How is diaphragm strength assessed by spirometry testing?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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Evaluation of diaphragm strength can be accomplished by measuring the vital capacity in an upright or sitting position followed by a measurement made in the supine position. A reduction in the vital capacity to less than 90% of the upright vital capacity suggests diaphragm weakness or paralysis. Interpreting an increased reduction in vital capacity in the supine position as diaphragm dysfunction should be made cautiously if the patient's body mass index is greater than 45 kg/m2. [4] Studies reporting the normal reduction of the vital capacity of less than 10% from upright to supine were conducted with individuals who were not obese. Slightly greater reductions in obese individuals in a supine position may not indicate diaphragm dysfunction, but rather an increase in the resistive forces against which the diaphragm descends. Reductions in the supine vital capacity more than 20% of baseline indicate hemidiaphragm or diaphragm dysfunction or paralysis.

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