Evaluation of a Method for Measuring Minute Volumes during Bias-Flow Ventilation

Background Gas exchange measurements made in patients mechanically ventilated with a bias-flow ventilator may require an in-line isolation valve to maintain accuracy, which may impose additional dead space and work of breathing. Evaluation Methods Two linear pneumotachographs (P-tachs) were attached to a custom-made T-piece without isolation valves. The inspiratory and expiratory limbs of the ventilator circuits were then attached to the P-tachs. The bias flow and compressible volume measured by the inspiratory P-tach was subtracted from the total expiratory flow measured by the expiratory P-tach to obtain the measurement of expired volume from a test lung. We ventilated this single-lung model with a bias-flow ventilator while volume measurements, made by the computerized dual-P-tach differential flow system, were simultaneously compared to volume measurements from a single P-tach positioned between the dual P-tachs and the test lung. Three sizes of P-tachs were tested at 3 tidal volumes and 2 ventilator frequencies with their corresponding bias flowrate. Inspiratory and expiratory volume measurements of each system were assessed with measures of agreement (bias ± precision) and Pearson product-moment correlation coefficient. Evaluation Results The accuracy (bias) and repeatability (precision or limits of agreement) of the dual-P-tach system measurements as compared to the single P-tach measurements were clinically acceptable under the conditions tested with both dry and humidified air (±3% of tidal volume). The degree of correlation was high for both the inspiratory volumes (r=0.999, p < 0.001) and expiratory volumes (r=0.998, p < 0.001). Conclusions The dual-P-tach differential flow system is as accurate and precise as a single P-tach during bias-flow ventilation. This dual-P-tach differential flow system requires no isolation valves and may be used with patients of all sizes who are ventilated with a bias-flow ventilator.