Inferring Respiratory Muscle Contribution To Breathing From Chest Wall Motion

Scientific Basis

Thoracoabdominal displacements such as those displayed on a Konno-Mead diagram demonstrate the action, dysfunction, or paralysis of specific respiratory muscles. For example, with diaphragmatic paralysis there may be "paradoxical" inward motion of the abdominal wall during early inhalation (Figure 4, mainly rib cage). As the active rib cage expands, pleural pressure decreases, and without diaphragmatic contraction pleural pressure is transmitted to the abdominal compartment, where it causes a passive inward movement of the abdominal wall. Likewise, rib cage muscle paralysis causes characteristic paradoxical movement of the rib cage during inhalation.

Patients with lung disease often have abnormal chest wall motions, as well. In patients with chronic obstructive pulmonary disease, alternation between rib cage breathing and abdominal breathing ("respiratory alternans") and paradoxical motion of the abdomen during inspiration can herald inspira-tory muscle fatigue and impending respiratory failure (19). This is not, however, pathognomonic of fatigue, because it can be elicited voluntarily. Furthermore, abdominal pain can trigger such patterns after upper abdominal surgery.

The Konno-Mead diagram is only one possible motionmotion diagram of the chest wall, and other plots such as an-teroposterior-transverse diameter plots can give additional information about specific respiratory muscle use and describe distortions of rib cage shape that occur with contraction of specific respiratory muscles. For example, inspiratory efforts made principally with the cervical accessory muscles of inspiration can cause the lower rib cage cross-section to become more elliptical (19). Compared with normal breathing, contraction of the diaphragm by itself causes predominance of abdominal wall over rib cage displacement, increased ellipticity of the lower rib cage, and predominance of lower over upper rib cage anteroposterior displacement (16).

Advantages

Chest wall motion is easy to measure and provides information about respiratory muscle activity.

Disadvantages

Motion-motion plots such as Konno-Mead diagrams, when used by themselves to infer specific muscle action, can be ambiguous, because a given motion may be produced by several different muscular actions. For example, paradoxical motion of the abdominal wall in inspiration is not necessarily an indication of diaphragm paralysis because of the influence of the zone of apposition (20) on abdominal displacements (see subsequent passages). When the rib cage expands in a normal inhalation, the outward displacement of the lower rib cage, which is part of the abdominal container, tends to lower abdominal pressure and draw the abdominal wall inward even when the diaphragm is shortening normally (21) Therefore, small paradoxical inward motions of the abdominal wall during inhalation do not necessarily indicate diaphragmatic paralysis (Figure 4). Conversely, diaphragm paralysis may be missed if one uses only motions to infer muscle action. The paradoxical in ward motion of the abdominal wall expected during inhalation with a paralyzed diaphragm may be abolished if inspiration occurs with simultaneous relaxation of abdominal muscles that were activated during expiration (22). Unambiguous evidence of muscle action is best achieved by combining displacement and pressure measurements.

Actions of specific rib cage muscles are difficult to infer from rib cage motions, because numerous muscles have specific actions at numerous sites on the rib cage. Furthermore, the simultaneous activity of inspiratory and expiratory muscles can obscure effects of individual muscles. Therefore, assessing the actions of specific rib cage muscles usually involves electromyography and measurements of several rib cage dimensions and pressures (see Inferring Respiratory Muscle Contribution to Breathing from Pressure-Volume Relationships and Inferring Diaphragm Activation and Electromechanical Effectiveness from EMG).

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