Morning all...Newcomer here who is based in the UK. I was recently diagnosed with a bicupsid aortic valve causing severe aortic stenosis. I have surgery booked for December to have it replaced and to have a section of the ascending aorta also replaced but to dilation. Surgeon has recommended a mechanical valve (Medtronic or On-X) based on my age and relatviely active life style. Until my doctor found a heart murmur 6 weeks ago I had no idea I had this problem so the last few weeks have been a real roller coaster. Other than this I am a fit and healthy 52 year old. The difficulty I have at the moment is valve type choice. It seems to boil down to going bio valve now and facing a re-operation in 15 years (if I am lucky) or go mechanical now, expect never to need a re-operation but be on warfarin and have a significant risk of a stroke or bleed hanging over me for the rest of my life. From what I have read online it seems risk of stroke or bleed with mechanical valves is around 2% per patient year. So if I hope to live another 30 years this to me means I have a 60% chance of stroke or bleed in this time. Have I understood 'per patient year' correctly? Is the risk really that high? Cheers, John.
'per patient year' question
- Thread starter JW1970
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pellicle
Professional Dingbat, Guru and Merkintologist
Basically the risk of bleeding or stroke is entirely up to you and how well you manage your INR.
The risk is not far away from the general age related population if you keep your INR within range most of the time. The risk is directly related to that "time in range". This is well demonstrated in this graph
where you can see that between INR = 2.0 and INR = 4.4 the expected indicence of events is about 2 per 100 patient years.
Please excuse the amount of data presented in this blog post I put together back in 2014, but its pretty much most of what you'll ever need to know
https://cjeastwd.blogspot.com/2014/09/managing-my-inr.html
But before you deep dive into that its perhaps worth revisiting your choices briefly (as to tissue or mechanical) because at 52 some factors are worth considering. Can you tell us a bit more about your activity and if you have any conditions which may make managing AntiCoagulation Therapy (or blood thinners as they're wrongly called) a problem or as they call it "contra-indicated"??
Myself I'm of the view that managing INR is pretty simple and there is any number of board members here who would also attest to that.
Lastly as you're in the UK, you may find (depending on your region) that you can get a test meter (as pictured in my blog post) given to you for free by the NHS.
Off the cuff I'd ask why a St Jude wasn't mentioned as a mechanical valve as it pretty much has the longest track record. If you do go down the path of an On-X then please, don't follow their (misleading) guidelines and swallow the bait of their sale pitch towards its benefit being lower AntiCoagulation Therapy ... this is not without risks and would arguably be without benefits.
Best Wishes
Superman
Well-known member
First, welcome to the forum and good luck with your decision!
As far as stats, they aren’t cumulative like that. I’ve been event free for 32 years on warfarin. There is still a 2% chance this year of an event just like there was the first year I received my valve. But that’s a 2% chance among all patients, not me individually.
If they were cumulative, our friend @dick0236 would be having adverse events annually right now (55 years on warfarin) at a 110% chance! He’s at the same 2% risk.
And as @pellicle points out, the risk is probably even lower for those with a well managed INR since adverse events happen when you’re outside of range.
Think if it this way. X% of people die in car accidents annually. Now that you’ve been driving 30 plus years, do you feel like any time now you’re going to die when you get in a car? Also, how many of that X% weren’t wearing seatbelts? Or we’re under the influence? So while it’s a % of all people, those that died actually made choices that contributed to them being in that statistic. I’d be curious what % of stone cold sober, well rested, non-texting, seatbelt wearing people (that weren’t passengers in cars driven by irresponsible people) died in vehicle accidents annually?
Hi John,
I am also in the UK and recently had my second surgery. I am 41 and received an ON-X mechanical valve. The surgeon I chose only offered Carbomedics or ON-X valves, and because this surgeon had an outstanding track record, including in complex re-operations, I ended up choosing the ON-X.
With respect to the risk, @pellicle is absolutely right that the risk can be significantly lowered with good INR manangement. There are many studies to support this. One recent UK survey has shown that you can reduce the risk of bloodclots with self-management by 50% or more: Self-monitoring of warfarin is safe and cost-effective
If you look at the prose trial:
https://www.sciencedirect.com/science/article/pii/S2666273622003084
in that trial, the risk of a bloodclot with either ON-X or St Jude is 0.5%, while the risk of bleeding is around 1%. Now the estimates of a stroke have gone from 1.5-2% per patient year in early studies from 20-30 years ago to 0.5% in this latest study. As this study shows, this is not a function of a change in the valve itself (On-X vs St Jude), but rather medical understanding of anticoagulation has advanced tremendously in the past couple of decades.
The paper below suggests that the life-time incidence of stroke in someone aged 50 is roughly 20% in the general population.
https://pubmed.ncbi.nlm.nih.gov/32090315/
In the PROSE study, the average age of patients was round 52.3. So the lifetime incidence of thromboembolic event by the time this person is 92.3 is 20%.
So with modern anticoagulation methods, the probability of a mechanical heart valve replacement patient to experience a stroke, is the same as in the general population, at least according to this study.
Finally, I am in the UK as well, and if you want to talk, you can always PM me.
Good luck in your choice.
I am also in the UK and recently had my second surgery. I am 41 and received an ON-X mechanical valve. The surgeon I chose only offered Carbomedics or ON-X valves, and because this surgeon had an outstanding track record, including in complex re-operations, I ended up choosing the ON-X.
With respect to the risk, @pellicle is absolutely right that the risk can be significantly lowered with good INR manangement. There are many studies to support this. One recent UK survey has shown that you can reduce the risk of bloodclots with self-management by 50% or more: Self-monitoring of warfarin is safe and cost-effective
If you look at the prose trial:
https://www.sciencedirect.com/science/article/pii/S2666273622003084
in that trial, the risk of a bloodclot with either ON-X or St Jude is 0.5%, while the risk of bleeding is around 1%. Now the estimates of a stroke have gone from 1.5-2% per patient year in early studies from 20-30 years ago to 0.5% in this latest study. As this study shows, this is not a function of a change in the valve itself (On-X vs St Jude), but rather medical understanding of anticoagulation has advanced tremendously in the past couple of decades.
The paper below suggests that the life-time incidence of stroke in someone aged 50 is roughly 20% in the general population.
https://pubmed.ncbi.nlm.nih.gov/32090315/
In the PROSE study, the average age of patients was round 52.3. So the lifetime incidence of thromboembolic event by the time this person is 92.3 is 20%.
So with modern anticoagulation methods, the probability of a mechanical heart valve replacement patient to experience a stroke, is the same as in the general population, at least according to this study.
Finally, I am in the UK as well, and if you want to talk, you can always PM me.
Good luck in your choice.
Apparently I screwed my earlier post to comment on the question of 2% stroke risk.....so I'll try again.
Stroke risk with a mechanical valve is about 2%/year but it is NOT cumulative.....in the first year it may be 2% and in the second year it is 2%......not 4%......etc. The risk has a LOT to do with patient compliance. If you take your pill as prescribed and test routinely you probably will have no issues.....screw around with this drug and you will have a problem....bin there, and dun that.
Stroke risk with a mechanical valve is about 2%/year but it is NOT cumulative.....in the first year it may be 2% and in the second year it is 2%......not 4%......etc. The risk has a LOT to do with patient compliance. If you take your pill as prescribed and test routinely you probably will have no issues.....screw around with this drug and you will have a problem....bin there, and dun that.
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pellicle
Professional Dingbat, Guru and Merkintologist
I notice that I forgot to mention this bit and as a few others have I'll throw in an answer which is different.
In probability theory there is dependent probability and independent probability.
- a coin toss is independent and each toss has no influence on the previous
- drawing a card from a deck is dependent, as long as the cards are not returned (as would be normal in a game of blackjack)
Basically 98:2 means that you have a 98% chance of being event free in the first year, the general formula for this is the probability raised to the power of the amount of years.
The excel formula is basically chances =A2^B2
risk | years | chances |
0.98 | 1 | 0.98 |
0.98 | 2 | 0.9604 |
0.98 | 3 | 0.941192 |
0.98 | 4 | 0.92236816 |
0.98 | 5 | 0.9039207968 |
0.98 | 6 | 0.885842380864 |
0.98 | 7 | 0.86812553324672 |
0.98 | 8 | 0.850763022581785 |
0.98 | 9 | 0.83374776213015 |
0.98 | 10 | 0.817072806887547 |
0.98 | 11 | 0.800731350749796 |
0.98 | 12 | 0.7847167237348 |
0.98 | 13 | 0.769022389260104 |
0.98 | 14 | 0.753641941474902 |
0.98 | 15 | 0.738569102645404 |
0.98 | 16 | 0.723797720592496 |
0.98 | 17 | 0.709321766180646 |
0.98 | 18 | 0.695135330857033 |
0.98 | 19 | 0.681232624239892 |
0.98 | 20 | 0.667607971755094 |
0.98 | 21 | 0.654255812319992 |
0.98 | 22 | 0.641170696073592 |
0.98 | 23 | 0.628347282152121 |
0.98 | 24 | 0.615780336509078 |
0.98 | 25 | 0.603464729778897 |
0.98 | 26 | 0.591395435183319 |
0.98 | 27 | 0.579567526479652 |
0.98 | 28 | 0.567976175950059 |
0.98 | 29 | 0.556616652431058 |
0.98 | 30 | 0.545484319382437 |
0.98 | 31 | 0.534574632994788 |
0.98 | 32 | 0.523883140334892 |
0.98 | 33 | 0.513405477528195 |
0.98 | 34 | 0.503137367977631 |
0.98 | 35 | 0.493074620618078 |
Running the numbers we find that it takes 35 years before you have a 50:50 chance of having had an event in that number years and the reason why most of us haven't had any events (sorry @Superman , this means you're not the outlier you may appear to be). So assuming that your number in your username (1970) is your DoB then the chances of you making another 30 years is reasonable (IMO). This means that you're likely to be dead of other causes before you've had your chance at an event even on those 2% per year odds.
If you reduce that to 1% per year it starts to take about a hundred years to get to the fifty:fifty chances you'll have had an event.
If someone out there has a better grasp of probability than me, then please do step in and correct my figures.
So I say "roll those dice", but do as much polishing and breathing on them as possible by keeping your INR in the range.
Superman
Well-known member
I didn’t intend to claim to be an outlier. Quite the contrary. I expect my experience is probably quite typical to be event free assuming a well managed INR over the years. And I wasn’t even that well managed for the first 20 years. Clinics being what they are and my lack of getting to them anyway.
pellicle
Professional Dingbat, Guru and Merkintologist
I knew I shoulda done a wink ;-) on how I approached that. I was writing to the OP and in case the OP thought you seemed like an outlier wanted to mention that you aren't; just a good example of good management.
WRT clinics: I'd say that the 2% is probably an example of clinics and that if we in our management improve that by 50% then its even better.
Hi Pellicle
Thanks for the detailed information. Can I ask please, where did you get the graph from?
In terms of my activity and any other conditions. I guess I am a fairly normal 50+ year old. My work is office based, away from work I enjoy DIY, gardening, hill walking, tinkering with my old car and when the sun is out and there is a bit of a breeze, sailing a single handed dinghy on local lakes. I like a beer or glass of wine but would not describe my self as a 'binge drinker'! My medical history to date has two points of interest, a detatched retina 4 years ago followed by a bleed 2 years ago. My eye doctors are OK with me taking wafarin. I have also had 'friendly' polyps removed in 2 of the last 3 colonoscopies I have had in the last 15 years.
I'm seeing a second surgeon next week so will ask about St Judes also. Then it will be decision time.....which surgeon and ultimately which valve type. On the warfarin topic, I have a couple of extra questions. #1 What happens if I need unrelated surgery that the warfarin may cause a problem with, can you come off warfarin for a few days? #2 Have you ever known of a case when the patient either reacts badly to warfarin or they cannot mantain the required theraputic range? Wondering what happens then.
Thanks for taking the time to read and any replies.
John.
Martin1962
Member
Hi John, I'm a relative newcomer having had mitral valve replacement with On-X mechanical in Jan this year. Also in the UK. I was 59 at the time and didn't fancy further ohs's in my 70's and then possibly again in my 80's. I'm happy with my decision and haven't found warfarin to be a big issue (I too enjoy a beer and wine, and more or less eat what I want). On the NHS I wasn't given a choice regarding brand of valve or surgeon, so was surprised that you have unless you're going private. If you do go mechanical I would recommend getting into home testing your INR if you can.
Regarding unrelated procedures on warfarin, I had my first last week (removal of polyps) and have to admit it is a bit of a kerfuffle! You need to come off warfarin a couple of days before, and inject yourself with a heparin type medication for a few days until you come back into range (known as "bridging").
Not sure about your 2nd question, but like I said I'm a relative newcomer to all this.
Let me know if you would any further info on my experiences on the above.
Regarding unrelated procedures on warfarin, I had my first last week (removal of polyps) and have to admit it is a bit of a kerfuffle! You need to come off warfarin a couple of days before, and inject yourself with a heparin type medication for a few days until you come back into range (known as "bridging").
Not sure about your 2nd question, but like I said I'm a relative newcomer to all this.
Let me know if you would any further info on my experiences on the above.
Superman
Well-known member
I’ve had a few procedures while on warfarin. They manage through in a few ways. By far the biggest was a second open heart surgery due to an aneurysm that showed up 19 years after my first surgery. I went off warfarin and bridged with Lovenox (self administered shots of a different kind of anticoagulant). I also had my gallbladder and appendix removed. That was more of a check into the emergency room in severe pain and you aren’t leaving until it’s done situation. I had to stay a couple nights until my INR came down and they did laparoscopic surgery. They used a heparin drip while in the hospital. I also had a vasectomy while in warfarin. That they just waited until I was below 2.0 and did the procedure. I did have to reschedule once because I wasn’t low enough. So now we have five great kids instead of four, so warfarin was really a blessing there.
pellicle
Professional Dingbat, Guru and Merkintologist
Good Morning (from OzTrayliYa)
most certainly the URL for study it came from is this:
http://jamanetwork.com/journals/jamainternalmedicine/fullarticle/415179
I'm sorry that I didn't include it in my first iteration of this discussion, sheer laziness (and probably the experience that few actually ask for reference sources), I'll reference my citations (which I normally do) with you from now on.
you sound a lot like me, although mostly I love my 1989 Mitsubishi Pajero because I never need to tinker with it. I do however need to tinker with my 2006 Yamaha quite a lot due to its age and some design flaws. Sadly however I've not done any sailing since leaving my little coastal village many years ago to "go to the big smoke" and do my degree. There have been many compensations however.
this is definitely me (or was until just this year for reasons I can go into but will refrain from here), so warfarin therapy is no obstacle to that.
Some good reading on why (not having one myself) I have come to recommend the St Jude
https://www.valvereplacement.org/threads/aortic-valve-choices.887840/post-902334
and of course many other good threads. However (lets use sailing here for your convenience) there is always a difference between what a maker (interested in sales) will claim about their product and then there is what happens to that in the water, does it win competitions and more importantly if its your daily hack how well will it last.
As you know, new comers need to attract someone, and may make claims not actually lived up to.
On the warfarin topic, I have a couple of extra questions. #1 What happens if I need unrelated surgery that the warfarin may cause a problem with, can you come off warfarin for a few days?
Ok, well on my blog I have a lot written on this topic, it boils down to the idea of "management". Some people "couldn't manage to get wet sailing" but others can manage to win races. The choice of a mechanical valve come down to how you manage Warfarin, so lets address two procedures I've had.
http://cjeastwd.blogspot.com/2017/12/perioperative-management-of-inr.html
and
http://cjeastwd.blogspot.com/2020/10/another-example-small-procedure.html
this of course should imply something about the needs for measurement and ways to deal with that, which this artile of mine touches on:
http://cjeastwd.blogspot.com/2022/05/rapid-dust-off-inr-management.html
That was of course written up (by me) in conjunction with another member here (@Chuck C ) who took my management ideas to a higher level (because unlike me he had a more pressing need) of accuracy. I'm both indebted to his diligence and honestly flattered by his recognition.
Naturally you'll notice that a model of modelling has been built in those articles, but not clarified with precision because I yet hold hopes of selling the idea.
In the (a little over ten) years of being here I can say
not on this forum but one or two cases appear in the literature where people have apparently been allergic to warfarin and in each case its either been to the dye or the excipient, in all cases I've read this has been addressed with changing brand.
you're welcome ... balls in your court, so feel free to digest all that and hit back with questions.
Lastly everyone who starts warfarin (and doesn't just sit around whinging about it) begins to develop a feel for managing it. Not least because of my background in biochem and data modelling I decided (back in about 2014) to develop a data based INR management decision making system to facilitate dose choice with a simple matrix. If you wish when you start managing INR reach out and I can set you up with that and can work with you to teach you how. Over the years here I've worked with quite a number of people and you can find a number of current members who know my model and work with it. Oh, I don't charge and I prefer to teach a man to fish, not become a supplier of fish; so the emphasis will be on making you self sufficient.
I'm sort of retired now and live out in the country to avoid large groups of people and enjoy my life
and ride my bike
as it happens at Mt Alford there is a great micro brewery too (which makes an unannounced appearance at the end.
https://www.scenicrimbrewery.com.au/
which never fails to impress me after a nice ride through the mountains to get there.
(and that front disc still has that song bird caught in it
)
Best Wishes
most certainly the URL for study it came from is this:
http://jamanetwork.com/journals/jamainternalmedicine/fullarticle/415179
I'm sorry that I didn't include it in my first iteration of this discussion, sheer laziness (and probably the experience that few actually ask for reference sources), I'll reference my citations (which I normally do) with you from now on.
you sound a lot like me, although mostly I love my 1989 Mitsubishi Pajero because I never need to tinker with it. I do however need to tinker with my 2006 Yamaha quite a lot due to its age and some design flaws. Sadly however I've not done any sailing since leaving my little coastal village many years ago to "go to the big smoke" and do my degree. There have been many compensations however.
this is definitely me (or was until just this year for reasons I can go into but will refrain from here), so warfarin therapy is no obstacle to that.
sorry to hear about the issue there but as you have identified, doctors do not suggest warfarin is contra indicated.
Some good reading on why (not having one myself) I have come to recommend the St Jude
https://www.valvereplacement.org/threads/aortic-valve-choices.887840/post-902334
and of course many other good threads. However (lets use sailing here for your convenience) there is always a difference between what a maker (interested in sales) will claim about their product and then there is what happens to that in the water, does it win competitions and more importantly if its your daily hack how well will it last.
As you know, new comers need to attract someone, and may make claims not actually lived up to.
On the warfarin topic, I have a couple of extra questions. #1 What happens if I need unrelated surgery that the warfarin may cause a problem with, can you come off warfarin for a few days?
Ok, well on my blog I have a lot written on this topic, it boils down to the idea of "management". Some people "couldn't manage to get wet sailing" but others can manage to win races. The choice of a mechanical valve come down to how you manage Warfarin, so lets address two procedures I've had.
http://cjeastwd.blogspot.com/2017/12/perioperative-management-of-inr.html
and
http://cjeastwd.blogspot.com/2020/10/another-example-small-procedure.html
this of course should imply something about the needs for measurement and ways to deal with that, which this artile of mine touches on:
http://cjeastwd.blogspot.com/2022/05/rapid-dust-off-inr-management.html
That was of course written up (by me) in conjunction with another member here (@Chuck C ) who took my management ideas to a higher level (because unlike me he had a more pressing need) of accuracy. I'm both indebted to his diligence and honestly flattered by his recognition.
Naturally you'll notice that a model of modelling has been built in those articles, but not clarified with precision because I yet hold hopes of selling the idea.
In the (a little over ten) years of being here I can say
- Harriet was inappropriately advised to have a mechanical (in Canada iirc) and her medical condition was entirely and strongly a contraindication for warfarin therapy
not on this forum but one or two cases appear in the literature where people have apparently been allergic to warfarin and in each case its either been to the dye or the excipient, in all cases I've read this has been addressed with changing brand.
you're welcome ... balls in your court, so feel free to digest all that and hit back with questions.
Lastly everyone who starts warfarin (and doesn't just sit around whinging about it) begins to develop a feel for managing it. Not least because of my background in biochem and data modelling I decided (back in about 2014) to develop a data based INR management decision making system to facilitate dose choice with a simple matrix. If you wish when you start managing INR reach out and I can set you up with that and can work with you to teach you how. Over the years here I've worked with quite a number of people and you can find a number of current members who know my model and work with it. Oh, I don't charge and I prefer to teach a man to fish, not become a supplier of fish; so the emphasis will be on making you self sufficient.
I'm sort of retired now and live out in the country to avoid large groups of people and enjoy my life
and ride my bike
as it happens at Mt Alford there is a great micro brewery too (which makes an unannounced appearance at the end.
https://www.scenicrimbrewery.com.au/
which never fails to impress me after a nice ride through the mountains to get there.
(and that front disc still has that song bird caught in it
)Best Wishes
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oo0My_Valve0oo
Well-known member
Hi John, have they said how you got to this point? I had similar problems but my murmur was detected 6 years earlier. After lots of tests and consulting a Cardiologist and my wonderful PCP I had at that time I anticipated living with a murmur for the rest of my life. I had routine physicals with nothing alarming. Then 6 years later I had no stamina and shortness of breath. Normally getting extra sleep brought me back but it was no longer helping. They had me in surgery as fast as I could make arrangements. However, I was able to delay it a month waiting for the new fiscal year. Mine was a genetic defect. They said there was nothing I did to cause it. I guess its durability was also genetically defective because initially the murmur was due to the valve being too thick. I think it is a common side effect to have surrounding tissue weaken from the abnormal pressure build up caused by the inefficiency so they also reinforced the nearby tissue. I am in the minority here and have a bio tissue valve. I didn't want the added risk of stroke and to be on anticoagulant medication for the rest of my life. But had I found this forum I might have gone mechanical. So much is in the presentation and so much information and experience is here at this forum that could not be covered in the brief introduction to the options I received from my cardiologist. "Stroke" and "anticoagulants for life" vs. a 2nd surgery later when I had too many other things concerning me than fear of OHS.
Here's to a successful resolution and a happy healthy future!
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Pretty late to the conversation but I saw this early on:
The difficulty I have at the moment is valve type choice. It seems to boil down to going bio valve now and facing a re-operation in 15 years (if I am lucky) or go mechanical now
That was my plan - pig valve failed after 8 years so now at 54 had the re-do with mechanical valve - On-X with dilation. From this site I’ve learned that the INR is not a big deal - stay compliant with warfarin and I have little concern.
Quality of life took a hit for the summer but now I am fortunate to be back to pre-surgery activity.
I would encourage first timer open heart to go one & done.
The difficulty I have at the moment is valve type choice. It seems to boil down to going bio valve now and facing a re-operation in 15 years (if I am lucky) or go mechanical now
That was my plan - pig valve failed after 8 years so now at 54 had the re-do with mechanical valve - On-X with dilation. From this site I’ve learned that the INR is not a big deal - stay compliant with warfarin and I have little concern.
Quality of life took a hit for the summer but now I am fortunate to be back to pre-surgery activity.
I would encourage first timer open heart to go one & done.
Had an On-X stitched in about 6 months ago. Managing my own INR (here in the states). I was worried and concerned that my life might change dramatically with Warfarin. It hasn't changed much at all 
. I still lift weights, run, drink, work on my vehicles, do housework up on ladders, go shooting, walk my dogs a lot, etc. And yes @Chuck C , I even still watch Hallmark with my wife even though that particular activity can be very demanding (lol).
Planning on some deer hunting in a month or so too.
I'd like to echo what @pellicle said above about both checking out the St. Jude valve (please do so) as well as to target standard and proven INR guidelines for mechanical aortic valves (2.0 -> 3.0)
. I still lift weights, run, drink, work on my vehicles, do housework up on ladders, go shooting, walk my dogs a lot, etc. And yes @Chuck C , I even still watch Hallmark with my wife even though that particular activity can be very demanding (lol). Planning on some deer hunting in a month or so too.
I'd like to echo what @pellicle said above about both checking out the St. Jude valve (please do so) as well as to target standard and proven INR guidelines for mechanical aortic valves (2.0 -> 3.0)
pellicle
Professional Dingbat, Guru and Merkintologist
2 e’s mate. Two!
Cheerios!
And yes, I had them this AM. 2 cups of cheerios, sliced banana, and … almond milk. (Ducks. Runs for cover at the suggestion of using Almond “milk”)
Daniel758
Well-known member
The answer is:
1 minus the probability of the event NOT happening
(1-(.98^30))100= 45% over 30 years at 2% per year.
So a little less than half of people receiving a mechanical valve should be expected to encounter a “stroke or bleed” event at some point over a 30 year period.
If the risk of stroke or bleed is lowered to 1% the answer would be roughly 26%, much better and why it’s important to keep your INR in range.
Note: Your probability of having an “event” is not 50-50 on the specific year 35, your probability of having an “event” is 50-50 by the time you reach year 35.
The longer the time period, the higher the probability that the event will happen within that time period.
