Back to Basics – Rhythms Review

Animated Rhythms Course

To take this month’s ECG Challenge Quiz, click below.  Then scroll down to review the quiz discussion.

Welcome to this month’s Quiz discussion.  Further down this page we will review the tracings in point from the previous quiz.  First, let us turn back the clock to review some basics of rhythm analysis.

Below is a sample taken from the Animated Rhythms Course of a Normal Sinus Rhythm animated tutorial.  Click on numbers 1 – 8 to view the animation of the electrical activity in the heart with the resultant wave form.  Each region of the heart has been color coordinated to the rhythm strip for signal origin analysis.  You may also click on “Play Rhythm” and “All Labels” to play they entire rhythm and review a detailed breakdown of the signal origins.

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The electrical signal initiates in the SA node, depolarizes the Atrial myocardium, depolarizes the AV node, propagates through the Bundle of HIS, and terminates in the Purkinje cells, depolarizes the Ventricular Myocardium (endocardium to the epicardium). The tissues then recharge (repolarize) and the SA node discharges again and the cycle repeats itself.

1 =time at depolarization of SA node
2 =time at depolarization of AV node
The resultant electrical signal is as follows:

P = Depolarization of the Atria
QRS = Depolarization of the Ventricles
T = Repolarization of the Ventricles

Note: The depolarization of the SA node (at 1) and the AV node (at 2) produce only a tiny voltage and will not be visible on the standard rhythm strip.

It is essential that you understand the electrical characteristics of the heart tissues. This will aid you in understanding the normal rhythm characteristics as well as the common arrhythmias that you will encounter in your practice.
The heart is composed of two basic tissue types:
Tissue 1. The myocardium (muscle).
Tissue 2. The special electrical conduction system*.

Electrically speaking, the two tissue types are characterized as follows:
Tissue 1.  The myocardium does not possess automaticity and subsequently will not depolarize until stimulated
by an external stimulus.  This stimulus is usually the adjacent Purkinje Cell or an adjacent myocardial
cell in the active process of depolarizing.

Tissue 2. The special electrical conduction system has two important unique properties:
a) It possesses automaticity, i.e., it will spontaneously depolarize and repolarize without an external
b) It conducts the electrical signal very rapidly as compared to other tissues.

Tissues with Automaticity:

  • SA Node (60-100 bpm)
  • Internodal Pathway (Atria; 60-80 bpm)
  • AV Junction Node (40-60 bpm)
  • Bundle of HIS (& all branches; 40-60 bpm)
  • Purkinje Cells (20-40 bpm)

Important properties of Automaticity Tissues:
The automaticity tissue of the heart is designed to provide an automatic electrical stimulus to ultimately depolarize
the myocardial tissue. The system is designed with a built in demand backup as its fail safe mechanism.  The design
function is as follows:

If normal Sino-Atrial Node function…
SA Node discharges at a rate of 60-100 bpm.  This signal will suppress all the other automaticity tissues.

If the SA Node fails…
The next in line automaticity tissue takes over as follows:
Atrial Internodal Pathways- discharge rate 60 – 80 bpm. (This signal will suppress all the lower hierarchy automaticity tissues.)

If the Atrial Tissue fails…
The next in line automaticity tissue takes over as follows:
Junctional (including Bundle of HIS and branches) – discharge rate 40-60 bpm. (This signal will suppress all the lower hierarchy automaticity tissues)

If the Junctional Tissue fails…
The next in line automaticity tissue takes over as follows:
Terminal Purkinje Fibers – discharge rate 20-40 bpm.

If the Purkinje Fibers fail…
Ventricle myocardium will receive no electrical signal.  Flatline will follow, then death.

Now to review the rhythms strips from the quiz (in order from “A” through “E”).

This is a benign rhythm and usually requires no treatment (unless hemodynamically indicated or patient is symptomatic).

The signal travels in a normal pathway from the SA node to the AV node to the Purkinje Cells, then across the
myocardium.  The rhythm strips will appear normal except that the rate is rapid.
P-P interval = constant • QRS-QRS interval = constant

Characterized by: 

  • Rate > 100 beats per minute
  • Otherwise all other parameters are normal.

Sinus Arrest with Junctional Escape Beat

These are usually benign beats and most often require no emergency treatment unless a patient is symptomatic or is hemodynamically unstable.

Occurs after long hesitation (pause).  Spontaneous discharge of automaticity tissue from:
Atrium ……………… Atrial Escape Beat
AV-Junction ……… Junctional Escape Beat
Ventricle …………… Ventricular Escape Beat

Common Error
An ectopic beat originating from the ventricle (because of its wide, bizarre shape) is sometimes referred to as a PVC, even if the beat does not occur early.  This is obviously incorrect.
If the beat occurs later than expected, it is not a premature beat, but an escape beat.

Escape Beat characterized by:

  • Spontaneous beats which occur after long pause of electrical activity.
  • Named according to origin of beat: Atrial, Junctional or Ventricular

Junctional Rhythm

This is a benign rhythm and usually requires no treatment (unless hemodynamically unstable).

There are 4 possible scenarios of the P-wave presentation

  1.  P-wave precedes QRS complex
  2.  P-wave coincident with QRS complex
  3.  P-wave follows QRS complex
  4.  P-wave is absent (is isoelectric)

Signal initiates at junction, travels retrograde in Atrium (therefore P-wave inverted); travels in a normal pathway direction (antegrade) through Ventricles– QRS complex is therefore normal appearing.

Junctional Rhythm characterized by:

  • Inverted or sometimes absent P-waves
  •  Normal QRS complexes
  •  Regular rhythm
  •  Rate 40-60

Atrial Flutter 2:1 (Ventricular Rate = 160)

Malfunctioning area of the Atria discharges at a rapid rate (commonly 300 beats per minutes) and takes on a sawtooth pattern.  Some of the flutter signals (f-waves) will be blocked at the AV node. You must therefore address this when describing the rhythm, e.g., Atrial Flutter with 2:1 Block, 3:1 Block or Variable Block.

Pearl regarding A Flutter:  If ventricular rate of a cardiogram is 150 – consider Atrial Flutter with 2:1 block. If you look at the 12-Lead, the flutter pattern may only be apparent in one or two leads.

Atrial Flutter is generally a more serious finding and sometimes requires more aggressive treatment.  However, you
must be diligent and correlate clinically, including comparison with the old ECG – the pattern might be chronic for this patient and the treatment may be none.

Atrial Flutter Characterized by:

  • Atrial flutter waves (sawtooth pattern)
  • Rhythm might be regular or irregular depending on block

SupraVentricular Tachycardia (SVT); Rate = 160 bpm

Spontaneous focus of irritability of Atrium (supraventricular) that discharges at a rapid rate. (Please note that the term SVT is typically reserved for regular rhythms, either atrial (with p-wave often hidden) or junctional.

Pearl regarding the SVT:  Often the P-waves of SVT cannot be discerned (sometimes may appear as a tiny notch), since the rate is so rapid that it blends with the T-wave.

If clinically indicated, treatment consists of vagal maneuvers and IV Adenosine.

SVT Characterized by:

  • Rate = 150-250 bpm
  • Rhythm = Regular
  • QRS complexes are narrow (unless a conduction defect of LBBB, RBBB or IVCD is present)