Effect of the Rate of Ventilation and Inspired Gas Concentration

Frequently it is desirable to overcome the slow rate of rise of alveolar tension associated with such factors as the high blood solubility of some anesthetics and increased pulmonary blood flow. Since both of these factors retard tension development by increasing the uptake of anesthetic, the most effective way to alleviate the problem is to accelerate the input of gas to the alveoli. A useful technique to increase the input of anesthetic to the lung is to elevate the minute alveolar ventilation. This maneuver, which causes a greater quantity of fresh anesthetic gas to be delivered to the patient per unit of time, is most effective with highly soluble agents (Fig. 25.4).

Increasing the inspired tension of an anesthetic gas above the maintenance tension (i.e., near the MAC value) is also an effective means of quickly establishing effective alveolar tension. This maneuver, frequently referred to as overpressure, parallels the concept of loading dose. As the desired depth of anesthesia or level of alveolar tension is achieved, the delivered tension of anesthetic must be returned to the maintenance (MAC) level to avoid overdosing the patient.

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