Awareness+Under+GA

= = toc
 * =Introduction=
 * Awareness is a serious consequence of relaxant anesthesia with incidence of around 1:1000, many incidents are due to errors in anesthetic administration and can be prevented by careful technique In an ideal situation, anesthesiologists could monitor conscious level. Technology to do this reliably has proved difficult to develop, and at present, depth of anesthesia is determined in the following ways:
 * =Clinical Parameters =
 * HR, BP, Pupil size, sweating, etc.rely on the parasympathetic nervous system and can be affected by factors like hypothermia, arrhythmias, B-blockers, antihypertension agents and epidual/subarachnoid block.
 * ==Isolated Forearm Technique: ==
 * A tourniquet on the upper arm is inflated above systolic BP before muscle relaxation. Spontaneous movementsor hand squeezing on command indicates impending or actual awareness. not all patients who respond have explicit recall postoperatively.
 * ==Minimum Alveolar Concentration (MAC): ==
 * is the minimum alveolar concentration of anesthetic at 1 atomsphere pressure producing immobility in 50% of subjects exposed to a standard painful stimuli (skin incison).
 * MAC awake: the minimum alveolar conentration of anesthetic producing unconsciousnessin 50% of subjects.
 * MACbar: the minimum alveolar concentrationof anesthetic blocking the sympathetic nervous system response to a painful stimulus in 50% of subjects.
 * ===Factors increasing MAC: ===
 * hyperthermia
 * hyperthyroidism
 * Alcoholism
 * ===Factors decreasing MAC: ===
 * Increasing Age
 * Hypothermia
 * <span style="font-family: Arial,Helvetica,sans-serif;">Hypoxia- CNS Depressants ( Opioid )
 * <span style="font-family: Arial,Helvetica,sans-serif;">N2O/Volatile Agents
 * <span style="font-family: Arial,Helvetica,sans-serif;">Alpha2 Agonist
 * <span style="font-family: Arial,Helvetica,sans-serif;">Opioids reduce MAC, particularly MACbar, however,they are not anestheticthemselves. It is, therefore, essential to administer enough volatile agent to prevent awareness, ( > MACawake ), even when painful surgical stimuli are blocked by high dose of opioids or regional anesthesia.

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 * =<span style="font-family: Arial,Helvetica,sans-serif;">Electroencephalographic Monitoring (EEG) =
 * <span style="font-family: Arial,Helvetica,sans-serif;">A standard EEg is impractical-- time consuming, electrical interference ( diathermy ), poor electrode contacts. The use of fewer frontal electrodes and fast-Fourier analysis identifying component waveforms frequencies and corresponding amplitudes has made the interpretation and application of the EEG easier. Techniques include:
 * =<span style="font-family: Arial,Helvetica,sans-serif;">Bispectral Index (BIS) =
 * <span style="font-family: Arial,Helvetica,sans-serif;">Combines compressed spectral array information with phase relationships of EEG`s component sine waves and calculates a value between 0 ( electrical silence ) to 100 ( awake ). Evidence that BIS reduces awareness is limited.
 * <span style="font-family: Arial,Helvetica,sans-serif;">100 Awake
 * <span style="font-family: Arial,Helvetica,sans-serif;">65-85 Sedation
 * <span style="font-family: Arial,Helvetica,sans-serif;">45-65 GA
 * <span style="font-family: Arial,Helvetica,sans-serif;">< 40 Burst Suppression
 * <span style="font-family: Arial,Helvetica,sans-serif;">0 No electrical activity

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