Assessment Of The Function Of The Active Chest Wall Campbell Diagram

Scientific Basis

To evaluate respiratory muscle action, a Campbell diagram, in which lung volume on the ordinate is plotted against pleural pressure on the abscissa, can be constructed. In this diagram, pleural pressure has differing significance depending on the maneuver. Consider a subject who is slowly inflated and deflated passively by a syringe connected to the airway while the respiratory muscles are relaxed (passive inflation; Figure 2). The pleural pressure, which is equal to transthoracic pressure, rises and falls, describing the characteristic PV curve for the relaxed chest wall, which is the same as that in the Rahn diagram (Figure 1). Alternatively, during active slow inhalation and exhalation with an open glottis, the pleural pressure (in this case equal to transpulmonary pressure with a negative sign) becomes more subatmospheric as the lungs inflate (active inflation; Figure 2), describing the lungs' characteristic PV curve, which appears as a mirror image of the lungs' curve in Figure 1. In the Campbell diagram the two curves intersect at relaxation volume at a pleural pressure of about —5 cm H2O. The intersection represents the equal and opposite elastic recoils of the lung and chest wall.

During inhalation, the pleural pressure is the pressure across the active chest wall. The pressure generated by the in-spiratory muscles is simply the pressure difference between the active chest wall characteristic and the relaxed chest wall characteristic. Work done by the inspiratory muscles,

J Pmus dv, is represented by the hatched area in Figure 2. The horizontally hatched area represents the work done to overcome elastic recoil of the lung and chest wall. Additional pressure is necessary to overcome airway resistance and lung tissue resistance; this work is shown with vertical hatching. Total work is therefore the sum of elastic and resistive work per inhalation, and is usually multiplied by breathing frequency and expressed as g ■ cm/ml. Work of breathing was found to average 2.2 ± 0.92 g ■ cm/ml at a respiration frequency of 15 breaths/minute (6), and was independent of age or sex. This is a test of great physiological interest and is widely used in research. It is seldom used for clinical evaluations. Reference 6 gives a full account of the complexities of how the work is done by the coordination of the various respiratory muscles.

Figure 3 shows a Campbell diagram with the addition of maximal static inspiratory pressure (MIP) and maximal expiratory pressure (MEP) during efforts against an occlusion (outer dashed lines). The MIP is greatest (most subatmo-spheric) at low lung volumes, whereas the MEP is greatest (most positive) at high lung volumes, largely because of the length-tension characteristics of the inspiratory and expiratory muscles. At high lung volume, the diaphragm and other inspiratory muscles are shorter, whereas expiratory muscles are longer (4). The inner dashed and dotted line in Figure 3 indicates the pleural pressure required to balance the elastic recoil of the lungs. TLC is at the intersection of these lines, where maximal inspiratory pleural pressure is balanced by the lungs' elastic recoil. The innermost loop represents resting breathing at FRC, as is shown for inspiration only in Figure 3.

The pressure-volume relationship during a maximal forced inspiration and expiration are shown as the inner solid line loop in Figure 3. Pressures at every volume are reduced from maximal static pressures because the muscle is shortening. The loss of maximal inspiratory pressure (the difference in pressure between the dashed and solid lines at a given lung volume in

Campbell Diagram

Figure 2. Campbell diagram. Graphical analysis of the work done during a breathing cycle by the inspiratory muscles. Vertical hatching: Work done to overcome flow resistance of the lungs. Horizontal hatching: Work done to overcome elastance of the lungs and chest wall. Modified by permission from Macklem PT, Mead J, editors. Handbook of physiology. Vol. 3: The respiratory system, Part 3. Bethesda, MD: American Physiological Society; 1986. p. 495.

Figure 2. Campbell diagram. Graphical analysis of the work done during a breathing cycle by the inspiratory muscles. Vertical hatching: Work done to overcome flow resistance of the lungs. Horizontal hatching: Work done to overcome elastance of the lungs and chest wall. Modified by permission from Macklem PT, Mead J, editors. Handbook of physiology. Vol. 3: The respiratory system, Part 3. Bethesda, MD: American Physiological Society; 1986. p. 495.

Figure 3) was estimated to be approximately 7%/L/second of flow at a volume of FRC + 1 L (7). The muscle pressure at this volume represents the maximal capacity (Pcap) of the inspira-tory muscle to generate pressure while shortening maximally. During submaximal exercise, for example, healthy subjects may require esophageal pressures in the range of —30 cm H2O to ventilate the lungs, or approximately 40% of Pcap. When pressures in that range are achieved while breathing for several minutes with a duty cycle of 0.5, muscle fatigue may result (see Section 5 of this Statement). At higher levels of ventilation (maximal voluntary ventilation), at which peak flow can reach up to 10 L/second, there is a decrease in maximal inspiratory muscle pressure within 15-20 seconds, attributed to fatigue.

Equipment

Measurements of pressure and volume are described in Sections 1 and 2 of this Statement.

Applications

The following properties of the chest wall and lung are found by analysis of Campbell diagrams:

1. The quasistatic (or static) PV characteristic of the lung is obtained from esophageal pressure (Pes = — PL,el) during a slow (or halting) inhalation and exhalation with the glottis held open.

2. The PV characteristic of the relaxed chest wall is described above as in the Rahn diagram.

3. Pressure generated by respiratory muscles (Pmus) is the horizontal distance (i.e., change in pressure) between the relaxation characteristic of the passive chest wall and the active one.

4. The Campbell diagram is also used to depict the maximal static inspiratory pressure (MIP) and maximal static expiratory pressure (MEP) measured with the airway occluded. The values of MIP in healthy young males are shown as the dashed outer loop in Figure 3. Section 2 of this Statement discusses the technique used to perform the MIP test and gives normal values in health and disease.

Advantages

The Campbell diagram is a convenient tool for calculating the elastic and resistive work of inspiratory and expiratory mus-

-f Y

1

\ MEP('

1

I

Vvc

: \ -1 MIP \ 1

1

Figure 3. Campbell diagram with reduced scale to show maximal static inspiratory pressures (MIP) and maximal expiratory pressures (MEP) at various lung volumes (dashed lines), pressures during maximal dynamic inspiration and expiration in a forced vital capacity (FVC) maneuver (outermost loop), and spontaneous breathing at rest (innermost loop).

cles. The relationship between work and oxygen consumption makes it possible to calculate the efficiency of respiratory muscles (see Figure 3 in Section 4 of this Statement). The difference between maximal static inspiratory pressures and peak actual pleural pressure measured during breathing indicates the muscle force reserve, an index that helps assess the likelihood of fatigue. This is a well-established method, often used in clinical research.

Disadvantages

To infer respiratory muscle action, the pleural pressure must be referred to the PV curve of the "relaxed" chest wall as in the example above. For example, a given positive pleural pressure may reflect either expiratory muscle activity at FRC or slight inspiratory muscle activity at a high lung volume. Furthermore, difficulties in measuring the relaxation characteristic in untrained subjects can make estimates of Pmus uncertain. Although the Campbell diagram is easy to plot, its interpretation requires some practice.

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