Emergency (ED placement) Transvenous Pacer appears to be working perfectly. What might go wrong?

Thanks to our Electrophysiologists Omar Iqbal and Rehan Karim for the below strips and the explanations I will be giving on this post.A patient had 3rd degree AV block in the ED.  A transvenous pacer was placed in the Emergency Department by the emergency physicians.  Capture was obtained and a 12-lead ECG was recorded:What do you see here and what complication can occur?The 5th beat (the one with the 4th visible pacer spike) comes early (PAC or PVC), and the pacing spike occurs AFTER initiation of the QRS.   (Ken Grauer points out that this 5th beat appears to be due to an early atrial beat and that these early beats continue for a few beats, suggesting a short run of atrial tachycardia.) The 6th beat (the one with the 5th visible pacer spike)has a pacing spike in the midst of the QRS.  What does it mean that the pacing spike comes in the midst of the 5th and 6th native beats?This means that the pacer is either failing to " sense " the native beats (usually due to less-than-perfect placement) or that it is on " asynchronous " mode in which it will fireregardless of the native beats (it is set so that even if it senses the native beat, it will pace regardless).  Asynchronous pacingseems like the safe setting, EXCEPT that it means that the pacemaker can trigger on the T-wave, which can result in Ventricular Fibrillation.Our electrophysiologists give an example of this here:" Asynchronous " mode means that the pacemaker will pace reg...
Source: Dr. Smith's ECG Blog - Category: Cardiology Authors: Source Type: blogs