According to the American Society of Anesthesiologists (ASA), a monitored anesthesia care (MAC) is a planned procedure during which the patient undergoes local anesthesia together with sedation and analgesia. The three fundamental components and purposes of a conscious sedation during a MAC are: a safe sedation, the control of the patient anxiety and the pain control.The patient should be consciously sedated and remain without pain or discomfort.This procedure can be performed with patient controlled sedation techniques or with continued intravenous infusion or with target controlled infusion.using anaesthetic agents which have a fast half life.Just connecting to monitor and asking surgeon to give blocks cannot be called MAC,in the ideal sense. But how conscious sedation monitored? The Internet Journal of Health, suggests bi spectral index for this purpose.Drugs to control PONV and other drugs for co morbid conditions like DM etc are advised to be given and their activity monitored.The patient may be observed in PACU for about thirty minutes, eventhough fast tracking is possible. Anaesthetic agents used are midazolam, propofol, remifentanyl,and alpha2adrenergic drugs.
Minimum alveolar concentration means,the concentration of inhalational anaesthetic agent that prevents movement in response to a skin incision in 50 % of individuals at 1 atmosphere in 100 % oxygen.The anaesthtic potency can be calculated from MAC based on the fact that, at equilibrium the alveolar concentration equates brain concentration and alveolar concentration can be easily measured.It has been observed that sex, weight, height and anaesthetic duration will not affect MAC . MAC values of different agents are additive, and it has been observed that 1.3 times MAC of any anaesthetic agent prevents movement in 90% of individuals. MAC values
* Halothane 0.74 %
* Enflurane 1.68 %
* Isoflurane 1.15 %
* Desflurane 6.3 %
* Sevoflurane 2.0 %
* Nitrous oxide 104 %
FACTORS AFFECTING MAC
INCRASE IN MAC
Hyperthermia
Hypernatraemia
Sympathoadrenal stimulation
Chronic alcohol abuse
? Chronic opioid abuse
Increases in ambient pressure
Hypercapnia
Decreasing age
Thyrotoxicosis
DECREASE IN MAC
Nitrous oxide Hypothyroid/myxoedema
Hypocapnia
Hypothermia-decrease is roughly linear
Hyponatraemia
Increasing age
Hypotension
Anaemia
Pregnancy
Hypoxia
CNS depressant drugs including alcohol,lithium, magnesium,clonidine etc
Ref: a brief history of MAC, anaesthesiology 2002,96(1), & anaesthesia uk, Intnt journl of health,2000,vol.1,no1
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INFORMATIVE***other terminology,. for MAC are MAC-BAR (1.7-2.0 MAC), which is the concentration required to block autonomic reflexes to nociceptive stimuli, and MAC-AWAKE (0.3-0.5 MAC), the concentration required to block voluntary reflexes and control perceptive awareness.NITROUS OXIDE WHEN USED THE MAC OF INHALATIONAL AGENTS CAN BE DECREASED
roughly 3*MAC is required for induction unless there is air dilution
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