Pulmonary ventilation-blood flow disproportion

Introduction There are two basic forms of imbalance in blood flow in alveolar ventilation: a decrease in the partial alveolar V/Q ratio and an increase in the partial alveolar V/Q ratio. Effective ventilation depends not only on the alveolar membrane area and thickness, alveolar total ventilation and blood flow, but also on the coordination of alveolar ventilation and blood flow. In the case of lung disease, the total ventilation and total blood flow of the lungs may be normal, but the uneven distribution of ventilation and blood flow and the proportional ventilation-perfusion imbalance may prevent the patient from performing effective ventilation. This is the most common mechanism of respiratory failure caused by lung disease.

Cause

In a resting state, the alveolar per minute ventilation (VA) is about 4L per minute, and the pulmonary blood flow (Q) is about 5L per minute. The ratio (VA/Q) is about 0.8. But even in healthy people, the distribution of ventilation and blood flow in the various parts of the lungs is not uniform. In the straight position, the alveolar ventilation and blood flow are increased from top to bottom, and the blood flow is more and more different. As a result, the V/Q ratio of each alveolar decreases from top to bottom. In the normal young people, the VA/Q ratio varies from 0.6 to 3 from top to bottom; as the age increases, the range of variation increases.

Despite this, PaO2 and PaCO2 will eventually remain in the normal range. In the case of lung disease, if the alveolar ventilation is insufficient and the blood flow is reduced at the same site (such as lobectomy, lobar pneumonia, gray liver metastasis), its function can be enhanced by the remaining lungs in appropriate proportions to enhance ventilation and blood flow. Reimbursement, and thus the impact on the ventilation function can be small. However, most of the respiratory diseases, alveolar ventilation and blood flow changes do not parallel, so that some alveolar V / Q ratio decreased or increased, and the range of VA / Q changes also expanded, thus causing a serious proportion of alveolar ventilation blood flow Disorders do not guarantee effective ventilation and lead to respiratory failure.

an examination

Related inspection

Pulmonary ventilation function pulmonary ventilation imaging

(1) Partial alveolar V/Q ratio reduction

Airway obstruction or stenosis caused by bronchial asthma, chronic bronchitis, obstructive emphysema, etc., and the reduction of lung and thoracic compliance in various parts of the lung are often not uniform, but not heavy One. Therefore, it can lead to severe uneven distribution of alveolar ventilation. If the alveolar ventilation is significantly reduced and the blood flow is not reduced or even increased, that is, the VA/Q ratio is decreased, the venous blood flowing through the part of the alveoli is incorporated into the arterial blood without sufficient oxygenation. This condition is similar to a pulmonary artery-venous short circuit, so the functional shunt is said to increase. Functional diversion due to uneven distribution of intrapulmonary ventilation in normal adults accounts for only 3% of pulmonary blood flow. When the chronic obstructive pulmonary disease is severe, the functional shunt is obviously increased, which is equivalent to 30-50% of the pulmonary blood flow, so it can seriously affect the ventilation function and lead to respiratory failure.

(2) Increased partial alveolar V/Q ratio

In some lung diseases, such as decreased pulmonary arterial pressure, pulmonary embolism, distortion of pulmonary vascular compression, and reduction of capillary wall in the lung wall, the VA/Q ratio is increased. There is less alveolar blood flow and more ventilation in the affected part. The inhaled air has little or no involvement in gas exchange, so it is similar to the airway, which means that the amount of alveolar dead space is increased, so this condition is also called dead cavity-like ventilation. At this time, the total effective ventilation of the lungs is inevitably reduced, which may also cause abnormal blood gas. The physiological dead volume (VD) of a normal person accounts for about 30% of the tidal volume (VT). The above-mentioned diseases can significantly increase the dead space volume, and the VD/QT can be as high as 60-70%.

Differential diagnosis

Differential diagnosis of pulmonary ventilation-blood flow imbalance:

1, a lot of pulmonary blood flow: the length of survival of patients with tricuspid atresia is closely related to pulmonary blood flow. Pulmonary blood flow is close to normal, the longest survival period can be more than 8 years; many patients with pulmonary blood flow can only survive for 3 months after birth. Tricuspid atresia is a kind of cyanotic congenital heart disease, the incidence rate is about It accounts for 1 to 5% of congenital heart disease. In the cyanotic congenital heart disease, the fourth place was after the tetralogy of Fallot and the aortic dislocation. The main pathological changes were tricuspid atresia or tricuspid valve loss, patent foramen ovale or atrial septal defect, mitral and left ventricular hypertrophy, right. Ventricular dysplasia.

2, pulmonary blood flow is very small: symptoms of tricuspid atresia patients with longevity and pulmonary blood flow are closely related. Pulmonary blood flow is close to normal, the survival period can be up to 8 years or more; those with many pulmonary blood flow can only survive for 3 months after birth; if the pulmonary blood flow is less than normal, the survival time after birth is in the above two cases. between. Keith et al reported that 50% of patients with tricuspid atresia can survive to 6 months, 33% survive to 1 year old, and only 10% can survive to 10 years. Cases with small septum in the room showed clinical systemic venous congestion, jugular vein engorgement, hepatomegaly and peripheral edema. Due to the small amount of blood in the pulmonary circulation, most cases can appear purpura from the neonatal period, and they are anxious after exertion, and can take the position of sputum or hypoxic fainting. The clubbing (toe) often occurs in patients over 2 years of age. In cases of increased pulmonary blood flow, the degree of purpura is reduced, but often there is shortness of breath, rapid breathing, prone to pulmonary infection, often present with congestive heart failure.

3, atelectasis: Lung insufficiency refers to the whole lung or part of the lung is contracted and airless. Lack of atelectasis may be acute or chronic, and there are often lung-free, infection, bronchiectasis, tissue destruction and fibrosis in chronic atelectasis lesions.

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