Differentiation of Impaired From Preserved Hemodynamics in Patients With Fontan Circulation Using Real-time Phase-velocity Cardiovascular Magnetic Resonance

Abstract

Purpose: Progressive impairment of hemodynamics in patients with Fontan circulation is common, multifactorial, and associated with decreased quality of life and increased morbidity. We sought to assess hemodynamic differences between patients with preserved (preserved Fontans) and those with impaired circulation (impaired Fontans) after pulmonary vasodilation using oxygen and under forced breathing conditions.

Materials and methods: Real-time phase-contrast cardiovascular magnetic resonance was performed using non-ECG triggered echo-planar imaging (temporal resolution=24 to 28 ms) in the ascending aorta (AAo) and superior vena cava (SVC)/inferior vena cava (IVC) on room air, after 100% oxygen inhalation (4 L/min; 10 min) and on forced breathing in 29 Fontan patients (17.2±7.3 y) and in 32 controls on room air (13.4±3.7 y). The simultaneously recorded patients' respiratory cycle was divided into 4 segments (expiration, end-expiration, inspiration, and end-inspiration) to generate respiratory-dependent stroke volumes (SVs). The imaging data were matched with physiological data and analyzed with home-made software.

Results: The mean SVi (AAo) was 46.1±11.1 mL/m in preserved Fontans versus 30.4±6.2 mL/m in impaired Fontans (P=0.002) and 51.1±6.9 mL/m in controls (P=0.107). The cutoff value for differentiation of Fontan groups was SVi (AAo, end-expiratory) of 32.1 mL/m. After hyperoxygenation, the mean SVi (AAo) increased to 48.7±12.7 mL/m in preserved Fontans (P=0.045) but remained unchanged in impaired Fontans (31.1±5.8 mL/m, P=0.665). Simultaneously, heart rates decreased from 75.2±15.9 to 70.8±16.4 bpm (preserved; P=0.000) but remained unchanged in impaired circulation (baseline: 84.1±9.8 bpm, P=0.612). Compared with physiological respiration, forced breathing increased the maximum respiratory-related cardiac index difference (ΔCImax) in preserved Fontans (SVC: 2.5-fold, P=0.000; and IVC: 1.8-fold, P=0.000) and to a lower extent in impaired Fontans (both veins, 1.5-fold; P(SVC)=0.011, P(IVC)=0.013). There was no impact on mean blood flow.

Conclusions: Oxygen affected the pulmonary vascular system by vasodilation and increased SVi in preserved Fontans but had no effect on impaired Fontans. Forced breathing increased ΔCImax but did not change the mean blood flow by sole activation of the ventilatory pump. End-expiratory aortic SVi represents a valuable measure for classifying the severity of Fontan hemodynamics impairment.

FIGURE 1

Parasagittal cine-steady-state free precession image in a 31 year-old woman delineating the extracardial Fontan tunnel (A, asterisk) for planning of quantitative real-time PC-CMR in the upper (B, C) and lower caval vein (D, E). End-systolic magnitude images (B, D) and corresponding phase-contrast velocity maps (C, E). DAo indicates descending aorta; RA, right atrium; RPA, right pulmonary artery.

FIGURE 2

A, Subdivision of the patient’s breathing curve into 4 respiratory phases (green=inspiration; blue=end-inspiration; yellow=expiration; red=end-expiration). As demonstrated in the small picture detail, heart intervals belong typically to different respiratory phases. B, Generation of respiratory-dependent stroke volume (SV) curves by accordingly assigning flow data to the appropriate respiratory phases. Data taken from the inferior vena cava (IVC) of a 25-year-old woman with preserved hemodynamics under normal physiological breathing (room-air condition).

FIGURE 3

Flow rates (A, B) and stroke volumes (C, D) obtained in the IVC from a volunteer (left, in a 15 year-old woman, room air) and a Fontan patient (right, 28 year-old man) by real-time PC-CMR demonstrating the considerable physiological variation between different heart cycles. Red lines represent average across all heartbeats. E and F, Respiratory-dependent stroke volumes reveal highest flow toward the lungs during end-inspiration (blue) and inspiration (green). Compared with controls an occasionally weak retrograde flow was detected during end-expiration (red) and expiration (yellow) in Fontans.

FIGURE 4

Dot diagram inclusive mean±SD (A) and ROC plot (B) to test the diagnostic accuracy for distinguishing preserved Fontans and impaired Fontans. Data taken from aortic end-expiratory SV_i under room-air condition. The AUC was 0.920 and thus represents excellent diagnostic accuracy.