Why isn’t PAO2 measured during DLCO tests?

Most pulmonary technicians and physicians are aware that a patient’s hemoglobin and carboxyhemoglobin levels are factors that affect DLCO test results. What may be less well appreciated is the degree to which DLCO varies with the alveolar oxygen concentration.

Oxygen and carbon monoxide both compete for uptake by hemoglobin so when alveolar O2 is low or high the measured DLCO will also vary accordingly. The relationship between DLCO and PAO2 has been studied both in a regards to FIO2 and to barometric pressure. Both empirically and mathematically DLCO is estimated to change by approximately 0.35% per mmHg PAO2 change.

Although PAO2 in exhaled DLCO samples has not been studied to any real degree a table of baseline patient information in the 1986 study by Kanner and Crapo showed a range of 109.8 mm Hg to 121.6 mm Hg in a small group of healthy subjects although testing was performed with an FiO2 of 25% at an altitude of 6000 feet.

I have been unable to find anyone who has studied the effect of the length of time spent exhaling to RV may have on PAO2. The ATS/ERS statement on DLCO testing recommends that exhalation to RV should take less than 6 seconds but in practice this depends on the patient and will probably range from 3 to 15 seconds more or less routinely. Simple math using the alveolar air equation and normal resting values for oxygen consumption produces an estimated change in PAO2 of approximately 10 mm Hg during a 6-second breath-hold at RV.

Because PFT Labs in the USA routinely use an inspired oxygen concentration of 21% and European PFT Lab routinely use a concentration of 17%-18% the results from same patient will vary according to where they are measured. DLCO measured at these two FIO2 leads to an approximately 5% difference in measured values so labs need to be careful when comparing results and when selecting the appropriate predicted equations.

The ATS/ERS statement on DLCO testing acknowledges the effect alveolar oxygen has on DLCO results by recommending that patients be removed from supplemental oxygen ten minutes before testing (and is there anybody that has actually followed this guideline for our oxygen-dependent patients?) and by adjusting predicted values for altitude. The ATS/ERS statement also gives a correction factor if PAO2 is measured but PAO2 is not measured by any commercially-available test system at present.

Since PAO2 has not been studied extensively it is difficult to gauge what level of error it contributes to the DLCO measurement. Back-of-the-envelope calculations suggests that a range of at least 20 mmHg in PAO2 is probably normal which means that PAO2 contributes at an error level of at least 7% to the measurement of DLCO and that is likely a conservative estimate.

Overall DLCO accuracy would likely be improved and both individual and laboratory variability would likely be decreased if PAO2 was routinely measured in the exhaled DLCO alveolar sample. PAO2 isn’t a routine measurement partly because its effect on DLCO has been overlooked and partly because of the added cost and complexity it would bring to testing systems. The latter is no longer the case, however. Any testing system that measures lung volumes by nitrogen washout has a rapid-response, high precision oxygen analyzer already connected to the breathing manifold that could also be used to measure PAO2. Adding PAO2 measurements in these test systems is now only a matter for software and there is no obvious reason that any of these systems couldn’t be upgraded to measure PAO2 with just a software revision.

So why haven’t manufacturers already offered this option? Conservatism is part of it, but also probably because enough customers haven’t asked for it. So, take a step for higher DLCO accuracy and if you already have a test system that measures lung volumes by nitrogen washout, ask for a software upgrade. If you are instead looking for a new test system make it a point to ask if it can measure PAO2.

References:

Frey TM, Crapo RM, Jensen RL, Kanner RE, Kass JE, Castriotta RJ, Mohsenifar Z. Adjustment of DLCO for varying COHb and alveolar PO2 using a Theoretical adjustment equation. Respir Physiol 1990; 81: 303-312

Kanner RE, Crapo RO. The relationship between alveolar oxygen tension and the single-breath carbon monoxide diffusing capacity. Am Rev Respir Dis 1986; 133: 676–678.

MacIntyre N, et al. Standardisation of the single-breath determination of carbon monoxide uptake in the lung. Eur Respir J 2005; 26: 720–735

Normand N, Marie C, Mouadil A. Single breath transfer factor in young health adults: 21% or 17.5% inspired oxygen? Eur Respir J 2004; 23: 927-931

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