Immediately before the inspiratory phase of spontaneous ventilation occurs, continuous fresh gas flows into the reservoir bag and the circuit (Figure 193-2). As the patient inhales, the reservoir bag begins to empty. The lower the fresh-gas flow, or the higher the tidal volume, the emptier the reservoir bag becomes. During the expiratory phase, the reservoir bag completely fills with fresh gas, and, when the fresh-gas flow exceeds 70% of minute ventilation, enough pressure develops to vent alveolar and fresh gas through the APL valve. At the last stage of the expiratory phase, a pause occurs before the initiation of the next cycle. During the expiratory pause, fresh-gas flow further drives alveolar gas through the APL valve and virtually eliminates rebreathing.

Figure 193-2 Mapleson A circuit: Spontaneous ventilation. A, Prior to the inspiratory phase, continuous fresh gas flows into the reservoir bag and the circuit. B, The reservoir bag empties during inspiration. C, During expiratory phase, the reservoir bag fills with fresh gas, and when it exceeds 70% of minute ventilation, alveolar gas is pushed through APL valve. D, During the last phase of expiration, fresh gas further pushes alveolar gas through the APL and virtually eliminates rebreathing. EXP, Expiratory valve; FGF, fresh-gas flow. (Redrawn from Ward CS. Anaesthetic Equipment: Physical Principles and Maintenance. 2nd ed. London: Bailliere Tindall (WB Saunders); 1985:122-126.)

How can a fresh gas flow that is only 70% of minute ventilation prevent rebreathing? The answer is “dead-space gas.” The gas in the reservoir tubing immediately before exhalation is dead-space gas because it is has not been exchanged within the patient’s lung and, therefore, does not contain alveolar gases. During the expiratory pause, all of the alveolar gases in the reservoir tubing and some of the dead-space gases are pushed by the fresh-gas flow and expelled through the APL valve. However, not all of the dead-space gas is expired before the next cycle. Because of this residual dead-space gas, the amount of fresh gas required to eliminate rebreathing in the Mapleson A circuit during spontaneous ventilation is less than the minute ventilation.

In contrast with spontaneous ventilation, controlled ventilation (hand ventilation) of the Mapleson A circuit empties the reservoir bag at the end of the inspiratory phase (Figure 193-3). The reservoir bag refills with a mixture of alveolar gases, fresh gas, and dead-space gases during the expiratory phase. During controlled ventilation, the expiratory pause is minimal, which increases the likelihood of alveolar gas retention in the reservoir tubing and increases the amount of alveolar gases present with the initiation of the next inspiratory phase. Among the Mapleson circuits, the Mapleson A circuit under controlled ventilation is considered to be the least efficient at preventing rebreathing; rebreathing is overcome by increasing fresh-gas flow far exceeding minute ventilation.

Figure 193-3 Mapleson A circuit: Controlled ventilation. A, At the end of the inspiratory phase, the reservoir bag is empty. B, During expiration, the reservoir bag refills with alveolar gas, fresh gas, and dead-space gas. C, The expiratory pause is minimal, and **(D)** the alveolar gas retention in the circuit is quite high. EXP, Expiratory valve; FGF, fresh-gas flow. (Redrawn from Ward CS. Anaesthetic Equipment: Physical Principles and Maintenance. 2nd ed. London: Bailliere Tindall (WB Saunders); 1985:122-126.)

Mapleson D circuit

In the Mapleson D circuit, compared with the Mapleson A circuit, the positions of the APL valve and the fresh-gas-flow nipple are reversed; the fresh-gas-flow nipple is located at the patient’s end of the circuit, and the APL valve is next to the reservoir bag at the opposite end. The Mapleson D circuit is considered to be a modification of a T-piece circuit; the T-piece circuit is modified into a Mapleson D circuit by adding a reservoir bag and APL valve to the distal end of the reservoir tubing. This circuit requires slightly more fresh-gas flow to eliminate rebreathing than does the Mapleson A circuit. However, for controlled mechanical ventilation, circuit D is the most efficient of the Mapleson circuits.

During spontaneous ventilation with the Mapleson D circuit (Figure 193-4), the alveolar gases are immediately mixed with fresh-gas flow as the gases pass down the reservoir tubing and fill the reservoir bag. When the reservoir bag is filled with a mixture of alveolar and fresh gases, the mixed gas is vented out the APL valve. The first gas to exit through the APL is the dead-space gas, followed by the mixture of alveolar and fresh gases. During the expiratory pause, fresh-gas flow expels most of the alveolar mixed gas if the minute ventilation is adequate. Therefore, to prevent rebreathing, the fresh-gas flow has to be twice the minute ventilation, and the expiratory pause has to be sufficiently long to allow all of the alveolar mixed gases to be expelled.

Figure 193-4 Mapleson D circuit: Spontaneous ventilation. A, The fresh gas flow nipple is at the patient’s end of the circuit, and the APL is at the opposite end with the reservoir bag, and therefore **(B)** alveolar and fresh gas mix in the circuit during expiration and **(C)** the alveolar gas is not completely evacuated before **(D)** the next inspiratory phase of spontaneous respiration. EXP, Expiratory valve; FGF, fresh-gas flow. (Redrawn from Ward CS. Anaesthetic Equipment: Physical Principles and Maintenance. 2nd ed. London: Bailliere Tindall (WB Saunders); 1985:122-126.)

During the expiratory phase of controlled ventilation with the Mapleson D circuit (Figure 193-5), the fresh-gas flow drives the mixed alveolar gases and dead-space gases out of the APL valve. Furthermore, during the inhalation phase, the mixed alveolar gases are pushed and expelled not only by the continuous fresh-gas flow, but also by the positive pressure of controlled ventilation. The amount of fresh-gas flow necessary to minimize rebreathing is greater than the patient’s minute ventilation.