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MAC An Essential Measure for Patient Safety

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With tens of millions of surgical procedures being performed in the United States each year, the delivery and monitoring of anesthesia is a ubiquitous necessity; without which the vast majority of procedures would not be possible. With the first systemic inhaled vapor anesthetic being delivered in the Ether Dome at Massachusetts General hospital in 1846, a revolution in surgical care was started which continues today in every operating room around the world.

The primary goals of general anesthesia are to produce a state in the patient that optimizes surgical conditions while rendering the patient safely insensate and unaware of the surgical trauma. Achievement of these goals is typically accomplished through the use of an anesthetic drug or drugs combined with an analgesic and often a muscle relaxant. It became evident early on that a measure specifically suited to determine the amount of an anesthetic necessary to achieve surgical immobility would be an invaluable tool. To that end, in the early 1960s, Drs. Eger and Merkel set out to determine the potency of volatile anesthetics in humans while studying at the University of California San Francisco.

In 1963 while Drs. Eger and Merkel were fellows at UCSF, they designed an animal study comparing inhalational anesthetics administered to canine subjects utilizing a tail clamp as the simulated surgical stimulus. Their research was extended into humans looking at the potency of the early volatile anesthetic halothane. The measure they derived from the experiment was the minimum alveolar concentration (MAC) or the alveolar concentration of anesthetic at 1 atmosphere, which produces immobility in 50% of subjects exposed to a one-inch skin incision. Prior to MAC the adequacy of anesthetic dosing relied on qualitative and highly variable metrics.1 MAC affords a quantitative representation of anesthetic depth by measuring the partial pressure of exhaled or end-tidal anesthetic at which 50% of people will not move in response to a surgical stimulus.1

MAC is based on the fat solubility of the anesthetic or more appropriately its ability to cross the lipid bilayer.2 Following the Meyer-Overton theory that governs the behavior of fat-soluble agents MAC can be derived using the formula:

MAC x λ ≈ 1.82

Where λ = the olive oil/gas partition coefficient.3

Different anesthetics have different pharmacokinetic properties based on their respective brain/blood coefficients, which leads to volatile anesthetics like Sevoflurane, and Desflurane reaching equilibrium between the alveolar and brain tissues quickly in comparison to isoflurane. This relationship remains true during elimination of the anesthetic with brain concentration reaching equilibrium with fresh gas (with no anesthetic) leading to awakening.

MAC values are additive; meaning that when more than one agent is in use their MAC values are combined. For instance 0.5 MAC of nitrous oxide along with 0.5 MAC of isoflurane yields a MAC of 1.0. Multiple factors decrease MAC (acute alcohol intoxication, hyponatremia, hypothermia, anemia, hypercarbia, hypoxia, pregnancy) while others increase it (chronic alcohol, infancy, red hair, hypernatremia, hyperthermia).4

The anesthesiologist uses MAC, or rather interprets MAC, taking into account all of the factors that may raise or lower it. MAC is used as a guide to deliver the correct amount of anesthetic, which may prevent over or under-dosing and the complications associated with both. Overdosing may lead to severe cardiovascular instability and hypotension while under-dosing may lead to movement or awareness during surgery.

The use of MAC to guide anesthetic titration helps increase safety and efficiency; which may help the clinician to achieve an adequate depth of anesthesia; to aid in the prediction of wake-up times with greater accuracy; and to limit side effects associated with higher anesthetic concentrations.5

Modern gas sampling technology has allowed the continuous monitoring of inspired and expired gases and is considered essential to patient safety under anesthesia. GE Healthcare offers a range of CARESCAPETM respiratory modules that are compact, feature-rich, and easily integrated; allowing the clinician to optimize workflow efficiently with fewer connections and less equipment while saving time on system checks, calibrations, and service. The CARESCAPE module is able to automatically detect, identify and continuously display anesthetic gases, concentrations and MAC. By supplying the patient’s age MAC is automatically adjusted rendering a more accurate MAC value.

With an enhanced suite of features that go beyond inspired and expired O2, CO2, and anesthetic gas concentrations, GE Healthcare's CARESCAPE modules are capable of supplying advanced information and graphics on lung mechanics and spirometry. Key features of the GE Healthcare respiratory modules include:

  • Very compact size, low weight and low power consumption
  • Sampling of all parameter values proximal at the patient’s airway with a single gas sampling line, D-lite(+)* or Pedi- lite(+) flow sensor, along with an additional Spirometry tube
  • Breath-by-breath updating of Et and Fi values
  • Fast oxygen measurement for accurate EtO2 and FiO2 values
  • Automatic identification of anesthetic agents
  • Automatic detection of end-inspiratory and end-expiratory occlusions
  • Display of values for Statis Plat, Static PEEPi+e and Static Compliance
  • Calculated balanced gas value for estimating the N2-concentration

Nothing can replace the knowledge and training of the anesthesiologist supported by advanced tools for the OR. Patients deserve the safest, most accurate, and appropriate anesthetic. GE Healthcare's CARESCAPE respiratory modules support clinicians in delivering safe and efficient anesthesia.

 

References:

  1. Lobo SA, Lopez J. Minimum Alveolar Concentration. [Updated 2018 Nov 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532974/
  2. Alkire MT, Gorski LA. Relative amnesic potency of five inhalational anesthetics follows the Meyer-Overton rule. Anesthesiology. 2004 Aug;101(2):417-29.
  3. Aranake A, Mashour GA, Avidan MS. Minimum alveolar concentration: ongoing relevance and clinical utility. Anaesthesia. 2013 May;68(5):512-22. 
  4. https://www.openanesthesia.org/inhaled_anesthetics_pharmacology/
  5. David White, Uses of MAC, BJA: British Journal of Anaesthesia, Volume 91, Issue 2, August 2003, Pages 167–169, https://doi.org/10.1093/bja/aeg160