Quantitative morphometric analysis of progressive infundibular obstruction in tetralogy of Fallot. A prospective longitudinal echocardiographic study.

BACKGROUND

The morphological hallmark of tetralogy of Fallot is controversial, with much disagreement as to whether the subpulmonary infundibulum in this lesion is hypoplastic. In addition, few quantitative data are available regarding the morphometry of the subpulmonary infundibulum, what anatomic characteristics are acquired in the postnatal period, and at what rate they progress. We also sought to determine whether echocardiographic morphometric analysis of the infundibulum can predict clinical course in infants with tetralogy of Fallot.

METHODS AND RESULTS

Twenty-one infants with tetralogy of Fallot (median age at initial study, 1.6 months) were prospectively followed with serial echocardiograms until the time of first surgical intervention (median age at surgery, 10 months). Selected video still frames were digitized off-line with a computerized system. Compared with age-matched normal control infants (n = 37), the following indexed infundibular dimensions in patients with tetralogy of Fallot were significantly smaller: length (1.86 +/- 0.54 versus 2.7 +/- 0.56 cm/BSA0.5, P < .0001), cross-sectional area (1.6 +/- 0.49 versus 4.7 +/- 1.3 cm2/BSA, P < .0001), and volume (1.24 +/- 0.62 versus 7.2 +/- 3 mL/BSA1.5, P < .0001). The following measurements were increased in tetralogy patients: infundibular septal thickness (0.83 +/- 0.21 versus 0.54 +/- 0.06 cm/BSA0.5, P = .0002) and infundibular free-wall thickness (0.62 +/- 0.13 versus 0.49 +/- 0.06 cm/BSA0.5, P = .006). The angle between infundibular septum and ventricular septum had a greater degree of anterosuperior deviation in tetralogy patients, resulting in a larger infundibuloventricular septal angle (77 +/- 8.2 degrees versus 31 +/- 6.5 degrees, P < .0001). During follow-up, infundibular volume in tetralogy patients decreased from 1.24 +/- 0.62 to 0.81 +/- 0.47 mL/BSA1.5 (P = .002), correlating with infundibular septal thickness (r = -.63, P < .003). The mean rate of decrease of indexed infundibular volume was 0.1 +/- 0.13 mL.BSA-15.mo-1. Correlation analysis revealed a nonlinear correlation between the degree of infundibular septal malalignment and indexed infundibular volume (r = .93, P < .0001). Tetralogy patients who required early surgical intervention (4.8 +/- 0.9 versus 10.7 +/- 1.7 months, P < .0001) had a smaller infundibulum at presentation (0.92 +/- 0.35 versus 1.41 +/- 0.67 mL/BSA1.5, P = .04) and an accelerated rate of infundibular narrowing (0.17 +/- 0.18 versus 0.06 +/- 0.08 mL.BSA-1.5.mo-1, P = .04).

CONCLUSIONS

Compared with normal infants, the subpulmonary infundibulum in tetralogy of Fallot is characterized by a smaller volume, shorter and thicker infundibular septum, and anterosuperior deviation of the infundibular septum. Infundibular obstruction in tetralogy patients is progressive, with an average rate of decrease in indexed infundibular volume of 0.1 +/- 0.13 mL.BSA-1.5.mo-1. Infants who are likely to require early therapeutic intervention may be identified on their initial echocardiogram as having an infundibular volume of < 0.9 to 1.0 mL/BSA1.5.