Welcome to Veterinary ECG Learning

A self-directed module covering ECG interpretation in small animals.
Work through each module in order — lessons first, then graded practice.

📋 Learning Outcomes

  • Identify rate, rhythm regularity, and P–QRS relationship on any ECG
  • Distinguish sinus rhythm, sinus arrhythmia, atrial fibrillation, SVT, VPCs, and 2nd/3rd degree AV block
  • Recognise life-threatening patterns requiring urgent action (sustained VT, high-grade AV block)
Species focus: Primarily canine, with feline values provided for comparison throughout. Reference values differ significantly — always note the species before interpreting.

📏 Module 1 — How to Read Any ECG

Paper speed, gain, heart rate calculation, rhythm, and interval measurements. Interactive simulated ECGs.

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🔬 Module 2 — Waveform ID & Conduction

Identify P waves, QRS complexes and T waves. Understand bundle branch blocks, supraventricular vs ventricular origin, and measure intervals.

6 levels

💡 Module 3 — Supraventricular vs Ventricular

Use QRS width and P wave presence to distinguish supraventricular from ventricular rhythms. Includes RBBB, LBBB and practice ECGs.

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🔍 Module 4 — Normal vs Abnormal

Classify rhythms as sinus vs non-sinus. Identify abnormal beats. Is it normal or not?

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🧬 Module 5 — Abnormal Beats

QRS width, P wave presence, PR relationship. Characterise individual abnormal beats.

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🌿 Module 6 — Rhythm Classification

Tachycardia vs bradycardia → SVT/VT → sinus/AV block/atrial standstill.

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🩺 Module 7 — Case-Based Practice

8 clinical cases with signalment, history, ECG, and 4 questions each. The main event.

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1

How to Read Any ECG

Paper speed · Heart rate · Rhythm · P waves · Intervals · Amplitudes

0/20Practice score
📚 Lesson
📝 Practice

Step 1 · Paper Speed & Gain

Before measuring anything, confirm paper speed (mm/s) and gain (cm/mV). These determine what every square represents.

Paper speed

SpeedSmall square (1mm)Large square (5mm)
50 mm/s ★ vet0.02 s0.10 s
25 mm/s0.04 s0.20 s

Gain (amplitude)

Gain1 cm (10mm)
1 cm/mV ★ standard= 1 mV
2 cm/mV (double/high gain — for small signals)1 cm box = 0.5 mV
0.5 cm/mV (half gain — for tall signals)1 cm box = 2 mV
⚠ Common trap: Using the wrong paper speed formula gives completely wrong heart rates. Always check the speed annotation on the strip before calculating.

Visual comparison — same dog, same heart rate, two paper speeds

50 mm/s (standard veterinary)

Activate caliper to measure

Beats spread out — easier to measure PR, QRS, P wave width

25 mm/s (compressed)

Activate caliper to measure

More beats visible — useful for rhythm assessment over time

Step 2 · Calculate Heart Rate

Method 1 — Large square counting (fast)

Count large squares between two adjacent R peaks, then use:

At 50 mm/s: HR = 600 ÷ (large squares)
At 25 mm/s: HR = 300 ÷ (large squares)

Example at 50mm/s: RR spans 6 large squares → 600÷6 = 100 bpm

Method 2 — Small box counting (precise)

Count small boxes in the RR interval:

At 50 mm/s: HR = 3000 ÷ (small boxes)
At 25 mm/s: HR = 1500 ÷ (small boxes)

Best for: borderline rates, irregular rhythms, bradycardia where large squares don't align to R peaks cleanly.

🐕 Normal canine HR: 60–180 bpm at rest (giant breeds 40–80 bpm; toy breeds up to 180 bpm)
🐈 Normal feline HR: 140–240 bpm (in-hospital; often higher due to stress)

🎯 Try It — Interactive HR Measurement

The green ▼ markers show R peaks. Click on any R peak, then click on the next R peak to measure the RR interval and calculate heart rate with a step-by-step explanation.

Paper speed: ECG 1 of 4
Click on an R peak (tall spike) to place first arm Click first R peak → click next R peak → step-by-step HR calculation appears below

Step 3 · Rhythm Regularity

Place caliper tips (or mark with paper) on consecutive R peaks. Step across the strip. Ask:

  • Regular — all R-R intervals equal (or within 10%)
  • Regularly irregular — R-R varies in a repeating pattern (e.g., sinus arrhythmia, 2nd degree AV block)
  • Irregularly irregular — no pattern to the variation (e.g., atrial fibrillation)
🐕 Dog-specific: Sinus arrhythmia (regular irregularity linked to breathing) is completely normal in dogs and is actually a sign of good vagal tone. Do not confuse it with a pathological arrhythmia.
→ Module 2: Waveform ID covers wave identification (P, QRS, T), normal intervals, and interactive practice marking waves on real ECG strips. Continue there once you're comfortable with HR and rhythm.
3

Supraventricular vs Ventricular

QRS width · P waves · Bundle branch blocks · Heart enlargement

0/6Practice score
📚 Lesson
📝 Practice

The Fundamental Question: Where Did the Beat Come From?

Every abnormal beat or rhythm can be traced back to one question: did the impulse originate above or below the bundle of His? This determines everything — QRS shape, width, and the appropriate treatment.

⬆️ Supraventricular Origin

Impulse arises in the SA node, atria, or AV node — above the ventricles. Conduction proceeds normally through the His-Purkinje system.

  • QRS is narrow (<0.06 s in dogs; <3 small boxes)
  • Normal QRS morphology
  • P wave usually present (may be hidden in fast rates)
  • Examples: sinus rhythm, SVT, AF, APCs
⬇️ Ventricular Origin

Impulse arises in the ventricular myocardium or Purkinje fibres — below the bundle of His. Slow cell-to-cell spread produces a wide, bizarre QRS.

  • QRS is wide (>0.06 s; >3 small boxes)
  • Bizarre, often notched QRS morphology
  • No preceding P wave (or AV dissociation)
  • Examples: VPCs, VT, ventricular escape
🔑 The key measurement: QRS duration.
• Narrow QRS (<0.06 s / <3 small boxes at 50 mm/s) = supraventricular
• Wide QRS (>0.06 s / >3 small boxes) = ventricular OR bundle branch block (see below)

The Wide QRS Problem: Not Always Ventricular

A wide QRS does not automatically mean ventricular origin. When one bundle branch is blocked, even a supraventricular impulse produces a wide QRS — this is called a bundle branch block (BBB). The key to telling them apart is the P wave.

Wide QRS — decision tree:
├── Is there a P wave before every QRS?
│    ├── Yes → Bundle Branch Block (supraventricular + conduction delay)
│    └── No (P absent or dissociated) → Ventricular origin (VPC, VT, escape)
└── Is the QRS narrow?
      └── Yes → Definitely supraventricular

Bundle Branch Blocks

A bundle branch block occurs when one of the two main branches of the His-Purkinje system fails to conduct. The affected ventricle depolarises late, via slow myocardial spread from the healthy side — producing a wide, abnormal QRS that is still supraventricular in origin.

Right Bundle Branch Block (RBBB)
Pt's L Pt's R AV LBB ✓ RBB ✗ LV 1st ✓ RV slow ✗
ECG findings (Lead II):
• Wide QRS (>70 ms / >3.5 boxes)
• Broad terminal S wave in Lead II (rS pattern — net biphasic or negative deflection)
• Negative/broad S wave also visible in Lead I
• P wave before every QRS (sinus origin)
• Common with right ventricular enlargement / pressure overload
Left Bundle Branch Block (LBBB)
Pt's R Pt's L (viewer L) (viewer R) AV RBB ✓ LBB ✗ RV 1st ✓ LV slow ✗
ECG findings (Lead II):
• Wide QRS (>70 ms / >3.5 boxes)
• Predominantly POSITIVE in Lead II
• P wave before every QRS (sinus origin)
• Common with left ventricular enlargement / cardiomegaly
⚠️ Critical clinical point: A dog with dilated cardiomyopathy (e.g. Dobermann) may develop LBBB. The ECG shows wide, positive QRS complexes in Lead II — superficially identical to VT. Always check for P waves. If every wide QRS is preceded by a P wave at a consistent PR interval, it is BBB, not VT. Treating BBB as VT is potentially dangerous.

Heart Enlargement and Bundle Branch Blocks

BBB in veterinary patients is frequently caused by underlying structural heart disease rather than idiopathic conduction system degeneration. The anatomical relationship matters clinically:

RBBB — Right Bundle Branch Block
  • Right bundle branch blocked
  • Wide, predominantly negative QRS in Lead II
  • Associated with right ventricular enlargement
  • Clinical contexts: pulmonic stenosis, pulmonary hypertension, right-sided heart failure, tricuspid dysplasia
  • Also seen as incidental finding or post-trauma
LBBB — Left Bundle Branch Block
  • Left bundle branch blocked
  • Wide, predominantly positive QRS in Lead II
  • Associated with left ventricular enlargement / cardiomegaly
  • Clinical contexts: dilated cardiomyopathy (especially Dobermann), severe mitral valve disease with LV dilation
  • Most important mimic of VT in the dog
Clinical pearl: In a dog presenting with a wide-complex tachycardia, finding regular P waves at a normal rate preceding each wide QRS is strong evidence of LBBB with sinus tachycardia — not VT. This distinction changes management completely.

Summary Table

Feature Supraventricular BBB (SV + block) Ventricular
QRS width Narrow (<0.06 s) Wide (>0.06 s) Wide (>0.06 s)
P wave Usually present before QRS Present before every QRS Absent or dissociated
PR interval Normal, consistent Normal, consistent Not measurable
Rate range Wide range Wide range Often fast (VT) or slow (escape)
Associated pathology Atrial/AV node disease RV or LV enlargement Ventricular myocardial disease
4

Normal vs Abnormal

Classify the overall rhythm · Identify abnormal beats

0/12Practice score
📚 Lesson
📝 Practice

What is Sinus Rhythm?

Sinus rhythm means every heartbeat is initiated by the sinoatrial (SA) node — the heart's natural pacemaker. For a rhythm to be sinus, all of the following must be true:

  • Every QRS is preceded by a P wave
  • All P waves have the same morphology (shape)
  • The PR interval is consistent
  • QRS complexes are narrow (≤0.06s in dogs)
Sinus rhythm includes:
• Normal sinus rhythm — regular, HR within normal range for species/breed
• Sinus tachycardia — sinus rhythm, HR elevated (stress, pain, fever, anaemia)
• Sinus bradycardia — sinus rhythm, HR low (athletic, vagal, hypothyroidism, drug effect)
• Sinus arrhythmia — sinus rhythm, HR varies with breathing (normal dog finding)

When is it NOT Sinus?

A rhythm is non-sinus if any of the following are present:

  • Absent P waves — atrial fibrillation, junctional/ventricular escape, atrial standstill
  • P waves present but not conducting — AV block (P waves not followed by QRS)
  • Wide QRS complexes — ventricular origin (VPC, VT) or bundle branch block
  • Extra beats — premature beats (atrial or ventricular) interrupt the sinus pattern
  • Irregularly irregular rhythm with no P waves — atrial fibrillation
Dog trap: Sinus arrhythmia can look irregular and alarming. Before calling something abnormal, confirm that every QRS still has a preceding P wave with identical morphology. If yes → sinus arrhythmia, not AF.

A Simple Two-Step Approach

Step 1: Is there a P wave before every QRS?
Step 2: Do all QRS complexes look the same (narrow and uniform)?

If yes to both → likely sinus. If no to either → look further.

5

Abnormal Beats

QRS width · P waves · PR relationship · Origin of ectopy

0/15Practice score
📚 Lesson
📝 Practice

The Two Most Important Questions About Any Abnormal Beat

1. Is the QRS narrow or wide?

Narrow QRS (≤0.06s in dogs):
Impulse travels through the normal His-Purkinje system. Origin is supraventricular (SA node, atria, or AV junction).

Wide QRS (>0.06s in dogs):
Slow cell-to-cell conduction in ventricular muscle. Origin is ventricular — OR a supraventricular impulse with aberrant conduction (bundle branch block).

2. Is there a P wave, and is it related to the QRS?

P before QRS, constant PR: Sinus or atrial origin

No P wave before QRS: Junctional or ventricular origin, or P buried in preceding T wave

P after QRS (retrograde): Junctional (AV nodal) origin

P present but NOT followed by QRS: AV block

Common Abnormal Beat Types

Beat typeQRS widthP wavePrematurityCompensatory pause
VPC (ventricular premature complex)Wide, bizarreAbsent (or dissociated)YesFull compensatory pause
APC (atrial premature complex)NarrowPresent (different P morphology)YesIncomplete (no full pause)
Junctional beatNarrowAbsent or retrogradeUsually escapeN/A (escape)
Ventricular escapeWideDissociatedLate (escape)N/A (escape)
Premature vs escape — critical distinction:
Premature beats come early (before expected next sinus beat). They are ectopic and usually indicate irritability.
Escape beats come late (after a pause where sinus failed to fire). They are protective — the ventricle is rescuing the heart. Never suppress escape beats without first treating the cause of the underlying bradycardia.

T Wave Changes

In dogs, T wave polarity in Lead II is variable — it may be positive, negative, or biphasic, and any of these is normal in a healthy dog. Do not flag a negative T wave as pathological in dogs based on polarity alone; consider it in clinical context. After a ventricular beat (VPC), the T wave is typically opposite in polarity to the QRS — this is a secondary T wave change and is expected.

6

Rhythm Classification

Tachycardia · Bradycardia · SVT · VT · AV Block · Atrial Standstill

0/15Practice score
📚 Lesson
📝 Practice

The Classification Tree

Start with rate, then narrow it down:

Abnormal rhythm
↙ Fast                    ↘ Slow
Tachyarrhythmia
Bradyarrhythmia
Narrow QRS tachycardia
→ Sinus tachycardia (appropriate)
→ Atrial fibrillation
→ SVT (AV nodal re-entry, etc.)
→ Atrial tachycardia

Wide QRS tachycardia
→ Ventricular tachycardia (VT) ⚠
→ SVT + bundle branch block (rare)
Regular bradycardia
→ Sinus bradycardia
→ 2nd degree AV block
→ 3rd degree (complete) AV block ⚠
→ Sinus node dysfunction

Regular bradycardia + no P waves
→ Atrial standstill ⚠ (hyperkalemia?)

Key Rhythms — Tachycardias

RhythmRateQRSP wavesRegularity
Sinus tachycardia↑ but variableNarrowPresent, normalSlightly variable
Atrial fibrillation100–250+NarrowAbsent (f waves)Irregularly irregular
SVT200–300NarrowBuried/retrogradeRegular
Ventricular tachycardia150–400Wide, bizarreDissociated (AV dissociation)Regular
⚠ Sustained VT is a life-threatening emergency. Wide complex tachycardia at high rate = assume VT until proven otherwise.

Key Rhythms — Bradycardias

RhythmRateP wavesPR intervalQRS
Sinus bradycardia<60 (dog)Present, normalNormal, consistentNarrow
2nd degree AV block
Mobitz I (Wenckebach)
Mobitz II (more serious)		Slow–normalPresentMobitz I: Progressive PR lengthening → dropped QRS
Mobitz II: Constant PR → sudden dropped QRS (infra-Hisian; higher risk of progression to 3rd degree)		Narrow
3rd degree AV blockVery slow (20–60)Present, regular, but no relationship to QRSNo consistent PRWide (ventricular escape)
Atrial standstillVery slowAbsent (flat baseline)NoneWide (ventricular escape)
⚠ Atrial standstill alert: Think hyperkalemia (urinary obstruction, Addison's disease, renal failure, massive muscle trauma). Treat the potassium, not just the bradycardia.
3rd degree AV block vs atrial standstill:
Both produce slow wide escape rhythms. The key difference: in 3rd degree AV block, P waves are visible (just dissociated from QRS). In atrial standstill, the baseline is completely flat — no P waves at all.
7

Case-Based Practice

8 clinical cases — signalment, history, ECG, and questions

0/26Practice score
📊

My Progress

Overall performance across all modules

Overall Score
0/97
2

Waveform ID & Conduction

Identify waves · Measure intervals · Understand BBB vs ventricular origin

📚 Lesson
🎯 Practice

Step 4 · Identify Waves & Measure Intervals

P Wave
Represents atrial depolarization — the electrical signal that spreads across the atria and triggers atrial contraction. Normally small and rounded, it precedes each QRS complex.
QRS Complex
Represents ventricular depolarization — rapid electrical activation of both ventricles, triggering ventricular contraction (systole). It is the tallest, narrowest complex on the ECG.
T Wave
Represents ventricular repolarization — electrical recovery of the ventricles, resetting them for the next beat. Broader and lower-amplitude than the QRS complex.

The interactive ECG below is labelled. Hover over each marker to see normal values.

Activate caliper to measure Click once → place first arm  |  Click again → measure distance  |  Click a third time → start new measurement
P wave: ≤0.04s | ≤0.4mV PR interval: 0.06–0.13s QRS: ≤0.06s (large breeds ≤0.07s) QT: 0.15–0.25s R wave: varies (up to 3mV lead II, large dogs)

Key intervals — dog

MeasurementNormal range
P wave duration≤ 0.04 s
P wave amplitude≤ 0.4 mV
PR interval0.06–0.13 s
QRS duration≤ 0.06 s (≤0.07 large)
R amplitude (lead II)≤ 2.5 mV (small/med) · ≤ 3.0 mV (large breeds)
ST segmentIsoelectric (±0.2 mV)

Key intervals — cat (comparison)

MeasurementNormal range
P wave duration≤ 0.04 s
P wave amplitude≤ 0.2 mV
PR interval0.05–0.09 s
QRS duration≤ 0.04 s
R amplitude (lead II)≤ 0.9 mV
T wavePositive, ≤ 0.3 mV

🔎 Waveform Abnormalities — Clinical Reference

Recognising abnormal waveforms is the foundation of ECG interpretation. The table below links ECG findings to their physiological cause.

P Wave Abnormalities

Wide/notched P (>0.04 s)
→ Left atrial enlargement
= P mitrale — classic in mitral valve disease (CKCS, Cavaliers)
Tall P (>0.4 mV dogs, >0.2 mV cats)
→ Right atrial enlargement
= P pulmonale — pulmonic stenosis, pulmonary hypertension
Absent P waves (flat baseline)
→ Atrial standstill
Hyperkalemia (Addison's, urinary obstruction), atrial myopathy (English Springer Spaniel)
Absent P + irregular baseline
→ Atrial fibrillation
Fibrillatory f-waves replace P waves; irregularly irregular R-R intervals
Negative/biphasic P (Lead II)
→ Ectopic atrial focus or retrograde conduction
P axis shifted; impulse not originating from SA node

QRS & T Wave Abnormalities

Wide QRS (>0.06 s dogs / >0.04 s cats)
→ BBB or ventricular origin
Always check: is there a P before every QRS? Yes=BBB, No=ventricular
Tall R wave (>3.0 mV large dogs, >0.9 mV cats)
→ Left ventricular enlargement
DCM, significant mitral regurgitation, hypertrophic CM
Tall peaked T wave (>25% of QRS)
→ Hyperkalemia (classic)
Addison's disease, urethral obstruction, renal failure, rhabdomyolysis
Negative T wave (Lead II)
→ Variable / myocardial hypoxia
Normal variant in dogs (T wave polarity is physiologically variable). Clinically significant if: previously positive → now negative, or associated with severe anaemia, cardiac tamponade, or severe LV enlargement
ST elevation or depression
→ Myocardial injury or ischaemia
Elevation: pericarditis, acute MI-equivalent. Depression: ischaemia, digitalis effect
🔑 Quick recognition rule:  Always assess the ECG systematically: Rate → Rhythm (regular?) → P waves present? → P:QRS ratio → QRS width → intervals. A single abnormal finding should prompt full systematic assessment — never stop after spotting one problem.

📋 Quick Reference Values

ParameterDogCat
Heart rate (bpm)60–180 (giant: 40–80)140–240
P wave duration≤ 0.04 s≤ 0.04 s
P wave amplitude≤ 0.4 mV≤ 0.2 mV
PR interval0.06–0.13 s0.05–0.09 s
QRS duration≤ 0.06 s (large ≤0.07)≤ 0.04 s
R amplitude lead II≤ 3.0 mV≤ 0.9 mV
QT interval0.15–0.25 s0.12–0.18 s
T wavePos/neg/biphasic (variable)Positive, ≤0.3 mV

Heart rate formula (at paper speed)

Paper speedLarge square methodSmall square method
50 mm/s ★HR = 600 ÷ large squaresHR = 3000 ÷ small squares
25 mm/sHR = 300 ÷ large squaresHR = 1500 ÷ small squares