REGIONAL INTRATHORACIC PRESSURES AND THEIR BEARING ON CALCULATION OF EFFECTIVE VENOUS PRESSURES

Abstract

This investigation was concerned with the natural pressures which exist in various regions of the chest; with the forces concerned in producing regional pressure differences, and with the most favorable region for recording pressures which enable one to follow trends of effective venous pressure. Technical errors introduced by various procedures employed to measure intrathoracic pressures such as creation of air pockets or introduction of balloons are discussed. Intrathoracic pressures were recorded by optical capsules from 8 separate regions of the chest by a method which requires introduction of very minimal quantities of air into mechanically created pockets. Records of intra-pleural pressure from the upper regions of the left side and lower regions of the right side of the thorax consist of smooth curves of sub-atmospheric pressure which decrease further during inspiration. Changes in these pressures are occasioned by modifications in tonus and contraction of respiratory muscles, blood content of the chest, and elasticity of lung tissue. Records from artificial pockets around the right heart and in the left lower pleural spaces adjacent to the apex of the heart show superposition of conspicuous cardiac variations on respiratory variations. It is suggested that these super-added variations rather than distortion of lung tissue by the heart and mediastinum are responsible for the existence of somewhat higher pressures around the heart than in the pleural cavities. Special methods are needed to compare variable pressures around the heart with constant pressures found in some pleural areas. Comparison of instantaneous pressures at the end of a selected diastole or so-called Z pressure is an adequate and simple procedure which is easily related to atrial pressures as well. Since Z pressures recorded from the right lower thoracic cavity are only about 10 mm. H2O higher than those derived around the right heart and change directionally with them, records from the former may be used in calculating trends of effective venous pressure. Intrathoracic pressures recorded from the upper regions of the chest, from the whole left side or regions above the diaphragm and from the mediastinum, are unreliable for one reason or another.