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Searching for Ways to Prevent Life-Threatening Blood Clots in COVID-19

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At Home with Gary Gibbons

Six months into the coronavirus disease 2019 (COVID-19) pandemic, researchers still have much to learn about the many ways in which COVID-19 can wreak devastation on the human body. Among the many mysteries is exactly how SARS-CoV-2, which is the novel coronavirus that causes COVID-19, triggers the formation of blood clots that can lead to strokes and other life-threatening complications, even in younger people.

Recently, I had a chance to talk with Dr. Gary Gibbons, Director of NIH’s Heart, Lung, and Blood Institute (NHLBI) about what research is being done to tackle this baffling complication of COVID-19. Our conversation took place via videoconference, with him connecting from his home in Washington, D.C., and me linking in from my home just up the road in Maryland. Here’s a condensed transcript of our chat:

Collins: I’m going to start by asking about the SARS-CoV-2-induced blood clotting not only in the lungs, but in other parts of the body. What do we know about the virus that would explain this?

Gibbons: It seems like every few weeks another page gets turned on COVID-19, and we learn even more about how this virus affects the body. Blood clots are one of the startling and, unfortunately, devastating complications that emerged as patients were cared for, particularly in New York City. It became apparent that certain individuals had difficulty getting enough oxygen into their system. The difficulty couldn’t be explained entirely by the extent of the pneumonia affecting the lungs’ ability to exchange oxygen.

It turned out that, in addition to the pneumonia, blood clots in the lungs were compromising oxygenation. But some patients also had clotting, or thrombotic, complications in their veins and arteries in other parts of the body. Quite puzzling. There were episodes of relatively young individuals in their 30s and 40s presenting with strokes related to blood clots affecting the arterial circulation to the brain.

We’re still trying to understand what promotes the clotting. One clue involves the endothelial cells that form the inner lining of our blood vessels. These cells have on their surface a protein called the angiotensin-converting enzyme 2 (ACE2) receptor, and this clue is important for two reasons. One, the virus attaches to the ACE2 receptor, using it as an entry point to infect cells. Two, endothelial-lined blood vessels extend to every organ in the body. Taken together, it seems that some COVID-19 complications relate to the virus attaching to endothelial cells, not only in the lungs, but in the heart and multiple organs.

Collins: So, starting in the respiratory tree, the virus somehow breaks through into a blood vessel and then gets spread around the body. There have been strange reports of people with COVID-19 who may not get really sick, but their toes look frostbitten. Is “COVID toes,” as some people call it, also part of this same syndrome?

Gibbons: We’re still in the early days of learning about this virus. But I think this offers a further clue that the virus not only affects large vessels but small vessels. In fact, clots have been reported at the capillary level, and that’s fairly unusual. It’s suggestive that an interaction is taking place between the platelets and the endothelial surface.

Normally, there’s a tightly regulated balance in the bloodstream between pro-coagulant and anticoagulant proteins to prevent clotting and keep the blood flowing. But when you cut your finger, for example, you get activation for blood clots in the form of a protein mesh. It looks like a fishing net that can help seal the injury. In addition, platelets in the blood stream help to plug the holes in that fishing net and create a real seal of a blood vessel.

Well, imagine it happening in those small vessels, which usually have a non-stick endothelial surface, almost like Teflon, that prevents clotting. Then the virus comes along and tips the balance toward promoting clot formation. This disturbs the Teflon-like property of the endothelial lining and makes it sticky. It’s incredible the tricks this virus has learned by binding onto one of these molecules in the endothelial lining.

Collins: Who are the COVID-19 patients most at risk for this clotting problem?

Gibbons: Unfortunately, it appears right now that older adults are among the most vulnerable. They have a lot of the risks for the formation of these blood clots. What’s notable is these thrombotic complications are also happening to relatively young adults or middle-aged individuals who don’t have a lot of other chronic conditions, or comorbidities, to put them at higher risk for severe disease. Again, it’s suggestive that this virus is doing something that is particular to the coagulation system.

Collins: We’d love to have a way of identifying in advance the people who are most likely to get into trouble with blood clotting. They might be the ones you’d want to start on an intervention, even before you have evidence that things are getting out of control. Do you have any kind of biomarker to tell you which patients might benefit from early intervention?

Gibbons: Biomarkers are being actively studied. What we do know from some earlier observations is that you can assess the balance of clotting and anticlotting factors in the blood by measuring a biomarker called D-dimer. It’s basically a protein fragment, a degradation product, from a prior clot. It tells you a bit about the system’s activity in forming and dissolving clots.
If there’s a lot of D-dimer activity, it suggests a coagulation cascade is jazzed up. In those patients, it’s probably a clue that this is a big trigger in terms of coagulation and thrombosis. So, D-dimer levels could maybe tell us which patients need really aggressive full anticoagulation.

Collins: Have people tried empirically using blood thinners for people who seem to be getting into trouble with this clotting problem?

Gibbons: There’s a paper out of the Mount Sinai in New York City that looked at thousands of patients being treated for COVID-19 [1]. Based on clinical practice and judgments, one of the striking findings is that those who were fully anticoagulated had better survival than those who were not. Now, this was not a randomized, controlled clinical trial, where some were given full anticoagulation and others were not. It was just an observational study that showed an association. But this study indicated indirectly that by giving the blood thinners, changing that thrombotic risk, maybe it’s possible to reduce morbidity and mortality. That’s why we need to do a randomized, controlled clinical trial to see if it can be used to reduce these case fatality rates.

Collins: You and your colleagues got together and came up with a design for such a clinical trial. Tell us about that.

Gibbons: My institute studies the heart, lung, and blood. The virus attacks all three. So, our community has a compelling need to lean in and study COVID-19. Recently, NIH helped to launch a public-private partnership called Accelerating COVID-19 Therapeutic Interventions and Vaccines (ACTIV). As the name spells out, this initiative provides is a clinical platform to generate life-saving treatments as we wait for the development of a vaccine.

Through ACTIV, a protocol is now in the final stages of review for a clinical trial that will involve a network of hospitals and explore the question: is it sufficient to try a low-dose thrombo-prophylactic, or clot preventative, approach versus full anticoagulation? Some think patients ought to have full anticoagulation, but that’s not without risk. So, we want to put that question to the test. As part of that, we’ll also learn more about biomarkers and what could be predictive of individuals getting the greatest benefit.

If we find that fully anticoagulating patients prevents clots, then that’s great. But it begs the question: what happens when patients go home? Is it sufficient to just turn off the drip and let them go their merry way? Should they have a low dose thrombo-prophylactic regimen for a period of time? If so, how long? Or should they be fully anticoagulated with oral anticoagulation for a certain period of time? All these and other questions still remain.

Collins: This can make a huge difference. If you’re admitted to the hospital with COVID-19, that means you’re pretty sick and, based on the numbers that I’ve seen, your chance of dying is about 12 percent if nothing else happens. If we can find something like an anticoagulant that would reduce that risk substantially, we can have a huge impact on reducing deaths from COVID-19. How soon can we get this trial going, Gary?

Gibbons: We have a sense of urgency that clearly this pandemic is taking too many lives and time is of the essence. So, we’ve indeed had a very streamlined process. We’re leveraging the fact that we have clinical trial networks, where regardless of what they were planning to do, it’s all hands on deck. As a result, we’re able to move faster to align with that sense of urgency. We hope that we can be off to a quick launch within the next two to three weeks with the anticoagulation trials.

Collins: This is good because people are waiting on the vaccines, but realistically we won’t know whether the vaccines are working for several more months, and having them available for lots of people will be at the very end of this year or early 2021 at best. Meanwhile, people still are going to be getting sick with COVID-19. We want to be able to have as many therapeutic options as possible to offer to them. And this seems like a pretty exciting one to try and move forward as quickly as possible. You and your colleagues deserve a lot of credit for bringing this to everybody’s attention.

But before we sign off, I have to raise another issue of deep significance. Gary, I think both of us are struggling not only with the impact of COVID-19 on the world, but the profound sorrow, grief, frustration, and anger that surrounds the death of George Floyd. This brings into acute focus the far too numerous other circumstances where African Americans have been mistreated and subjected to tragic outcomes.

This troubling time also shines a light on the health disparities that affect our nation in so many ways. We can see what COVID-19 has done to certain underrepresented groups who have borne an undue share of the burden, and have suffered injustices at the hands of society. It’s been tough for many of us to admit that our country is far from treating everyone equally, but it’s a learning opportunity and a call to redouble our efforts to find solutions.

Gary, you’ve been a wonderful leader in that conversation for a long time. I want to thank you both for what you’re doing scientifically and for your willingness to speak the truth and stand up for what’s right and fair. It’s been great talking to you about all these issues.

Gibbons: Thank you. We appreciate this opportunity to fulfill NIH’s mission of turning scientific discovery into better health for all. If there’s any moment that our nation needs us, this is it.


[1] Association of Treatment Dose Anticoagulation With In-Hospital Survival Among Hospitalized Patients With COVID-19. Paranjpe I, Fuster V, Lala A, Russak A, Glicksberg BS, Levin MA, Charney AW, Narula J, Fayad ZA, Bagiella E, Zhao S, Nadkarni GN. J Am Coll Cardiol. 2020 May 5;S0735-1097(20)35218-9.


Coronavirus (COVID-19) (NIH)

Rising to the Challenge of COVID-19: The NHLBI Community Response,” Director’s Messages, National Heart, Lung, and Blood Institute/NIH, April 29, 2020.

Accelerating COVID-19 Therapeutic Interventions and Vaccines (ACTIV) (NIH)

Bringing Needed Structure to COVID-19 Drug Development

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SARS-Cov-2 Molecular Map
Caption: Molecular map showing interaction between the spike protein (gold) of the novel coronavirus and the peptidase domain (blue) of human angiotensin-converting enzyme 2 (ACE2). Credit: Adapted from Yan R., Science, 2020.

With so much information swirling around these days about the coronavirus disease 2019 (COVID-19) pandemic, it would be easy to miss one of the most interesting and significant basic science reports of the past few weeks. It’s a paper published in the journal Science [1] that presents an atomic-scale snapshot showing the 3D structure of the spike protein on the novel coronavirus attached to a human cell surface protein called ACE2, or angiotensin converting enzyme 2. ACE2 is the receptor that the virus uses to gain entry.

What makes this image such a big deal is that it shows—in exquisite detail—how the coronavirus attaches to human cells before infecting them and making people sick. The structural map of this interaction will help guide drug developers, atom by atom, in devising safe and effective ways to treat COVID-19.

This new work, conducted by a team led by Qiang Zhou, Westlake Institute for Advanced Study, Hangzhou, China, took advantage of a high-resolution imaging tool called cryo-electron microscopy (cryo-EM). This approach involves flash-freezing molecules in liquid nitrogen and bombarding them with electrons to capture their images with a special camera. When all goes well, cryo-EM can solve the structure of intricate macromolecular complexes in a matter of days, including this one showing the interaction between a viral protein and human protein.

Zhou’s team began by mapping the structure of human ACE2 in a complex with B0AT1, which is a membrane protein that it helps to fold. In the context of this complex, ACE2 is a dimer—a scientific term for a compound composed of two very similar units. Additional mapping revealed how the surface protein of the novel coronavirus interacts with ACE2, indicating how the virus’s two trimeric (3-unit) spike proteins might bind to an ACE2 dimer. After confirmation by further research, these maps may well provide a basis for the design and development of therapeutics that specifically target this critical interaction.

The ACE2 protein resides on the surface of cells in many parts of the human body, including the heart and lungs. The protein is known to play a prominent role in the body’s complex system of regulating blood pressure. In fact, a class of drugs that inhibit ACE and related proteins are frequently prescribed to help control high blood pressure, or hypertension. These ACE inhibitors lower blood pressure by causing blood vessels to relax.

Since the COVID-19 outbreak, many people have wondered whether taking ACE inhibitors would be helpful or detrimental against coronavirus infection. This is of particular concern to doctors whose patients are already taking the medications to control hypertension. Indeed, data from China and elsewhere indicate hypertension is one of several coexisting conditions that have consistently been reported to be more common among people with COVID-19 who develop life-threatening severe acute respiratory syndrome.

In a new report in this week’s New England Journal of Medicine, a team of U.K. and U.S. researchers, partly supported by NIH, examined the use of ACE inhibitors and other angiotensin-receptor blockers (ARBs) in people with COVID-19. The team, led by Scott D. Solomon of Brigham and Women’s Hospital and Harvard Medical School, Boston, found that current evidence in humans is insufficient to support or refute claims that ACE inhibitors or ARBs may be helpful or harmful to individuals with COVID-19.

The researchers concluded that these anti-hypertensive drugs should be continued in people who have or at-risk for COVID-19, stating: “Although additional data may further inform the treatment of high-risk patients … clinicians need to be cognizant of the unintended consequences of prematurely discontinuing proven therapies in response to hypothetical concerns.” [2]

Research is underway to generate needed data on the use of ACE inhibitors and similar drugs in the context of the COVID-19 pandemic, as well as to understand more about the basic mechanisms underlying this rapidly spreading viral disease. This kind of fundamental research isn’t necessarily the stuff that will make headlines, but it likely will prove vital to guiding the design of effective drugs that can help bring this serious global health crisis under control.


[1] Structural basis for the recognition of the SARS-CoV-2 by full-length human ACE2. Yan R, Zhang Y, Li Y, Xia L, Guo Y, Zhou Q. Science. 27 March 2020. [Epub ahead of publication]

[2] Renin–Angiotensin–Aldosterone System Inhibitors in Patients with Covid-19. Vaduganathan M, Vardeny O, Michel T, McMurray J, Pfeffer MA, Solomon SD. 30 NEJM. March 2020 [Epub ahead of Publication]


Coronavirus (COVID-19) (NIH)

COVID-19, MERS & SARS (National Institute of Allergy and Infectious Diseases/NIH)

Transformative High Resolution Cryo-Electron Microscopy (Common Fund/NIH)

Qiang Zhou (Westlake Institute for Advanced Study, Zhejiang Province)

Scott D. Solomon (Brigham and Women’s Hospital, Boston)

NIH Support: National Center for Advancing Translational Sciences; National Heart, Lung, and Blood Institute

3D Printing a Human Heart Valve

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It is now possible to pull up the design of a guitar on a computer screen and print out its parts on a 3D printer equipped with special metal or plastic “inks.” The same technological ingenuity is also now being applied with bioinks—printable gels containing supportive biomaterials and/or cells—to print out tissue, bone, blood vessels, and, even perhaps one day, viable organs.

While there’s a long way to go until then, a team of researchers has reached an important milestone in bioprinting collagen and other extracellular matrix proteins that undergird every tissue and organ in the body. The researchers have become so adept at it that they now can print biomaterials that mimic the structural, mechanical, and biological properties of real human tissues.

Take a look at the video. It shows a life-size human heart valve that’s been printed with their improved collagen bioink. As fluid passes through the aortic valve in a lab test, its three leaf-like flaps open and close like the real thing. All the while, the soft, flexible valve withstands the intense fluid pressure, which mimics that of blood flowing in and out of a beating heart.

The researchers, led by NIH grantee Adam Feinberg, Carnegie Mellon University, Pittsburgh, PA, did it with their latest version of a 3D bioprinting technique featured on the blog a few years ago. It’s called: Freeform Reversible Embedding of Suspended Hydrogels v.2.0. Or, just FRESH v2.0.

The FRESH system uses a bioink that consists of collagen (or other soft biomaterials) embedded in a thick slurry of gelatin microparticles and water. While a number of technical improvements have been made to FRESH v. 2.0, the big one was getting better at bioprinting collagen.

The secret is to dissolve the collagen bioink in an acid solution. When extruded into a neutral support bath, the change in pH drives the rapid assembly of collagen. The ability to extrude miniscule amounts and move the needle anywhere in 3D space enables them to produce amazingly complex, high-resolution structures, layer by layer. The porous microstructure of the printed collagen also helps for incorporating human cells. When printing is complete, the support bath easily melts away by heating to body temperature.

As described in Science, in addition to the working heart valve, the researchers have printed a small model of a heart ventricle. By combining collagen with cardiac muscle cells, they found they could actually control the organization of muscle tissue within the model heart chamber. The 3D-printed ventricles also showed synchronized muscle contractions, just like you’d expect in a living, beating human heart!

That’s not all. Using MRI images of an adult human heart as a template, the researchers created a complete organ structure including internal valves, large veins, and arteries. Based on the vessels they could see in the MRI, they printed even tinier microvessels and showed that the structure could support blood-like fluid flow.

While the researchers have focused the potential of FRESH v.2.0 printing on a human heart, in principle the technology could be used for many other organ systems. But there are still many challenges to overcome. A major one is the need to generate and incorporate billions of human cells, as would be needed to produce a transplantable human heart or other organ.

Feinberg reports more immediate applications of the technology on the horizon, however. His team is working to apply FRESH v.2.0 for producing child-sized replacement tracheas and precisely printed scaffolds for healing wounded muscle tissue.

Meanwhile, the Feinberg lab generously shares its designs with the scientific community via the NIH 3D Print Exchange. This innovative program is helping to bring more 3D scientific models online and advance the field of bioprinting. So we can expect to read about many more exciting milestones like this one from the Feinberg lab.


[1] 3D bioprinting of collagen to rebuild components of the human heart. Lee A, Hudson AR, Shiwarski DJ, Tashman JW, Hinton TJ, Yerneni S, Bliley JM, Campbell PG, Feinberg AW. Science. 2019 Aug 2;365(6452):482-487.


Tissue Engineering and Regenerative Medicine (National Institute of Biomedical Imaging and Bioengineering/NIH)

Regenerative Biomaterials and Therapeutics Group (Carnegie Mellon University, Pittsburgh, PA)

FluidForm (Acton, MA)

3D Bioprinting Open Source Workshops (Carnegie Mellon)

Video: Adam Feinberg on Tissue Engineering to Treat Human Disease (YouTube)

NIH 3D Print Exchange

NIH Support: National Heart, Lung, and Blood Institute; Eunice Kennedy Shriver National Institute of Child Health and Human Development; Common Fund

Using Artificial Intelligence to Catch Irregular Heartbeats

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ECG Readout
Credit: gettyimages/enot-poloskun

Thanks to advances in wearable health technologies, it’s now possible for people to monitor their heart rhythms at home for days, weeks, or even months via wireless electrocardiogram (EKG) patches. In fact, my Apple Watch makes it possible to record a real-time EKG whenever I want. (I’m glad to say I am in normal sinus rhythm.)

For true medical benefit, however, the challenge lies in analyzing the vast amounts of data—often hundreds of hours worth per person—to distinguish reliably between harmless rhythm irregularities and potentially life-threatening problems. Now, NIH-funded researchers have found that artificial intelligence (AI) can help.

A powerful computer “studied” more than 90,000 EKG recordings, from which it “learned” to recognize patterns, form rules, and apply them accurately to future EKG readings. The computer became so “smart” that it could classify 10 different types of irregular heart rhythms, including atrial fibrillation (AFib). In fact, after just seven months of training, the computer-devised algorithm was as good—and in some cases even better than—cardiology experts at making the correct diagnostic call.

EKG tests measure electrical impulses in the heart, which signal the heart muscle to contract and pump blood to the rest of the body. The precise, wave-like features of the electrical impulses allow doctors to determine whether a person’s heart is beating normally.

For example, in people with AFib, the heart’s upper chambers (the atria) contract rapidly and unpredictably, causing the ventricles (the main heart muscle) to contract irregularly rather than in a steady rhythm. This is an important arrhythmia to detect, even if it may only be present occasionally over many days of monitoring. That’s not always easy to do with current methods.

Here’s where the team, led by computer scientists Awni Hannun and Andrew Ng, Stanford University, Palo Alto, CA, saw an AI opportunity. As published in Nature Medicine, the Stanford team started by assembling a large EKG dataset from more than 53,000 people [1]. The data included various forms of arrhythmia and normal heart rhythms from people who had worn the FDA-approved Zio patch for about two weeks.

The Zio patch is a 2-by-5-inch adhesive patch, worn much like a bandage, on the upper left side of the chest. It’s water resistant and can be kept on around the clock while a person sleeps, exercises, or takes a shower. The wireless patch continuously monitors heart rhythms, storing EKG data for later analysis.

The Stanford researchers looked to machine learning to process all the EKG data. In machine learning, computers rely on large datasets of examples in order to learn how to perform a given task. The accuracy improves as the machine “sees” more data.

But the team’s real interest was in utilizing a special class of machine learning called deep neural networks, or deep learning. Deep learning is inspired by how our own brain’s neural networks process information, learning to focus on some details but not others.

In deep learning, computers look for patterns in data. As they begin to “see” complex relationships, some connections in the network are strengthened while others are weakened. The network is typically composed of multiple information-processing layers, which operate on the data and compute increasingly complex and abstract representations.

Those data reach the final output layer, which acts as a classifier, assigning each bit of data to a particular category or, in the case of the EKG readings, a diagnosis. In this way, computers can learn to analyze and sort highly complex data using both more obvious and hidden features.

Ultimately, the computer in the new study could differentiate between EKG readings representing 10 different arrhythmias as well as a normal heart rhythm. It could also tell the difference between irregular heart rhythms and background “noise” caused by interference of one kind or another, such as a jostled or disconnected Zio patch.

For validation, the computer attempted to assign a diagnosis to the EKG readings of 328 additional patients. Independently, several expert cardiologists also read those EKGs and reached a consensus diagnosis for each patient. In almost all cases, the computer’s diagnosis agreed with the consensus of the cardiologists. The computer also made its calls much faster.

Next, the researchers compared the computer’s diagnoses to those of six individual cardiologists who weren’t part of the original consensus committee. And, the results show that the computer actually outperformed these experienced cardiologists!

The findings suggest that artificial intelligence can be used to improve the accuracy and efficiency of EKG readings. In fact, Hannun reports that iRhythm Technologies, maker of the Zio patch, has already incorporated the algorithm into the interpretation now being used to analyze data from real patients.

As impressive as this is, we are surely just at the beginning of AI applications to health and health care. In recognition of the opportunities ahead, NIH has recently launched a working group on AI to explore ways to make the best use of existing data, and harness the potential of artificial intelligence and machine learning to advance biomedical research and the practice of medicine.

Meanwhile, more and more impressive NIH-supported research featuring AI is being published. In my next blog, I’ll highlight a recent paper that uses AI to make a real difference for cervical cancer, particularly in low resource settings.


[1] Cardiologist-level arrhythmia detection and classification in ambulatory electrocardiograms using a deep neural network. Hannun AY, Rajpurkar P, Haghpanahi M, Tison GH, Bourn C, Turakhia MP, Ng AY.
Nat Med. 2019 Jan;25(1):65-69.


Arrhythmia (National Heart, Lung, and Blood Institute/NIH)

Video: Artificial Intelligence: Collecting Data to Maximize Potential (NIH)

Andrew Ng (Palo Alto, CA)

NIH Support: National Heart, Lung, and Blood Institute

Modeling Hypertrophic Cardiomyopathy in a Dish

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Image of cardiac fibers

Credit: Zhen Ma, University of California, Berkeley

Researchers have learned in recent years how to grow miniature human hearts in a dish. These “organoids” beat like the real thing and have allowed researchers to model many key aspects of how the heart works. What’s been really tough to model in a dish is how stresses on hearts that are genetically abnormal, such as in inherited familial cardiomyopathies, put people at greater risk for cardiac problems.

Enter the lab-grown human cardiac tissue pictured above. This healthy tissue comprised of the heart’s muscle cells, or cardiomyocytes (green, nuclei in red), was derived from induced pluripotent stem (iPS) cells. These cells are derived from adult skin or blood cells that are genetically reprogrammed to have the potential to develop into many different types of cells, including cardiomyocytes.

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