Mechanical Vent Test 4 Practice

During a spontaneous breath, at which part of the pressure-time curve is transdiaphragmatic pressure greatest?

End-expiration Mid-inspiration Peak inspiration During expiration

Correct Answer:

Mid-inspiration

Explanation:

Transdiaphragmatic pressure reflects how hard the diaphragm is working, calculated as the difference between abdominal pressure and pleural (intrapleural) pressure. When you take a spontaneous breath, the diaphragm contracts, pushing the abdominal contents downward (increasing abdominal pressure) and making the pleural space more negative (decreasing pleural pressure). The largest difference between these two pressures occurs when the diaphragm is contracting most vigorously, which is during mid-inspiration. As inspiration continues toward its end, the contraction eases off and the pressures begin to change, so the transdiaphragmatic pressure falls. End-expiration and expiration involve little to no diaphragmatic contraction, so Pdi remains small.

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