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How do we know what the PR interval is in this ECG? One large square on the ECG is 200 msec. In this ECG image, only the large squares are visible. But, we don’t have to measure the PR interval ourselves. Unlike the QT interval, the automatic measurement of the PR interval by the ECG machine is usually reliable. In the ECG image above, the PR interval is printed out in the top right corner—210 msec.]
Some sources may say that the normal duration of the QRS complex is 60 to 100 msec.
ECG machines are also good at identifying right bundle branch block, [Optional to read: The reason that both the ECG machine and the cardiologist read this ECG as showing right bundle branch block is not only that the QRS is wide (which can also be non-specific) but in leads V1 and V2, it has an rSR pattern. That is, the first wave of the QRS complex is POSITIVE (an r wave) rather than negative (an s wave) as is usually found. I put a red rectangle around one QRS complex in lead V1. Do you see how it starts with a positive wave followed by a negative and then another positive wave? That’s the rSR pattern.]
I said above that ECG machines are good at automatic reading of the PR interval and the QRS duration. But, ECG machines are NOT as good at automatically measuring the QT interval.
Why not? This is because the QT interval is measured from the beginning of the QRS complex to the end of the T wave and it often hard to identify on ECGs exactly where the T waves end.
This fact is not optional for psychopharmacologists to know because we have a lot of interest in the QT interval and should know that we cannot simply trust the number given by the ECG machine. We need to either make sure a cardiologist has confirmed the value of the QT interval OR if the ECG was done but has not been read by the cardiologist yet, we know how to measure the (approximate) QT interval ourselves.
Also, the QT interval seen on the ECG also needs to be “corrected” for the heart rate. That is why we have two different values for each ECG—the QT interval and the QTc interval. If we think that the machine has read the uncorrected QT interval approximately correctly, then we can also trust the QTc interval since that is only a mathematical calculation using the uncorrected QT interval and the heart rate.
In this particular patient, the ends of the T waves are relatively well-defined and the cardiologist agreed with the QT interval being around 482 msec, which is what the machine read it as.
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