Vena Cava Responsiveness to Controlled Isovolumetric Respiratory Efforts

Objectives—Respirophasic variation of inferior vena cava (IVC) size is affected by large variability with spontaneous breathing. This study aims at characterizing the dependence of IVC size on controlled changes in intrathoracic pressure. Methods—Ten healthy subjects, in supine position, performed controlled isovolumetric respiratory efforts at functional residual capacity, attaining positive (5, 10, and 15mmHg) and negative (25, 210, and 215 mmHg) alveolar pressure levels. The isovolumetric constraint implies that equivalent changes are exhibited by alveolar and intrathoracic pressures during respiratory tasks. Results—The IVC cross-sectional area equal to 2.8860.43 cm2 at baseline (alveolar pressure50 mmHg) was progressively decreased by both expiratory and inspiratory efforts of increasing strength, with diaphragmatic efforts producing larger effects than thoracic ones: 255615% decrease, at 115 mmHg of alveolar pressure (P<.01), 280633612% at 215 mmHg diaphragmatic (P<.01), 233612% at 215 mmHg thoracic. Significant IVC changes in size (P<.01) and pulsatility (P<.05), along with non significant reduction in the response to respiratory efforts, were also observed during the first 30 minutes of supine rest, detecting an increase in vascular filling, and taking place after switching from the standing to the supine position. Conclusions—This study quantified the dependence of the IVC cross-sectional area on controlled intrathoracic pressure changes and evidenced the stronger influence of diaphragmatic over thoracic activity. Individual variability in thoracic/diaphragmatic respiratory pattern should be considered in the interpretation of the respirophasic modulations of IVC size.