[Modification of facial mask on the dead space effect in non-invasive mask ventilation]

There were reports concerning the CO2 rebreathing during non-invasive positive pressure ventilation (NIPPV) with full face mask. It is our hypothesis that modification of the mask from one way connection to two ways connection by making a side hole in the mask makes it possible that CO2 inside the mask could be washed out by a constant flow through the mask.

A randomized self-control study on CO2 rebreathing was conducted in 7 COPD patients to compare the modified set-up with the conventional one. A BiPAP-30 ventilator and a plateau exhaustion valve (Respironics USA) were employed in the study. In the modified two ways set-up, the exhaustion valve (with distal end blocked) was connected to the side hole of the mask, so that a constant base flow could pass through the mask to the exhaustion valve. The average base flow was 0.43 LPS. The parameters were set as following: S/T mode, f: 15 BPM, pressure support level: 8 cm H2O. Three different levels of PEEP (2, 3 and 5 cm H2O) were used to investigate the PEEP level on CO2 rebreathing. Flow and CO2% were constantly recorded with computer data acquisition and analysis system (Microcal Origin).

In conventional set-up, there was obvious CO2 rebreathing (rebreathing volume: 83.1 +/- 32.9 ml). In modified connection, the rebreathing volume was only (0.1 +/- 0.4) ml (P < 0.001).

There was obvious CO2 rebreathing during full face mask NIPPV in conventional set-up. A modified two ways connection could reduce CO2 rebreathing to be near zero, which might be important in the management of hypercapnic respiratory failure with NIPPV.