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

Abstract

Rationale: Sitting-to-supine fall in vital capacity (∆VC) can be used to help identify diaphragm dysfunction (DD), but its optimal predictive threshold value is uncertain. Our aim was to evaluate the diagnostic performance of ∆VC in identifying the presence of unilateral or bilateral DD.

Methods: Patients referred to the diaphragm dysfunction clinic of our center (2017-2018) were included. All subjects had lung function testing (including measurement of ∆VC) and an ultrasound assessment of diaphragm thickening fraction (TFdi). Unilateral DD was defined as a single hemidiaphragm with TFdi ≤30% and bilateral DD as a mean TFdi value of both hemidiaphragms ≤30%. Clinical and physiological characteristics were compared across groups, and sensitivity/specificity analyses of ∆VC to identify DD were performed.

Results: 84 patients were included (31 unilateral DD, 17 bilateral DD and 36 without significant DD). DD groups had similar age, gender and BMI (all p>0.05), but patients with bilateral DD had lower FVC, FEV1, MIP, TLC, ∆VC and more frequent orthopnea than patients with unilateral DD (all p<0.05). There was a significant correlation between TFdi and ∆VC (rho=-0.56, p<0.001). The optimal ∆VC value to identify bilateral DD was ≤-14% [AUC 0.97 (95%CI 0.93-1.00), p<0.001, with sensitivity and specificity of 100% and 86%, respectively]. No single threshold of ∆VC could accurately predict unilateral DD [AUC 0.57 (95%CI 0.43-0.71), p=0.35].

Conclusion: ∆VC performs poorly in identifying patients with unilateral DD. However, a ∆VC value ≤-14% is strongly associated with the presence of bilateral DD. These findings should be taken into account when using ∆VC in the evaluation of patients with suspected DD.