tag:blogger.com,1999:blog-1199060416202745408.post365253527794204106..comments2024-03-19T13:40:12.355+08:00Comments on ECG of the Week: ECG of the Week - 1st April 2013 - InterpretationAnonymoushttp://www.blogger.com/profile/09026589956407176781noreply@blogger.comBlogger1125tag:blogger.com,1999:blog-1199060416202745408.post-35945084815761678562013-04-03T21:35:11.743+08:002013-04-03T21:35:11.743+08:00The amount the PR interval prolongs becomes less w...The amount the PR interval prolongs becomes less with each beat, leading to the shortening R-R interval (presuming a fixed P-P).<br /><br />I found it to be an abstract concept to wrap my head around at first!<br /><br />If you think of the R-R in terms of the PRi, it is a bit easier. With a fixed PRi you have a fixed R-R (we'll assume a fixed P-P). If you simply have a longer PRi the R-R does not change, and the same goes for a shorter PRi.<br /><br />The PRi dictates <i>when</i> the R's occur relative to the P's.<br /><br />If you vary the PRi from beat-to-beat, then you'll change when the R's occur from beat-to-beat. If you change when the R's occur, you'll change the R-R.<br /><br />Going back, constant PRi's do not affect the R-R. Thus the amount which <i>varies from beat-to-beat</i> is the change in the R-R.<br /><br />In AV Wenckebach the amount of beat-to-beat prolongation which occurs lessens (or stays the same), with the largest delta-PRi occurring in the first beat (e.g. 200ms, 260ms, 290ms, 310ms, drop; deltas of 60, 30, and 20). So, if the amount we vary decreases with each beat, the R-R will decrease with each beat.<br /><br />Put in math form, assuming k is our baseline PRi and we have a repeating series of PRi's:<br /><br />PRi := { k, k60, k90, k110, k, k60, ... }<br /><br />The n'th R-wave (we'll ignore drops, and P is the PP interval):<br />R_n := nP + PRi_n<br /><br />With an R-R:<br /><br />RR_n := R_n+1 - R_n<br /><br /> := ((n+1)P + PRi_n+1) - (nP + PRi_n)<br /><br /> := (n+1)P - nP + PRi_n+1 - PRi_n<br /><br /> := P + (PRi_n+1 - PRi_n)<br /><br />If you plug in the PRi's in order you'll find you get a decremental series:<br /><br />RR := { P + 59k, P + 30k, P + 20k, ... }<br /><br />I hope that helps others understand it, I had to draw it a lot when I first noticed the decreasing R-R's.<br /><br />(<i>apologies if the math formats oddly</i>)Christopherhttps://www.blogger.com/profile/11415988855392944633noreply@blogger.com