Accuracy of the MAAS Method (Minimal-flow Auto-control Anesthesia System) for the Administration of Desflurane and Sevoflurane in the Anesthetic Maintenance Phase. Prospective and Paired Observational Study.

Halogenated anesthetics (HAs) are the gaseous drugs commonly used in inhalation anesthesia to ensure, among other effects, loss of consciousness or induction of "anesthetic sleep". They are administered in their gaseous state through the anesthesia vaporizers inserted in the anesthesia workstations. Like other gaseous hydrocarbons, HAs contribute to global warming, with the particularity that their global warming potential (GWP) is hundreds of times higher than that of the reference molecule, CO2, especially in the case of desflurane. For this reason, professionals and administrations must be especially sensitive when handling this type of drugs. Indeed, the investigators consider that its use should be questioned in the absence of administration devices capable of minimizing atmospheric emissions, provided that there are adequate therapeutic alternatives.

Currently, there are "universally" available tools that guarantee that these minimum contamination principles can be met in daily practice. The fundamental tool would be the anesthesia circle breathing circuit. Circle circuits are conceived as artificial breathing systems that allow the rebreathing of exhaled gases. Rebreathing exhaled gases, once the CO2 has been removed by means of gas neutralization systems incorporated in anesthesia stations (CO2 absorbers), is safe and effective, and today this mode of administration of the inhalation anesthesia has become the gold standard. The usefulness and efficiency of circle circuits is anesthesiologist-dependent, since the reuse of exhaled gases will depend on the amount of "new" (fresh) medicinal gases that the anesthesiologist allows to enter the anesthesia workstation every minute, by means of the regulation of the fresh gas flow (FGF): the higher FGF, the less reuse of exhaled gases and at the greater surplus of "waste" gases to be evacuated and expelled into the atmosphere. On the contrary, the fewer new gases the anesthesiologist allows to enter the anesthesia workstation, the greater the reuse of exhaled gases, and the smaller the volume of gases expelled into the atmosphere. The administration of inhalation anesthesia through the use of low FGF, known as low flow anesthesia (LFA) techniques, would therefore constitute one of the main contributors to the reduction of HA emissions into the atmosphere.

The majority of anesthesia workstations used in daily practice allow anesthesiologists to work with FGF lower than 500 milliliters per minute (ml/min) or reaching to the closed-circuit anesthesia mode, that modality of inhalation anesthesia in which it is supplied to the system (anesthesia workstation and patient) the amount of gases (ml/min) that are transferred ("lost") from the central compartment (blood and highly perfused organs- HPO) to other compartments due to the principle of partial pressures equilibrium between different compartments (uptake phenomenon by muscle and fat), or metabolic consumption in the case of O2.

Different techniques for administering LFA have been described. The MAAS method (Minimal-flow Autocontrol Anesthesia System) proposes a didactic and easy way to estimate the needs for HA supplementation during the anesthetic maintenance phase, and includes a feasible formulation when adjusting the supply of HA to the system. The maintenance phase would comprise from the moment in which the target concentration of HA in the central compartment is reached, until the moment in which HA stops being administered to the system. During this period, HA keeps on being transferred towards muscle and fat from the central compartment (highly perfused organs (HPO) compartment) following the principle of partial pressures equilibrium. The estimation of HA uptake (HAup) by muscle and fat once the equilibrium state between the anesthesia workstation (respiratory circuit) and the HPO is reached, is based on the calculation HAup (ml/min) =HAfi-HAfe*MV(ml/min), where HAfi and HAfe state for inspired and expired fraction of HA, respectively; and MV for volume minute.

The estimate of the ml of HA to be supplied to the anesthetic circuit through the FGF, would be done following the formula HAdel (from HA delivered)= HAup (ml/min )*100% /FGF(ml).