CardioBird

Estimated reading time: 4.13 minutes

 

Welcome back to our series, The ECG Decoded: A Veterinarian’s Guide to the Heart’s Rhythm. In our first issue, we laid the foundation by exploring the heart’s electrical blueprint. Now, let’s build on that knowledge and dive into the practical skill of interpreting the ECG tracing itself. This installment will guide you through understanding the core components of the ECG: the waves, normal sinus rhythm, and the insightful metric of heart rate variability (HRV).

 

The Language of the Heart: A Closer Look at ECG Waves

An electrocardiogram is a storybook of the heart’s electrical activity. Each wave, segment, and interval corresponds to a specific event. Let’s break down the key characters in this story:

• The P Wave: This small, rounded wave represents atrial depolarization. It’s the electrical signature of the impulse spreading from the sinoatrial (SA) node throughout the right and left atria, causing them to contract. A normal P wave indicates that the electrical impulse originated in the SA node—the heart’s natural pacemaker.
• The QRS Complex: This is the most prominent feature on the ECG, representing ventricular depolarization. It corresponds to the rapid spread of the electrical impulse through the Bundle of His, bundle branches, and Purkinje fibers, leading to the contraction of the main ventricular muscle mass. The atria are repolarizing during this time, but that signal is masked by the much larger ventricular activity.
• The T Wave: This wave represents ventricular repolarization—the resetting of the ventricular muscle cells as they prepare for the next cycle. Its shape and direction can be influenced by various factors, including electrolyte imbalances and certain medications.

The PR interval (from the start of the P wave to the start of the QRS complex) reflects the time it takes for the impulse to travel from the SA node, through the atria, and pause at the AV node before proceeding to the ventricles. The QT interval (from the start of the QRS complex to the end of the T wave) represents the total time for ventricular depolarization and repolarization.

 

The Gold Standard: Recognizing Normal Sinus Rhythm

When we say a heart is in “normal” rhythm, what we typically mean is Normal Sinus Rhythm (NSR). This is the ideal rhythm, indicating a healthy, well-functioning conduction system. For a rhythm to be classified as NSR, it must meet these criteria:

1. Consistent P Waves: Each P wave should have a uniform shape and size, indicating a single, stable site of origin (the SA node).
2. Consistent PR Interval: The time from the P wave to the QRS complex should be constant from beat to beat, confirming stable conduction through the AV node.
3. A P Wave for Every QRS: Every P wave should be followed by a QRS complex, and every QRS complex should be preceded by a P wave. This demonstrates that every atrial impulse is successfully being conducted to the ventricles.
4. Regular Rhythm: The R-R intervals (the time between consecutive QRS complexes) should be essentially regular.
5. Appropriate Heart Rate: The rate should fall within the normal species-specific range (e.g., ~70-160 bpm for a dog, depending on size and breed).

Identifying NSR is the first and most critical step in ECG interpretation. It establishes a baseline from which all deviations (arrhythmias) can be recognized and assessed.

 

The Rhythm Beneath the Rhythm: Understanding Heart Rate Variability

While a metronome is perfectly regular, a healthy heart is not. Heart Rate Variability (HRV) refers to the beat-to-beat variation in the R-R interval. Contrary to what one might think, high HRV (more variation) is generally a sign of good cardiovascular health and robust autonomic nervous system function. It indicates a strong balance between the sympathetic (“fight-or-flight”) and parasympathetic (“rest-and-digest”) inputs to the heart.

Low HRV, on the other hand, can be a marker of stress, illness, pain, or underlying cardiac disease, as it often reflects dominant sympathetic tone. Monitoring HRV can provide valuable insights into a patient’s overall physiologic status, response to treatment, and even prognosis.

 

Putting It All Together in Practice

Mastering the interpretation of these fundamental elements transforms the ECG from a confusing squiggle into a powerful diagnostic narrative. By systematically assessing the waves, confirming sinus rhythm, and noting the characteristics of HRV, you can confidently identify the hallmarks of a healthy heart or the early signs of dysfunction.

At CardioBird, we are dedicated to empowering your practice with this level of deep understanding. Our AI-ECG analysis doesn’t just provide an answer; it is built upon this foundational physiology to give you a detailed, evidence-based report. We translate these complex waveforms into clear, actionable insights, helping you make confident diagnoses and provide the best possible care for your patients.

In our next issue, we will move from the normal to the abnormal, exploring the ECG signs of chamber enlargement and the pathogenesis of arrhythmias.

 

The CardioBird Team