Inferring Respiratory Muscle Contribution To Breathing From Pressurevolume Relationships

Scientific Basis

Pressure-volume measurements of rib cage and abdominal compartments can provide the basis for mechanical analysis of the diaphragm, rib cage, and abdominal muscles, allowing inferences about which muscles contribute to a particular breath or respiratory maneuver. Whereas the Campbell diagram can be used to infer inspiratory and expiratory activity of all the respiratory muscles, other PV diagrams can be used to infer action of specific respiratory muscles. Pressure-volume diagrams of rib cage and abdomen were introduced by Konno and Mead (24), who showed their respective relaxation characteristics. By comparing pressure-volume data during breathing with those obtained during relaxation, these diagrams can be used to infer the action of specific respiratory muscles when pressure data alone or volume data alone could be misleading. An example of the pressure-volume diagram for the abdomen is shown in Figure 6. Contraction of the abdominal muscles tends to displace the abdominal wall inward and increase abdominal pressure. Therefore, a movement down and to the right of the relaxation line in Figure 6 reliably indicates abdominal muscle contraction. similarly, diaphragm contraction (or paralysis) can be inferred from deviations of the diaphragm's PV data in relation to its relaxation characteristic. If the diaphragm shortens or remains relatively isometric and transdiaphragmatic pressure increases, the diaphragm must be actively contracting. For examples of the use of such plots, see Goldman and coworkers (25) and Mead and coworkers (26).

The rib cage presents a challenge in pressure-volume analysis because no single pressure difference characterizes the pressures acting on the whole structure. The internal pressure applied to the lung-apposed surface of the rib cage can be characterized by pleural (esophageal) pressure, but pressure in the caudal rib cage apposed to abdominal viscera and diaphragm (the "zone of apposition") is closer to abdominal (gastric) pressure than pleural pressure. in addition, the diaphragm itself can have an inspiratory effect on the lower rib cage when it contracts. Therefore, the pressure acting on or contributed by the rib cage is generally neither pleural nor abdominal pressure but some combination of the two (27). This complication can be handled by more elaborate analysis of chest wall motion and pressures (28).

Advantages

PV data can provide unambiguous evidence of diaphragm and abdominal muscle activity and, with careful analysis, evidence of rib cage muscle inspiratory and expiratory action.

Disadvantages inferring contributions of specific rib cage muscles generally requires the addition of electromyographic data and a more complex analysis. The method remains, however, a powerful research tool, which can be perfected.

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