Sheep Heart Research Articles

Restoration of Function Following in Utero Fetal MI Is Associated with Increased Cardiac Progenitor Cell Recruitment

Background: We have previously shown that following myocardial infarction (MI), the fetal heart has the capability to heal by regeneration with restoration of normal tissue architecture and left ventricular (LV) function. Early in the fetal healing response, there are proliferating cells that surround the infarct; however, it is unknown whether cardiac progenitor cells play a significant role in fetal regenerative cardiac healing. We hypothesized that following fetal MI; there would be migration of nkx 2.5+ cardiac progenitor cells to the area of infarction. Methods: Fetal anteroapical MI encompassing approximately 20% of the LV mass was created in early gestation fetal sheep. The fetal heart was harvested at 3 days following MI. Immunohistochemistry utilizing confocal microscopy was used to identify the presence of nkx 2.5+ cells in the infarct, the border zone, and the uninfarcted myocardium. Age matched uninfarcted fetal sheep heart was the control. Results: Three days following fetal MI, the presence of nkx 2.5+ cells was preferentially increased at the endocardial, the epicardial, and perivascular borders within the anteroapical infarct. Nkx 2.5+ cells were also seen in the border zone and the uninfarcted myocardium; however, the numbers decreased significantly away from the infarct. Nkx 2.5+ cells were identified in control hearts; however, there was no evidence of increased numbers in the apex or clustering around blood vessels within the fetal myocardium. Conclusions: Following fetal MI, nkx 2.5+ cells preferentially migrate to the area of infarction, suggesting that active recruitment of cardiac progenitor cells is partially responsible for the regenerative capacity of the fetal heart following MI. In addition, the increased numbers of nkx 2.5+ cells surrounding blood vessels suggests that some of these cardiac progenitor cells are recruited from the peripheral circulation. Understanding the mechanisms driving recruitment of cardiac progenitor cells to the fetal infarct may help us alter the post-natal response to MI.

Patterns of Spatiotemporal Evolution of Atrial Fibrillation in Sheep Heart Assessed by High-density Mapping

Patterns of Spatiotemporal Evolution of Atrial Fibrillation in Sheep Heart Assessed by High-density Mapping

Isoform- and Species-Specific Proteolysis of Cardiac Pacemaker Channels

Proteolysis of cardiac pacemaker channels affects biophysical properties of functional channels. Hyperpolarization-activated channels HCN2 and HCN4 can form homomeric or heteromeric functional pacemaker channels in cardiac ventricles. Employing Western blot and immunoprecipitation techniques with antibodies against N- or C- terminus of HCN2 or HCN4, respectively, we investigated protein expression patterns of endogenous HCN2 and HCN4 in cardiac ventricles of small (mouse, rat) and large (sheep, canine) animals and human. Using an antibody against N-terminus of HCN2, more full length protein at 100kD and less cleaved bands around 50kD were detected in small than in large animals. An additional cleaved band around 60kD was exclusively expressed in human. HCN2 C-terminal antibody could not detect any full length protein in all species tested. A 75kD cleaved band was detected in mice, rat, canine and substantially higher in sheep heart ventricles. A 60kD band was observed in human only. Using an N-terminal HCN4 antibody, the full length protein signals (at 160kD) were present in sheep and canine only. The cleaved bands near 100kD predominated in small animals but absent in large animals. With a C-terminal HCN4 antibody, the full length protein was observed in mice, barely detectable in rat, and clearly present in sheep, canine and human. A cleaved band around 100kD predominated in all animals. A minor cleaved band around 50kD appeared in all tested species except human. Overall, there was less HCN2 and more HCN4 proteolysis in small than in large animal cardiac ventricles. Endogenous myocardial HCN2 and HCN4 underwent intensive proteolysis at both N- and C- termini in an isoform- and species-specific pattern. In conclusion, results obtained from HCN2 and HCN4 protein expression in small animals may not be directly applied to large ones including human.

Annual rhythms of some physiological parameters in Ovis aries and Capra hircus

The aim of this study was to evaluate the annual variation of some physiological parameters (total lipid, NEFA, phospholipids, triglycerides, creatinine, β-idrossibutirrate, albumin, total protein, total cholesterol, urea, uric acid, glucose, rectal temperature and heart rate) in five female goats and five female sheep housed under natural photoperiod and natural ambient temperature in a 12 m2 sound proof box equipped with 50 × 100 cm opening window, that allowed natural ventilation. ANOVA was used to determine significant differences (p values < 0.05 were considered statistically significant). A trigonometric statistical model was applied to describe the periodic phenomenon analytically, by individuating the main rhythmic parameters according to the single cosinor procedure. Serum total lipid, NEFA, phospholipids, triglycerides, rectal temperature and heart rate, both in sheep and goats, showed an annual rhythm with acrophases in autumn. The results show that sheep and goats present similar annual patterns, probably influenced by various environmental factors.

Regional stiffening of the mitral valve anterior leaflet in the beating ovine heart

Left atrial muscle extends into the proximal third of the mitral valve (MV) anterior leaflet and transient tensing of this muscle has been proposed as a mechanism aiding valve closure. If such tensing occurs, regional stiffness in the proximal anterior mitral leaflet will be greater during isovolumic contraction (IVC) than isovolumic relaxation (IVR) and this regional stiffness difference will be selectively abolished by β-receptor blockade. We tested this hypothesis in the beating ovine heart. Radiopaque markers were sewn around the MV annulus and on the anterior MV leaflet in 10 sheep hearts. Four-dimensional marker coordinates were obtained from biplane videofluoroscopy before (CRTL) and after administration of esmolol (ESML). Heterogeneous finite element models of each anterior leaflet were developed using marker coordinates over matched pressures during IVC and IVR for CRTL and ESML. Leaflet displacements were simulated using measured left ventricular and atrial pressures and a response function was computed as the difference between simulated and measured displacements. Circumferential and radial elastic moduli for ANNULAR, BELLY and EDGE leaflet regions were iteratively varied until the response function reached a minimum. The stiffness values at this minimum were interpreted as the in vivo regional material properties of the anterior leaflet. For all regions and all CTRL beats IVC stiffness was 40–58% greater than IVR stiffness. ESML reduced ANNULAR IVC stiffness to ANNULAR IVR stiffness values. These results strongly implicate transient tensing of leaflet atrial muscle during IVC as the basis of the ANNULAR IVC–IVR stiffness difference.

Septic shock induces distinct changes in sympathetic nerve activity to the heart and kidney in conscious sheep

Sepsis and septic shock are the chief cause of death in intensive care units, with mortality rates between 30 and 70%. In a large animal model of septic shock, we have demonstrated hypotension, increased cardiac output, and tachycardia, together with renal vasodilatation and renal failure. The changes in cardiac sympathetic nerve activity (CSNA) that may contribute to the tachycardia have not been investigated, and the changes in renal SNA (RSNA) that may mediate the changes in renal blood flow and function are unclear. We therefore recorded CSNA and RSNA during septic shock in conscious sheep. Septic shock was induced by administration of Escherichia coli, which caused a delayed hypotension and an immediate, biphasic increase in heart rate (HR) associated with similar changes in CSNA. After E. coli, RSNA decreased for over 3 h, followed by a sustained increase (180%), whereas renal blood flow progressively increased and remained elevated. There was an initial diuresis, followed by oliguria and decreased creatinine clearance. There were differential changes in the range of the arterial baroreflex curves; it was depressed for HR, increased for CSNA, and unchanged for RSNA. Our findings, recording CSNA for the first time in septic shock, suggest that the increase in SNA to the heart is not driven solely by unloading of baroreceptors and that the increase has an important role to increase HR and cardiac output. There was little correlation between the changes in RSNA and renal blood flow, suggesting that the renal vasodilatation was mediated mainly by other mechanisms.

Mechanisms of stretch-induced atrial fibrillation in the presence and the absence of adrenocholinergic stimulation: Interplay between rotors and focal discharges

Both atrial stretch and combined adrenocholinergic stimulation (ACS) have been shown to favor initiation and maintenance of atrial fibrillation (AF). Their respective contributions to the electrophysiological mechanism remains, however, incompletely understood. This study endeavored to determine the mechanism of maintenance of stretch-related AF (SRAF) in the presence and absence of ACS and to assess how focal discharges interact with rotors to modify the level of complexity in the activation patterns to perpetuate AF. Video imaging of AF dynamics was carried out using a SRAF model in isolated sheep hearts (n = 24). Pharmacological approaches were used to (1) mimic ACS with acetylcholine (1 microM) plus isoproterenol (0.03 microM), and (2) abolish triggered activity, in response to sarcoplasmic reticulum calcium release, with caffeine (5 mM, CA) or ryanodine (10 to 40 microM, RYA). In the absence of ACS, on perfusion of CA or RYA, focal discharges were abolished and SRAF was terminated in most of the cases (10 of 13 experiments). In the presence of ACS, multiple drifting rotors as well as a large number of focal discharges were identified and only 1 of 11 AF episodes was terminated. In the absence of ACS, SRAF is maintained by high-frequency focal discharges that generate fibrillatory conduction and wave breaks. In the presence of ACS, SRAF dynamics is characterized by multiple high frequency rotors that are rendered unstable by spatially distributed focal discharges.

Fetal heart growth: IGF-1 and sex

Evaluation of: Lumbers ER, Kim MY, Burrell JH et al. Effects of intrafetal IGF-1 on growth of cardiac myocytes in the late gestation sheep. Am. J. Physiol. 296(3), E513–E519 (2009).A range of growth factors, including IGF-1, play a role in the prenatal growth and maturation of the heart in humans and sheep. IGF-1 was infused into the sheep fetus for 4 days from 120 days of gestation to test the hypothesis that IGF-1 increases fetal heart weight and cardiomyocyte growth. However, IGF-1 infusion had no effect on heart weight. IGF-1 infusion increased cardiomyocyte volume in male fetuses only, leading the authors to investigate the effect of sex on cardiomyocyte growth and development. At 128 days of gestation, females had a lesser proportion of mononucleated cardiomyocytes than males. The volume of these cardiomyocytes was greater in females than in males. Lumbers et al. were the first to investigate the effect of sex on cardiomyocyte growth and development during fetal life.

Oxidative Stress Implication After Prolonged Storage Donor Heart with Blood Versus Crystalloid Cardioplegia and Reperfusion Versus Static Storage

Several factors are known to limit cardiac transplantation, such as number of donors, quality of cardiac graft preservation, and ischemia-reperfusion injury. Some mechanisms of reperfusion injury are now recognized; they include oxygen free radical (OFR), white blood cells activation, changes in calcium influx, alteration of microvascular blood flow, and sympathetic activation. The goal of this study was to assess the effects of two types of cardioplegia with long-term storage, either static or continuous perfusion, in 30 isolated sheep hearts as a model for heart transplantation. We examined myocardial function, histology, ischemic damage, and markers of oxidative stress. Two types of cardioplegia and storage conditions using a Langendorff reperfusion were studied in a combined approach: crystalloid (CP) [groups I and III] or cold oxygenated autologous blood (BC) [groups II and IV], immediate storage during 8h in profound hypothermia (groups I and II), or reperfused with crystalloid (group III), or blood cardioplegia (group IV). All perfusate samples were drawn from the coronary sinus. Lactate levels increased progressively in groups I, II, and IV, but not in group III, as no significant elevation was shown [90 min: 13.6+/-1.7 versus 5.2+/-1.0 mmol/L (P<0.01)]. Arrhythmias were more frequent when using BC (n=5) than CP (n=0). For plasma thiobarbituric acid-reactive substances (TBARS) levels a significant difference was found between group III and the other groups since 15 to 90 min (P<0.05). Vitamin E concentration decreased significantly from 5 min for groups II and IV, 15 min for group I, and 30 min for group III, with a significant difference between groups II and IV (P<0.05) but not between groups I and III. CP followed by a reperfusion with the same solution showed a significantly lower ischemic injury and OFR production, less frequent ventricular arrhythmias while stable hemodynamic parameters carried on. However, this protocol did not act on the early postoperative contractile function.

Transmural Strains in the Ovine Left Ventricular Lateral Wall During Diastolic Filling

Rapid early diastolic left ventricular (LV) filling requires a highly compliant chamber immediately after systole, allowing inflow at low driving pressures. The transmural LV deformations associated with such filling are not completely understood. We sought to characterize regional transmural LV strains during diastole, with focus on early filling, in ovine hearts at 1 week and 8 weeks after myocardial marker implantation. In seven normal sheep hearts, 13 radiopaque markers were inserted to silhouette the LV chamber and a transmural beadset was implanted into the lateral equatorial LV wall to measure transmural strains. Four-dimensional marker dynamics were obtained 1 week and 8 weeks thereafter with biplane videofluoroscopy in closed-chest, anesthetized animals. LV transmural strains in both cardiac and fiber-sheet coordinates were studied from filling onset to the end of early filling (EOEF, 100 ms after filling onset) and at end diastole. At the 8 week study, subepicardial circumferential strain (ECC) had reached its final value already at EOEF, while longitudinal and radial strains were nearly zero at this time. Subepicardial ECC and fiber relengthening (Eff) at EOEF were reduced to 1 compared with 8 weeks after surgery (ECC:0.02+/-0.01 to 0.08+/-0.02 and Eff:0.00+/-0.01 to 0.03+/-0.01, respectively, both P<0.05). Subepicardial ECC during early LV filling was associated primarily with fiber-normal and sheet-normal shears at the 1 week study, but to all three fiber-sheet shears and fiber relengthening at the 8 week study. These changes in LV subepicardial mechanics provide a possible mechanistic basis for regional myocardial lusitropic function, and may add to our understanding of LV myocardial diastolic dysfunction.

Iron (II) Modulation of the Cardiac Ryanodine Receptor (RyR2)

Cardiomyopathies and arrhythmias are major causes of mortality in chronic iron overload. There is evidence that iron overload impairs cardiomyocytes Ca2+ homeostasis (Baptista-Hon et al, 2005). However its molecular substrates remain unknown. Cardiac ryanodine receptors (RyR2) dysfunction is implicated in several diseases where Ca2+ homeostasis is lost. We therefore wanted to investigate RyR2 role as a potential target for iron-induced cardiomyopathies.We isolated heavy sarcoplasmic reticulum (HSR) vesicles containing RyR2 from sheep hearts. RyR2 were reconstituted into L-α-phosphatidylethanolamine (PE) bilayers. Unitary currents were measured under voltage-clamp with 250mM Cs+ as the charge carrier and 10μM activating Ca2+. High affinity [3H]ryanodine binding of the native vesicles was detected by liquid scintillation counting. Non-specific binding determined by incubations with 100x cold ryanodine.Fe2+ reduced RyR2 open probability and conductance in a dose dependent manner. Lifetime analysis revealed 5 shut times components and 3 open times components in control. Fe2+ caused an extra-shut component. Furthermore, there was a dose dependent shift in the open time constants towards the faster components. The binding assays revealed a [Fe2+] dependent, co-operative reduction in [3H]ryanodine binding to HSR vesicles. Preliminary data of [3H]ryanodine binding in increasing [Ca2+] showed a rightward shift in the presence of Fe2+.The results presented here show for the first time that Fe2+ is a potent inhibitor of RyR2. The mechanism of this inhibition may be due to competition with Ca2+ for RyR2 activation sites. Suppression of RyR2 activity by Fe2+ may therefore be one of the mechanisms involved in iron-induced cardiomyopathies.ReferencesBaptista-Hon, D, Diaz, M. E., and Elliot, A. C. Acute exposure to iron (II) alters calcium handling in isolated rat ventricular myocytes. Journal of Molecular and Cellular Cardiology 39, 179. 2005.

The Effect of Volatile Anaesthetics on the Cardiac Ryanodine Receptor

Although volatile anaesthetics serve a crucial role in preventing pain they continue to have side effects such as their ability to excite Ca2+ release from the sarcoplasmic reticulum (SR) via the ryanodine receptor calcium release channels (RyR). Here we report the first detailed investigation on the effects of volatile anaesthetics on the function of cardiac RyRs.RyRs were isolated from sheep hearts and incorporated into artificial lipid bilayers and subjected to single channel recording. Clinical doses of halothane and isoflurane increased RyR activation by luminal and cytoplasmic Ca2+ by increasing channel open time and opening frequency. The Ka's for halothane and isoflurane were 1 mM and 3 mM, respectively. However, the maximal effect of halothane (5-fold increase in placePo) was ∼3-fold larger than that for isoflurane. These agents activated RyRs by interacting with cytoplasmic domains distinct from the ATP activating sites.The effects of halothane on RyR regulation by luminal and cytoplasmic Ca2+ and Mg2+ were accurately fitted by a luminal-triggered calcium feedthrough model involving four Ca2+ sensing mechanisms on each RyR subunit; two activation sites (luminal L-site, 40 μM affinity; cytoplasmic A-site, 1 μM affinity) and two cytoplasmic inactivation sites (I1-site, 10 mM affinity; I2-site, 1 μM affinity). Halothane did not appear to alter the ion binding affinities for these sites. Rather, it increased channel opening rate and decreased the channel closing rate associated with Ca2+ binding to the two activation sites.The potentiating effect of halothane on luminal Ca2+ activation of cardiac RyRs was due to 1) an increase on opening frequency because of synergistic actions of the luminal and cytoplasmic (L and A) sites and 2) an increase in open time because of Ca2+ feedthrough to the A-site.

Ultrastructure and Cytoarchitecture of Bachmann's Bundle in the Mammalian Heart

Recently, the Bachmann's bundle (BB) has been examined in connection with atrial fibrillation. However, the morphological properties of the BB remain to be clarified.In this study, the BB in hearts of monkeys and sheep was investigated by immunohistochemistry, scanning (SEM) and transmission (TEM) electron microscopy.Immuno‐histochemically, BB myocytes showed a strong positive reaction for desmin antibody and that connexin (Cx) 40 and Cx43 were distributed at the intercalated disks. BB myocytes were characterized by a dense network of intermediate filaments which enveloped nucleus, myofibrils and mitochondria, respectively. The intercalated disks showed an irregular stair‐like profile. Microprojections on the steps were smaller in number and larger in size than those in auricular myocytes.In conclusion, BB myocytes were different ultrastructurally from auricular myocytes, showing morphological properties of the conduction system.

Intramural Purkinje Cell Network of Sheep Ventricles as the Terminal Pathway of Conduction System

To identify the anatomical basis for cardiac electrical signal conduction, particularly seeking the intramural terminals of conduction pathway within the ventricles, sheep hearts were examined compared with human hearts utilizing the characteristic morphology of Purkinje cells as a histological marker. In 15 sheep and five human autopsies of noncardiac death, prevalence of Purkinje or Purkinje-type cells were histologically examined in the atrioventricular node, its distal conduction pathway, the interventricular septum, and the right- and left-ventricular free walls. Myocardial tissue cleavages were examined in the transmural sections (along cardiac base-to-apex axis) obtained from the septum and ventricular free walls. Serial histological sections through virtually the entirety of the septum in selected sheep were used as the basis of a three-dimensional reconstruction of the conduction pathway, particularly of the intramural Purkinje cell network. Purkinje cells were found within the mural myocardium of sheep ventricles whereas no intramural Purkinje-type cell was detected within the human ventricles. In the sheep septum, every intramural Purkinje cell composed a three-dimensional network throughout the mural myocardium, which proximally connected to the subendocardial extension of the bundle branches and distally formed an occasional junction with ordinary working myocytes. The Purkinje-cell network may participate in the ventricular excitation as the terminal conduction pathway. Individual connections among the Purkinje cells contain the links of through-wall orientation which would benefit the signal conduction crossing the architectural barriers by cleavages in sheep hearts. The myocardial architectural changes found in diseased hearts could disrupt the network links including those with transmural orientation. Anat Rec, 2009. (c) 2008 Wiley-Liss, Inc.

Mitral Leaflet Adaptation to Ventricular Remodeling

Functional mitral regurgitation (MR) is caused by systolic traction on the mitral leaflets related to ventricular distortion. Little is known about whether chronic tethering causes the mitral leaflet area to adapt to the geometric needs imposed by tethering, in part because of inability to reconstruct leaflet area in vivo. Our aim was to explore whether adaptive increases in leaflet area occur in patients with functional MR compared with normal subjects and to test the hypothesis that leaflet area influences MR severity. A new method for 3-dimensional echocardiographic measurement of mitral leaflet area was developed and validated in vivo against 15 sheep heart valves, later excised. This method was then applied in 80 consecutive patients from 3 groups: patients with normal hearts by echocardiography (n=20), patients with functional MR caused by isolated inferior wall-motion abnormality or dilated cardiomyopathy (n=29), and patients with inferior wall-motion abnormality or dilated cardiomyopathy but no MR (n=31). Leaflet area was increased by 35+/-20% in patients with LV dysfunction compared with normal subjects. The ratio of leaflet to annular area was 1.95+/-0.40 and was not different among groups, which indicates a surplus leaflet area that adapts to left-heart changes. In contrast, the ratio of total leaflet area to the area required to close the orifice in midsystole was decreased in patients with functional MR compared with those with normal hearts (1.29+/-0.15 versus 1.78+/-0.39, P=0.001) and compared with patients with inferior wall-motion abnormality or dilated cardiomyopathy but no MR (1.81+/-0.38, P=0.001). After adjustment for measures of LV remodeling and tethering, a leaflet-to-closure area ratio <1.7 was associated with significant MR (odds ratio 23.2, 95% confidence interval 2.0 to 49.1, P=0.02). Mitral leaflet area increases in response to chronic tethering in patients with inferior wall-motion abnormality and dilated cardiomyopathy, but the development of significant MR is associated with insufficient leaflet area relative to that demanded by tethering geometry. The varying adequacy of leaflet adaptation may explain in part the heterogeneity of this disease among patients. The results suggest the need to understand the mechanisms that underlie leaflet adaptation and whether leaflet area can potentially be modified as part of the therapeutic approach.

Atrial Septopulmonary Bundle of the Posterior Left Atrium Provides a Substrate for Atrial Fibrillation Initiation in a Model of Vagally Mediated Pulmonary Vein Tachycardia of the Structurally Normal Heart

The posterior left atrium (PLA) and pulmonary veins (PVs) have been shown to be critical for atrial fibrillation (AF) initiation. However, the detailed mechanisms of reentry and AF initiation by PV impulses are poorly understood. We hypothesized that PV impulses trigger reentry and AF by undergoing wavebreaks as a result of sink-to-source mismatch at specific PV-PLA transitions along the septopulmonary bundle, where there are changes in thickness and fiber direction. In 7 Langendorff-perfused sheep hearts AF was initiated by a burst of 6 pulses (CL 80 to 150ms) delivered to the left inferior or right superior PV ostium 100 to 150 ms after the sinus impulse in the presence of 0.5 micromol/L acetylcholine. The exposed septal-PLA endocardial area was mapped with high spatio-temporal resolution (DI-4-ANEPPS, 1000-fr/s) during AF initiation. Isochronal maps for each paced beat preceding AF onset were constructed to localize areas of conduction delay and block. Phase movies allowed the determination of the wavebreak sites at the onset of AF. Thereafter, the PLA myocardial wall thickness was quantified by echocardiography, and the fiber direction in the optical field of view was determined after peeling off the endocardium. Finally, isochrone, phase and conduction velocity maps were superimposed on the corresponding anatomic pictures for each of the 28 episodes of AF initiation. The longest delays of the paced PV impulses, as well as the first wavebreak, occurred at those boundaries along the septopulmonary bundle that showed sharp changes in fiber direction and the largest and most abrupt increase in myocardial thickness. Waves propagating from the PVs into the PLA originating from a simulated PV tachycardia triggered reentry and vagally mediated AF by breaking at boundaries along the septopulmonary bundle where abrupt changes in thickness and fiber direction resulted in sink-to-source mismatch and low safety for propagation.

The Long and the Short of Long and Short Duration Ventricular Fibrillation

See related article, pages 1256–1264 The mechanisms behind the complex activation sequences during ventricular fibrillation (VF) have been the topic of intense research over many years. Experimental studies in animal models of VF, as well as clinical studies of human VF,1–8 have provided a wealth of data on the nature of electric activity in the fibrillating heart. Supplemented by computer simulations and a theoretical framework of wave propagation in excitable media,9,10 our community has amassed significant insights into the mechanisms that lead to the degradation of electric activity into VF and of VF maintenance. However, the majority of these studies have been devoted to VF mechanisms in the short time period after VF onset (short-duration VF, lasting less than 1 minute). In an episode of cardiac arrest outside of the hospital, the median time to delivery of the first defibrillation shock is 4.4 minutes,11 with VF episodes often lasting as long as 10 minutes (long-duration VF); over this period, the heart becomes ischemic while being subjected to excessively high excitation rates. Changes in the electrophysiological properties of the myocardium take place over these long-duration VF episodes. This is reflected in the altered outcome of a defibrillation shock, with survival rates decreasing rapidly with the increase in minutes spent in VF.12 Similarly, the guidelines for optimal resuscitation therapy differ depending on VF duration: immediate defibrillation is prescribed if time from VF onset is less than 4 or …

Rotor meandering contributes to irregularity in electrograms during atrial fibrillation

Radiofrequency ablation therapy of atrial fibrillation (AF) recently incorporated the analysis of dominant frequency (DF) and/or electrogram fractionation for guidance. However, the relationships between DF, fractionation, and spatiotemporal characteristics of the AF source remain unclear. We hypothesize that a meandering reentrant AF source contributes to the wave fractionation and is reflected in the power spectrum of local electrograms elsewhere in the rotor's surroundings. Meandering rotors as AF sources were simulated in 2-dimensional models of human atrial tissue and recorded in isolated sheep hearts. Nondominant elements of the signals were differentiated from the dominant elements using singular value decomposition, whereby the purely periodic constituent (PC) relating to the rotor's DF was eliminated rendering a residual constituent (RC) that consisted of all other activity. Spectral analysis of the decomposed constituents revealed peaks corresponding to the meandering frequency of the rotor tip, the magnitudes of which were proportional to the size of and the distance to the rotor core. Similar analyses on epicardial optical signals and electrograms from isolated sheep hearts, as well as human complex fractionated atrial electrograms, showed applicability of the approach. Increased meandering of the rotor driving AF reduces activation periodicity and increases fractionation. The spectral manifestation of the rotor activity beyond the meandering region makes it possible to characterize AF source stability, as well as DF in humans using electrode mapping.

Chronic right ventricular pressure overload results in a hyperplastic rather than a hypertrophic myocardial response

Myocardial hyperplasia is generally considered to occur only during fetal development. However, recent evidence suggests that this type of response may also be triggered by cardiac overload after birth. In congenital heart disease, loading conditions are frequently abnormal, thereby affecting ventricular function. We hypothesized that chronic right ventricular pressure overload imposed on neonatal hearts initiates a hyperplastic response in the right ventricular myocardium. To test this, young lambs (aged 2-3 weeks) underwent adjustable pulmonary artery banding to obtain peak right ventricular pressures equal to left ventricular pressures for 8 weeks. Transmural cardiac tissue samples from the right and left ventricles of five banded and five age-matched control animals were studied. We found that chronic right ventricular pressure overload resulted in a twofold increase in right-to-left ventricle wall thickness ratio. Morphometric right ventricular myocardial tissue analysis revealed no changes in tissue composition between the two groups; nor were right ventricular myocyte dimensions, relative number of binucleated myocytes, or myocardial DNA concentration significantly different from control values. In chronic pressure overloaded right ventricular myocardium, significantly (P < 0.01) more myocyte nuclei were positive for the proliferation marker proliferating cellular nuclear antigen than in control right ventricular myocardium. Chronic right ventricular pressure overload applied in neonatal sheep hearts results in a significant increase in right ventricular free wall thickness which is primarily the result of a hyperplastic myocardial response.

Ambulatory Oxygenator Right Ventricular Assist Device for Total Right Heart and Respiratory Support

Our goal is ambulatory total respiratory and right heart assistance allowing a bridge to lung transplant. To that end, we have coupled a compact paracorporeal gas exchange device with a right ventricular assist device (RVAD) to create an "OxyRVAD." Through a limited left thoracotomy, the main pulmonary artery (PA) and right atrium (RA) were exposed in 5 anesthetized sheep. After a systemic heparin bolus, a 12-mm outer diameter crimped graft glued to tubing was anastomosed (end to side) to the main PA and a VAD atrial cannula was placed through the RA appendage. The chest was drained and closed, then the PA graft flowed at 1 to 2 L/min as a shunt to the RA overnight. The next day, the animal was reanticoagulated, and the shunt cannulae clamped and divided. The OxyRVAD unit, consisting of commercially available components including an axial flow pump and low-resistance cardiopulmonary bypass gas exchange device, was interposed. Pumping from RA to PA was maintained at 3 L/min. Five consecutive sheep survived the implant, and stood and ate normally after initiation of the OxyRVAD. Three survived the full 2-week study, and a fourth was sacrificed on day 13 owing to a storm-related power failure. For these 4 sheep, pump flow was stable at 3 L/min. Carbon dioxide removal was constant and total during the experiment at 200 +/- 19 mL/min. Oxygen transfer was 144 +/- 44 mL/min. One sheep had progressive thrombocytopenia and was sacrificed on day 5 after implant. Our ambulatory OxyRVAD can provide total assistance for the right heart and lungs in normal awake sheep for 14 days.