Influence of lung volume, fluid and capillary recruitment during positional changes and exercise on thoracic impedance in heart failure

• Intrathoracic impedance increased with lung volume during slow vital capacity maneuvers.
• Positional changes (upright to supine) resulted in increased impedance, while pulmonary blood volume increased, and an index of extravascular lung water and lung volume decreased.
• During exercise, lung volume and pulmonary blood volume increased, and an index of extravascular lung water decreased. However, impedance was not altered significantly.

It is unclear how dynamic changes in pulmonary–capillary blood volume (Vc), alveolar lung volume (derived from end-inspiratory lung volume, EILV) and interstitial fluid (ratio of alveolar capillary membrane conductance and pulmonary capillary blood volume, Dm/Vc) influence lung impedance (ZT). The purpose of this study was to investigate if positional change and exercise result in increased EILV, Vc and/or lung interstitial fluid, and if ZT tracks these variables.Methods12 heart failure (HF) patients underwent measurements (ZT, EILV, Vc/Dm) at rest in the upright and supine positions, during exercise and into recovery. Inspiratory capacity was obtained to provide consistent measures of EILV while assessing ZT.ResultsZT increased with lung volume during slow vital capacity maneuvers (p < 0.05). Positional change (upright → supine) resulted in an increased ZT (p < 0.01), while Vc increased and EILV and Dm/Vc decreased (p < 0.05). Moreover, during exercise Vc and EILV increased and Dm/Vc decreased (p < 0.05), whereas, ZT did not change significantly (p > 0.05).ConclusionImpedance appears sensitive to changes in lung volume and body position which appear to generally overwhelm small acute changes in lung fluid when assed dynamically at rest or during exercise.