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Ann Thorac Surg 1999;67:153
© 1999 The Society of Thoracic Surgeons
a Department of Anesthesia, Yale University School of Medicine, 333 Cedar St, New Haven, CT 06520-8051 USA
Invited commentary
The authors of this report are to be commended for demonstrating the feasibility of obtaining load-independant estimates of right ventricular (RV) performance at various levels of inotropy (families of pressure–volume loops) with bedside technology. In this study, RV pressures were transduced using a high-fidelity, low-compliance transduction system. This pressure measurement technique simply represents the standard method for accurately measuring rapidly occurring changes in intracavity pressure. The originality of the paper lies in the application of clinically available, nonradionuclear technology to the estimation of the corresponding changes in RV volume. Thus, the report is a methods paper describing measurement technique logistics and validity. The RV volume measurement consisted of transesophageal echocardiographic (TEE) estimates of RV area in a single, standardized transgastric plane of interrogation. The term TEE is an all inclusive term used to describe a variety of ultrasound diagnostic techniques. In this study, the particular TEE modality used to acquire these data consisted of automated border detection, alternatively termed acoustic quantification. Automated border detection is an established ultrasound imaging method that has the ability to differentiate blood from surrounding tissues. In this situation, the blood consisted of the blood within the RV, and the surrounding tissues consisted of the underlying RV myocardium. The ability to differentiate between these tissues (blood versus myocardium) allowed continuous definition of the contour of the endocardium and, thus, can furnish continuously available RV area data.
The critical issue to be addressed concerns the validity of using a single two-dimensional measurement (RV area) to estimate the dimensions of the RV, a three-dimensional structure that varies markedly in shape at various volume states. Stated simplistically, the RV adopts a relatively triangular shape under conditions of normovolemia or hypovolemia. By contrast, the overloaded RV assumes a nonuniform globular appearance, in effect, an entirely different shape. Thus, RV area measurements obtained in one plane of interrogation, albeit standardized, may not accurately represent true dimensional changes. The authors of this paper have circumvented this limitation by the elegant expedient of excluding patients with any qualitative echocardiographic indication of hypervolemia from the study (exclusion of patients demonstrating the presence of tricuspid or pulmonic leaflets in the transgastric mid left ventricle level cross-sections). Although these exclusionary criteria provided a study population with a relatively uniform RV shape, it must be emphasized that this study design precludes extrapolation of these data beyond this restricted "within or below normal limits" RV volume measurement range. For this reason, it remains to be seen whether the technique can be validly applied to evaluation of patients with RV dysfunction.
Related Article
Ann. Thorac. Surg. 1999 67: 146-152.
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