The ventilation-perfusion ratio describes the quotient of pulmonary ventilation and pulmonary perfusion. Normal values of the ratio range from 0.8 to one in a healthy individual. Deviations are based on the principle of an intrapulmonary right-to-left shunt or increased alveolar dead space ventilation.
What is the ventilation-perfusion ratio?
The ventilation-perfusion ratio refers to the relationship between the total ventilation of the lungs and their perfusion. Perfusion refers to the flow of blood. Lung ventilation is also known as ventilation. Under this, medicine summarizes the ventilation of the entire respiratory tract during breathing. Gas exchange takes place via alveolar ventilation. However, the structures less involved in gas exchange are also ventilated. This is also referred to as dead space ventilation. The ventilation perfusion quotient refers to the relationship between the total ventilation of the lungs and their perfusion. Perfusion refers to the flow of blood. In the ventilation perfusion quotient, perfusion is equal to cardiac output, which is calculated as stroke volume times heart rate. The norm for cardiac output is about five liters. Perfusion of the lungs is between five and eight liters. Ventilation is about five to seven liters in a healthy adult. At rest, the ventilation-perfusion ratio averages between 0.8 and one. The quotient of the two volumes is a respiratory gas analysis parameter used in pneumology for diagnostic purposes.
Function and task
Pulmonary respiration is vital for humans. Gas exchange takes place in the alveoli of the paired organ. Oxygen is taken in with the air we breathe. Carbon dioxide is released into the environment at the same time. If too much CO were to remain in the body, this would result in symptoms of poisoning or even death. Similarly, death can also result from an undersupply of oxygen. Every tissue in the body is permanently dependent on a supply of O2 for maintenance. If the oxygen supply fails over a period of time, the tissue will die. In organs, organ failure is the result of this process. Oxygen is transferred from the air we breathe into the bloodstream in the alveoli. The blood serves as a transport medium during pulmonary respiration. Thus, oxygen reaches even the narrowest tissues via the bloodstream. Oxygen is transported in the blood in both dissolved and bound forms. The molecules of oxygen bind to the hemoglobin in human blood. Its binding affinity decreases in the increasing acidic environment of the rest of the body. In this way, oxygen separates from hemoglobin as it travels through the bloodstream, allowing it to be absorbed into the tissues. The norm of the ventilation-perfusion quotient describes the ideal of the ratio of blood flow and ventilation that the lungs need to supply oxygen to the body. Alveolar ventilation and perfusion differ from each other because of gravity in each lung segment. With the thorax (chest) upright, perfusion and ventilation gradually increase from the apex to the base of the lung. The vertical ventilation gradient is less pronounced than the perfusion gradient. The variable bronchial and vascular supply in the lung regions makes the ventilation-perfusion inhomogeneity even more extreme. For example, the regional ratio is as low as 0.5 in the basal sections, whereas it is as high as three at the top of the lungs. The average of these numbers gives a ventilation-perfusion ratio of about one. The areas above the mean are called hyperventilated districts and those below the mean are called hypoventilated districts. Hyperventilated areas are, for example, the alveoli. They contribute more to gas exchange than the hypoventilated districts. The inhomogeneity of perfusion and ventilation is increased in impaired lung function and worsens the gas exchange capacity of the lungs.
Diseases and medical conditions
Two different principles may underlie an aberrant ventilation-perfusion relationship. The first principle corresponds to a deviation due to an intrapulmonary right-to-left shunt. In this case, the alveoli are not ventilated but are perfused and mix mixed venous blood into the systemic circulation.Thus, right-to-left shunt is a disorder in the blood circulation that pumps deoxygenated blood from the venous legs into the arterial leg without passing through the pulmonary circulation. The cause of such a phenomenon may be an atrial or ventricular septal defect, which creates a direct connection of the large vessels of the body. Thus, right ventricular pressure exceeds left ventricular pressure. Surfactant deficiency may also trigger the phenomenon by causing inadequate ventilation in the basal districts of the lungs. The second principle for an abnormal ventilation-perfusion relationship corresponds to increased alveolar dead space ventilation. In this case, the alveoli are not perfused but are ventilated, thus reducing effective ventilation as the respiratory minute volume increases in a compensatory manner. The partial pressure of carbon dioxide thus remains unchanged despite respiration. Disturbances in pulmonary gas exchange with altered blood gas levels are also referred to as respiratory insufficiency. Such an insufficiency may be present in any imbalances in the ventilation-perfusion relationship. Partial respiratory insufficiency corresponds to arterial hypoxemia with a partial pressure of oxygen below 65 mmHg. In respiratory global insufficiency, hypercapnia is present in addition to hypoxemia. Thus, the partial pressure of carbon dioxide is above 45 mmHg. The most important symptoms of the insufficiency include shortness of breath, inner restlessness and palpitations. In severe cases, impaired consciousness and bradycardia may also occur. Abnormal breath sounds or rales also occur.
