Pulmonary hypertension (PH) as a result of pulmonary vein stenosis (PVS) is extremely difficult to treat. The ideal therapy should not target the high-pressure/low-flow (HP/LF) vasculature that drains into stenotic veins, but only the high-pressure/high-flow (HP/HF) vasculature draining into unaffected pulmonary veins, reducing vascular resistance and pressure without risk of pulmonary oedema. We aimed to assess the activity of the nitric oxide (NO) pathway in PVS during the development of PH, and investigate whether interventions in the NO pathway differentially affect vasodilation in the HP/HF vs. HP/LF territories. Swine underwent pulmonary vein banding (PVB; n = 7) or sham surgery (n = 6) and were chronically instrumented to assess progression of PH. Pulmonary sensitivity to exogenous NO (sodium nitroprusside, SNP) and the contribution of endogenous NO were assessed bi-weekly. The pulmonary vasodilator response to phosphodiesterase-5 (PDE5) inhibition was assessed 12 weeks after PVB or sham surgery. After sacrifice, 12 weeks post-surgery, interventions in the NO pathway on pulmonary small arteries isolated from HP/LF and HP/HF territories were further investigated. There were no differences in the in vivo pulmonary vasodilator response to SNP and the pulmonary vasoconstrictor response to endothelial nitric oxide synthase (eNOS) inhibition up to 8 weeks after PVB as compared to the sham group. However, at 10 and 12 weeks post-PVB, the in vivo pulmonary vasodilation in response to SNP was larger in the PVB group. Similarly, the vasoconstriction to eNOS inhibition was larger in the PVB group, particularly during exercise, while pulmonary vasodilation in response to PDE5 inhibition was larger in the PVB group both at rest and during exercise. In isolated pulmonary small arteries, sensitivity to NO donor SNP was similar in PVB vs. sham groups irrespective of HP/LF and HP/HF, while sensitivity to the PDE5 inhibitor sildenafil was lower in PVB HP/HF and sensitivity to bradykinin was lower in PVB HP/LF. In conclusion, both NO availability and sensitivity were increased in the PVB group. The increased nitric oxide sensitivity was not the result of a decreased PDE5 activity, as PDE5 activity was even increased. Some vasodilators differentially effect HP/HF vs. HP/LF vasculature.

Assessing right ventricular (RV) functional reserve is important for determining clinical status and prognosis in patients with pulmonary hypertension (PH). In this study, we aimed to establish RV oxygen (O2) delivery as a determinant for RV functional reserve during exercise in swine with chronic PH. Chronic PH was induced by pulmonary vein banding (PVB), with sham operation serving as control. RV function and RV O2 delivery were measured over time in chronically instrumented swine, up to 12 wk after PVB at rest and during exercise. At rest, RV afterload (pulmonary artery pressure and arterial elastance) and contractility ( Ees and dP/d tmax) were higher in PH compared with control with preserved cardiac index and RV O2 delivery. However, RV functional reserve, as measured by the exercise-induced relative change (Δ) in cardiac index, dP/d tmax, and end-systolic elastance ( Ees), was decreased in PH, and RV pulmonary arterial coupling was lower both at rest and during exercise in PH. Furthermore, the increase in RV O2 delivery was attenuated in PH during exercise principally due to a lower systolic coronary blood flow in combination with an attenuated increase in aorta pressure while arterial O2 content was not significantly altered in PH. Moreover, RV O2 delivery reserve correlated with RV functional reserve, Δcardiac index ( r2 = 0.85), ΔdP/d tmax ( r2 = 0.49), and Δ Ees ( r2 = 0.70), all P < 0.05. The inability to sufficiently increase RV O2 supply to meet the increased O2 demand during exercise is principally due to the reduced RV perfusion relative to healthy control values and likely contributes to impaired RV contractile function and thereby to the limited exercise capacity that is commonly observed in patients with PH. NEW & NOTEWORTHY Impaired right ventricular (RV) O2 delivery reserve is associated with reduced RV functional reserve during exercise in a swine model of pulmonary hypertension (PH) induced by pulmonary vein banding. Our data suggest that RV function and exercise capacity might be improved by improving RV O2 delivery.

The most widespread technique used to register sets of medical images consists of selecting one image as fixed reference, to which all remaining images are successively registered. This pairwise scheme requires one optimization procedure per pair of images to register. Pairwise mutual information is a common dissimilarity measure applied to a large variety of datasets. Alternative methods, called groupwise registrations, have been presented to register two or more images in a single optimization procedure, without the need of a reference image. Given the success of mutual information in pairwise registration, we adapt one of its multivariate versions, called total correlation, in a groupwise context. We justify the choice of total correlation among other multivariate versions of mutual information, and provide full implementation details. The resulting total correlation measure is remarkably close to measures previously proposed by Huizinga et al. based on principal component analysis. Our experiments, performed on five quantitative imaging datasets and on a dynamic CT imaging dataset, show that total correlation yields registration results that are comparable to Huizinga's methods. Total correlation has the advantage of being theoretically justified, while the measures of Huizinga et al. were designed empirically. Additionally, total correlation offers an alternative to pairwise mutual information on quantitative imaging datasets.

Key points Passive, isolated post‐capillary pulmonary hypertension (PH) secondary to left heart disease may progress to combined pre‐ and post‐capillary or ‘active’ PH This ‘activation’ of post‐capillary PH significantly increases morbidity and mortality, and is still incompletely understood. In this study, pulmonary vein banding gradually produced post‐capillary PH with structural and functional microvascular remodelling in swine. Ten weeks after banding, the pulmonary endothelin pathway was upregulated, likely contributing to pre‐capillary aspects in the initially isolated post‐capillary PH. Inhibition of the endothelin pathway could potentially stop the progression of early stage post‐capillary PH. AbstractPassive, isolated post‐capillary pulmonary hypertension (IpcPH) secondary to left heart disease may progress to combined pre‐ and post‐capillary or ‘active’ PH (CpcPH) characterized by chronic pulmonary vascular constriction and remodelling. The mechanisms underlying this ‘activation’ of passive pulmonary hypertension (PH) remain incompletely understood. Here we investigated the role of the vasoconstrictor endothelin‐1 (ET) in the progression from IpcPH to CpcPH in a swine model for post‐capillary PH. Swine underwent pulmonary vein banding (PVB; n = 7) or sham‐surgery (Sham; n = 6) and were chronically instrumented 4 weeks later. Haemodynamics were assessed for 8 weeks, at rest and during exercise, before and after administration of the ET receptor antagonist tezosentan. After sacrifice, the pulmonary vasculature was investigated by histology, RT‐qPCR and myograph experiments. Pulmonary arterial pressure and resistance increased significantly over time. mRNA expression of prepro‐endothelin‐1 and endothelin converting enzyme‐1 in the lung was increased, while ETA expression was unchanged and ETB expression was downregulated. This was associated with increased plasma ET levels from week 10 onward and a more pronounced vasodilatation to in vivo administration of tezosentan at rest and during exercise. Myograph experiments showed decreased endothelium‐dependent vasodilatation to Substance P and increased vasoconstriction to KCl in PVB swine consistent with increased muscularization observed with histology. Moreover, maximal vasoconstriction to ET was increased whereas ET sensitivity was decreased. In conclusion, PVB swine gradually developed PH with structural and functional vascular remodelling. From week 10 onward, the pulmonary ET pathway was upregulated, likely contributing to pre‐capillary activation of the initially isolated post‐capillary PH. Inhibition of the ET pathway could thus potentially provide a pharmacotherapeutic target for early stage post‐capillary PH.

Pulmonary vascular remodeling in pulmonary arterial hypertension involves perturbations in the nitric oxide (NO) and endothelin-1 (ET-1) pathways. However, the implications of pulmonary vascular remodeling and these pathways remain unclear in chronic thrombo-embolic pulmonary hypertension (CTEPH). The objective of the present study was to characterize changes in microvascular morphology and function, focussing on the ET-1 and NO pathways, in a CTEPH swine model. Swine were chronically instrumented and received up to five pulmonary embolizations by microsphere infusion, whereas endothelial dysfunction was induced by daily administration of the endothelial NO synthase inhibitor N ω -nitro-l-arginine methyl ester until 2 wk before the end of study. Swine were subjected to exercise, and the pulmonary vasculature was investigated by hemodynamic, histological, quantitative PCR, and myograph experiments. In swine with CTEPH, the increased right-ventricular afterload, decreased cardiac index, and mild ventilation-perfusion-mismatch were exacerbated during exercise. Pulmonary microvascular remodeling was evidenced by increased muscularization, which was accompanied by an increased maximal vasoconstriction. Although ET-1-induced vasoconstriction was increased in CTEPH pulmonary small arteries, the ET-1 sensitivity was decreased. Moreover, the contribution of the ET A receptor to ET-1 vasoconstriction was increased, whereas the contribution of the ET B receptor was decreased and the contribution of Rho-kinase was lost. A reduction in endogenous NO production was compensated in part by a decreased phosphodiesterase 5 (PDE5) activity resulting in an apparent increased NO sensitivity in CTEPH pulmonary small arteries. These findings suggest that pulmonary microvascular remodeling with a reduced activity of PDE5 and Rho-kinase may contribute to the lack of therapeutic efficacy of PDE5 inhibitors and Rho-kinase inhibitors in CTEPH.

Despite early revascularization, remodeling and dysfunction of the left ventricle (LV) after acute myocardial infarction (AMI) remain important therapeutic targets. Intermittent pacing therapy (IPT) of the LV can limit infarct size, when applied during early reperfusion. However, the effects of IPT on post-AMI LV remodeling and infarct healing are unknown. We therefore investigated the effects of IPT on global LV remodeling and infarct geometry in swine with a 3-day old AMI. For this purpose, fifteen pigs underwent 2 h ligation of the left circumflex coronary artery followed by reperfusion. An epicardial pacing lead was implanted in the peri-infarct zone. After three days, global LV remodeling and infarct geometry were assessed using magnetic resonance imaging (MRI). Animals were stratified into MI control and IPT groups. Thirty-five days post-AMI, follow-up MRI was obtained and myofibroblast content, markers of extracellular matrix (ECM) turnover and Wnt/frizzled signaling in infarct and non-infarct control tissue were studied. Results showed that IPT had no significant effect on global LV remodeling, function or infarct mass, but modulated infarct healing. In MI control pigs, infarct mass reduction was principally due to a 26.2 ± 4.4% reduction in infarct thickness (P ≤ 0.05), whereas in IPT pigs it was mainly due to a 35.7 ± 4.5% decrease in the number of infarct segments (P ≤ 0.05), with no significant change in infarct thickness. Myofibroblast content of the infarct zone was higher in IPT (10.9 ± 2.1%) compared to MI control (5.4 ± 1.6%; P ≤ 0.05). Higher myofibroblast presence did not coincide with alterations in expression of genes involved in ECM turnover or Wnt/frizzled signaling at 5 weeks follow-up. Taken together, IPT limited infarct expansion and altered infarct composition, showing that IPT influences remodeling of the infarct zone, likely by increasing regional myofibroblast content.

Background Intracoronary infusion of autologous bone marrow-derived mononuclear cells (BMMNC), after acute myocardial infarction (AMI), has been shown to improve myocardial function. However, therapeutic efficacy is limited, possibly because cell retention rates are low, suggesting that optimization of cell retention might increase therapeutic efficacy. Since retention of injected BMMNC is observed only within infarcted, but not remote, myocardium, we hypothesized that adhesion molecules on activated endothelium following reperfusion are essential. Consequently, we investigated the role of vascular cell adhesion molecule 1 (VCAM-1) in BMMNC retention in swine undergoing reperfused AMI produced by 120 min of percutaneous left circumflex coronary occlusion. Methods and results VCAM-1 expression in the infarct and remote region was quantified at 1, 3, 7, 14, and 35 days, post-reperfusion (n≥6 swine per group). Since expression levels were significantly higher at 3 days (2.41±0.62%) than at 7 days (0.98±0.28%; p<0.05), we compared the degree of cell retention at those time points in a follow-up study, in which an average of 43·106 autologous BMMNCs were infused intracoronary at 3, or 7 days, post-reperfusion (n = 6 swine per group) and retention was histologically quantified one hour after intracoronary infusion of autologous BMMNCs. Although VCAM-1 expression correlated with retention of BMMNC within each time point, overall BMMNC retention was similar at day 3 and day 7 (2.3±1.3% vs. 3.1±1.4%, p = 0.72). This was not due to the composition of infused bone marrow cell fractions (analyzed with flow cytometry; n = 5 per group), as cell composition of the infused BMMNC fractions was similar. Conclusion These findings suggest that VCAM-1 expression influences to a small degree, but is not the principal determinant of, BMMNC retention.

Myocardial infarction (MI) may result in pulmonary hypertension (PH). Inhibition of phosphodiesterase 5 (PDE5), the enzyme responsible for the breakdown of cGMP in vascular smooth muscle, has become part of the contemporary therapeutic armamentarium for pulmonary arterial hypertension and may also be beneficial for PH secondary to MI. Nitric oxide (NO) is an important activator of cGMP synthesis and can be enhanced in early PH and decreased in severe PH. In the present study, we investigated if PDE5 inhibition ameliorates pulmonary hemodynamics in swine with PH secondary to MI and whether NO is essential. The PDE5 inhibitor EMD360527 was administered in awake, chronically instrumented swine with or without MI. At rest, PDE5 inhibition produced pulmonary vasodilation as evidenced by a decrease in pulmonary vascular resistance, which was more pronounced in MI ( n = 5) compared with normal swine ( n = 10, P ≤ 0.01) and was accompanied by an increase in stroke volume in MI swine. Both pulmonary vasodilation and increased stroke volume were maintained during exercise, suggesting that this therapy may improve exercise capacity in patients with PH secondary to MI. Interestingly, prior inhibition of NO significantly enhanced ( P ≤ 0.01) pulmonary vasodilation by PDE5 inhibition in both normal ( n = 8) and MI swine ( n = 5, P ≤ 0.05 vs. normal). This suggests that the increased vasodilator responses to PDE5 inhibition after MI were not due to an increase in NO-induced cGMP production. These observations indicate that PDE5 inhibition represents an interesting pharmacotherapeutic approach in early PH after a recent MI to prevent overt PH. NEW & NOTEWORTHY This research article is the first to describe that pulmonary vasodilation to phosphodiesterase 5 inhibition is enhanced and nitric oxide independent in resting and exercising swine with pulmonary hypertension as a result of myocardial infarction. This suggests that phosphodiesterase 5 inhibition can normalize pulmonary hemodynamics in postcapillary pulmonary hypertension after a recent myocardial infarction and may improve exercise capacity.

The elegant study of Kiss et al.1 in this issue of Acta Physiologica entitled ‘Vagal nerve stimulation reduces infarct size via a mechanism involving the alpha-7 nicotinic acetylcholine receptor and down-regulation of cardiac and vascular arginase’ explores the role of arginase in myocardial ischaemia-reperfusion damage, and in the myocardial infarct size limitation produced by vagal nerve stimulation. Although the incidence of acute myocardial infarction has decreased over the last decades, acute myocardial infarction continues to be a major cause of morbidity and mortality, worldwide.2 Moreover, patients that survive an acute myocardial infarction are at increased risk of developing congestive heart failure, with infarct size as a major predictor of post-infarct cardiac remodelling and heart failure.3 The single most effective therapy to limit infarct size is timely and complete reperfusion. However, despite its overall benefits, reperfusion itself has been shown to result in additional cardiomyocyte death.4 Hence, adjunctive therapies in addition to reperfusion are still needed to further limit infarct size and improve clinical outcome.4 Myocardial necrosis and microvascular no-reflow are prominent features of ischaemia-reperfusion damage. The mechanisms underlying these events remain incompletely understood, but there is evidence that inflammation plays an important role, including regional influx of neutrophils and macrophages.5, 6 Evidence is accumulating that vagal nerve activity exerts an immunomodulatory influence, characterized by an anti-inflammatory effect that may be beneficial in a variety of cardiovascular disease states, including myocardial infarction and heart failure.6-8 Indeed, vagal nerve stimulation (VNS) has been reported to limit infarct size in a variety of experimental studies (see Table 4 of Uitterdijk et al.6). The exact mechanism by which VNS exerts its cardioprotective effect is not fully understood, but includes nicotinic8 and muscarinic7 receptor stimulation as well as activation of nitric oxide synthase,6 while the VNS-associated bradycardia does not appear to be mandatory for its cardioprotective effects.6-8 In the present issue of Acta Physiologica, Kiss et al.1 further explore the mechanism by which VNS reduces myocardial infarct size in the setting of an in vivo rat model of 30 min of coronary artery occlusion followed by 2 h of reperfusion. Specifically, the authors set out to test the hypothesis that VNS affords cardioprotection via inhibition of arginase activity. This hypothesis was based on the established anti-inflammatory actions of VNS,8, 9 and the critical importance of activation of NO synthase in the cardioprotection by VNS, in conjunction with evidence that inflammation-mediated upregulation of arginase activity contributes to ischaemia-reperfusion injury by reducing NO bioavailability.10 The authors first demonstrated that the ischaemia-reperfusion protocol resulted in ~67% of the area at risk to become infarcted, which was accompanied by a ~70% increase in arginase activity. Subsequently, the authors showed that VNS limited infarct size and reduced arginase activity, both of which were blocked by the alpha-7 nicotinic acetylcholine receptor antagonist methyllycaconitine. These findings not only demonstrate the importance of alpha-7 nicotinic receptor activation in the cardioprotection afforded by VNS, but also support a role for arginase inhibition in the protection by VNS. A critical role of arginase in myocardial necrosis was then demonstrated with the use of the arginase inhibitor N(omega)-hydroxy-nor-L-arginine (nor-NOHA) which resulted in marked reductions in infarct size and arginase activity. Interestingly, the reductions in infarct size and arginase activity produced by either VNS or nor-NOHA were virtually identical. Moreover, combined VNS in the presence of nor-NOHA failed to further reduce myocardial arginase activity and myocardial infarct size, consistent with the hypothesis that VNS afforded cardioprotection via inhibition of arginase activity. The importance of the study by Kiss et al. is that it provides further insight into the mechanism by which VNS exerts its cardioprotective effects, by providing evidence that the previously reported VNS-mediated stimulation of nicotinic8 and muscarinic receptors7 and the importance of NO bioavailability6 are likely interlinked through modulation of arginase activity. Notwithstanding its elegance, the study by Kiss et al.1 also raises a number of questions. First, definitive proof for a causal role of arginase inhibition in the cardioprotection by VNS requires demonstration that preventing the VNS-induced reduction in arginase activity also abrogates the VNS-induced cardioprotection. For this purpose, future studies should be aimed at specific strategies to increase myocardial arginase activity, for example using pharmacological or molecular strategies, to further test this interesting hypothesis. Second, in the study by Kiss et al.1 there was no assessment of coronary no-reflow, which has been proposed as an independent predictor of left ventricular remodelling and heart failure.11 As there is evidence that no-reflow can be mitigated, independent of a reduction in myocardial necrosis, it would be of great interest to assess the effects of arginase inhibition on no-reflow and to determine whether arginase is involved in the limitation of no-reflow produced by VNS.6 Third, we still do not know what the optimal VNS algorithm is to provide the greatest degree of cardioprotection. In the study by Kiss et al., only the right vagal nerve was stimulated, leaving the question open as to whether bilateral nerve stimulation might afford greater protection. This is important in view of a recent study in rats demonstrating differential hemodynamic responses between stimulation of the right and left cervical vagal nerve.12 Similarly, it remains unclear whether prolonged versus brief and whether continuous versus intermittent stimulation elicit the greatest degree of cardioprotection. Finally, and this is acknowledged by the authors, in the study by Kiss et al.1 VNS was initiated well before the start of myocardial ischaemia. In patients encountering an acute myocardial infarction, adjunctive therapy cannot be applied prior to ischaemia and is typically started just prior to reperfusion. Hence, from a clinical perspective, it would be important to study the effects of VNS (and of arginase inhibition) when started just prior to reperfusion. These scientific issues are excellent topics for future research. In conclusion, the article by Kiss et al.1 not only lends further support to the cardioprotective promise of VNS but also elucidates an important component of the mechanism by which VNS affords cardioprotection against irreversible ischaemia-reperfusion injury. Finally, the study shows that arginase is a bona fide target for further exploration in the ongoing quest for effective cardioprotective strategies in the treatment of acute myocardial infarction. There is no conflict of interest.

Vagal nerve stimulation (VNS) started prior to, or during, ischemia has been shown to reduce infarct size. Here, we investigated the effect of VNS when started just prior to, and continued during early, reperfusion on infarct size and no-reflow and studied the underlying mechanisms. For this purpose, swine (13 VNS, 10 sham) underwent 45 min mid-LAD occlusion followed by 120 min of reperfusion. VNS was started 5 min prior to reperfusion and continued until 15 min of reperfusion. Area at risk, area of no-reflow (% of infarct area) and infarct size (% of area at risk), circulating cytokines, and regional myocardial leukocyte influx were assessed after 120 min of reperfusion. VNS significantly reduced infarct size from 67 ± 2 % in sham to 54 ± 5 % and area of no-reflow from 54 ± 6 % in sham to 32 ± 6 %. These effects were accompanied by reductions in neutrophil (~40 %) and macrophage (~60 %) infiltration in the infarct area (all p < 0.05), whereas systemic circulating plasma levels of TNFα and IL6 were not affected. The degree of cardioprotection could not be explained by the VNS-induced bradycardia or the VNS-induced decrease in the double product of heart rate and left ventricular systolic pressure. In the presence of NO-synthase inhibitor LNNA, VNS no longer attenuated infarct size and area of no-reflow, which was paralleled by similarly unaffected regional leukocyte infiltration. In conclusion, VNS is a promising novel adjunctive therapy that limits reperfusion injury in a large animal model of acute myocardial infarction.

IntroductionFollowing myocardial infarction (MI), left ventricular (LV) remodeling aims to maintain cardiac function. Nevertheless, adverse remodeling results in thinning of the infarct and LV dilation which may subsequently lead to the development of heart failure (HF) with poor prognosis. Wnt/Frizzled signaling plays a key role in cardiac remodeling following MI. We have shown previously that blockade of Wnt/Frizzled signaling with UM206 suppresses adverse remodeling and prevents HF development in mice post‐MI. Here we investigated the effects of UM206 therapy in a clinically relevant large animal model.MethodsTwelve Yorkshire x Landrace swine were subjected to 2h of myocardial ischemia by ligation of the proximal left circumflex artery, followed by reperfusion. Starting 24h post‐MI, 6 animals were treated with UM206 (0.6 μg/kg/day i.v.) for 5 weeks. The 6 control swine were treated with vehicle. Cardiac dimensions and infarct mass (IM) were determined by echocardiography and dedicated markers, respectively.ResultsTreatment with UM206 for 5 weeks resulted in a significant decrease in IM compared to baseline, whereas IM remained unaffected in the control group. This was accompanied by progressive dilation of the LV in the control group between 3 and 5 weeks after MI while adverse remodeling was halted in the UM206 treated group.ConclusionUM206 treatment in a clinically relevant swine model of reperfused myocardial infarction attenuates adverse remodeling.Supported by the Dutch Heart Foundation (2010B196)

Angiogenesis induced by growth factor-releasing microspheres can be an off-the-shelf and immediate alternative to stem cell therapy for acute myocardial infarction (AMI), independent of stem cell yield and comorbidity-induced dysfunction. Reliable and prolonged local delivery of intact proteins such as VEGF is, however, notoriously difficult. Our objective was to create a platform for local angiogenesis in human-sized hearts, using polyethylene-glycol/polybutylene-terephthalate (PEG-PBT) microsphere-based VEGF165Adelivery. PEG-PBT microspheres were biocompatible, distribution was size dependent, and a regimen of 10 × 10615-μm microspheres at 0.5 × 106/min did not induce cardiac necrosis. Efficacy, studied in a porcine model of AMI with reperfusion rather than chronic ischemia used for most reported VEGF studies, shows that microspheres were retained for at least 35 days. Acute VEGF165Arelease attenuated early cytokine release upon reperfusion and produced a dose-dependent increase in microvascular density at 5 wk following AMI. However, it did not improve major variables for global cardiac function, left ventricular dimensions, infarct size, or scar composition (collagen and myocyte content). Taken together, controlled VEGF165Adelivery is safe, attenuates early cytokine release, and leads to a dose-dependent increase in microvascular density in the infarct zone but does not translate into changes in global or regional cardiac function and scar composition.

markdownabstractAbstract This thesis is dedicated to i) novel methods and optimization studies to improve the diagnosis of myocardial ischemia and myocardial infarction as well as fundamental studies that precede novel therapies for myocardial infarction. In part ii) 2 novel, adjunctive therapies for the early treatment of myocardial infarction and reperfusion injury are presented using promising drugs or neuro-humoral stimulation. The thesis is completed with part iii) which presents novel, adjunctive therapies for acute myocardial infarction that aim to improve long-term global and regional myocardial remodeling and attenuate infarct healing and geometry using novel drugs, mechanical stimulation or controlled regional angiogenesis. The exiting results of the thesis include, but are not limited to, significant reductions in infarct size and no-reflow, improved infarct geometry and infarct composition, and increased infarct microvasculature. These results will eventually contribute to improved prognosis of patients with acute myocardial infarction.

OBJECTIVES:In the absence of effective clinical pharmacotherapy for prevention of reperfusion-mediated injury, this study re-evaluated the effects of intracoronary adenosine on infarct size and no-reflow in a porcine model of acute myocardial infarction using clinical bolus and experimental high-dose infusion regimens. BACKGROUND:Despite the clear cardioprotective effects of adenosine, when administered prior to ischemia, studies on cardioprotection by adenosine when administered at reperfusion have yielded contradictory results in both pre-clinical and clinical settings. METHODS:Swine (54 ± 1 kg) were subjected to a 45-min mid-left anterior descending artery occlusion followed by 2 h of reperfusion. In protocol A, an intracoronary bolus of 3 mg adenosine injected over 1 min (n = 5) or saline (n = 10) was administered at reperfusion. In protocol B, an intracoronary infusion of 50 μg/kg/min adenosine (n = 15) or saline (n = 21) was administered starting 5 min prior to reperfusion and continued throughout the 2-h reperfusion period. RESULTS:In protocol A, area-at-risk, infarct size, and no-reflow were similar between groups. In protocol B, risk zones were similar, but administration of adenosine resulted in significant reductions in infarct size from 59 ± 3% of the area-at-risk in control swine to 46 ± 4% (p = 0.02), and no-reflow from 49 ± 6% of the infarct area to 26 ± 6% (p = 0.03). CONCLUSIONS:During reperfusion, intracoronary adenosine can limit infarct size and no-reflow in a porcine model of acute myocardial infarction. However, protection was only observed when adenosine was administered via prolonged high-dose infusion, and not via short-acting bolus injection. These findings warrant reconsideration of adenosine as an adjuvant therapy during early reperfusion.

Modulation of Wnt/Frizzled signaling with UM206 reduced infarct expansion and prevented heart failure development in mice, an effect that was accompanied by increased myofibroblast presence in the infarct, suggesting that Wnt/Frizzled signaling has a key role in cardiac remodeling following myocardial infarction (MI). This study investigated the effects of modulation of Wnt/Frizzled signaling with UM206 in a swine model of reperfused MI. For this purpose, seven swine with MI were treated with continuous infusion of UM206 for 5 weeks. Six control swine were treated with vehicle. Another eight swine were sham-operated. Cardiac function was determined by echo in awake swine. Infarct mass was estimated at baseline by heart-specific fatty acid-binding protein ELISA and at follow-up using planimetry. Components of Wnt/Frizzled signaling, myofibroblast presence, and extracellular matrix were measured at follow-up with qPCR and/or histology. Results show that UM206 treatment resulted in a significant decrease in infarct mass compared with baseline (−41±10%), whereas infarct mass remained stable in the Control-MI group (+3±17%). Progressive dilation of the left ventricle occurred in the Control-MI group between 3 and 5 weeks after MI, while adverse remodeling was halted in the UM206-treated group. mRNA expression for Frizzled-4 and the Frizzled co-receptor LRP5 was increased in UM206-treated swine as compared with Control-MI swine. Myofibroblast presence was significantly lower in infarcted tissue of the UM206-treated animals (1.53±0.43% vs 3.38±0.61%) at 5 weeks follow-up. This study demonstrates that UM206 treatment attenuates adverse remodeling in a swine model of reperfused MI, indicating that Wnt/Frizzled signaling is a promising target to improve infarct healing and limit post-MI remodeling.

Quantitative magnetic resonance imaging (qMRI) aims to extract quantitative parameters representing tissue properties from a series of images by modeling the image acquisition process. This requires the images to be spatially aligned but, due to patient motion, anatomical structures in the consecutive images may be misaligned. In this work, we propose a groupwise non-rigid image registration method for motion compensation in qMRI. The method minimizes a dissimilarity measure based on principal component analysis (PCA), exploiting the fact that intensity changes can be described by a low-dimensional acquisition model. Using an unbiased groupwise formulation of the registration problem, there is no need to choose a reference image as in conventional pairwise approaches. The method was evaluated on three applications: modified Look-Locker inversion recovery T 1 mapping in a porcine myocardium, black-blood variable flip-angle T 1 mapping in the carotid artery region, and apparent diffusion coefficient (ADC) mapping in the abdomen. The method was compared to a conventional pairwise alignment that uses a mutual information similarity measure. Registration accuracy was evaluated by computing precision of the estimated parameters of the qMRI model. The results show that the proposed method performs equally well or better than an optimized pairwise approach and is therefore a suitable motion compensation method for a wide variety of qMRI applications.

BACKGROUND:Our current understanding is that left ventricular (LV) remodeling after acute myocardial infarction (AMI) is caused by expansion of the infarcted myocardium with thinning of the wall and eccentric hypertrophy of the remote myocardium. To study the geometric changes in the remodeling process after reperfused AMI we used cardiac magnetic resonance imaging (CMR).METHODS:Nine juvenile swine underwent a 120-min occlusion of the left circumflex coronary artery followed by reperfusion. CMR was performed at 3 and 36 days post-infarction. Global and regional LV remodeling was assessed including geometric changes of infarcted and remote myocardium; infarct longitudinal length (mm), mean circumferential length (mm), total infarct surface (mm(2)), end-diastolic wall thickness (EDWT) (mm) and transmural extent of infarction (TEI).RESULTS:From 3 days to 36 days post-infarction end-diastolic volume increased by 43% (p<0.01). Infarct mass decreased by 36% (p<0.01), mainly by reduction of EDWT with 26%, while mean infarct circumferential length and longitudinal infarct length did not change. Remote myocardial mass increased by 23%, which was the result of an increase in its circumferential length from 95 ± 10 mm to 113 ± 11 mm (p<0.01), with no change in its EDWT. In contrast, EDWT in the infarct, peri-infarct and border zone decreased.CONCLUSIONS:Contrary to the widely held view the present, using CMR measurements, shows that post-infarction remodeling was not associated with expansion of the infarcted myocardium. These findings suggest that eccentric hypertrophy of the remote myocardium, but not expansion of the infarct region, is responsible for left ventricular dilatation after AMI.

The objective of this study was to compare heart-specific fatty acid binding protein (hFABP) and high-sensitivity troponin I (hsTnI) via serial measurements to identify early time points to accurately quantify infarct size and no-reflow in a preclinical swine model of ST-elevated myocardial infarction (STEMI). Myocardial necrosis, usually confirmed by hsTnI or TnT, takes several hours of ischemia before plasma levels rise in the absence of reperfusion. We evaluated the fast marker hFABP compared with hsTnI to estimate infarct size and no-reflow upon reperfused (2 h occlusion) and nonreperfused (8 h occlusion) STEMI in swine. In STEMI ( n = 4) and STEMI + reperfusion ( n = 8) induced in swine, serial blood samples were taken for hFABP and hsTnI and compared with triphenyl tetrazolium chloride and thioflavin-S staining for infarct size and no-reflow at the time of euthanasia. hFABP increased faster than hsTnI upon occlusion (82 ± 29 vs. 180 ± 73 min, P < 0.05) and increased immediately upon reperfusion while hsTnI release was delayed 16 ± 3 min ( P < 0.05). Peak hFABP and hsTnI reperfusion values were reached at 30 ± 5 and 139 ± 21 min, respectively ( P < 0.05). Infarct size (containing 84 ± 0.6% no-reflow) correlated well with area under the curve for hFABP ( r 2 = 0.92) but less for hsTnI ( r 2 = 0.53). At 50 and 60 min reperfusion, hFABP correlated best with infarct size ( r 2 = 0.94 and 0.93) and no-reflow ( r 2 = 0.96 and 0.94) and showed high sensitivity for myocardial necrosis (2.3 ± 0.6 and 0.4 ± 0.6 g). hFABP rises faster and correlates better with infarct size and no-reflow than hsTnI in STEMI + reperfusion when measured early after reperfusion. The highest sensitivity detecting myocardial necrosis, 0.4 ± 0.6 g at 60 min postreperfusion, provides an accurate and early measurement of infarct size and no-reflow.

AIMSOnly few preliminary experimental studies demonstrated the feasibility of adenosine stress CT myocardial perfusion imaging to calculate the absolute myocardial blood flow (MBF), thereby providing information whether a coronary stenosis is flow limiting. Therefore, the aim of our study was to determine whether adenosine stress myocardial perfusion imaging by Dual Source CT (DSCT) enables non-invasive quantification of regional MBF in an animal model with various degrees of coronary flow reduction.METHODS AND RESULTSIn seven pigs, a coronary flow probe and an adjustable hydraulic occluder were placed around the left anterior descending coronary artery to monitor the distal coronary artery blood flow (CBF) while several degrees of coronary flow reduction were induced. CT perfusion (CT-MBF) was acquired during adenosine stress with no CBF reduction, an intermediate (15-39%) and a severe (40-95%) CBF reduction. Reference standards were CBF and fractional flow reserve measurements (FFR). FFR was simultaneously derived from distal coronary artery pressure and aortic pressure measurements. CT-MBF decreased progressively with increasing CBF reduction severity from 2.68 (2.31-2.81)mL/g/min (normal CBF) to 1.96 (1.83-2.33) mL/g/min (intermediate CBF-reduction) and to 1.55 (1.14-2.06)mL/g/min (severe CBF-reduction) (both P < 0.001). We observed very good correlations between CT-MBF and CBF (r = 0.85, P < 0.001) and CT-MBF and FFR (r = 0.85, P < 0.001).CONCLUSIONAdenosine stress DSCT myocardial perfusion imaging allows quantification of regional MBF under various degrees of CBF reduction.