P Wave

As we discussed in the last chapter, depolarization in the atria is depicted on the ECG tracing by what is termed the P wave. Normally, the excitation of the heart starts in the sinus node. The sinus node is located in the right atrium. From here, the excitation first spreads throughout the right atrium itself, then into the left atrium, and then into the ventricles via the AV node. If this pattern of excitation formation and excitation propagation is visible in an ECG, we refer to the presence of a sinus rhythm. The assessment of the P waves plays a key role in the diagnosis of the cardiac rhythm. If every QRS complex is not preceded by a P wave, the patient does not have a normal sinus rhythm. A little piece of advice: P waves are usually best seen in leads II and v1.

Normally, the P wave is positive in all leads, except for aVR. The duration is generally a maximum of 100 milliseconds and the maximum amplitude is 0.25 millivolts. Since the sinus node lies in the right atrium, the first half of the P wave represents the excitation propagation in the right atrium. The second half of the P-wave represents the excitation propagation in the left atrium.

Abnormal P waves are seen when intra-atrial excitation propagation is disturbed, or when the excitation begins at a position in the atria other than the sinus node.

The most common cause of altered P wave morphology is atrial dilation.

Dilation of the atria can be caused by a number of factors.

Dilation of the left atrium is often seen in patients with systolic or diastolic heart failure, atrial fibrillation, or restrictive cardiomyopathy. In patients with an isolated dilation of the left atrium, the first --- or right-atrial --- part of the P wave is normal. However, in the second half of the P wave, various changes occur.

-          First, a widening of the P wave to more than the normal 100 ms is observed.

-          Second, in leads I and II, the P has two peaks, and in leads III, V1, and V2 it is biphasic, which means that it is first positive, then negative.

-          Third, the PQ interval is shortened due to the extension of the P wave.

When the ECG was first introduced, rheumatic mitral valve stenosis was the most common cause of left atrial dilation in the Western world. For this reason, this ECG change was termed "mitral P". You may still come across this expression in the medical literature.

Enlargement of the right atrium occurs secondary to an overload in the pressure and/or volume of the right heart. No widening of the P wave occurs, and the duration of the P wave is normal, at less than 100 milliseconds.

However, the P wave is pointed and high in II, III, v1 and v2, meaning that the amplitude exceeds the normal 0.25 millivolts.

However, observations aren’t always this straightforward. This is, because we commonly see dilation of both atria at the same time. When we do, the P wave morphology combines a double-peak, an increased amplitude, and a bi- or even more-phasic course. This condition mainly occurs in patients with advanced heart failure or pronounced cardiac ischemia.

The PR interval describes the period from the beginning of atrial depolarization to the beginning of ventricular depolarization. It is measured from the beginning of the P wave to the beginning of the Q wave, or, if there is no Q wave, until the beginning of the R. A PQ or PR interval of between 120 and 200 ms is normal. In the event of physical stress, or some other cause of sympathetic nervous system activation, the increase in heart rate causes a physiological shortening of the PQ interval.

When interpreting an ECG, you must always assess the PQ interval. It is important to recognize if it is prolonged or shortened. In addition, you should assess whether the PQ interval is the same in all successive P-QRS complexes, or whether it varies.

The PQ interval is measured in those leads in which the P wave is most visible and the PQ interval is the longest.

But watch out! In certain conditions, the PQ interval varies or does not exist at all. If you observe any irregularity in successive P-QRS complexes, an arrhythmia may be present! We will discuss how each particular rhythm affects the PQ segment in later chapters.

Another important point: A PQ interval can only be determined if there is a P-wave. If the patient has atrial fibrillation, for example, atrial depolarization is chaotic. No P waves are seen, and thus no PQ segment.