In a Mapleson D breathing system (Bain circuit) used for spontaneously breathing patients, which feature distinguishes it from the Mapleson A system?

• A The APL valve is at the patient end and the reservoir bag at the machine end
• B Rebreathing is eliminated by the parallel inner tube delivering fresh gas near the patient
• C The fresh gas inlet is near the patient and the APL valve is near the reservoir bag ✓
• D It requires the lowest fresh gas flow for spontaneous ventilation

Explanation

In the Mapleson D (Bain modification) system, fresh gas is delivered close to the patient via a narrow inner coaxial tube while the APL (pop-off/expiratory) valve is at the reservoir bag (machine) end. This arrangement makes it efficient for controlled ventilation (fresh gas flow ~70 mL/kg/min) but inefficient for spontaneous breathing (requiring ~200–300 mL/kg/min to prevent rebreathing). In contrast, the Mapleson A (Magill) circuit has the fresh gas inlet near the reservoir bag and the APL valve at the patient end, making it highly efficient for spontaneous ventilation (70 mL/kg/min is adequate).

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

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Written and medically reviewed by the StethoPrep medical team.