Medical brain notes 7

  The anesthesia machine As anesthetic agents and techniques have evolved, so have the delivery systems. Modern anesthesia requires the ability to administer gases and vapors in the desired combinations, often while mechanically ventilating the patient’s lungs. Here we present step-by-step the concepts on which anesthesia machines are based. We are starting with systems that sport neither valves nor means to store gases, advance to systems that can store gases – which requires a couple of valves, and advance to systems in which the patient rebreathes his exhaled gas – but without carbon dioxide. Thus, we will have set the stage for the modern anesthesia machine. 

  Systems without gas storage If we have to give anesthesia on the North Pole, and we have nothing but a can of diethyl ether, we can give a fine anesthetic by dripping the ether on a cloth held over the patient’s mouth. That will work also with halothane and isoflurane. Early anesthetists used masks (Fig.  8.1 ) on which to drape the cloth. Back from the North Pole, assume we have a patient who weighs 70 kg, has a tidal volume of 600 mL, a respiratory rate of 10 breaths/min and an I : E (inspiratory to expiratory) ratio of 1 to 2; that is, he spends twice as much time exhaling (and pausing between breaths) than inhaling. Assume his trachea to be intubated. All his respired gases flow through the tubing connecting his endotracheal tube to the source of oxygen. (Fig.  8.2 ). We wish to provide his lungs with 100% oxygen. To achieve this, his exhaled gas needs to be vented through a T near the mouth. This T will also allow him to pull in room air during inspiration, dilutin...

  Single-valve system with gas storage In order to save gas, we can provide for storage of the gas. Well known is the Mapleson system, often used during resuscitation and during transport of a  patient who requires mechanical ventilation.   The system shown in Fig.  8.3 is properly called a Mapleson  D  (as there are different arrangements lettered A through F). Figure  8.4 shows the real thing. It is light and deceptively simple. To prevent rebreathing of exhaled carbon dioxide requires a relatively high fresh gas flow, both to meet inspiratory demand and to wash exhaled CO 2  out of the tubing. Here, the excess gas escapes through an adjustable one-way (pop-off ) valve that prevents entrainment of room air. The pressure required to open the spring-loaded valve can be varied, enabling us to generate enough pressure (by squeezing the bag) to inflate the patient’s lungs. Slow respiratory rates help because, during a long pause between inspirat...

  Multi-valve system with gas storage A self-inflating bag provides an alternative that enables the resuscitator to venti-late the patient’s lungs with room air or, if oxygen is available, with air enriched with oxygen. For the latter to succeed, the self-inflating bag must have a reservoir in which oxygen can accumulate during inspiration (see Fig.  8.5 ). Without a self-inflating bag, gas has to be admitted to the breathing system under pressure. Figure  8.6 shows a simple arrangement. We incorporate a bag and two valves. Now the fresh gas accumulates in the bag during exhalation when the inspiratory valve closes and the expiratory valve opens (venting CO 2 -laden gas to the atmosphere). During inspiration the valves swap roles: the inspiratory valve opens and the expiratory one closes. Such valves have little resistance, perhaps 1 or 2 cm H 2 O, and thus will easily open during the respiratory cycle. This works for a patient breathing spontaneously. Again, an adjustabl...

  Systems with carbon dioxide absorption In anesthesia, because of the cost (and ozone-depleting qualities) of volatile anes-thetic agents, we prefer to conserve even more gas. Furthermore, the patient does not consume all inhaled oxygen (at rest, a patient consumes only a small por-tion of the inhaled oxygen, reducing the FiO 2  of 0.21 to an FeO 2  of 0.17). Thus, we save a lot if we have to do nothing more than replace the oxygen the patient consumes (for the average adult at rest – about 250 to 300 mL/min). We need to remove the carbon dioxide, of which our resting patient generates about as much (depending on his respiratory quotient) as he consumes oxygen. Figure  8.7 shows the arrangement. We simply formed a circle (conceptually, if not diagrammati-cally) by connecting the expiratory and inspiratory limbs. The two valves in the circle assure a one-way flow of gases in the circuit. We still have the APL valve and the breathing bag, but we have incorporated a ca...