How to calculate the STPD

   Spirometry measures tidal volumes (tidal volume, inspiratory and expiratory reserve volume, together vital capacity VC), pneumotachography, flow rates. The entire VC can be expired from the maximum inspiration position (the lungs contain the total capacity); if this is done as quickly as possible, the peak expiratory flow (PEV; for obstructive ventilation disorders <6 l / s) is determined. The proportion of the VC exhaled in one second is the 1 s value (should be at least 70-80% of the VC: Tiffeneau test). Inhaled and exhaled air volumes are of different sizes (temperature, pH2O). ~ 0.3 liters of oxygen are consumed per minute; The pO2 in the exhaled air is lower (~ 16 kPa) than in the inhaled air (~ 21 kPa), but the former contains ~ 5% CO2
 
   The respiratory time volume increases up to 20-fold with physical exertion (with exertion to> 120 l / min). The respiratory flow rate can be shown as a function of the tidal volume (flow-volume diagram): The maximum respiratory flow rate (PEF: 8-12 l / s) is reached quickly, then decreases linearly. Obstructive disorders result in reduced flow and indented expiratory curves; in the case of a restrictive ventilation disorder, the vital capacity is reduced
 
   The functional residual capacity is determined by means of indicator dilution (helium distribution or nitrogen washout method). Whole-body plethysmography also enables FRC to be determined (principle: [pressure x volume] constant). The volume of the plethysmograph is known, the pressure changes in it and in the airways when the breathing valve is closed are measured, FRC calculated). The residual volume is calculated from [FRC - ERV] (increases with restrictive ventilation disorders and with increasing age). Airway resistance is increased in obstructive ventilation disorders; it is calculated from the air flow (pneumotachograph) and alveolar pressure (whole-body plethysmography) (0.1-0.2 kPa per l / s). Whole-body plethysmography is also used to determine compliance values ​​(lungs: pleural vs. alveolar pressure, thorax: external vs. pleural pressure (esophageal probe), overall system: external vs. alveolar pressure (both with relaxed respiratory muscles). Restrictive ventilation disorders reduce the compliance of the respiratory system
 
   The tidal volume is ~ 0.6 l, the breathing rate ~ 14 / min, the residual volume 1.2-1.5 l, the compliance of the breathing apparatus in the resting position ~ 1 l / kPa. The breathing limit value (> 100 l / min) reflects the mechanical performance of the lung-thorax system. It is tested over 15 or 20 seconds and the value is extrapolated to one minute (hyperventilation → hypocapnia). The indirect breathing limit value is calculated from the vital capacity as [30 x VC]