Systems without gas storage - The 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, diluting his FiO₂. What oxygen flow rate will prevent such entrainment of room air? The easy answer: the oxygen flow rate must match his inspiratory flow rate (which, by the way, is not constant). The total amount of oxygen the system must deliver then will equal his minute ventilation of 6000 mL, but this volume must be given over only 1/3 of a minute (with an I : E of 1:2). The technology for such an arrangement, i.e., to flow oxygen only during inspiration, exists in ICU ventilators. 

In anesthesia machines, we instead have continuous gas flow throughout the respiratory cycle. In the example above, during exhalation, the continuing oxygen flow has nowhere to go but to escape, together with the patient’s exhaled gas, to the outside. We still must meet his inspiratory flow demand, delivering his minute volume during inspiration (in 1/3 minute), and we would lose all of the oxygen (2/3 of the total) flowing during expiration. Thus, using this simple system in a patient with an I : E ratio of 1:2, we would need a fresh gas flow three times as large as his minute volume.