Thoracoabdominal Motion

Rationale. The analysis of breathing movements gives some insight into the level of recruitment and function of the respiratory

Figure 1. Pressure-frequency relationships for the human diaphragm at various time points after a loaded breathing trial (i.e., a breathing trial in which an external resistive load was applied to increase the work done by the diaphragm and simulate the effects of lung disease). Diaphragm pressure generation was measured by determining the transdiaphragmatic pressure gradient (Pdi) generated in response to electrical stimulation of the phrenic nerves; Pdi is shown on the y axis and phrenic stimulation frequency is shown on the x axis. The time at which each set of curves was obtained is shown in the upper left-hand corner of each panel. For reference, each panel contains both a postload pressure-frequency curve obtained at the designated time (dashed line), and a preload pressure-frequency curve (solid line). At 2-4 minutes postloading, there was a large decrement in pressure generation in response to phrenic stimulation of 20-100 Hz (top left panel), indicating diaphragm fatigue. Over time, the pressures generated in response to high-frequency stimulation increased so that at 25-30 minutes postloading, pressure generation in response to 50- and 100-Hz stimulation had returned to preloading levels (bottom right panel). In contrast, however, response to 20Hz stimulation remained depressed 25-30 minutes after loading. In effect, both low- and high-frequency peripheral diaphragm fatigue was present 2-4 minutes after loading, with rapid resolution of the high-frequency component of fatigue and persistence of the low-frequency component at 25-30 minutes. Reprinted by permission from Reference 15.

muscles (in particular of the diaphragm, the rib cage inspiratory muscles, and the abdominal muscles). Two unusual patterns of muscle recruitment may be observed in healthy subjects subjected to fatiguing inspiratory loads (26). The first is an increased variability in compartmental contribution to tidal volume, with breaths characterized by clear rib cage predominance alternating with other breaths in which abdominal motion predominates. This pattern reflects alternatively predominant recruitment of the inspiratory rib cage muscles and of the diaphragm. Because fatigue may develop separately in the diaphragm and in the inspiratory rib cage muscles (27), such alternation may represent a way to postpone respiratory muscle failure. The second pattern is frank paradoxical movement of one compartment, generally the abdomen, that is, an inward movement of the abdominal wall during inspiration. Abdominal paradox indicates weak, absent, or inefficient contraction of the diaphragm. These two patterns may also be observed in patients showing signs of diaphragmatic fatigue during weaning trials from mechanical ventilation (2) (see Figure 5 in Section 6 of this Statement).

Methodology. Most anomalies of thoracoabdominal motion can be detected by visual inspection by a trained observer. This assessment is facilitated by placing the patient in a recumbent position and conducting a visual inspection for several minutes. Quantitative measurements of rib cage-abdominal motion can also be performed (see Estimation of Ventilation Based on Chest Wall Motion: Konno-Mead Diagram in Section 6 of this Statement).

Advantages. Visual inspection provides a simple bedside means of detecting alterations in respiratory muscle use.

Disadvantages. The abnormal patterns of thoracoabdomi-nal motion described above are not specific for respiratory muscle fatigue. indeed, respiratory alternans and abdominal paradox can appear immediately after the institution of loaded breathing and these abnormalities do not appear to become accentuated with the development of fatigue. Furthermore, these patterns can also occur, albeit to a lesser degree, during the application of low, nonfatiguing respiratory loads (28). Thus, abnormal thoracoabdominal motion should be viewed as reflecting an increased ventilatory load, which in itself may or may not induce respiratory muscle fatigue.

Applications. Analysis of thoracoabdominal motion is most useful to detect either specific forms of respiratory muscle dysfunction (e.g., diaphragmatic paresis) and/or an increase in the ventilatory load. This assessment is, therefore, of some routine clinical use, but lacks specificity for detecting respiratory muscle fatigue (see Breathing Pattern in Section 10 and Devices Used to Monitor Breathing: Pneumograph, Magnetometer, and Respiratory Inductive Plethysmograph in Section 6 of this Statement).

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Midsection Meltdown

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