Certified Pulmonary Function Technologist (CPFT) Practice Exam

Why does a high level of carbon monoxide bound hemoglobin (COHb) lead to a decrease in DLCO?

1. Increased oxygen saturation

2. Back pressure created by COHb levels

3. Increased lung compliance

4. Decreased lung volume

The correct answer is: Back pressure created by COHb levels

A high level of carbon monoxide bound hemoglobin (COHb) leads to a decrease in DLCO, or diffusion capacity of the lungs for carbon monoxide, primarily due to the back pressure created by elevated COHb levels. When carbon monoxide (CO) binds to hemoglobin in the blood, it forms COHb, which reduces the amount of hemoglobin available for oxygen transport. This binding is competitive, as CO has a much higher affinity for hemoglobin than oxygen, thus effectively reducing oxygen transport and altering the dynamics of gas exchange in the lungs. As CO competes with oxygen, the concentration of CO in the blood can lead to an increased partial pressure of carbon monoxide in the alveoli, which can create a back pressure against the diffusion of additional CO from the alveoli into the bloodstream. This reduced gradient limits the rate at which CO can diffuse across the alveolar-capillary membrane, resulting in a lower DLCO measurement. The other options do not accurately reflect the physiological impact of increased COHb levels on DLCO. Increased oxygen saturation does not occur due to the presence of CO; instead, oxygen delivery is impaired. Increased lung compliance and decreased lung volume do affect gas exchange but are not direct consequences of elevated COHb levels leading to DLCO measurement