Estimation Of Ventilation Based On Chest Wall Motion Konnomead Diagram

Scientific Basis

Quantitative measurements of respiratory system motion are usually based on measurements of lung volume and displacements of chest wall structures, including the abdominal wall. Because the tissues of the chest wall are essentially incompressible, volume changes (displacements) of the chest wall surfaces are nearly equal to volume changes of the lungs and can be used to "noninvasively" estimate lung volume, without the need for a mouthpiece, mask, or other connection to the airway. (Changes in intrathoracic blood volume cause differences between chest wall and lung volume displacements, but these are usually negligible.) The relevance of noninvasive measurements of ventilation is shown in Breathing Patterns in Section 10 of this Statement.

During quiet breathing in a subject at rest, the surface of the chest wall moves in a predictable way as lung volume increases and decreases. Various dimensions of the chest wall have been measured to estimate changes in chest wall (or lung) volume. The "chest pneumogram" is a record of changes in thoracic circumference that provides a qualitative measure of ventilation. However, because the major compartments of the chest wall, the rib cage and abdomen, move independently in most conscious subjects, measuring overall chest wall displacement accurately usually requires two or more simultaneous measurements of displacement.

Konno and Mead (8) established that chest wall volume change could be assessed by measuring displacements of rib cage and abdominal surfaces. In their subjects, who were standing still, a single anteroposterior diameter sufficed to indicate motion of the rib cage; this was also true of the abdomen. Thus, the chest wall can be described as having two principal degrees of freedom of motion: Overall chest wall displacements can be specified by knowing the rib cage displacement and the abdominal displacement. The two-compartment model of the chest wall introduced by Konno and Mead and the plot of rib cage displacement versus abdominal displacement (the Konno-Mead diagram) have been used in numerous studies.

Figure 4 shows a Konno-Mead diagram of a subject breathing quietly and performing an "isovolume maneuver." During quiet inspiration, the rib cage and abdomen move out synchronously, following the rib cage-abdomen relaxation characteristic. In the isovolume maneuver, the subject voluntarily shifts volume between rib cage and abdominal compartments by contracting and relaxing abdominal muscles with the glottis closed. Because lung volume is constant, the decrease in abdominal volume (i.e., the volume displaced by inward movement of the abdominal wall) must be equal to the increase in rib cage volume; two isovolume maneuvers performed at known lung volumes allow calibration of rib cage and abdominal displacements in terms of lung volume change.

Most methods of measurement of thoracoabdominal displacement require that the subject maintain a constant posture, as spinal flexion affects the relationships among lung volume, rib cage, and abdominal displacements (9). Therefore, postural change should, as a rule, be minimized. However, it is possible to measure spinal flexion in addition to rib cage and abdominal displacements to estimate lung volume in subjects whose posture changes (10). This method is widely used in clinical research to assess tidal volume and the relative displacement of rib cage-diaphragm.

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