Mar 25, 2019
Dr Carolyn Lam: Welcome to Circulation on the Run, your weekly podcast summary and backstage pass to the journal and its editors. We're your co-hosts, I'm Dr Carolyn Lam, associate editor from the National Heart Center and Duke National University of Singapore.
Dr Greg Hundley: I'm Greg Hundley, associated editor from the Pauley Heart Center at VCU Health Sciences in Richmond, Virginia.
Dr Carolyn Lam: A big number of acute ischemic stroke patients receiving endovascular therapy in the United States are receiving this therapy only after inter-hospital transfer. What are the temporal transient outcomes following this inter-hospital transfer? Very important discussion coming right up with our featured paper. But for now, sit back, relax with us. We're going to discuss a couple of papers that we found were interesting in this week's journal.
Dr Greg Hundley: Very good, so thanks Carolyn. I'll start off, and I'm going to talk a little bit about stress induced cardiomyopathy, and we also know it as takotsubo cardiomyopathy, looking at a paper from Dana Dawson from the University of Aberdeen in the United Kingdom. Takotsubo cardiomyopathy can result in a heart failure phenotype with a prognosis comparable to myocardial infarction.
In this study, the investigators hypothesize that inflammation is central to the pathophysiology in natural history of takotsubo cardiomyopathy. They prospectively recruited 55 patients with takotsubo cardiomyopathy, and 51 age, sex, and comorbidity match control subjects.
During the index event, and at five months of follow-up, the patients with takotsubo cardiomyopathy underwent a cardiac MRI study in which they looked at ultra-small, super paramagnetic particles of iron oxide, or USPIOs, enhancement for detection of inflammatory macrophages in the myocardium. What would the studies show? Patients with acute takotsubo cardiomyopathy had macrophage-mediated myocardial inflammation.
They also demonstrated modulation of peripheral monocyte subsets and increased systemic pro-inflammatory cytokines. This systemic inflammation persisted for five months, and then at that five-month time point, the cardiac MRI evidence of the macrophage presence was diminished.
Dr Carolyn Lam: Wow, Greg. So this is right up your wheelhouse, isn't it? Can you explain? What are the clinical implications of these MRI findings?
Dr Greg Hundley: It was really interesting. For the first time, they've linked an ongoing inflammatory process using the USPIO contrast agent with MRI actually going on or operative in the heart, and they associate that with systemic markers in the circulation.
They help us elucidate the mechanisms and the pathogenesis of takotsubo cardiomyopathy, and systemic and myocardial inflammation really may start to now serve as a therapeutic target for patients with acute takotsubo cardiomyopathy.
Dr Carolyn Lam: Very interesting. From stress-induced cardiomyopathy to early onset myocardial infarction. The first paper I chose really answers the question, "What is the relative prevalence and clinical importance of monogenic mutations, that is, a single mutation that significantly increases risk, versus a polygenic score, which really measures the cumulative impact of many common variants, in early onset myocardial infarction?"
The co-corresponding authors were Doctor Amit Khera and Sekar Kathiresan and both from Massachusetts General Hospital, and they performed deep coverage, whole genome sequencing of more than 2,000 patients from four racial subgroups hospitalized in the United States with early onset myocardial infarction defined as myocardial infarction before the age of 55 years, and compared this to 3,761 population base controls.
What they found was that a monogenic mutation related to familial hypercholesterolemia was identified in 1.7% of the patients, and associated with a 3.8-fold increased odd of myocardial infarction. In comparison, the high polygenic score, which was composed of 6.6 million common DNA variants and defined as the top 5% of the control population distribution, now, that was identified in 10 times as many patients, so 17% of patients, and associated with a similar 3.7-fold increased odds of myocardial infarction.
Dr Greg Hundley: Interesting. How do we apply this clinically, Carolyn?
Dr Carolyn Lam: These findings really lay the scientific foundation for the systematic identification of individuals born with a substantially increased risk of myocardial infarction. The important point is both familial hypercholesterol mutations and a high polygenic score are associated with more than three-fold increased odds of an early onset myocardial infarction.
However, the high polygenic score cannot be reliably identified on the basis of elevated LDL cholesterol, and yet has a 10-fold higher prevalence among patients presenting with early onset myocardial infarction. So very intriguing that both groups matter.
Dr Greg Hundley: Very good. My next paper is from Adrian Hobbs at the London School of Medicine, and is looking at the role of endothelial C type natriuretic peptide as a critical regulator of angiogenesis and vascular remodeling. We know that a central pathway coordinating both neovascularization and ischemic extremities in PAD is driven by vascular endothelial growth factor or VEGF-A4.
But preclinical studies and other large scale clinical trials have been disappointing because administering or using VEGF-A to promote angiogenesis or arteriogenesis in PAD really hasn't occurred. This group focused on endothelial-derived CMP. Why? Because it plays a fundamental role in regulating vascular homeostasis. It controls local blood flow and the resistance vasculature, and systemic blood pressure, and reduces the reactivity of leukocytes and platelets.
So, what were the results? Clinical vascular ischemia was associated with reduced levels of CMP and it's cognate NPR-C. Moreover, genetic and pharmacological inhibition of CNP and NPR-C reduced the angiogenic potential of the pulmonary microvascular endothelial cells and the human umbilical vein endothelial, and it isolated vessels ex vivo.
So, the study really defines a central pathophysiological role for endothelium-derived C type natriuretic peptide via activation of cognate natriuretic peptide receptor C in angiogenesis and in vascular remodeling. Moreover, the work demonstrates the therapeutic utility of pharmacologically targeting NPR-C to restore deficits in these processes following ischemia and injury.
Dr Carolyn Lam: Interesting, from new mechanisms and targets to good, old, major risk factors for coronary heart disease. Back to the basics but in a really, I think, nicely done paper from Dr Pencina and colleagues from Duke Clinical Research Institute.
Now, their objective in this next paper was to compare the associations of key, modifiable coronary heart disease risk factors with incident coronary heart disease events based on their prognostic performance, the attributable risk fractions and treatment benefits overall and by age.
And so really aiming at quantifying the importance of these major, modifiable risk factors for coronary heart disease. What they did is they used pool participant level data from four observational cohort studies sponsored by the NHLBI, and they created a cohort of more than 22,600 individuals ages 45 to 84 years old who are initially free of cardiovascular disease.
And these individuals were followed for 10 years from baseline evaluation and followed for incident coronary heart disease. They estimated that age, sex and race captured up to 80% of the prognostic performance of cardiovascular risk models. When we add either systolic blood pressure or non-HDL cholesterol, diabetes or smoking to model with the other risk factors, the prognostic performance, as measured by the C index, increased by only 0.004 to 0.013.
However, if you look at it from the attributable risk and absolute risk reduction standpoint, lowering the systolic blood pressure of all individuals to less than 130, or lowering LDL cholesterol by 30% would be expected to lower a baseline, 10-year coronary heart disease risk of 10% to 7% and 8% respectively.
Dr Greg Hundley: That's a lot of data, Carolyn. Help me synthesize all that.
Dr Carolyn Lam: This is a take-home message. Although the individual modifiable risk factors contribute only modestly to the overall model prognostic performance, when we eliminate or control these risk factors, they would actually lead to a substantial reduction in total population coronary heart disease.
That's because if we look at the attributable fraction and the absolute risk reductions, we see that they actually really matter. The take-home message too from Dr Pencina was that metrics used to judge the importance of these risk factors should therefore be tailored to the question being asked.
Dr Greg Hundley: Very good. That was a very nice summary, Carolyn.
Dr Carolyn Lam: Thanks. Let's move on now to our feature discussion, shall we?
Dr Greg Hundley: Very good.
Dr Carolyn Lam: Trials have established that endovascular thrombectomy dramatically reduces disability after acute ischemic stroke due to intracranial large vessel occlusion. In fact, guidelines almost immediately adopted endovascular thrombectomy as a standard of care. However, that has created some problems.
The main one being that hospitals equipped to carry out this procedure are largely limited to tertiary centers in urban areas. This is, of course, important because that means that patients may need to be transferred from another center to receive such treatment.
Today's feature paper discusses this very issue, a terribly important one, and I'm so pleased to have the author with us, Dr Shreyansh Shah from Duke University Medical Center. We have our editorialist, Dr James Grotta who's director of the Mobile Stroke Unit project at Memorial Herman Hospital.
And we have an associate editor, Dr Graeme Hankey from University of Western Australia. So, such an important topic. I think Shrey, could you just jump right in and tell us what your study showed.
Dr Shreyansh Shah: I'm very excited to present findings of our study, and as a Carolyn mentioned, this study is going to have a very important implication in our country here in US on the creation of systems of stroke. I think the findings are already applicable to other countries also where we are seeing endovascular care getting more and more used.
As Carolyn was talking, endovascular treatment is very important and lifesaving measure. But unfortunately, it is not available at every hospital. Patients are often transferred across different hospital or institution before they can receive this endovascular care.
What we did in our project was we looked at the data from the hospital that's participating in Get With The Guidelines®® Stroke, which is a quality improvement program here in US. It looked at the endovascular thrombectomy used especially in relation to inter-hospital transfer.
What we found was big proportion of patients receiving endovascular care, up to about 43% to 45% of patients, were getting the care after transferring across different hospital. The outcomes in this patient were worse compared to the patient who were receiving endovascular care if they had come directly to the hospital.
While there was no difference in mortality between these two groups, the endovascular care, after inter-hospital transfer, resulted in a higher rate of symptomatic ICH, patients are less likely to be discharged to home, which is the preferred outcome. And patient was also less likely to be able to ambulate independently prior to the hospital discharge.
There was also delay in endovascular care initiation for patient who received this after inter-hospital transfer. I think this particular study highlights the magnitude of this problem, and that's why it's going to be important for people who are studying systems of care. The fact that about 45% of patient had to get inter-hospital transfer before endovascular care tells us that we still need to take significant steps in increasing access to this lifesaving therapy.
Dr Carolyn Lam: Thank you and indeed James, I really love the editorial you wrote that accompanied this. I mean you highlighted its importance, and you also noted that what was unusual about the paper was that even after controlling for the delay in initiating endovascular thrombectomy, there was still worse outcomes in the patients who were transferred. Could you share some thoughts?
Dr James Grotta: It is a very timely issue. Now that we have a very effective treatment, the big challenge we have is getting it to the patients as fast as possible. Right now, our system, as is pointed out, means shuffling patients from one hospital to another.
I think that clearly with stroke treatment, any sort of stroke treatment, the faster we deliver it, the better. Other studies have shown that transferring patients is associated with a delay of treatment, and this study showed the same thing.
There was a substantial delay in getting the patients treated if they required a transfer. And as you pointed out, however, this did not explain the entire or was not at least the entire explanation for the worst outcome. So, it is a little bit of a mystery.
I do know from personal experience that transferring patients from hospital to hospital, it's not exactly a black hole, but you lose control of the patient when they're being transferred. These are patients who have large artery occlusions. That means they have their middle cerebral artery is blocked.
And so, the area of brain that's affected is in a very tenuous shape. So, any drop-in oxygen concentration from breathing problems or of any drop-in blood pressure might further worsen the stroke. So, this could happen in transit. So, it's possible that in the process of transfer, these sorts of things happen.
I do think that we do have to be a little bit careful in that by remembering that this was not a randomized comparison, so patients that were treated directly and those that were transferred were not randomized. And so, although they appear to be balanced in a lot of the important variables like their stroke severity, there may be other things that we can't account for that could explain some of the worst outcomes.
I'd like to ask Dr Shah whether he identified any things in ... well, he and his co-authors think might have contributed to some of the worst outcomes.
Dr Shreyansh Shah: To answer Dr Grotta's question about what other factors may have played a role in the worst outcome that we saw in patients who were getting inter-hospital transfer, I think as we correctly pointed out, transferring this very sick patient is very tricky. As we know, the hemodynamic instability or variability plays an important role in outcomes of stroke patient.
And it is very likely that during the transfer process, there is not adequate control of their blood pressure variability, their oxygen saturation, and this ends up affecting their brain leading to worst outcome. The other possibilities also, as Dr Grotta was explaining, this is not a randomized control trial.
And although we balance for number of important factors that can affect stroke outcome, there might be a selection bias in transferring patient who are more sicker and also patients who received thrombolysis with TPA but did not improve, while the patient who were directly arriving to the hospitals and getting endovascular care, they received the TPA.
It is possible that they started to improve and still received a thrombectomy at the same time. So that group may have been more favorable in that respect, which could have also played a role in better outcomes with patient who are directly arriving.
Dr Carolyn Lam: Interesting. And, you know, with the mention of TPA, I really have to bring James back. I loved your mention about potential solution using mobile stroke units. And since you direct one of them, could you tell us what you meant there?
Dr James Grotta: Yes, of course, I have to state at the outset that I have a little bit of a bias about mobile strokes, and so I do it every day. What a mobile stroke unit is, for those who don't know, it's basically taking the emergency department to the patient.
It's an ambulance with a CT scanner on board and the ability to treat with TPA in the field. But in addition, it's also the CT scanner. We can do CT angio and identify large vessel occlusions on the mobile stroke unit, not to mention the fact that you have a vascular neurologist either in-person or by telemedicine examining the patient.
So clinically, you can make the determination also much more accurately than any sort of pre-hospital stroke scale, whether the patient has a large artery occlusion. That way, you don't have to take the patient to the nearest hospital. You can bypass the nearest hospital, take them right to the thrombectomy center, therefore, avoiding the transfer process.
We've been implementing this in Houston, and there are now about 30 mobile stroke units around the world. The innovation actually started in Germany by Dr Fassbender about a decade ago in Hamburg, Germany. We are conducting a randomized trial, comparing mobile stroke unit care to standard management to see how much better outcomes occur as a result of this faster treatment.
We obviously can treat patients with TPA faster. For example, a similar study from the Get With The Guidelines® a few years ago showed that only 1% of patients treated with TPA in emergency departments get treated within the first hour after symptom onset simply because it takes an hour in the emergency room itself to do the evaluation of the patient and get them treated.
Whereas on our mobile stroke unit, at least a third and probably 40% of the patients we're treating with TPA, we can get treated within that first hour where there may be an exponential better benefit. But we don't yet know really how much that translates to better benefit, and also, of course, mobile stroke units are more intensive in terms of the amount of facilities on board and costs.
So, we need to look at the cost-effectiveness. If it produces only a marginal reduction in disability but costs a fortune, then it's not worth it. But in fact, in our experience, it's pretty practical. We can cover almost the entire City of Houston, which is the fourth largest city in the country, with one mobile stroke unit. When it's well-integrated, it requires careful integration with the fire department and other hospitals in the city.
Dr Shreyansh Shah: At those two conferences, I came across a very interesting talk from Dr Grotta's group about rendezvous with the EMS which allows extending their coverage area significantly. I think we definitely need more and more innovative solutions like this where we can identify patients by their origin, whether they have large vessel occlusion or not, and then triage them appropriately at the centers that can perform endovascular therapy. So as a result, we can provide them earlier therapy and hopefully, it will lead to better outcome.
Dr Carolyn Lam: Thank you Shrey and James for these incredible insights. Now, Graeme, I want you to have the last word and reflections from down under.
Dr Graeme Hankey: Firstly, just to congratulate Dr Shrey and colleagues on this terrific study that reports a contemporary United States experience, a very broad one across the country, really highlighting how since 2012, until a year ago, there's been a six-fold increase in the number of patients being transferred for endovascular therapy.
And we're all experiencing that around the world. And moreover, since the DAWN trial and the DEFUSE trial were published just over a year ago, which is when this study stopped, there's been an expansion of the window from six hours out to 24 hours.
So, in the last year, which this study doesn't cover, we've seen an exponential increase in the number of people being transferred from rural and remote areas who have had a stroke up to 24 hours ago being considered for endovascular therapy if their CT angiogram at the base hospital shows a large vessel occlusion.
This is likely to be not only internally valid, but externally valid to all of us around the world. It reflects our experience of this avalanche of cases coming. And it's provided a lot of challenges for those who are trying to deliver the service at the tertiary referral center.
And it highlights that nearly half of the cases who are having endovascular therapy are coming from external sites. As Jim has really highlighted in his editorial, it challenges us to reassess the current practice of inter-hospital transfer.
Dr Carolyn Lam: Thank you so much for publishing this paper with us and the editorial. And listeners, don't forget to tune in again next week. This program is copyright American Heart Association, 2019.