Diffusing Capacity
RET 2414
Pulmonary Function Testing
Module 4.0
Diffusing Capacity

Single Breath Method (DLcosb)
(Modified Krogh Technique)

DLco measures the transfer of a
carbon MONOXIDE (CO) across the
alveolocapillary membranes
Diffusing Capacity

DLcosb


CO combines with Hb 210 times
more readily than O2
DLco is expressed as:
ml of CO/minute/mm Hg (STPD)

STPD (0 C, 760 mm Hg, Dry)
Diffusing Capacity

DLcosb
Procedure

Unforced exhalation to RV
(limited to 6 seconds)

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
Diffusing Capacity

DLcosb
Procedure

Breath hold at TLC for 10
+/- 2 seconds

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)
Diffusing Capacity

DLcosb
Procedure

Sample gas volume should
be 0.50 – 1.0 L
(If VC <1.0L, a sample of <0.50L
can be analyzed if deadspace
volume has been cleared)

Sample is analyzed for the
fractional CO and He (tracer
gas) concentration
Change in He concentration reflects
dilution by gas in lungs at RV
This change is used to determine
the initial CO concentration
Diffusing Capacity

DLcosb
Diffusing Capacity

DLcosb

DLcosb is then calculated as
follows:
DLcosb=
VA x 60
x Ln FACO0
(PB – 47) x (T)
FACOT
MEMORIZE !
NOT !
Diffusing Capacity

DLcosb

Average DLcosb value
25 ml CO/min/mm Hg (STPD)
Diffusing Capacity

DLcosb

Pretest Patient Preparation

Should refrain from:


Smoking for at least 24 hours prior to
testing
Consuming alcohol 4 hours prior to test
(will reduce DLco)

Strenuous exercise before testing
(effects
pulmonary capillary blood volume)

Eating for at least two hours prior to
testing (digestion effects pulmonary capillary blood
volume)
Diffusing Capacity

DLcosb

Pretest Patient Preparation


Should sit for at lest 5 minutes before
test (effects pulmonary capillary blood volume)
Supplemental O2 should be
discontinued at least five minutes
prior to testing
Diffusing Capacity

DLcosb

Acceptability Criteria


Volume-Time tracing should show
smooth, rapid inspiration (<4 sec)
from RV to TLC
Expiration should be rapid but not
forced; 4 seconds or less
Diffusing Capacity

DLcosb

Acceptability Criteria



Dead space washout should be 0.75 –
1.00 L (0.5 L if VC is less than 2.0 L)
Alveolar sample volume should be 0.5
to 1.0 L
Inspired volume should be at least
85% of previously recorded best VC
Diffusing Capacity

DLcosb

Acceptability Criteria


Breath hold time should 10 sec +/- 2 sec
(No Valsalva or Mueller maneuver)
The average of two or more acceptable
test should be reported. Duplicate
determinations should be within 10% of
highest value or 3 ml CO/min/mm Hg
Diffusing Capacity

DLcosb
Significance and Pathology

Decreased in:

Restrictive Lung diseases







Asbestosis
Berylliosis
Silicosis
Idiopathic pulmonary fibrosis
Sarcoidosis
Systemic lupus erythematosus
Scleroderma
Diffusing Capacity

DLcosb
Significance and Pathology

Decreased in:







Inhalation of toxic gases (alveolitis)
Loss of lung tissue
Space occupying lesions (tumors)
Pulmonary edema
Lung resection
Radiation therapy (fibrotic changes)
Chemotherapy
Diffusing Capacity

DLcosb
Significance and Pathology

Decreased in:

Emphysema


Chronic Bronchitis , Asthma (may or may
not be decreased)
DLco sometimes used to differentiate
between emphysema and chronic
bronchitis
Diffusing Capacity

DLcosb
Significance and Pathology


In patients with COPD, DLco less than
50% of predicted is accompanied by O2
desaturation during exercise
Low resting DLco (<50% - 60% of
predicted) may indicate the need for
assessment of oxygenation during
exercise
Diffusing Capacity

DLcosb

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
Diffusing Capacity

DLcosb

DL/VA

DL/VA is useful in differentiating
between restrictive and obstructive
disease

Obstruction = Low DL/VA ratio

Restriction = DL/VA Ratio is preserved
Diffusing Capacity

DLcosb

DLco is affected by:

Hemoglobin (Hb)


Corrections for Hb concentrations should
be applied
 Low Hb reduces DLco
 High Hb elevates DLco
Carboxyhemoglobin (COHb)

Corrections for COHb should be applied
 High COHb reduces DLco
Diffusing Capacity

DLcosb

DLco is affected by:

Alveolar PCO2

Increased PCO2 elevates DLco


Hypoventilation
Pulmonary capillary blood volume

Increased blood volume causes increased
DLco



Mueller maneuver
Increased cardiac output
Decrease pulmonary capillary blood
volume causes decreased DLco

Valsalva maneuver
Diffusing Capacity

DLcosb

DLco is affected by:

Altitude above sea level


High altitude increases DLco
Poor inspiratory effort during testing

If less than 85% of VC will decrease DLco