This lecture covers diffusing capacity testing, specifically the single-breath carbon monoxide diffusing capacity (DLCO) test. DLCO measures the transfer of carbon monoxide across the alveolar-capillary membrane and is used to evaluate gas exchange ability. The single-breath method involves rapid inhalation of a test gas mixture containing carbon monoxide to total lung capacity, a 10 second breath hold, and analysis of exhaled gases. DLCO may be reduced in conditions involving decreased alveolar surface area or pulmonary capillary blood volume such as emphysema. Physiologic factors like hemoglobin, carboxyhemoglobin, and pulmonary blood volume also impact DLCO values.

1. RTS 474: Advanced Pulmonary
Function Testing
Lecture Four
Chapter 3: Diffusing Capacity Tests

2. LecturePlan
 This lecture covers part of the following course student’s
learning outcomes:
 1.1, 1.2, 1.3 & 2.1 (see your course syllabus)
 Lecture Outlines:
 Introduction
 Carbon Monoxide Diffusing Capacity
 Description
 Techniques
 Significance and pathophysiology
 Lecture/Chapter Objectives:
 Identify the steps for performing the single-breath DLCO.
 List at least two criteria for an acceptable single-breath
DLCO test.
 Describe why DLCO is often reduced in emphysema.
 Describe at least two non-pulmonary causes for reduced
DLCO.
 Explain the significance of a reduced Dl/VA.
 Compare diffusion limitation caused by membranes and
pulmonary capillary blood volume.
 Explain the significance of the TLC-VA relationship.

3. Introduction
What is the purpose of the Diffusing
Capacity Testing?
Diffusing capacity is performed to
evaluate the alveolar capillary interface
where gas exchange occurs.

4. Introduction
cont.,
Diffusing capacity (also referred to as transfer
factor) is usually measured using small
concentrations of carbon monoxide (CO) and is
referred to as DLCO or DCO.
DLCO is used to assess the gas-exchange ability of
the lungs, specifically oxygenation of mixed venous
blood.
Various methods, all of which use CO, have been
described. The most commonly used method is the
single-breath, or breath-hold technique.
The single-breath method is also the most widely
standardized.

5. Introduction
cont.,

6. CARBON
MONOXIDE
DIFFUSING
CAPACITY
DESCRIPTION
 DLCO measures the transfer of a diffusion-limited gas
(CO) across the alveolocapillary membranes.
 DLCO is reported in milliliters of CO/minute/millimeter
of mercury at 0°C, 760 mm Hg, dry (i.e.,[STPD]).

7. DLCO
TECHNIQUES
 In the presence of normal amounts of Hb and normal
ventilatory function, the primary limiting factor to
diffusion of CO is the status of the alveolocapillary
membranes.
 This process of conductance across the membranes can
be divided into two components:
 Membrane conductance (Dm)
 Dm reflects the process of diffusion across the
alveolocapillary membrane.
 The chemical reaction between CO and Hb
 Uptake of CO by Hb depends on the reaction rate (θ) and the
pulmonary capillary blood volume (Vc).
 DLCO will be affected by
 Changes in membrane component
 Alterations in Hb
 Capillary blood volumes

8. DLCOTECHNIQUES cont.,
 A small amount of CO in inspired gas produces measurable changes in the concentration
of inspired versus expired. Why?
 If the partial pressure of CO in the alveoli and the rate of uptake of the gas can be
measured, the DLCO of the lung can be determined.
 There are several methods for determining DLCO and all are based on the following
equation:
 Methods are:
 Single Breath-Hold Technique (Modified Krogh's Technique)
 Rebreathing Technique
 Slow Exhalation Single-Breath Intrabreath Method
 Membrane Diffusion Coefficient and Capillary Blood Volume

9. DLCOTECHNIQUES cont.,

10. TestGas for
DLCO
 One of three tracer gases is used with varies
concentration depending on gas used:
 Helium (He)
 Neon (Ne)
 Methane (CH4)
 CO concentration is 0.3%
 Balance air (O2 and N2)

11. Single Breath-Hold Technique
(Modified Krogh's Technique)

12. DLCOSb
 Procedure
 Unforced exhalation to RV limited to 6 sec
 Rapid inhalation of a diffusion gas mixture to TLC from
spirometer/demand valve/reservoir
 0.3% CO
 10% He (tracer gas)
 21% O2
 Balance Nitrogen
 Breath hold at TLC for 10 +/- 2 sec
 Rapid exhalation should not exceed 4 sec
 Alveolar gas is collected after a washout volume (0.75-1.0 L)
has been discarded
 If VC is <2.0 L, washout volume may be reduced to 0.50L
 Sample gas volume should be 0.50 to I L
 If VC <1.0L, a sample of <0.50L can be analyzed if dead-space
volume has been cleared
 Average value = 25 mL CO/min/mmHg (STPD)

13. DLCOSb

14. DLCOSb
CRITERIAFOR
ACCEPTABILITY
1. System has passed calibration and quality control procedures.
2. Inspiration from RV to TLC should be rapid and should occur within
4 seconds or less.
3. The volume inspired (VI or IVC) should be at least 85% of the best
previously recorded VC.
4. Breath-hold time should be between 8 and 12 seconds, with no
evidence of leaks, or Valsalva or Müller maneuver.
5. Exhalation should be rapid with total exhalation lasting 4 seconds or
less with appropriate clearance of VD and proper sampling/analysis of
alveolar gas
6. An interval of at least 4 minutes should elapse between repeated
tests. No more than five single-breath maneuvers should be
performed.
7. The VA is consistent with the clinical presentation. In a normal
subject, the TLC–VA relationship should be very close, if not the
same, and TLC performed by any method in any disease state should
be larger than VA.
8. The average of two or more acceptable tests should be reported.
Duplicate determinations should be within 3 mL/min/mm Hg of each
other or within 10% of the largest observed value.

15. Rebreathing
Technique
 The patient rebreathes from a reservoir containing a
mixture of 0.3% CO, tracer gas, and air (or an O2 mixture)
for 30–60 seconds at a rate of approximately 30
breaths/min.
 The final CO, tracer, and O2 concentrations in the
reservoir are measured after this interval. An equation
similar to that used for the single-breath technique is
used:
 The rebreathing method can also be implemented using a
rapidly responding analyzer (for CO and tracer gas) and
plotting the slope of the change in CO in relation to the
slope of the tracer gas to estimate the rate of CO uptake.
 For clinical & research application; Provides most
accurate DLCO. The rebreathing method can be used
during exercise.

16. SlowExhalation
Single Breath
Intrabreath
Method
 Patient Inspires a VC gas containing 0.3% CO, CH4
Methane and 21 % O2 the balance Nitrogen.
 Patient exhales slowly at approximately 0.5 l/sec from
TLC to RV.
 A rapidly responding infrared analyzer monitors CO
and CH4 gas concentrations
 Can be used during exercise

17. DLCO
INTERPRETIVE
STRATEGIES

18. Significanceand
Pathophysiology

19. Significanceand
Pathophysiology
 Decreases with
 Restrictive Lung diseases
 Asbestosis
 Silicosis
 Idiopathic pulmonary fibrosis
 Sarcoidosis
 Systemic lupus erythematosus
 Inhalation of toxic gases (alveolitis)
 Loss of lung tissue
 Space occupying lesions (tumors)
 Pulmonary edema
 Lung resection
 Radiation therapy (fibrotic changes)
 Chemotherapy

20. Significanceand
Pathophysiology
 Decreases in
 Emphysema
 Chronic Bronchitis , Asthma (may or may not be
decreased)
 DLco sometimes used to differentiate between
emphysema and chronic bronchitis
 In patients with COPD, DLco less than 50% of predicted
indicate O2 desaturation during exercise
 Low resting DLco (<50% - 60% of predicted) may indicate
the need for assessment of oxygenation during
exercise

21. DL/VA
 DLco is directly related to lung volume in healthy
individuals
 DL/VA is approximately 4-5 ml CO
transferred/minute/liter of lung volume
 DL/VA is useful in differentiating between restrictive
and obstructive disease
 Obstruction = Low DL/VA ratio
 Restriction = DL/VA Ratio is preserved

22. Physiologic
Factors
 Numerous physiologic factors can influence the
observed DLCO
 Hemoglobin and hematocrit (Hct):
 Decreased Hb or Hct reduces DLCO.
 Increased Hb and Hct elevate DLCO.
 DLCO may be corrected if the patient's Hb is known.
 CO uptake varies approximately 7% for each gram of Hb.
 The predicted DLCO may be corrected so that the value
reported is compared to a standardized Hb level of 14.6 g% for
men and 13.4 g% for women and children younger than 15
years.

23. Physiologic
Factors
 Carboxyhemoglobin (COHb)
 Increased COHb levels, often found in smokers, reduce
DLCO.
 Smokers may have COHb levels of 10% or greater,
causing significant CO back-pressure. (usually less than
2% COHb in non-smokers)
 Each 1% increase in COHb causes an approximate 1%
decrease in the measured DLCO CO back-pressure.
 Corrections for COHb should be applied

24. Physiologic
Factors
 Alveolar Pco2. Increased Pco2 elevates DLCO because the
alveolar Po2 is necessarily decreased. Significant
increases in alveolar Pco2 reduce the alveolar Po2.
 Body position. The supine position increases DLCO.
Changes in body position affect the distribution of
capillary blood flow.
 Altitude above sea level. DLCO varies inversely with
changes in alveolar oxygen pressure (PAO2). At altitudes
significantly greater than sea level, DLCO increases
unless corrections are made.
 Poor inspiratory effort during testing, if less than 85% of
VC will decrease DLCO.
 Asthma and obesity. Asthma and obesity have been
associated with an elevated DLCO.
 Increased pulmonary capillary blood volume may explain
these observations

25. Physiologic
Factors
 Pulmonary capillary blood volume. Increased blood
volume in the lungs (VC) causes increased DLCO.
 Increases in pulmonary capillary blood volume may
result from increased cardiac output as occurs during
exercise. Patients should be seated and resting for
several minutes before DLCO testing is performed.
 Pulmonary hemorrhage and left-to-right shunts may also
cause an increase in blood volume in the lungs.
 Excessive negative intrathoracic pressure during breath
holding can increase pulmonary capillary volume and
elevate the DLCO.
 Excessive positive intrathoracic pressure (Valsalva
maneuver) can reduce pulmonary blood flow and
decrease DLCO.

26. Diffusing
Capacity