Noninvasive quantification of systemic-to-pulmonary collateral flow: a major source of inefficiency in patients with superior cavopulmonary connections

Abstract

Background: Systemic-to-pulmonary collateral flow (SPCF) is common in single-ventricle patients with superior cavopulmonary connections (SCPC). Because no validated method to quantify that SPCF exists, neither its hemodynamic burden nor its clinical impact can be systematically evaluated. We hypothesize that (1) the difference in total ascending aortic (Ao) and caval flow (superior vena cava [SVC]+inferior vena cava [IVC]) and (2) the difference between pulmonary vein and pulmonary artery flow (PV-PA) provide 2 independent estimators of SPCF.

Methods and results: We measured Ao, SVC, IVC, right (RPA) and left (LPA) PA, and left (LPV) and right (RPV) PV flows in 17 patients with SCPC during routine cardiac MRI studies using through-plane phase-contrast velocity mapping. Two independent measures of SPCF were obtained: model 1, Ao-(SVC+IVC); and model 2, (LPV-LPA)+(RPV-RPA). Values were normalized to body surface area, Ao, and PV, and comparisons were made using linear regression and Bland-Altman analysis. SPCF ranged from 0.2 to 1.4 L/min for model 1 and 0.2 to 1.6 L/min for model 2, for an average indexed SPCF of 0.5 to 2.8 L/min/m(2): 11% to 53% (mean, 37%) of Ao and 19% to 77% (mean, 54%) of PV. The mean difference between model 1 and model 2 was 0.01 L/min (P=0.40; 2-SD range, -0.45 to 0.47 L/min).

Conclusions: We present a noninvasive method for SPCF quantification in patients with SCPC. It should provide an important clinical tool in treating these patients. Furthermore, we show that SPCF is a significant hemodynamic burden in many patients with bidirectional Glenn shunt physiology. Future investigations will allow objective study of the impact of collateral flow on outcome.

Figure 1

Top: Schematic showing the location of the phase contrast velocity used to calculate collateral flow. The yellow bars represent the location of the velocity maps. Note that often the left pulmonary veins could be obtained together as shown, but other times were acquired separately. SVC and IVC are superior and inferior vena cava, RPA and LPA (shown in both left panels) are right and left pulmonary artery, RUPV, RLPV, LUPV, and LLPV are the right and left upper and lower pulmonary veins, Ao is ascending aorta. Bottom: example bright blood images with the velocity mapping plane locations. Bottom right: example of resulting velocity maps of the aorta and left pulmonary vein.

Figure 1

Top: Schematic showing the location of the phase contrast velocity used to calculate collateral flow. The yellow bars represent the location of the velocity maps. Note that often the left pulmonary veins could be obtained together as shown, but other times were acquired separately. SVC and IVC are superior and inferior vena cava, RPA and LPA (shown in both left panels) are right and left pulmonary artery, RUPV, RLPV, LUPV, and LLPV are the right and left upper and lower pulmonary veins, Ao is ascending aorta. Bottom: example bright blood images with the velocity mapping plane locations. Bottom right: example of resulting velocity maps of the aorta and left pulmonary vein.

Figure 2

Top: Q_coll-pulm vs. Q_coll-syst demonstrating excellent correlation between the two methods of estimating systemic to pulmonary collateral flow. Bottom: Bland-Altman plot of the difference between the two systemic to pulmonary collateral flow estimators (Q_coll-syst − Q_coll-pulm) vs. the average of the two estimators (both in L/min). The thin line represents the mean difference between the estimators and the thick lines represent two standard deviations around the mean difference.

Figure 2

Top: Q_coll-pulm vs. Q_coll-syst demonstrating excellent correlation between the two methods of estimating systemic to pulmonary collateral flow. Bottom: Bland-Altman plot of the difference between the two systemic to pulmonary collateral flow estimators (Q_coll-syst − Q_coll-pulm) vs. the average of the two estimators (both in L/min). The thin line represents the mean difference between the estimators and the thick lines represent two standard deviations around the mean difference.

Figure 3

Venous return to the heart (measured total pulmonary vein plus IVC flow) vs. measured aortic output for SCPC pts, demonstrating excellent agreement. Bottom: Bland-Altman plot for SCPC pts of the difference between the measured venous return to the heart and the measured aortic output vs. the average of the venous return and aortic output (both in L/min). The thin line represents the mean difference between the estimators and the thick lines represent two standard deviations around the mean difference.

Figure 3

Venous return to the heart (measured total pulmonary vein plus IVC flow) vs. measured aortic output for SCPC pts, demonstrating excellent agreement. Bottom: Bland-Altman plot for SCPC pts of the difference between the measured venous return to the heart and the measured aortic output vs. the average of the venous return and aortic output (both in L/min). The thin line represents the mean difference between the estimators and the thick lines represent two standard deviations around the mean difference.

Figure 4

Indexed end-diastolic volume of the ventricle compared to the amount of indexed collateral flow, demonstrates significant correlation. Note that two outliers were deleted who had other reasons for significant RV dilation (severe tricuspid regurgitation, moderate ventricular dysfunction).

Figure 5

With one outlier deleted who had a large amount of collateral flow soon after surgery, mean collateral flow (½ (Q_coll-syst + Q_coll-pulm)) indexed to BSA vs. time from SCPC surgery demonstrates a weak but significant positive correlation, suggesting collateral flow increases with increasing time with SCPC physiology.