Comparative Noninvasive Measurement of Cardiac Output Based on the Inert Gas Rebreathing Method (Innocor^®) and MRI in Patients with Univentricular Hearts

There are many complex cardiac malformations that are characterized by a functionally univentricular physiology. Staged surgical repair according to the Fontan principle separates the systemic and pulmonary circulations by connecting the systemic venous return to the pulmonary arteries. However, long-term follow-up studies demonstrate a gradual deterioration of cardiac function, particularly from the second or third decade. Noninvasive evaluation of the cardiac function is therefore important in the follow-up of these patients. The cardiac index (CI) is a reliable hemodynamic parameter and represents an important marker of cardiac function.

We compared CI values determined by cardiac MRI (CMRI) with values obtained by noninvasive inert gas rebreathing (IGR; Innocor^® system). Sixteen patients (age range: 7.2–32.7 years) with functionally univentricular hearts (UVH) following total cavopulmonary connection (TCPC) were compared with 12 healthy subjects (age range: 8.5–18.6 years). The standard treadmill protocol of the German Society of Pediatric Cardiology was used for exercise testing. CI was determined at rest and at two standardized submaximal exercise levels.

In all subjects, CI increased under exercise conditions, but the values were significantly lower in patients with UVH. There was no significant difference between patients with UVH and predominantly right- or left-ventricular morphology. In comparison with CMRI measurements, the CI values obtained by the IGR method tended to be lower, with a mean difference of 1.02 l/min/m².

Noninvasive measurement of CI with the IGR method is feasible at rest and during exercise, and appears to be suited for routine determination of CI in patients with UVH following TCPC.

No conflict of interest has been declared by the author(s).