An evaluation of the multiple-breath nitrogen washout as a pulmonary function test in horses.

Multiple-breath nitrogen washouts (MBNW) were performed with 29 light horses. Seven normal horses were used to examine the reproducibility, and 22, ranging from normal to severely diseased, were used to examine the changes in chronic obstructive pulmonary disease (COPD) and the effect of a bronchodilator, salbutamol, on the distribution of ventilation. The MBNW were analyzed using the functional residual capacity (FRC), end-tidal N2 concentration of the final breath of the MBNW (FETN2,fb), end-tidal N2 concentration when the cumulative expired volume was equal to body weight (FETN2,bw), lung clearance index (LCI), Becklake index (BI), mixing ratio (MR), index of distribution of inspiration (IDI), pulmonary N2 clearance delay (PCD) and ventilatory efficiency (EFF). The LCI, MR, IDI, PCD and EFF were calculated at end-tidal N2 concentrations (FETN2), equal to 3%, 2% and FETN2,fb. The EFF was also calculated at FETN2 = 8%, as was BI. The within day coefficient of variation for most indices of the MBNW was 10-15%. None of the indices varied significantly from day-to-day. The FETN2,fb, FETN2,bw, MR, IDI, PCD, and EFFfb, EFF3% and EFF2% differed between the horses with COPD and the normal horses, and all of the indices, except FRC, were correlated with a histopathological score of the small airways. The FETN2,bw appeared to be the most sensitive index in detecting the horses with COPD. In the horses with COPD, salbutamol caused a decrease in FRC and FETN2,fb indicating increased ventilation of the lung. However, LCI increased suggesting a less efficient distribution of ventilation.