In recent years huge strides have been made in the fields of interventional cardiology and cardiac surgery which now allow physicians and surgeons to repair or replace cardiac valves with greater success in a larger demographic of patients. Pivotal to these advances has been significant improvements in cardiac imaging and improved fundamental understanding of valvular anatomies and morphologies. We describe here a novel series of techniques utilized within the Visible Heart® laboratory by engineers, scientists, and/or anatomists to visualize and analyze the form and function of the four cardiac valves and to assess potential repair or replacement therapies. The study of reanimated large mammalian hearts (including human hearts) using various imaging modalities, as well as specially prepared anatomical specimens, has enhanced the design, development, and testing of novel cardiac therapies.
interventional cardiology; cardiac surgery; Visible Heart®; heart valve
Fluorescent optical mapping of electrically active cardiac tissues provides a unique method to examine the excitation wave dynamics of underlying action potentials. Such mapping can be viewed as a bridge between cellular level and organ systems physiology, e.g., by facilitating the development of advanced theoretical concepts of arrhythmia. We present the design and use of a high-speed, high-resolution optical mapping system composed entirely of "off the shelf" components. The electrical design integrates a 256 element photodiode array with a 16 bit data acquisition system. Proper grounding and shielding at various stages of the design reduce electromagnetic interference. Our mechanical design provides flexibility in terms of mounting positions and applications (use for whole heart or tissue preparations), while maintaining precise alignment between all optical components. The system software incorporates a user friendly graphical user interface, e.g., spatially recorded action potentials can be represented as intensity graphs or in strip chart format. Thus, this system is capable of displaying cardiac action potentials with high spatiotemporal resolution. Results from cardiac action potential mapping with intact mouse hearts are provided. It should be noted that this system could be readily configured to study isolated myocardial biopsies (e.g., isolated ventricular trabeculae). We describe the details of a versatile, user-friendly system that could be employed for a magnitude of study protocols.
optical mapping; cardiac mapping; signal processing; voltage sensitive dye
Localization of the source of cardiac ectopic activity has direct clinical benefits for determining the location of the corresponding ectopic focus. In the present study, a recently developed current-density-based localization approach was experimentally evaluated in noninvasively localizing the origin of the cardiac ectopic activity from body surface potential maps (BSPMs) in a well-controlled experimental setting. The cardiac ectopic activities were induced in 4 well-controlled intact pigs by single-site pacing at various sites within the left ventricle (LV). In each pacing study, the origin of the induced ectopic activity was localized by reconstructing the current density (CD) distribution on the endocardial surface of the LV from the measured BSPMs and compared with the estimated single moving dipole (SMD) solution and precise pacing site (PS). Over the 60 analyzed beats corresponding to ten pacing sites (six for each), the mean and standard deviation of the distance between the locations of maximum CD value and the corresponding PSs were 16.9 mm and 4.6 mm, respectively. In comparison, the averaged distance between the SMD locations and the corresponding PSs was slightly larger (18.4 ± 3.4 mm). The obtained CD distribution of activated sources extended from the stimulus site also showed high consistency with the endocardial potential maps estimated by a minimally invasive endocardial mapping system. The present experimental results suggest that the CD method is able to locate the approximate site of origin of a cardiac ectopic activity, and that the distribution of the current density can portray the propagation of early activation of an ectopic beat.
Electrocardiophysiology; body surface potential mapping; electrocardiographic inverse problem; current density reconstruction; single moving dipole; cardiac pacing
Three-dimensional mapping of the ventricular activation is of importance to better understand the mechanisms and facilitate management of ventricular arrhythmia. The goal of the present study was to develop and evaluate a three-dimensional cardiac electrical imaging (3DCEI) approach for imaging myocardial electrical activation from the intracavitary electrograms and heart-torso geometry information over the 3-dimensional (3D) volume of the heart. The 3DCEI was evaluated in a swine model undergoing intracavitary non-contact mapping (NCM). Each animal’s pre-operative MRI data were acquired to construct the heart-torso model. NCM was performed with the Ensite® 3000 system during acute ventricular pacing. Subsequent 3DCEI analyses were performed on the measured intracavitary electrograms. The estimated initial sites (ISs) were compared to the precise pacing locations, and the estimated activation sequences (ASs) and electrograms (EGs) were compared to those recorded by the NCM system over the endocardial surface. In total, 6 ventricular sites from 2 pigs were paced. The averaged localization error of IS was 6.7 ± 2.6 mm. The endocardial ASs and EGs as a subset of the estimated 3-dimensional solutions were consistent with those reconstructed from the NCM system. The present results demonstrate that the intracavitary-recording-based 3DCEI approach can well localize the sites of initiation and can obtain physiologically reasonable activation sequences as well as electrograms in an in vivo setting; under control/paced conditions. The present study suggests the feasibility of tomographic imaging of 3D ventricular activation and 3D electrograms from intracavitary recordings.
Activation imaging; Cardiac electrical imaging; Intracavitary recordings; Intramural potential mapping
Therapeutic hypothermia (TH) is considered to improve survival with favorable neurological outcome in the case of global cerebral ischemia after cardiac arrest and perinatal asphyxia. The efficacy of hypothermia in acute ischemic stroke (AIS) and traumatic brain injury (TBI), however, is not well studied. Induction of TH typically requires a multimodal approach, including the use of both pharmacological agents and physical techniques. To date, clinical outcomes for patients with either AIS or TBI who received TH have yielded conflicting results; thus, no adequate therapeutic consensus has been reached. Nevertheless, it seems that by determining optimal TH parameters and also appropriate applications, cooling therapy still has the potential to become a valuable neuroprotective intervention. Among the various methods for hypothermia induction, intravascular cooling (IVC) may have the most promise in the awake patient in terms of clinical outcomes. Currently, the IVC method has the capability of more rapid target temperature attainment and more precise control of temperature. However, this technique requires expertise in endovascular surgery that can preclude its application in the field and/or in most emergency settings. It is very likely that combining neuroprotective strategies will yield better outcomes than utilizing a single approach.
hypothermia; stroke; traumatic brain injury; neuroprotection
Localization of the initial site of cardiac ectopic activity has direct clinical benefits for treating focal cardiac arrhythmias. The aim of the present study is to experimentally evaluate the performance of the equivalent moving dipole technique on non-invasively localizing the origin of the cardiac ectopic activity from body surface potential maps (BSPMs) in a well-controlled experimental setting. The cardiac ectopic activities were induced in 4 well-controlled intact pigs by either single-site pacing or dual-site pacing within the ventricles. In each pacing study, the initiation sites (ISs) of cardiac ectopic activity were localized by estimating the locations of a single moving dipole (SMD) or two moving dipoles (TMDs) from the measured BSPMs, and compared with the precise pacing sites (PSs). For the single-site pacing, the averaged SMD localization error was 18.6 ± 3.8 mm over 16 sites, while the averaged distance between the TMD locations and the two corresponding pacing sites was slightly larger (24.9 ± 6.2 mm over 5 pairs of sites), both occurring at the onset of the QRS complex (10–25 ms following the pacing spike). The obtained SMD trajectories originated near the stimulus site and then traversed across the heart during the ventricular depolarization. The present experimental results show that the initial location of the moving dipole can provide the approximate site of origin of a cardiac ectopic activity in vivo, and that the migration of the dipole can portray the passage of an ectopic beat across the heart.
Body surface potential mapping; electrocardiographic inverse problem; equivalent moving dipole; cardiac pacing; non-invasive localization
To study the effect of the extra-cranial portion of a deep brain stimulation (DBS) lead on radiofrequency (RF) heating with a transmit and receive 9.4 tesla head coil.
Material and Methods
The RF heating was studied in four excised porcine heads (mean animal head weight = 5.46 ± 0.14 kg) for each of the following two extra-cranial DBS lead orientations: one, parallel to the coil axial direction; two, perpendicular to the coil axial direction (i.e., azimuthal). Temperatures were measured using fluoroptic probes at four locations: one, scalp; two, near the second DBS lead electrode-brain contact; three, near the distal tip of the DBS lead; and four, air surrounding the head. A continuous wave RF power was delivered to each head for 15 minutes using the coil. Net, delivered RF power was measured at the coil (mean whole head average specific absorption rate = 2.94 ± 0.08 W/kg).
RF heating was significantly reduced when the extra-cranial DBS lead was placed in the axial direction (temperature change = 0-5 °C) compared to the azimuthal direction (temperature change = 1-27 °C).
Development of protocols seems feasible to keep RF heating near DBS electrodes clinically safe during ultra-high field head imaging.
Safety; MRI; RF heating; DBS; high field
Atrial fibrillation (AF) causes a continuum of atrial anatomical remodeling.
Using a library of perfusion-fixed human hearts, specimens with AF were compared to controls. During this preliminary assessment study, direct measurements were taken of atrial volume, pulmonary vein (PV) circumference, and left atrial (LA) wall thicknesses.
Hearts with AF typically had larger atrial volumes, as well as a much larger variation in volume compared to controls (range of 59.6–227.1 mL in AF hearts compared to 65.1–115.9 mL in controls). For all hearts, right PVs were larger than left PVs (mean: 171.4 ± 84.6 mm for right and 118.2 ± 50.1 mm for left, P < 0.005). LA wall thicknesses ranged from 0.7 mm to 3.1 mm for both AF and control hearts.
Hearts with AF had a large range of sizes which is consistent with the progression of atrial remodeling during AF. The large range of thicknesses will influence the amount of energy needed to create transmural lesions during ablation procedures.
left atrial dimensions; volumes; pulmonary vein ostia; atrial fibrillation
Bears are among the most physiologically remarkable mammals. They spend half their life in an active state and the other half in a state of dormancy without food or water, and without urinating, defecating, or physical activity, yet can rouse and defend themselves when disturbed. Although important data have been obtained in both captive and wild bears, long-term physiological monitoring of bears has not been possible until the recent advancement of implantable devices.
Insertable cardiac monitors that were developed for use in human heart patients (Reveal® XT, Medtronic, Inc) were implanted in 15 hibernating bears. Data were recovered from 8, including 2 that were legally shot by hunters. Devices recorded low heart rates (pauses of over 14 seconds) and low respiration rates (1.5 breaths/min) during hibernation, dramatic respiratory sinus arrhythmias in the fall and winter months, and elevated heart rates in summer (up to 214 beats/min (bpm)) and during interactions with hunters (exceeding 250 bpm). The devices documented the first and last day of denning, a period of quiescence in two parturient females after birthing, and extraordinary variation in the amount of activity/day, ranging from 0 (winter) to 1084 minutes (summer). Data showed a transition toward greater nocturnal activity in the fall, preceding hibernation. The data-loggers also provided evidence of the physiological and behavioral responses of bears to our den visits to retrieve the data.
Annual variations in heart rate and activity have been documented for the first time in wild black bears. This technique has broad applications to wildlife management and physiological research, enabling the impact of environmental stressors from humans, changing seasons, climate change, social interactions and predation to be directly monitored over multiple years.
Electrophysiology; Hibernation; Cardiac Physiology
This study demonstrates the capabilities of MR imaging in the assessment of cardiac pacing induced ventricular dyssynchrony, and findings support the need for employing more physiological pacing. A human donor heart deemed non-viable for transplantation, was reanimated using an MR compatible, four-chamber working perfusion system. The heart was imaged using a 1.5T MR scanner while being paced from the right ventricular apex (RVA) via an epicardial placed lead. Four-chamber, short-axis, and tagged short-axis cines were acquired in order track wall motion and intramyocardial strain during pacing. The results of this study revealed that the activation patterns of the left ventricle (LV) during RVA pacing demonstrated intraventricular dyssynchrony; as the left ventricular mechanical activation proceeded from the septum and anterior wall to the lateral wall, with the posterior wall being activated last. As such, the time difference to peak contraction between the septum and lateral wall was ∼125 ms. Likewise, interventricular dyssynchrony was demonstrated from the four-chamber cine as the time difference between the peak LV and RV free wall motion was 180 ms. With the ongoing development of MR safe and MR compatible pacing systems, we can expect MRI to be added to the list of imaging modalities used to optimize cardiac resynchronization therapy and/or alternate site pacing.
Cardiac pacing; dyssynchrony; magnetic resonance imaging; myocardial contraction
In vivo thermoregulatory temperature response to RF heating at 9.4 T was studied by measuring temperatures in nine anesthetized swine. Temperatures were measured in the scalp, brain, and rectum. The RF energy was deposited using a four loop head coil tuned to 400.2 MHz. Sham RF was delivered to three swine to understand thermal effects of anesthesia (animal weight = 54.16 kg, SD = 3.08 kg). Continuous wave RF energy was delivered to the other six animals for 2.5–3.4 hours (animal weight = 74.01 ± 26.0 kg, heating duration = 3.05 ± 0.29 hours). The whole head specific absorption rate (SAR) varied between 2.71 W/kg and 3.20 W/kg (SAR = 2.93 ± 0.18 W/kg). Anesthesia caused the brain and rectal temperatures to drop linearly. Altered thermoregulatory response was detected by comparing the difference in the temperature slopes before and after the RF delivery from zero. RF heating statistically significantly altered the rate of cooling down of the animal. The temperature slope changes correlated well with the RF energy per unit head weight and heating duration, and the maximum rectal temperature change during heating in heated animals. The temperature slope changes did not correlate well to the whole head average SARs.
RF safety; RF heating; MR safety; ultra high field MRI; MR heating; 9.4 T
Gap junction expression has been studied in the atrioventricular junction (AVJ) of many species, however, their distribution in the human AVJ is unknown. The AVJ expression of the gap junction protein connexin 43 (Cx43) is species dependent; therefore we investigated its distribution in the human AVJ. Using Masson trichrome histology, we reconstructed the AVJ of three normal human hearts and one with dilated cardiomyopathy in three dimensions. Cx43 was immunolabeled with vimentin and α-actinin to determine the cellular origin of Cx43 and was quantified in the following structures: interatrial septum (IAS), His bundle, compact node (CN), lower nodal bundle (LNB), leftward and rightward nodal extensions (LE and RE), and inferior, endocardial, and left-sided transitional cells. Histology revealed two nodal extensions in three of four hearts. Cx43 was found in the myocytes, but not fibroblasts, of the AVJ. LE and CN Cx43 was lower than the IAS (P < 0.05) and the RE, LNB, and His all expressed Cx43 similarly, with approximately half of IAS expression (RE: 44 ± 36%; LNB: 50 ± 26%; His: 48 ± 12%, P = NS compared with IAS). Cx43 levels in transitional cells were similar to the IAS (P = not significant). Cx43 was found in myocytes of the human AVJ, and its expression pattern delineates two separate continuous structures: one consists of the LE and CN with little Cx43, and the other consists of the His, LNB, and RE expressing approximately half the Cx43 of the IAS. The differential Cx43 expression may provide each structure with unique conduction properties, contributing to arrhythmias arising from the AVJ.
atrioventricular node; dual-pathway electrophysiology; connexin43; slow pathway; AVNRT; fibroblasts
In vivo temperatures were correlated to the whole head average specific absorption rate (SARavg) at 9.4T using 12 anesthetized swine (mean animal weight = 52 kg, standard deviation = 6.7 kg). Correlating the temperatures and SARavg is necessary to ensure safe levels of human heating during ultra-high field MR exams. The temperatures were measured at three depths inside the brain, in the rectum, and at the head-skin of swine. A 400 MHz, continuous wave RF power was deposited to the head using a volume coil. The SARavg values were varied between 2.7–5.8 W/kg. The RF power exposure durations were varied between 1.4 –3.7 hr. To differentiate the temperature response caused by the RF from that of the anesthesia, the temperatures were recorded in four unheated swine. To study the effect of the spatial distribution of the RF and tissue properties, the temperature probes were placed at two brain locations (n = 4 swine for each location). Results showed that the in vivo brain temperatures correlated to the SARavg in a geometry-dependent manner. Additionally, 1) the skin temperature change was not the maximum temperature change; 2) the RF heating caused an inhomogeneous brain temperature distribution; and 3) the maximum temperature occurred inside the brain.
RF safety; RF heating; bioheat thermal model; high field MRI
A novel non-invasive imaging technique, the heart-model-based 3-dimensional cardiac electrical imaging (3DCEI) approach was previously developed and validated to estimate the initiation site (IS) of cardiac activity and the activation sequence (AS) from body surface potential maps (BSPMs) in a rabbit model. The aim of the present study was to validate the 3DCEI in an intact large mammalian model (swine) during acute ventricular pacing.
Methods and results
The heart-torso geometries were constructed from pre-operative MR images acquired from each animal. Body surface potential mapping and intracavitary non-contact mapping (NCM) were performed simultaneously during pacing from both right ventricular (intramural) and left ventricular sites (endocardial). Subsequent 3DCEI analyses were performed from the measured BSPMs. The estimated ISs were compared to the precise pacing locations and estimated ASs were compared to those recorded by the NCM system. In total, 5 RV and 5 LV sites from control and heart failure animals were paced and sequences of 100 paced beats were analyzed (10 for each site). The averaged localization error of the RV and LV sites were 7.3 ± 1.8 mm (n=50) and 7.0 ± 2.2 mm (n=50), respectively. The global 3-dimensional activation sequences throughout the ventricular myocardium were also derived. The endocardial ASs as a subset of the estimated 3-dimensional ASs were consistent with those reconstructed from the NCM system.
The present experimental results demonstrate that the noninvasive 3DCEI approach can localize the IS and estimate AS with good accuracy in an in vivo setting; under control, paced and/or diseased conditions.
3D Cardiac Electrical Imaging; Pacing; Electrocardiophysiology; body surface potential mapping; Noncontact mapping
Ricin-mAb35 is an immunotoxin targeted against skeletal muscle. Previously, we have shown that injection of ricin-mAb35 into rabbit extraocular muscle results in long-term muscle loss, and we have proposed this as a potential treatment for strabismus. In this study, we assessed the effects of ricin-mAb35 injection on extraocular muscle force generation.
Ricin-mAb35, 0.2 μg/kg in a volume of 0.1 mL, was injected into 1 superior rectus muscle in 16 adult rabbits. The contralateral superior rectus was injected with an equal volume of normal saline. Muscle force generation was assessed in vivo at 1, 6, and 12 weeks. Isometric length-tension curves were developed. Single-twitch tension, peak tetanic force generation, and fatigue rate were determined at optimal preload. Data from treated and control muscles were compared with the paired t test.
Force generation declined in ricin-mAb35 treated muscles at each postinjection interval. At 12 weeks, mean tetanic tension (200 Hz) in treated muscles was 13.8 mN/cm3 compared with 27.7 mN/cm3 in saline-injected controls (P = .02), a reduction of 50%. Single-twitch tension at 12 weeks was reduced 33% compared to controls (P = .04). Similar effects were noted at 1 and 6 weeks. Fatigue rate was not greater in treated muscles at any postinjection intervals.
Injection of ricin-mAb35 results in sustained weakness in extraocular muscle, although additional studies will be required to determine the duration of physiologic effect. These results confirm our histological analysis and suggest that ricin-mAb35 may be a more long-term alternative to botulinum toxin A for the treatment of strabismus.