The heart’s electrical activity is recorded by an electrocardiogram (EKG), which translates muscle contractions into a series of waves and complexes. The P wave represents the initial electrical activation of the heart’s upper chambers. When the heart beats too quickly (tachycardia), the appearance and timing of these waves change dramatically, complicating EKG interpretation. This is relevant in supraventricular tachycardia (SVT), a fast rhythm originating above the lower chambers. The presence of P waves in SVT is fundamental to understanding this common arrhythmia.
The P Wave in Normal Sinus Rhythm
In a healthy heart, the rhythm is established by the sinoatrial (SA) node, the heart’s natural pacemaker, which generates a regular electrical impulse. This impulse spreads across the atria, causing them to contract and push blood into the lower chambers. The P wave on the EKG is the visual representation of this electrical event, known as atrial depolarization.
Under normal circumstances, the heart maintains Normal Sinus Rhythm (NSR), typically between 60 and 100 beats per minute. During NSR, the P wave has a predictable morphology and position on the EKG tracing. It appears as a small, smooth, and rounded deflection that is upright in most leads (such as lead II) and inverted in lead aVR.
Each P wave is consistently followed by a QRS complex, which represents the electrical activation of the ventricles. The time interval between the start of the P wave and the QRS complex, the PR interval, remains constant. This reliable sequence confirms that the impulse travels correctly from the atria, through the atrioventricular (AV) node, and down to the ventricles.
What Defines Supraventricular Tachycardia
Supraventricular tachycardia (SVT) describes a rapid heart rhythm originating at or above the bundle of His (in the atria or the AV node). The term “supraventricular” means “above the ventricles,” indicating the starting point of the electrical disturbance. SVT is characterized by a fast heart rate, often ranging from 150 to 250 beats per minute.
This rapid rate is caused by an electrical short circuit, often involving a reentrant loop or an abnormal focus. A reentrant circuit traps the electrical impulse in a looping pathway, repeatedly stimulating the atria and ventricles. Since the signal travels through the AV node and down the normal ventricular pathways, SVT usually presents as a narrow-complex tachycardia (QRS complex less than 120 milliseconds wide).
The rapid firing overrides the normal, slower rhythm set by the SA node. This abnormal activity prevents the atria and ventricles from filling completely, which reduces the heart’s pumping efficiency. The abrupt onset can cause symptoms like palpitations, dizziness, or lightheadedness. The P wave appearance provides insight into the specific underlying mechanism of the rapid rhythm.
The Mechanism of P Wave Obscuration in SVT
While the atria must contract during any heart rhythm, the visibility of the P wave in SVT is often severely compromised. The primary reason for this obscuration is the speed of the heart rate. At rates exceeding 150 beats per minute, the time between heartbeats (the R-R interval) becomes extremely short.
This shortened interval means the P wave (atrial depolarization) occurs almost simultaneously with or immediately following the QRS complex (ventricular depolarization) or the T wave of the preceding beat. When the small P wave signal occurs at the same time as the much larger QRS complex or T wave, it becomes buried and is not discernible on the EKG tracing. This makes it appear as though P waves are absent.
A second factor is retrograde conduction, common in many reentrant SVTs. In these arrhythmias, the electrical impulse travels backward from the AV node up to the atria, rather than in the normal downward direction. This backward activation causes the P wave to be inverted in leads that normally show an upright P wave (e.g., lead II, III, and aVF).
When the atria are activated almost simultaneously with the ventricles, the resulting inverted P wave is often hidden within the QRS complex itself. If atrial activation occurs immediately after ventricular activation, the inverted P wave may be visible as a small notch or distortion at the end of the QRS complex or the beginning of the ST segment. This altered timing and inverted morphology make the P wave unrecognizable compared to a normal heart rhythm.
Variations in P Wave Appearance Across SVT Subtypes
The specific location and morphology of the P wave, or its complete absence, is a powerful tool for distinguishing between the different types of SVT. The appearance depends entirely on the pathway the electrical impulse follows.
Atrioventricular Nodal Reentrant Tachycardia (AVNRT)
AVNRT is the most common form of SVT, where the atria and ventricles are activated almost simultaneously. This tight timing causes the P wave to be completely hidden within the QRS complex in most cases. When visible, it often appears as a small distortion immediately following the QRS complex, known as a “pseudo-S wave” in the inferior leads or a “pseudo-R wave” in lead V1.
Atrial Tachycardia
In Atrial Tachycardia, the impulse originates from an ectopic focus, an abnormal spot in the atria outside of the SA node. The P waves are usually visible but have an abnormal shape or morphology compared to the normal sinus P wave. This abnormal shape reflects the non-standard path the impulse takes, and the P wave will precede the QRS complex.
Atrioventricular Reentrant Tachycardia (AVRT)
AVRT involves an extra, abnormal electrical connection between the atria and ventricles, known as an accessory pathway. In the most common form of AVRT, the P wave is retrograde and appears after the QRS complex, separated by a distinct short interval. This separation helps differentiate AVRT from AVNRT, where the P wave is usually buried or immediately adjacent to the QRS complex.