Fundamentals of Reading EKGs

Fundamentals >Normal Sinus Rhythm

Normal Sinus Rhythm

Molecules move in and out of the cell, electrical currents and waves of depolarization are formed that ultimately spread throughout the heart.  Contraction of cardiac myocytes is coupled to the electrical phenomenon, and all of this activity can be recorded and graphed as vectors using external electrodes.  So what do the lines and waves mean, and how do we make sense of these patterns?

For those of you working though the entire tutorial, rejoice.  We are finally going to start examining EKGs.  In this section, we will cover the basic rhythm of the heart in its normal healthy state and correlate it with the EKG findings.

Sinus Rhythm with Labels

Naming the Waves

The EKG consists of a series of waves and spaces which reflect underlying cardiac electrical activity.  In this section, we will review the conduction pathway again, labeling each stage as we go.  The waves are labeled alphabetically, starting with the letter P.  The baselines between waves are also given specific names, labeled as either segments or intervals.  A segment connects two waves.  Intervals are a segment with one of the waves included.

Atrial Depolarization and the P Wave

In the normal state, the sinoatrial node (SA node), located in the high right atrium, acts as the pacemaker for the entire heart.  As the SA node spontaneously depolarizes, a wave of positive electrical current flows through the atria.  From the SA node, electrical current travels quickly along the four conducting paths that comprise the atrial conduction system.  They are the Anterior, Posterior, and Middle Internodal tracts, as well as Bachmann's Bundle.

The three internodal tracts speed conduction throughout the right atrium, connecting the SA node to the AV node.  Bachmann's Bundle is the only pathway that crosses the interatrial septum, allowing depolarization to flow into the left atrium and depolarize the chamber. 

At a slower pace, waves of depolarization are spreading throughout the atrial wall from the SA node and atrial conduction system, much like the wake of a boat.  The end result is an organized contraction of the left and right atrium. 

Importantly, the EKG is not sensitive enough to detect depolarization of the conducting pathways.  Instead, it is recording depolarization of the much larger myocardium.  As the right and left atrium depolarize, the EKG records a small symmetrical wave, termed the P Wave.  The first half of the P wave represents Right Atrial depolarization and the second half of the P wave represents the slightly delayed Left Atrial depolarization.  Occasionally, leads will show a slight notching of the P-wave, further delineating right and left atrial activation.

AV Node and the PR Interval

Conduction is delayed as the wave of depolarization reaches the AV-node, which is located on the inferior aspect of the interatrial septum within the right atrium.   The AV-node is the only electrical connection between the atria and ventricles.  This delay allows the atria to fully depolarize and contract, pumping blood into a relaxed ventricle.  If the ventricles were to contract at the same time as the atria, blood flow would be impeded, and cardiac output would decrease. 

As the wave of myocardial depolarization approaches the ventricles, it is impeded by the Atrio-Ventricular (AV) valves, which serve as both an electrical and mechanical barrier between the two chambers.  As you may recall, the AV-node (and SA node) differs from the rest of the cardiac conduction system.  The two nodes consist of slow response cells, which utilize calcium for depolarization and have a different response to depolarizing current.  The rest of the conduction system and myocardium relies on sodium for depolarization, and is collectively referred to as fast response cells.  The difference in depolarizing ions allows for targeted pharmacological intervention, such as calcium channel blockers to specifically target the slow response cells.

This delay in conduction between the atria and the ventricles is reflected on the EKG as a flat line between the P wave and the QRS complex, which we will talk about shortly.  Again, a flat line can be named as either a segment or an interval.  In this particular case, the PR segment represents the period between the end of atrial depolarization and the beginning of ventricular depolarization (QRS complex), which correlates directly with conduction speed of the AV-node. 

The PR Interval, which includes both the PR segment and the P wave, reflects the period from the beginning of atrial depolarization to the beginning of ventricular depolarization and is also affected by changes in AV-conduction.  Shorter conduction times reduce the length of both the PR-segment and interval while prolonged conduction times lengthen the PR segment and interval. 

Ventricular Depolarization and the QRS Interval

Following a brief delay through the AV node, the wave of depolarization arrives at the ventricles and begins the rapid journey through the ventricular conduction system.  Connected to the AV node is the Bundle of His, which, like its name implies, is a large bundle of rapidly conducting Purkinje fibers.  The Bundle of His enters the interventricular septum, where it quickly divides into the Left and Right Bundle Branches (LBB and RBB) respectively). 

The RBB carries the depolarizing currents to the Right Ventricle, periodically giving off short Purkinje fibers that spread throughout the myocardium, speeding up conduction in the thick, slower conducting  muscle.

The LBB is a bit more involved.  Shortly after separating from the RBB, it divides into three fascicles.  The Septal Fascicle innervates the interventricular septum, depolarizing in a left to right manner. The Left Anterior Fascicle supplies the anterior LV and the Left Posterior Fascicle supplies the posterior LV.  Just like the RBB, the LBB and its fascicles give off numerous Purkinje fibers that help speed conduction through the myocardium, facilitating a coordinated contraction. 

Ventricular depolarization is represented on the EKG by the QRS complex, which consists of multiple waves.  Again, this complex is recording myocardial depolarization, not depolarization of the conduction system itself.  The QRS complex has its own nomenclature, and not every portion of the QRS complex must be present.  Working in a left-to-right fashion, each deflection is given its own unique name.  The rules are:

QRS Labels


Every rule has its exception, and the QRS is our exception to the interval rule.  As mentioned earlier, an interval is a segment that includes one wave.  However, the QRS Interval does not include a flat line.  It is strictly the QRS complex, and the two terms may be used interchangeably. 

There is a wealth of information within the QRS Interval.  EKG waves are actually vectors with direction that reflect the amount of underlying electrical activity through amplitude and voltage measurements.  The first portion of the QRS complex may show depolarization of the interventricular septum, which is recorded as a small, negatively inscribed Q-wave.  Buried within the QRS complex is also a small recording that reflects atrial repolarization.  However, it is not usually visible, except during pathologic conditions.  The rest of the QRS complex represents ventricular depolarization, especially the large left ventricle.  Changes in direction, voltage, or pattern can be an early clue to underlying abnormalities.

Following the QRS complex is the J point, which is the connection between the end of the QRS and beginning of the ST segment.

Ventricular Repolarization and T waves

Following our alphabetical order, the wave after the QRS complex is the T-wave, and represents Phase 3 or the rapid phase of ventricular repolarization. 

Phase 3 was specified because repolarization actually begins at the end of the QRS complex.  The ST- segment, which connects the QRS complex to the T-wave, represents Phase 2, also known as the plateau phase. 
Following ventricular repolarization is another flat line until the SA node depolarizes, repeating the entire process again. 

U Waves

In the vast majority of cases, you will find an baseline connecting the T wave to the next P wave initiating depolarization.  Occasionally, there will be another deflection between the T and P wave, predictably named the U wave.

U waves are often low amplitude waves that travel in the same direction as the associated T waves, and are most often identified in the precordial leads.  There is not a consensus on what they represent, although the current belief is that U waves reflect either repolarization of the His-Purkinje system, or relaxation of the ventricles.  Prominent U waves are often associated with hypokalemia and hypercalcemia.

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