Impact of Respiratory Mechanics on Venous Return in Single Ventricle Physiology

Figure 1. Study Overview We used real-time magnetic resonance imaging to evaluate the impact of respiratory effort and rate on flow reversal in the Fontan IVC. IVC = inferior vena cava; PA = pulmonary artery; SVC = superior vena cava.

Figure 2. Breathing Protocol The protocol is subdivided into 5 sequential breathing patterns: baseline, shallow, deep, slow, and fast. Although the first 3 breathing patterns (baseline, shallow, deep) account for different respiratory efforts at a constant respiratory rate, the remaining 2 breathing patterns (slow and fast) account for different respiratory rates at a constant respiratory effort. MRI = magnetic resonance imaging.

Figure 3. Imaging Plane As verified by the (A) coronal, (B) sagittal, and (C) axial views, the imaging plane (blue) of the magnetic resonance imaging was positioned perpendicular to the inferior vena cava (IVC), 20 mm above the entrance of the hepatic veins. (D) The IVC (magenta) and aorta (yellow) were identified in the same imaging plane (blue).

Figure 4. Impact of Respiratory Effort on the Fontan Inferior Vena Cava (IVC) Flow Normalized retrograde flow ratio decreases with shallow (P < 0.001) breathing and increases with deep breathing (P < 0.001) (A), whereas absolute values representing this change in reverse flow ratio are shown in (B). Compared to baseline, both forward (P = 0.007) and retrograde (P < 0.001) flow volume decrease with shallow breathing and increase (forward P = 0.011, retrograde P < 0.001) with deep breathing (C). Augmenting the respiratory effort also increases the pulsatility index in the Fontan IVC (shallow P < 0.001, deep P < 0.001) (D). Data are presented as median with interquartile range; dots represent individual participants. Overall differences across breathing conditions were assessed using the Friedman test, followed by post hoc Wilcoxon signed-rank tests vs baseline with Holm correction for multiple comparisons. Reported P-values reflect Holm-adjusted post hoc comparisons.

Figure 5. Impact of Respiratory Rate on the Fontan Inferior Vena Cava (IVC) Flow Normalized retrograde flow ratio decreases with slow (P = 0.006) breathing and increases with faster breathing (P < 0.001) (A), whereas absolute values of the reverse flow ratio are shown in (B). Compared to baseline, forward flow volume increases with slow breathing (P < 0.001) and decreases with fast breathing (P < 0.001), whereas the reverse flow volume remains stable during slow breathing (P = 0.45) and increases slightly during fast breathing (P < 0.001) (C). Increasing the respiratory rate slightly increases the pulsatility index in the Fontan IVC (slow P = 0.001, fast P < 0.001) (D). Data are presented as median with interquartile range; dots represent individual participants. Overall differences across breathing conditions were assessed using the Friedman test, followed by post hoc Wilcoxon signed-rank tests vs baseline with Holm correction for multiple comparisons. Reported P-values reflect Holm-adjusted post hoc comparisons.

Figure 6. Effect of Baseline Inferior Vena Cava (IVC) Flow Increased baseline IVC flow was associated with less retrograde flow in the Fontan IVC (Spearman's ρ = −0.59; P = 0.004) (A). Concurrently, baseline IVC flow was associated with a lower pulsatility index in the Fontan IVC (Spearman's ρ = −0.51; P = 0.015) (B).