62 Effective diffusion area of the erythrocyte flow (EPR)
The effective diffusion area of the erythrocyte flow (EPR) is the total membrane surface of the red blood cells that pass through the lung capillaries in one minute and are available for the diffusion of oxygen and carbon dioxide. It is expressed in m²/min. Each red cell is a flexible biconcave disc about 7.8 µm in diameter with a surface of about 135 µm². EPR therefore combines three things: how many red cells the blood contains, how large and deformable they are, and how much blood the heart pumps through the lungs per minute (cardiac output). It is a calculated index of the method used by this application, not a standard laboratory test.
How it is calculated
EPR ≈ red cell count (per litre) × surface of one red cell × cardiac output (L/min)
For example, 5 × 10¹² cells/L × 135 µm² × 5 L/min ≈ 3400 m²/min. At rest each red cell spends about 0.75 s in a lung capillary; during exercise cardiac output rises several times, so many more red cells, and a much larger membrane area, pass through the lungs each minute. The word "effective" means that only red cells that actually reach ventilated alveoli and can deform in the narrow capillaries take part in gas exchange.
Physiological background
Oxygen uptake in the lungs depends on two steps: diffusion across the alveolar–capillary membrane and binding to haemoglobin inside the red cells. Roughton and Forster showed that the red-cell component, the volume of blood in the lung capillaries multiplied by the rate at which red cells take up the gas, is a large part of the total diffusing capacity of the lung. Anaemia, fewer red cells in the capillaries or reduced pulmonary blood flow therefore lower the lung's ability to load oxygen. In clinical practice this is measured as the diffusing capacity for carbon monoxide (DLCO), which is corrected for haemoglobin.
Clinical significance
Reduced EPR
Anaemia (fewer red cells), low cardiac output (heart failure, bleeding, dehydration), spherocytosis and other conditions with small or rigid red cells, pulmonary embolism or loss of lung capillaries. Possible results: breathlessness and fatigue on exertion, low oxygen saturation during exercise.
Increased EPR
Physical exercise (higher cardiac output), high red cell count in adaptation to high altitude or chronic hypoxia (lung disease, smoking), polycythaemia vera. A very high haematocrit increases blood viscosity and the risk of thrombosis.
EPR should be interpreted together with haemoglobin, the red cell count and indices (MCV), cardiac output and oxygen saturation.
Fatigue, pallor, breathlessness on exertion or a very high red cell count should be investigated with a full blood count and, if needed, lung function tests including DLCO and echocardiography.
References
- Roughton FJW, Forster RE. Relative importance of diffusion and chemical reaction rates in determining rate of exchange of gases in the human lung, with special reference to true diffusing capacity of pulmonary membrane and volume of blood in the lung capillaries. J Appl Physiol. 1957;11(2):290–302.
- Evans E, Fung YC. Improved measurements of the erythrocyte geometry. Microvasc Res. 1972;4(4):335–347.
- West JB, Luks AM. West’s Respiratory Physiology: The Essentials. 10th ed. Philadelphia: Wolters Kluwer; 2016.
This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.
Published on 29 April 2024