ECG or Echocardiogram: Which Test to Order (and Why They Are Not Substitutes)

The electrocardiogram (ECG) and the echocardiogram answer different questions about the heart. The ECG records electrical activity and is the test of choice to assess rhythm and conduction: it detects and classifies arrhythmias, blocks and conduction disturbances. The echocardiogram uses ultrasound to show structure and function: chamber size, wall thickness, valves and contractility. Because they assess distinct domains, they are complementary and not substitutes.

What the ECG actually assesses

The ECG is a recording of the heart's electrical activity over time. Its value lies in showing how the heart is activated: the origin of the impulse, the depolarization sequence and the presence of abnormal beats. It is the reference tool to identify and characterize arrhythmias, such as atrial fibrillation, tachycardias, premature complexes and atrioventricular blocks.

Typical indications include rhythm irregularity or a pulse deficit on auscultation, syncope, episodes of weakness or exercise intolerance, and screening before anesthetic procedures. When intermittent arrhythmia is suspected, a short recording may miss the event, and prolonged ambulatory monitoring (Holter) is often indicated to document and quantify the disturbance.

It is important to understand the ECG's limits: electrical criteria for chamber enlargement (for example, P wave or QRS complex changes) have low sensitivity. In dogs with mitral valve disease, P wave parameters used to detect left atrial enlargement showed a median sensitivity around 60 percent, meaning a normal ECG does not rule out chamber enlargement.

What the echocardiogram actually assesses

The echocardiogram is an ultrasound imaging test that shows the heart in motion. It allows measurement of chamber size and wall thickness, assessment of valve anatomy and function, myocardial contractility and, with Doppler, blood flow. It is the test that defines the structural cause behind a murmur or an enlarged cardiac silhouette.

In the ACVIM consensus guidelines, echocardiography is the recommended method to definitively identify the cause of a murmur in canine myxomatous mitral valve disease, and it is considered the gold-standard test for diagnosing cardiomyopathies in cats. Examples of measurements used for staging include: in canine mitral valve disease (stage B2), a left atrium to aorta ratio of 1.6 or greater and a normalized left ventricular diameter of 1.7 or greater; in cats, an end-diastolic left ventricular wall thickness below 5 mm is considered normal and 6 mm or greater indicates hypertrophy.

What the echocardiogram does not do well is characterize arrhythmias over time. It provides a snapshot of the rhythm during the exam, but it does not replace the ECG or the Holter in detecting and classifying electrical disturbances, especially intermittent ones.

Side-by-side comparison

What it measures: ECG, electrical activity (rhythm and conduction) · echocardiogram, structure and function (imaging).

Question it answers: ECG, is the heart beating in a normal rhythm? · echocardiogram, is the heart normal in shape and contraction?

Best for: ECG, arrhythmias, blocks and conduction disturbances · echocardiogram, valves, wall thickness, chamber size and contractility.

Chamber enlargement: ECG, electrical criteria with low sensitivity · echocardiogram, direct measurement and reference standard.

Main limitation: ECG, does not show structure · echocardiogram, captures only a snapshot of the rhythm.

Technology: ECG, electrodes and an electrical tracing · echocardiogram, ultrasound and Doppler.

How they complement each other in practice

In cardiology practice, the two tests usually go together. The echocardiogram defines the structural disease; the ECG and, when indicated, the Holter characterize the electrical component that disease may generate. A dog with advanced mitral valve disease, for example, may develop atrial fibrillation, and diagnosing the rhythm depends on the ECG, while quantifying the remodeling depends on the echocardiogram.

The initial choice depends on the clinical question. When facing rhythm irregularity, a pulse deficit, syncope or suspected arrhythmia, the ECG is the first step. When facing a murmur, signs of heart failure, suspected cardiomyopathy or staging of valvular disease, the echocardiogram is the indicated test. In many patients, both are needed for a complete assessment.

This combination translates well into a connected ecosystem. ECG devices such as the INcardio X and the INcardio Agile, integrated with the INpulse One platform and the cloud history on INcloud, make it easier to document the rhythm and follow its evolution over time, complementing the imaging exam. The decision on which tests to order and how to interpret them always belongs to the attending veterinarian, in light of each patient's clinical picture.

Summary: which one to order

Order an ECG when the question is about rhythm: auscultated arrhythmia, pulse deficit, syncope, episodic weakness, exercise intolerance or pre-anesthetic screening. Consider a Holter for intermittent arrhythmias.

Order an echocardiogram when the question is about structure and function: investigating the cause of a murmur, staging valvular disease, suspected cardiomyopathy or signs of heart failure.

When heart disease is confirmed or relevantly suspected, the two tests tend to be complementary. Neither replaces the other: the ECG does not see the structure and the echocardiogram does not track the rhythm over time. This page is informational and does not replace assessment by a veterinarian.