This ECG is presented as an example of INCORRECT MACHINE INTERPRETATION. While there are many abnormalities in this ECG, it does not represent a paced rhythm. While there are exceptions, most paced rhythms represent either AV sequential pacing, right ventricular pacing, or bi-ventricular pacing.
RECOGNITION OF A PACED RHYTHM
Recognizing a paced rhythm can be difficult in some cases. Because pacemakers now have so many programmable features, there is a wide variety of ECG changes associated with them. Pacer “spikes” can be difficult to see in all leads. Finding evidence of the device on the patient’s chest or via patient history is a big help in reminding us to scrutinize the ECG for paced rhythm.
An AV sequential pacemaker or a right ventricular pacemaker will pace the ventricles via the right ventricle. This produces a WIDE QRS and a leftward axis, often causing Leads II, III, and aVF to be negative and aVL and aVR to be positive. Along with the wide QRS, we will see DISCORDANT ST CHANGES. That is, there will be ST depression and T wave inversion in leads with positive QRS complexes and ST elevation and upright T waves in leads with negative QRS complexes.
Bi-ventricular pacing can be a little more complicated to recognize, as the QRS can be narrow, with signs of fusion between the wave produced by the LV electrode and the RV electrode.
The frontal plane axis is usually far right – aVR will be positive. Lead I will be negative.
The machine is wrong: there is no indication of a pacemaker, and P waves are present, even though they are not noted in the "PR Interval" or "P Axis".
SO, THIS IS NOT A PACED RHYTHM – WHAT IS IT?
There are many abnormalities in this ECG, and they can be due to many different conditions. All ECGs should be evaluated in a clinical setting, with the patient’s symptoms, signs, and medical history all considered. That being said, I will point out what I see to be abnormal, and await our readers’ and experts’ opinions.
The rhythm is sinus, at a rate of 62 bpm. The PR interval is not given by the machine, but P waves are very obvious, and the PR interval is about .24 seconds, a first-degree AV block. The QRS complex is measured by the machine as 114 ms wide (.11 seconds). This is barely under the 120 ms usually required for diagnosis of wide-complex rhythm, and many would consider it adequate for a wide QRS. If we accept that this is a wide-complex QRS, then we should look for the ECG criteria for RIGHT BUNDLE BRANCH BLOCK and LEFT BUNDLE BRANCH BLOCK. In RBBB, there will be an rSR’ pattern in V1 and a small s wave in Leads I and V6. In LBBB, Leads I, V5 and V6 should have a broad, monomorphic, upright QRS. In this ECG, V5 and V6 have small s waves that contribute to the total width of the QRS, while the R waves are narrow in appearance. The term for a wide-complex, supraventricular rhythm that does not meet the criteria for either right or left BBB is INTRAVENTRICULAR CONDUCTION DELAY. (IVCD)
The frontal plane axis is -17 degrees, which is normal, but slightly to the left. That would be typical of left bundle branch block. However, the pronounced S wave in Lead II and the deep S wave in Lead III are not typical. In a study of S waves in these two leads, they have been found to be very rare in healthy hearts, and often associated with M.I. and cardiomyopathy. The abnormal S waves studied were deeper than the R wave was tall, however. S waves can be seen in Leads II and III in ventricular conduction defects that cause wide QRS – like LBBB and IVCD .
The ST segments and T waves in this ECG are clearly abnormal. T waves are inverted in Leads II, III, aVF; Lead I; Leads V4, V5, and V6; and they are biphasic in V3 as they transition from V2 (positive) to V4 (negative). The inverted T waves are very shallow in most leads, so it is hard to determine if they are symmetrical (indicating ischemia) or asymmetrical (with many causes). In cases of wide QRS, we expect to see DISCORDANT ST AND T WAVE CHANGES. That is, the ST and T waves will go opposite the main direction of the QRS. Right chest leads like V1 and V2, which have negative QRSs will have some ST elevation and upright T waves. The opposite is true over the left side, where we expect upright QRS complexes (I, aVL, V5, V6) and, in the case of wide QRS, depressed ST segments with inverted T waves. This is seen for the most part in this ECG, but Leads III and aVF seem to have CONCORDANT STs. It would be very helpful to know this patient’s current symptoms and recent medical history.
There are also Q waves in V1 and V2 that meet the criteria for "pathological Q waves", a sign of necrosis. However, large Q waves (or more accurately, loss of initial R waves) are common in V1 and V2, and may not be related to acute M.I. This is where knowing the patient's presentation and history would be very helpful.
As stated, the MAIN REASON for posting this ECG is to show students, beginners and advanced alike, that the machine’s interpretation should be taken with a “grain of salt”, and the interpreter should never rely solely on that interpretation. But we must also make the point that every ECG should be interpreted, when possible, in the setting of the patient’s presentation.
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