In the measurement of pulmonary diffusing capacity (DLCO), carbon monoxide is used as the test gas. The primary reason CO is ideal for this measurement is:

• A CO has an extraordinarily high affinity for hemoglobin (240× O₂), so mixed venous PCO ≈ 0 mmHg, maintaining maximum diffusion gradient throughout transit ✓
• B CO is completely soluble in blood; the rate-limiting step is purely membrane diffusion
• C CO does not bind to plasma proteins, making its uptake entirely dependent on red blood cell transit time
• D CO is inert at low concentrations and does not alter pulmonary vascular resistance

Explanation

The driving force for CO diffusion is the alveolar-to-capillary partial pressure gradient. Because hemoglobin binds CO with ~240 times the affinity of O₂ (forming carboxyhemoglobin), essentially all CO diffusing into blood is immediately bound, keeping the dissolved (free) PCO in capillary plasma extremely close to zero. This maintained near-zero back-pressure ensures the alveolar-capillary gradient remains constant throughout the pulmonary capillary, making CO uptake limited only by diffusion and membrane conductance — not by perfusion (unlike O₂ at rest). DLCO is thus a sensitive indicator of alveolar-membrane disease.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.