(USMLE topics, cardiology) (USMLE topics, cardiology). This part covers heart rate, heart rhythm, P-wave. See part 2 for more. This is an update of a previously published video.
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For heart rate: Identify the QRS complex, usually the biggest on an ECG; count the number of small squares between two consecutive QRS complexes and calculate the heart rate with this formula. If this number is variable, count the number of QRS complexes on a 6-second strip and multiply by 10. A normal heart rate is between 60 and 100 beats per minute.
For rhythm: measure the intervals between the R waves. If these intervals vary by less than 1.5 small squares, the rhythm is regular; if the variation is greater than 1.5 small squares, the rhythm is irregular.
P wave represents depolarization of the atria initiated by the SA node. Presence of a normal P wave therefore indicates sinus rhythm. P waves are most prominent in leads II, III, aVF and V1.
Absence of P waves indicates non-sinus rhythms. Absence of P waves and presence of irregular narrow QRS complexes are the hallmark of atrial fibrillation. The baseline may be undulating or totally flat.
A sawtooth pattern instead of regular P waves signifies atrial flutter. These are called flutter waves. The number of flutter waves preceding a QRS complex corresponds to number of atrial contractions to one ventricular contraction.
P wave is the summation of 2 smaller waves resulting from depolarization of the right atrium followed by that of the left atrium. Normal P waves are rounded, smooth and upright in most leads. In V1, P wave is biphasic, with an initial positive deflection corresponding to activation of the right atrium, and a subsequent negative deflection resulting from activation of the left atrium.
Unusual morphology of P waves is indicative of atrial enlargement. In right atrial enlargement, depolarization of the right atrium lasts longer than normal and its waveform extends to the end of that of the left atrium. This results in a P wave that is taller than normal, more than 2.5 small squares. Its duration remains unchanged, less than 120 miliseconds. In V1, this is seen as a taller initial positive deflection of the P wave, more than 1.5 small squares. Right atrial enlargement is usually due to pulmonary hypertension.
In left atrial enlargement, depolarization of the left atrium lasts longer than normal. This results in a wider P wave, of more than 3 small squares. The waveform may also be notched. In V1 the negative portion of P wave is deeper and wider. Left atrial enlargement is commonly due to mitral stenosis.
P-wave inversion in the inferior leads indicates a non-sinus rhythm. When this happens measure the PR interval. If the PR interval is less than 3 small squares, the rhythm is started in the AV junction. If the PR interval is more than 3 small squares, the origin of the rhythm is within the atria.