The impact of three dimensional imaging in the characterisation of patients with volume overload

Background

Volume overload is a common finding in several cardiac disorders, and it is usually caused by different pathologies such as hypertension, valvular disease, myocardial infarction and cardiomyopathies. In each disease, major therapeutical interventions that may control symptoms and improve outcome still show a significant number of non-responders, and the identification of those patients who will benefit from high cost treatments could optimise clinical resources and limit costs of patients’ care.

Cardiac volumes and ejection fraction

In clinical practice, end-diastolic and end-systolic volumes are not easily measured and few cardiologists use them as markers of the efficacy of treatment. Their difference, or stroke volume, is the basis for calculating the ejection fraction as ratio versus the relative end-diastolic volume.

Despite the fact that ejection fraction is an important prognostic factor, its nature, e.g. a ratio between absolute or normalised volumes, areas or counts, causes a loss of accuracy. If both volumes double, ejection fraction remains constant; if the end-diastolic volume increases more than the end-systolic one, the ejection fraction may paradoxically increase. Thus, except when it is generated by “volumetric” techniques such as MRI, gated SPECT or Fast CT, ejection fraction does not linearly reflect cardiac volumes. When it is obtained geometrically (contrast ventriculography, 2D echocardiography), ejection fraction is referred to a ratio of areas rather than volumes and results less accurate in characterising ventricular dynamics.

In fact, it is not uncommon to observe “inappropriate” ejection fractions in some patients with volume overload in whom the apparent overstimation is due, beyond methodological considerations, to the absence of volume determination. Similarly, patients with low ejection fraction in whom an aggressive medical or surgical treatment has been chosen, frequently show the same value at follow-up, but probably at a lower volume overload.

These considerations indicate that selection of patients undergoing major treatments should be based on volumes rather than on ejection fraction, and that these volumes should be monitored in the follow-up to identify responders and to measure the magnitude of the therapeutical effect.

The volumetric “NO RETURN” behaviour

MRI, Gated SPECT and fast CT allow the determination of cardiac volumes with high accuracy and reproducibility. The clinical use of these volumetric measurements changes in different diseases but evidence exists that volume overload is one of the most important predictors of therapeutical effect.

In patients with viable myocardium undergoing coronary revascularisation, benefits are limited by the degree of volume overload (1). In patients with heart failure, massive LV dilation is an independent contributor to poor outcome that may be stabilised by aggressive vasodilator and diuretic therapy (2). Similar findings have been found in patients with valvular disease in whom the degree of volume overload conditions the effect on cavity size after the intervention.

Recently, the problem of volume overload has been demonstrated to affect also the response to biventricular pacing in patients with left bundle branch block in whom the percentage of non responders has been described to be as high as 30% - 50% (3), revealing that clinical and echocardiographic criteria are far from providing an optimal selection of candidates for resynchronisation therapy.

In the field of cardiac surgery, reliable quantification of left ventricular functional parameters is mandatory to predict clinical outcome in patients undergoing aneurysmectomy. Experience in patients with severe volume overload undergoing aneurysmectomy suggests that advanced preoperative left ventricular remodeling may result in postoperative diastolic dysfunction, promoting later redilation with mitral regurgitation (4). In such patients, MRI proved to be superior as a sensitive and non-invasive tool compared to conventional 2D echocardiography in assessing volume and ejection fraction changes after intervention.

The appropriate use of 3D imaging techniques

3D imaging techniques provide many data, and in some cases the information is redundant. In the absence of a cross standardisation, the clinician should select those measurements that will result useful in clinical decision-making. Thus, cardiac volumes first, followed by ejection fraction and by the specific parameters of each technique.

For MRI, volumes can be extended to mass, 4 chambers, wall tickness and valvular function, while for Gated SPECT scan, the regional perfusion/function match will be the added value to volumetric assessment. Fast CT may combine volumes with coronary calcifications. Of course these techniques should be used in selected patients with advanced heart failure undergoing major interventions such as PCI, CABG, left ventricular surgical remodelling, valvular surgery or resynchronisation. Before interventions, for similar ejection fractions, “big hearts” should be managed with extreme care since recovery of function is less common and outcome is poor. Large databases of patients with low ejection fraction and various degrees of volume overload will identify responders with higher accuracy, and hopefully the “volumetric” approach will change some aspects of the current therapeutical approach. Finally, in patients with good image quality, 3D echocardiography may be an attractive alternative to Gated SPECT, MRI or fast CT for the rapid and accurate low-cost quantitation of LV volumes at the bedside.

The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology.