ANESTHESIA MACHINE 

GLOSSARY

· Vital capacity (CV): Is the amount of air that is possible to expel from the lungs after having inspired completely. They are around 4.6 liters. CV = VRI + VC + VRE

·  Residual volume: the amount of air left in a person's lungs after exhaling completely.

· Respiratory frequency: is the number of breaths a person makes per minute. The frequency is usually measured when a person is at rest and simply consists of counting the number of breaths for one minute each time the chest is raised.

· Tidal volume (VT, or TV): is the amount of air that is displaced along normal inhalation and exhalation, in other words, the amount of air that is breathed throughout normal breathing. In a healthy young adult, the tidal volume is approximately 500 ml per inspiration or 7 ml / kg body mass.

PHARMACOLOGICAL FUNDAMENTALS OF INHALATORY ANESTHETICS

The pharmacokinetics of inhalation anesthetics describe their absorption from the socket to the systemic circulation, its distribution in the organism and its elimination mainly through the lungs and, to a lesser extent, by liver metabolism. Inhalation anesthetics are administered with the aim of achieve a concentration in the central nervous system that allows a adequate pain control in surgical interventions. To do this, it makes the lungs reach through the ventilation system a certain inspiratory partial pressure (Pi).

From here the anesthetic is captured by blood and transported to organs and tissues. After a certain saturation period cerebral partial pressure is reached (Pcerb) Suitable for anesthesia. Throughout the anesthesia a gradient of partial pressures of the anesthetic, so that all tissues tend to match their partial pressure with alveolar partial pressure (PA). By controlling the BP, we indirectly control the Pcerb. The PA of an inhalation anesthetic is a reflection of your Pcerb and is the reason why with the BP we define the speed of induction and recovery of anesthesia, and it is measure of its power.

PHYSICAL-CHEMICAL PROPERTIES OF ANESTHETICS

INHALATORIES

Except nitrous oxide and xenon, which are gases, the rest of inhalation anesthetics, which are halogenated ethers, are liquids to room temperature and atomospheric pressure. Currently, we have in our environment of sevofluoran and desfluorano, which have displaced the rest of the halogenated anesthetics that were traditionally used (halothane, isofluorane).

The defluoran has a high vapor pressure and a low point of boiling 23º C, so you need special vaporizers that keep constant pressure and temperature. Sevofluoran, unlike the defluoran, has a pleasant smell and is not irritating because of what it does ideal for inhalation induction.

FACTOR THAT DETERMINE THE ALVEOLAR PARTIAL PRESSURE.

The BP, and therefore the Pcerb of an inhalation anesthetic, comes determined, on the one hand, by the entry of said anesthetic into the socket through alveolar ventilation with a certain Pi. Moreover, the Blood and tissue uptake opposes the maintenance of BP. This in this way, the factors that determine BP are:

·         * Inspiratory partial pressure (Pi)

·         * Alveolar ventilation

·         * Blood and tissue uptake

Pi and alevolar ventilation are factors that favor the increase in anesthetic concentration to match BP, while uptake he opposes this increase. We will review each of these three factors.

PHARMACODYNAMICS OF INHALATORY ANESTHETICS

 1.- MECHANISM OF ACTION The exact mechanism by which these compounds produce the anesthetic effect is unknown. Probably through direct interaction with cellular proteins causing changes in their configuration and altering neuronal transmission. In any case, no specific receptor is involved in its mechanism, so there is no antagonist for these drugs.

2.- CAM CONCEPT (or MAC) CAM is the minimum alveolar concentration, at atmospheric pressure, that suppresses the motor response in 50% of individuals. In a pure inhalation anesthesia it is necessary to reach 1.2 - 1.3 CAM to prevent movement in 95% of patients. CAM decreases as age increases, and with the addition of some drugs such as opiates, clonidine, magnesium sulfate or nitrous oxide.

3.- NITROUS OXIDE Nitrous oxide is a gas at room temperature, the first one that was used in anesthesia in 1846. It is a poorly potent anesthetic: its CAM is 104, which means that a total inhalation anesthetic with nitrous oxide would lead to carried out under anoxia conditions, which is why it has been used as an adjuvant in anesthesia with halogenates in clinical practice (50-70%). Its low power, together with the problem it presents in the gaseous cavities of the organism due to its different solubility with nitrogen (increases the pressure in closed cavities and the volume in distensible cavities), diffusion hypoxia (at the end of administration of nitrous oxide, access to the alveolus of a large amount of the gas, decreasing the PAO2) and its relative contraindication in ventilation with minimal flows, makes this gas currently in disuse. From now on, we will refer only to halogenated inhalation anesthetics.

I.                    EFFECTS ON THE NERVOUS SYSTEM

Halogenated anesthetics produce hypnosis, analgesia and anesthesia. In this involves both supra and spinal structures, but they do not have action in the peripheral nerve. Inhalation anesthetics cause decreased consumption of oxygen and cerebral flow but  not in parallel, producing a decoupling between brain metabolism and blood flow. Equally, interfere with self-regulation, and increase intracranial pressure.

II.                  EFFECTS ON THE CARDIOCIRCULATORY SYSTEM

l halogenates lower blood pressure doce dependent. Sevofluorano and desfluorano do it by descent in the systemic vascular resistance, as opposed to halothane, which produced direct myocardial depression. They also produce tachycardia, in especially defluoran when administered quickly.
Neither sevofluorane and defluorane are arrhythmogenic (as opposed to halothane, which sensitized the myocardium to endogenous catecholamines) or They cause coronary theft. On the contrary, they produce cardioprotection in surgery cardiac.

III.                    EFFECTS ON THE RESPIRATORY SYSTEM

Halogenated anesthetics, in spontaneous ventilation, descend the volume / minute and increase respiratory rate. They all are bronchodilatarores Sevofluorane, unlike defluorane, is not irritating to airway so it is the ideal anesthetic for anesthetic induction inhalation.

IV.                  HEPATIC EFFECTS

Sevofluorane as a defluoroane are safe in terms of flow and liver function However, special mention deserves the case of the halothane since could lead to severe liver pathology by two mechanisms: the first of them due to a hypersensitivity reaction by a process autoimmune that gave rise to fulminant hepatitis. The other, due to a change in the metabolism of halothane which, under hypoxic conditions, became reductive instead of oxidative. This fact, together with the high degree of halothane metabolism (up to 20%) could cause liver ne.

V.  EFFECTS ON RENAL FUNCTION.

Sevofluoran reacts with CO2 absorbers producing the called compound A (fluoromethyl 2,2-difluoro-1-vinyl ether), which was demonstrated nephrotoxic in rats but has never been demonstrated in humans at doses clinics Therefore, low flow ventilation with sevofluorane can be considered safe as far as renal function is concerne. 

ANESTHESIA MACHINE MAINTENANCE

• Cylinder storage

They should be stored in a dry and well ventilated place, they should not be placed near flammable materials. Non-flammable oxidizing gases that sustain combustion, such as nitrous oxide and oxygen should be stored separately from flammable gases.

Cylinders containing Carbon Dioxide should be stored in the same area of flammable gases, as this gas acts as a fire extinguisher.

• Canister: The soda that it contains has to be changed when at most ¾ of this turn violet and be replaced by a new one (white soda).

• For the anesthesia machine it is advisable to make a review every 3 months (preventive maintenance) of the following components:

§  Oxygen Failure Safety Valves and Oxygen Alarms: The valves must be closed while the pipe is supplied, check the equipment manual according to the supplier.

§  Flow Control Valves or Buttons: Ensure that the indicator moves smoothly and that the indicator reaches zero when closing the valve.

§   Flowmeters: Ensure that the indicator moves smoothly and freely, it must be disassembled and change the gaskets.

§   Oxygen "FLUSH" valve: Check that they are connected to the maximum flow indicator at the machine outlet.

§  The flow should not be less than 35 liters per minute. It must be determined with an oxygen analyzer, which only exits oxygen when this mechanism is activated.

§   Hoses and Connections in General: The degree of deterioration and if there are bends that hinder the flow or leaks should be checked.

After each anesthesia the hoses should be washed with soap and water, left hanging in order to drain the water and ventilate. After a septic procedure should be sterilized.

§   Vaporizers: It should be checked that the buttons can be moved and are not out of adjustment.

These must be emptied after each use, if the flush key is opened, zero the vaporizer; In general, handle them very delicately, making them assess by the technician when their malfunction is suspected.

Video explanation about anesthesia machine, pharmacological principles and function.