introduction to the ECG Flashcards
what is syncytium?
one large cell having many nuclei that are not separated by cell membrane
what is a function syncytium?
many cells functioning as one
what are the 3 types of cardiac monocyte & their functions?
- pacemaker cells = for setting heart rhythms
- conducting cells = for transmitting rhythms throughout the body
- contractile cells = for contracting to that rhythm (most numerous)
at what speed do the atrial and ventricular myocytes usually contract?
0.3-0.5 m/s
what speed do the Purkinje fibres contract at?
5m/s
what speed does the AV node contract at?
0.05m/s
how are cardiomyocytes linked together?
linked together by low resistance pathways associated with gap junctions at the intercalated disks
what ensures that the atria contract at the same time?
internodal bundles conduct the impulse from the SA node to the AV node
what do the 4 specialised bundles in the atria consist of?
contain Purkinje-like cells and the cardiomyocytes are modified to conduct
*these bundles are also in direct contact with the atrial muscle
what is the only point where the wave of depolarisation passes from atria to ventricles?
AV nodes
what are ventricles insulated by?
insulated by connective tissue from atria
how many seconds does the AV node delay the excitation from atria to ventricles by?
0.1-0.2s
why is there a delay in the electrical conduction from atria to ventricles?
to permit longer and more effective ventricular filling
what is the AV node composed of?
small modified myocytes
what order does the depolarisation occur?
first part of ventricular wall is depolarised in the septum, then the apex and then finally the atrioventricular groove
what is an ECG?
a gross electrical measurement of the heart
why is it possible to measure the ECG from the skin surface?
due to the heart being a functional syncytium which large groups of cells all make electrical changes simultaneously
what are 2 positives about ECGs?
they are very fast and affordable
what is a lead?
a configuration of electrodes
where is lead II placed?
- positive electrode is on the left leg
- negative electrode is on the right arm
- ground electrode is on the right leg
what are the 12 standard leads?
- 3 bipolar leads (I, II, III) frontal plane
- 3 augmented leads
- 6 precordial leads
what does the P-wave on an ECG show?
depolarisation of atria in response to SA node triggering
what does the PR segment show on an ECG?
the delay of the AV node allowing the ventricles to fill
what does the QRS complex show on an ECG?
depolarisation of the ventricles, triggering the main pumping contractions
what does the ST segment show on an ECG?
the beginning of the ventricle repolarisation
what does the T wave show on an ECG?
ventricular repolarisation
what is present on an ECG if the ventricular conduction is abnormal?
the QRS is wide
what is a sign of dead tissue on an ECG?
large (deep) Q waves
what generates the heart rhythm?
the SA node
what is sinus tachycardia?
a tachycardia driven by the SA node beating too quickly: it has normal PR waves and each P is matched with QRS
what is the normal duration of a PR interval on an ECG?
3-5 boxes (120-200ms)
what is the normal duration of a QRS complex on an ECG?
2-3 boxes (80-120ms)
what is the normal duration of a QT interval on an ECG?
9-11.5 boxes (360-460ms)
what does the parasympathetic muscarinic stimulus lead to?
decreased heart rate, decrease in contractility and decrease in conduction velocity
what does parasympathetic withdrawal lead to?
increased heart rate, increased contractility and increase in conduction velocity
what is atropine?
a muscarinic antagonist
what is not innervated by the parasympathetic system?
vasculature
what does sympathetic stimulation lead to?
increased heart rate, increased contractility and increased conductance velocity
what nerve is involved with parasympathetic input?
vagus nerve
what nerve is involved with sympathetic input?
stellate nerve
what effect do beta-agonists have on the heart rate?
increase rate
what effect to beta-blockers have on the heart rate?
decreased rate
what is heart block?
a type of dysrhythmia caused by any kind of impulse contraction block of the heart
what causes a heart block?
- ischaemia of AV node or AV bundle
- compression of AV bundle by scar or calcified tissue
- inflammation of the AV node or bundle
what is an AV heart block?
a delay or failure of atrial signal stimulating ventricle
what are the symptoms of heart block?
- can be asymptomatic
- palpitations
- hypotension-like: dizziness, malaise, syncope
- risk of sudden death
what is first degree heart block?
- when PR interval >5 little boxes
- but all Ps followed by QRS
- always almost asymptomatic
- often young people
- delayed AV node transmission
- rarely treated
what is mobitz type 1 second degree heart block?
- PR interval gets longer until QRS wave fails to follow P wave
- likely cause is AV node damage
- usually no treatment given
what is second degree heart block?
some P waves are blocked and are not followed by QRS so some QRS complexes are missing
what is mobitz type II second degree heart block?
- some P waves are blocked and are not followed by QRS
- PR interval remains the same
- likely problem in bundle of His
- high risk: can progress to 3rd degree heart block
- treatment: implant pacemaker
what triggers premature (early) beats?
irritable tissue
what triggers escape (late) beats?
natural rhythmicity of non-atrial tissue
what are the characteristics of premature ventricular contractions?
- usually wide and weird looking ventricular electrical activity
- no S wave, instead wide negative dip where the T wave should be
- often beat triggered in middle of myocardium
- the 2 ventricles will be electorally unsynchronized
- width is determined by slowed conduction velocity
what is atrial fibrillation?
- no P wave instead flat or wiggly line
- ventricular rate is fast and irregular so many signals reach the AV node
- it is very common in the elderly
what can atrial fibrillation lead to?
thrombus formation in atrium due to slow flow of blood leads to stroke risk
what is respiratory sinus arrhythmia?
- heart beat is slightly faster during inspiration, slightly slower during expiration
- sign of a healthy heart
- usually only present in children and athletes
- caused by respiratory centres in brains medulla
- observe ventricular rate: opposite of RR interval
what is an interval?
duration from start of one mark to the end of another mark
what is a segment?
the wave form between 2 other waves
what is the ST segment elevation a sign of?
an acute MI
what is an iso-electric baseline?
from end of T to next P
