ECG Flashcards
Rhythm
the place where the action potential started
Sinus rhythm - physiology
action potential originates from the SA node, through the atria and to the ventricles via the AV node, bundle of His and Purkinje fibres
Sinus rhythm - ECG
- Normal rate (50-100bpm)
- Regular PQRST pattern
- Constant PR interval
- QRS complex duration < 100ms wide
- Normal P wave morphology
Sinus arrhythmia - Physiology
Heart rate varies due to reflex changes in vagal tone during the different stages of the respiratory cycle
Normal phenomenon, most common in young healthy people
Sinus arrhythmia - ECG
Variation in P-P interval is more than 120ms (3 small boxes) - changes in length normally correspond with the respiratory cycle
P wave has normal morphology and P-R interval is constant
Sinus arrhythmia - ECG
Variation in P-P interval is more than 120ms (3 small boxes) - changes in length correspond with the respiratory cycle
P wave has normal morphology and PR interval is constant
Sinus tachycardia - Physiology
Action potential originates from the SA node, through the atria and to the ventricles via the AV node, bundle of His and Purkinje fibres (happens too fast due to an increased rate of depolarisation in the SA node)
Sinus tachycardia - ECG
Sinus rhythm with a resting rate > 100bpm
P waves can be hidden within preceding T waves
Sinus tachycardia - causes
Exercise, pain, anxiety, sepsis, anaemia, pulmonary embolism (blocked vessel in lungs), cardiac tamponade (space around heart fills with fluid causing pressure on heart), hyperthyroidism, hypoxia (low oxygen levels), hypovolaemia (loss of fluid)
Pharmacological causes… Beta-agonists (adrenaline, salbutamol, isoprenaline), sympathomimetic, antihistamines, tricyclic antidepressants, caffeine.
Inappropriate sinus tachycardia = If patient is symptomatic and these things are ruled out
Sinus tachycardia - treatment
Treat underlying cause, may require beta-blockers or calcium channel blockers to lower heart rate
Can be due to re-entry phenomenon in the SA node (abrupt onset/termination with a rate of 130-140bpm) – vagal manoeuvres may stop it.
Sinus bradycardia - physiology
Action potential originates from the SA node, through the atria and to the ventricles via the AV node, bundle of His and Purkinje fibres (happens too slow)
Sinus bradycardia - ECG
Sinus rhythm with a rate of <60bpm
P-R interval is at least 0.12s
Common to see prominent U waves (a small deflection immediately following the T wave) in the precordial leads
Sinus bradycardia - causes
normal when sleeping and athletes with increased vagal tone
pathological causes - inferior MI, hyperthyroidism, anorexia, hyperkalemia, myocarditis, sick sinus syndrome, hypothermia
drugs - beta-blockers, calcium channel blockers, digoxin, amiodarone, GABA-ergic agents
Beta blockers =
used to slow down heart rate and reduce blood pressure
Propranolol, Bisoprolol
calcium-channel blockers =
used to reduce blood pressure
Verapamil, Diltiazem
Digoxin =
used to control arrythmias
Amiodarone =
anti-arrhythmic medication that slows conduction rate and prolongs refractory period of SA + AV nodes = used to treat VT, VF, AF
Sick sinus syndrome - physiology
dysfunction of the SA node with impairment of its ability to generate impulses (associated with myocardial ischaemia, digoxin toxicity, myocarditis and cardiac surgery) =
Sick sinus syndrome - ECG
Severe sinus bradycardia
Periods of sinoatrial block
Sinus arrest
Junctional or ventricular escape rhythms
Tachycardia-bradycardia syndrome
Paroxysmal atrial flutter and atrial fibrillation.
Tachycardia-bradycardia syndrome =
Common in sick sinus syndrome
Characterised by bursts of atrial tachycardia interspersed with periods of bradycardia
Paroxysmal atrial flutter or fibrillation may also occur
Cardioversion may be needed
Absolute bradycardia:
< 40 bpm
Sinus exit block - physiology
caused by failed propagation of pacemaker impulses beyond the SA node – the SA node depolarises normally but some impulses are blocked before leaving the SA node = intermitted failure of atrial depolarisation
Sinus exit block - ECG
Dropped P waves caused by SA node dysfunction
The pauses are the length of two or more P-P intervals.
Failure of SA pace-making cells causes…
sinus pause/sinus arrest
Failure of SA transitional cells (that transmit the impulse) causes…
SA exit block
Sinus arrest - physiology
Sinus arrest occurs when there is transient cessation of impulse formation at the sinoatrial node
Sinus arrest - ECG
prolonged pause without P wave activity (>3s)
the pause is unrelated to the length of the P-P cycle.
Escape rhythms =
the result of spontaneous activity from another pacemaker in the atria, atrioventricular junction or ventricles, which take over when normal impulse formation in the SA node fails
Escape beats/rhythms - physiology
Groups of pacemaker cells throughout the conducting system are capable of spontaneous depolarisation
The rate of depolarisation decreases from top to bottom: fastest at the sinoatrial node; slowest within the ventricles
Ectopic impulses from subsidiary pacemakers are normally suppressed by more rapid impulses from above
However, if an ectopic focus depolarises early enough (before the arrival of the next sinus impuls) it may produce a premature contraction
Premature contractions (“ectopics”) are classified by their origin — atrial, junctional or ventricular
The rate of spontaneous depolarisation of pacemaker cells decreases down the conducting system:
- SA node (60-100 bpm)
- Atria (< 60 bpm)
- AV node (40-60 bpm)
- Ventricles (20-40 bpm)
Atrial premature beats (atrial ectopics) - physiology
premature beat arising from ectopic pacemaking tissue within the atria
Atrial premature beats (atrial ectopics) - ECG
- Abnormal (non-sinus) P wave
- Normal QRS complex
- Pauses may be present after ectopic
Atrial premature beats (atrial ectopics) - symptoms and treatment
Ectopics are a normal electrophysiological phenomenon not usually requiring treatment
Frequent ectopics may cause palpitations
In patients with underlying predispositions (e.g. left atrial enlargement, ischaemic heart disease, WPW), ectopics may trigger the onset of a re-entry tachyarrhythmia — e.g. Atrial fibrillation, atrial flutter, AVNRT, AVRT
Supraventricular tachycardia =
any tachyarrhythmia arising from the atria or atrioventricular junction (above the Bundle of His)
SVT is classified based onsite of origin (atria or AV node) and regularity (irregular or regular):
Regular atria:
- Sinus tachycardia
- Atrial tachycardia
- Atrial flutter
- Inappropriate sinus tachycardia = elevated rate without a reason
- SA node re-entrant tachycardia
Irregular atria
- AF
- Multifocal atrial tachycardia = A rapid, irregular atrial rhythm arising from multiple ectopic foci within the atria.
Regular atrioventricular
- AV nodal re-entry tachycardia (AVNRT)
- AV re-entry tachycardia (AVRT)
- Junctional tachycardia = arrhythmias arising from the region of the atrioventricular junction as a result of a re-entry mechanism
Atrial flutter - physiology
A form of supraventricular narrow complex tachycardia
Caused by a single re-entry circuit in the right atrium with secondary activation in the left atrium = produces atrial contractions at a rate of 300bpm = looks like F waves
The AV node only lets electrical activity through at certain points = regular ventricle rhythm.
Atrial flutter - ECG
Loss of isoelectric baseline - Jagged pattern (Flutter waves = saw-toothed and best seen in inferior leads and V1)
Regular atrial (~300bpm) and ventricular rhythm (depends on AV conduction ratio - can be 2:1, 3:1, 4:1) – in contrast, AF will be completely irregular
Atrial fibrillation - physiology
P waves are not visible due to abnormal atrial depolarisation = irregular atrial rhythm (300-600bpm)
The AV node is unable to transmit beats as quickly as this, and thus does so intermittently, resulting in an irregular ventricular rhythm.
Atrial fibrillation - causes
o Idiopathic
o Heart failure
o MI
o Hypertension
o valve disease
o Congenital heart disease
Atrial fibrillation - management
- Beta-blockers
- Cardioversion
- Ablation
AF is a potential risk for stroke…
the atria are not contracting properly (because there is no depolarisation) so blood can pool in the atria and cause thrombus formation which can embolise and go to the brain = patients with AF need anti-coagulants to prevent this.
AV nodal (junctional) rhythm - physiology
If rhythm starts at AV node the action potential would spread upwards and downwards to activate the heart = heart would beat in rhythm with AV node.
Because the AV node is at the junction between the atria and ventricles its often described as a junctional rhythm
AV nodal (junctional) rhythm - ECG
rate of 40-60bpm
QRS morphology is typically narrow (< 120ms)
no relationship with P waves or inverted P waves