ECG

The most important contractile element for the heart is —- while its opposing ion —- relaxes.

Ca2+

Mg2+

True or False: An ECG is the surface recording of average electrical activity of the heart, where contraction and electrical activity are measured.

False! ECG does NOT measure contraction, just electrical activity

True or False: Each portion of ECG arises from a specific anatomic area of the heart, where lead systems allow you to look at the heart from different angles.

True!

Define upward, downward, and isoelectric deflections.

Upward: impulse towards positive electrode

Downward: impulse towards negative electrode

Isoelectric: electric forces equal or there is no electrical activity

What are the P and QRS waves? What follow them?

P wave: atrial depolarization; atrial contraction follows

QRS wave: ventricular depolarization; ventricular contraction follows

What ECG segment tells us about the activity of the conduction system (AV node, bundle branches, Purkinje fibers)?

P-R segment

———— events always precede ———- events.

Electrical (depolarization)

Mechanical (contraction)

What does the T wave represent? When does it occur?

Ventricular repolarization

Just before the end of ventricular contraction

Why is the Q-T interval clinically important?

Drugs can prolong and cause V-fib

What is the heart muscle contracting in response to electrical stimulus called?

How does this occur?

Where?

Depolarization

Electrolytes traveling across cell membrane via Na/K pump

Inside myocardial cell negative relative to outside

Explain the action potential cascade beginning with the fast Na+ channels in a negatively charged cell and ending with the production of an action potential.

Fat Na+ channels open -> Na+ comes in -> Cell becomes more positive -> Slow/L-type Ca2+ channels open -> small amount of Ca2+ comes in -> SR and T tubules release Ca2+ -> Ca2+ causes actin and myosin to interact -> contraction -> K+ leaves the cells -> Cell becomes more negative -> Contraction ends and action potential occurs

True or False: Action potentials can excite adjacent cells and spread over the heart.

True!

What is the heart muscle relaxing as electrolytes move back across the cell membrane called?

Repolarization

Every QRS wave must have what?

An associated T wave

Describe the automaticity of the heart from fastest to slowest.

SA node -> AV node -> Purkinje fibers (fastest rate always wins)

True or False: Any cells of the conduction system can initiate their own impulses.

True!

In terms of automaticity, it is very important to identify where impulses originated. What are the TWO major rhythms outside of the normal system and how are they different?

Escape: protective; something above not working

Aberrant: irritable; rapid rhythm causes takeover (ventricular tachy)

What happens in terms of excitability of the heart when the electrical stimulus reduces?

Resting potential to threshold

What do we know about the refractoriness of cardiac muscle?

Cardiac muscle won't respond to a stimulus during contraction

During what period does it take a greater stimulus to achieve an action potential?

This is why the heart does not remain in systole (effectively stopped) during —-, where the skeletal muscles remain contracted.

Relative refractory period

Tetanus

What is meant by the conductivity of the heart?

Where is its velocity fastest? Slowest?

Activation of an individual muscle cell produces activity in neighboring cell

Fastest: Purkinje fibers

Slowest: AV node

Since the ECG can only measure the stimulus for contraction and not the contraction itself, what can be used to measure contraction?

Echocardiogram

Describe the standard ECG protocol in terms of patient position and paper speed.

Patient in right lateral recumbency

Run paper strip at 25mm/sec for 1 minute to get more complexes on paper to check for arrhythmias, then run at 50mm/sec to measure the complexes (spreads out; easier to interpret)

What is the standard sensitivity of an ECG?

1cm = 1mV

1 mV produces deflection on ECG of 1 cm (10 small boxes)

True or False: When sensitivity changes, it changes the complexes without any changes in the animal.

True!

How would the ECG change if set to 1/2 cm? 2cm?

1/2: complexes will be very small

2: complexes will be very large

How many leads are there in a standard ECG? Which ones are bipolar? Which are unipolar?

6

Bipolar: I, II, III

Unipolar (positive pole only): aVR (right arm), aVL (left arm), aVF (left foot)

The R wave should always be (positive/negative) in lead I.

What does it indicate if it is the other and its not an issue with the leads themselves?

Positive

If negative, true cardiac abnormality

Positive and negative ends of the 6 leads define axes every —- degrees in the frontal plane. The more leads we add, the more detailed the picture of the heart.

30

Describe the TWO methods used to calculate heart rate based upon an ECG.

Count number of beats (R-R intervals) between 2 sets of marks on ECG for 3 seconds at 50mm/sec and multiply by 20

Lay pen on ECG and count number of beats taken up by pen and multiply by 20

What does the normal mean electrical axis (MEA) point to?

What is the normal MEA for dogs and cats?

Left ventricle

Dogs: 40-100

Cats: 0-160

True or False: Because MEA is towards an area of block, it takes longer to depolarize that area.

True!

Describe how to calculate MEA.

Select isoelectric lead -> Find lead on diagram -> Follow to 90 degree perpendicular lead -> Upright complexes = positive side and negative complexes = negative side

What are the left axis shift ranges in dogs? Cats? What are the TWO major causes of this rare shift?

Dog: 40 to -90

Cat: 0 to -90

LBBB and LAFB (cat)

What would you expect to see on ECG with LBBB and LAFB?

LBBB: prolonged QRS

LAFB: marked left axis deviation

What usually causes the LAFB abnormality in cats, giving them a marked left axis deviation?

Describe how it looks on ECG.

HCM

Deep S waves lead II shift MEA to -90 (left axis shift)

What is the most common ECG abnormality in cats?

Deep S waves in lead II

What does an increased width/height of P wave indicate?

What about increased height of R wave?

What about increased width of R wave?

Atrial enlargement

LV enlargement

LBBB (left bundle branch block)

What does an increased S wave height indicate?

What does increased S wave width indicate?

What does increased T wave height indicate?

RV enlargement

RBBB

Hyperkalemia/myocardial hypoxia

Name the THREE major types of impulse formation arrhythmias.

Sinus (sinus bradycardia, sinus tachy)

Supraventricular (APCs, atrial tachy, a-fib, AVP, etc.)

Ventricular (VPCs, V-tach, V-fib, ventricular asystole)

Sinus arrest/block, sick sinus syndrome (SSS), atrial standstill, ventricular pre-excitation, AV block, and BBB are all examples of what kind of arrhythmia?

Abnormal impulse conduction

What site of origin arrhythmia category has positive P waves, constant P-R interval, and normal QRS duration?

What is important to know about the atrial ectopic beats here?

Atrial

Atrial ectopic beats have same features as normal SA node but occur at early or late times

What site of origin arrhythmia category has negative or absent P wave and normal QRS, where the impulse may be originating in the AV node?

What is the exception that has an abnormal QRS?

Is this slower or faster than atrial arrhythmias?

Junctional

Bundle branch block

Slower

What site of origin arrhythmia category has no P waves and QRS complexes that are wide and bizarre?

What does the rate look like if the rhythm is an escape or irritable?

Ventricular

Escape: slow rate

Irritable: fast rate (must be faster than SA node to take over as pacemaker)

What tool do you need for accurate rhythm assessment?

Calipers

What is the difference between regularly irregular and irregularly irregular rhythms?

Regularly irregular: pattern to irregularity

Irregularly irregular: random

Which arrhythmia is most classified as irregularly irregular?

A-fib (myocytes in atria beatings on own without coordination)

What indication should you get from a normal, abnormal, inverted, and absent P wave?

Normal: impulse from SA node

Abnormal: ectopic focus in atria

Inverted: impulse around AV junction

Absent: A-fib, standstill, or buried in QRS

What does a normal duration QRS indicate?

Where are these complexes formed in the heart?

Normal activation of ventricles

SA node or above ventricles (above bundle of His)

What does a wide QRS indicate?

Impulse below bundle of His (ventricular impulse formation) or intraventricular conduction (bundle branch block)

True or False: You should always assess the relationship of P wave to QRS and see if there is a P for every QRS and if there is a constant P-R interval.

True!

True or False: There is a QRS wave for every P wave, but not a P wave for every QRS.

False! There is a P wave for every QRS, but NOT a QRS wave for every P wave.

What does the P-R interval reflect?

How do you measure it?

AV node activation

Beginning of P wave to beginning of Q (or R if no Q)

What is the ST segment?

Time interval from end of QRS to onset T

What is the first major deflection following QRS wave that can be positive, negative, notched, or biphasic?

It is normally less than —-% of — wave height.

T wave

Less than 25% of R wave height

What interval is the summation of ventricular depolarization and repolarization that is important in people and congenital arrhythmias?

Is the ventricular in diastole or systole at this time?

QT interval

Systole