USMLE® Step 1 style questions USMLE
A 25-year-old woman comes to the clinic for an evaluation of intermittent palpitations. The patient has not had any episodes of syncope, chest pain, shortness of breath, or vomiting. An electrocardiogram (ECG) is performed that shows an electrical abnormality representing the slowing of the conduction through the AV node. Which of the following is most likely to be abnormal in this patient’s ECG tracing?
Contributors:Tanner Marshall, MS
An electrocardiogram is also known as an ECG; the Dutch and German version of the word, elektrokardiogram, is shortened to EKG. It is a tool used to visualize, or “gram,” the electricity, or “electro,” that flows through the heart, or “cardio.” Specifically, an ECG tracing shows how the depolarization wave, which is a wave of positive charge, moves during each heartbeat by providing the perspectives of different sets of electrodes. This particular set of electrodes is called lead II; one electrode is placed on the right arm and the other on the left leg. Essentially, when the wave’s moving toward the left leg electrode, you get a positive deflection. This big, positive deflection corresponds to the wave moving down the septum. When reading an ECG, there are a few key elements to keep in mind; one of them is looking at the intervals.
In a typical waveform, there’s a p-wave, QRS complex, and t-wave. In addition, there are certain intervals, including the PR interval, the QRS complex itself, and the QT interval.
The PR interval spans from the beginning of the p-wave to the beginning of the QRS complex, and it represents the time from the beginning of atrial depolarization to the beginning of ventricular depolarization. It’s normally 0.12-0.20 seconds, which is three to five little boxes, since each little box is 0.04 seconds. Therefore, the PR interval shown is about four boxes or 0.16 seconds.
Any deviation in the normal depolarization pathway from the SA node to the ventricles can change the PR interval. For example, consider if the atria are depolarized by an ectopic atrial focus, such as an irritable atrial cell outside of the SA node. If it was farther away from the AV node, it’d result in a longer PR interval, because the signal has to travel a greater distance. Alternatively, if it was really close to the AV node, the PR interval might be super short. Another example is first degree heart block, which is when the electrical signal travels more slowly through the AV node than it normally does, causing the PR interval to lengthen beyond 0.2 seconds.
The QRS complex represents depolarization of the ventricles; it’s normally less than 100 milliseconds or two and a half little boxes. Just like the PR interval, the QRS duration can differ in its path from the av node to the ventricles. For example, if an ectopic ventricular focus, such as an irritated ventricular cell, fires off, the resulting depolarization wave will move slowly from muscle cell to muscle cell, instead of traveling quickly through the electrical conduction system. Therefore, it takes a longer time to depolarize the ventricles, and the QRS is wider. It’s considered intermediate if it’s 100 to 120 milliseconds, and prolonged if it’s over 120 milliseconds, or three little boxes.
The QT interval spans from the beginning of the QRS complex to the end of the t-wave. It represents ventricular systole, which is the entire span from depolarization through repolarization. Normally, the QT interval should be less than half of a cardiac cycle. In fact, for a heart rate of 60 beats per minute, the QT interval is generally considered to be abnormally long when it’s greater than 440 milliseconds in men, or 460 milliseconds in women. If you measure someone’s QT interval at a different rate, say 90 beats per minute, and it was 400 milliseconds, you might think that that’d be considered normal; however, you can’t really use these values to compare to the normal QT interval at 60 beats per minute, or bpm, because the QT interval changes depending on the rate.
ECG intervals are major elements to look at when reading an ECG strip. These include the PR interval, the QRS complex itself, the QT, and the RR intervals. The PR interval is the time from the beginning of the P wave to the beginning of the QRS complex. It represents the time between the beginning of atrial depolarization and the beginning of ventricular depolarization, which normally lasts about 0.12-0.20 seconds. The QRS complex represents ventricular depolarization and typically lasts less than 100 milliseconds. Next, the QT interval is measured from the beginning of the QRS complex to the end of the T wave. It represents the ventricular systole and typically lasts less than half of the individual's cardiac cycle. Finally, the RR interval is measured from one R wave peak to the next. It helps to calculate the heart rate and tell if it's regular.