This ECG is taken from an elderly woman who complains of feeling weak and tired. We have no other clinical information, unfortunately.
There is an obvious bradycardia, with more P waves than QRS complexes. Here is what we see:
* Atrial rate is around 115/min. and P waves are regular and all alike.
* Ventricular rate is around 35/min. and QRS complexes are regular and all alike.
* PR intervals, when they occur, are all the same at 162 ms.
* QRS duration is wide at 122 ms.
* QTc interval is prolonged at 549 ms.
What does this mean? There is sinus tachycardia with second-degree AV block because the atrial rate is over 100/min, but not all P waves are conducted. The AV block looks like a Type II (Mobitz II) block because the PR intervals are all the same. This is a reliable indicator of conduction. (Not third-degree AVB). The wide QRS complexes are due to right bundle branch block. The ECG signs of RBBB are: 1) wide QRS; 2) supraventricular rhythm; and 3) rSR’ pattern in V1 and Rs, with a wide little s wave, in Leads I and V6.
Type II AV blocks are almost always blocks of the intraventricular conduction system. That is, they occur in the region of the bundle branches. A second-degree, Type II AVB is an “intermittent tri-fascicular block”. That is, one or two of the three main fascicles of the bundle branches is constantly blocked, and the remaining fascicle(s) is intermittently blocked. When all three fascicles are blocked, there is no QRS following the P wave. When the intermittently-blocked fascicle conducts, we see a QRS. Often, that QRS will be conducted with a bundle branch block pattern.
In this case, there is a constant right bundle branch block. The left bundle branch appears to be intermittently blocked, resulting in no conduction for two beats. So, we would call this a 3:1 AV block.
What about the QTc interval? The QT interval (corrected to a rate of 60) is 549 ms. This is prolonged in any age or gender. QTc intervals over 500 ms are associated with an increased risk of Torsades de Pointes.
Additional teaching points. This is a great ECG to show students how P waves can “hide” in T waves. By carefully marching out the P waves, we can find the hidden ones, and also see how they affect the shapes of the T waves in each lead. V3 shows the P wave occurring on the upslope of the T wave. It is also a good case for discussion of treatment of bradycardias. At this rate, it is very likely that the patient is hemodynamically compromised. Generally, emergency transthoracic pacing is used until a temporary transvenous or permanent transvenous pacemaker can be applied. Patients with Type II blocks do not often respond well to atropine because the problem lies in the intraventricular conduction system. Atropine exerts it’s rate-increasing effect in the SA and AV nodes and by blocking the vagus nerve. Type II AV blocks are generally considered serious and prone to worsening. A complete heart block occurring at this anatomic level would have a ventricular escape rhythm rather than a junctional escape rhythm. AV blocks occurring at the level of the AV node, such as second-degree, Type I (Wenckebach) blocks, would be likely to have junctional escape. Prolonged QT intervals can be very serious, and the patient should be evaluated for reversible causes of the prolonged QT interval, while medications known to prolong the QT interval should be avoided.
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