An electrocardiogram, or EKG, records the electrical activity of the heart over time. Electrodes placed on the skin detect the electrical changes that occur as the heart muscle depolarizes and repolarizes with each beat. This activity is translated into a wave pattern on a monitor, composed of deflections such as the P wave, QRS complex, and T wave. These waves represent the sequential contraction and relaxation of the heart’s upper and lower chambers. Rhythms are systematically categorized based on where the electrical impulse originates and how it travels through the heart’s conduction system.
The Baseline Normal Sinus Rhythm
All other rhythms are measured against the standard of Normal Sinus Rhythm (NSR), which represents the standard electrical pattern of the heart. In NSR, the electrical impulse begins correctly in the sinoatrial (SA) node, often called the heart’s natural pacemaker, located in the right atrium. This impulse then follows the heart’s specialized conduction system, causing the atria and ventricles to contract in a coordinated sequence. On an EKG, NSR is characterized by a regular rhythm and a heart rate typically ranging between 60 and 100 beats per minute in a resting adult.
Every beat in NSR is preceded by an upright P wave, which signifies the electrical activation of the atria, followed by a narrow QRS complex, which represents the activation of the ventricles. This consistent, one-to-one relationship between the P wave and the QRS complex confirms that the electrical signal is originating and traveling along the correct path. The coordinated contraction sequence ensures efficient blood flow.
Primary Classification Rhythms by Site of Origin
Cardiac rhythms are organized into four anatomical groups based on the region of the heart where the electrical signal originates. Sinus rhythms, such as the normal baseline, all begin in the SA node and maintain the characteristic P wave preceding the QRS complex. Variations like sinus bradycardia (slow) and sinus tachycardia (fast) maintain the correct electrical pathway. These rhythms usually indicate a normal response to external factors like sleep, exercise, or fever.
Atrial Rhythms
Atrial rhythms originate from an ectopic focus somewhere in the atria but outside of the SA node. Because the impulse starts from a different location, the P wave morphology becomes abnormal, often appearing flattened, notched, or inverted. Atrial fibrillation is characterized by chaotic electrical activity, leading to an absence of distinct P waves and an irregularly irregular heartbeat. Atrial flutter shows rapid, regular atrial activity that produces a distinctive “sawtooth” pattern on the EKG tracing.
Junctional Rhythms
Junctional rhythms arise from the atrioventricular (AV) node area, with an intrinsic rate of 40 to 60 beats per minute. Since the impulse originates here, it travels backward (retrograde) to activate the atria, often resulting in an inverted P wave. This P wave may appear before, be hidden within, or follow the QRS complex, depending on the timing of the retrograde conduction. The resulting QRS complex is typically narrow because the impulse still follows the lower, specialized conduction system.
Ventricular Rhythms
Rhythms that originate in the ventricles bypass the heart’s natural conduction pathways entirely. Ventricular rhythms, which have an intrinsic rate of only 20 to 40 beats per minute, are identified by a wide and bizarre-looking QRS complex. This wide appearance results from the slow, cell-to-cell spread of electricity through the ventricular muscle. Ventricular tachycardia is an example where a rapid rate combined with wide complexes can severely compromise the heart’s pumping ability.
Disturbances Based on Rate and Regularity
Rate disturbances are defined by a heart rate outside the normal range of 60 to 100 beats per minute. Tachycardia is the term used for any rhythm exceeding 100 beats per minute. Conversely, bradycardia describes any rhythm slower than 60 beats per minute, which can be a benign finding in conditioned athletes or a symptom of a failing SA node.
Fibrillation is a specific type of irregularity characterized by rapid, chaotic electrical activity that prevents the chambers from contracting effectively. Atrial fibrillation is a common example where the atria merely quiver. Ventricular fibrillation is a life-threatening emergency.
Ectopic beats are premature heartbeats that interrupt the normal rhythm. These impulses arise from an abnormal focus in the atria (Premature Atrial Contractions or PACs) or the ventricles (Premature Ventricular Contractions or PVCs). These premature beats are often perceived as a “skipped beat” or a “thump” in the chest.
Understanding Critical and Lethal Rhythms
Ventricular fibrillation (V-fib) is immediately life-threatening because it leads to a complete loss of the heart’s ability to pump blood. V-fib is characterized by a disorganized tracing on the EKG. The electrical impulses are so rapid and random that the ventricles merely quiver, resulting in no effective circulation. This rhythm causes immediate cardiac arrest and requires prompt defibrillation to reset the heart’s electrical system.
Asystole represents a complete absence of electrical activity in the heart. In this state, there is no electrical impulse to stimulate any muscle contraction, and the heart is completely silent. Unlike V-fib, asystole cannot be treated with defibrillation and requires immediate cardiopulmonary resuscitation (CPR) and specific medications.
Pulseless Electrical Activity (PEA) occurs when the EKG tracing shows an organized electrical rhythm, but the patient has no measurable pulse. In PEA, the electrical system is working, but the heart muscle is failing to contract or pump blood effectively. Like Asystole, this condition requires immediate CPR and treatment of the underlying cause.