I. Content of pulmonary function measurement
(A) Resting pulmonary function: the subject’s pulmonary ventilation function (lung volume, lung ventilation, small airway function, respiratory kinetics, inhalation gas distribution, respiratory muscle function) and pulmonary ventilation function (diffusion function, ventilation blood flow ratio) are measured and evaluated under resting conditions.
(B) Cardiopulmonary exercise test: It refers to the simultaneous measurement and comprehensive assessment of energy metabolism, cardiac and pulmonary function of the subject during loading exercise.
(C) Other: respiratory regulation function, airway responsiveness measurement, pulmonary blood flow measurement, etc.
B. Commonly used lung function indicators
(A), pulmonary ventilation function
Pulmonary ventilation refers to the gas exchange between the lungs and the external environment.
1.Lung volume
Lung volume refers to the amount of gas that the lung can hold at different respiratory levels. It consists of eight parts, namely tidal volume (TV), compensatory expiratory volume (ERV), compensatory inspiratory volume (IRV), residual volume (RV), deep inspiratory volume (IC), functional residual volume (FRC), lung volume (VC) and total lung volume (TLC).
(1) Lung volume (VC): refers to the maximum volume of air that can be exhaled after maximum inspiration. Normal VC % > 80%. Reflects the ability of the lungs to expand. Decrease is seen in: restricted lung expansion (e.g., interstitial lung disease), restricted thoracic expansion (e.g., scoliosis), respiratory muscle fatigue (e.g., severe COPD) and neuromuscular pathology (e.g., poliomyelitis).
(2) Residual air volume (RV): refers to the volume of air remaining in the lungs after maximum expiration. Normal RV% is 80% to 120%. Increase is seen in obstructive lung disease (e.g. COPD) and decrease is seen in restrictive lung disease (e.g. interstitial lung disease).
(3) Total lung volume (TLC): refers to the amount of gas contained in the lungs after maximum inspiration. Normal TLC% is 80% to 120%. Increases are seen in obstructive lung disease and decreases are seen in restrictive lung disease.
(4) Residual total ratio (RV/TLC): refers to the ratio of residual gas volume to total lung volume, normal RV/TLC <35%. RV/TLC increases in emphysema.
2.Ventilation volume
(1) forceful lung volume (FVC), one-second volume (FEV1.0) and one-second rate (FEV1.0%): FVC refers to the expiratory lung volume obtained by maximum inspiration followed by maximum effort and fastest exhalation. FEV1.0 refers to the volume of air exhaled in the first second when doing FVC, the ratio of measured value to the expected value > 80% is normal. the ratio of FEV1.0 to FVC is the one-second rate ( FEV1.0%), FEV1.0% is an indicator of whether the airway is obstructed or not, normal >70%, decreased in airway obstruction and/or emphysema.
(2) Maximum voluntary ventilation volume (MVV): the ventilation volume obtained by repetition of maximum voluntary effort breathing at the fastest possible rate and as deep as possible per unit time. Normal MVV % > 80%. It is a comprehensive index reflecting the pulmonary ventilation function, reduced seen in: restricted lung expansion, restricted thoracic expansion, respiratory muscle fatigue, neuromuscular lesions, airway obstruction and emphysema, etc.
3.Small airway function
The main measurement method of small airway function is the maximum expiratory flow-volume curve. That is, the subject in the process of maximum expiratory force, the volume of exhaled gas and the corresponding expiratory flow traced into a curve. It mainly reflects the effect of intrathoracic pressure, pulmonary elastic retraction pressure, and airway resistance on expiratory flow during forceful expiration. The maximum expiratory flow rate in the ascending branch of the curve is related to the subject’s expiratory effort, while the maximum expiratory flow rate in the descending branch depends on the alveolar elastic retraction force and the peripheral airway resistance, independent of the exertion.
Small airway function was evaluated based on curve morphology and expiratory flow rates at different lung volume levels. The normal flow-volume curve is steep and straight in the ascending branches and decreases obliquely in the descending branches, with a gradual decrease in maximum flow. In small airway pathology, the descending branch of the curve is concave toward the volume axis and the slope becomes smaller. In COPD patients, with the progression of slow branch → emphysema → pulmonary heart disease, the maximum respiratory flow decreases progressively and the slope of the descending branch of the curve decreases progressively.
Commonly used indicators are.
①V50: the maximum expiratory flow rate at 50% of exhaled lung volume.
②V75: the maximum expiratory flow rate at 75% of the exhaled lung volume. The ratio of the measured value to the expected value > 80% is normal. v50 and v75 decrease indicates that the small airway function is reduced, which is seen in smoking, early COPD, early occupational disease and air pollution, etc.
4.Respiratory mechanics
Respiratory mechanics determines the pressure, volume and flow during respiration, so as to study the dynamics and resistance of the respiratory process.
(1) Respiratory muscle function
The power of breathing comes from the respiratory muscle. The maximum inspiratory pressure (MIP) and maximum expiratory pressure (MEP) are commonly used to evaluate the function of respiratory muscles. The minimum value of MIP for normal male is 7.25 kPa and the minimum value of MEP is 9.67 kPa; the minimum value of MIP for normal female is 4.84 kPa and the minimum value of MEP is 7.74 kPa. Among them, MIP is an index to evaluate the function of inspiratory muscle, and when it is less than 30% of the normal expected value, it is easy to develop respiratory failure. In addition, it is one of the important indicators for withdrawal of mechanical ventilation, and MEP can evaluate coughing ability. A decrease in the second index indicates respiratory muscle hypofunction or respiratory muscle fatigue, which is common in COPD.
(2) Respiratory resistance (R)
According to the physical properties, the respiratory resistance is divided into viscous, elastic and inertial resistance, and the sum of the three is called total respiratory resistance. Among them, viscous resistance comes from the airway and lung tissue, with airway resistance dominating; elastic resistance is distributed in lung tissue and expandable fine bronchi. Inertial resistance is mainly distributed in the large airways and thorax. Respiratory resistance is divided into airway resistance, lung resistance and thoracic resistance according to anatomical sites.
Airway resistance is usually measured by the body tracer method. The normal value is 0.0196~0.196kPa/L/s . Total respiratory resistance and its components are usually determined by the pulse oscillometry method. The percentage of measured and expected values of total respiratory impedance (Zrs) and total airway resistance (R5) in normal subjects is >120%; the percentage of measured and expected values of upper airway resistance (R35) is >130%.
Increased viscous resistance or airway resistance was seen in various causes of airway obstruction or narrowing and emphysema. Increased pulmonary elastic resistance is seen in restricted lung expansion and emphysema due to various causes. An increase in either of these resistances can lead to an increase in total respiratory resistance.
(3) Compliance (C)
Respiratory organ compliance refers to the change in lung volume caused by a change in unit pressure. It includes pulmonary compliance, chest wall compliance and total compliance. Pulmonary compliance is often measured clinically, and it refers to the change in lung volume caused by a change in unit pressure via pulmonary pressure, as measured by simultaneous measurement of respiratory flow rate and intraesophageal pressure. The lung compliance measured during the respiratory cycle, when airflow is temporarily blocked, is called static lung compliance. The lung compliance measured when the airflow is not blocked is called dynamic lung compliance. Normal male dynamic lung compliance is 1.7±0.6L/kPa, static lung compliance is 2.3±0.6L/kPa; normal female dynamic lung compliance is 1.1±0.3L/kPa, static lung compliance is 1.5±0.6L/kPa. lung compliance reflects the elasticity of the lung. In emphysema, static lung compliance increases and dynamic lung compliance decreases. In diffuse pulmonary fibrosis, both dynamic and static lung compliance are reduced.
Pulmonary ventilation function
Pulmonary gas exchange refers to the gas exchange between alveoli and pulmonary capillaries.
1, pulmonary diffusion function
Diffusion refers to the tendency of molecules to move from areas of high concentration to areas of low concentration. Pulmonary diffusion refers to the process of oxygen and carbon dioxide passing through the alveolar capillary membrane.
Commonly used evaluation indexes are.
(1) DLCO: the amount of CO that passes through the alveolar capillary membrane into the capillary blood per unit time and per unit pressure difference, the percentage of measured value and the expected value > 80% is normal.
(2) Diffusion coefficient (DLCO/VA): the ratio of carbon monoxide diffusion volume to alveolar air volume, the percentage of measured value and the expected value > 80% is normal.
The normal or abnormal pulmonary diffusion function depends mainly on the following factors.
(1) Thickness of the respiratory membrane: its thickening prolongs the diffusion distance, resulting in a decrease in both DLCO and DLCO/VA. It is common in interstitial lung disease.
(2) Respiratory area: Its reduction decreases the diffusion area, leading to a decrease in DLCO, while DLCO/VA may be normal. Commonly seen in post-pneumonectomy, damaged lung, etc.
(3) Hemoglobin volume: A decrease in hemoglobin reduces its binding to CO or O2, resulting in a decrease in both DLCO and DLCO/VA. Seen in anemia.
(4) Ventilation flow ratio: When the ventilation flow ratio is imbalanced or the ventilation flow is unevenly distributed, the pressure difference between CO or O2 on both sides of the alveolar membrane increases, resulting in a decrease in both DLCO and DLCO/VA. This is commonly seen in obstructive pulmonary disease.
(5) Pulmonary capillary blood volume: A decrease in pulmonary capillary blood volume reduces the respiratory area, resulting in a decrease in both DLCO and DLCO/VA. It is commonly seen in pulmonary artery embolism.
2.Ventilation flow ratio (V/Q)
The ventilation flow ratio refers to the ratio of pulmonary ventilation to pulmonary blood flow, with a normal value of 0.8 and some reports close to 1. Clinically, the ventilation flow ratio is generally evaluated indirectly through the measurement of physiological dead space and fractional flow.
(1) Physiological dead space: the volume of air that enters the airways and alveoli but cannot come into contact with pulmonary capillary blood and thus does not receive gas exchange. The former refers to the volume of gas that remains in the airway and cannot be exchanged, which is normally about 150 ml and increases when the bronchus is dilated; the latter refers to the volume of gas that has entered the alveoli but cannot be exchanged due to insufficient local blood flow, and increases when the pulmonary artery is embolized. The ratio of physiological dead space to tidal volume (VD/VT) is generally used to indicate the size of the physiological dead space, with a normal range of 0.25 to 0.35. Its increase indicates an increase in V/Q. In healthy subjects, VD/VT reflects the amount of anatomical dead space, while in patients with increased V/Q, an increase in VD/VT means an increase in the amount of alveolar dead space.
(2) Physiologic shunt: refers to the blood flow from venous blood that enters directly into the arterial segment of the body circulation without arterialization. The former is the direct entry of blood flow into the body circulation such as cardiac minimal veins and bronchial veins; the latter refers to venous blood flow through poorly ventilated alveoli that cannot be arterialized and forms a static-arterial shunt when it mixes with arterialized blood. This is generally expressed as the ratio of shunt flow to cardiac output (Qs/Qt). Normal is 3.65± 1.69%. Qs/Qt increases in pulmonary atelectasis, severe chronic bronchitis, etc. An increase in Qs/Qt implies a decrease in V/Q. In healthy individuals, Qs/Qt reflects the amount of anatomical fractional flow, while in patients with pulmonary disease with reduced V/Q, increased Qs/Qt means increased alveolar fractional flow.
IV. Exercise cardiopulmonary function measurement
Exercise cardiopulmonary function test is a combined measurement and comprehensive assessment of the subject’s cardiopulmonary function under exercise load. It combines the application of respiratory gas monitoring technology, electronic computer technology, and activity plate or bicycle technology to detect 12-lead ECG, blood pressure, energy metabolism, pulmonary function and cardiac function in real time during exercise. Its physiological basis is the oxidative reaction in the cellular mitochondria mediated by the cardiopulmonary coupling of O2 and CO2, which provides energy for exercise. Abnormalities in any of these components can result in reduced exercise capacity and abnormal exercise cardiopulmonary function in subjects. Commonly used evaluation indicators are.
1.Energy metabolism parameters
(1) Maximum oxygen consumption (VO2max): refers to the maximum amount of oxygen per minute that is inhaled and used by the body at the time of maximum load in increasing exercise, and the ratio of the measured value to the expected value > 84% is normal. It reflects whether the body’s gas transport system (cardiovascular, lung, hemoglobin) and muscle cell aerobic metabolism is normal, and any abnormality in the body’s gas transport system can reduce VO2max.
(2) Kilogram oxygen consumption (VO2/kg): refers to the maximum oxygen consumption per unit body weight. Normal >20ml/min/kg. >15ml/min/kg is feasible for pneumonectomy.
(3) Metabolic equivalent (MET): the basic unit of work, 1 MET is equivalent to 3 or 5 ml/min/kg of VO2/kg, normal >7 MET, often used as an index to evaluate cardiac function.
(4) Anaerobic threshold (AT): The maximum oxygen consumption before the blood lactate concentration rises sharply during exercise. Normal is greater than 40% of the expected value of maximum oxygen consumption. When the anaerobic threshold is reached, aerobic metabolism can no longer meet the energy demand of the exercising muscles, and anaerobic metabolism needs to be used to supplement the energy deficit of aerobic metabolism, and abnormal cardiopulmonary function can cause the anaerobic threshold to appear earlier.
2.Cardiac function parameters
(1) Heart rate reserve (HRR): It refers to the difference between the expected maximum heart rate during exercise and the maximum heart rate when the subject reaches the maximum load. Under normal conditions, HRR is <15 beats/min. HRR is increased in coronary artery disease and pulmonary disease due to early termination of exercise. HRR is also increased in patients with sick-sinus syndrome.
(2) Oxygen pulse (VO2/HR): It refers to the ratio of VO2 to HR. It represents the oxygen supply capacity of the heart per ejection and the reserve function of the heart, and indirectly reflects the cardiac output. Normal measured/expected〉80%. Cardiac lesions, serious pulmonary lesions and abnormal metabolic lesions can all reduce VO2/HR.
3.Pulmonary function parameters
(1) Ventilation volume during exercise (VE): refers to the ventilation volume per minute during exercise. The increase of VE depends on the compensatory capacity of the lungs, thus VE is a key indicator of exercise limitation in patients with respiratory diseases.
(2) Respiratory reserve (BR): refers to the difference between MVV and VE during exercise. Normal BR should be >15 ml/min. BR is reduced in patients with lung disease.
(3) respiratory rate (f): in the maximum load exercise, normal < 50 times / min. Restrictive lung disease, respiratory rate 〉 50 times / min.
(4) dyspnea index (DI): the ratio of VE to MVV during exercise, normal 50 times / min.
V. Airway responsiveness measurement
Airway reactivity refers to the contractile response of the airway to various physical, chemical, pharmacological or biological stimuli.