An Electrocardiogram (ECG or EKG) is a non-invasive medical test that records the electrical activity of the heart. Electrodes placed on the skin detect the tiny electrical changes that occur as the heart muscle depolarizes and repolarizes with each beat. This process translates the heart’s electrical signals into a visual tracing on paper or a screen. The sensation commonly described as a “skipped heartbeat” is a frequent experience that can refer to various electrical patterns within the heart, and an ECG serves as the primary tool for identifying these underlying patterns.
Understanding a Normal Heartbeat on ECG
To understand what a “skipped heartbeat” looks like, it is helpful to first visualize a normal heart rhythm on an ECG. A typical heartbeat originates from the sinoatrial (SA) node, the heart’s natural pacemaker, generating an electrical impulse that spreads through the heart in an organized manner. This orderly electrical activity produces a characteristic waveform on the ECG, known as normal sinus rhythm.
The normal ECG tracing comprises three main components, each representing a specific electrical event in the heart’s cycle. The P wave signifies the electrical activation of the atria, the heart’s upper chambers. The QRS complex illustrates the rapid depolarization of the ventricles, the heart’s lower pumping chambers. The T wave represents the electrical recovery of the ventricles, preparing them for the next beat. In a normal sinus rhythm, these P waves, QRS complexes, and T waves occur with consistent timing and appearance, indicating a regular and coordinated heartbeat.
Visualizing Premature Beats
One common reason for the sensation of a “skipped heartbeat” is the occurrence of premature beats, which are extra heartbeats that arise earlier than expected in the heart’s rhythm. These premature beats can originate from either the atria or the ventricles, each presenting distinct visual characteristics on an ECG. The perceived “skip” often results from a brief pause that follows the premature beat, allowing the heart to reset before resuming its regular rhythm.
Premature Atrial Contractions (PACs) occur when an electrical impulse originates from an abnormal location in the atria, rather than the SA node, triggering an early beat. On an ECG, a PAC is characterized by an abnormally shaped P wave that appears earlier than anticipated. This early P wave is typically followed by a normal, narrow QRS complex, as the electrical impulse usually conducts normally through the heart’s electrical system. The P wave associated with a PAC may have a different shape, such as being wider, taller, or even inverted, depending on its origin within the atria.
Premature Ventricular Contractions (PVCs) originate from an ectopic focus within the ventricles, causing the lower chambers to contract prematurely. On an ECG, PVCs have a distinctive appearance, marked by a wide, unusually shaped QRS complex that occurs early and is not preceded by a P wave. This wide and “bizarre” QRS complex reflects the abnormal and slower electrical conduction through the ventricles, bypassing the normal pathways. A PVC is often followed by a full compensatory pause, meaning the interval between the PVC and the subsequent normal beat is longer than usual, which contributes to the “skipped” sensation.
Identifying Pauses and Missed Beats
Beyond premature contractions, a “skipped heartbeat” can also refer to an actual absence of a beat or a significant delay in the heart’s rhythm, which appear differently on an ECG. These events are distinct from premature beats, as they involve a temporary cessation or interruption of the electrical activity rather than an extra beat.
Sinus pauses or sinus arrest occur when the sinoatrial node, the heart’s natural pacemaker, temporarily fails to generate an electrical impulse. On an ECG, this manifests as a flat line without any P waves or QRS complexes for a period, indicating a halt in electrical activity. A sinus pause is typically defined as a delay of two seconds or more between P waves, while sinus arrest can refer to a more prolonged absence of electrical activity. Following such a pause, the normal rhythm usually resumes, or an escape beat from another part of the heart may take over to maintain a heart rate.
Atrioventricular (AV) blocks represent another category of missed beats, where the electrical signal from the atria is either delayed or completely blocked from reaching the ventricles. In second-degree AV block, some P waves are not followed by a QRS complex, meaning some atrial impulses fail to conduct to the ventricles, resulting in “dropped” beats.
Mobitz Type I (Wenckebach)
Mobitz Type I (Wenckebach) second-degree AV block is identified by a progressive lengthening of the PR interval—the time between the P wave and the QRS complex—until a P wave is completely blocked, and no QRS complex appears.
Mobitz Type II
In contrast, Mobitz Type II second-degree AV block shows a consistent PR interval for conducted beats, but P waves are intermittently and unpredictably blocked without prior PR interval prolongation, leading to sudden dropped QRS complexes.
Common Underlying Causes and Significance
Many factors can contribute to the occurrence of “skipped heartbeats,” ranging from lifestyle influences to underlying medical conditions. Everyday stimulants like caffeine, alcohol, and nicotine can trigger premature beats in some individuals. Stress, anxiety, and fatigue are also frequently associated with these sensations, as they can influence the heart’s electrical stability. Dehydration and electrolyte imbalances, such as low potassium or magnesium levels, can disrupt the electrical signals in the heart, leading to irregularities.
While many instances of “skipped heartbeats” are benign and occur even in otherwise healthy individuals, some can indicate more significant underlying issues. Conditions like thyroid imbalances, sleep apnea, or existing heart diseases, including structural abnormalities or prior heart attacks, can increase the likelihood of experiencing these arrhythmias. Certain medications, including some decongestants and antihistamines, may also contribute to their occurrence.
It is important to understand when a “skipped heartbeat” warrants medical attention. Occasional, isolated skipped beats are typically not a concern. However, if these sensations are frequent, persistent, or accompanied by other symptoms, consulting a healthcare professional is advisable. Symptoms such as dizziness, lightheadedness, chest pain, shortness of breath, or fainting alongside skipped beats indicate a need for prompt medical evaluation. A healthcare provider can determine the cause and recommend appropriate management or further testing if necessary.