A historical perspective and recent advances on the evolution of the relationship between acute and chronic pain and cardiovascular disease.

The relationship between acute pain and the cardiovascular system was recognized approximately 50 years ago following the initial observation, along with several subsequent experimental studies, that hypertension can result in decreases in the perception of pain. These studies provided a strong impetus to study potential mechanisms to clarify commonalities between the regulatory pathways associated with pain and the cardiovascular system. Attention subsequently shifted to an emphasis on the impact of chronic pain on cardiovascular diseases and mortality with several large meta-analyses of longitudinal studies providing clear evidence that chronic widespread pain increases the risk for developing cardiovascular disease and is associated with excess morbidity and mortality. Cardiovascular associated mortality from myocardial infarction and stroke appears to be directly related to the duration and severity of chronic pain, a result often characterized as a 'dose-response' relationship. The availability and reproducibility of extensive large-scale observational and retrospective studies have emphasized the critical need for more research, including prospective studies, along with the need for the development of preclinical animal models, to better understand the relationship(s) and underlying mechanisms between chronic pain, associated comorbidities, and cardiovascular disease. Elucidation and a deeper understanding of these relationships, including a focus on the link between chronic pain, cardiovascular disease, and depression, could provide valuable information to guide the development of potential treatment interventions to aid in attenuating pain while preventing pain-associated cardiovascular disease, comorbidities, and mortality.

CLIC4 localizes to mitochondrial-associated membranes and mediates cardioprotection.

Mitochondrial-associated membranes (MAMs) are known to modulate organellar and cellular functions and can subsequently affect pathophysiology including myocardial ischemia-reperfusion (IR) injury. Thus, identifying molecular targets in MAMs that regulate the outcome of IR injury will hold a key to efficient therapeutics. Here, we found chloride intracellular channel protein (CLIC4) presence in MAMs of cardiomyocytes and demonstrate its role in modulating ER and mitochondrial calcium homeostasis under physiological and pathological conditions. In a murine model, loss of CLIC4 increased myocardial infarction and substantially reduced cardiac function after IR injury. CLIC4 null cardiomyocytes showed increased apoptosis and mitochondrial dysfunction upon hypoxia-reoxygenation injury in comparison to wild-type cardiomyocytes. Overall, our results indicate that MAM-CLIC4 is a key mediator of cellular response to IR injury and therefore may have a potential implication on other pathophysiological processes.

Inhibition of BKCa channels protects neonatal hearts against myocardial ischemia and reperfusion injury.

BKCa channels are large-conductance calcium and voltage-activated potassium channels that are heterogeneously expressed in a wide array of cells. Activation of BKCa channels present in mitochondria of adult ventricular cardiomyocytes is implicated in cardioprotection against ischemia-reperfusion (IR) injury. However, the BKCa channel's activity has never been detected in the plasma membrane of adult ventricular cardiomyocytes. In this study, we report the presence of the BKCa channel in the plasma membrane and mitochondria of neonatal murine and rodent cardiomyocytes, which protects the heart on inhibition but not activation. Furthermore, K+ currents measured in neonatal cardiomyocyte (NCM) was sensitive to iberiotoxin (IbTx), suggesting the presence of BKCa channels in the plasma membrane. Neonatal hearts subjected to IR when post-conditioned with NS1619 during reoxygenation increased the myocardial infarction whereas IbTx reduced the infarct size. In agreement, isolated NCM also presented increased apoptosis on treatment with NS1619 during hypoxia and reoxygenation, whereas IbTx reduced TUNEL-positive cells. In NCMs, activation of BKCa channels increased the intracellular reactive oxygen species post HR injury. Electrophysiological characterization of NCMs indicated that NS1619 increased the beat period, field, and action potential duration, and decreased the conduction velocity and spike amplitude. In contrast, IbTx had no impact on the electrophysiological properties of NCMs. Taken together, our data established that inhibition of plasma membrane BKCa channels in the NCM protects neonatal heart/cardiomyocytes from IR injury. Furthermore, the functional disparity observed towards the cardioprotective activity of BKCa channels in adults compared to neonatal heart could be attributed to their differential localization.

Selective Enhancement of Swine Myocardium with a Novel Ultrasound Enhancing Agent During Transthoracic Echocardiography.

Ultrasound enhancing agents are approved to delineate the endocardial border and opacify the left ventricle cavity (LVC). We present a nested phase change agent (NPCA) designed to enable selective myocardial enhancement without enhancing the LVC by employing a dual-activation mechanism dependent on sufficient ultrasound intensity and the microenvironment of the myocardium. Swine received bolus injections of NPCA while echocardiograms were collected and processed to determine background-subtracted acoustic intensities (AI) in the LVC and septal myocardium. At mechanical index (MI) ≥ 0.8, the NPCA enhanced the myocardium selectively (p < 0.001) while the LVC remained at baseline AI. A 5-mL bolus of NPCA enhanced swine myocardium and enhancement persisted for > 5 min at 1.4 MI, while hemodynamics and EKG remained normal. Our findings demonstrate that the NPCA enhances swine myocardium selectively without enhancing the LVC. The NPCA could have utility for functional and structural echocardiographic studies with clinical ultrasound using standard settings.

Use of Speckle Tracking Echocardiography to Detect Induced Regional Strain Changes in the Murine Myocardium by Acoustic Radiation Force.

BACKGROUND: It is difficult to simulate the abnormal myocardial strain patterns caused by ischemic coronary artery disease (CAD) which are a precursor to heart failure (HF) within an animal model. Simulation of these strain changes could contribute to better understanding of the early formative stages of HF. This is especially important in investigating the poorly understood pathogenesis of heart failure with preserved ejection fraction (HFpEF). Here, we discuss delivery of high intensity focused ultrasound (HIFU) in a murine model to alter left ventricular (LV) regional longitudinal strain (RLS), and use of speckle tracking echocardiography to detect these changes. METHODS: HIFU pulses (pressure amplitude 1.7 MPa) were generated by amplifying a sinusoidal waveform from a function generator into a piezoelectric transducer. These pulses were then directed extracorporeally towards the anterior LV surface of C57BI6 mice during three time periods (early, mid, and late diastole). Speckle tracking echocardiography was then used to quantify changes in RLS within six segments of the LV. RESULTS: We observed an increase in LV RLS with acoustic augmentation during all three time periods. This augmentation was most prominent near the anterior apical region in early diastole and near the posterior basilar region during late diastole. CONCLUSIONS: Our findings demonstrate the application of HIFU to non-invasively induce changes in RLS within a murine model. Our results also reflect the capability of speckle tracking echocardiography to analyze and quantify these changes. These findings represent the first demonstration of ultrasound-induced augmentation in LV RLS within a small animal model.

Correction of iron deficiency in hospitalized heart failure patients does not improve patient outcomes.

Heart failure (HF) has an estimated prevalence of 1-2% in the world's population and up to 10% of patients age 65 and above. Iron deficiency (ID) in HF has been shown to be an independent contributor of increased mortality and poorer quality of life and has been associated with increased rates of hospitalization. Estimates are varied, but it is believed that as many as 30-83% of HF patients have ID, often without overt anemia, therefore making diagnosis more difficult. Well-established large studies have shown intravenous iron (IVFe) supplementation in HF patients is superior to an oral route, though these guidelines were developed for the chronic HF patients in the outpatient setting. For patients who are frequently hospitalized for HF, their inpatient stays may present an opportunity to diagnose ID. We previously showed that ID is underdiagnosed in the inpatient setting. To date, limited studies investigate long-term outcomes in hospitalized HF patients diagnosed with ID who are treated with IVFe compared to those who are not. In this retrospective analysis, we assessed 1-year readmission rates and mortality outcomes in patients who were diagnosed with ID while admitted for HF and subsequently received IVFe versus those who did not on their initial admission. These data suggest that there is no significant reduction in readmissions for HF or mortality between those patients who received IVFe and those who did not.

An Ultrasound Enhancing Agent with Nonlinear Acoustic Activity that Depends on the Presence of an Electric Field.

The nonlinear acoustic properties of microbubble ultrasound enhancing agents have allowed for the development of subharmonic, second harmonic, and contrast-pulse sequence ultrasound imaging modes, which enhance the quality, reduce the noise, and improve the diagnostic capabilities of clinical ultrasound. This study details acoustic scattering responses of perfluorobutane (PFB) microbubbles, an un-nested perfluoropentane (PFP) nanoemulsion, and two nested PFP nanoemulsions-one comprising a negatively charged phospholipid bilayer and another comprising a zwitterionic phospholipid bilayer-when excited at 1 or 2.25 MHz over a peak negative pressure range of 200 kPa to 4 MPa in the absence and presence of a 1-Hz, 1-V/cm electric field. The only sample that exhibited an increase in nonlinear activity in the presence of an electric field at both excitation frequencies was the negatively charged nested PFP nanoemulsion; the most pronounced effect was observed at an excitation of 2.25 MHz. Interestingly, the application of an electric field not only increased the nonlinear acoustic activity of the negatively charged nested PFP nanoemulsion but increased it beyond that seen when the nanoemulsion is un-nested and on the same scale as PFB microbubbles.

A Voltage-Sensitive Ultrasound Enhancing Agent for Myocardial Perfusion Imaging in a Rat Model.

Echocardiographers with specialized expertise sometimes perform myocardial perfusion imaging using U.S. Food and Drug Administration-approved microbubbles in an off-label capacity, correlating microbubble replenishment in the near field with blood flow through the myocardium. This study reports the in vivo clinical feasibility of a voltage-sensitive ultrasound enhancing agent (UEA) for myocardial perfusion imaging. Four UEAs were injected into Sprague-Dawley rats while ultrasound images were collected to quantify brightness in the left ventricular (LV) cavity, septal wall, and posterior wall in systole and diastole. Formulation IV, a phase change agent nested within a negatively charged phospholipid bilayer, increased the tissue-to-cavity ratio in both systole and diastole in the septal wall, 6 dB, and in the posterior wall, 5 dB, while leaving the LV cavity at baseline. This outcome improves the signal of the myocardium relative to the LV cavity and shows promise as a myocardial perfusion UEA.

Iron deficiency in heart failure, an underdiagnosed and undertreated condition during hospitalization.

Heart failure (HF) is a chronic medical condition affecting an estimated 1-2% of the world's population, and as many as 10% of patients age 65 and above. Among patients with HF, iron deficiency (ID) has an estimated prevalence of 30-83%, often without concomitant anemia. Thus, ID in HF is often underdiagnosed unless actively sought after. ID in HF has been shown to be an independent contributor of increased mortality, hospitalization, and early readmission compared with HF patients without ID or HF patients with anemia without ID. Previous trials suggest that intravenous iron supplementation for patients with chronic HF and ID with or without anemia has resulted in improved functional outcomes and quality of life; however, the role of iron supplementation in patients hospitalized with HF has not been well characterized. In this retrospective analysis conducted in a large urban health system, we show that of the greater than 10,000 patients admitted for HF in 1 year, only 158 patients underwent screening for ID. Of these, 109 met criteria for ID. Despite intravenous iron being the standard of care for treatment of ID in HF patients, only 23 patients received this therapy. These data suggest that iron deficiency, despite having major implications in HF, is not being adequately evaluated during hospitalizations for HF. Further, if ID is identified, it is not being appropriately addressed, as per current treatment guidelines.

Anti-thrombotic therapy strategies with long-term anticoagulation after percutaneous coronary intervention - a systematic review and meta-analysis.

Background: Long-term oral anticoagulants (OAC) increases bleeding risk after the percutaneous coronary intervention (PCI) with dual antiplatelet therapy (DAPT) with Aspirin and P2Y12 inhibitors. We hypothesize that dual anti-thrombotic therapy (DATT) reduces bleeding without increased cardiovascular events. Objectives: DATT does not increase adverse cardiovascular events compared to triple anti-thrombotic therapy (TATT). Method: We searched MEDLINE, PUBMED, Google Scholar, Cochrane and EMBASE from inception to 6 April 2019 for randomized control trials (RCTs) comparing DATT to TATT after PCI. Results: We identified 641 citations (411 after excluding duplicates). Four RCTs with 5,317 patients (3,039 on DATT vs 2,278 on TATT) were included. DATT arm showed significantly reduced [total bleeding, 731 vs. 784, odds ratio [OR] = 0.51, Confidence Interval [CI] = 0.39-0.67, p < 0.00001, I2 = 71% (I2 = 0% without WOEST study)], [TIIMI major bleeding 60 vs. 80, OR = 0.56, CI = 0.4-0.79, p = 0.0009, I2 = 0%], and [TIIMI minor bleeding, 70 vs 126, OR = 0.43, CI = 0.32-0.59, p < 0.00001, I2 = 0%]. There was no difference in subsequent strokes, myocardial infarction, stent thrombosis, and mortality. A trend towards decreased non-cardiac deaths with DATT was observed, 14 vs 26, OR = 0.55, CI = 0.27-1.10, p = 0.09, I2 = 6%. Conclusions: DATT is associated with significantly reduced bleeding and a trend towards reduced non-cardiac death with no difference in adverse cardiovascular outcomes.

Inhibition of BKCa negatively alters cardiovascular function.

Large conductance calcium and voltage-activated potassium channels (BKCa ) are transmembrane proteins, ubiquitously expressed in the majority of organs, and play an active role in regulating cellular physiology. In the heart, BKCa channels are known to play a role in regulating the heart rate and protect it from ischemia-reperfusion injury. In vascular smooth muscle cells, the opening of BKCa channels results in membrane hyperpolarization which eventually results in vasodilation mediated by a reduction in Ca2+ influx due to the closure of voltage-dependent Ca2+ channels. Ex vivo studies have shown that BKCa channels play an active role in the regulation of the function of the majority of blood vessels. However, in vivo role of BKCa channels in cardiovascular function is not completely deciphered. Here, we have evaluated the rapid in vivo role of BKCa channels in regulating the cardiovascular function by using two well-established, rapid-acting, potent blockers, paxilline and iberiotoxin. Our results show that BKCa channels are actively involved in regulating the heart rate, the function of the left and right heart as well as major vessels. We also found that the effect on BKCa channels by blockers is completely reversible, and hence, BKCa channels can be exploited as potential targets for clinical applications for modulating heart rate and cardiac contractility.

Recent technological advancements in cardiac ultrasound imaging.

About 92.1 million Americans suffer from at least one type of cardiovascular disease. Worldwide, cardiovascular diseases are the number one cause of death (about 31% of all global deaths). Recent technological advancements in cardiac ultrasound imaging are expected to aid in the clinical diagnosis of many cardiovascular diseases. This article provides an overview of such recent technological advancements, specifically focusing on tissue Doppler imaging, strain imaging, contrast echocardiography, 3D echocardiography, point-of-care echocardiography, 3D volumetric flow assessments, and elastography. With these advancements ultrasound imaging is rapidly changing the domain of cardiac imaging. The advantages offered by ultrasound imaging include real-time imaging, imaging at patient bed-side, cost-effectiveness and ionizing-radiation-free imaging. Along with these advantages, the steps taken towards standardization of ultrasound based quantitative markers, reviewed here, will play a major role in addressing the healthcare burden associated with cardiovascular diseases.

Readmission rate after ultrafiltration in acute decompensated heart failure: a systematic review and meta-analysis.

Significance of ultrafiltration in acute decompensated heart failure remains unclear. We performed meta-analysis to determine its role in reducing readmissions after acute decompensated heart failure. MEDLINE was searched using PUBMED from inception to March 22, 2017 for prospective randomized control trials comparing ultrafiltration to diuretics in acute decompensated heart failure. Five hundred ninety studies were found; nine studies with 820 patients were included. Studies with renal replacement therapy bar ultrafiltration, chronic decompensated heart failure, and non-English language were excluded. RevMan Version 5.3 was used for analysis. The primary outcomes analyzed were cumulative and 90 days readmissions secondary to heart failure and all-cause readmissions. Baseline characteristics were similar. One hundred eighty-eight patients were readmitted with heart failure, 77 vs 111 favoring ultrafiltration; risk ratio (RR) = 0.71 (95% confidence interval (CI), 0.49-1.02, p = 0.07, I 2  = 47%). Ninety days readmissions were 43 vs 67 favoring ultrafiltration; RR = 0.65 (95%CI, 0.47-0.90, p = 0.01, I 2  = 0%). Ultrafiltration showed significantly higher fluid removal and weight loss. Hypotension was common in ultrafiltration (24 vs 13, OR = 2.06, 95%CI = 0.98-4.32, p = 0.06, I 2  = 0%). Ultrafiltration showed reduced 90 days heart failure readmissions and trend towards reduced cumulative hospital readmissions. Renal and cardiovascular outcomes and hospital stay were similar.

Comprehensive Echocardiographic Assessment of the Right Ventricle in Murine Models.

BACKGROUND: Non-invasive high-resolution echocardiography to evaluate cardiovascular function of small animals is increasingly being used due to availability of genetically engineered murine models. Even though guidelines and standard values for humans were revised by the American Society of Echocardiography, evaluations on murine models are not performed according to any standard protocols. These limitations are preventing translation of preclinical evaluations to clinical meaningful conclusions. We have assessed the right heart of two commonly used murine models according to standard clinical guidelines, and provided the practical guide and sample values for cardiac assessments. METHODS: Right heart echocardiography evaluations of CD1 and C57BL/6 mice were performed under 1-3% isoflurane anesthesia using Vevo® 2100 Imaging System with a high-frequency (18-38 MHz) probe (VisualSonics MS400). We have provided a practical guide on how to image and assess the right heart of a mouse which is frequently used to evaluate development of right heart failure due to pulmonary hypertension. RESULTS: Our results show significant differences between CD1 and C57BL/6 mice. Right ventricle structural assessment showed significantly larger (p < 0.05) size, and pulmonary artery diameter in CD1 mice (n = 11) compared to C57BL/6 mice (n = 15). Right heart systolic and diastolic functions were similar for both strains. CONCLUSION: Our practical guide on how to image and assess the right heart of murine models provides the first comprehensive values which can be used for preclinical research studies using echocardiography. Additionally, our results indicate that there is a high variability between mouse species and experimental models should be carefully selected for cardiac evaluations.

Magnetic Nanoparticle-Mediated Targeting of Cell Therapy Reduces In-Stent Stenosis in Injured Arteries.

Although drug-eluting stents have dramatically reduced the recurrence of restenosis after vascular interventions, the nonselective antiproliferative drugs released from these devices significantly delay reendothelialization and vascular healing, increasing the risk of short- and long-term stent failure. Efficient repopulation of endothelial cells in the vessel wall following injury may limit complications, such as thrombosis, neoatherosclerosis, and restenosis, through reconstitution of a luminal barrier and cellular secretion of paracrine factors. We assessed the potential of magnetically mediated delivery of endothelial cells (ECs) to inhibit in-stent stenosis induced by mechanical injury in a rat carotid artery stent angioplasty model. ECs loaded with biodegradable superparamagnetic nanoparticles (MNPs) were administered at the distal end of the stented artery and localized to the stent using a brief exposure to a uniform magnetic field. After two months, magnetic localization of ECs demonstrated significant protection from stenosis at the distal part of the stent in the cell therapy group compared to both the proximal part of stent in the cell therapy group and the control (stented, nontreated) group: 1.7-fold (p < 0.001) less reduction in lumen diameter as measured by B-mode and color Doppler ultrasound, 2.3-fold (p < 0.001) less reduction in the ratios of peak systolic velocities as measured by pulsed wave Doppler ultrasound, and 2.1-fold (p < 0.001) attenuation of stenosis as determined through end point morphometric analysis. The study thus demonstrates that magnetically assisted delivery of ECs is a promising strategy for prevention of vessel lumen narrowing after stent angioplasty procedure.

The potential of ultrasound in cardiac pacing and rhythm modulation.

INTRODUCTION: This review examines the potential for ultrasound to induce or otherwise influence cardiac pacing and rhythm modulation. AREAS COVERED: Of particular interest is the possibility of developing new, truly non-invasive, nonpharmacological, acute and chronic, ultrasound-based arrhythmia treatments. Such approaches would not depend upon implanted or indwelling devices of any kind and would use ultrasound at diagnostic exposure levels (so as not to harm the heart or surrounding tissues). It is known that ultrasound can cause cardiomyocyte depolarization and a variety of underlying mechanisms have been proposed. Expert commentary: Questions still remain regarding the effect of exposure parameters and work will also be necessary to identify the optimal target regions within the heart if ultrasound energy is to be used to induce safe and reliable pacing in a clinical setting.

Ultrasound-induced modulation of cardiac rhythm in neonatal rat ventricular cardiomyocytes.

Isolated neonatal rat ventricular cardiomyocytes were used to study the influence of ultrasound on the chronotropic response in a tissue culture model. The beat frequency of the cells, varying from 40 to 90 beats/min, was measured based upon the translocation of the nuclear membrane captured by a high-speed camera. Ultrasound pulses (frequency = 2.5 MHz) were delivered at 300-ms intervals [3.33 Hz pulse repetition frequency (PRF)], in turn corresponding to 200 pulses/min. The intensity of acoustic energy and pulse duration were made variable, 0.02-0.87 W/cm(2) and 1-5 ms, respectively. In 57 of 99 trials, there was a noted average increase in beat frequency of 25% with 8-s exposures to ultrasonic pulses. Applied ultrasound energy with a spatial peak time average acoustic intensity (Ispta) of 0.02 W/cm(2) and pulse duration of 1 ms effectively increased the contraction rate of cardiomyocytes (P < 0.05). Of the acoustic power tested, the lowest level of acoustic intensity and shortest pulse duration proved most effective at increasing the electrophysiological responsiveness and beat frequency of cardiomyocytes. Determining the optimal conditions for delivery of ultrasound will be essential to developing new models for understanding mechanoelectrical coupling (MEC) and understanding novel nonelectrical pacing modalities for clinical applications.

Superior vena cava echocardiography as a screening tool to predict cardiovascular implantable electronic device lead fibrosis.

BACKGROUND: Currently there is no noninvasive imaging modality used to risk stratify patients requiring lead extractions. We report the novel use of superior vena cava (SVC) echocardiography to identify lead fibrosis and complex cardiac implantable electronic device (CIED) lead extraction. With an aging population and expanding indications for cardiac device implantation, the ability to deal with the complications associated with chronically implanted device has also increased. METHODS: This was a retrospective analysis of Doppler echocardiography recorded in our outpatient Electrophysiology/Device Clinic office over 6 months. Images from 109 consecutive patients were reviewed. RESULTS: 62% (68/109) did not have a CIED and 38% (41/109) had a CIED. In patients without a CIED, 6% (4/68) displayed turbulent color flow by Doppler in the SVC, while 22% (9/41) of patients with a CIED displayed turbulent flow. Fisher's exact test found a statistically significant difference between the two groups (p value < 0.05). The CIED group was subdivided into 2 groups based on device implant duration (< 2 years vs. ≥ 2 years). Of the CIED implanted for ≥ 2 years, 27% (9/33) had turbulent flow in the SVC by Doppler, while no patients (0/8) with implant durations < 2 years demonstrated turbulent flow. Nine patients underwent subsequent lead extraction. A turbulent color pattern successfully identified all 3 patients that had significant fibrosis in the SVC found during extraction. CONCLUSION: Our data suggests that assessing turbulent flow using color Doppler in the SVC may be a valuable noninvasive screening tool prior to lead extraction in predicting complex procedures.

Extracorporeal acute cardiac pacing by high intensity focused ultrasound.

Ultrasound has been shown to produce Premature Ventricular Contractions (PVC's). Two clinical applications in which acute cardiac pacing by ultrasound may be valuable are: (1) preoperative patient screening in cardiac resynchronization therapy surgery; (2) Emergency life support, following an event of sudden death, caused by cardiac arrest. Yet, previously the demonstrated mean success rate of extra-systole induction by High Intensity Focused Ultrasound (HIFU) in rats is below 4.5% (Miller et al., 2011). This stands in contrast to previous work in rats using ultrasound (US) and ultrasound contrast agents (UCAs), where success rates of close to 100% were reported (Rota et al., 2006). Herein, bi-stage temporal sequences of accentuated negative pressure (rarefaction) and positive pressure HIFU transmission (insonation) patterns were applied to anaesthetized rats under real-time vital-signs monitoring and US imaging. This pattern of insonation first produces a gradual growth of dissolved gas cavities in tissue (cavitation) and then an ultrasonic impact. Results demonstrate sequences of successive successful HIFU pacing. Triggering insonation at different delays from the preceding ECG R-wave demonstrated successful HIFU pacing induction from mid ECG T-wave till the next ECG complex's PR interval. Spatially focusing the beam at different locations allows cumulative coverage of the whole left ventricle. Analysis of the acoustic wave patterns and temporal characteristics of paced PVCs is suggested to provide new insight into the mechanisms of HIFU cardiac pacing. Specifically, the observed HIFU pacing temporal success rate distribution suggests against sarcomere length modulation current being the dominant cellular level mechanism of HIFU cardiac pacing and may allow postulating that membrane deformation currents are dominant at the applied insonation conditions.