ECG - electrocardiogram

How an ECG is recorded

• 1 electrode on each limb (limb leads)

  • (chest leads are human)

• Machine compares potential difference between 4 points on skin → derives ECG

• 3 main electrodes attached to patients skin:

  • Right forelimb

  • Left forelimb

  • LEFT HINDLIMB

• Only require 1 lead to interpret heart rhythm

Lead 1 trace

Right forelimb → left forelimb

Lead 2 trace (MOST IMPORTANT)

Right forelimb → left hindlimb

• Axis of potential difference overlies heart

• Deflections and deviations of heart most obvious

Lead 3 trace

Left forelimb → left hindlimb

Different components of ECG

• P wave → atrial depol

• PQ segment (‘PR’) → length of atrioventricular conduction

  • Straight line

  • [AV node only structure A → V]

• PR interval → time from atrial depol → ventricular depol

• QRS → ventricular depol

• ST segment → ventricles completely depolarised

  • Straight line

• QT interval

  • Length of ventricular depol and repol

• T wave

  • Ventricular repol

• RR interval → measure of cardiac cycle length

Electrical signal origination on ECG

• Potential differences generated → waves of depol spread across myocardium

• PD across cell membrane detected

• Depolarisation → intracellular more +ve, outside more -ve

  • Detecting -ve to +ve extracellular charge from depol to repol } depolarising wave

    • Excited myocyte → -ve extracellular charge

    • Repol/resting myocyte → +ve extracellular charge (Na+/Ca2+ cannot enter cell → vg channels closed)

• Big QRS complex → difference between depol and repol sections of ventricular myocardium

Order of cardiac conduction

• All detected by +ve pole of lead 2 at heart apex

1. SAN spontaneously depolarises

2. Depol spreads across atrial muscle

3. Slow conduction through AVN

4. Rapid conduction through bundle of His

5. Rapid conduction through Purkinje fibres

6. Complete depolarisation of ventricles

7. Ventricular contraction after depol

8. Repolarisation

Cardiac conduction relating to +ve pole

• Atrial depolarisation

• P wave (+ve deflection) → depol spreads towards +ve pole of lead 2

• PR segment → no signal → AV conduction delay → no muscle depol

Cardiac conduction relating to +ve pole

• Ventricular depolarisation

• Septum depolarises → depolarises AWAY from +ve pole

  • Small -ve deflection → Q wave

• Large depolarisation of left ventricle myocardium

  • Large +ve deflection → R wave

• Depolarisation of top of ventricular muscle wall (ventricular contraction bottom → top)

  • Small -ve deflection → S wave

  • Sometimes large R wave obscures S wave → still called QRS complex

Cardiac conduction relating to +ve pole

• Repolarisation

• Intrinsic function of each cell → NOT A WAVE

• Impulses cancel each other out → gives rise to T wave

• Lead arrangement + species differences → inverted T waves

  • Not pathology in dog or cat

Characteristics of normal ECG

• P wave preceeds every QRS

• Fixed PR segment + interval

• Consistent QRS → no ectopic/abnormal depolarisation

• Normal HR (RR interval)

Normal species heart rates

• Dog → 70-120

• Cat → 120-140

• Horse → 28-40

Possible explanation for hidden P waves

• Beats overlap → covered by QRS

  • SAN fires before ventricles finish depolarisation

• Atrial contraction overriden

• Atrial contraction hidden in ST segment

Info obtained from ECG (top 3)

• Rate

• Rhythm

• Nature of cardiac depol/repol

Info obtained from ECG (other tests better at confirming)

• Change in myocardial mass

  • May indicate ventricular hypertrophy

  • Not very successful in mammals

  • Use ultrasound instead

• Indication of metabolic abnormalities may manifest

  • Due to electrolyte abnormalities

  • Measure blood electrolytes instead

• Indications of alterations in conductivity

  • Pericardial effusion → ultrasound

Diagnosis of abnormal rhythm → lost P wave

• No corresponding P for every QRS → disorganised atrial depolarisation

  • Atrial fibrillation

    • Enlarged atria + multiple waves of depolarisation simultaneously spreading in a disorganised fashion

    • F waves → rapid and irregular contraction, R-R waves not constant (coarse oscillation)

• No atrial activity → atrial standstill

  • Myocardial disease or electrolyte disturbances

  • Life threatening

  • No P waves

  • Regular, relatively normal but infrequent QRS

Diagnosis of abnormal rhythm - QRS

(signals not reliant on atria)

• Ectopic/irregular premature complexes

  • Premature = early QRS before SAN fires

  • Typically quite wide → takes longer for signal to spread

  • Can spread left → right (-ve deflection)

• Ventricular → from Purkinje fibres

  • Pattern of depolarisation different

  • Signal can spread left → right (-ve deflection)

• Junctional → from AVN

  • QRS complex looks normal (+ve deflection)

  • Known as supraventricular complex

    • Narrow upright complexes in appropriate leads unless concurrent hypertrophy or conduction disturbances

• Escape → ectopic heartbeat due to sinus arrest

  • No SAN impulse

  • Junctional or ventricular escape

Abnormal QRS complexes:

• Ventricular/junctional prematures

• Ventricular/junctional escapes

AV heart block (atrioventricular block)

No P wave for every QRS

Issue with AV conduction

1. First degree heart block → AV conduction delayed

   • QRS still present

   • lengthened PR interval

2. Second degree heart block → intermittent failure of AV conduction

   • QRS sometimes not present

   • Pathological when P waves consistently blocked → slow heart rate

3. Third degree heart block → complete failure of AV conduction

   • Complete AV dissociation

   • Ventricles resort to spontaneous depolarisation typically at low rate (low escape rate)

   • Enough to keep animal alive

   • No relationship between P waves and QRS complexes

     • Many frequent P waves but infrequent QRS complexes → atria depolarise but ventricles do not synchronise

     • Ventricles depolarise spontaneously

     • Escape frequencies

When is second degree heart block normal in horses and dogs?

• Sleeping dogs

• Heart slowing down in horses

When is rhythm disturbance significant?

• Clinical signs are present

• Rhythm disturbances → secondary effect from underlying problem

• Treat root cause

Supraventricular complex - P wave association

Atrial origin:

• There is a P wave, usually abnormal in shape (compared to sinus P wave).

• The P wave precedes the QRS but may be hidden in the preceding T wave.

2. Junctional origin (e.g., junctional premature beats):

• P wave may be:

  • Absent (if depolarization is retrograde and slow).

  • Inverted and after the QRS, or

  • Before the QRS but inverted.

• Still technically a P wave association, just not always visible or typical.

Complexes of ventricular origin

• Wide

• Bizarre

• Atrial ventricular dissociation (heartblock) → no associated P waves

P wave variation

• Normal in dog (wandering pacemakers)

• May indicate atrial ectopy

• P wave should be positive

QRS variation

• Can be normal

• Indicates variable conduction routes

• Can indicate electrical alternans → ectopic heartbeats

What to look for in an ECG

• P for every QRS

• QRS for every P

• P and QRS present all the time

• Consistently and reasonably related P and QRS

• Regularly irregular rhythm

  • Normal waves, irregular R-R

• Irregularly irregular rhythm

  • Irregular waves and R-R

Causes in cardiac rhythm alterations

• Intrinsic cardiac disease

• Hypoxia

• Autonomic influence

• Mechanical abnormalities

• Metabolic abnormalities

• Electrolyte disturbances

• Drugs