One of the methods to evaluate lung function is body plethysmography. Body plethysmography allows to measure indicators that cannot be obtained during spirometry, that is: total lung capacity (TLC), bronchial resistance (Raw), and functional residual capacity (FRC). FRC measured by body plethysmography reflects thoracic gas volume (TGV or VTG), and includes both ventilated and poorly ventilated volume. Plethysmographic measurements are based on the principle of the relationship between pressure and volume at constant temperature and fixed amount of gas. This principle states that the absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system (Boyle-Mariotte Law). In body plethysmography, the following parameters: TGV, vital capacity (VC), expiratory reserve volume (ERV), tidal volume (TV), inspiratory reserve volume (IRV), inspiratory capacity (IC), Raw are obtained. TLC, residual volume (RV) and its share in total lung capacity can be calculated. Reduced TLC allows to diagnose restrictive disorders of pulmonary ventilation. The RV and its share in TLC increase together with airway obstruction indicates air trapping. The TGV increase together with airway obstruction indicates hyperinflation. In the analysis of lung volumes predictive values, which are calculated by the formulas recommended by ATS/ERS statement, are used. The results are expressed as a percentage of the predicted value. Thus, body plethysmography is an important addition to spirometry and image methods of research, allowing to assess the functional state of the lungs more objectively; it helps to conduct the differential diagnosis and control the course of the disease.

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At present, there are a number of problems associated with the diagnosis of restrictive type of ventilation disorders: first, to identify restriction, it is necessary to perform body plethysmography, which is a laborious technique and requires the active cooperation of the patient with medical personnel; secondly, methodological issues that concern the criteria for the diagnosis of restriction are still under development. Impulse oscillometry (IOS) is a non-invasive and effortless technique for the patient to identify ventilation disorders, especially obstructive ones. In order to assess the informative value of IOS in the diagnosis of the restrictive ventilation disorders, respiratory impedance and its parameters were measured in 82 patients with various bronchopulmonary pathologies. The restriction was established on the basis of spirometric and bodyplethysmography studies. The results showed that IOS is a low-sensitivity method for detecting the restrictive type of ventilation disorders with mild reduced total lung capacity (TLC≥70%pred.), since in this case, the basic parameters of the IOS, such as the resistive component of the respiratory impedance (Rrs) at 5 and 20 Hz (Rrs5 and Rrs20, respectively), as well as the reactive component of the respiratory impedance at 5 Hz (Xrs5) remain within the normal values. When TLC was less than 70%pred., there was a decrease of Xrs5 with an increase in the resonance frequency and the preservation of the normal values of Rrs5 and Rrs20. The abnormal absolute frequency dependence of Rrs was determined at any degree of change in the TLC, increasing as it decreased.

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