How to assess diffusing capacity for gas exchange impairment?

Assessment of Diffusing Capacity for Gas Exchange Impairment

The single-breath diffusing capacity of the lung for carbon monoxide (DLCO) is the primary test to assess gas exchange impairment, and should be measured alongside spirometry and lung volumes, with results reported as absolute values, z-scores, and percent predicted using current reference equations (GLI 2017), adjusted for hemoglobin when measured. 1

Primary Testing Method

• Single-breath DLCO measurement is the standard technique for assessing gas exchange efficiency, as it provides an integrated picture of alveolar-capillary membrane function 1, 2
• The test must include measurement of alveolar volume (VA) during the maneuver, with results corrected to standard barometric pressure (particularly important at altitude) 1
• Hemoglobin concentration must be measured and reported, with predicted DLCO values adjusted accordingly, as anemia artificially lowers DLCO while polycythemia increases it 1, 3
• Results should display CO and tracer gas washout curves graphed versus exhaled volume (not time), showing the sample volume used in calculations 1

Reporting and Interpretation Standards

• Report DLCO in absolute values (mL/min/mmHg), z-scores, and percent predicted using GLI 2017 reference equations, with the lower limit of normal (LLN) at z-score of -1.64 1
• The carbon monoxide transfer coefficient (KCO) may be reported optionally, but avoid using the term "DLCO/VA ratio" as it is commonly misunderstood 1
• Include test quality grading (Grade A, B, or C) and any technical comments that may affect interpretation 1
• Document barometric pressure at time of testing, as this affects DLCO values 1

Alternative Methods When Standard Testing Is Not Feasible

• Rebreathing DLCO technique should be used when patients cannot perform single-breath maneuvers due to severe neuromuscular dysfunction or coordination problems 1
• In patients unable to perform any DLCO measurement, monitor alternative endpoints: respiratory rate and effort, auscultation findings, transcutaneous oxygen saturation at rest, exercise oximetry, and overnight oximetry 1
• Impulse oscillometry can serve as a noninvasive alternative in patients unable to perform standard pulmonary function tests reliably 1

Comprehensive Assessment Algorithm

• Always measure DLCO alongside complete pulmonary function testing including pre- and post-bronchodilator spirometry, static lung volumes (by body plethysmography or helium dilution), and arterial blood gas analysis 1
• Body plethysmography typically yields higher lung volumes than gas dilution methods due to trapped air in poorly ventilated regions, which is clinically relevant in emphysema and obstructive disease 1
• DLCO and FEV1 correlate poorly—reduction in DLCO can occur even with normal spirometry, and both should be assessed independently when evaluating disease severity 1, 4
• More than 40% of patients with normal FEV1 (>80% predicted) may have reduced DLCO (<80% predicted), making DLCO essential for detecting early parenchymal or vascular disease 4

Clinical Patterns of DLCO Impairment

• Reduced DLCO with normal lung volumes suggests early interstitial lung disease, pulmonary vascular disease (including pulmonary hypertension), early emphysema, or anemia 4
• Reduced DLCO with restrictive pattern (low TLC) indicates interstitial lung disease, with severity correlating with extent of fibrosis on high-resolution CT 1, 4
• Reduced DLCO with obstructive pattern suggests emphysema, with the degree of reduction reflecting alveolar-capillary membrane destruction 1
• Increased DLCO occurs with polycythemia, pulmonary hemorrhage, or increased pulmonary capillary blood volume 3

Advanced Assessment: DLCO Partitioning

• DLCO can be partitioned into alveolar-capillary membrane conductance (Dm) and pulmonary capillary blood volume (Vc) using combined single-breath DLNO and DLCO measurements 5
• DLNO is more sensitive than DLCO for detecting early functional impairment before radiologic or hemodynamic changes become apparent 5
• Disproportionate reduction of Dm relative to Vc suggests thickening of the blood-gas barrier (as in early interstitial disease), while proportional reduction of both suggests combined parenchymal and vascular involvement 5

Follow-Up Testing Strategy

• Repeat DLCO at yearly intervals for monitoring disease progression in chronic conditions like alpha-1 antitrypsin deficiency, connective tissue disease-associated ILD, and other progressive lung diseases 1
• Progressive pulmonary fibrosis requires documentation of physiological decline: ≥10% relative decline in FVC or ≥15% relative decline in DLCO within one year (along with worsening symptoms and/or radiological progression) 1
• Exercise testing with simultaneous DLCO measurement can reveal exertional gas exchange impairment not apparent at rest, particularly in heart failure with preserved ejection fraction and early ILD 6

Critical Pitfalls to Avoid

• Never interpret DLCO in isolation—always consider in context of spirometry, lung volumes, and clinical presentation 4, 2
• Ensure proper breath-hold time (approximately 10 seconds) and adequate inspired volume (≥85% of VC) for valid single-breath measurements 1
• Laboratory error should only be considered after ruling out pathological causes of abnormal DLCO 4
• The relationship between DLCO and lung volume is not linear, so interpret KCO with caution, especially in patients with reduced alveolar volume 4