COPD causes polycythemia because damaged lungs can’t deliver enough oxygen to the blood, and the body compensates by producing more red blood cells. This compensation is driven by the kidneys, which detect low oxygen levels and release a hormone called erythropoietin (EPO) that tells the bone marrow to ramp up red blood cell production. While this response makes biological sense as a survival mechanism, it eventually backfires by thickening the blood and straining the heart and lungs even further.
How Low Oxygen Triggers Red Blood Cell Production
In healthy lungs, oxygen passes easily from the air sacs into the bloodstream. In COPD, chronic inflammation and structural damage to those air sacs reduce this gas exchange, leaving the blood chronically low in oxygen. This state, called chronic hypoxemia, is the starting point of the chain reaction that leads to polycythemia.
When blood oxygen drops, specialized cells in the kidneys activate proteins called hypoxia-inducible factors (HIFs). These proteins function like oxygen sensors. One type in particular, HIF-2, directly controls the gene responsible for producing erythropoietin. The kidney responds to ongoing low oxygen by increasing the number of cells that produce EPO, effectively turning up the volume on its distress signal to the bone marrow.
EPO travels through the bloodstream to the bone marrow, where it stimulates the growth and maturation of red blood cell precursors. More red blood cells enter circulation, raising both the hemoglobin concentration and the hematocrit (the percentage of blood volume occupied by red cells). This is the body’s attempt to pack more oxygen-carrying capacity into each unit of blood. The response can be triggered within minutes of oxygen deprivation, though the measurable increase in red blood cells takes days to weeks of sustained low oxygen.
Research on oxygen thresholds suggests that EPO production increases meaningfully when arterial oxygen saturation falls below about 90%. Many COPD patients live with saturations in this range or lower, particularly during sleep, physical activity, or acute flare-ups. The longer and more severely oxygen levels stay depressed, the stronger the EPO signal becomes.
Why This Compensation Becomes a Problem
At first, having more red blood cells helps. Extra hemoglobin means each pass of blood through the lungs picks up a bit more oxygen than it otherwise would. But past a certain point, the rising red blood cell count thickens the blood significantly. Blood viscosity increases in an exponential fashion as hematocrit climbs, with particularly steep increases once hematocrit exceeds about 54%.
Thicker blood is harder to push through the lungs’ tiny blood vessels. Pulmonary vascular resistance rises, forcing the right side of the heart to work harder to pump blood through the lungs. Over time, this extra workload can enlarge and weaken the right ventricle, a condition called cor pulmonale. For someone whose lungs are already compromised by COPD, this creates a vicious cycle: the body’s attempt to fix low oxygen ends up further straining the cardiovascular system that delivers it.
Thrombosis and Other Risks
The most dangerous consequence of thicker, more viscous blood is clotting. Elevated hematocrit increases the risk of both arterial events (like stroke and heart attack) and venous events (like deep vein thrombosis and pulmonary embolism). The venous risk appears to be especially pronounced. One study of patients with blood cancers that cause high red cell counts found that those who also had COPD or asthma had a dramatically higher rate of venous clotting events compared to those without lung disease.
Beyond clotting, hyperviscosity can cause headaches, dizziness, blurred vision, and a feeling of mental fogginess. Some patients notice their skin takes on a ruddy or bluish-red appearance, particularly in the face and hands. These symptoms often develop so gradually that patients attribute them to their COPD getting worse rather than recognizing a separate, treatable problem.
How Polycythemia Is Identified
A standard complete blood count (CBC) is all that’s needed to spot the problem. In healthy adults, normal hematocrit runs around 47% for men and 40% for women. When a COPD patient’s hematocrit creeps well above these ranges, secondary polycythemia is the likely explanation, though doctors will typically rule out other causes like dehydration (which can falsely elevate hematocrit) and primary blood disorders like polycythemia vera.
The distinction between primary and secondary polycythemia matters for treatment. In secondary polycythemia, the EPO level is elevated because the kidneys are responding to genuinely low oxygen. In polycythemia vera, the bone marrow overproduces red cells on its own due to a genetic mutation, and EPO levels are typically low. A blood test measuring EPO concentration helps distinguish the two.
How It’s Managed
The most effective treatment targets the root cause: low oxygen. Long-term supplemental oxygen therapy, typically prescribed for at least 15 to 18 hours per day, raises blood oxygen levels enough to quiet the kidneys’ EPO signal. Over weeks to months, the bone marrow slows red blood cell production and hematocrit gradually falls toward a safer range. This is one of the key reasons oxygen therapy improves survival in COPD patients with chronic hypoxemia, not just because it delivers more oxygen directly, but because it interrupts the polycythemia cycle.
In cases where hematocrit is dangerously high (generally above 55 to 60%), phlebotomy, the controlled removal of blood, can provide faster relief. Removing a unit of blood lowers hematocrit immediately, reducing viscosity and easing the strain on the heart. Studies of COPD patients with severe secondary polycythemia found that phlebotomy produced noticeable improvement in most patients, with the most dramatic benefits seen in those whose hematocrit exceeded 60% or who had signs of heart failure. However, phlebotomy is considered a short-term fix. Since it doesn’t address the underlying hypoxemia, red blood cell counts will climb again without adequate oxygen therapy.
Staying well hydrated also helps. Dehydration concentrates the blood and pushes an already elevated hematocrit even higher, compounding the viscosity problem. For COPD patients with polycythemia, something as simple as consistent fluid intake can meaningfully affect how thick their blood runs on any given day.