Tag Archives: DL/VA

Using DL/VA (no, no, no, it’s really KCO!) to assess PFT results

Posted on by
10

Last week I was discussing the use of DL/VA to differentiate between the different causes of gas exchange defects with a physician. DL/VA is DLCO divided by the alveolar volume (VA). It is an often misunderstood value and the most frequent misconception is that it is a way to determine the amount of diffusing capacity per unit of lung volume (and therefore a way to “adjust” DLCO for lung volume). This is not the case because dividing DLCO by VA actually cancels VA out of the DLCO calculation and for this reason it is actually an index of the rate at which carbon monoxide disappears during breath-holding.

[Note: The value calculated from DLCO/VA is related to Krogh’s constant, K, and for this reason DL/VA is also known as KCO. The term DL/VA is misleading since the presence of ‘VA’ implies that DL/VA is related to a lung volume when in fact there is no volume involved. The use of the term DL/VA is probably a major contributor to the confusion surrounding this subject and for this reason it really should be banned and KCO substituted instead.]

I’ve written on this subject previously but based on several conversations I’ve had since then I don’t think the basic concepts are as clear as they should be.

DLCO_Model

When you know the volume of the lung that you’re measuring, then knowing the breath-holding time and the inspired and expired carbon monoxide concentrations allows you to calculate DLCO in ml/min/mmHg.  When you remove the volume of the lung from the equation however (which is what happens when you divide DLCO by VA), all you can measure is how quickly carbon monoxide decreases during breath-holding (KCO). 
Continue reading →

DLCO, by the numbers

Posted on by
Reply

I was recently contacted by a physician looking for an illustration or diagram to help make gas exchange and DLCO more understandable. We’ve all seen the diagram of the alveoli with a capillary stretched around it and with oxygen and carbon dioxide exchanging across the membrane. I think I first saw it in Comroe’s ‘The Lung” published in the 1960’s, but it may well be older than that. It’s hard to improve on this and dozens of versions have been made of it.

Alveoli

He said something that got me thinking “for instance in a preoperative setting … all we know is a number on a seedy print out and all we use is a DLco % to tell us what to do!”. When I review reports I can access all of the raw data from all of a patient’s efforts so there’s a lot I can see about test quality that doesn’t show up on the final report. So what is a reduced DLCO test trying to tell you when all you have are numbers to look at?

Continue reading →

DL/VA is really K in disguise

Posted on by
2

One misconception about DLCO results that I’ve heard over and over is that DLCO/VA is DLCO “adjusted” for lung volume and that using it is a way of “normalizing” DLCO for lung volume. This is not true and the information that DL/VA can give you about a patient’s lung without also considering VA as well is very limited and can be misleading.

WARNING, MATH AHEAD!

The equation for calculating DLCO is: 

Continue reading →