Study of the function of external respiration.
Respiratory disorders in bronchial asthma are caused by reversible airway obstruction, which is manifested primarily by a decrease in FEV1 and PIC . These indicators usually quickly normalize after the use of bronchodilators . An increase in FEV1 after the use of bronchodilators by more than 20% indicates a reversible bronchospasm. With obstruction of the bronchi with mucous plugs and swelling of the mucosa, the effect of bronchodilators is slower. It should be remembered that the absence of a significant increase in FEV1 after the use of bronchodilators does not exclude the diagnosis of bronchial asthma. The lack of reaction may be due to the following reasons:
– the absence or slight obstruction of the airways in the interictal period;
– the action of bronchodilators applied shortly before the study;
– improper use of inhaled bronchodilators;
– bronchospasm caused by irritating substances that make up inhaled bronchodilators;
– bronchospasm caused by diagnostic procedures, in particular spirometry.
In the interictal period, FEV1 is usually normal. Indicators FEV1 and PIC reflect the condition of large bronchi. With a narrowing of small (less than 2-3 mm in diameter) bronchi FEV1 and POS are often normal (FEV is reduced only with severe obstruction of the small bronchi). To assess the condition of the small bronchi, another indicator is used – SOS25-75% . To determine the SOS25-75%, a graph of the dependence of the air flow on the forced expiratory volume is constructed — the flow-volume curve ( Fig. 7.3 ). It should be remembered that an isolated decrease in SOS25-75% can be observed in the interictal period.
The dependence of the forced expiratory flow rate on the gas density allows a more accurate determination of the diameter of the bronchi subject to obstruction. To identify this dependence, two flow-volume curves are constructed: the 1st — by inhalation of air, the 2nd — by inhalation of a low-density gas mixture consisting of 80% helium and 20% oxygen. If, when breathing a low-density gas mixture, the forced expiratory flow is not less than 20% higher than when breathing air, the main flow restriction occurs in large bronchi. The absence of a dependence of the forced expiratory flow rate on the density of the inhaled gas indicates the predominant obstruction of the small bronchi. With a mild course of bronchial asthma, obstruction is observed mainly in large bronchi . In severe bronchial asthma, especially accompanied by persistent cough and frequent respiratory infections, as well as smokers , predominant obstruction of the small bronchi is observed . Obstruction of the small bronchi is usually more persistent than obstruction of the large bronchi.
During an attack of bronchial asthma, FEV1 , POS and SOS 25-75% decrease in proportion to the degree of bronchial obstruction, the improvement is accompanied by a gradual normalization of these indications. Faster normalization indicates that obstruction of the large bronchi is more treatable than obstruction of the small bronchi. Obstruction of the small bronchi is often associated with edema and clogging with mucous plugs, so it requires longer treatment.
Jelly decreases during seizures, as well as with a prolonged course of bronchial asthma. The decrease in VC is due primarily to OO , ( Fig. 7.5 ), which is due to the effect of an air trap. The effect of an air trap and an increase in airway resistance lead to an increase in FOE and OEL , which, in turn, is accompanied by overstretching of the lungs. It is of a compensatory nature, since it is accompanied by an increase in the diameter of the bronchi. However, with an increase in volume, the electric traction of the lungs and the mobility of the diaphragm decrease, which leads to an increase in the work of breathing. With an increase in bronchial obstruction, a decrease in VC, an increase in OO, FOE, OEL become more pronounced. After a prolonged attack of bronchial asthma, pulmonary volumes recover slowly. In some cases, treatment of severe bronchial asthma leads to subjective improvement without a marked increase in FEV1. This is due to the fact that initially elevated OO, OEL and FOE significantly decrease during treatment, which leads to a decrease in lung overstretching, narrowing of the bronchi and, as a result, to an increase in airway resistance. The decrease in dyspnea in such cases is due to a decrease in FOE to normal values, an increase in the mobility of the diaphragm and a decrease in the work of inspiration.
The diffusion capacity of the lungs with bronchial asthma, in contrast to that with emphysema , is usually normal, the alveolar-capillary membrane is not affected in bronchial asthma. Measurement of the diffusion capacity of the lungs leads to a differential diagnosis of bronchial asthma and emphysema .
VC , FEV1, and СОС25-75% ( Fig. 7.3 ) are usually measured using water or dry spirographs. Using a pneumotachograph, you can build a flow-volume curve. However, pneumotachographs have almost no advantages over spirographs. PIC can be determined using a pneumatograph (according to the maximum angle of inclination of the curve of the dependence of forced expiratory volume versus time) or measured using a pneumotachometer. The reliability of the indicators depends on the accuracy of the device and the patient’s instructions from the doctor. During a severe attack of bronchial asthma, it is often impossible to reliably measure the VC and indicators of forced expiration. To avoid bronchospasm during the study of the function of external respiration, you can ask the patient to take an incomplete exhalation, and then a forced exhalation. On the basis of the data thus obtained, the so-called incomplete flow-volume curves are constructed.
Resistance to the airways is measured using general plethysmography. This study is shown if, when prescribing bronchodilators, FEV1 remains the same or even decreases. The latter is due to the fact that inhalation and forced exhalation can cause narrowing of the bronchi. During general plethysmography, bronchial obstruction can be avoided, since this does not require forced breathing. With bronchial asthma, airway resistance is increased. After the use of bronchodilators, it usually decreases by no less than 35%.
OO , OEL , FOE can be measured by dilution of an inert gas, leaching of nitrogen from the lungs, as well as using general plethysmography. These studies are conducted only in specialized laboratories.
The objectives of the study of the function of external respiration:
– Identification of reversible bronchial obstruction to confirm the diagnosis of bronchial asthma;
– assessment of bronchial obstruction and the effectiveness of bronchodilators;
– observation of patients with severe bronchial asthma during treatment with bronchodilators and corticosteroids;
– assessment of the risk of surgery.
For outpatient treatment of bronchial asthma and for monitoring the patient’s condition during emergency care, it is sufficient to measure FEV1 and VC or only PIC. A complete study of the function of external respiration, sometimes with an assessment of the diffusion capacity of the lungs, is carried out only for diagnostic purposes and after emergency care in case of an attack of bronchial asthma. To diagnose bronchial asthma in the interictal period, a provocative test with metacholine is performed.