doses while keeping awareness as a concern may increase the risk of hypotension and delay in recovery time and has the potential to increase other complications, thus requiring in-depth monitoring of anesthesia intraoperatively.

What Are the Various Methods Used to Monitor the Depth of Anesthesia?

A. Subjective Methods or Clinical and Conventional Techniques:

1. Autonomic response.

• Hemodynamic changes.

• Lacrimation (production of tears).

• Pupillary dilation.

• Sweating.

2. Isolated forearm technique.

B. Objective Methods:

1. Pharmacological principles of depth monitoring.

2. Spontaneous surface electromyogram (SEMG).

3. Lower oesophageal contractility (LOC).

4. Heart rate variability (HRV).

5. Electroencephalogram (EEG) and derived indices.

• Spectral edge frequency.

• Median frequency.

• Bispectral index.

6. Evoked potentials.

• Auditory evoked potentials.

• Visual evoked potentials.

• Somatosensory evoked potentials.

• Auditory evoked potential index.

C. Subjective Methods or Clinical and Conventional Techniques:

1. Autonomic Response: The most commonly used scoring system, PRST score or Evan’s score, assesses autonomic activity relating to P (systolic blood pressure), R (heart rate), S (sweating), and T (tears).

• Advantages: Simple to use and does not require specialized equipment.

• Disadvantages: Gives subjective values and is highly variable and not reliable.

2. Isolated Forearm Technique: A tourniquet is applied to the upper arm of the patient and inflated above the systolic blood pressure before administering muscle relaxants. Movement of the arm either spontaneously or to the command indicates wakefulness. It is commonly used during cesarean surgeries. The disadvantage is the patient has very little time to respond to tourniquet-induced ischemia.

D. Objective Methods:

1. Pharmacological Principles of Anesthetic Depth Monitoring: The purposeful moment of any body part in response to noxious perioperative stimuli is one of the most useful clinical signs of the depth of anesthesia. The minimum alveolar concentration (MAC) of inhaled anesthetics is the concentration required to prevent 50 percent of subjects from responding to painful stimuli. It depends on the equilibrium of the drug’s concentration in plasma with the concentration of the drug at its site of action and with the measured drug effect. It relies on the relationship between drug concentration and drug effect and also on the influence of noxious stimuli.

• MAC - Intubation: Inhibit movement and coughing during endotracheal intubation.

• MAC - Incision: Prevent movement during initial surgical incision.

• MAC - Bar: Prevents adrenergic response to skin incision, as measured by the venous concentration of catecholamine.

• MAC - Awake: Allows opening of the eyes on verbal command during the recovery from anesthesia.

2. Spontaneous Surface Electromyogram (SEMG): In patients with incomplete paralysis, SEMG can be recorded from various muscle groups like facial, abdominal, and neck muscles. Supplied by a branch of the facial nerve, the frontalis muscle is less likely to be affected by the neuromuscular blockade. An electrode placed over the frontalis muscle can record FEMG (frontalis electromyogram). The FEMG levels fall during anesthesia and rise back to pre-anesthetic levels just before awakening.

3. Heart Rate Variability (HRV): Anesthetic agents, either directly or indirectly, first act on the brain stem and then (probably) inhibit the cerebral cortex via ascending efferent projections from the midbrain. Objective measurement of mediated autonomic tone, which is not affected by any other factor other than anesthetic depth, is a good indicator of the depth of anesthesia. A special analysis of HRV revealed three components:

1. Low-frequency fluctuations are believed to be circadian.

2. Medium frequency fluctuations attributed to baroreceptor reflex.

3. High-frequency fluctuations coincide with the frequency of ventilation, in which heart rate increases during inspiration and decreases during expiration. This is called RSA (respiratory sinus arrhythmia), which is typically characterized by greater than ten percent variation in the ECG (electrocardiograph) P-wave interval over five minutes.

Some monitors use HRV at a respiratory frequency or respiratory sinus arrhythmia (RSA) as a method of assessing the depth of anesthesia. It depends on an intact autonomic nervous system and a healthy myocardial conduction system.

4. Electroencephalogram (EEG): Brain electrical activity to assess anesthetic effects 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 that are auditory evoked potential (AEP). An EEG can be obtained using the standard 19-electrode method. From raw EEG or AEP, the analog signal is amplified and converted into a digital domain. Various algorithms are applied to the frequency, amplitude, latency, and phase relationship data, and a single number is generated, referred to as an “index,” typically scaled between 0 and 100.

The generic problems associated with processed EEG technologies are:

1. Dissimilar anesthetic agents generate different EEG patterns or signatures.

2. Various pathophysiological events also affect the EEG (for example, hypotension, hypoxia, hypercarbia).

A value of 100 is associated with the awake state, and values of 0 occur with an isoelectric EEG. The sudden appearance of frontal (forehead) EMG (electromyogram) activity suggests a somatic response to noxious stimulation resulting from inadequate analgesia and may give impending warning arousal. For this reason, separate information on the level of EMG activity should be given.

EEG Derived Bispectral Index: The bispectral index converts a single channel of frontal EEG into an index of hypnotic level. Bispectral index range:

1. 100 - Awake, respond to a normal voice.

2. 80 - Light to moderate sedation, respond to loud commands or mild shaking.

3. 60 - General anesthesia, low probability of explicit recall, unresponsive to verbal stimulus.

4. 40 - Deep hypnotic state.

5. 20 - Burst suppression.

6. 0 - Flatline EEG.

It correlates well with the level of responsiveness and provides an excellent prediction of the level of consciousness with Propofol, Midazolam, and Isoflurane. Also correlates with the hemodynamic response to intubation, the patient’s response to skin incision, and verbal command during inhalation as well as total intravenous anesthesia.

5. Lower Oesophageal Contractility (LOC): Spontaneous and provoked lower esophageal contractions both reduce latency and amplitude during general anesthesia. These are measured using a balloon in the esophagus. Its uses for monitoring the depth of anesthesia monitor are limited.

6. Evoked Potentials:

• Somatosensory Evoked Potential (SSEP): A supramaximal stimulus is applied to peripheral nerves while a recording scalp electrode is placed over the appropriate sensory area. Most anesthetic agents increase the latency and decrease the amplitude in a dose-dependent manner. Etomidate consistently increases the amplitude.
• 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 anesthetic agents increase the latency and decrease the amplitude in a dose-dependent manner. Less reliable than AEP (auditory evoked potential). Used during the surgery for lesions involving the pituitary gland, optic nerve, and chiasma.
• Auditory Evoked Potential (AEP): It is defined as the passage of electrical activity from the cochlea to the cortex, which produces a waveform consisting of fifteen waves.

The waveform is divided into three parts:

1. Brainstem auditory evoked potential (BAEP).

2. Middle latency evoked potential (MLAEP).

3. Long latency auditory evoked potential (LLAEP).

These parts indicate the sites in the brain from which various waves originate. Early cortical responses (MLAEPs) change predictably with increasing concentrations of both volatile and intravenous anesthetics.

Conclusion:

Quantifying the effect of anesthetics on brain activity is an attractive proposition for monitoring both volatile and intravenous anesthesia. In time, EEG-based monitoring may prove to be a sensitive indicator of the depth of anesthesia to use in conjunction with conventional means to prevent intraoperative awareness.