Name 4 indications for ECG analysis
- evaluation of arrhythmias + HR abnormalities
- history of syncope or weakness
- cardiac monitoring (GA/ICU)
- Electrolyte abnormalities
How does an ECG work?
The heart generates an electrical field during depolarization and repolarization of the myocardium that can be detected by a surface ECG. The ECG records the sum of all the elctrical impulses generated by the individual myocates during heart cardiac cycle. These impulses are then plotted over time.
What represents a positive deflection in ECG?
The sum of the heart’s electrical impulses was moving toward the positive electrode
What represents a negative deflection in ECG?
The sum of the heart’s electrical impulses was moving away from the positive electrode
What happens to impulses that are traveling perpendicular to the electrode?
Do not cause a deflection in the tracing
What are the standard leads used in veterinary medicine?
Biploar: lead I, II and III - record electrical activity between two limb electrodes
unipolar: augmented leads aVR, aVL, aVF - electrical activitiy between electrode and common terminal
In which lead should measurements be done?
lead II
Explain the leads I, II and III?
lead I: R arm to L arm
lead II: R arm to L hind
lead III: L arm to L hind
Explain the leads aVR, aVL and aVF
aVR: R arm to common terminal
aVL: L arm to common terminal
aVF: L hind to common terminal
Explain the ECG tracing
- P wave = atrial depolarization
- PQ interval = beginning of P wave to start of QRS = time it took to travel from SN to ventricular myocardium (via AV node)
- QRS = ventricular deplolarization
–> Q = interventricular septum depolarization (from L–>R)
–> S = basal ventricular depolarization - T wave = ventricular repolarization
- QT interval = start of QRS to end of T-wave –> time required for ventricular deploarization and reploarization
Discuss normal canine and felines lead II ECG values
What ECG tracing is often altered in electrolyte abnormalities?
QT interval
BUT also strongly affected by HR
How much RR variation can you usually have in a regular rhythm?
<10%
How can you determine the mean electrical axis (MEA) when using the isoelectric lead method?
- identify the iso-electric lead:
–> This is the lead where the QRS complex is neither strongly positive nor strongly negative (as if they cancel eachother out) - find the perpendicular leads
- Check the polarity of the perpendicular lead –> If aVF is positive (R wave is up) –> heart’s electrical signal is moving towards the positive pole of aVF –> MEA is +90°
What are the two methods for determining mean electrical axis (MEA)?
- isoelectric lead method
- pie method
What is the mean electrical axis?
overall average direction of all the tiny electrical signals that happen in sequence as the ventricles of the heart become electrically active during depolarization
It normally points towards the caudal half of the body in the dog
What is a normal MEA in a dog and a cat?
dog: +40 to +100
cat: 0 to +160
How can you determine the mean electrical axis (MEA) when using the pie method?
Think of the heart’s electrical field as a pie divided into 4 slices, based on the polarity (positive or negative) of those 2 leads.
–> Look at the QRS polarity in:
Lead I: - Lead aVF: -
Name 5 common causes of a right axis deviation?
–> Right ventricular hypertrophy
- Pulmonic stenosis
- Tetralogy of Fallot
- Pulmonary hypertension
- Tricuspid valve dysplasia
- RBBB (cardiomyopathy, myocarditis, hyperkalemia, neoplasia)
Name 5 common causes of a left axis deviation?
- Partial left BBB
- left anterior fascicular block
- Cardiomyopathy
- Hyperthyroidism
- Hyperkalemia
What makes Magnesium unique regarding its confirmation?
has a double shell of hydrating water molecules that require a large amount of free energy to be shed –> Magnesium must shed this shell before entering divalent cation channel
–> acts as endogenous calcium channel blocker
Which electrolyte can act as an endogenous calcium channel blocker and why?
Magenesium:
has a double shell of hydrating water molecules that require a large amount of free energy to be shed –> Magnesium must shed this shell before entering divalent cation channel
How does progressive hyperkalemia affect the ECG tracing?
What electrophysiological changes occur in hyperkalemia?
Decreases resting membrane potential and moves it closer to threshhold potential –> cell is easier to excite
BUT: it also reduces the number of Na+ channels available for activation –> less excitable
