You have asked…How do I interpret ECGs with confidence?
The expert says…
Arrhythmias are clinically important because of their ability to compromise cardiac output and oxygen delivery to the body. The level of cardiac performance during an arrhythmia depends on rate, site of origin, and duration of the arrhythmia, as well as the presence of underlying cardiac or systemic diseases that may adversely affect the patient. Consequences of an arrhythmia may be clinically undetectable, produce signs of inadequate cardiac output (eg, weakness, fainting, shock), or lead to complete collapse of the circulatory system and sudden death.
The 4 general steps for ECG assessment include evaluation of heart rate, cardiac rhythm, mean electrical axis, and assessment for patterns of chamber enlargement. The following review assesses and interprets common canine and feline arrhythmias.
A systematic approach to interpreting ECGs will ensure an accurate diagnosis. The initial steps should include evaluating paper speed and sensitivity followed by labeling the individual waveforms recorded on the ECG. Afterward, the following questions should be addressed:
This is the first installment of a two-part series outlining ECG interpretation. Part 2 will address bradyarrhythmias and tachyarrhythmias.
Is there a P wave for every QRS complex?
Atrial depolarization normally propagates through the AV node and subsequently produces depolarization of the ventricles, resulting in a fixed relationship of one P wave preceding every QRS complex.
Meanwhile, ectopic QRS complexes often originate without a preceding atrial event. Premature ectopic complexes occur <1 R-R interval from the preceding sinus beat, while escape complexes generally occur 1.5 to 2 R-R intervals from the preceding sinus beat.
Is there a QRS complex for every P wave?
After a brief delay (PR interval) ventricular depolarization normally follows every atrial depolarization, resulting in a QRS complex for every P wave. However, in some instances the electrical impulse may be blocked at the level of the AV node or His-Purkinje system, preventing propagation and subsequent depolarization of the ventricles.
AV = atrioventricular, SV = supraventricular, VPC = ventricular premature complex
Are P waves associated with QRS complexes?
Normally there is a fixed or nearly fixed physiologic PR interval between the P waves and QRS complexes, but in some instances there appear to be P waves that contribute to generating ventricular depolarization while they are instead merely fortuitous and QRS complexes are unrelated to the preceding atrial activity.
Are the QRS complexes supraventricular or ventricular?
If the electrical impulse originates from the sinus node, atrial myocardium, or atrioventricular (AV) node, the QRS complex will generally be tall, upright, and narrow (ie, supraventricular [SV]) because it utilizes the normal His-Purkinje system. Circumstances in which the QRS complex is SV but appears abnormal include ventricular enlargement patterns or bundle branch blocks.
If the electrical impulse originates from the ventricular myocardium, the QRS complex will be wide and bizarre (ie, ventricular) because it does not utilize the normal His-Purkinje system.
What is the underlying rhythm of P waves & QRS complexes?
A regular rhythm generally has <10% variation in the R-R intervals, while a regularly irregular rhythm has QRS complexes that vary by >10% but there is a repeating pattern to rate variation. An irregularly irregular rhythm is chaotic, usually fast, and without pattern to the irregular nature.
A paroxysmal rhythm disturbance is usually characterized by a sudden outburst of arrhythmia that may last for as short as 3 to 4 beats or as long as minutes to hours.
What are the most common rhythms with normal heart rates?
The most common cardiac rhythms that occur while the heart rate is normal include sinus rhythm, sinus arrhythmia, SV premature complexes, ventricular premature complexes (VPCs), or accelerated idioventricular rhythm. Sinus rhythm, sinus arrhythmia, and SV premature complexes tend to have tall, upright, narrow QRS complexes. Sinus rhythm with VPCs or accelerated idioventricular rhythm may have a normal heart rate with intermittent or sustained ventricular morphology QRS complexes; because these impulses originate from the ventricular myocardium, they are conducted aberrantly and have abnormal morphology.
Requirements for Success
- Obtaining accurate ECG diagnosis
- Considering possible underlying mechanism or cause of arrhythmia
- Determining presence of underlying heart disease
- Understanding the goals of arrhythmia treatment:
- In patients with clinical signs: Reducing hemodynamic compromise caused by arrhythmia and preventing death
- In asymptomatic patients: Preventing arrhythmia from progressing to cause clinical signs or death
See the Common Cardiac Arrhythmias Management Tree for an algorithm outlining the diagnosis and management of rhythms associated with normal heart rates (dogs, 70-160 bpm; cats, 140-220 bpm).
Sinus rhythm & Cardiac arrhythmia
Sinus rhythm represents the normal sequence of cardiac electrical activity. Pacemaker cells within the sinoatrial node display automaticity and serve as the origin for the normal cardiac impulse. The P wave is recorded on the ECG as the wave of electrical activity depolarizes the atrial myocardium. The electrical impulse then traverses the AV node and His-Purkinje system, recorded as the PR interval, followed by depolarization of the ventricular myocardium producing the QRS complex. The T wave is recorded during ventricular repolarization (Figure 1). The average heart rate (ie, ventricular heart rate) can be determined on an ECG strip recorded at 50 mm/sec by counting the number of QRS complexes in 15 large boxes (75 mm) and multiplying by 40.