Monitoring of end-tidal carbon dioxide partial pressure during high frequency jet ventilation

A new method has been developed to measure end-tidal carbon dioxide partial pressure (PECO2) during high frequency jet ventilation (HFJV). A digital flow controller incorporated in a computerized high frequency jet ventilator was used to deliver either a single deep breath or a series of three deep breaths. On user request, HFJV was interrupted and the deep breaths delivered, after which HFJV was resumed. Using a mathematical model, we were able to predict accurately the pressures to which the lungs would be inflated during deep breaths. The effect of varying the deep breath pressure (Pdb) on the ratio of end-tidal PCO2 to arterial (PCO2 (PECO2:PaCO2) was studied in three dogs. In all the dogs, within an optimum Pdb range of 5-10 cm H2O, PECO2 during the first deep breath was found to be similar (+/- 0.2 kPa) to the PaCO2 immediately before the onset of deep breaths. Deep breaths delivered above or below the optimum Pdb range resulted in a decrease in the ratio PECO2:PaCO2. The frequency of jet ventilation (12-200 b.p.m.) before the onset of the deep breaths did not affect PECO2:PaCO2.