Before use, a Bain coaxial circuit is tested by occluding the patient end with a syringe plunger and squeezing the reservoir bag. Gas is seen escaping through the expiratory valve, after which the bag suddenly deflates and the plunger is pushed out. This manoeuvre (Pethick's test) primarily assesses:
- A Integrity of the inner fresh gas delivery tube ✓
- B Patency of the outer transparent tubing
- C Function of the unidirectional valves
- D Calibration of the pressure manometer
Correct answer: A. Integrity of the inner fresh gas delivery tube
Explanation
In Pethick's test, occluding the patient end and squeezing the bag diverts gas down the outer tube, opening the expiratory valve. If the inner fresh gas tube is intact, the resulting pressure drop makes the bag collapse and pushes the occluding object away. B persistently distended bag suggests an inner tube leak or disconnection, which would cause hypoxia from inadequate fresh gas delivery. The test specifically detects inner tube defects, not outer tube patency.
Reference: Miller's Anesthesia, 9th ed.
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Written and medically reviewed by the StethoPrep medical team.