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DLCO Adjustment for Altitude


Diffusing Capacity of the Lung for CO - Carbon Monoxide

DLCO (Diffusing Capacity of the Lung for Carbon Monoxide) is an indication of the rate of transfer of CO in the air of the lung into the  pulmonary capillary blood.  In Europe it is commonly referred to as TLCO (Transfer factor of the Lung for Carbon Monoxide). This transfer process requires several steps including:
  1. Bulk flow delivery of CO to the airways and alveolar spaces;
  2. Mixing and diffusion of CO in the alveolar ducts, air sacs and alveoli
  3. Transfer of CO across the gaseous to liquid interface of the alveolar membrane
  4. Mixing and diffusion of CO in the lung parenchyma and alveolar capillary plasma
  5. Diffusion across the red cell membrane and within the interior of the red blood cell and
  6. Chemical reaction with constituents of blood Hemoglobin (Hb)
The most common methodology to measure carbon monoxide (CO) uptake is with the single breath technique, although there are several other techniques that can be used (e.g., steady-state, intra-breath and rebreathing techniques). Equipment needed to measure DLCO is varies widely in complexity, but the basic principles are the same. All systems have a source of test gas (bag-in-box, spirometer, compressed gas cylinder), a method for measuring inspired and expired volume over time (spirometers with kymographs, pneumotachometers near the mouthpiece or near a bag-in-box), and gas analysers (single-sample analysers or continuous high-speed analysers).

A number of pathological states can affect DLCO (e.g., anemia, pumonary emboli, emphysema, Interstitial lung disease, etc.). Measurement of DLCO is indicated when any of these pathological processes are suspected or need to be ruled out. Moreover, measuring changes in DLCO over time in these processes is a useful way of following the course of disease.4

See an online calculator for DLCO predicted normal values

Units of measurement

The European Respiratory Society (ERS) recommends expressing DLCO in the SI units (mmol/(min kPa), while the ATS prefers the traditional units mL (standard temperature, pressure and dry (STPD))/(min·mmHg). Care must be taken to ensure the same set of units is used throughout all calculations. Values in SI units should be multiplied by 2.987 to obtain values in traditional units.


Adjustments to the measurement of DLCO prior to interpretation

DLCO depends upon a number of physiological factors. Besides varying with age, sex, height and possibly race, DLCO also changes with hemoglobin, lung volume, carboxyhaemoglobin (COHb), PIO2 - inspired oxygen partial pressure (e.g. altitude), exercise and body position. Although these effects may cause changes in DLCO in opposite directions  all should be considered in interpreting the observed CO uptake. Moreover, specific adjustments for three of these factors (Hb, COHb and PIO2) should always be made to ensure appropriate interpretation. The formula for the fully corrected DLCO is the multiple of the correction factors for Hb, COHb, and and altitude.

See an online calculator to correct DLCO for hemoglobin.

See an online calculator to correct DLCO for carboxyhemoglobin (COHb).

Adjustment of DLCO due to altitude

Given a constant FI02 (fraction of inspired Oxygen), increases in altitude will result in a decreasing PI02, and an increasing DLCO of about 0.35% per mmHg decrease in alveolar sample PAO2. Adjustments to a standard PA02 of 120mmHG



DLCO altitude adjusted = DCLO measured x (1.0 + 0.0035(PAO2 - 120))

Assuming that the average PI02 is 150 mmHg at sea level an average adjustment for interpretive purposes can be made for altitude.


DLCO altitude adjusted = DCLO measured x (1.0 + 0.0031(PIO2 - 150))
 

This is based on the knowledge that DLCO decreases by 0.31% per mmHg decrease in PI02. Altitude effects on increasing Hb concentration will also have an  impact on the measured DLCO. If is not known if increasing altitude will have any other efects on DLCO.4

PIO2 can be estimated from barometric pressure in mmHg with the following formula:

PIO2 = (Barometric pressure - 47) x 0.21

The online calculator here will calculate the DLCO adjusted for altitude based on the inputs of DLCO measured and  barometric pressure via the formulas above.
These calculations are based on the method of Cotes1,2,3 as recommended by the American Thoracic Society.



CALCULATOR INPUT
Required inputs:
DLCO measured

units

Barometric pressure

pressure units








Reference

1.
American Thoracic society, Single Breath Carbon Monoxide Diffusing Capacity (transfer factor) Recomendations for a Standard Technique- 1995 Update, Am J Respir Crit Care Med. 152 pp 2185-2198 (1995).
2.
J.E. Cotes 1993, Lung Function, 5th Edition., Blackwell Scientific Publications, London
3.
J.E. Cotes, J.M. Dabbs, P.C. Elwood, A.M. Hall, A. McDonald, and M.J. Saunders.  Iron-deficiency anaemia: its effects on transfer factor for the lung (diffusing capacity) and ventilation and cardiac frequency during submaximal exercise. Clin. Sci. 42:325-33 (1972).
4.
Macintyre N, Crapo RO, Viegi G et al. (October 2005). "Standardisation of the single-breath determination of carbon monoxide uptake in the lung". Eur. Respir. J. 26 (4): 720–35.



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