Intravascular ultrasound catheter evaluation of the left ventricle in mice: A feasibility study

With the advent of transgenic technology, it has become increasingly important to find a method for evaluating left ventricular (LV) anatomy and function in intact wild type, intervened, and transgenic mice. Mice are 1/10th the size of rats, and have body masses of 10-60 g, LV masses of 40- 150 mg, LV wall thicknesses of 0.5-2 mm, and LV internal dimensions of 1-3 mm. Although the murine LV has been imaged by transthoracic (TTE) two- dimensional directed M-mode echocardiography, we explored the use of intravascular ultrasound (IVUS) catheters, with imaging from various positions, to see if better two-dimensional images of the LV could be obtained by IVUS than TTE. Eight normal mice were anesthetized using pentobarbital or avertin. The mice were studied using a commercially available IVUS system (Endosonics, Inc.). Two IVUS catheters (3.5 and 5.0 Fr) with 20-MHz multielement array transducers were used. Each catheter had a 4.0-mm imaging depth of field in all directions (360°) from the mid-point of the catheter core. Multiple imaging approaches were attempted: transesophageal (TEE); transjugular (TJ); transperitoneal (TP); and open chest, from both epicardial surface (Ep) and via direct LV puncture. TEE and TJ approaches afforded insufficient depth of field to image the entire LV in cross section. TP and Ep approaches resulted in poor images, related both to inadequate depth of field and to relatively small sector angles subtended by imaging elements. LVP (intracavitary imaging) was capable of satisfactorily imaging the LV epicardium, but was unable to image the endocardium, probably because the latter was within the 1.9-mm 'ringdown' catheter artifact. All IVUS approach studies lacked sufficient temporal resolution (10 frames/sec) to reliably display systolic and diastolic frames necessary for evaluation of LV function. In contrast, as previously reported, transthoracic two- dimensionally directed M-mode echocardiograms have sufficient temporal and spatial resolution to permit accurate estimates of L V mass and systolic function. Currently, IVUS catheter-based approaches are not feasible for imaging murine L V anatomy and function. Limitations include: (1) inadequate temporal resolution might be improved by ECG gating; (2) limited depth of field possibly resolvable by lower frequency transducers; and (3) relatively large catheter size.