EFFECT OF BREATHING CIRCUIT RESISTANCE ON THE MEASUREMENT OF VENTILATORY FUNCTION

Background-The American Thoracic Society (ATS) has set the acceptable resistance far spirometers at less than 1.5 cm H2O/1/s over the flow r ange 0-14 1/s and for monitoring devices at less than 2.5 cm H2O/1/s ( 0-14 1/s). The aims of this study were to determine the resistance cha racteristics of commonly used spirometers and monitoring devices and t he effect of resistance on ventilatory function. Methods-The resistanc e of five spirometers (Vitalograph wedge bellows, Morgan rolling seal, Stead Wells water sealed, Fleisch pneumotachograph, Lilly pneumotacho graph) and three monitoring devices (Spiro 1, Ferraris, mini-Wright) w as measured from the back pressure developed over a range of known flo ws (1.6-13.1 1/s). Peak expiratory flow. (PEF), forced expiratory flow in one second (FEV1), forced vital capacity (FVC), and mid forced exp iratory flow (FEF25-75%) were measured on six subjects with normal lun g function and 13 subjects with respiratory disorders using a pneumota chograph. Ventilatory function was then repeated with four different s ized resistors (approximately 1-11 cmH(2)O/1/s) inserted between the m outhpiece and pneumotachograph. Results-All five diagnostic spirometer s and two of the three monitoring devices passed the ATS upper limit f or resistance. PEF, FEV1 and FVC showed significant (p < 0.05) inverse correlations with added resistance with no significant difference bet ween the normal and patient groups. At a resistance of 1.5 cm H2O/1/s the mean percentage falls (95% confidence interval) were: PEF 6.9% (5. 4 to 8.3); FEV1 1.9% (1.0 to 2.8), and FVC 1.5% (0.8 to 2.3). Conclusi ons-The ATS resistance specification for diagnostic spirometers appear s to be appropriate. However, the specification for monitoring devices may be too conservative. PEF was found to be the most sensitive index to added resistance.