Detection of inner tube defects in co‐axial circle and Bain breathing systems: a comparison of occlusion and Pethick tests

Abstract

SummaryThe performance of the occlusion and Pethick tests in detecting faulty inner tubes in co-axial circle and Bain systems was compared. Twelve co-axial circle and 12 Bain anaesthetic breathing systems were tested using the occlusion and the Pethick tests. For each system, three tubes were intact, and the remaining nine had a defect deliberately created in the inner tube (three proximal, three middle and three distal). The investigators were blinded to which of the tubes were defective, and to each other's results. The results showed 100% specificity for both tests. The sensitivity of the occlusion test for detecting faulty breathing systems was found to be good (98%). Our results suggest that the occlusion test should be performed in preference to the Pethick test when testing co-axial circle and Bain systems.

Hypercapnia resulting from a faulty co-axial (Bain) circuit

“…[89]Pethick test: This tests the low-pressure system and the integrity of the inner tube. The procedure is as follows: Occlude the patient end of the circuit.…”

“…Collapse of the reservoir bag due to the creation of venturi effect in the outer tube is an indication that the inner tube is intact. [8] This test will not detect a system in which inner tube is omitted or does not extend to the patient port or one that has holes at the patient end of the inner tube.Foex-Crampton-Smith manoeuvre: This manoeuvre assesses the gas flow line from the flowmeters of the machine to the patient end of the circuit. With an oxygen flow at 2 litres/minute, the patient end of the inner tube is occluded briefly for 2–3 seconds using the forefinger.…”

“…Collapse of the reservoir bag due to the creation of venturi effect in the outer tube is an indication that the inner tube is intact. [8] This test will not detect a system in which inner tube is omitted or does not extend to the patient port or one that has holes at the patient end of the inner tube.…”

Comparison of the Coaxial Circle Circuit with the Conventional Circle Circuit

“…Fresh gas flow was then reduced to the anesthesia machine's minimum (250 mL/min). The breathing system was considered intact if pressure was maintained at 30 cmH 2 O for 5 seconds; the breathing system was deemed faulty if the pressure failed to reach 30 cmH 2 O or was not maintained for five seconds [15]. Relative humidity and temperature were measured at the endotracheal tube at 5 minutes and again at 30 minutes with a hygrometer (Drager, Serial No: 044936, model 5204644-A4).…”

An unusual cause of apparent rebreathing associated with anaesthesia delivered by a coaxial circle breathing system

“…Although coaxial systems are convenient to use, problems have been associated with a failure of the integrity of the inner lumen, resulting in re-breathing and consequently potential patient harm [3,4]. In order to ensure integrity of the inner tube specific occlusion tests have been recommended [5,6].…”