On the mechanisms of ultrasound contrast agents-induced arrhythmias

Acoustic radiation force impulse under clinical conditions with single infusion of ultrasound contrast agent evoking arrhythmias in rabbit heart

PURPOSE

We previously reported that acoustic radiation force impulse (ARFI) with concomitant administration of perfluorobutane as an ultrasound contrast agent (UCA)-induced arrhythmias at a mechanical index (MI) of 1.8 or 4.0 in a rabbit model. The present study identified the location of arrhythmias with a MI < 1.8 using a new system that can transmit ARFI with B-mode imaging.

METHODS

Under general anesthesia, six male Japanese white rabbits were placed in a supine position. Using this system, we targeted ARFI to the exact site of the heart. ARFI exposure with MI 0.9-1.2 was performed to the right or left ventricle of the heart 2 min after UCA injection.

RESULTS

ARFI with a MI lower than previously reported to rabbit heart evoked extrasystolic waves with single UCA infusion. Arrhythmias were not observed using ARFI without UCA. Extrasystolic waves were observed significantly more frequently in the right ventricle group than in the left ventricle group, with arrhythmias showing reversed shapes. No fatal arrhythmias were observed.

CONCLUSION

ARFI applied to simulate clinical conditions in rabbit heart evoked extrasystolic waves with single UCA infusion. The right ventricle group was significantly more sensitive to ARFI exposure, resulting in arrhythmias, than the left ventricle group. The shapes of PVCs that occurred in the right ventricle group and the left ventricle group were reversed. Ultrasound practitioners who use ARFI should be aware of this adverse reaction, even if the MI is below the previously determined value of 1.9.

Non-invasive cardiac pacing with image-guided focused ultrasound

Currently, no non-invasive cardiac pacing device acceptable for prolonged use in conscious patients exists. High Intensity Focused Ultrasound (HIFU) can be used to perform remote pacing using reversibility of electromechanical coupling of cardiomyocytes. Here we described an extracorporeal cardiac stimulation device and study its efficacy and safety. We conducted experiments ex vivo and in vivo in a large animal model (pig) to evaluate clinical potential of such a technique. The stimulation threshold was determined in 10 different ex vivo hearts and different clinically relevant electrical effects such as consecutive stimulations of different heart chambers with a single ultrasonic probe, continuous pacing or the inducibility of ventricular tachycardia were shown. Using ultrasonic contrast agent, consistent cardiac stimulation was achievable in vivo for up to 1 hour sessions in 4 different animals. No damage was observed in inversion-recovery MR sequences performed in vivo in the 4 animals. Histological analysis revealed no differences between stimulated and control regions, for all ex vivo and in vivo cases.

Bioeffects of ultrasound-stimulated microbubbles on endothelial cells: gene expression changes associated with radiation enhancement in vitro

Ultrasound can be used to target endothelial cells in cancer therapy where the destruction of vasculature leads to tumor cell death. Here, we demonstrate ultrasound bioeffects in which the levels of genes in endothelial cells can be significantly altered by ultrasound-stimulated microbubble exposure. These were compared with established effects of radiation on endothelial cells at a gene level. Human-endothelial cells were exposed to ultrasound and microbubbles, radiation or combinations of ultrasound, microbubbles and radiation. Gene expression analyses revealed an up-regulation of genes known to be involved in apoptosis and ceramide-induced apoptotic pathways, including SMPD2, UGT8, COX6B1, Caspase 9 and MAP2K1 with ultrasound-stimulated microbubble exposure but not SMPD1. This was supported by immunohistochemistry and morphologic changes examined with cell microscopy, which showed changes in SMPD1 gene product in cells with microbubble exposure. This supports the hypothesis that ultrasound-stimulated microbubbles can induce significant bioeffect-related changes in gene expression and can affect ceramide signaling pathways in endothelial cells, leading to apoptosis.

Are ECG premature complexes induced by ultrasonic cavitation electrophysiological responses to irreversible cardiomyocyte injury?

The objective of this study was to explore the relationship between premature complexes (PCs) in the electrocardiogram (ECG) and lethal injury of cardiomyocytes induced by ultrasound exposure of the heart with contrast-agent gas bodies in the circulation. Anesthetized rats were exposed in a heated water bath to 1.55 MHz focused ultrasound with bursts triggered at end systole during contrast agent infusion. PCs were detected in ECG recordings and cardiomyocyte necrosis was scored by identifying Evans blue-stained cells in multiple frozen sections. With 0.1 μL/kg/min infusion of contrast agent for 5 min, both effects increased strongly for 2-ms bursts with increasing peak rarefactional pressure amplitude >1 MPa. At 8 MPa, statistically significant effects were found even for no agent infusion relative to sham tests. For 2-ms bursts at 2 MPa, the highly significant bioeffects seen for 10-, 1- and 0.1-μL/kg/min infusion became marginally significant for 0.01 μL/kg/min, which indicated a lower probability of cavitation nucleation. Burst duration variation from 0.2-20 ms produced no substantial trends in the results. Overall, the two effects were well correlated (r(2) = 0.88). The PCs occurring during contrast-enhanced ultrasound therefore appear to be electrophysiological responses to irreversible cardiomyocyte injury induced by ultrasonic cavitation.

Rayleigh theory of ultrasound scattering applied to liquid-filled contrast nanoparticles

We present a novel modified theory based upon Rayleigh scattering of ultrasound from composite nanoparticles with a liquid core and solid shell. We derive closed form solutions to the scattering cross-section and have applied this model to an ultrasound contrast agent consisting of a liquid-filled core (perfluorooctyl bromide, PFOB) encapsulated by a polymer shell (poly-caprolactone, PCL). Sensitivity analysis was performed to predict the dependence of the scattering cross-section upon material and dimensional parameters. A rapid increase in the scattering cross-section was achieved by increasing the compressibility of the core, validating the incorporation of high compressibility PFOB; the compressibility of the shell had little impact on the overall scattering cross-section although a more compressible shell is desirable. Changes in the density of the shell and the core result in predicted local minima in the scattering cross-section, approximately corresponding to the PFOB-PCL contrast agent considered; hence, incorporation of a lower shell density could potentially significantly improve the scattering cross-section. A 50% reduction in shell thickness relative to external radius increased the predicted scattering cross-section by 50%. Although it has often been considered that the shell has a negative effect on the echogeneity due to its low compressibility, we have shown that it can potentially play an important role in the echogeneity of the contrast agent. The challenge for the future is to identify suitable shell and core materials that meet the predicted characteristics in order to achieve optimal echogenity.

Intracellular delivery and calcium transients generated in sonoporation facilitated by microbubbles

Ultrasound application in the presence of microbubbles is a promising strategy for intracellular drug and gene delivery, but it may also trigger other cellular responses. This study investigates the relationship between the change of cell membrane permeability generated by ultrasound-driven microbubbles and the changes in intracellular calcium concentration ([Ca(2+)](i)). Cultured rat cardiomyoblast (H9c2) cells were exposed to a single ultrasound pulse (1MHz, 10-15cycles, 0.27MPa) in the presence of a Definity(TM) microbubble. Intracellular transport via sonoporation was assessed in real time using propidium iodide (PI), while [Ca(2+)](i) and dye loss from the cells were measured with preloaded fura-2. The ultrasound exposure generated fragmentation or shrinking of the microbubble. Only cells adjacent to the ultrasound-driven microbubble exhibited propidium iodide uptake with simultaneous [Ca(2+)](i) increase and fura-2 dye loss. The amount of PI uptake was correlated with the amount of fura-2 dye loss. Cells with delayed [Ca(2+)](i) transients from the time of ultrasound application had no uptake of PI. These results indicate the formation of non-specific pores in the cell membrane by ultrasound-stimulated microbubbles and the generation of calcium waves in surrounding cells without pores.