Danger of low pressure alarm failure in preterm infants on continuous positive airway pressure.

In certain settings of conventional continuous positive airway pressure (CPAP) application, the ventilator may not be able to detect dislodgement of the prongs. This occurs especially in settings with high flow and small prongs. We investigated the relation between ventilator flows, size of the nasal prongs, and pressure generated within the ventilator circuit due to the flow resistance of the prongs. We studied a Baby-flow CPAP connected to a Babylog 8000plus ventilator. Five prongs of increasing size (x-small, small, medium, large, x-large) and one nose mask were connected to the CPAP in turn. Starting at 30 lpm, the flow was reduced in 2 lpm steps. The dynamic pressure caused by the flow resistance of the prongs within the ventilator circuit was recorded. For all devices, we observed a correlation between the reduction of the flow and the reduction in pressure within the ventilator circuit. However, the flow resistance of the x-small prongs generated the highest dynamic pressure (30 mbar at 22 lpm) within the ventilator circuit while the mask gave rise to the lowest pressure (9 mbar at 30 lpm). The pressure value generated with x-small prongs at low flow rate was observed at high flow rate with x-large prongs or with a mask. We conclude that in settings with high flow rates, low CPAP levels, and small prongs, the resistance of the prongs will create enough dynamic pressure within the ventilator circuit to permit the ventilator to compensate a large leakage flow by closing the expiratory valve. Thus, in case of dislodgement of the prongs, the pressure within the ventilator circuit will not decrease below the alarm level, and the machine will not be able to generate an alarm.