DLCO VA abnormality

The interpretation of a low DLCO can be confusing when the alveolar volume of gas (VA) is reduced as in this patient. The major determinants of CO gas transfer are gas exchange surface area, alveolar–capillary membrane function, and capillary blood (hemoglobin) volume. VA is also important because it represents the volume of CO available for transfer during a breath hold. VA can be reduced either by extrapulmonary factors (eg, a poor inspiration from weakness, chest wall abnormalities, or poor effort) or by intrinsic pulmonary disease.

These effects can be sorted out by assessing the DLCO/VA relationship (sometimes referred to as Kco). Importantly, the observed DLCO/VA is often referenced to a predicted DLCO/VA that would occur with a normal VA. In the setting of a low VA, however, its use as a “normal” value loses meaning and must be viewed differently. When VA is reduced only because of a poor inspiration, the losses of surface area and blood volume are small despite the loss of gas volume (the alveolar–capillary interface tends to fold like an accordion). As a consequence, DLCO falls less than VA, and the DLCO/VA ratio rises substantially, often above the upper limit of normal for the predicted DLCO/VA. In contrast, with intrinsic pulmonary disease, VA loss is usually associated with both surface area loss and derangements of the alveolar–capillary interface. Under these circumstances, both DLCO and VA decrease simultaneously (a DLCO/VA ratio near what would be predicted with a normal VA), or else the DLCO decreases more than the VA (a low DLCO/VA ratio), suggesting more severe alveolar–capillary abnormalities than volume loss.

In this patient, the inspired volume is adequate (92% of the known vital capacity) and both DLCO and VA are reduced proportional to each other, yielding a DLCO/VA ratio near what would be predicted at a normal VA (ie, 98%). As noted, this suggests that intrinsic pulmonary disease is present (choice D is correct; choice B incorrect). However, because this DLCO/VA is within the “normal” range for DLCO/VA, a common misinterpretation is to instead label this as a DLCO that is normal when corrected for lung volume. Again, as noted, this is not physiologically accurate, is misleading, and should not be done (choice A is incorrect). Interpreting DLCO also requires consideration of hemoglobin concentration. A low hemoglobin can significantly reduce DLCO, and hemoglobin correction formulas should be used on the predicted value, as was done in this patient (choice C is incorrect).1

Footnotes

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