Lung perfusion refers to the flow of blood through the small capillaries that surround the air sacs, or alveoli, in the lungs. This blood flow, known as “Q,” is distinct from ventilation, or “V,” which is the movement of air into and out of the alveoli. Perfusion delivers deoxygenated blood to the gas exchange surfaces, allowing carbon dioxide to be offloaded and oxygen to be absorbed. The effectiveness of this process relies on the continuous, balanced delivery of both air and blood. If blood flow is inadequate, the body cannot complete the necessary gas exchange to sustain life.
The Pulmonary Circulation System
The circulatory system serving the lungs is unique, functioning as a high-flow, low-pressure circuit. This contrasts with the high-pressure systemic circulation that supplies the rest of the body. This low pressure protects the fragile alveoli from damage while processing the entire cardiac output, which is approximately five liters of blood per minute.
The mean pressure within the pulmonary artery is low, typically around 15 millimeters of mercury (mmHg), compared to the mean systemic arterial pressure of approximately 93 mmHg. This low pressure is maintained because the pulmonary arteries are thin-walled, highly compliant, and contain minimal smooth muscle. The lung’s vast network of capillaries forms a parallel circuit that lowers the overall resistance to blood flow.
The journey of deoxygenated blood begins when the right ventricle pumps it into the main pulmonary artery. This artery branches into a dense microvascular bed surrounding the alveoli. After the blood exchanges carbon dioxide for oxygen across the alveolar-capillary membrane, it collects into the pulmonary venules and veins. This newly oxygenated blood then returns to the left atrium of the heart, ready to be distributed to the rest of the body.
The Principle of Ventilation-Perfusion Matching
The lung’s purpose is to ensure that the volume of air reaching the alveoli (V) is matched with the volume of blood flowing past them (Q). This relationship is quantified as the Ventilation/Perfusion ratio, or V/Q ratio. An ideal V/Q ratio is 1.0, meaning that air entering an alveolus is perfectly matched by blood flow to pick up the oxygen.
In a healthy person, the average V/Q ratio is closer to 0.8 because the body typically has slightly more blood flow (Q) than total ventilation (V). Localized imbalances in this ratio represent two physiological extremes. A high V/Q ratio, or “wasted ventilation,” occurs when there is air flow but no blood flow. The air delivered is useless for gas exchange and is termed alveolar dead space.
The opposite extreme is a low V/Q ratio, often called a “shunt,” where there is blood flow but no air flow. The blood passes by unventilated alveoli, returning to the left side of the heart without being oxygenated. This significantly lowers the body’s oxygen levels. The body attempts to correct this mismatch through a protective mechanism called hypoxic pulmonary vasoconstriction, which constricts blood vessels in poorly ventilated areas, diverting flow to healthier lung segments.
Major Causes of Impaired Lung Perfusion
Impaired lung perfusion occurs when blood delivery to the gas exchange surface is compromised, disrupting the V/Q match. One significant cause is a mechanical blockage in the pulmonary arteries, most commonly a pulmonary embolism (PE). A PE is a blood clot, often originating in the legs, that travels to the lungs and physically obstructs a branch of the pulmonary artery.
Because the clot blocks blood flow (Q), the affected lung segment still receives air (V), resulting in a high V/Q ratio and creating dead space. The air inhaled into this region is wasted, as there is no blood to accept the oxygen. Another impairment mechanism is increased vascular resistance, such as in pulmonary hypertension.
Pulmonary hypertension is defined by an abnormally high mean pulmonary artery pressure, typically exceeding 20 mmHg at rest. This condition develops when pulmonary vessels narrow, thicken, or become destroyed, increasing the resistance against which the right ventricle must pump. The resulting strain on the heart impairs perfusion efficiency and can lead to heart failure.
Circulatory re-routing, or functional shunting, is a challenge where perfusion is wasted because the alveoli are collapsed or filled with fluid. Conditions like pneumonia or atelectasis (lung collapse) fill the air sacs, preventing ventilation. Although blood flow (Q) may be normal, the lack of air (V) means the blood returns deoxygenated, acting as a shunt and reducing systemic blood oxygen content.
How Lung Perfusion is Measured
Physicians utilize specialized imaging techniques to visualize and quantify blood flow distribution within the lungs. The most direct method for assessing the V/Q relationship is the Ventilation-Perfusion Scan (V/Q scan). This nuclear medicine test has two phases: the ventilation phase, where the patient inhales a radioactive gas to show air distribution, and the perfusion phase, where a radioactive tracer is injected to map blood flow.
By comparing the two images, physicians identify V/Q mismatches, such as a segment with normal ventilation but a lack of perfusion, which suggests a pulmonary embolism. A second, more anatomically detailed method is the CT Pulmonary Angiography (CTPA). This procedure involves injecting an iodine-based contrast agent intravenously while a computed tomography scan is performed.
The contrast agent rapidly fills the pulmonary arteries, making them appear bright white on the scan. If a blood clot is present, it appears as a dark “filling defect” against the bright contrast, visualizing the location and extent of the blockage. CTPA is effective at diagnosing large vessel blockages, while the V/Q scan remains useful for functional assessment, especially in patients who cannot tolerate the contrast agent required for a CTPA.