Nitric Oxide Diffusing Capacity on Exercise

Abstract

The excellent article by Zavorsky et al (March 2004)1Zavorsky GS Quiron KB Massarelli PS et al.The relationship between single-breath diffusion capacity of the lung for nitric oxide and carbon monoxide during exercise intensifies.Chest. 2004; 125: 1019-1027Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar expands our knowledge of lung diffusing capacity on exercise, but it is not quite true to state that no study has compared the diffusion capacity of the lung for nitric oxide (Dlno)/diffusion capacity of the lung for carbon monoxide (Dlco) ratio using a single-breath technique during various exercise intensities. In our original study,2Borland CDR Higenbottam TW A simultaneous single-breath measurement of pulmonary diffusing capacity with nitric oxide and carbon monoxide.Eur Respir J. 1989; 2: 56-63PubMed Google Scholar we measured single-breath Dlno and Dlco in three male patients at rest and at 75 W, 125 W, and 175 W of bicycle ergometer-induced exercise (the 50 W in the methods of that article is a typographic error). The results can be deduced from Figure 2 in our study, but in contrast to Zavorsky et al,1Zavorsky GS Quiron KB Massarelli PS et al.The relationship between single-breath diffusion capacity of the lung for nitric oxide and carbon monoxide during exercise intensifies.Chest. 2004; 125: 1019-1027Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar we found that single-breath Dlno/Dlco declined from 4.4 (rest), 3.8 (75 W), 3.9 (125 W), to 3.7 (175 W). In agreement with their study,1Zavorsky GS Quiron KB Massarelli PS et al.The relationship between single-breath diffusion capacity of the lung for nitric oxide and carbon monoxide during exercise intensifies.Chest. 2004; 125: 1019-1027Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar however, we observed a linear rise in Dlno with workload, and I have today performed a linear regression of Dlno on alveolar volume (Va) and workload for our data, which yields the model Dlno = 12.3 × Va + 0.27 × workload + 66.2, which is agreeably close to their result. Why did we find a decline in Dlno/Dlco with increasing exercise? A decline in Dlno/Dlco has been observed in mild hypoxia,3Borland CDR Cox Y Effect of varying alveolar oxygen partial pressure on diffusing capacity for nitric oxide and carbon monoxide, membrane diffusing capacity and lung capillary blood volume.Clin Sci (Colch). 1991; 81: 759-765Crossref Scopus (49) Google Scholar which increases the specific blood transfer conductance for carbon monoxide but not the specific blood transfer conductance for nitric oxide. In our study,2Borland CDR Higenbottam TW A simultaneous single-breath measurement of pulmonary diffusing capacity with nitric oxide and carbon monoxide.Eur Respir J. 1989; 2: 56-63PubMed Google Scholar subjects held their breath for 7.5 s rather than 5 s, as done by Zavorsky et al,1Zavorsky GS Quiron KB Massarelli PS et al.The relationship between single-breath diffusion capacity of the lung for nitric oxide and carbon monoxide during exercise intensifies.Chest. 2004; 125: 1019-1027Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar whose inspired gas mixture contained 21% oxygen prior to addition of nitric oxide, whereas ours contained 17%. Our subjects could therefore have been more hypoxic. The other cause of reduced Dlno/Dlco is a reduction in membrane diffusion capacity (Dm)/pulmonary capillary blood volume (Vc). This has been demonstrated for measurements at reduced lung volume that will affect Dm and hence Dlno to a greater extent than Vc and hence Dlco.2Borland CDR Higenbottam TW A simultaneous single-breath measurement of pulmonary diffusing capacity with nitric oxide and carbon monoxide.Eur Respir J. 1989; 2: 56-63PubMed Google Scholar The mechanism of increased lung diffusion on exercise is believed to be recruitment of closed pulmonary capillaries and further dilatation of those already open. Recruitment will merely increase the number of available alveoli of uniform Dm/Vc without affecting the overall ratio. Dilatation, however, will increase capillary volume in proportion to the cube of capillary diameter, but surface area and hence Dm only by the square. It is conceivable that by virtue of subject selection or training (in our case lack of!) that dilatation rather than recruitment effected our observed increase in Dlno and decline in Dlno/Dlco.