Flow resistance of coaxial breathing systems: investigation of a circuit disconnect.

A clinical incident involving an undetected disconnection occurred during the use of a CPRAM coaxial breathing circuit. The flow resistance of this circuit was evaluated and compared with that of a Bain circuit to determine the factors involved. A differential pressure transducer was used to monitor the pressure drop across each circuit during simulation of controlled ventilation with a fresh gas flow of 6 L.min-1. An Ohio V5 anaesthesia ventilator was adjusted to different flow rates and tidal volumes and a test lung simulated an airway resistance of 2.7 cm H2O.L-1.sec and a compliance of 0.05 L.cmH2O-1. Absolute pressure at the ventilator was also monitored when the circuit was disconnected from the test lung. The CPRAM circuit displayed a pressure drop from the ventilator to the endotracheal tube of 6.3 cm H2O at 60 L.min-1, about twice that found with the Bain. A disconnection at the tracheal connector produced an absolute pressure at the ventilator of 9.6 cm H2O with the CPRAM and 5.5 cm H2O with the Bain. Since the ventilator low pressure alarm was preset to 9.2 cm H2O, the alarm provided a warning with the Bain but not the CPRAM. The elevated flow resistance of the CPRAM circuit was attributed to a restriction in the flow area at the patient end of the circuit. Capnographs or adjustable low-pressure alarms provide more reliable monitoring for breathing circuit disconnects.