Predictive value of positional change in vital capacity to identify diaphragm dysfunction

Area of Science:

• Respiratory Medicine
• Pulmonary Physiology
• Diagnostic Imaging

Background:

• Diaphragm dysfunction (DD) diagnosis can be aided by measuring the sitting-to-supine fall in vital capacity (ΔVC).
• The optimal threshold for ΔVC in identifying unilateral or bilateral DD remains uncertain.
• Ultrasound assessment of diaphragm thickening fraction (TFdi) is a key diagnostic tool for DD.

Purpose of the Study:

• To evaluate the diagnostic performance of ΔVC in identifying unilateral and bilateral diaphragm dysfunction.
• To determine the optimal predictive threshold value of ΔVC for DD.

Main Methods:

• Patients referred to a diaphragm dysfunction clinic underwent lung function testing (including ΔVC) and ultrasound (TFdi).
• Unilateral DD was defined as TFdi ≤30% in one hemidiaphragm; bilateral DD as mean TFdi ≤30% in both.
• Sensitivity and specificity analyses were performed for ΔVC thresholds to identify DD.

Main Results:

• 84 patients were analyzed; 31 had unilateral DD, 17 bilateral DD, and 36 no significant DD.
• Bilateral DD patients showed significantly lower FVC, FEV1, MIP, TLC, ΔVC, and higher orthopnea compared to unilateral DD.
• A ΔVC threshold of ≤-15% accurately identified bilateral DD (AUC 0.97, sensitivity 100%, specificity 89%).
• ΔVC showed poor performance in predicting unilateral DD (AUC 0.58).

Conclusions:

• A ΔVC value of ≤-15% is a strong indicator of bilateral diaphragm dysfunction.
• ΔVC is not a reliable predictor for unilateral diaphragm dysfunction.
• Clinical interpretation of ΔVC for suspected DD should consider these findings, particularly differentiating unilateral from bilateral cases.