Pressures In The Chest Wall

In respiratory mechanics, it is important to distinguish between the two uses of the word "pressure." In one case it denotes a pressure measured at a given location, as in "pleural pressure." In the other case it denotes a difference in pressure between two points, usually on opposite sides of a structure, such as "transpulmonary pressure," defined as the difference between pressure at the airway opening (Pao) and pressure in the pleural space (Ppl). Pressures are usually measured relative to barometric pressure (i.e., they are taken to be zero when they are equal to barometric pressure).

Pressures at a point are usually assumed to be representative of the pressure in that space (see Figure 1 in Section 2 of this Statement). This simplification must be qualified when variations of pressure within a space are to be expected (1). In particular, gravity causes vertical gradients in pressure related to the density of the semisolid or liquid contents of a space: in the thorax, this gradient is approximately 0.2 cm H2O/cm height and is affected by lung density; in the abdomen, the gradient is nearly 1 cm H2O/cm height. Temporal fluctuations in pressure, as in tidal breathing, are little affected by gravitational gradients. Shear stress resulting from the deformation of elastic, shape-stable organs can cause local variations in pressure, such as those that occur just below the diaphragm when it displaces the liver during a large forceful contraction (2).

Pressure differences across structures, as opposed to pressures measured at a point, are relevant for characterizing those structures. The schematic drawing in Figure 1 of Section 1 of this Statement shows relationships among locations where pressures can be measured (within circles) and intervening respiratory structures and equipment (within rectangles). Pleural and abdominal pressures are usually estimated by measuring esophageal and gastric pressures (Pes and Pga), respectively. Table 1 and Figure 1 in Section 2 of this Statement list pressures measured at a point and pressure differences across structures. These differences are usually taken in a direction such that positive pressure differences expand the structure (e.g., lung). An exception to this rule is transdiaphragmatic pressure (Pdi), which has been defined both as pleural pressure minus abdominal pressure, Pdi = Ppl — Pab, and as its reverse, Pdi = Pab — Ppl. The complicating effects of gravity must be considered when pleural pressure is estimated from esophageal pressure (Pes) and abdominal pressure is estimated from gastric pressure (Pga). When the diaphragm itself is completely relaxed and the actual pressure difference across the diaphragm is nil, the measured transdiaphragmatic pressure has a minimum value, usually approximately 10 cm H2O, which is attributed mostly to the gravitational hydrostatic difference between esophageal and gastric pressures. This hydrostatic transdiaphragmatic pressure, which changes only slightly with breathing (3)), is usually subtracted from reported measurements of Pdi.

A pressure difference between two points may characterize two or more different structures or groups of structures. For example, the pressure difference between the pleural space and the body surface in a spontaneously breathing person is both the transthoracic (transchest wall pressure, Pcw) and the negative of transpulmonary pressure (—Pl).

0 0

Post a comment