Pediatric breathing circuits

Published on 07/02/2015 by admin

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Last modified 07/02/2015

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Pediatric breathing circuits

Dawit T. Haile, MD

Anesthetic breathing circuits function to deliver O2 and anesthetic gases to patients and to eliminate CO2 from patients. They are classified according to (1) the presence or absence of unidirectional valves, (2) the presence and the position of a reservoir bag, (3) the means by which CO2 is eliminated, (4) the ability of the circuit to permit or prevent rebreathing, and (5) the efficiency of the circuit at preventing rebreathing.

Mapleson circuits

The first anesthesia breathing systems delivered NO2-O2 mixtures for dental anesthesia via a reservoir bag directly connected to an expiratory valve and a facemask. Sir Ivan Magill improved this circuit by distancing the reservoir bag from the expiratory valve and facemask with a reservoir tube to improve surgical access for facial operations. The Magill attachment, also referred to as Mapleson A, was popular for more than 50 years.

By the 1950s, several types of semiclosed circuits were used to deliver anesthetic gases. Semiclosed circuits under optimal conditions prevent rebreathing of alveolar gases. In 1954, the physicist William W. Mapleson analyzed five of these circuits and proposed optimal conditions that would prevent rebreathing. The efficiency of a nonrebreather is determined by the amount of fresh-gas flow, as well as by the positions of the inflow of fresh gas, the expiratory valve, and the reservoir bag. Mapleson labeled these circuits A, B, C, D, and E (Figure 193-1); subsequently, these circuits have been referred to as the Mapleson circuits, and Mapleson’s theoretical analyses have been verified empirically by others.

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Figure 193-1 Mapleson breathing circuits A through E. Note that E is a T-piece system. 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.)

The Mapleson (A, B, C, D, and E) circuits lack unidirectional valves and a CO2 absorber. They have the advantage of reduced airflow resistance, which is ideal for use in pediatric patients. The Mapleson circuit removes CO2 by venting exhausted gas to the atmosphere, in contrast with circle systems, in which CO2 is removed by a CO2 absorber. Because Mapleson circuits lack a unidirectional valve, the fresh gas and alveolar gases mix, and significant rebreathing occurs if the fresh-gas flow is not adequate. The Mapleson A and D circuits have been analyzed most extensively, the B and the C circuits are rarely used, and the E circuit is basically a T-piece system. The D circuit is the most commonly used Mapleson circuit, and the A circuit is infrequently used but has a historical and a functional significance.

Mapleson A circuit

The Mapleson A circuit, as described earlier, comprises a reservoir tubing (corrugated tubing) separating, at one end, the fresh-gas flow passing through a reservoir bag and, at the opposite end, an adjustable pressure-limiting valve (APL valve) near the facemask. The system is the most efficient and, with spontaneous ventilation, requires less fresh-gas flow than with controlled ventilation. To explain these differences, the breathing cycle can be artificially divided into three phases: the inspiratory, the expiratory, and the expiratory-pause phase.

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.

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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,

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