...MAC has acceptable pragmatic utility in preventing awareness because generally greater MAC is used to prevent mobility - to achieve surgical anaesthesia!
P.S. MAC (Eger et al, 1965) Minimum Alveolar Concentration, is a misnomer, rather it should be called MEDIAN Alveolar Concentration because only 50% of the patients are likely to remain immobile to a standard surgical stimulus. For 95% have to deliver 1.2 MAC and for 99%, 1.3 MAC.
Vapouriser dial setting (%) corresponds with the MAC number of an inhalational agent, for an example MAC 6 of Desflurane implies that a dial setting of 6% will deliver 1 MAC and 2% of Sevoflurane, 1 MAC. But, this setting reflects the delivered concentration and not the effect site concentration.
MAC number better equates with the Expired Concentration (%) of inhalational agent (shown alongside EtCO2), which reflects effect site Concentration in the Brain (spinal cord) and a single end point - IMMOBILITY - compared to Guedel's assessment with regard to Breathing, Muscle relaxation, Pupil size, Lacrimation and Eyelid reflex to describe the changing stages and phases (surgical anaesthesia - third stage, third phase) to denote insensibility (mental - brain) and inactivity (motor - spinal cord) responses. Effect on airway reactivity, blood pressure, heart rate, respiratory rate and depth, muscle tone are pragmatically controlled by adjusting the dial setting (delivered %) and more accurately with end tidal concentration (effect site %) - these percentages correspond with the MAC number, e.g. 2% for Sevoflurane (MAC 2).
Amnesia (0.06 - 0.3 MAC) and "inability to explicitly recall" (upon probing with questions) - loss of awareness is monitored using BIS, bispectral index and maintaining the number between 40 and 60. Below 40 (MAC 1.2-2.0) would obtund autonomic reflexes and above 60 (MAC <0.5) would cause awareness.
Meyer-Overton hypothesis states that potency is directly proportional to lipid solubility and therefore, product of MAC and olive oil/gas partition coefficient (lamda) = 1.82 atmospheres, a constant. It implies that higher the MAC, lower the potency, faster the onset-offset time, lower the lipid solubility, for an example, MAC of N2O is 105 but its potency is so low; induction and emergence is faster!
MAC awake signifies eye opening to verbal command during emergence - Halothane, 0.41 (MAC 0.75), Isoflurane 0.49 ( MAC 1.2), Sevoflurane 0.62 (MAC 2.0), Desflurane 2.5 (MAC 6.0) and Nitrous oxide 68 (MAC 105). Quite interestingly, generally MAC awake 68% Nitrous Oxide is given along with Oxygen! About 33% oxygen is generally enough to saturated the haemoglobins unless comorbidities exist.
MAC is highest at 6 months of age, thereafter declines with advancing age. Hypothermia decreases cerebral oxygen consumption and therefore reduced MAC is needed. CSF and serum sodium level is inversely proportional with MAC; higher sodium requires less MAC and vice versa. Hypoxia, hypercarbia and anaemia, pregnancy, postpartum decrease MAC. Genetic background like melanocortin-1 gene mutation can increase MAC but sex and obesity do not appear to affect MAC significantly. Alzheimer's disease requires more.
Controversially, the "triple low" - low MAC (<0.7), low BP (<75 mm Hg), low BIS (<40) - is associated with increased mortality and higher MAC with delirium and cognitive decline.
Minimum alveolar concentration: ongoing relevance and clinical utility: https://pdfs.semanticscholar.org/09ff/5b39513579874c7044ed2afaa5b1ba329d36.pdf
Uses of MAC:
https://academic.oup.com/bja/article/91/2/167/371166
Inhaled Anesthetics (Pharmacology)
Overview/General Principles:
https://www.openanesthesia.org/inhaled_anesthetics_pharmacology/
Minimum alveolar concentration:
https://www.sciencedirect.com/topics/medicine-and-dentistry/minimum-alveolar-concentration
P.S. MAC (Eger et al, 1965) Minimum Alveolar Concentration, is a misnomer, rather it should be called MEDIAN Alveolar Concentration because only 50% of the patients are likely to remain immobile to a standard surgical stimulus. For 95% have to deliver 1.2 MAC and for 99%, 1.3 MAC.
Vapouriser dial setting (%) corresponds with the MAC number of an inhalational agent, for an example MAC 6 of Desflurane implies that a dial setting of 6% will deliver 1 MAC and 2% of Sevoflurane, 1 MAC. But, this setting reflects the delivered concentration and not the effect site concentration.
MAC number better equates with the Expired Concentration (%) of inhalational agent (shown alongside EtCO2), which reflects effect site Concentration in the Brain (spinal cord) and a single end point - IMMOBILITY - compared to Guedel's assessment with regard to Breathing, Muscle relaxation, Pupil size, Lacrimation and Eyelid reflex to describe the changing stages and phases (surgical anaesthesia - third stage, third phase) to denote insensibility (mental - brain) and inactivity (motor - spinal cord) responses. Effect on airway reactivity, blood pressure, heart rate, respiratory rate and depth, muscle tone are pragmatically controlled by adjusting the dial setting (delivered %) and more accurately with end tidal concentration (effect site %) - these percentages correspond with the MAC number, e.g. 2% for Sevoflurane (MAC 2).
Amnesia (0.06 - 0.3 MAC) and "inability to explicitly recall" (upon probing with questions) - loss of awareness is monitored using BIS, bispectral index and maintaining the number between 40 and 60. Below 40 (MAC 1.2-2.0) would obtund autonomic reflexes and above 60 (MAC <0.5) would cause awareness.
Meyer-Overton hypothesis states that potency is directly proportional to lipid solubility and therefore, product of MAC and olive oil/gas partition coefficient (lamda) = 1.82 atmospheres, a constant. It implies that higher the MAC, lower the potency, faster the onset-offset time, lower the lipid solubility, for an example, MAC of N2O is 105 but its potency is so low; induction and emergence is faster!
MAC awake signifies eye opening to verbal command during emergence - Halothane, 0.41 (MAC 0.75), Isoflurane 0.49 ( MAC 1.2), Sevoflurane 0.62 (MAC 2.0), Desflurane 2.5 (MAC 6.0) and Nitrous oxide 68 (MAC 105). Quite interestingly, generally MAC awake 68% Nitrous Oxide is given along with Oxygen! About 33% oxygen is generally enough to saturated the haemoglobins unless comorbidities exist.
MAC is highest at 6 months of age, thereafter declines with advancing age. Hypothermia decreases cerebral oxygen consumption and therefore reduced MAC is needed. CSF and serum sodium level is inversely proportional with MAC; higher sodium requires less MAC and vice versa. Hypoxia, hypercarbia and anaemia, pregnancy, postpartum decrease MAC. Genetic background like melanocortin-1 gene mutation can increase MAC but sex and obesity do not appear to affect MAC significantly. Alzheimer's disease requires more.
Controversially, the "triple low" - low MAC (<0.7), low BP (<75 mm Hg), low BIS (<40) - is associated with increased mortality and higher MAC with delirium and cognitive decline.
Minimum alveolar concentration: ongoing relevance and clinical utility: https://pdfs.semanticscholar.org/09ff/5b39513579874c7044ed2afaa5b1ba329d36.pdf
Uses of MAC:
https://academic.oup.com/bja/article/91/2/167/371166
Inhaled Anesthetics (Pharmacology)
Overview/General Principles:
https://www.openanesthesia.org/inhaled_anesthetics_pharmacology/
Minimum alveolar concentration:
https://www.sciencedirect.com/topics/medicine-and-dentistry/minimum-alveolar-concentration
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