Pulmonary function tests are a test of the respiratory physiology of the lungs. The lungs exchange oxygen (O2) from the inhaled air with carbon dioxide (CO2) from the venous blood at the alveolar level. The four steps of gas volume, flow rate (gas flow per unit time), diffusion (gas exchange between alveoli and blood) and gas transport in the lungs ensure smooth gas exchange.
Pulmonary function tests include: lung volume, pulmonary ventilation, physiological dead space, alveolar gas distribution, small airway ventilation, airway resistance, pulmonary compliance, diffusion, blood gas analysis, exercise and other measurements. In clinical practice, lung function is measured by lung volume, ventilation function and blood gas analysis as a routine examination.
I. Lung volume in the respiratory movement, breathing amplitude can cause changes in the volume of air held in the lungs
1, the basic volume of the lung
(1) tidal volume: in calm breathing, the volume of air inhaled or exhaled each time.
(2) Complementary inspiratory volume: the maximum volume of air that can be inhaled after calm inspiration.
(3) Complementary expiratory volume: the maximum volume of air that can continue to be exhaled after a calm exhalation.
(4) residual volume: the residual volume of air that cannot be exhaled from the lungs after compensatory exhalation.
2, the lungs of the four volumes
(1) deep expiratory volume: the maximum amount of air that can be inhaled after a calm exhalation, consisting of tidal volume and compensatory inspiratory volume.
(2) lung volume: the maximum amount of air that can be exhaled after maximum inspiration, composed of deep inspiratory volume and compensatory expiratory volume.
(3) Functional residual air volume: the volume of air contained in the lungs after calm expiration, consisting of compensatory expiratory volume and residual air volume.
(4) Total lung volume: the total volume of air contained in the lungs after deep inspiration, consisting of lung volume and residual air volume.
Tidal volume, deep inspiratory volume, compensatory expiratory volume and lung volume can be directly measured by spirometer. Functional residual air volume and residual air volume cannot be directly determined by spirometry, but only by indirect methods, and total lung volume can be determined by adding spirometry and residual air volume.
Decreased spirometry is seen in thorax, restricted lung expansion, lung tissue damage, and airway obstruction. Changes in functional residual air volume often coexist with changes in residual air volume. Residual air volume increases in obstructive lung diseases such as bronchial asthma and emphysema. Restrictive lung diseases such as diffuse interstitial lung fibrosis, pulmonary occupational disease, and post-pneumonectomy lung tissue compression reduce the volume of residual air. Clinically, the residual air volume/total lung volume is used as the assessment index.
Pulmonary ventilation
Pulmonary ventilation function is measured as the volume of air inhaled or exhaled by the lungs per unit time. The resting ventilation volume per minute is the product of tidal volume and respiratory frequency, the number of breaths per minute in a normal adult at rest is about 15, the tidal volume is 500ml, the ventilation volume is 7,5L/min. 140ml of gas in the tidal volume remains in the airway without gas exchange, called anatomical dead space, so the alveolar ventilation volume is only 5,5L/min. If the breathing is shallow and fast, the anatomical dead space The ventilation volume is relatively higher, which affects the alveolar ventilation volume. The amount of gas entering the alveoli can be due to insufficient local blood flow resulting in gas exchange with blood. This part of the gas is called the alveolar dead space volume. The alveolar dead volume plus the anatomical dead volume together is called the physiological dead volume.
1, alveolar ventilation = (tidal volume – physiological dead space volume) × respiratory rate
Insufficient alveolar ventilation is commonly seen in emphysema; increased alveolar ventilation is seen in hyperventilation syndrome.
2, maximum ventilation volume: the ventilation volume obtained by breathing as fast and as deep as possible per unit time.
It is a simple stress test to measure the patency of the airway, the elasticity of the lungs and thorax and the strength of the respiratory muscles usually used as an indicator of whether thoracic surgery can be performed.
3.Exertional spirometry: the expiratory spirometry made with the fastest speed.
This can be calculated from the volume exhaled in the first second and the ratio of the volume exhaled in the first second to the exertional lung volume. Force spirometry is the best current measurement to reflect the expiratory resistance of the larger airways. It can be used as an auxiliary diagnostic tool for chronic bronchitis, bronchial asthma and emphysema, and can also assess the efficacy of bronchodilators.
4, peak expiratory flow: in the total lung volume position, blow hard and fast to the highest expiratory flow meter, observe the highest expiratory flow rate.
The measurement method is simple and easy to use. It is widely used in the epidemiological investigation of respiratory diseases, especially for the judgment of bronchial asthma disease condition and efficacy. In the 24-hour dynamic observation of asthma patients, it is found that the lowest value of peak expiratory flow rate often occurs at 0-5 am.
Pulmonary ventilation blood flow ratio
The inhaled air exchanges oxygen and carbon dioxide with the blood in the alveolar capillaries after reaching the alveoli. If pulmonary ventilation and blood flow per minute can be maintained at a certain ratio (4:5) on average, the gas exchange can be carried out normally.
If pulmonary ventilation is normal and pulmonary capillary blood flow is reduced or obstructed, which increases the amount of alveolar dead space, the ventilation/blood flow ratio increases; if pulmonary bronchial obstruction, local blood flow cannot be fully oxygenated, forming a physiological shunt, the ventilation/blood flow ratio decreases. Pulmonary function tests that reflect the ventilation/flow ratio include physiologic dead space measurement, alveolar arterial oxygen partial pressure difference measurement, and physiologic shunt measurement. Increased physiologic dead space can be seen in diseases such as red emphysema or pulmonary embolism. Increased physiological fractional flow is seen in cyanosis bloated emphysema or adult respiratory distress syndrome and other disorders.
Fourth, small airway ventilation function
The small airway resistance accounts for only 20% of the total airway resistance. It is difficult to detect with conventional pulmonary function measurements that reflect large airway resistance. Small airway resistance can be measured at low lung volume levels; small airway lesions are reversible in the early stages. There are 2 commonly used methods of examining small airway function.
1, maximum expiratory flow-volume curve: is to observe the expiratory flow at each instant during the period from the total lung volume position to the residual air volume. When small airway function is impaired, more than 50% of the flow of exhaled lung volume is affected, which is especially evident when 75% of the exhaled lung volume.
2. Closure volume measurement: the volume of air that can continue to be exhaled when the total lung volume is uniformly exhaled from the total lung position, when it reaches a position close to the residual air, and when the small airways at the bottom of the lung begin to close. An increase in the closure volume / lung volume % indicates early closure of the small airways at the base of the lungs. It can be caused by small airway pathology or by a decrease in the elastic retraction of the lung.
Impairment of small airway function is common in patients with atmospheric pollution, long-term heavy smokers, long-term exposure to volatile chemicals, early pneumoconiosis, fine bronchial virus infection, asthma in remission, early emphysema, interstitial fibrosis, etc.
V. Diffusion function
The main function of the lung is gas exchange, that is, the exchange of oxygen and carbon dioxide. The site of gas exchange in the lungs is in the alveoli, and the principle of diffusion is followed, that is, gas molecules are diffused from high partial pressure through the alveolar capillary membrane (blood gas barrier) to low partial pressure, until the pressure balance of the gas on both sides of the membrane. The partial pressure is the percentage of the total pressure of a gas in the gas mixture. The partial pressure of oxygen in the alveolar gas is higher than the partial pressure of oxygen in the capillaries of the alveolar membrane, so oxygen diffuses from the alveoli through the alveolar membrane to the capillaries and binds to hemoglobin in the red blood cells. The partial pressure of blood carbon dioxide is higher than that of gas in the alveoli, so carbon dioxide diffuses from the blood to the alveoli. Since the diffusing capacity of carbon dioxide is 20 times greater than that of oxygen, once diffusion impairment occurs, it is mainly an impairment of oxygen diffusion, and in severe cases, hypoxia can occur. Decreased diffusion is mainly seen in interstitial lung disorders, such as diffuse interstitial fibrosis, others, such as emphysema, where the diffusion area is reduced due to destruction of the alveolar wall, or anemia, where hemoglobin is reduced, can reduce lung diffusion.