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Mapleson A FGF Spontaneous Ventilation — FRCA Final MCQ

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ModerateAnaesthetic Equipment & SafetyMapleson A FGF Spontaneous VentilationFRCA Final

A 40-year-old man (ASA I) is undergoing a laparoscopic hernia repair. You are using the Lack breathing circuit (a Mapleson A system). What is the minimum fresh gas flow required to prevent rebreathing during spontaneous ventilation with a Mapleson A system?

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Correct answer: B — Equal to alveolar ventilation (approximately 70% of minute ventilation)

The Mapleson A (Lack) circuit is the most efficient circuit for SPONTANEOUS ventilation, requiring a fresh gas flow equal to only the alveolar ventilation (approximately 70% of minute ventilation) to prevent rebreathing. This is because during expiration, alveolar gas (high CO2) is exhaled first and displaced out through the APL valve by the continuous fresh gas flow, while the dead space gas (low CO2) remains in the tubing and is re-inhaled first during the next breath. For CONTROLLED (IPPV) ventilation, the Mapleson A is the LEAST efficient, requiring 2-3× minute ventilation.

Reference: RCOA – 2023 – Anaesthetic equipment: breathing systems; Davis PD – 2019 – Basic Physics and Measurement