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It’s important that you recognize it when you see it on an ECG, but also that you know what the potential causes could be and how to solve them.",{"id":215,"name":216},1,"Course previews",[218,229,234,239,247],{"type":219,"data":220},"video",{"readDurationInSeconds":221,"video":222},339,{"platform":223,"name":224,"url":225,"size":226},"Vimeo","PM_promo_2.mp4","https:\u002F\u002Fvimeo.com\u002F139341218",{"width":227,"height":228},480,270,{"type":230,"data":231},"free_text",{"readDurationInSeconds":232,"text":233},9.333333333333334,"\u003Cp>Undersensing can be a life-threatening problem in our pacemaker patients, so it’s important that you recognize it when you see it on an ECG. In this video, you’ll learn everything you need to know about how to recognize this problem, potential causes, and how to solve them.\u003C\u002Fp>",{"type":230,"data":235},{"readDurationInSeconds":236,"title":237,"text":238},15.2,"Join our Pacemaker Essentials course now!","\u003Cp>This hands-on course takes the mystery out of pacemakers and gives clinicians a solid grounding in pacemaker therapy. You'll learn how pacemakers work, what can be done in order to optimize a patient’s pacemaker therapy, and how to recognize and troubleshoot common problems. By the end, you’ll feel comfortable addressing the most common pacemaker issues without having a pacemaker expert on hand.\u003C\u002Fp>",{"type":240,"data":241},"cta",{"readDurationInSeconds":242,"text":243,"buttonText":244,"buttonUrl":245,"teachIllustration":246},3.466666666666667,"Start the first chapter of our Pacemaker Essentials course for free","Start chapter 1 now","https:\u002F\u002Fwww.medmastery.com\u002Fuser\u002Fregister?target=%2Fcourses%2Fpacemaker-essentials","https:\u002F\u002Fmedmastery-backend-prod-kjbeds.s3.eu-west-1.amazonaws.com\u002Fmedia\u002Fimages\u002F99f55e6b-af25-4eb8-9d82-e2d14ce90eca",{"type":230,"data":248},{"readDurationInSeconds":249,"title":250,"text":251},156.53333333333333,"Video Transcript","\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:00:00] \u003C\u002Fb>Undersensing is where the pacemaker fails to register depolarization of a chamber. Let's have a look at how this may appear on an ECG. In this example, the pacemaker is sensing the atrial depolarization. It starts its AV delay, and then the intrinsic natural depolarization of the ventricles occurs. The pacemaker sees this and registers it as a V sensed \u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:00:30] \u003C\u002Fb>event. The same occurs, here. However, on the next heartbeat, we can see the atria depolarizes. This is sensed by the pacemaker, it then starts its AV delay but this time it hasn't registered that a ventricular depolarization has occurred. So what does it do? It tries to trigger a depolarization itself. This is what a pacemaker is programmed to do. You can see, here, \u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:01:00] \u003C\u002Fb>it delivers its output pulse, which is signified by this pacemaker spike. But actually, it fails to depolarize the tissue. The reason is that the tissue has only just depolarized and is now repolarizing and is refractory. What this means is, no matter how much electrical energy you deliver to tissue, when it is only just depolarized, it will not be able to depolarize again. This is like a flushing \u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:01:30] \u003C\u002Fb>toilet. Once you flush the toilet, you have to wait again before the toilet is ready to flush once more and it's the same with cardiac tissue. So, here, although it's tried, the tissue is not ready and so it doesn't trigger a ventricular depolarization. Normal service is resumed on these next two heartbeats so we can say that this is intermittent undersensing. In the example below, we can see the atria depolarizes, \u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:02:00] \u003C\u002Fb>the AV delay is started in the pacemaker, and again, the ventricular depolarization isn't sensed. So, just like the previous example, the pacemaker sends an output pulse. This fails to capture the now refractory ventricular tissue. This happens on all of the heartbeats on this electrogram. And so we would say that this is permanent undersensing, absolute chronic undersensing. Just to highlight these once more, \u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:02:30]\u003C\u002Fb> I've marked all the events where the pacemaker fails to sense on the electrograms. So what is happening? Well, for whatever reason, the pacemaker is not sensing the ventricular activity. This is quite often because the ventricular signal has become too small in amplitude. And here, we can see that the pacemaker sensitivity is set at 4 mV, therefore, is ignoring these ventricular events, which are \u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:03:00]\u003C\u002Fb> less than 4 mV in amplitude. If this was the case, we could resolve this by making the pacemaker more sensitive. Once more, we are detecting all these ventricular events and normal service is resumed. However, it can be indicative of bigger problems that aren't so easy to solve. This includes lead displacement, lead insulation break, or lead fracture. Let's break them down. Lead displacement \u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:03:30] \u003C\u002Fb>is where the lead has come away from the myocardium or the position that it is meant to be in. This occurs most commonly in the first four to six weeks after implant, where the pacing lead has not had a chance to fully fixate itself to the myocardium. So, the lead is displaced and the electrode is no longer in direct contact with the myocardium. This makes it much more difficult for the pacemaker to sense the depolarization \u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:04:00]\u003C\u002Fb> of that tissue. The signal becomes much quieter and much smaller. So, the pacemaker will not recognize this signal as a ventricular depolarization. It could also be indicative of a problem with the lead insulation. Now, pacemaker leads have a metal wire that is insulated by a non-conductive material. This ensures that the electrical current makes it all the way down to lead before leaving at the exposed \u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:04:30] \u003C\u002Fb>electrode, the metallic electrode. Unfortunately, the insulation of a lead can sometimes break. If it does break, the point at which the insulation has broken, the wire essentially becomes a new electrode. So because of the insulation break, we have a new sensing circuit and it is not sensing the tissue that we're interested in. In this example, it may pick up a small amount of atrial activity again. \u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:05:00] \u003C\u002Fb>This will not be detected by the pacemaker. Finally, it could be indicative of a lead fracture. Essentially, a fracture in the wire of the lead has meant that there is no connection between the two parts within the lead. So, any electrical activity that is given off when the tissue depolarizes may enter the lead but can never make it back to the pacemaker So, your takeaway message—undersensing can often\u003C\u002Fp>\u003Cp dir=\"ltr\">\u003Cb id=\"docs-internal-guid-691f49de-7fff-9ab4-0bea-74244a1d78f5\">[00:05:30] \u003C\u002Fb>be resolved by making the pacemaker more sensitive. However, a faulty lead will need to be replaced or repositioned.\u003C\u002Fp>",{"courses":253,"showAwardsBelow":48},[254],{"id":255,"isFreeCmeCourse":256,"title":257,"type":258,"specialization":259,"teachers":261,"cmeCredits":263,"teacherIllustration":246,"backgroundImage":264,"relativeUrl":265,"lastChanged":266,"created":267,"description":268,"relativeURL":265,"meta":269},2176,false,"Pacemaker Essentials","course",[260],"Cardiology",[262],"Kristian Webb",5,"https:\u002F\u002Fmedmastery-backend-prod-kjbeds.s3.eu-west-1.amazonaws.com\u002Fmedia\u002Fimages\u002F99f55e6c-8db5-4a40-8e74-aed3c99bae3f","\u002Fcourses\u002Fpacemaker-essentials","1763556346","1442566649","This hands-on course takes the mystery out of pacemakers and gives clinicians a solid grounding in pacemaker therapy. You'll learn how pacemakers work, what can be done in order to optimize a patient’s pacemaker therapy, and how to recognize and troubleshoot common problems. 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Learn how to treat mild and moderate COPD exacerbations—and when to escalate care.",{"alt":336,"title":290,"size":337,"location":338},"Older patient using a handheld nebulizer to inhale bronchodilator medication during an acute COPD exacerbation",{"width":319,"height":319},{"bucket":294,"key":339},"\u002F2026-04\u002FC133(4)_ exacerbations_magazine image.png",{"id":341,"title":342,"relativeURL":343,"image":344,"professionLong":345,"profession":346,"weight":348},5365,"Siamak Moayedi, MD","\u002Fteachers\u002Fsiamak-moayedi-md","https:\u002F\u002Fmedmastery-backend-prod-kjbeds.s3.eu-west-1.amazonaws.com\u002Fmedia\u002Fimages\u002F9c43cf02-1064-4074-817b-0eb5de6ded74","Professor and Director of Medical Student Education, University of Maryland and Course Director, Essential and Critical Procedures, Emergency Medicine.",{"name":347},"Emergency medicine physician",1533,"\u002Fmagazine\u002Facute-copd-exacerbation-treatment",191,{"created":352,"updated":353,"published":354},1776705362,1776710540,1776710541,{"seo":356,"og":358},{"title":357,"description":213},"Common pacemaker problems (part 2): Undersensing | Medmastery",{"title":357,"description":213,"image":359},{"alt":290,"title":290,"size":360,"location":362},{"width":361,"height":361},220,{"bucket":294,"key":363},"\u002Fmigrated-images\u002Fmagazine-150916.jpg",{"id":365,"created":366,"updated":367,"published":366},65,1442389701,1735572763]