Effect of breathing circuit resistance on the measurement of ventilatory function

• Published in: Thorax Vol. 53; no. 11; pp. 944 - 948
• Main Authors: Johns, David P, Ingram, Corrie M, Khov, Souvanny, Rochford, Peter D, Walters, E Haydn
• Format: Journal Article
• Language: English
• Published: London BMJ Publishing Group Ltd and British Thoracic Society 01-11-1998; BMJ; BMJ Publishing Group LTD; BMJ Group
• Subjects: Adult; Aged; Aged, 80 and over; Airway Resistance; back pressure; Biological and medical sciences; Chronic obstructive pulmonary disease; flow resistance; Forced Expiratory Flow Rates; Humans; instrument resistance; Investigative techniques of respiratory function; Investigative techniques, diagnostic techniques (general aspects); Laboratories; Medical sciences; Middle Aged; Original; Peak Expiratory Flow Rate; Respiratory Function Tests - instrumentation; Respiratory Function Tests - methods; Spirometry - instrumentation; Transplants & implants; Values; Vital Capacity; United Kingdom > UK; Lung volume; Human; Resistance; Spirometry; Relation; Forced vital capacity; Maximum expiratory flow rate; Exploration; Circuit; Spirometer; Maximal expiratory volume per second; Respiratory system
• ISSN: 0040-6376; 1468-3296
• DOI: 10.1136/thx.53.11.944

BACKGROUND The American Thoracic Society (ATS) has set the acceptable resistance for spirometers at less than 1.5 cm H2O/l/s over the flow range 0–14 l/s and for monitoring devices at less than 2.5 cm H2O/l/s (0–14 l/s). The aims of this study were to determine the resistance characteristics of commonly used spirometers and monitoring devices and the effect of resistance on ventilatory function. METHODS The resistance of five spirometers (Vitalograph wedge bellows, Morgan rolling seal, Stead Wells water sealed, Fleisch pneumotachograph, Lilly pneumotachograph) and three monitoring devices (Spiro 1, Ferraris, mini-Wright) was measured from the back pressure developed over a range of known flows (1.6–13.1 l/s). Peak expiratory flow (PEF), forced expiratory flow in one second (FEV1), forced vital capacity (FVC), and mid forced expiratory flow (FEF25–75%) were measured on six subjects with normal lung function and 13 subjects with respiratory disorders using a pneumotachograph. Ventilatory function was then repeated with four different sized resistors (approximately 1–11 cmH2O/l/s) inserted between the mouthpiece and pneumotachograph. RESULTS All five diagnostic spirometers and two of the three monitoring devices passed the ATS upper limit for resistance. PEF, FEV1 and FVC showed significant (p<0.05) inverse correlations with added resistance with no significant difference between the normal and patient groups. At a resistance of 1.5 cm H2O/l/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). CONCLUSIONS The ATS resistance specification for diagnostic spirometers appears 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.