What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Normal ECG: How to Read a Normal Tracing

Name: Normal ECG: How to Read a Normal Tracing
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

A normal ECG is not a single picture. It is a set of measurements that only make sense when you know the paper speed, the calibration, the species, the body size, and the patient's state of mind. This guide walks you through a repeatable seven-step method for reading a normal tracing in dogs, cats, and horses. By the end you will be able to calculate heart rate, confirm sinus rhythm, measure each wave and interval in milliseconds and millivolts, estimate the mean electrical axis, and recognize the artifacts that most often make a normal ECG look abnormal.

You need a six-lead or at least a single-lead ECG recorder, electrode gel or alcohol, flat clip electrodes or alligator clips, a table or mat for right lateral recumbency, and a printed or on-screen tracing with a visible calibration mark. A ruler graduated in millimeters is helpful for manual measurement. Most modern systems display measurements automatically, but you should still know how to verify them by hand.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

What a Normal ECG Actually Shows

The electrocardiogram records the sum of electrical activity generated by cardiac muscle cells as they depolarize and repolarize. Each deflection on the tracing corresponds to a specific event in the cardiac cycle. The P wave represents atrial depolarization. The QRS complex represents ventricular depolarization. The T wave represents ventricular repolarization. Atrial repolarization is hidden inside the QRS complex and is not visible as a separate wave.

A normal tracing shows a regular sequence of P waves, each followed by a QRS complex, each followed by a T wave, at a rate appropriate for the species and the patient's physiologic state. The intervals between these events fall within published reference ranges. The amplitudes of the waves fall within expected limits for the lead being examined.

The word "normal" carries two caveats. First, normal values differ by species. A normal feline ECG looks different from a normal canine ECG, and a normal equine ECG looks different from both. Second, normal values differ by body size within a species. A Chihuahua and a Great Dane can both have normal ECGs, but the amplitudes and durations will not be identical.

Paper Speed and Calibration: The Foundation of Every Measurement

Every measurement you make depends on two settings that must be confirmed before you interpret anything.

Paper speed determines the horizontal scale. At the standard small animal speed of 25 mm/s, one small box (1 mm) equals 0.04 seconds, and one large box (5 mm) equals 0.20 seconds. At 50 mm/s, one small box equals 0.02 seconds and one large box equals 0.10 seconds. Some equine recordings use 25 mm/s, while others use 50 mm/s to spread out the rapid complexes. If you measure intervals at the wrong assumed speed, every duration will be off by a factor of two.

Calibration determines the vertical scale. The standard is 1 mV = 10 mm, meaning a 1 mV signal produces a deflection of 10 mm (two large boxes). Some systems use 1 mV = 5 mm for large animals or for patients with very tall QRS complexes, and some use 1 mV = 20 mm for very small patients with low-voltage complexes. If you measure amplitudes at the wrong assumed calibration, every amplitude will be off by a factor of two or more.

The practical rule is simple. Before you measure anything, look at the calibration mark on the tracing. Confirm the paper speed printed on the recording. Write both values at the top of your interpretation. If they are not visible, repeat the recording.

A study comparing two digital electrocardiographs in endurance-trained sled dogs found clinically important differences in lead II and III Q- and R-wave amplitudes, with the veterinary-adapted device measuring 11% to 23% higher than the human-adapted device [1]. This finding reinforces that even the machine you choose can shift your numbers. The takeaway is not that one machine is wrong. The takeaway is that you must know your equipment and use consistent reference intervals derived from comparable equipment.

Step-by-Step Interpretation of a Normal Tracing

The following seven steps should be performed in order. Each step builds on the previous one. If you find an abnormality at any step, note it and continue, because a complete interpretation requires all seven.

Step 1: Confirm Signal Quality and Artifact-Free Recording

Before you measure anything, scan the entire tracing for artifact. Common artifacts include:

Muscle tremor artifact: fine, rapid oscillations superimposed on the baseline. This is common in tense or shivering patients.
Movement artifact: large, irregular baseline swings that can mimic P waves or QRS complexes.
Poor electrode contact: a wandering or drifting baseline, often caused by insufficient gel, dry skin, or loose clips.
Electrical interference: a regular, high-frequency oscillation at 50 or 60 Hz, depending on your power supply. This is called 60-cycle interference in the United States.

A normal ECG must have a stable baseline in all leads. If the baseline wanders, dry the skin, reapply gel, and secure the electrodes. If the patient is moving, wait for a calm moment or gently restrain without causing distress. Do not interpret a tracing that is full of artifact.

Step 2: Calculate the Heart Rate

Heart rate is the first measurement because it frames everything else. You can calculate it by several methods.

Method A: Count large boxes between R waves.

At 25 mm/s:

Heart rate (bpm) = 300 / number of large boxes between consecutive R waves

At 50 mm/s:

Heart rate (bpm) = 600 / number of large boxes between consecutive R waves

Method B: Count small boxes between R waves.

At 25 mm/s:

Heart rate (bpm) = 1500 / number of small boxes between consecutive R waves

At 50 mm/s:

Heart rate (bpm) = 3000 / number of small boxes between consecutive R waves

Method C: Count R waves in a 6-second strip.

At 25 mm/s, a 6-second strip is 150 mm long (30 large boxes). Count the R waves in that strip and multiply by 10.

At 50 mm/s, a 6-second strip is 300 mm long (60 large boxes). Count the R waves and multiply by 10.

Worked example. You record a dog at 25 mm/s. You count 4 large boxes between two consecutive R waves.

Heart rate = 300 / 4 = 75 bpm

You count 20 small boxes between two consecutive R waves.

Heart rate = 1500 / 20 = 75 bpm

Both methods agree. The dog's heart rate is 75 bpm.

For a cat at 25 mm/s with 2 large boxes between R waves:

Heart rate = 300 / 2 = 150 bpm

For a horse at 25 mm/s with 10 large boxes between R waves:

Heart rate = 300 / 10 = 30 bpm

Normal resting heart rates vary widely by species and by the patient's demeanor. In dogs, a study of 243 ECG recordings and 153 clinical examinations found no significant correlation between heart rate and body weight, but dogs under one year of age had significantly higher heart rates than older dogs, and relaxed dogs had lower rates than excited or nervous dogs [2]. This means you should not assume a small dog must have a fast heart rate or a large dog must have a slow one. Assess the patient's attitude and age before deciding whether a rate is normal.

Step 3: Assess Rhythm

Rhythm assessment asks two questions. Is there a P wave before every QRS complex? Is there a QRS complex after every P wave?

In sinus rhythm, the answer to both questions is yes. The P waves are consistent in morphology, and the PR interval is constant. The rhythm is regular or shows a predictable pattern of variation.

Respiratory sinus arrhythmia is a normal finding in dogs. The heart rate increases during inspiration and decreases during expiration. This occurs because vagal tone fluctuates with the respiratory cycle. On the tracing, you see gradual shortening and lengthening of the R-R interval in a cyclic pattern. The P waves remain present and constant in morphology. The PR interval remains constant. This is not a disease. It is a normal vagal phenomenon, and it is most prominent in young, healthy, relaxed dogs.

A study of 239 dogs found that the incidence of sinus arrhythmia was mostly seen in older dogs, which may reflect the population studied or the recording conditions [3]. Regardless of age, respiratory sinus arrhythmia is common and should not be mistaken for atrial fibrillation or another arrhythmia.

In cats, sinus arrhythmia is less common and usually less pronounced. In horses, sinus arrhythmia is common, and you may also see sinus block or sinus arrest, which are normal vagal phenomena in resting horses.

Sinus bradycardia is a slow sinus rhythm. In dogs, this can be normal in large breeds, in athletic dogs, or in relaxed patients. In horses, resting heart rates of 28 to 40 bpm are normal.

Sinus tachycardia is a fast sinus rhythm. This can be normal during exercise, excitement, or pain. It can also be a compensatory response to anemia, fever, or shock.

A Holter study of healthy puppies found median minimum, average, and maximum heart rates of 51 bpm, 99 bpm, and 274 bpm, respectively [4]. This wide range illustrates that normal heart rate in dogs is not a single number. It is a range that depends on activity, age, and autonomic tone.

Step 4: Examine P-Wave Morphology

The P wave represents atrial depolarization. In a normal tracing, the P wave should be:

Upright (positive) in lead II. This is the standard lead for rhythm assessment in small animals.
Consistent in shape from beat to beat.
Within normal duration and amplitude for the species.

P-wave duration is measured from the start of the P wave to the end of the P wave. P-wave amplitude is measured from the baseline to the peak of the P wave.

In dogs, normal P-wave duration is generally less than 40 milliseconds (0.04 seconds, or one small box at 25 mm/s). Normal P-wave amplitude in lead II is generally less than 0.4 mV (4 mm at standard calibration).

In cats, normal P-wave duration is generally less than 40 milliseconds. Normal P-wave amplitude in lead II is generally less than 0.2 mV (2 mm).

In horses, the P wave is often biphasic or notched, and its duration can be longer. Normal equine P-wave duration is generally less than 140 milliseconds, and amplitude varies with the lead.

A study of Labrador retrievers found no significant effect of gender or body weight on P-wave amplitude or duration [5]. A study of American Staffordshire Terriers found that P-wave duration was longer than general population reference intervals [6]. This suggests that breed-specific variation exists and should be considered when interpreting P-wave measurements in predisposed breeds.

Step 5: Measure the PR Interval

The PR interval is measured from the start of the P wave to the start of the QRS complex. It represents the time for the electrical impulse to travel from the sinus node through the atria, the AV node, and the His-Purkinje system to the ventricular myocardium.

In dogs, normal PR interval is generally 60 to 130 milliseconds (0.06 to 0.13 seconds).

In cats, normal PR interval is generally 50 to 90 milliseconds (0.05 to 0.09 seconds).

In horses, normal PR interval is generally 200 to 500 milliseconds, with wide variation depending on heart rate and autonomic tone.

A Holter study of healthy dogs found that PR intervals were positively correlated with body weight [7]. This means larger dogs tend to have slightly longer PR intervals than smaller dogs. The same study reported reference intervals for minimum, maximum, and mean heart rates of 15 to 42 bpm, 194 to 294 bpm, and 50 to 93 bpm, respectively [7].

The PR interval should be constant from beat to beat in sinus rhythm. If it varies, consider a wandering pacemaker or second-degree AV block.

Step 6: Measure QRS Duration and Amplitude

The QRS complex represents ventricular depolarization. It is the most visually prominent part of the tracing.

QRS duration is measured from the start of the Q wave (or the start of the R wave if no Q wave is present) to the end of the S wave. In dogs, normal QRS duration is generally less than 70 milliseconds (0.07 seconds). In cats, normal QRS duration is generally less than 60 milliseconds. In horses, normal QRS duration is generally less than 140 milliseconds.

QRS amplitude varies by lead and by species. In lead II, the R wave is typically the tallest positive deflection. In dogs, normal R-wave amplitude in lead II is generally less than 3.0 mV. In cats, normal R-wave amplitude in lead II is generally less than 1.0 mV. In horses, the QRS amplitude varies widely with lead placement and body size.

A study of Labrador retrievers noted deep Q waves in leads I, II, and aVF, and variation in QRS pattern [5]. This is a breed-specific finding and should not be misinterpreted as pathologic. A study of American Staffordshire Terriers found longer QRS duration compared to general population reference intervals [6]. Again, breed matters.

The QRS complex should be narrow and should not vary in duration from beat to beat. A wide QRS complex suggests ventricular origin or conduction disturbance.

Step 7: Measure the QT Interval and Estimate the Mean Electrical Axis

The QT interval is measured from the start of the QRS complex to the end of the T wave. It represents the total duration of ventricular depolarization and repolarization.

In dogs, normal QT interval varies with heart rate. A common correction formula is Bazett's formula:

QTc = QT / sqrt(RR interval in seconds)

In cats, a study of 20 healthy cats developed prediction equations for QT interval duration as a function of heart rate, with R² values of 0.81 for the full model and 0.71 for a simplified model [8]. The study found that heart rate, age group, and their interactions were significant predictors of QT interval duration. Sex, individual cat, and time of day were of little value in predicting QT interval duration [8].

In horses, normal QT interval is generally 400 to 600 milliseconds, again varying with heart rate.

The mean electrical axis (MEA) is the average direction of ventricular depolarization in the frontal plane. It is estimated using leads I and III, or leads I and aVF. The standard method is to find the lead with the tallest net QRS deflection, then find the lead perpendicular to it. The axis lies between these two leads.

In dogs, normal MEA is generally between 40 and 100 degrees. In cats, normal MEA is generally between 0 and 120 degrees. In horses, normal MEA is generally between -30 and 120 degrees, though wide variation exists.

A study of wild rooks found a mean electrical axis of -93 ± 2.2 degrees [9]. This is far outside the normal range for dogs and cats, which illustrates that MEA reference ranges are highly species-specific.

Normal Intervals and Amplitudes by Species

The following table summarizes commonly cited normal values for dogs, cats, and horses. These values are derived from standard veterinary cardiology references and from the studies cited in this article. Individual patients may fall slightly outside these ranges and still be normal, especially if they are athletic, young, or excited.

Parameter	Dog	Cat	Horse
Heart rate (bpm)	60 to 160 (adult, resting)	140 to 240	28 to 40 (resting)
P-wave duration (ms)	< 40	< 40	< 140
P-wave amplitude, lead II (mV)	< 0.4	< 0.2	Variable
PR interval (ms)	60 to 130	50 to 90	200 to 500
QRS duration (ms)	< 70	< 60	< 140
R-wave amplitude, lead II (mV)	< 3.0	< 1.0	Variable
QT interval (ms)	Rate-dependent	Rate-dependent	400 to 600
Mean electrical axis (degrees)	40 to 100	0 to 120	-30 to 120

These ranges are guides, not absolutes. A study of 239 dogs found that heart rate did not vary significantly among breeds, sex, or age groups, but the highest heart rate was reported in Dobermans and the lowest in Beagles [3]. A separate study found no significant correlation between heart rate and body weight in dogs [2]. This means you should not adjust your heart rate interpretation solely on the basis of breed or body size. Assess the patient.

Common Pitfalls in ECG Interpretation

Artifact from Movement

Movement artifact is the most common reason a normal ECG looks abnormal. The baseline swings wildly, and you may see deflections that resemble P waves or QRS complexes. The fix is to ensure the patient is comfortable and still. In dogs, right lateral recumbency is standard. In cats, right lateral recumbency is also standard, but some cats do better in sternal recumbency. In horses, standing recordings are standard, and a quiet environment is essential.

Poor Electrode Contact

Poor electrode contact causes a wandering baseline, low-amplitude complexes, or both. The fix is to clip hair if necessary, clean the skin with alcohol, apply fresh electrode gel, and secure the clips firmly without causing pain. In horses, electrode placement under the saddle area or on the chest may require careful preparation to avoid movement artifact.

Respiratory Sinus Arrhythmia in Dogs

Respiratory sinus arrhythmia is normal in dogs, but it can be mistaken for an arrhythmia by an inexperienced reader. The key features are that the P waves are present and constant in morphology, the PR interval is constant, and the R-R interval varies in a cyclic pattern with respiration. If you are unsure, ask the patient to hold still and record a longer strip. The cyclic pattern will become obvious.

Species and Body Size Differences

Normal values differ by species and by body size within a species. A study of American Staffordshire Terriers found longer P-wave and QRS durations compared to general population reference intervals [6]. A study of Labrador retrievers found deep Q waves in leads I, II, and aVF [5]. These breed-specific findings are normal for those breeds. If you apply general population reference intervals to every dog, you will over-diagnose abnormalities in some breeds and under-diagnose them in others.

Equipment Differences

Different electrocardiographs can produce different measurements. A study comparing two digital electrocardiographs found statistically significant differences in the majority of global ECG indices and lead-specific amplitudes [1]. The veterinary-adapted device measured lead II and III Q- and R-wave amplitudes 11% to 23% higher than the human-adapted device [1]. This means you should use reference intervals derived from equipment comparable to your own, and you should be cautious about comparing measurements across different machines.

Smartphone-Based ECG Limitations

Smartphone-based ECG devices are convenient, but they have limitations. A study comparing a smartphone-based single-lead recorder to a standard 6-lead ECG in dogs and cats found that heart rate values were within 1 beat of each other when mean heart rates were calculated [10]. However, there was frequent disagreement in the polarity of depolarization in cats [10]. This means smartphone ECGs can be useful for heart rate and rhythm assessment, but they may not be reliable for axis determination or for detecting subtle changes in QRS morphology in cats.

How to Check Your Interpretation Is Correct

After you complete the seven steps, perform a consistency check.

Verify the rate. Calculate the heart rate using two different methods. If the results disagree by more than 10%, recheck your measurements.
Verify the rhythm. Confirm that every P wave is followed by a QRS complex and every QRS complex is preceded by a P wave. Confirm that the PR interval is constant.
Verify the intervals. Re-measure the PR interval, QRS duration, and QT interval in at least three consecutive beats. If the values vary by more than 10%, consider whether the variation is physiologic or pathologic.
Verify the axis. Confirm that the mean electrical axis falls within the expected range for the species. If it does not, recheck your lead placement and your measurement of net QRS deflection in leads I and III.
Verify the calibration. Confirm that the calibration mark is 10 mm tall for 1 mV. If it is not, adjust your amplitude measurements accordingly.

If all five checks pass, your interpretation is internally consistent. If any check fails, repeat the measurement or repeat the recording.

Clinical Relevance, Limitations and Common Mistakes

A normal ECG is a powerful tool. It confirms that the cardiac electrical system is functioning within expected limits. It provides a baseline for future comparisons. It can rule out certain arrhythmias and conduction disturbances.

However, a normal ECG does not rule out structural heart disease. A dog with dilated cardiomyopathy may have a normal ECG in the early stages. A cat with hypertrophic cardiomyopathy may have a normal ECG until the disease is advanced. A horse with valvular disease may have a normal ECG at rest.

A normal ECG also does not rule out intermittent arrhythmias. A 30-second recording captures only a brief window of time. If the patient has paroxysmal arrhythmias, a Holter monitor or event recorder may be necessary. A study of 44 healthy puppies found that ventricular and supraventricular arrhythmias were rare, with a median of 0 [4]. A study of 44 healthy adult dogs found that ventricular ectopic complexes were rare when recordings with more than 100 ventricular ectopic complexes were excluded, with a median of 0 and a range of 0 to 17 [7]. These findings suggest that frequent arrhythmias in a resting ECG should prompt further investigation.

Common mistakes in ECG interpretation include:

Measuring at the wrong paper speed. Always confirm the paper speed before measuring.
Using the wrong calibration. Always confirm the calibration mark before measuring amplitudes.
Applying canine reference intervals to cats or horses. Species-specific reference intervals are essential.
Ignoring breed-specific variation. Some breeds have normal ECG features that differ from general population reference intervals.
Mistaking respiratory sinus arrhythmia for atrial fibrillation. The presence of consistent P waves and a constant PR interval rules out atrial fibrillation.
Overinterpreting artifact. A tracing full of movement artifact cannot be interpreted. Repeat the recording.
Assuming a normal ECG rules out heart disease. A normal ECG is one piece of the puzzle. Echocardiography, radiography, and clinical examination provide additional information.

This article is educational and is not a substitute for veterinary diagnosis or treatment. Individual cases require assessment by a veterinarian who can integrate the ECG with the patient's history, physical examination, and other diagnostic findings.

Frequently Asked Questions

What is a normal heart rate for a dog on an ECG?

A normal resting heart rate for an adult dog is generally 60 to 160 bpm. Puppies and excited dogs can have rates well above 200 bpm. Relaxed dogs can have rates below 60 bpm. A study of healthy puppies found a median maximum heart rate of 274 bpm [4].

What is a normal PR interval in a cat?

A normal PR interval in a cat is generally 50 to 90 milliseconds. This is shorter than the normal canine PR interval because the feline heart is smaller and the electrical impulse travels a shorter distance.

Can respiratory sinus arrhythmia be normal in dogs?

Yes. Respiratory sinus arrhythmia is a normal vagal phenomenon in dogs. The heart rate increases during inspiration and decreases during expiration. The P waves remain present and constant in morphology, and the PR interval remains constant.

How do I calculate heart rate from an ECG at 25 mm/s?

Count the number of large boxes between consecutive R waves and divide 300 by that number. Alternatively, count the number of small boxes between consecutive R waves and divide 1500 by that number. Both methods give the heart rate in beats per minute.

What is a normal QRS duration in a dog?

A normal QRS duration in a dog is generally less than 70 milliseconds. A study of American Staffordshire Terriers found longer QRS duration compared to general population reference intervals, so breed-specific variation should be considered [6].

What is a normal mean electrical axis in a dog?

A normal mean electrical axis in a dog is generally 40 to 100 degrees. Values outside this range may indicate ventricular enlargement or conduction disturbance, but breed-specific variation exists.

Why does my dog's ECG show a wandering baseline?

A wandering baseline is usually caused by poor electrode contact, movement artifact, or respiratory motion. Check that the electrodes are firmly attached, that the skin is clean and dry, and that the patient is comfortable and still.

Can a smartphone ECG replace a standard ECG?

A smartphone ECG can be useful for heart rate and rhythm assessment, but it has limitations. A study comparing smartphone and standard ECGs found frequent disagreement in the polarity of depolarization in cats [10]. A standard multi-lead ECG is preferred for complete interpretation.