ECGs Flashcards
What is the pacemaker of the heart and why?
Most of the cells within the heart have the ability to
generate their own electrical activity, but the sinoatrial
node is the fastest to do so and is, therefore, the ‘rate
controller’ or pacemaker of the heart.
The rate of the sinoatrial node is influenced by the balance in autonomic tone, involving the sympathetic (which increases the rate) and parasympathetic (which decreases the rate) systems.
How does conduction in the heart occur?
The sinoatrial node normally initiates the electrical
discharge for each cardiac cycle. Depolarisation spreads through the atrial muscle cells. The depolarisation wave then spreads through the atrioventricular node, but it does so more slowly, thereby creating a time delay.
Conduction passes through the atrioventricular ring
(from the atria into the ventricles) through a narrow
pathway called the bundle of His. This then divides in
the ventricular septum into left and right bundle branches (going to the left and right ventricles). The left bundle branch divides further into anterior and posterior fascicles. The conduction tissue spreads into the myocardium as very fine branches called Purkinje fibres.
What is the T wave?
Repolarisation of the ventricles
Outline a normal sinus rhythm
In normal sinus rhythm, the stimulus originates regularly
at a constant rate from the sinoatrial node (dominant pacemaker), depolarising the atria and ventricles normally and producing a coordinated atrioventricular contraction.
The ECG shows a normal P wave followed by normal
QRS and T waves. The rhythm is regular (constant) and
the rate is normal for age, breed and species. The size
of the ECG complexes are typically small in cats and, therefore, obtaining an artefact-free tracing is important
in order to clearly identify the ECG complexes.
What is a wandering pacemaker?
slight variation in P wave amplitude, can be positive/ negative/ isoelectric and v hard to see
Occurs as a result of the dominant pacemaker shifting from the SA node to other pacemaker cells with a high intrinsic rate within the atria
What is sinus arrhythmia
In the case of sinus arrhythmia, the stimulus originates
from the sinoatrial node, but the rate varies (increases
and decreases) regularly. This is usually associated with the variation in autonomic tone which is often synchronous with respiration and is therefore sometimes called respiratory sinus arrhythmia.
CLINICAL FINDINGS
The heart rhythm varies with some regularity, increasing and decreasing in rate, and there is a pulse for every heartbeat.
ECG FEATURES
The ECG shows a normal P wave followed by normal
QRS and T waves. The rhythm varies in rate, often associated with respiration. The rhythm is sometimes described as being regularly irregular (ie, the variation in rate is fairly regular). The rate is normal for age, breed and species
What is sinus tachycardia?
In the case of sinus tachycardia, the sinoatrial node generates an impulse and depolarisation which occurs faster than normal.
Rhythm is still normal, as is ECG and PE findings (pulses may be weak if really fast)
What is sinus brady cardia?
In the case of sinus bradycardia, the sinoatrial node generates an impulse and depolarisation which occurs more slowly than normal. This can be a normal feature in some giant-breed dogs and in athletically fit animals.
How do you describe the number of ectopic beats in an ECG?
Premature ectopic complexes may occur singly, in pairs or in runs of three or more; the last is referred to as tachycardia.
Tachycardia may be continuous, in which case it is known as persistent or sustained, or may be intermittent, which is termed paroxysmal
How do you describe the timing of ectopic beats?
Ectopic complexes that occur before the next normal complex would have been due are termed premature; those that occur following a pause, such as a period of sinus arrest or in the case of complete heart block, are termed escape complexes
How do you describe the frequency of ectopic beats?
The number of premature ectopic complexes in a tracing may vary from occasional to very frequent. When there is a set ratio, such as one sinus complex to one ectopic complex, this is known as bigeminy; when there is one ectopic to two sinus complexes, this is termed trigeminy
How may the T wave appear after a VPC?
Often opposite to the QRS and large
What happens in a ventricular premature complex
From an ectopic focus/ foci within the ventricular myocardium
Conduction occurs cell to cell rather than through conduction tissue
What would you find on exam with VPCs?
N.B this would be the same for supraventricular premature complexes
May sound like a trip in the rhtyhm, may be silent with a gap in the rhtyhm, depends on when it occurs
Pulse deficit
If they are frequent, will sound like an irregular rhythm
If premature beats are very frequent, the heart will sound v chaotic, with a much slower pulse rate than heart rate
Sustained ventricular tachycardia will sound relatively normal, pulses palpable but reduced in strength, becoming weaker with faster heart rates
What are the ECG features of VPCs?
Wide and bizarre
T wave large and opposite
What would you call a short burst of ventricular premature complexes which are of different shapes?
multiform paroxysmal ventricular tachycardia
What is R-on-T phenomenom?
a ventricular premature complex occurs so early that it is superimposed on the T wave of the preceding ventricular premature complex
What is ventricular tachycardia?
run of three or more ventricular premature complexes
How do you divide supraventricular complexes?
Those which occur in the atrial muscle mass (atrial
ectopics); and
* Those which arise from within the atrioventricular
node or bundle of His (junctional or nodal ectopics).
How can you identify a supraventricular complex?
occurs as a premature beat, which means that it is
primarily recognised by its premature timing
Looks like a normal QRS as has had to go through the bundle of his to get to the ventricles
P wave morphology is often different
What are the ECG features of supraventricular tachycardia
QRS-T complexes, which have a normal morphology,
are seen to occur prematurely. The ECG features are:
Normal QRS morphology (except with bundle branch
block);
The QRS complex is seen to occur prematurely;
P waves may or may not be identified;
If P waves are seen, they are usually of an abnormal
morphology (ie, non-sinus) and the P-R interval may
differ from that seen in a normal sinus complex.
What would you find clinically with atrioventricular dissocaition?
The heart rhythm will sound fairly normal and the pulse should match the heart rate.
What are the ECG features of atrioventricular dissociation?
The ECG shows a ventricular rate that is usually very
slightly faster than the atrial rate. The P waves may
occur before, during or after the QRS complex. The
P waves and QRS complexes are independent of each
other, with the QRS complexes appearing to ‘catch up’
on the P waves. Atrioventricular dissociation should be
differentiated from complete heart block. In the case of
heart block, the ventricular rate is slow and much lower than the atrial rate; in atrioventricular dissociation, the atrial and ventricular rates are not dissimilar (and usually at a normal or faster rate).
What occurs in atrial fibrillation?
Fibrillation means rapid irregular small movements of
fibres. In atrial fibrillation, one of the most common
arrhythmias seen in small animals, depolarisation waves occur randomly throughout the atria. As atrial fibrillation originates above the ventricles, it can also be classified as a supraventricular arrhythmia.
