Measurement of exhaled nitric oxide concentration using nasal continuous negative pressure

Contamination of nasal nitric oxide (NO) is a major obstacle when one needs to sample exhaled NO originating only from the lungs. To eliminate nasal NO, we used the nasal continuous negative pressure (nasal CNP) technique which, we verified, caused closure of the vellum. Exhaled gas was sampled from six healthy volunteers into fraction 1 (initially exhaled 200 mL) and fraction 2 (remainder of the gas) under three conditions; while subjects were wearing a noseclip, using nasal CNP at -5, -10 and -20 cm H2O, and under endotracheal intubation. Exhaled NO concentration ([NO]) obtained with nasal CNP was significantly lower, regardless of the pressure applied, than that measured with a noseclip, and was similar to and closely correlated to that obtained under intubation (F1, r = 0.90; F2, r = 0.88; P < 0.05). Real-time recorded [NO] obtained with nasal CNP of -5 cm H2O was again lower than that measured with a noseclip at any expiratory flow rate examined, indicating nasal NO contamination was eliminated irrespective of the flow rate. In conclusion, because a nasal CNP of -5 cm H2O was easily tolerated without any discomfort, this technique is a simple, easy and effective technique to eliminate nasal NO which should be widely applicable for the measurement of exhaled [NO].