The remote collection and storage of exhaled gas is complicated by production of NO by exhaled bacteria: A solution to this problem
Kharitonov S., Barnes PJ.
Exhaled nitric oxide (NO) is recognised as a marker of inflammation in pulmonary disease. Other gaseous products are now being investigated e.g. carbon monoxide (CO), ethane and pentane. There are limitations in the collection of the samples. Analysis and sample collection are simultaneous, so the subject has to go to the analyser. As this may not be possible we have investigated remote sample collection using polyethylene bags and a specially designed collection device. We have validated this method for NO and CO for collection 24 hours before analysis. This method has been described previously (Paredi et al, Thorax 1998 in press). Briefly, subjects exhale into the device at a constant flow to prevent contamination by upper airway gas. The effect of dead space on the sample was examined: one subject undertook successive exhalations and compared direct measurement with expirations from which a portion of breath was discarded. To investigate the effects of storage for 24 hours, successive breaths were collected and stored in bags containing Milton, 100% alcohol, with and without a bacterial filter, refrigerated and frozen. Results:1)Dead space: direct measurement mean NO 8.02ppb (95% CI 7.5-8.6), simple bag collection mean NO 9.4ppb (7.4-11.2), bag collection with 10 second discard NO 7.8ppb (7.4-8.2) and with 20 second discard NO 8.1ppb (7.1-9.1).2). Storage: both alcohol and Milton (antiseptic agents) damaged the analyser. Gas samples were measured immediately at collection and at 24 hours (table). In stored breath samples there is a dramatic rise in NO due to bacterial NO production. CO is unaffected by storage method or collection technique. Time 0 Time 24 hrs Sample Mean NO 95% CI Mean NO 95% CI p value Untreated 7.9 7.4-8.4 21.6 183-25.0 0.063 Frozen 7.7 7.1-8.3 10.3 9.1-11.5 0.0625 Filtered 7.6 7.1-8.2 10.4 8.7-12.1 0.0625 Froz. + Filt. 7.6 7.1-8.1 8.4 7.5-9.1 0.125 Conclusions: Gas samples can be collected at a location distant from the analyser using polyethylene bags and our device. Samples should have a ten second breath discard to minimise the effect of airway dead space. Samples that are to be analysed later should be stored at -20°C and collected using a bacterial filter to prevent contamination and bacterial release of NO.