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

Multiple-breath nitrogen washouts (MBNW) were performed with 21 adult dairy cows (seven normal cows tested on two days each, four clinically normal cows tested on six days, and ten cows with respiratory disease tested on one day each). The functional residual capacity (FRC), lung clearance index (LCI), Becklake index (BI), mixing ratio (MR), pulmonary N2 clearance delay (PCD), index of distribution of inspiration (IDI) and ventilatory efficiency (EFF) were calculated for each MBNW. The LCI, MR, PCD, IDI and EFF were calculated at an end-tidal N2 concentration of 3% (FETN2,3%) and the end-tidal N2 concentration of the final breath of the MBNW (FETN2,fb), while BI was calculated and EFF was also calculated at FETN2 = 8% (i.e. after washout of 90% of the N2 in the lung). All of the MBNW were irregular due to breath-to-breath variability in the breathing pattern of cows and variability in FRC. The within day coefficients of variation for FRC, LCI, MR, IDI, EFF3% and EFFfb were less than 10%, but in the four cows tested on six days there was significant day-to-day variability for individuals for FRC, LCI, BI, MR, PCD and IDI. The shape of the MBNW curves differed between the normal cows and those with respiratory disease. The FRC, EFF3% and EFFfb were the most sensitive indices in detecting the cows with respiratory disease, and when EFF3%, EFFfb or FETN2fb, was used in conjunction with FRC it was possible to detect all of the cows with respiratory disease.