Diffusing Capacity

RET 2414Pulmonary Function TestingModule 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