Involuntary breath-stacking: An alternative method for vital capacity estimation in poorly cooperative subjects

Performed correctly, the vital capacity (VC) is a useful indicator of the mechanical properties of the thorax and of neuromuscular performance. Unfortunately, its use is often limited by impaired comprehension, altered mental status, or inability to sustain forceful effort. Our purpose was to develop a measure of VC independent of subject cooperation. We estimated the subcomponents of VC (inspiratory capacity (IC) and expiratory reserve volume (ERV)), using one-way valving of an external circuit to enforce cumulation or elimination of the tidal breaths stimulated by endogenous ventilatory drive. When configured to measure IC, gas entered the chest incrementally, until tidal effort became insufficient to overcome thoracic recoil. Valve rearrangement permitted analogous estimation of ERV. We tested the validity of this method in cooperative but naive subjects by comparing the VC measured in standard fashion (VC(C)) to the breath-stacked estimate (VC(S)). Thirty normal subjects and 20 ambulatory patients with diverse causes for respiratory impairment were studied. Peak and mean values of VC(S) correlated strongly with the corresponding values of VC(C) (r ≥ 0.91). The coefficient of variation for sequential VC(S) determinations (≃5.5%) was comparable to that observed for VC(C) (≃3.5%) in both subject groups, indicating acceptable reproducibility of the involuntary VC(S) measurement. VC(S) maneuvers were quickly completed and well tolerated. Involuntary breath-stacking may provide a useful estimate of VC in clinical settings where conventional methodology cannot be confidently applied.