Inferior vena cava revisited - Real-time flow MRI of respiratory maneuvers.

Recent MRI studies of blood flow in the inferior vena cava (IVC) resulted in findings which are inconsistent with earlier observations by invasive procedures - most likely because ECG-gated MRI techniques are unable to resolve dynamic adjustments due to respiration. The purpose of this work was to apply real-time phase-contrast MRI at 50 ms resolution to re-evaluate IVC flow in response to normal and deep breathing as well as breath holding and Valsalva maneuver (11 young healthy subjects). Real-time flow MRI relied on highly undersampled radial gradient-echo sequences and a model-based nonlinear inverse reconstruction. A frequency analysis of the predominant pulsatility classified IVC flow in individual subjects as "cardiac", "respiratory" or "mixed" type. Peak flow velocities during free breathing ranged from 30 to 58 cm s-1 , while flow rates varied from 15 to 37 ml s-1 . The subject-specific IVC flow pattern persists during deep breathing although the enhanced respiratory influence may shift subjects form "cardiac" to "mixed" or from "mixed" to "respiratory" type. Peak velocities increased relative to normal breathing but led to similar flow rates of 16 to 34 ml s-1 . Inspiration during deep breathing elicited brief periods of flow reversal in all subjects with mean peak velocities of -21 cm s-1 . The observation of only mildly flattened parabolic velocity distributions within the IVC indicated mostly laminar flow. Breath holding reduced blood flow velocities and rates by more than 40% on average, while Valsalva maneuvers completely abolished venous return. In conclusion, IVC blood flow is dominated by the acquired respiratory behavior of individual subjects and its pressure-induced alterations relative to cardiac pulsation. The responses to breath holding and Valsalva maneuver are in full agreement with previous invasive observations of reduced or even ceased flow, respectively.