tag:blogger.com,1999:blog-549949223388475481.post7107725428777151710..comments2020-07-13T07:52:48.170-05:00Comments on Dr. Smith's ECG Blog: QT Correction Formulas Compared to The Rule of Thumb ("Half the RR")Unknownnoreply@blogger.comBlogger17125tag:blogger.com,1999:blog-549949223388475481.post-18259700772270907422018-10-30T08:43:30.699-05:002018-10-30T08:43:30.699-05:00All those figures show exactly what happens when s...All those figures show exactly what happens when square root is used vs. simple inverse. Just study those figures!Steve Smithhttps://www.blogger.com/profile/08027289511840815536noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-59299171418484780512018-10-28T20:30:46.871-05:002018-10-28T20:30:46.871-05:00Hello Dr. Smith
Thank you very much for the intere...Hello Dr. Smith<br />Thank you very much for the interesting and very relevant blog. I have a question which is more related to algebra and mathematical construction of Bazett formula. Why in the denominator of Bazett formula, we use square root of R-R interval rather than the absolute R-R value itself? I understand that using the square root will result in a horizontal line when we plot the corrected QT against heart rate which in turns corrects for HR of 60 at any given HR. This is opposed to the slope which we get if we use the absolute R-R interval. I just need to visualize the formula mathematically/algebraically, what does it mean conceptually to divide by the square root? it is easier to absorb the division by the whole number. Thank youAnonymoushttps://www.blogger.com/profile/02072114760351277662noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-16047335974400563382018-10-17T13:14:01.655-05:002018-10-17T13:14:01.655-05:00Ilya,
Here is Ari's answer:
So this is cool an...Ilya,<br />Here is Ari's answer:<br />So this is cool and I applaud his enthusiasm. I think this is mainly a problem of wording and interpretation. "Over-correct" and "under-correct" don't mean it's u-shaped, they mean it's not brought enough towards the line. This is apparent from the diagram that he made. I think he's reading the words as implying that QTcF/B are u-shaped and that should not be the case.<br /><br />AriSteve Smithhttps://www.blogger.com/profile/08027289511840815536noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-3800226022509039362018-10-15T15:54:34.903-05:002018-10-15T15:54:34.903-05:00Dear Dr. Smith,
Thank you for your blog notes on ...Dear Dr. Smith,<br /><br />Thank you for your blog notes on QT correction. This is the most<br />comprehensive guide I have ever read on the problem and I am rereading<br />it for the second time with ever more interest.<br /><br />My background is not medical, so I do not understand intuitively most<br />of the formulas. However, I tried to apply common sense when reasoning<br />about them. By doing so, I found that there are several<br />inconsistencies I cannot overcome myself. Hence, I really hope for<br />your help in this.<br /><br />1. The Bazett and Fridericia formulas over-correct at high HR (RR <<br />1000 ms) and, moreover, Bazett more so than Fridericia. Hence, QTc > QT.<br /><br />2. At low HR (RR > 1000 ms) Bazett and Fridericia under-correct and do<br />not shorten enough, that is QTc < QT, but not enough so.<br /><br />3. You mention (referencing "2017 article by Vandenberk B et al. in J<br />of the American Heart Association") that the QTc/RR slope close to<br />zero indicates better correction (in fact, the slope of zero is the<br />perfect correction). This is due to the fact that the correction is<br />supposed to remove the QT dependence on RR.<br /><br />4. Finally, another purpose of correction is to normalize QT to that<br />of HR=60 bpm (or RR=1000 ms). Thus, the comparison between patients is<br />possible.<br /><br />My problem is:<br /><br />- the QTc line (of independence) over RR cannot be a line if 1. and<br /> 2. are to be satisfied together.<br /><br />In other words:<br /><br />- From 3. it goes that QTc as a function of RR is a line.<br /><br />- From 4. it goes that QTc=QT at RR=1000ms, that is, the line passes<br /> the raw QT value at RR=1000 ms.<br /><br />- From 1. it goes that (perfect) QTc is below QTcF and QTcB curves on<br /> the QT-RR plane for RR < 1000 ms.<br /><br />- From 2. it goes that (perfect) QTc is again below QTcF and QTcB<br /> curves on the QT-RR plane for RR > 1000 ms.<br /><br />- Hence, the (perfect) QTc cannot be a line.<br /><br />To better illustrate this point, let us consider a real example of the<br />QT-RR plane with the real measurements<br />(https://drive.google.com/file/d/1jPIJ8QEz5PdllS7E-QIuWgjb4ZIXs_iq/view?usp=sharing).<br /><br />The line of independence drawn for two distinct QT values (350 and 450<br />ms) cannot be simultaneously below QTcF/QTcB on both sides of<br />RR=1000ms. This is due to nonlinear nature of the formulas. Otherwise,<br />it is not a line anymore.<br /><br />Moreover, the line can still be a line before and up to RR=1000 ms,<br />but for RR > 1000 ms it cannot continue as the same line and be below<br />QTcF/QTcB curves.<br /><br />So either normalization of QT on HR=60 bpm (or RR=1000 ms) or the<br />independence condition must be violated for propositions 2. and 3. to<br />be valid simultaneously.<br /><br />And this I found puzzling for myself. I really hope for your help and<br />support in resolution of this dilemma in my head.<br /><br />I thank you once more for your best of guides to the QT-correction<br />problem. Looking forward in hearing from you.<br /><br />with kind regards,<br />Ilya PotapovAnonymoushttps://www.blogger.com/profile/15717393286390397036noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-77661232721281789782018-01-13T06:34:54.842-06:002018-01-13T06:34:54.842-06:00Thanks, Peter!Thanks, Peter!Steve Smithhttps://www.blogger.com/profile/08027289511840815536noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-35354615212815845972018-01-12T18:14:04.968-06:002018-01-12T18:14:04.968-06:00This is amazing, thank you!!
I will inform my coll...This is amazing, thank you!!<br />I will inform my collegues on this<br /><br />Best wishes<br />Peter, DenmarkPeter Tagmose Thomsenhttps://www.blogger.com/profile/12137489436968282949noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-67092917241067900452017-11-27T03:09:09.191-06:002017-11-27T03:09:09.191-06:00Sounds good, thanks for clarifying that for me!
A...Sounds good, thanks for clarifying that for me!<br /><br />AndrewAndyJhttps://www.blogger.com/profile/08742812483232241614noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-51186879650739991152017-11-25T08:00:00.234-06:002017-11-25T08:00:00.234-06:00Andrew,
Excellent question.
You can generally trus...Andrew,<br />Excellent question.<br />You can generally trust the QT when it is in the range found in such cases: 370-440.<br />It is when the QT is long that the computer falsely measures it as shorter than it is.<br />OK?<br />SteveSteve Smithhttps://www.blogger.com/profile/08027289511840815536noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-20398269348090135202017-11-24T23:15:16.184-06:002017-11-24T23:15:16.184-06:00Excellent work Dr Smith!
Given the potential inacc...Excellent work Dr Smith!<br />Given the potential inaccuracies of the computer-measured QTc, should that change our thinking when using the calculated QTc for the LAD occlusion vs early repol calculation?<br />Should we be hand measuring the QTc to ensure a more accurate calculation? or is it appropriate to just eyeball the QTc and only hand measure it if it seems off??<br /><br />Thanks!<br />AndrewAndyJhttps://www.blogger.com/profile/08742812483232241614noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-61362811261766568802017-11-24T15:51:03.061-06:002017-11-24T15:51:03.061-06:00Mark,
Email me at: smith253@umn.edu and let's ...Mark,<br />Email me at: smith253@umn.edu and let's discuss it. OK?<br />Thanks,<br />Steve SmithSteve Smithhttps://www.blogger.com/profile/08027289511840815536noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-14141914066140625972017-11-24T14:16:44.510-06:002017-11-24T14:16:44.510-06:00Thanks, Ken!Thanks, Ken!Steve Smithhttps://www.blogger.com/profile/08027289511840815536noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-47086531601601845842017-11-24T14:15:28.391-06:002017-11-24T14:15:28.391-06:00Thanks!Thanks!Steve Smithhttps://www.blogger.com/profile/08027289511840815536noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-43309842399491138572017-11-24T13:52:20.430-06:002017-11-24T13:52:20.430-06:00Strong work. I have always wondered why the machin...Strong work. I have always wondered why the machine reading is out of sorts with the rule of thumb. Consider publishing in the peer review literature. I would be happy to accept the submission as a review for the Western Journal of emergency medicine. Mark langdorf editorAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-7498200847268304512017-11-24T12:56:22.158-06:002017-11-24T12:56:22.158-06:00Our thanks to Drs. Stephen Smith and Ari Friedman ...Our thanks to Drs. Stephen Smith and Ari Friedman for their impressive work sifting through the large amount of data regarding clinical use of the QT interval. As one who has used the QT Rule of Thumb for decades (including this approach in my QT chapters, since the first of my ECG books was published in the 1980s) — I wanted to add a few supportive comments. First, as emphasized in this post — judgment is essential for assessing the significance of the QTc in any given patient. Distinction between a QTc interval that is acceptable, vs one that is likely to be problematic is not dependent on any single specifically defined upper-limit numerical value, but rather is a function of a series of clinical and ECG features such as heart rate and rate variation; serial QTc measurements; ongoing QTc-affecting medication use; and other clinical factors that may influence the relative likelihood of the patient in question developing Torsades de Pointes. Optimal use of the “QTc Rule of Thumb” recognizes the imperfection of this method, and therefore seeks simply to classify the QTc interval into one of 3 general groupings: i) “Normal” QTc; ii) a “Prolonged” QTc; or iii) a “Borderline” QTc. Rather than stating that “if the QTc is less than half the R-R interval, it is not prolonged” — optimal wording of this Rule of Thumb is, “that for the QTc to be prolonged, it must clearly be MORE than half the R-R interval”. If the QTc is half the R-R — it may be borderline. Caution is essential in using the Rule of Thumb for “faster heart rates” (ie, once heart rate exceeds 90-100/minute). The Rule of Thumb should clearly not be used for cases such as the 1st ECG shown in this post, in which the heart rate is ~150/minute (and the Rule should probably not be used once heart rate exceeds 120/minute). And, as emphasized in this post — providers should recognize the difficulty inherent in estimating what constitutes a problematic QTc with marked bradycardia. With these caveats in mind — the Rule of Thumb works great! It allows me to teach a group of providers how to quickly and accurately recognize into which of the 3 general groupings a given patient falls most of the time. As emphasized in this post — when more accurate determination of a specific QTc value is clinically important to the case at hand — then the provider needs to more accurately verify the QTc value obtained.ECG Interpretationhttps://www.blogger.com/profile/02309020028961384995noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-74795912180911391852017-11-24T12:22:55.101-06:002017-11-24T12:22:55.101-06:00thought* about submitting it to a journal
Not tho...thought* about submitting it to a journal<br />Not though aboutAnonymoushttps://www.blogger.com/profile/10627297504661393180noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-36949744166088699522017-11-24T10:19:15.060-06:002017-11-24T10:19:15.060-06:00Thanks for the feedback!Thanks for the feedback!Steve Smithhttps://www.blogger.com/profile/08027289511840815536noreply@blogger.comtag:blogger.com,1999:blog-549949223388475481.post-46653533489227967042017-11-24T10:04:53.468-06:002017-11-24T10:04:53.468-06:00This is an amazing and needed piece of work, thank...This is an amazing and needed piece of work, thank you to Dr. Smith and all involved for sharing this freely Blwebbnoreply@blogger.com