Transesophageal ultrasonographic imaging in rat hearts: visualization of aortic valve vegetations in non-bacterial thrombotic endocarditis

An initial application of transesophageal Doppler echocardiography in experimental small animal models

This study examined whether an intracardiac echocardiography catheter could be used for transesophageal echocardiography (TEE) examinations in normal rats, and intraoperative TEE in small animal models of disease. The study used 30 Sprague-Dawley normal rats, 10 rats undergoing coronary artery ligation, and 10 rats with experimentally induced mitral regurgitation. The rats were anesthetized with isoflurane and intubated. An intracardiac echocardiographic catheter was inserted into the esophagus. M-mode, 2-dimensional, and Doppler studies were performed in multiple views. TEE probe insertions were successful in all animals. Intraoperative TEE was safely performed in the rat models of myocardial infarction or mitral regurgitation. Mitral regurgitation was well assessed using color Doppler and pulmonary venous flow. This study demonstrates that TEE (including intraoperative TEE) can be safely performed in rats using an intracardiac echocardiographic catheter. It provides a new approach to the assessment of cardiac function and valvular regurgitation in small animals.

Ultrasound imaging: principles and applications in rodent research

Ultrasound imaging utilizes the interaction of sound waves with living tissue to produce an image of the tissue or, in Doppler-based modes, determine the velocity of a moving tissue, primarily blood. These dynamic, real time images can be analyzed to obtain quantitative structural and functional information from the target organ. This versatile, noninvasive diagnostic tool is widely used and accepted in human and veterinary medicine. Until recently its application as a research tool was limited primarily to larger, nonrodent species. Due to advances in ultrasound imaging technology, commercially available ultrasound systems now have the spatial and temporal resolution to obtain accurate images of rat and mouse hearts, kidneys, and other target tissues including tumor masses. As a result, ultrasound imaging is being used more frequently as a research tool to image rats and mice, and particularly to evaluate cardiac structure and function. The developing technology of ultrasound biomicroscopy has even greater spatial resolution and has been used to evaluate developing mouse embryos and guide site-specific injections into mouse embryos. Additional ultrasound imaging technologies, including contrast-enhanced imaging and intravascular ultrasound transducers adapted for transesophageal use, have been utilized in rats and mice. This paper provides an overview of basic ultrasound principles, equipment, and research applications. The use of noninvasive ultrasound imaging in research represents both a significant refinement as a potential replacement for more invasive techniques and a significant advancement in research techniques to study rats and mice.

Transrectal ultrasound for monitoring murine orthotopic prostate tumor

BACKGROUND

The mouse orthotopic prostate tumor model has been recognized as an ideal preclinical animal model simulating the anatomical and biological milieu of the prostate. In comparison with the subcutaneous tumor model, the only disadvantage of this model is the difficulty of chronological tumor growth monitoring. We have applied recent endoluminal ultrasound technology, transrectal ultrasonography (TRUS), to the monitoring of mouse orthotopic prostate tumors.

METHODS

A 6 Fr. 20 MHz catheter-based radial scan probe was used and TRUS was performed without any prior preparation including anesthesia. Orthotopic tumors were initiated by inoculation of 5000 RM-9 cells into the dorsal prostate of 12-week-old C57BL/6 male mice. The tumor growth was monitored by TRUS from day 3 to day 21. In addition, TRUS was performed to detect tumor growth suppression after intraperitoneal administration of cis-diamminedichloroplatinum (CDDP).

RESULTS

By ultrasound, tumors became detectable 7 days after tumor cell inoculation. TRUS images were clear and parallel to actual tumor growth. The tumor volume (X) calculated by TRUS correlated significantly with the actual tumor weight (Y) measured at autopsy; Y = 101.653 + 1.174X (R = 0.930, P < 0.001). Similarly, tumor growth suppression induced by CDDP was clearly detected by TRUS with reasonable accuracy.

CONCLUSIONS

A high resolution TRUS allows simple and reliable monitoring of in situ tumor growth and growth suppression, making the mouse orthotopic prostate tumor model more efficient.

Comparative efficacies of liposomal amikacin (MiKasome) plus oxacillin versus conventional amikacin plus oxacillin in experimental endocarditis induced by Staphylococcus aureus: microbiological and echocardiographic analyses

Optimal treatment strategies for serious infections caused by Staphylococcus aureus have not been fully characterized. The combination of a beta-lactam plus an aminoglycoside can act synergistically against S. aureus in vitro and in vivo. MiKasome, a new liposome-encapsulated formulation of conventional amikacin, significantly prolongs serum half-life (t1/2) and increases the area under the concentration-time curve (AUC) compared to free amikacin. Microbiologic efficacy and left ventricular function, as assessed by echocardiography, were compared in animals administered either oxacillin alone or oxacillin in combination with conventional amikacin or MiKasome in a rabbit model of experimental endocarditis due to S. aureus. In vitro, oxacillin, combined with either free amikacin or MiKasome, prevented the bacterial regrowth observed with aminoglycosides alone at 24 h of incubation. Rabbits with S. aureus endocarditis were treated with either oxacillin alone (50 mg/kg, given intramuscularly three times daily), oxacillin plus daily amikacin (27 mg/kg, given intravenously twice daily), or oxacillin plus intermittent MiKasome (160 mg/kg, given intravenously, a single dose on days 1 and 4). The oxacillin-alone dosage represents a subtherapeutic regimen against the infecting strain in the endocarditis model (L. Hirano and A. S. Bayer, Antimicrob. Agents Chemother. 35:685-690, 1991), thus allowing recognition of any enhanced bactericidal effects between oxacillin and either aminoglycoside formulation. Treatment was administered for either 3 or 6 days, and animals were sacrificed after each of these time points or at 5 days after a 6-day treatment course (to evaluate for posttherapy relapse). Left ventricular function was analyzed by utilizing serial transthoracic echocardiography during treatment and posttherapy by measurement of left ventricular fractional shortening. At all sacrifice times, both combination regimens significantly reduced S. aureus vegetation counts versus control counts (P < 0.05). In contrast, oxacillin alone did not significantly reduce S. aureus vegetation counts after 3 days of therapy. Furthermore, at this time point, the two combinations were significantly more effective than oxacillin alone (P < 0.05). All three regimens were effective in significantly decreasing bacterial counts in the myocardium during and after therapy compared to controls (P < 0.05). In kidney and spleen abscesses, all regimens significantly reduced bacterial counts during therapy (P < 0.0001); however, only the combination regimens prevented bacteriologic relapse in these organs posttherapy. By echocardiographic analysis, both combination regimens yielded a significant physiological benefit by maintaining normal left ventricular function during treatment and posttherapy compared with oxacillin alone (P < 0.001). These results suggest that the use of intermittent MiKasome (similar to daily conventional amikacin) enhances the in vivo bactericidal effects of oxacillin in a severe S. aureus infection model and preserves selected physiological functions in target end organs.

Feasibility study: transesophageal echocardiography with a 10F (3.2-mm), multifrequency (5.5- to 10-MHz) ultrasound catheter in a small rabbit model

Transesophageal echocardiography (TEE) is useful in children with congenital heart defects. However, because of available probe size (>/=7 mm diameter), its use is limited to patients weighing more than 3 kg. The aim of this study was to determine the feasibility of TEE in a small animal model by using a 10F (3.2-mm) intravascular ultrasound tipped catheter with a monoplane (longitudinal) 5.5- to 10-MHz phased vector array transducer. Ten New Zealand White rabbits (400 to 3400 g; mean 1580 g) underwent TEE. With animals under general sedation, the probe was blindly introduced into the esophagus. All intracardiac and extracardiac structures were examined, and the images were stored and independently reviewed. All pertinent intracardiac and extracardiac structures were identified except in the 3 smallest rabbits (400 to 600 g). Doppler hemodynamics and color Doppler were possible in each animal. Frequency agility (5.5 to 10 MHz) facilitated optimization of image resolution and penetration. Certain transgastric, 4-chamber, and short-axis views were limited because of the monoplane array. No overt adverse effects were associated with the procedure. Diagnostic TEE can be performed in a small animal model with a 10F, 5.5- to 10-MHz phased vector array ultrasound catheter. Our study suggests that this system has potential in performing diagnostic TEE safely in small, even premature, neonates.