ECG Flashcards
What is an ECG?
An electrocardiogram is a graphic representation of electrical potential difference changes, against time, of the myocardium throughout the cardiac cycle.
How are cardiac muscle contractions visible on ECGs?
The myocardium has electrical currents that sequentially depolarise each individual cell and result in a change in cellular morphology that allows muscle contraction
The electro / chemical changes that cause myocardial contractions are shown by the cardiac action potentials
What must occur for the myocyte to contract?
P.d. across the cell membrane must change from negative to positive
How do changes in p.d. occur across the cell membrane?
Changes in P.d. occur through flow of ions through specialised ion channels in cell membrane and relatively freely through gap junctions
How is the cardiac dipole created?
When cardiomyocytes re/depolarise, different currents flow across the cell membrane at various points creating a p.d. between one and another part of the cell => dipole
How is an electrical field formed in the heart?
Flow of current along cell surface sets up an electrical field around the dipole
What happens to the dipole during the resting potential?
Once the cell has re/depolarised and at resting potential, there is no membrane p.d. so no electric field, despite a p.d inside and outside cell.
Describe the features of the electric dipole
The electric dipole consists of 2 equal and opposite charges =q and -q, separated by a distance, d.
The dipole is a vector, with direction as well as magnitude
What are the 12 leads of an ECG?
6 Frontal plane leads (6 limb leads)
6 Horizontal / precordial / chest leads
Describe the 6 frontal plane leads used
6 frontal plane / limb leads consist of:
- 3 bipolar leads (I,II and III) derived from einthoven lead
- 3 unipolar leads (aVL, aVR, aVF) from I,II and III
What are the 6 horizontal / precordial / chest leads?
V1 - 4th IC, right sternal edge
V2 - 4th IC, left sternal edge
V3 - equidistant from V2 - V4
V4 - 5th IC, midclavicular line
V5 - left anterior axillary line horizontal with V4
V6 - midaxillary line, horizontal with V4 and V5
Describe features of the electrical anatomy
Initiated by SAN, traverses through atria into AV node and His / Purkinje rapid conduction occurs through to ventricular myocardium
Describe the features of the ECG complex
components - PQRSTU
specific order in sinus rhythm
each relates to a cardiac cycle phase
What occurs during the P wave?
Depolarisation of both atria
Onset of P wave activates QRS complex
Measurement of AV conduction time
from SAN -> ventricles usually (120-200 ms)
What is the QRS complex?
Q - 1st wave of negative deflection
R - 1st wave of positive deflection
S - 2nd wave of negative deflection or 1st after positive
occurs when ventricles depolarise following traversing AVN / His / Purkinje system
Specialised conduction cells for fast conduction
From the beginning to end of QRS = 80 - 110 ms
What events occur during the QT interval?
Typically lasts 350 -420 ms
Onset of QRS to end of T wave
Ventricular depolarisation and repolarisation
Long QT syndrome causes sudden death
What happens during ST segment / T wave?
Isoelectric (flat) segment between QRS and T wave Repolarisation of ventricles Ischaemia manifests here - depression - ischaemia - elevation - myocardial infarction some elevation is normal
What does an ECG tell us?
more tissue = higher voltage
atria - small & ventricles - large
Outline the features of a typical ECG
ECG is standardised via calibration signals Typical voltage scale - 10 mm = 1 mV 1 small square = 0.1 mV Speed = 25 mm/s 1 small square = 0.04 s Unit of measurement = bpm so more beats = faster rate
What is Tachycardia?
fast heart rate
e.g. sinus tachycardia >100 bpm
What is bradycardia?
slow heart rate
e.g. sinus bradycardia <50 bpm
What components are visible on ECG from a normal sinus rhythm?
- Positive P wave before each QRS
- Normal PR interval
- Normal QRS duration
- Normal QT interval
What are the effects of ventricular tachycardia?
Can impair CO
-> hypotension, collapse and acute cardiac failure
What does the presence of a Y wave tell us?
Y complex seen due to extreme heart rates & lack of coordinated atrial contraction
List some pathological conditions producing characteristic ECG changes
- premature atrial & ventricular contractions (ectopics)
- atrial / ventricular tachycardia
- atrial / ventricular flutter
- sinus arrest & Sinoatrial block
- AV blocks
- Bundle Branch Blocks (BBB)
- ventricular pre-excitation
- supraventricular arrhythmias
- AV nodal re-entrant tachycardia
What are the 3 arrhythmogenic mechanisms responsible for initiating tachyarrhythmia?
- Altered automaticity (normal / abnormal)
- triggered activity (abnormal re/depolarisation
- ectopics)
- re entry
What are arrhythmogenic mechanisms caused by?
alterations in ionic currents
Describe supraventricular tachycardia effects
- ST depression
- AVNRT typically paroxysmal - occurs spontaneously or
on provocation (exertion, caffeine, beta blockers
(salbutamol) etc. ) - more common in women
How is angina and infarcts caused?
due to insufficient oxygen
What is angina?
chest pain caused during exercise due to chronic arterial narrowing
what is an ST depression caused by?
Caused by the injury potential difference
What is the Injury current effect?
ischaemic myocytes have reduced membrane potentials compared to normal. The difference in p.d. between the ischaemic and healthy regions, displaces the ST segment => this is named the ‘injury current’
Describe the ECG of an ischaemic patient
ST depression
+/- T wave inversion - due to altered repolarisation
What does an injury ECG look like?
ST elevation
+/- elevation
loss of R wave ‘current of injury’
Describe the ECG of an infarction
deep Q waves resulting from no depolarisation from necrosis
receding currents from opposite side of hear as ‘electrical window’ formed
