Determine the following:
P waves: _______________________________________
PR interval: _______________________________________
QRS complex: _______________________________________
What's your interpretation?
(Answers on next page)
Name that strip: Answers
Rhythm: Basic rhythm regular; irregular with PAC
Rate: Basic rhythm rate 60 beats/minute
P waves: Sinus P waves with basic rhythm; premature, abnormal P wave with PAC
PR interval: 0.12 to 0.16 second (basic rhythm); 0.16 second (PAC)
QRS complex: 0.08 second (basic rhythm and PAC)
Comments: ST segment depression and a noncompensatory pause are present.
Interpretation: Normal sinus rhythm with a premature atrial contraction (PAC).
A PAC is an early beat originating from an ectopic pacemaker site in the atria. The early beat interrupts the regularity of the basic rhythm, which is usually a sinus rhythm. PACs may have the same appearance when they originate from a single ectopic site in the atria, or different appearances when they originate from multiple pacemaker sites in the atria. The premature beat occurs in addition to the underlying rhythm. So, it is important to identify the underlying rhythm along with the premature ectopic beat (for example, normal sinus rhythm with a PAC). The premature beat is characterized by a premature, abnormal P wave followed by a normal duration QRS and a pause.
When P waves are not easily seen, it is important to compare the T waves preceding each PAC with those of the underlying rhythm. If the contour of the T wave preceding the PAC is different (taller or different in shape) from the underlying rhythm, there is a hidden P wave in it. This occurs commonly with PACs.
The PR intervals of the PACs are usually normal, like those of the underlying rhythm. Occasionally the PR interval may be prolonged if the PAC is very early and finds the atrioventricular junction still partially refractory and unable to conduct at a normal rate. The PR interval will be unmeasurable if the abnormal P wave is obscured in the preceding T wave.
The QRS of the PAC usually resembles that of the underlying rhythm because the impulse is conducted normally through the bundle branches into the ventricles. The ventricles depolarize simultaneously, resulting in a normal duration QRS complex. If the PAC occurs very early, it is possible the bundle branches may not be repolarized sufficiently to conduct the premature electrical impulse normally. If the bundle branches are not sufficiently repolarized, the electrical impulse is conducted down one bundle branch (usually the left because it depolarizes quicker) and not conducted down the other. The left ventricle is depolarized first, followed by depolarization of the right ventricle (sequential depolarization). Sequential ventricular depolarization is slower, resulting in a wide QRS complex of 0.12 second or more. A PAC associated with a wide QRS complex is called a PAC with aberrancy, indicating that conduction through the ventricles is abnormal (aberrant). Aberrantly conducted PACs must be differentiated from a premature ventricular contraction, especially if the abnormal P wave associated with the PAC is hidden in the preceding T wave.
The pause associated with the PAC is usually a noncompensatory pause (the measurement from the R wave before the premature beat to the R wave after the premature beat is less than two R-R intervals of the underlying regular rhythm). This pause is called an incomplete pause because it does not equal two R-R intervals. Rarely, the PAC may occur with a pause that is longer than compensatory.
PACs may appear as a single beat, as every other beat (bigeminal PACs), as every third beat (trigeminal PACs), as every fourth beat (quadrigeminal PACs), in pairs (couplets), or in runs of three or more. Frequent PACs may initiate more serious atrial dysrhythmias, such as paroxysmal atrial tachycardia, atrial flutter, or atrial fibrillation.
PACs are common. They can occur in individuals with a normal heart or in those with heart disease. PACs may be seen during times when the patient is experiencing emotional stress (caused by an increase in sympathetic tone), or after ingestion of certain substances such as alcohol, caffeine, or tobacco. Other causes include hypoxemia, electrolyte imbalances, myocardial ischemia or injury, atrial enlargement, heart failure, and the administration of certain drugs that increase sympathetic tone, such as epinephrine or norepinephrine. PACs may also occur without apparent cause.
PAC: Identifying ECG features
Rhythm: Underlying rhythm usually regular; irregular with PACs
Rate: That of underlying rhythm
P waves: P wave associated with PAC is premature and abnormal in size, shape, and direction (commonly appears small, upright, and pointed; may be inverted); abnormal P wave commonly found hidden in preceding T wave, distorting the T-wave contour
PR interval: Usually normal duration; not measurable if hidden in T wave
QRS complex: Premature; normal duration (0.10 second or less)