CARDIOLOGY
3 Studies In : Fontan Circulation Flow Propogation
What can 4D MRI reveal about single ventricle flow patterns and how they correlate to clinical outcomes ?
The three operations that rebuild the circulatory system for kids with hypoplastic left heart syndrome — the Norwood , the Glenn and the Fontan — have saved thousands of lives . But it ’ s not a perfect solution . Fontan circulation creates a host of problems that affect every system of the body , but it ’ s largely a mystery how and why . “ This work is a first attempt to push back the veil and understand what happens downstream in these patients after the blood leaves the heart ,” says pediatric cardiologist Michael Di Maria , MD . “ That ’ s never really been looked at before using these MRI techniques .”
To look at it , Dr . Di Maria partnered with biomedical engineer Michal Schäfer , PhD , who , three years into medical school , boasts a lengthy and growing list of publications for his work in 4D flow MRI , which has produced novel measurements and biomarkers in dozens of diseases affecting the vasculature .
Drs . Schäfer and Di Maria looked at Fontan propagation through two novel lenses : wave intensity analysis and principal component analysis . They then squared their findings against a registry , developed in part by Dr . Di Maria and unique to Children ’ s Hospital Colorado , that collects a huge amount of prospective data on single ventricle patients in the course of care , allowing them to correlate their measurements with clinical outcomes .
The results lay the groundwork for a new way of understanding Fontan circulation . In fact , a third paper deconstructing some of their earliest findings has the potential to change the way surgeons perform the Norwood .
PRINCIPAL COMPONENT ANALYSIS ( 1 )
The same technique that powers facial recognition software , principal component analysis deciphers morphological patterns based on how often they occur in a data set , such as MRI images .
“ Then we correlated those features with functional metrics we already use ,” says Dr . Schäfer . “ Boom , we got a correlation .”
The flow patterns they identified using MRI associated with the degree of stiffness and dilation in the functional ventricle , which in turn correlated to good or poor patient exercise output , a standard measure of system capacity . Notably , relaxation turned out to be just as important as contraction : A compliant ventricle not only helps to push blood through the system , but to pull blood out of the lungs as it descends back toward the heart .
WAVE INTENSITY ANALYSIS ( 2 )
“ The heart and blood vessels have this dance called coupling ,” says Dr . Di Maria . “ The heart delivers a beat and the blood vessels receive it , and what you want is good cooperation between the two .”
Dr . Schäfer broke that reciprocal action into three basic components : forward compression , backward compression , and forward decompression . Of particular interest was backward compression , referring to the vessels ’ ability to receive flow . The stiffer the vessel , the greater the wave . A lead pipe , for example , would have greater backward compression than a rubber hose .
“ The patterns that we saw were way different from the control population ,” Dr . Di Maria says . “ Off the grid .”
Not surprisingly , a lower backward compression wave correlated with better exercise output in the patient data . As a next step , Drs . Schäfer and Di Maria hope to gather real time exercise data using a specially built supine bike they can roll right up to the MRI .
Another key variable proved to be the stiffness of the aorta , which is surgically reconstructed during the Norwood procedure with a patch . But it wasn ’ t just the stiffness . It was also the shape .
NORWOOD : BRIEF RESEARCH REPORT ( 3 )
A close examination of flow through the aorta revealed that hemodynamics were in large part dictated by geometry . Specifically , svelte , tubular aorta performed more efficiently than a wide , cylindrical one — suggesting that a reconstruction mirroring the shape and geometry of the natural aorta is not only a good aesthetic ideal , but is also physiologically worth the effort .
The response was immediate . The Journal of Thoracic and Cardiovascular Surgery published editorials with titles like
4 | CHILDREN ’ S HOSPITAL COLORADO