Assessment Of The Properties Of The Relaxes Human Chest Wall Rahn Diagram

Scientific Batit

Pressure differences across viscoelastic, plastoelastic structures such as the lungs and chest wall depend on the structure's volume, volume history, and rate of change of volume. Accordingly, pressure differences across respiratory structures are often represented as characteristic pressure-volume (PV) curves (4). For the relaxed respiratory system, transpulmo-nary, transthoracic, and transrespiratory pressures are usually plotted against lung volume in a Rahn diagram (Figure 1). The PV characteristics shown in Figure 1 are of a relaxed subject slowly inflated or deflated by a pressure source at the airway opening. All the passive structures show an increase in volume with an increase in the pressure difference across them. When two pneumatic structures are in series, for example, the lung and the chest wall, the pressure difference across both structures (the transrespiratory pressure) is the sum of the pressure differences across each, and the volume displacements of the whole are equal to the volume displacements of each part. The PV curve can be locally described by the volume at a given pressure and the slope (compliance) at that point. The compliance of a passive structure at a given volume is the ratio of volume change to pressure change (i.e., the slope of the characteristic PV curve at that volume).

Mathodology

Lung volume displacements and pressures are measured as described in Sections 1 and 2 of this Statement. The following properties of the chest wall and lung are found by analysis of Rahn diagrams:

1. In trained subjects, the static PV characteristic of lung compliance is obtained from transpulmonary pressure (Pl = Pao — Pes) during an interrupted exhalation from total lung capacity (TLC) to residual volume (RV) with the glottis held open. The quasistatic deflation curve is similar, and is measured during a slow exhalation (expiring at flows less than 0.3 L/second). For subjects who cannot satisfactorily perform these maneuvers, Pl can be measured during intermittent airway occlusions, 2-4 seconds long, in an expiration from TLC to RV, or during interrupted deflation of the respiratory system with a supersyringe, valve, or other device.

2. The PV characteristic of the relaxed chest wall (Pcw) is obtained from esophageal pressure (Pes) during a slow relaxed exhalation through pursed lips or other high resistance from TLC to functional residual capacity (FRC) and during passive inhalation with relaxation against an intermittently occluded airway between RV and FRC. Alternatively, it can be measured during exhalation from TLC to RV with periodic airway occlusions with relaxation. Relaxation above FRC, however, may be difficult for untrained subjects, and relaxation below FRC can usually be achieved only by highly trained subjects. Normal compliance at FRC is approximately 0.2 L/cm H2O. 3. The PV characteristic of the relaxed respiratory system is obtained by plotting Pao versus Vl during the maneuvers described above (Prs = Pao — Pbs [body surface pressure]). Normal values are approximately 0.1 L/cm H2O at volumes of 40—60% of the vital capacity (VC).

Advantages

The Rahn diagram is useful for describing the elastic properties of passive systems. Each curve reflects the pressure difference developed by this structure for a range of volumes. Static compliance determined from these curves can be used for diagnosis. For example, the compliance of the lung is decreased in interstitial lung disease and increased in emphysema. Chest wall compliance is decreased in ankylosing spondylitis and obesity.

Disadvantages

Whereas the lungs' PV characteristic (i.e., the plot of elastic recoil pressure of the lung, PL,el, versus lung volume) is relatively easy to obtain in untrained subjects, the elastic recoil pressure of the chest wall (Pcw,el versus VL) is difficult to measure because it requires complete relaxation of the respiratory muscles at various lung volumes. Relaxation can be monitored via surface EMG of the chest wall. Failure to relax the respiratory muscles is also revealed when repeated measurements of passive PV curves of the chest wall are not identical. In subjects who can achieve relaxation at FRC but not at volumes above FRC, a relaxation curve can be approximated by extending a line from the relaxation point, assuming a normal chest wall compliance of 0.1 L/cm H2O. The chest wall compliance can also be estimated in untrained subjects by the weighted spirometer technique (5). These estimations may, however, be unreliable in hyperinflated patients (e.g., in chronic obstructive pulmonary disease) who never reach a true (static) relaxation volume even when they are relaxed during exhalation. This is a well-established test, although it is seldom used for clinical diagnosis.

Rahn diagram

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Chest Wall /

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Pcw = Pes - Pbs """*"/

A Prs = Pao - Pbs

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Pressure

Figure 1. The Rahn diagram shows the pressure-volume relationships of the relaxed chest wall (Pcw), lungs (Pl), and the total respiratory system (Prs). Ordinate: Lung volume expressed as a percentage of vital capacity (VC). Abscissa: Pressures, each defined by the respective equations. Horizontal dashed line: Relaxation volume of the respiratory system. Pes = esophageal pressure; Pbs = body surface pressure; Pao = airway opening pressure. Modified from Campbell EJM, Agostoni E, Davis JN, editors. The respiratory muscles: mechanics and neural control, 2nd ed. Philadelphia, PA: W.B. Saunders; 1970. p. 55.

Pressure

Figure 1. The Rahn diagram shows the pressure-volume relationships of the relaxed chest wall (Pcw), lungs (Pl), and the total respiratory system (Prs). Ordinate: Lung volume expressed as a percentage of vital capacity (VC). Abscissa: Pressures, each defined by the respective equations. Horizontal dashed line: Relaxation volume of the respiratory system. Pes = esophageal pressure; Pbs = body surface pressure; Pao = airway opening pressure. Modified from Campbell EJM, Agostoni E, Davis JN, editors. The respiratory muscles: mechanics and neural control, 2nd ed. Philadelphia, PA: W.B. Saunders; 1970. p. 55.

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