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Circulation on the Run

Dec 11, 2017

Dr. Carolyn Lam:               Welcome to Circulation on the Run, your weekly podcast summary and backstage pass to the journal and its editors. I'm Dr. Carolyn Lam, associate editor from the National Heart Centre and Duke-National University of Singapore. Our feature discussion today centers on patients with acute stroke due to large vessel occlusion, and asks the question, "Does interhospital transfer prior to thrombectomy relate to delayed treatment and worse outcomes?" Well, stay tuned for more right after these summaries.

                                                Our first original paper this week tells us that cardio protection is alive, and mitochondrial cardiomyocyte calcium-activated potassium channels of the BK type may be a promising target. In this study from first author Dr. Frankenreiter, corresponding author Dr. Lukowski, from University of Tuebingen in Germany, the authors used a combination of transgenic, pharmacologic and electrophysiological approaches to show that mice with a cardiomyocyte-specific knockout of BK channels had larger infarct size after 30 minutes of coronary occlusion, and 120 minutes of reperfusion, and were less protected by ischemic pre- and post-conditioning maneuvers, such as guanylate cyclase stimulators or activators and phosphodiesterase-5 inhibitors.

                                                In a chronic infarct model, mice with cardiomyocyte-specific knockout of BK channels had more fibrosis and lower left ventricular function. Mechanistically, the activation of BK channels in the inner mitochondrial membrane by cyclic GMP and protein kinase G was identified by patch clamping, and resulted in reduced formation of reactive oxygen species and activation of cardioprotective signaling. In summary, deficiency of BK channels in cardiomyocyte mitochondria rendered the heart highly vulnerable to ischemic and reperfusion injury, whereas the beneficial effects of cardioprotective agents known to target the nitric oxide cyclic GMP pathway required these cardiomyocyte BK channels. This thus establishes these cardiomyocyte mitochondrial BK channels as a promising target for limiting acute cardiac damage and adverse long-term events following myocardial infarction.

                                                The next study suggests that integration of maximal myocardial blood flow and coronary flow reserve, termed coronary flow capacity, may be helpful in predicting cardiovascular mortality in patients with stable coronary artery disease. First author Dr. Gupta, corresponding author Dr. Di Carli, and colleagues from Brigham and Women's Hospital, quantify myocardial blood flow and coronary flow reserve in more than 4,000 consecutive patients referred for myocardial perfusion PET scans from 2006 to 2013.

                                                Maximal myocardial blood flow of less than 1.8 mLs per gram per minute, and coronary flow reserve of less than two, were considered impaired. Four patient groups were then identified based on the concordant or discordant impairment of maximal myocardial blood flow, or its coronary flow reserve. The authors found that in patients with known or suspected coronary artery disease, impaired coronary flow reserve with preserved maximal myocardial blood flow identifies patients at an increased risk of cardiovascular mortality, despite a lack of myocardial ischemia. Patients who may be targeted for initiation or intensification of lifestyle preventive therapies for cardiovascular risk reduction. Conversely, preserved coronary flow reserve, even in the absence of impaired myocardial blood flow, identifies patients at low risk, in whom the need for revascularization should be reevaluated.

                                                The next study provides insights into cardiac regeneration, particularly with regards to using resident cardiac progenitor cells expressing the tyrosine kinase receptor c-Kit, which is being tested in clinical trials. In this study from first authors Dr. Chen and Zhu, corresponding authors Dr. van Berlo from University of Minnesota and colleagues, the authors used single-cell sequencing and genetic lineage tracing to show that there was innate heterogeneity within these c-Kit positive cardiac cells, where some have either endothelial or mesenchymal identity. Cardiac pressure overload resulted in a modest increase in c-Kit derived cardiomyocytes, with significant increases in the number of endothelial cells and fibroblasts. On the other hand, doxorubicin-induced acute cardio toxicity did not increase c-Kit derived endothelial cell fates, but instead induced cardiomyocyte differentiation.

                                                Although the overall rate of cardiomyocyte formation from c-Kit positive cells was below clinically-relevant levels, the authors further showed an important role for p53 in the differentiation of c-Kit positive cells to cardiomyocytes. Thus, this paper shows that different pathologic stimuli induced different cell fates in c-Kit positive target cells. These are novel findings that could aid in the development of strategies to preferentially regenerate cardiomyocytes.

                                                Since December 2014, a series of pivotal trials have shown that endovascular thrombectomy was highly effective in acute stroke management, prompting calls for reorganization of stroke systems of care. But how have these trials influenced the frequency of endovascular thrombectomy in clinical practice? Well, the last original paper in this week's journal tells us how. First and corresponding author, Dr. Smith from University of Calgary in Alberta, Canada, and colleagues, used data from the Get With The Guidelines stroke program to determine how the frequency of endovascular thrombectomy has changed in U.S. practice. They analyzed prospectively-collected data from a cohort of more than two million ischemic stroke patients, admitted to more than 2,000 participating hospitals between 2003 and the third quarter of 2016.

                                                The authors found that the use of endovascular thrombectomy for acute ischemic stroke accelerated sharply after the publication of pivotal randomized control trials beginning in December 2014. The endovascular thrombectomy case volume doubled at hospitals providing therapy. In the third quarter of 2016, endovascular thrombectomy was provided to 3.3% of all ischemic stroke patients. This represented 15.1% of all patients who were potentially eligible for endovascular thrombectomy based on stroke duration and severity. In summary, endovascular thrombectomy use is increasing rapidly, however there are still opportunities to treat more patients. Reorganizing stroke systems to route patients to adequately resourced endovascular thrombectomy-capable hospitals might increase treatment of eligible patients, improve outcomes, and reduce disparities.

                                                Coming right up, we will be discussing even more about endovascular thrombectomy in acute stroke management. Just hang on, our feature discussion is coming right up.

                                                Endovascular treatment with mechanical thrombectomy is beneficial for acute stroke patients suffering a large vessel occlusion. And that is in the guidelines, however we also know that treatment efficacy is highly time-dependent. And so, will interhospital transfer to an endovascular-capable center help in cases of acute large vessel stroke? Well, today's feature paper really helps to present novel data to answer that question. And it is from the STRATIS study. I'm so delighted to have with us the first and corresponding author, Dr. Michael Froehler from Vanderbilt University Medical Center, who will tell us about his findings, as well as Dr. Graeme Hankey, associate editor from University of Western Australia, joining us today. Welcome, gentlemen.

Dr. Michael Froehler:     Hello Carolyn.

Dr. Graeme Hankey:       Thank you Carolyn.

Dr. Carolyn Lam:               Thanks for making the time. Mike, tell us about the STRATIS study. What inspired it, what you found.

Dr. Michael Froehler:     Well, the STRATIS study was actually a large registry of the use of the Solitaire device for large vessel occlusion. Those results, the primary results, were published separately. But what we did in this study is look at one key aspect of the system of care for stroke delivery, in terms of its effect on time to treatment and patient outcomes.

                                                And so in short, what we found is that patients that are transferred from one hospital to another for mechanical thrombectomy take longer to receive treatment, and do worse in terms of functional outcome, compared to the patients that present directly to that thrombectomy center.

Dr. Carolyn Lam:               Wow. Could you put some numbers to that?

Dr. Michael Froehler:     Well, so we looked at 984 patients, almost a thousand patients. And what we found was that the time from stroke onset to revascularization, until the time the vessel was actually opened, was 202 minutes on average, for patients that presented directly to the thrombectomy center. Compared to over 311 minutes for patients that were transferred from one hospital to another. So that's a difference, on average, of over 100 minutes.

Dr. Carolyn Lam:               And I really was impressed with this other analysis you did. So I was wondering if you could share, where you did a hypothetical bypass modeling. Could you tell us about that? Because I thought that was really practical with a feasible message as well.

Dr. Michael Froehler:     I'm excited about that, and I should also share with you that we're working on a more in-depth bypass analysis, to really understand the implications of going to one center directly versus another. But the model that is built in to this publication is really designed to answer one or two questions. And the first is, how much time would we save if we went directly to the thrombectomy-capable center, compared to what actually happened? Meaning the patient was taken to a regional hospital and then subsequently transferred to the thrombectomy-capable center. And this was basically an ideal scenario.

                                                So if they were taken to one hospital and then transferred to another, we simply calculated what the maximum driving time from the starting position to the thrombectomy-capable center would be. And that did rest on the assumption that you actually had to drive past the first hospital. We didn't take any shortcuts in terms of the driving, and probably that small amount of driving time is actually shorter than the number that we found in our calculation.

                                                So the first question was, how much time would we save with that bypass? And the second question was, what kind of impact would that have on IV-tPA? Because, as a lot of us are thinking right now, with strong evidence in support of endovascular therapy for large vessel occlusion, if necessary how should we prioritize getting to endovascular treatment versus the standard therapy that we've known for 20 years, which is IV-tPA? And if you've got a choice, which one is more important?

                                                I don't know the answer to that question, but to try and help lead up to it, we did this hypothetical bypass analysis to look at the impact of bypass, driving directly to the thrombectomy center, the impact of that on the time to delivery of IV-tPA. And so that was really the second question that we asked with this hypothetical bypass analysis.

Dr. Carolyn Lam:               Yeah. I love that analysis, because I agree with you, it's a very, very practical question, and it's the way we clinicians think, right? So, tell us, what's the bottom line?

Dr. Michael Froehler:     So, the bottom line is, you're gonna save about an hour and a half if you bypass the regional hospital and go directly to the thrombectomy-capable center. On average, you're gonna get to the ultimate treatment center 91 minutes sooner, compared to the transferred group. Contrast that 91-minute time savings with a delay of IV-tPA delivery of 12 minutes. So yes, tPA will be delivered a little bit later, but endovascular therapy will be delivered much sooner.

                                                Now, that solution is probably not going to work everywhere, depending on your geography. So one of the other things we did within the hypothetical bypass analysis was limit that analysis only to patients who were transferred within a 20-mile radius. And that doesn't seem like a long distance, but actually there's a lot of patients in that group, that are still taken to the nearest hospital and then need to be transferred to another hospital that may be less than 20 miles away.

                                                So if we looked at that group of patients, then thrombectomy is still performed an hour and a half earlier, in that analysis it was 94 minutes earlier, but IV-tPA was delayed by only seven minutes. So certainly, there is a large group of patients out there that are perhaps being taken to hospitals that are not necessary, it's not a necessary stop.

Dr. Carolyn Lam:               Wow, Mike, this is really amazing results, it's starting to make me think of the old days of acute myocardial infarction treatment, when we were thinking of intravenous thrombolytics, comparison to primary PCI, an analogy and comparison that was also mentioned in the accompanying editorial that you invited. Graeme, would you like to share some of your thoughts on the implication of all this?

Dr. Graeme Hankey:       Just to take a step back, of course this begins with a stroke occurring out in the field. And unlike acute coronary syndromes, where chest pain is the major symptom, there are many symptoms of stroke. And the first problem is trying to identify the patient who has actually had a stroke, and in particular, one of the 15% or so who's had a large vessel occlusion, who's amenable to large vessel mechanical thrombectomy. So in the field we have an issue with clinical triage, and trying to work out who's the one in six who really need endovascular therapy, and who are the five in six who perhaps don't.

                                                And we're trying to develop clinical triage scales like the RACE scale to work out in the ambulance where someone should go. But we still haven't nailed that yet. Then you have scales that are very sensitive but not very specific, and have a high sort of false-positive rate. So then the question at the ambulance is, where does it go, to the hospital, the primary stroke center nearby, and give the patient the earliest opportunity to get tPA?

                                                And that's the potential benefit of early transfer to a primary center, but tPA is not very effective in dissolving these big clots in large arteries. And so, of course the trials have shown a substantial benefit of endovascular therapy to remove the clots via thrombectomy. But those resources, they're only really limited to comprehensive stroke units, and that's what this paper was about. So the trade-off is early transfer to the primary center so you can get some tPA, versus delaying, as Michael has shown, by 1 1/2 to two hours on average, to get to a comprehensive center that can access the expertise of endovascular thrombectomy experts.

                                                And this paper is really taking us forward in emphasizing again that time is brain, and we really don't want to delay. Perhaps there's a small trade-off in driving a little bit further, another 20 miles at the most perhaps, to get to a comprehensive center directly. And there may be some who are not shown to have a large vessel occlusion at that comprehensive stroke center, but the overall benefit is probably offset, the few who might miss out on tPA. And so this is a really important study, the largest registry of large vessel occlusion patients to observe and compare the outcomes after adjusting for all the different factors. And give us some clues, that perhaps we really need to be trying to focus on building our resources in comprehensive stroke centers, and also being able to more accurately identify those who are likely to benefit and go directly there.

Dr. Michael Froehler:     I agree with everything Graeme said, and I would just amplify one thing that he said, that it does depend on distance, and those distances in turn depend on your own geography. We did an analysis of all our transferred patients and then limited it to those that were within a 20-mile radius. For Graeme in Western Australia, you know Graeme's mailbox is probably 20 miles away. And so there are huge distances in Western Australia to account for. And it may not be possible.

Dr. Carolyn Lam:               Contrast that to me in Singapore. I think if I drive any bit more, and I'll be driving out of my country already.

Dr. Michael Froehler:     I think that you make a great point though, Carolyn, that the solution that works for metro Singapore is not what's going to work for rural Western Australia. And we've seen this in New York City, for example. My colleagues at Mount Sinai are looking at different ways to deliver care across metro New York, which obviously is very different compared to myself in Nashville, Tennessee. So the right solution is not gonna be the same solution for everyone.

Dr. Graeme Hankey:       And that's right Carolyn, because in rural places like out in Western Australia, we are learning now that another important message is to try and help upscale and reorganize our primary stroke centers, or just our medical centers out in the rural and remote areas. Because as Mike's paper shows, the delays once someone comes to a primary stroke center or a rural center, is about 30 minutes for diagnosis, about 30 minutes to arrange the transport, and about 30 minutes to actually do the transport.

                                                So we need to once trying to develop comprehensive stroke units, also build up those peripheral hub and spoke centers to be more slick with their diagnosis, arrangement of transport, and transport times. And one of the important things I think is, we need our primary centers, when a stroke does come, to not just do a plain CT to exclude hemorrhage, but to do a CT angiogram at the time. And find out those who really do have an occlusion, rather than putting them all on the plane and sending them down, and quite a few of them don't actually have an occlusion by the time that they've got here. They haven't been fully investigated, it's just an extra five minutes to do the contrast CT angiogram at the time in the primary center if they're gonna go there.

Dr. Michael Froehler:     I think the one other thing I should add, and this is just to reflect back on something Graeme said a minute ago, is that one of the differences we found that really came out of that bypass analysis is the impact on tPA was smaller than we expected. Because the door-to-needle times are actually much longer at the regional hospitals that are not thrombectomy-capable, compared to the thrombectomy centers themselves, that are not only obviously delivering mechanical thrombectomy, but are actually delivering IV-tPA much sooner in terms of door-to-needle times.

Dr. Carolyn Lam:               So, room for improvement even for non-endovascular-capable centers, isn't it?

Dr. Michael Froehler:     Right, I think it's another area where there's room for improvement.

Dr. Carolyn Lam:               Please don't forget to tune in again next week.