ECGs Flashcards
What does an ECG represent?
-the electrical events of the cardiac cycle
-each event has a distinct waveform
-used to evaluate cardiac pathophysiology
Cardiac cycle review
1) atrial contraction - D
2) isovolumetric contraction - S
3) rapid ejection - S
4) reduced ejection - S
5) isovolumetric relaxation - D
6) rapid filling - diastole - D
7) reduced filling - D
CC - atrial contraction
AV open, SV closed
-P wave
-SAN impulse
-electrical depolarisation of atria
-atrial contraction
-atrial pressure increase
-atrial pressure > ventricular pressure
-blood flows -> ventricles
-atrial booster
-atrial contraction complete
-maximal ventricular volume - EDV
-LAP - a-wave - atrial contraction causes small increase in venous pressure
CC - isovolumetric contraction
All valves closed
-QRS complex
-SAN impulse
-ventricular depolarisation - excitation-contraction coupling, myocyte contraction
-ventricular contraction
-ventricular pressure increase, ventricular blood volume constant = ISOVOLUMETRIC CONTRACTION
-LAP - c-wave - bulging of mitral valve leaflets back into atrium (x-descent follows peak of c-wave)
CC - rapid ejection
AV closed, SV open
-intraventricular pressure > aortic/pulmonary artery pressure -> SV open
-rapid ejection of blood from ventricles -> aorta/pulmonary artery
-low pressure gradient across valve (due to large opening) -> low resistance
-maximal outflow velocity (early in ejection phase), maximal systolic aortic/pulmonary artery pressures
-LAP - initial decrease as atrial base is pulled down so chamber expands -> BF -> atria continued -> atrial pressure increases (increasing pressure is maintained until AV valves open at end of isovolumetric relaxation)
CC - reduced ejection
AV closed, AV open
-T wave
-beginning of ventricular repolarisation
-decline in ventricular active tension & pressure generation
-rate of ejection falls
-Vp < outflow tract pressure - however outward flow still occurs due to kinetic/inertial energy of blood
-LAP/RAP - gradual pressure increase due to continued venous return from the lungs/systemic circulation
CC - isovolumetric relaxation
All valves closed
-intraventricular pressures fall
-aortic/pulmonary valves close (aortic valve closure precedes pulmonary - 2nd heart sound - beginning of isovolumetric relaxation) - (small backflow of blood into ventricles - dicrotic notch in aortic/pulmonary pressure tracings)
-aortic & pulmonary pressures rise slightly - dicrotic wave
-slow aortic/pulmonary pressure decline
-ventricular pressures decrease however volume is constant = ISOVOLUMETRIC RELAXATION
-volume of blood remaining in ventricle is ESV
(SV = EDV - ESV)
-LAP - continues to rise due to venous return - peak LAP at end of isovolumetric relaxation - v-wave
Ap > Vp -> AV open
CC - rapid filling
AV open, SV closed
-end of phase 5 - ventricles continue to relex
-Vp < Ap -> AV valves open
-passive ventricular filling begins
-brief decline in ventricular pressure due to continued ventricular relaxation
-ventricles completely relaxed
-pressure rises as ventricles are filled with blood from atria (blood moves down pressure gradient)
-LAP - opening of mitral valve causes rapid fall in LAP - before mitral valve opens - peak of LAP = v-wave - mitral valve opens, atrial pressure decreases = y-descent
CC - reduced filling
AV open, SV closed
-ventricles continue to fill with blood & expand
-ventricles become less compliant & intraventricular pressure rises
-increase in intraventricular pressure
-pressure gradient across AV valves reduces
-rate of ventricular filling falls = REDUCED FILLING
-90% of ventricular filling is complete by end of this phase
-aortic/pulmonary pressures continue to fall during this phase
Co-ordination of the heart review
-SAN pacemaker cells depolarise
-electrical impulse generated
-impulse spreads across atria
-atria contract
-annulus fibrosus (cardiac skeleton, fibrous tissue band) separates atria from ventricles -> prevents impulse travelling directly from atria -> ventricles
-impulse spreads through atria -> AVN
-AVN delays impulse
-AVN transmits impulse down bundle of His
-impulse reaches end of bundle of His
-impulse spreads through ventricle walls via Purkinje fibres
-impulse spreads up ventricle walls
-ventricles contract
Diastole
0.3s
When the ventricles relax & fill with blood
Systole
0.5s
When the ventricles contract to pump blood into pulmonary/systemic circulations
What type of wave is produced by an electrical impulse that travels towards the electrode?
Upright positive deflection
What conditions can ECGs identify?
Arrhythmias
Myocardial ischaemia/infarction
Pericarditis
Chamber hypertrophy
Electrolyte disturbances (hyper/hypokalaemia)
Drug toxicity
Name the 3 pacemakers of the heart
Sinoatrial node
Atrioventricular node
Ventricular cells
What is the dominant pacemaker?
SAN
SAN intrinsic rate
60-100bpm
What are the back up pacemakers?
AVN
Ventricular cells
AVN intrinsic rate
40-60bpm
Standard calibration of ECG machine
25mm/s
0.1mV/mm
What does an ECG look like?
Heart order of impulse conduction
SAN -> AVN -> Bundle of His -> Bundle branches -> Purkinje fibres
What does the P wave represent
Atrial depolarisation
What does QRS represent?
Ventricular depolarisation
What does T wave represent?
Ventricular repolarisation
What does the PR interval represent?
Atrial depolarisation, delay in AV junction (delay allows time for atria to contract before the ventricles contract)
What do ECG leads measure?
Difference in electrical potential between 2 points
Bipolar leads
-2 points on body
-positive & negative
Unipolar leads
-1 point on body, 1 virtual reference point with zero electrical potential
-centre of heart
(only require positive electrode for monitoring)
How many leads are calculated using the 10 electrodes?
12 leads using 10 electrodes
(6 precordial, 4 limbs (1 neutral))
How many chest electrodes and chest leads are there?
6 electrodes
6 leads
What information do chest electrodes give?
Info. about heart’s horizontal plane
How many limb electrodes & limb leads are there?
4 electrodes
6 frontal leads
What information do limb electrodes give?
Info. about heart’s vertical plane
Einthoven’s triangle
Placement of limb electrodes & frontal leads
- Lead I
- Lead II
- Lead III
- Augmented vector right (aVR)
- Augmented vector left (aVL)
- Augmented vector foot (aVF)
Leads 1-3 - bipolar
Augmented leads - unipolar
Common abnormalities of the P wave
P wave = atrial depolarisation
Right atrial enlargement - tall > 2.5mm - P pulmonale
Left atrial enlargement - notched (M-shaped) - P mitrale
Long PR interval - first degree heart block
First degree heart block
-split-second delay in the time that it takes electrical impulses to move through the AVN
Common abnormalities of the QRS complex
Depth of the S wave should not excess 30mm
Pathological Q wave
- >2mm deep, >1mm wide
- >25% amplitude of the subsequent R wave
What does the QRS axis represent?
Overall direction of the heart’s electrical activity
What do abnormalities of the QRS axis suggest?
Ventricular enlargement or conduction blocks
Common abnormalities of the ST segment
-usually flat (isoelectric)
-elevation/depression of ST segment by 1mm or more can be pathological
Common abnormalities of the T wave
-should be at least 1/8 but less than 2/3 the amplitude of R
-T wave amplitude rarely exceeds 10mm
-abnormal T waves - symmetrical, tall, peaked, biphasic or inverted
How does the QT interval change when heart rate increases?
Decreases
Common abnormalities of the QT interval
-QT interval decreases when heart rate increases
-regular interval = 0.35s-0.45s
-should not be more than half of the interval between adjacent R waves
U waves
-small, round, symmetrical and positive in lead II
-amplitude <2mm (regular)
-U wave should be same direction as T wave
How do you determine heart rate from an ECG?
For regular rhythms - rule of 300/1500
-count number of big/small boxes between two QRS complexes
-divide this into 300/1500 for regular rhythms
For irregular rhythms - 10 second rule
-count number of beats present on the ECG
-multiply by 6
Explain the quadrant approach for the QRS axis
-QRS complex in leads I & aVF
-determine if they are positive/negative
-the combination should place the axis into 1/4 of the quadrants
LAD = left axis deviation
RAD = right axis deviation
