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The TLNO/TLCO ratio in pulmonary function test interpretation

2013; European Respiratory Society; Volume: 43; Issue: 1 Linguagem: Inglês

10.1183/09031936.00041313

1399-3003

Judith M. Hughes, I. van der Lee,

Ultrasound in Clinical Applications

We are grateful to C. Borland for his comments on our review of the TLNO/TLCO ratio (the ratio of the transfer factor of the lung for nitric oxide versus carbon monoxide) [1] and its clinical applications.Undoubtedly, TLNO, on its own, has a role to play in accessing alveolar function in terms of the membrane diffusing capacity, DM.The calculation of pulmonary capillary volume (Vc), on the other hand, requires a value for HblCO, the specific conductance of blood for carbon monoxide, and there is some doubt about its correct value.The TLNO/TLCO ratio does not require knowledge of HblCO.An important difference between TLNO and TLCO is that TLNO is not affected by changes in alveolar oxygen tension (PAO 2 ).Thus, as Borland points out, the TLNO/TLCO ratio will fall if mean PAO 2 falls because of its dependence on the DMCO/HblCO?Vc ratio and the increase in the value of HblCO.But, with the standard single breath technique, with a maximal inspiration and subsequent breath hold, there is not likely to be much variation in PAO 2 in those units ''accessible'' to the inspired test gas, whose oxygen fraction is between 0.18 and 0.21 depending on the test gas composition.In routine lung function no corrections have been proposed for PAO 2 inhomogeneity in the single breath TLCO test.C. Borland argues that units with a high alveolar ventilation to perfusion (V9A/Q9) ratio have a high DM/Vc since pulmonary capillary blood flow (and presumably capillary volume) is low in relation to alveolar surface area and volume, and this will increase the TLNO/TLCO ratio.Because of its dependence on DMCO/ HblCO?Vc, the TLNO/TLCO ratio will be further increased in high V9A/Q9 units by the rise in PAO 2 and fall in HblCO; thus, a rise in TLNO/TLCO could be a marker for pathological processes involving the pulmonary circulation, including emphysema.We thank C. Borland for pointing this out.But, because of the nature of the single breath test, the rise in DM/Vc is likely to be more important than the fall in HblCO.The converse, that low V9A/Q9 (poorly ventilated but overperfused) regions will have a low DM/Vc ratio seems less likely, since V9A/Q9 and DM/Vc ratios are not directly related.V9A/Q9 depends on the ratio of flow rates, measured during tidal breathing; DM/Vc ratios, on the other hand, measured at full inflation during a breath hold, reflect, more or less, the ratio of alveolar membrane to capillary and red cell surface areas.As we point out in our review [1], this surface area ratio decreases as lung expansion declines (equivalent to a fall in the ratio of transfer coefficients for NO versus CO) and is the reason for the fall in TLNO/TLCO in extrapulmonary restriction.Finally, in cases of severe anaemia with very low haemoglobin levels, we take C. Borland's point that TLNO may be reduced.