diffusing capacity ret 2414 pulmonary function testing module 4.0
TRANSCRIPT
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