In a Mapleson D breathing system (Bain circuit), the fresh gas flow required to prevent rebreathing during spontaneous ventilation is approximately:

• A 1× minute ventilation
• B 0.5× minute ventilation
• C 2–3× minute ventilation ✓
• D Equal to tidal volume in mL/breath

Explanation

In a Mapleson D circuit (and its co-axial modification, the Bain circuit), the fresh gas inlet is at the patient end and the APL (adjustable pressure-limiting) valve is at the machine end. During spontaneous ventilation, exhaled gas passes down the corrugated outer tube toward the APL valve. To prevent CO2 rebreathing, fresh gas flow must be sufficient to flush exhaled gas from the system before the next inspiration — requiring approximately 2–3 times the minute ventilation (approximately 150–200 mL/kg/min, or 200–250 mL/kg/min in children). During controlled ventilation, a fresh gas flow of 70–100 mL/kg/min is sufficient as the clinician controls the ventilatory pattern.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.