damianrasch.com

Understanding TAVR: A Cardiologist's Guide to Aortic Valve Replacement

Medically Reviewed & Edited

Board-Certified Invasive Cardiologist
Encinitas and La Jolla, CA

Developed with digital research and writing assistance, then medically reviewed and edited by Dr. Rasch to ensure clinical accuracy and adherence to current evidence-based guidelines.

A different kind of conversation

Not that long ago, if I sat down with a patient in their seventies who had developed severe aortic stenosis, the conversation went roughly like this. “You have a narrowed valve. It’s getting tighter. At some point it’s going to cause problems, and eventually you’ll need open-heart surgery to fix it.” I’d watch their face, and the face of whatever family member had come with them, fall. Open-heart surgery, for someone in their late seventies, is a lot to process. The sternum is split. The heart is stopped. A bypass machine takes over. Recovery is measured in months. Some people bounced back beautifully. Others didn’t.

That conversation has changed. For most patients with severe aortic stenosis who sit in my clinic today, the answer isn’t open-heart surgery. It’s a procedure called TAVR — transcatheter aortic valve replacement — where we deliver a new valve through a catheter in the groin, the patient is usually awake or lightly sedated, and they often go home the next day. It’s one of the most significant shifts cardiology has seen in a generation, and if you or someone you love is facing an aortic stenosis diagnosis, it’s worth understanding what it is, who it’s for, and, just as importantly, who it isn’t for.

This is the longer version of the conversation I have with patients in my office when severe aortic stenosis has been confirmed and we’re deciding what to do next. It’s meant to demystify TAVR without overselling it.

What’s actually going wrong inside

Your aortic valve is the one-way door between your heart’s main pumping chamber and the rest of your body. Every time your heart squeezes, it pushes blood through that valve into the aorta, and from there out to your brain, your kidneys, your muscles. The valve is a thin, flexible structure with three little leaflets that flap open to let blood through and flap closed to keep it from washing back.

Over time, and almost always slowly, that valve can stiffen and narrow. Calcium deposits build up on the leaflets. They become less flexible. The opening — which should be about the size of a quarter when the valve is wide open — starts to look more like a dime, then a nickel, then a smaller coin. Your heart has to push harder and harder to get the same amount of blood through a shrinking hole. This is aortic stenosis, and by the time it becomes severe, the pressure the heart muscle has to generate is two or three times what it should be.

For a long stretch of years, the heart compensates. The muscle thickens. Most people don’t feel anything. This is the stage where I find it on a routine exam, or where it shows up on an echocardiogram that was ordered for some other reason. At this stage we watch and wait. We do another echocardiogram in six months or a year. We don’t intervene, because the risks of the procedure outweigh the benefits in a patient without symptoms.

But severe aortic stenosis, once it starts causing symptoms, is one of the more dangerous conditions in cardiology. The three classic symptoms are chest pain with exertion, fainting or near-fainting, and shortness of breath — the signs that the heart can no longer compensate for the narrowed valve. From the point of symptom onset, the survival numbers for untreated severe aortic stenosis are sobering. We’re talking about a two-year life expectancy for some patterns, a few years longer for others, but the trajectory is downward and it’s steep. This is not a condition where we can keep watching indefinitely once symptoms start. It has to be fixed.

The good news is that when the valve is replaced, the heart, in most cases, recovers remarkably well. Patients often tell me they didn’t realize how tired they’d been until a month after the procedure, when they noticed they could walk up a hill without stopping. The valve was the brake. Once we release it, the engine does what it’s supposed to do.

How TAVR actually works

TAVR is, at its core, a plumbing trick. The old, stiff, calcified valve stays right where it is. We don’t cut it out. Instead, we squeeze a new valve onto the end of a long, thin tube — a catheter — thread the catheter up through an artery, and deploy the new valve inside the frame of the old one. The new valve expands, shoves the old leaflets out of the way, and immediately takes over the job.

The artery we use, in about ninety-five percent of cases, is the femoral artery at the top of the leg. A small puncture — not a cut — is made in the groin. The catheter is threaded up through the aorta and positioned across the diseased valve using live X-ray and, often, ultrasound guidance. When everything is lined up, the new valve is deployed. Some valves expand on their own, shape-memory metal springing back into position. Others are expanded with a balloon. Either way, the process of actually placing the valve takes a few minutes. The whole procedure, from sterile prep to closure, is typically somewhere between one and two hours.

Compare that to surgical aortic valve replacement — SAVR, the traditional open-heart operation. In SAVR, the surgeon opens the chest by splitting the breastbone, connects the patient to a heart-lung bypass machine, stops the heart, cuts out the old valve, and sews in a new one. It’s a masterful operation when it’s done well, and for the right patient it’s an excellent choice. But it’s still open-heart surgery. Recovery from SAVR means weeks of activity restriction, careful protection of the healing breastbone, and a return to full activity that’s typically measured in months rather than weeks.

TAVR avoids all of that. No sternotomy, no stopped heart, no bypass machine. The heart keeps beating the whole time. For many patients, the difference in how they feel on day three or day seven after the procedure is the single biggest reason this operation has changed cardiology.

What the day actually looks like

I want to walk through a typical TAVR day as concretely as I can, because the picture patients have in their heads is often stitched together from a prior family member’s open-heart experience, and it’s usually scarier than the reality.

You arrive at the hospital in the morning, having not eaten or had anything to drink since midnight. A nurse starts an IV. Someone shaves a small area in the groin. You change into a gown. A member of the team comes by to confirm everything. Most centers, including mine, now perform TAVR with what we call conscious sedation — medication that makes you sleepy and comfortable, but not fully unconscious. You aren’t intubated. You don’t have a breathing tube. Some patients doze through the whole thing. Others are awake enough to chat quietly with the team. In certain cases, for anatomical or other reasons, a general anesthetic is still the right choice, and that decision is made ahead of time.

In the procedure room, you lie flat on a table under an imaging arm. The groin area is numbed. Over the next forty-five minutes or so, the catheter is worked up through the aorta, the new valve is positioned, and it’s deployed. You may feel pressure in your chest for a moment as everything is seated. Most patients feel very little.

At the end of the procedure, the puncture in the groin is closed, usually with a closure device rather than stitches. You’re moved to a recovery area and lie flat for a few hours so the artery can seal. A temporary pacing wire is often placed as a precaution, and depending on how your heart’s electrical system behaves afterward, it’s removed either shortly after the procedure or the next morning.

At many centers now, patients go home the next day. Some stay a second night, particularly if they needed a permanent pacemaker or had another reason to stay. The average hospital stay for a modern transfemoral TAVR is somewhere between one and three days, a dramatic change from the old SAVR standard of five to seven days in the hospital with an ICU stay in there somewhere.

When you go home, you go home on a baby aspirin and usually a second mild blood thinner for a short period. You’re walking. You can eat whatever you want. A week of taking it easy, no lifting anything heavier than a gallon of milk, no long drives, and most patients feel more or less back to themselves by two weeks.

Who it’s for — and why that keeps changing

TAVR wasn’t always for everyone. When the technology was first approved about fifteen years ago, it was reserved for the sickest patients — the ones who had severe aortic stenosis but were too fragile, too old, or too medically complicated to survive open-heart surgery. For that first group of patients, TAVR wasn’t being compared to surgery. It was being compared to nothing, because no one thought they could tolerate surgery. And compared to nothing, TAVR was dramatically better. People who would otherwise have died on a palliative care trajectory lived.

Once the technology proved itself there, the trials stepped up. The next group studied was patients at high surgical risk — people who could in theory tolerate open-heart surgery, but whose risk of dying from it was meaningfully elevated. Again, TAVR held up. It was at least as good as surgery in that population, with fewer of the complications associated with sternotomy and bypass.

Then came intermediate risk — the majority of people in their seventies and early eighties who come to the clinic with severe aortic stenosis. Two large randomized trials compared TAVR to open-heart surgery in that group. In both, patients who got TAVR had comparable or better survival and stroke outcomes, shorter hospital stays, faster recovery, and better quality of life in the weeks right after the procedure. After those results came out in the middle of the last decade, the guidelines caught up. TAVR became a standard-of-care option for intermediate-risk patients.

The biggest and most recent shift was the move into low-risk patients — healthier, younger people for whom open-heart surgery used to be the unquestioned first choice. Two large randomized trials, both published around 2019 and now with five-year follow-up, compared TAVR to SAVR in low-risk patients. The short version: TAVR held its own. Survival was equivalent. Stroke rates were comparable. Hospital stays and quality-of-life measures favored TAVR, sometimes substantially. The valve itself, five years out, was functioning as well as the surgical valve. These trials led directly to an expansion of the guidelines, and TAVR is now considered a reasonable option for most patients with severe aortic stenosis who are in the right age range — essentially, anyone roughly 65 and older whose anatomy is favorable.

The practical result: around two-thirds of all aortic valve replacements in the United States are now done by catheter rather than by surgery. A decade ago, that number was essentially zero. It has been one of the fastest shifts in cardiac care in my professional lifetime.

When open-heart surgery still makes more sense

I want to be careful not to leave the impression that TAVR is always the right answer, because it isn’t. There are real situations where surgical aortic valve replacement remains the better operation, and a good heart team has no trouble identifying them. Understanding these exceptions is part of what makes shared decision-making honest.

The first is age and durability. A surgical bioprosthetic valve has a track record stretching back decades. We know how long these valves last. In a 50-year-old patient with severe aortic stenosis, we can predict with reasonable confidence how the valve will behave over the next twenty years. The TAVR valves haven’t been around long enough to have that same depth of long-term data. Five-year data looks excellent. Ten-year data is just beginning to arrive and so far is reassuring. But if a patient is young enough that they’re likely to outlive the first valve, the question of how to handle the second or third valve later in life becomes part of the initial decision. For a relatively young patient with a long life expectancy, a surgical valve — which typically lasts well over a decade, often longer — may give more strategic flexibility. We can plan to do a TAVR inside the surgical valve later, when the patient is older and surgery has become less attractive, rather than backing ourselves into a corner.

The second is bicuspid aortic valve anatomy. About one in a hundred people are born with an aortic valve that has two leaflets instead of three. Those bicuspid valves often wear out faster, which is why we see them present with severe stenosis in patients in their fifties and sixties rather than their eighties. The valve shape is different, the calcium pattern tends to be asymmetric, and in some bicuspid patients the aorta itself just above the valve is enlarged in a way that surgery can address and TAVR cannot. For many bicuspid patients TAVR still works very well, but for a meaningful minority — particularly those with extensive asymmetric calcium, certain bicuspid subtypes, or a dilated ascending aorta that needs to be replaced — surgery is the better operation.

The third situation is when the aortic valve isn’t the only thing that needs to be fixed. If a patient has severe aortic stenosis and severe coronary artery disease that requires bypass grafting, doing a single combined operation — bypass plus valve replacement — is often the cleanest path. If a patient has severe disease in another valve, such as the mitral valve, a surgical approach that repairs or replaces both in one operation often makes more sense than treating each separately. If the valve has been infected — endocarditis — surgery is almost always necessary, because the infected tissue needs to be physically removed and cleaned.

Finally, there are anatomical issues specific to the individual patient. If the femoral arteries are severely diseased or too small to accommodate the catheter, transfemoral TAVR may not be possible. Alternative access sites exist, but in some patients surgery is simply a smoother operation. The CT scan we do before TAVR tells us most of what we need to know about this, and if there’s a roadblock, we find it before the day of the procedure, not during it.

For every patient I see with severe aortic stenosis, there’s a real conversation about which operation fits. The default has shifted, but the decision is still individual.

What the heart team is actually doing

When you’re referred for a TAVR evaluation, you become the subject of what’s called a heart team review. That phrase can sound bureaucratic, but in practice it’s one of the best things modern cardiology does.

A heart team is, minimally, an interventional cardiologist who performs TAVRs, a cardiac surgeon who performs SAVRs, and imaging specialists who interpret the detailed scans. It often also includes a general cardiologist, a nurse navigator who walks the patient through the process, and, depending on the patient, a geriatrician or other specialist. The team’s job is to look at the whole picture — the valve, the anatomy, the coronary arteries, the other organ systems, the patient’s functional status, the patient’s own preferences and goals — and reach a recommendation together.

Before that happens, a few things need to be done. An echocardiogram confirms the diagnosis and measures the severity. A coronary angiogram looks at whether there’s significant disease in the heart arteries that needs to be addressed at the same time. A CT scan of the heart and the arteries from the chest all the way down into the legs gives us the precise measurements of the aortic valve, the aorta, and the femoral arteries — the information that tells us exactly which valve to use, what size to pick, and whether transfemoral access will work.

The other piece, which matters more than patients sometimes expect, is a functional assessment. Frailty is not the same as age. I have 85-year-olds who hike every day and 68-year-olds who struggle to walk a block. What we’re really trying to estimate is whether a patient is going to bounce back from the procedure and actually get the benefit — the return of energy, the recovery of exercise tolerance, the quality of life — or whether other health issues are going to dominate whatever we do with the valve. This assessment involves straightforward things: how far can you walk in six minutes, how fast can you walk a short distance, how’s your grip strength, how’s your nutrition, how independent are you at home. It isn’t about disqualifying anyone. It’s about predicting, honestly, what the patient has to gain.

The output of this process is a recommendation, but it’s genuinely a recommendation. It comes back to the patient, who gets to weigh in on what they want. Some patients who could medically handle either TAVR or SAVR prefer surgery because the long-term valve data is more established. Some strongly prefer the shorter recovery of TAVR. Both are reasonable positions. The point of the heart team is that the decision is made with all the information on the table, not under time pressure in a hospital room.

Recovery, and what to expect

Recovery from TAVR, for a typical transfemoral case, is dramatically gentler than recovery from open-heart surgery.

On the day of discharge — often the morning after the procedure — you walk out of the hospital. You’ll have a small puncture site in the groin with a bandage on it. You’ll have some bruising there, sometimes a lot of bruising; it looks worse than it is and resolves over a week or two. Your energy will be lower than usual for a few days, partly from the procedure itself and partly from the sedation and the interrupted sleep of a hospital stay.

The main restrictions in the first week are around the access site. No heavy lifting, typically nothing over about ten pounds. No vigorous activity. No long drives, because sitting with your hip flexed for hours isn’t great for a freshly closed femoral artery. Walking, on the other hand, is encouraged from day one. I want patients walking short distances around the house the day they get home, longer walks by the end of the first week.

Most patients are cleared to drive again at about a week, sometimes sooner depending on how the groin is healing. Most are back to their normal daily activity — walking, light chores, returning to work for those who work — within two weeks. Flying short distances is usually fine after a week or two, though we generally ask patients to wait longer for long-haul flights.

The real return is a few weeks out. Most of my TAVR patients tell me that somewhere around the three- or four-week mark, they realize they have more energy than they’ve had in a year or two. The symptoms that pushed us toward the procedure — the breathlessness walking up the stairs, the lightheadedness, the chest tightness — have usually faded well before that. What patients notice later is the return of stamina. They can walk farther. They can garden for an hour instead of twenty minutes. Their sleep improves. A spouse often notices the change before the patient does.

Cardiac rehabilitation — a structured exercise and education program, typically twice or three times a week for a couple of months — is something I recommend to virtually every TAVR patient who is a reasonable candidate for it. The evidence that it improves exercise capacity, quality of life, and the durability of the recovery is strong. Medicare covers it for patients after valve replacement, and it works. Patients who do cardiac rehab after TAVR come through the recovery faster, feel better longer, and tend to keep moving after the program ends. Home-based and virtual versions exist now for patients who can’t easily get to a center-based program, and they work too. If your team doesn’t bring up cardiac rehab, ask about it.

The honest answer about durability

Durability — how long the new valve will last — is the most common question I get from patients considering TAVR, and it deserves a careful answer.

Here’s what we know. The TAVR valves that are in use today are made of biological tissue — typically cow or pig pericardium — mounted on a metal frame. They don’t wear out abruptly. They wear out gradually, the way a surgical biological valve does, with the leaflets slowly thickening, stiffening, or becoming leaky over the course of many years. The same is true of the surgical bioprosthetic valves that have been in use for decades.

What we know for certain: at one year, the TAVR valve and the surgical valve perform equivalently. At five years, in randomized trial data comparing TAVR to SAVR in lower-risk patients, the two are still essentially indistinguishable. Rates of reintervention — the need to do anything to the valve again — are low and comparable in both groups at five years. The patients who got a TAVR are not doing worse than the patients who got a surgical valve, across every outcome we measure.

What we don’t fully know yet: what happens at ten years and beyond in low-risk patients who got TAVR in their late sixties and early seventies. Those patients are out there, being followed carefully, and the data is starting to come in. Registry data from hundreds of thousands of TAVR patients suggests the ten-year durability is holding up. But we don’t have as deep a bench of evidence at that time point as we do for surgical valves, which have a fifty-year track record.

What this means in practice: for a patient in their late seventies or eighties, durability concerns are largely irrelevant. The valve will, with near certainty, outlast them. For a patient in their late sixties, durability matters more, but the evidence we have is reassuring. For a patient in their late fifties or early sixties, durability becomes central to the decision, and this is where a candid conversation about whether surgery might serve the patient better over the long haul needs to happen.

There’s another layer to this, which is the concept of lifetime management. A patient who gets a TAVR at 70 may, at 82, develop valve dysfunction and need another intervention. The options at that point are usually to do another TAVR inside the first one — a valve-in-valve TAVR, which is a routine procedure — or, less commonly, surgery. If a patient has a surgical valve placed first, they can later have a TAVR inside it when the surgical valve wears out. Thinking about which valve comes first, and what the second or third valve might look like, is part of the planning, and it’s one of the things a heart team is explicitly trained to weigh.

My own take, after years of doing this: the durability question is real, but the vast majority of patients who are candidates for TAVR today will do very well with it over the life span that matters. I don’t hide uncertainty from patients. But I also won’t steer a 75-year-old away from a procedure that will almost certainly give them back several excellent years of life because of a question about what a valve looks like in 2040.

What to ask your cardiologist

If TAVR is being considered for you or for someone in your family, a handful of questions tend to move the conversation forward.

There are no wrong answers to most of these. You’re looking for a team that can have the conversation openly, not one that pushes a single answer.

Where this leaves us

Cardiology has, over the last fifteen years, rewritten the way we handle a narrowed aortic valve. What used to mean six months of recovery from open-heart surgery now, for the majority of patients, means a procedure that takes an hour or two, a night in the hospital, and a return to normal life within a couple of weeks. Patients who would once have been told they were too old or too fragile for any operation now have an option that can give them years of good quality of life back. The data, at every risk tier in which TAVR has been tested, has held up, and the technology keeps improving.

The catch, which I want to be honest about, is that none of this changes the underlying biology of aortic stenosis. Severe aortic stenosis still needs to be fixed, and it still needs to be fixed at the right time — after symptoms begin, ideally before the heart has been damaged by years of pumping against the narrowed valve. The best outcomes, in my experience, come from patients who don’t wait. If an echocardiogram has shown severe aortic stenosis and you’re starting to notice that you can’t do quite as much as you used to, don’t write it off as getting older. See a cardiologist who works with a heart team. Let the team look at the whole picture. The procedure, whichever one fits your anatomy and situation, is much better than the alternative of letting untreated severe aortic stenosis take its course.

The conversation I have in my clinic today is not the conversation I had ten years ago. For most patients, the news is much better than it used to be. The valve can be fixed, the fix is gentler, and the recovery is a fraction of what it used to be. If that’s where you are right now, please know the options are better than you may think. Come in. Let’s talk.

References

  1. Otto CM, Nishimura RA, Bonow RO, et al. 2020 ACC/AHA guideline for the management of patients with valvular heart disease: a report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines. Circulation. 2021;143(5):e72–e227.

  2. Vahanian A, Beyersdorf F, Praz F, et al. 2021 ESC/EACTS Guidelines for the management of valvular heart disease. European Heart Journal. 2022;43(7):561–632.

  3. Mack MJ, Leon MB, Thourani VH, et al. Transcatheter aortic-valve replacement with a balloon-expandable valve in low-risk patients (PARTNER 3). New England Journal of Medicine. 2019;380(18):1695–1705.

  4. Popma JJ, Deeb GM, Yakubov SJ, et al. Transcatheter aortic-valve replacement with a self-expanding valve in low-risk patients (Evolut Low Risk). New England Journal of Medicine. 2019;380(18):1706–1715.

  5. Mack MJ, Leon MB, Thourani VH, et al. Transcatheter aortic-valve replacement in low-risk patients at five years (PARTNER 3 five-year). New England Journal of Medicine. 2023;389(21):1949–1960.

  6. Forrest JK, Deeb GM, Yakubov SJ, et al. 5-year outcomes after transcatheter or surgical aortic valve replacement in low-risk patients with aortic stenosis (Evolut Low Risk five-year). Journal of the American College of Cardiology. 2025;85(15):1523–1532.

  7. Leon MB, Smith CR, Mack MJ, et al. Transcatheter or surgical aortic-valve replacement in intermediate-risk patients (PARTNER 2A). New England Journal of Medicine. 2016;374(17):1609–1620.

  8. Reardon MJ, Van Mieghem NM, Popma JJ, et al. Surgical or transcatheter aortic-valve replacement in intermediate-risk patients (SURTAVI). New England Journal of Medicine. 2017;376(14):1321–1331.

  9. Smith CR, Leon MB, Mack MJ, et al. Transcatheter versus surgical aortic-valve replacement in high-risk patients (PARTNER 1A). New England Journal of Medicine. 2011;364(23):2187–2198.

  10. Leon MB, Smith CR, Mack M, et al. Transcatheter aortic-valve implantation for aortic stenosis in patients who cannot undergo surgery (PARTNER 1B). New England Journal of Medicine. 2010;363(17):1597–1607.

  11. Jørgensen TH, Thyregod HGH, Savontaus M, et al. Transcatheter versus surgical aortic valve replacement in patients ≤75 years old with symptomatic severe aortic stenosis (NOTION-2). European Heart Journal. 2024;45(37):3672–3683.

  12. Carroll JD, Mack MJ, Vemulapalli S, et al. STS-ACC TVT Registry of transcatheter aortic valve replacement. Journal of the American College of Cardiology. 2020;76(21):2492–2516.

  13. Sharma T, Krishnan AM, Lahoud R, et al. Trends in transcatheter aortic valve replacement outcomes: insights from the STS/ACC TVT Registry. JAMA Cardiology. 2024;9(11):1034–1042.

  14. Thourani VH, Mack MJ, Chikwe J, et al. Outcomes of transcatheter aortic valve replacement in low-risk patients in the United States: a report from the STS/ACC TVT Registry. Circulation. 2025;151(15):1125–1138.

  15. Blanke P, Weir-McCall JR, Achenbach S, et al. Computed tomography imaging in the context of transcatheter aortic valve implantation/transcatheter aortic valve replacement: an expert consensus document of the Society of Cardiovascular Computed Tomography. JACC: Cardiovascular Imaging. 2019;12(1):1–24.

  16. Afilalo J, Lauck S, Kim DH, et al. Frailty in older adults undergoing aortic valve replacement: the FRAILTY-AVR study. Journal of the American College of Cardiology. 2017;70(6):689–700.

  17. Patel KP, Treibel TA, Scully PR, et al. Frailty and outcomes after transcatheter aortic valve implantation: a systematic review. JACC: Advances. 2024;3(4):100877.

  18. Villablanca PA, Mohananey D, Nikolic K, et al. Comparison of local versus general anesthesia in patients undergoing transcatheter aortic valve replacement: a meta-analysis. Catheterization and Cardiovascular Interventions. 2018;91(2):330–342.

  19. Butala NM, Chung M, Secemsky EA, et al. Conscious sedation versus general anesthesia for transcatheter aortic valve replacement: variation in practice and outcomes. JACC: Cardiovascular Interventions. 2020;13(11):1277–1287.

  20. Ribeiro RVP, Yanagawa B, Légaré JF, et al. Cardiac rehabilitation after transcatheter aortic valve replacement: a systematic review and meta-analysis. Canadian Journal of Cardiology. 2021;37(7):1074–1083.

  21. Patel KV, Omar W, Gonzalez PE, et al. Expansion of TAVR into low-risk patients and who to consider for SAVR. Cardiology and Therapy. 2020;9(2):377–394.

  22. Makkar RR, Yoon SH, Leon MB, et al. Association between transcatheter aortic valve replacement for bicuspid vs tricuspid aortic stenosis and mortality or stroke. JAMA. 2019;321(22):2193–2202.

  23. Tarantini G, Fabris T, Nai Fovino L. TAVR in bicuspid aortic valve disease. JACC: Cardiovascular Interventions. 2023;16(17):2093–2113.

  24. Sondergaard L, Ihlemann N, Capodanno D, et al. Durability of transcatheter and surgical bioprosthetic aortic valves in patients at lower surgical risk. Journal of the American College of Cardiology. 2019;73(5):546–553.