Body plethysmography

The body plethysmography (often called whole body plethysmography or large lung function hereinafter) is a method of Pulmonary for the measurement of lung and respiratory parameters. A large number of the respiratory physiological variables measured in body plethysmography - such as breathing resistance , residual volume or total lung capacity - can not be accessed with other measurement techniques, such as spirometry .

Modern body plethysmograph

The body plethysmography method used today goes back to the introduction of the method by DuBois in 1956. Body plethysmography is the diagnostic method of choice in the clinical area and in the practices of established pulmonologists. The analysis by body plethysmograph is the ideal lung function testing method because it has the measures of spirometry can calculate the specific airway resistance including breathing loop, the thoracic gas volume and all derived from these parameters in the same examination process also. This method is less dependent on cooperation than with spirometry and the additional time required is minimal. However, the high expenditure on equipment and the high acquisition costs explain why body plethysmographs can practically only be found in clinics and with specialists.

Measuring principle

Determination of breathing resistance

The body plethysmograph is designed as a cabin with a (largely) closed air volume, it looks like a small telephone booth. The cabin has minimal leakage in order to compensate for an increase in cabin pressure caused by the patient's body heat. During the exam, the patient breathes through a spirometer to determine flow properties. To inhale, the patient's chest is raised, which minimally reduces the volume of air in the cabin and thus increases the pressure. In resting breathing you can now observe which pressure change is necessary to generate a certain flow. The analysis of this so-called breathing loop allows conclusions to be drawn about the resistance of the lungs and can thus easily and quickly reveal clinical pictures such as simple obstruction (flatter breathing loop) or COPD (triangular protuberance in the expiratory part of the breathing loop). The specific airway resistance sRAW then corresponds to the slope of the breathing loop. To determine the total respiratory resistance, the thoracic gas volume (TGV) is required: which is determined in the next step. ${\displaystyle RAW={\frac {sRAW}{(TGV)}}}$

Determination of the thoracic gas volume

The measurement of the thoracic (often also: intrathoracic) gas volume is based on the physical law of Boyle and Mariotte , according to which the product of pressure and volume remains constant at constant temperature. As described, the breathing movements result in a compression or expansion of the gas volume enclosed in the thorax. While in the resting position, i.e. at the end of an exhalation, the air pressure in the lungs corresponds to the external pressure, when inhaling, the volume of the lungs increases when the chest is raised, which in turn reduces the air pressure. Therefore, Boyle and Mariotte's law can be applied here as follows:

${\displaystyle P\cdot V_{l}=(V_{l}+\Delta V)\cdot (P-\Delta P)}$

The pressure here corresponds to the normal external pressure at rest. The change in volume can be determined by the change in cabin pressure during inhalation. The change in pressure is determined by a pressure gauge on the patient's mouthpiece. A shutter briefly closes the mouthpiece, which is why no more respiratory flow can be measured for a short moment and thus no pressure drop over the airway resistance exists. The measured pressure then corresponds solely to the pressure in the pulmonary alveoli, the intrapulmonary pressure. Thus, the above equation (neglecting ) can be converted to the unknown lung volume, which corresponds to the thoracic gas volume: ${\displaystyle P}$ ${\displaystyle \Delta V}$ ${\displaystyle \Delta P}$ ${\displaystyle \Delta V\cdot \Delta P}$

${\displaystyle V_{l}=P\cdot {\frac {\Delta V}{\Delta P}}}$

With further parameters determined by simple spirometry, conclusions can now be drawn about the total lung capacity and the residual volume .

Although the physics of this method does not seem too complex, only a few manufacturers have mastered this technology. Only about a dozen companies worldwide offer body plethysmographs. This is due on the one hand to the extremely small pressure differences that arise during inspiration, but also to the large disruptive influences such as cabin heating, phase shift of the pressure or susceptibility to external pressure influences of the sensitive sensors.

indication

The Spirometry provides the first evidence for the presence of lung disease and is suitable for monitoring the disease and treatment history. For a definitive diagnosis, however, body plethysmography is essential. In addition to the basic distinction between z. B. asthma and COPD allows a body plethysmograph a quick classification of the obstruction in z. B. homogeneous obstruction, COPD or extrathoracic stenosis. A body plethysmography is also required for the reliable diagnosis of restrictions , emphysema or pulmonary fibrosis .

Web links

• Body plethysmography on Vimeo (examination procedure / German Respiratory League eV )
• Recommendations of the German Respiratory League on body plethysmography (PDF; 1.39 MB) Year of publication: 2009

Individual evidence

1. ↑ Recommendations of the German Respiratory League for body plethysmography: Chapter 3.1 Measurement using occlusive pressure maneuvers (PDF)
2. ^ Diagnosis of lung diseases

This article is about a health issue. It is not used for self-diagnosis and does not replace a diagnosis by a doctor. Please note the information on health issues !