What Are the Causes of Ventilation Perfusion Mismatch?

Throughout the lung, hundreds of millions of microscopic structures, known as alveoli, create the functional link between the circulation and the atmosphere. Within these specialized gas exchange structures lies the interface between environmental influx of oxygen and efflux of metabolically produced carbon dioxide. There are a number of pathological processes that may impair lung function by reducing either ventilation of air, perfusion of blood or by both mechanisms.

Normal V/Q

• The rate of gas exchange between the lungs and the blood is determined by two factors: lung ventilation and blood perfusion. For example, the movement of oxygen from the environment into the blood depends on how much oxygen is inhaled and how much blood is reaching the lung capillaries. For gas exchange to occur in an efficient manner, the perfusion of blood to a given lung unit must be matched to the ventilation of that unit. If areas of the lung are receiving only one or the other, this can have significant effects on respiratory gas levels.

Alveolar Dead Space

• The concept of dead space is useful when discussing several aspects of respiratory physiology. Alveolar dead space, specifically, is the absence of proper ventilation of the alveoli, or gas exchange structures, in a given section of lung. Importantly, this area of the lung may still receive normal blood flow, thus leading to inefficient exchange of gas in the lung, overall. When blood goes to a region of the lung that is not receiving ventilation, it will not be able to absorb oxygen, or remove much carbon dioxide. This is because the gas pressure gradient is not in favor of the appropriate movement of gas. The natural diffusion of oxygen into the blood and carbon dioxide into the lungs only occurs when ventilation of a lung region closely matches the perfusion of deoxygenated blood to that region.

Right-to-Left Shunt

• Also known as an arterial-venous shunt, this form of ventilation-perfusion mismatch may result from disease processes affecting blood vessels. For example, cardiovascular abnormalities that permit large amounts of venous blood to bypass the lungs will effectively reduce gas exchange by decreasing blood perfusion to the lungs. Also known an interatrial septal defect, this form of congenital heart disease allows deoxygenated blood to pass from the right to the left side of the heart without entering the lungs and participating in gas exchange. This leads to arterial blood gas abnormalities as the lungs are unable to oxygenate blood and remove carbon dioxide from blood that it never receives.

Low Ventilation and Perfusion

• In some cases, both ventilation and perfusion will be reduced, resulting in low blood oxygen and high carbon dioxide, also known as hypercapnia.

Applications

• The absorptive surface area of the lungs is enormous; when spread out flat, the alveoli participating in gas exchange would cover 70 to 80 square meters, or one tennis court. This incredible organ has evolved to meet the metabolic demands of the body by developing mechanisms for maximizing gas exchange with the environment. By precisely matching alveolar ventilation with lung perfusion, the respiratory system can most efficiently absorb oxygen and expel carbon dioxide. Ventilation-perfusion mismatch is a major cause of disturbances in blood gas levels, often leading to clinical hypoxia, or low blood oxygen. Physicians will commonly use test results and observations from the physical exam to determine the mechanism of the V/Q mismatch, which will help guide therapeutic decisions.