Comparison of the performance of linear resistance and ultrasonic pneumotachometers at rest and during lobeline-induced hyperpnoea

The performance of a Fleisch No. 5 pneumotachometer (F), and two commercial ultrasonic pneumotachometers, the BRDL (B) and the Spiroson (s) systems we re compared in respect to their use for determination of ventilatory parame ters at rest and during lobeline-induced hyperpnoea. Five clinically health y Thoroughbred horses were tested with the three pneumotachometers in rando m order. Respiratory airflow, respired gas concentrations, oesophageal pres sures, pressures within the mask systems and arterial blood gases were dete rmined before and during lobeline-induced hyperpnoea. Because measured peak expiratory airflow rates exceeded the stated linear range of the Fleisch p neumotachometer (similar to +/- 25 1 s(-1)) differential pressure-flow curv es were determined in vitro over the range of flows recorded in vivo. Expir ed flows greater than the linear range were corrected according to the deri ved regression equation. No differences in any of the measured variables am ong the three systems were present at rest. At peak ventilation of lobeline -induced hyperpnoea mask pressures [Delta P-mask (mean (SEM)): F: 9.6 (2.8) cm H2O, B: 0.8 (0.4) cm H2O, s: 1.4 (0.8) cm H2O] and end tidal carbon dio xide [ET CO2 (mean (SEM)): F: 2.6 (0.1)%, B: 2.1 (0.2)%, S: 2.1 (0.1)%] wer e significantly higher in system F. Despite a tendency for respiratory freq uency and peak inspired and expired flows, to be lower with system F, no si gnificant differences in the measurements of ventilatory mechanics were det ected. In conclusion, the ultrasonic flowmeters pose significantly lower re sistive loads onto the respiratory system during ventilation above resting levels than Fleisch No 5 pneumotachometers. However, at the flowrates achie ved during lobeline-induced hyperpnoea an in vitro calibration of the diffe rential pressure-flow relationship allows correction for expiratory alinear ity in system F. In addition, the performance of the Spiroson flowmeter is accurate in determining ventilatory mechanics at rest and during lobeline-i nduced hyperpnoea. (C) 2000 Harcourt Publishers Ltd.