Why CO isused for Dlco. What's the application of DLCO

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Why CO is Used for DLCO

The Problem with Oxygen

To measure diffusing capacity, you need a gas whose uptake rate across the alveolar-capillary membrane can be accurately calculated. The formula is:

D = Gas uptake / (Alveolar partial pressure - Capillary partial pressure)

For oxygen, this is impossible to use clinically because the pulmonary capillary PO2 rises continuously as blood passes through the capillary - it cannot be accurately measured at any single moment. Therefore, Pc'O2 cannot be determined, making DLO2 unmeasurable in practice.

Morgan & Mikhail's Clinical Anesthesiology, 7e, p. 952

Why CO Solves This Problem

Carbon monoxide has two properties that make it ideal:

Extremely high affinity for hemoglobin - CO binds hemoglobin approximately 200-240 times more avidly than oxygen. This means that virtually all CO that crosses into the blood is immediately bound to hemoglobin and almost none remains dissolved as free CO in capillary plasma.
Near-zero back pressure - Because dissolved CO in capillary blood is essentially zero (Pc'CO ≈ 0), the driving pressure for CO diffusion equals the alveolar partial pressure alone:

DLCO = CO uptake / PACO

This makes the denominator fully measurable and the calculation straightforward.

Barash Clinical Anesthesia, 9e, p. 1138
Morgan & Mikhail's Clinical Anesthesiology, 7e, p. 952
Miller's Anesthesia, 10e, p. 1304

What DLCO Actually Measures

DLCO is not a single membrane property - it collectively reflects all factors affecting gas transfer across the alveolar-capillary interface:

Factor	Effect on DLCO
Alveolar surface area	Reduced area → lower DLCO
Alveolar-capillary membrane thickness	Thickening → lower DLCO
Pulmonary capillary blood volume	Less blood → lower DLCO
Hemoglobin concentration	Anemia → lower DLCO
V/Q matching	Mismatch → lower DLCO
Body position	Supine → higher DLCO
Alveolar PCO2	Elevated PCO2 → higher DLCO

Normal resting DLCO (single-breath method): ~25 mL CO/min/mmHg. This can increase 2-3 times during exercise.

Barash Clinical Anesthesia, 9e, p. 1138

How DLCO is Measured (Single-Breath Technique)

Patient exhales fully to residual volume
Rapidly inhales a gas mixture containing a known concentration of CO + an inert tracer gas (helium or methane) to total lung capacity
Holds breath for 10 seconds - CO diffuses across the membrane and binds hemoglobin; tracer gas remains in alveolar space
Exhales - the inert gas calculates alveolar volume (VA); CO concentration drop calculates CO uptake

Goldman-Cecil Medicine, p. 868

Clinical Applications of DLCO

1. Differentiating Obstructive Lung Diseases

Disease	DLCO	Reason
Emphysema	Markedly reduced	Destruction of alveolar walls and capillary bed; loss of surface area
Asthma	Normal (or mildly reduced)	Airway problem, parenchyma intact
Chronic bronchitis	Normal or mildly reduced	Airway secretions, not alveolar destruction

DLCO distinguishing emphysema from asthma in an obstructed patient is one of its most practical clinical uses.

Goldman-Cecil Medicine, p. 868 | Barash Clinical Anesthesia, 9e, p. 1138

2. Detecting and Monitoring Interstitial Lung Disease (ILD)

Reduced DLCO is one of the earliest functional abnormalities in ILD. Diseases include:

Idiopathic pulmonary fibrosis
Sarcoidosis
Asbestosis
Berylliosis
Connective tissue disease-associated ILD (e.g., systemic sclerosis - DLCO is used as a monitoring marker in SSc)
Oxygen toxicity, pulmonary edema

3. Identifying Pulmonary Vascular Disease

An isolated reduction in DLCO (with normal TLC, VC, FEV1) is a red flag for pulmonary vascular disease (e.g., pulmonary hypertension, pulmonary emboli). The vascular bed destruction reduces the available surface area for gas exchange.

4. Distinguishing Parenchymal from Chest Wall Restriction

In a restrictive pattern (low TLC), DLCO helps separate:

Low DLCO + restriction = parenchymal (ILD, sarcoid) disease
Normal DLCO + restriction = chest wall / neuromuscular cause (scoliosis, obesity, neuromuscular disease)

See the algorithm below:

Goldman-Cecil Medicine - Algorithm for interpreting pulmonary function tests. DLCO features prominently at multiple branch points.

5. Pre-operative Risk Assessment

DLCO below 60% of predicted is associated with increased risk of postoperative pulmonary complications. It is an indication for further preoperative workup, often exercise testing (cardiopulmonary exercise testing / CPET).

Miller's Anesthesia, 10e, p. 1304

Similarly, for thoracic surgery (lung resection), a predicted postoperative DLCO (ppo-DLCO) <40% predicts high perioperative risk.

Barash Clinical Anesthesia, 9e, p. 1138

6. Exercise-Induced Dyspnoea

DLCO can increase 2-3x during exercise in normal individuals due to alveolar microvascular recruitment. Failure to augment DLCO with exercise is seen in pulmonary hypertension and other vascular diseases.

Summary Table

Clinical Question	How DLCO Helps
Emphysema vs. Asthma	Low in emphysema, normal in asthma
Is ILD present?	Early reduction even before symptoms
Pulmonary hypertension screen	Isolated DLCO reduction
Parenchymal vs. chest wall restriction	Low vs. normal DLCO
Preoperative lung surgery	ppo-DLCO predicts complications
Anemia correction	Adjust DLCO for Hb (low Hb falsely lowers DLCO)

Bottom line: CO is used precisely because its near-zero back pressure in the blood (due to extreme Hb affinity) makes the diffusion equation solvable. DLCO is one of the most clinically informative pulmonary function tests because it reflects the integrated health of the alveolar membrane, capillary bed, and ventilation-perfusion matching - not just airflow.

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