ECG
Cardiff and Vale ECG Department
Electrocardiogram (ECG)
Clinical Skills
ECG
Aims & Outcomes
The aim of this module is to produce a technically accurate, artefact free 12 lead ECG in accordance with AHA/ SCST guidelines. To be able to recognise the basic components of the ECG.
Learning OutcomesAt the end of the session the student should be able to:
Recognise the anatomy of the conduction system of the heart.Identify the waveforms of the cardiac cycle, as seen on the ECG.Calculate the heart rate from the ECG.Outline the equipment and specifications required for recording a 12 lead ECG.State and demonstrate the anatomical positions for electrode placement.Recognise and minimise interference patterns on an ECG.Identify Einthoven’s Triangle, and its uses in practical electrocardiography.Produce a technically accurate ECG.
Aims & Outcomes
ECG
An ECG is a transthoracic interpretation of the electrical
activity of the heart.
A typical ECG tracing of the cardiac cycle (heartbeat)
consists of a P wave, QRS complex, T wave and sometimes
a U wave.
William Einthoven chose the letters P,Q,R, S, T to identify
the tracing.
What is an ECG?
WHAT IS AN ECG?
ECG
Conduction System
The heart is influenced by the autonomic nervous system which can increase or decrease the heart rate in line with the requirements of the body.
However, due to an intrinsic regulating system, called the conduction system it is possible for the heart to go on beating without any direct stimulus from the nervous system. This system is composed of specialised muscle tissue that generates and distributes the conduction that causes contraction of the cardiac muscle. These tissues are found in the sinus (or sinoatrial) node, atrioventricular node, bundle of His, bundle branches, and conduction myofibres.
When stimulated by electrical activity, muscle fibres contract and produce motion. In the heart, this electrical activity is referred to as depolarisation. The contraction causes the blood to be pumped around the body.
Relaxation of the heart muscle is caused by electrical repolarisation.
Conduction System
ECGConduction System
DEPOLARISATION
REPOLARISATION
ECG
Principle of how the ‘ECG’ works
In this example we are recording the potential difference between two points. As depolarisations (positive charge entering the cell from outside) move across the cell, the voltage difference increases. A movement of positive charges into a cell is recorded as a positive deflection and a movement of positive charges out of the cell is recorded as a negative deflection.
Principles of how the ‘ECG’ works
ECG
Principle of how the ‘ECG’ works
Principles of how the ‘ECG’ works
To understand the morphology of the ECG waveforms one needs to appreciate only one biophysical fact: if a wave front of depolarisation travels towards the electrode attached to the + input terminal of the ECG amplifier and away from the electrode attached to the – terminal, a positive-going deflection will result. If the waveform travels away from the + electrode towards the – electrode, a negative-going deflection will be seen.
If the waveform is travelling in a direction perpendicular to the line joining the sites where the two electrodes are placed, no deflection or a biphasic deflection will be produced.
ECG
Views of Depolarisation
It can then be seen that the voltage recorded along a particular lead axis (the vector joining the - to the + electrode) at a particular time is obtained by taking a projection onto that axis of the vector representing the magnitude and direction of depolarization at that time. Thus, when the lead axis in the figure alongside points from left to right, parallel to the direction of movement of depolarization, a positive-going complex results. When the two directions are anti-parallel, a negative-going complex is produced.
Views of Depolarisation
ECG
Atrial Depolarisation
The electrical activity of the heart originates in the sino-atrial node. The impulse then rapidly spreads through the right atrium to the atrioventricular node. It also spreads through the atrial muscle directly from the right atrium to the left atrium. The P-wave is generated by activation of the muscle of both atria.
Atrial Depolarisation
ECG
Septum Repolarisation
The impulse travels very slowly through the AV node, then very quickly through the bundle of His, then the bundle branches, the Purkinje network, and finally the ventricular muscle.
The first area of the ventricular muscle to be activated is the interventricular septum, which activates from left to right. This generates the Q-wave.
Septum Repolarisation
ECG
Ventricular Depolarisation
Next, the left and right ventricular free walls, which form the bulk of the muscle of both ventricles, gets activated by an action potential from the bundles of His and begin to cause depolarisation from the septum towards the apex (bottom) of the ventricles.
This generates the R-wave on the ECG.
Ventricular Depolarisation
ECG
Late Ventricular Depolarisation
A few small areas of the ventricles are activated at a rather late stage. This generates the S-wave
Late Ventricular Depolarisation
ECG
Ventricular Repolarisation
Finally, the ventricular muscle repolarises. This generates the T-wave
Ventricular Repolarisation
ECG
Not All Cardiac Actions Potentials Are The Same!!
Action potentials at the bottom edge of the ventricle are SHORTER than at the top. This helps to explain why the repolarisation of the ventricles (the T wave) gives a positive deflection on the ECG trace.
You can see that the action potentials at the bottom of the ventricle repolarise before those at the top.
Therefore the wave of repolarisation moves in the opposite direction to the wave of depolarisation.
Cardiac Action Potentials
ECG
The ECG
The ECG
P
QRS
T
U
Atrial Depolarisation
Ventricular Depolarisation
Ventricular Repolarisation
Though to be Septal Repolarisation
ECG
The P Wave
The P Wave
P Atrial Depolarisation
Represents atrial depolarisation Small, rounded wave
ECG
The QRS Wave
The QRS Wave
Represents ventricular depolarisation Large, “pointed” wave
Q Wave:The first negative deflection.
R Wave:Any positive deflection.
S Wave:Any negative deflection after an R wave.
QRS Ventricular Depolarisation
ECG
The T Wave
The T Wave
T Ventricular Repolarisation
Large, rounded wave Represents ventricular repolarisation
ECG
• Measured from the beginning of the P wave to the beginning of the QRS complex
• Normal value: 0.12 – 0.2 secs3 – 5 small squares
PR Interval
ECG
QRS Duration
• Measured from initial deflection of the QRS from the isoelectric line to the end of the QRS complex
• Normal value: < 0.12 secs
less than 3 small squares
ECG
Sinus Rhythm
Quick Check: Sinus Rhythm
Rate Rhythm
Ventricular 60 - 100 Regular
Atrial Same as ventricular Regular
P-R Interval Normal
QRS Complex Normal
ECG
ATRIO-VENTRICULAR BLOCKS
ECG
1st Degree Heart Block
Measured from the beginning of the P wave to the beginning of the QRS complex
Normal value: 0.12 – 0.2 secs3 – 5 small squares
Atrio-Ventricular Heart Block
PR Interval
> 5 small squares
ECG
1st Degree Heart Block
Prolonged PR interval: >0.2 secs Constant PR interval Regular ventricular rhythm
Atrio-Ventricular Heart Block
ECG
2nd Degree Heart Block
Atrio-Ventricular Heart Block
Mobitz Type 1 Wenckebach
Mobitz Type 2 ConstantPeriodic
ECG
2nd Degree Heart Block – Mobitz Type 1 - Wenckebach
Progressive Lengthening of PR interval, until a non-conducted P wave occurs. Usually occurs in a cyclic pattern
Atrio-Ventricular Heart Block
ECG
2nd Degree Heart Block – Mobitz Type 1 - Wenckebach
Atrio-Ventricular Heart Block
ECG
2nd Degree Heart Block – Mobitz Type 2 – Constant Block
P wave not followed by a QRS plus P wave normally conducted. (May be 2:1, 3:1, etc) PR interval of the conducted beat is constant. (May be normal or prolonged) Atrial rate is regular and normal. Ventricular rate may be regular (eg. 2:1)
Atrio-Ventricular Heart Block
ECG
2nd Degree Heart Block – Mobitz Type 2 – Constant Block
Atrio-Ventricular Heart Block
ECG
2nd Degree Heart Block – Mobitz Type 2 – Periodic Block
Normal 1 P:1 QRS conduction with occasional non-conducted P waves.
PR interval of the conducted beat is regular. (May be normal or prolonged)
Atrio-Ventricular Heart Block
ECG
2nd Degree Heart Block – Mobitz Type 2 – Periodic Block
Atrio-Ventricular Heart Block
ECG
3rd Degree Heart Block – Complete Heart Block
Regular P waves at a normal rate Regular QRS complexes at a slow rate. (30 – 40 beats / min) No correlation between P waves and QRS complexes QRS may have abnormal shape
Atrio-Ventricular Heart Block
ECG
MYOCARDIAL ISCHAEMIA & INFARCTION
ECG
What is an MI?
…is when part of the HEART muscle dies because it has been starved of OXYGEN
Myocardial Ischaemia & Infarction
Myocardial Infarction
Heart Attack
Coronary Thrombosis
ECG
Coronary Arteries
There are 3 Coronary Arteries:
1.LAD (Left main coronary artery)
2.Circumflex artery (originates off the LAD)
3.Right coronary artery
All these sub-divide into smaller branches
Myocardial Ischaemia & Infarction
ECG
Myocardial Infarction
Myocardial Ischaemia & Infarction
Myocardial Infarction as a result of a blocked left anterior descending coronary artery
ECG
Coronary Arteries Territory
Myocardial Ischaemia & Infarction
ARTERY ECG LEADS TERRITORY
RCA II, III & aVF Inferior
LAD V1 – V6 Anterior
Cx I, aVL, V5 & V6 Lateral
ECGMyocardial Ischaemia & Infarction
V1 – V6
I, aVL, V
5 & V6
ECG
ST Segments
Myocardial Ischaemia & Infarction
Note: “Upsloping” ST depression is not an ischaemic abnormality
ECG
ST Segments
Myocardial Ischaemia & Infarction
ECG
Evolution of Acute MI
A. Normal ECG prior to MI
B. Hyperacute T wave change increased T wave amplitude and width may also see ST elevation
C. Marked ST elevation with hyperacute T wave changes (transmural injury)
D. Pathologic Q waves, less ST elevation, terminal T wave inversion (necrosis)
E. Pathologic Q waves, T wave inversion (necrosis and fibrosis)
F. Pathologic Q waves, upright T waves (fibrosis)
Myocardial Ischaemia & Infarction
ECG
A 55 year old man with 4 hours of “crushing” chest pain
• Acute inferior myocardial infarction
• ST elevation in the inferior leads II, III and aVF
• Reciprocal ST depression in the anterior leads
Myocardial Ischaemia & Infarction
ECG
• Acute antero-lateral infarction
• ST elevation in I, aVL, V2 – V6
• Reciprocal ST depression in inferior leads
Myocardial Ischaemia & Infarction
ECG
ECG Taken @ 11:01 am
Myocardial Ischaemia & Infarction
ECG
ECG Taken @ 11:12 am
Myocardial Ischaemia & Infarction
ECG
Narrowed Proximal LAD
Myocardial Ischaemia & Infarction
ECG
Interventional Procedure
Myocardial Ischaemia & Infarction
PTCAPercutaneous Transluminal Coronary Angioplasty
PCIPercutaneous Coronary Intervention
ECG
Evolution of Acute MI
ST elevationOften within minutes of onset of MI, but may be delayed for several hours
T Wave InversionSlightly later than ST elevation
ST elevation returns to iso-electric lineUsually within 48-72 hours
Q wavesMay take several hours to develop after onset of MI
Myocardial Ischaemia & Infarction
ECG
ELECTRODE POSITIONING
ECG
Limb Leads
RA – Right Wrist
LA – Left Wrist
LL – Left Ankle
RL – Anywhere on Body
(Records I, II, III, aVR, aVL, aVF)
Electrode Positioning
ECG
Chest Leads
V1 – 4th intercostal space, right sternal border - (Always locate by using sternal angle)
V2 – 4th intercostal space, left sternal border
V4 – 5th intercostal space, left mid-clavicular line
V3 – Diagonally midway between V2 and V4
V5 – Left anterior axillary line, horizontal with V4*
V6 – Left mid axillary line, horizontal with V4 and V5*
(*Not 5th intercostal space!)
Electrode Positioning
ECG
Einthoven’s Triangle
Gives the first three limb leads on the ECG:I, II & III.
Utilises the RA, LA & LL, recording the potential difference between two points.
Can be used to identify origin of interference.
Electrode Positioning
ECGElectrode Positioning
ECG
Recording Procedure
Identify patient and explain the procedure.
Ask patient to lie on bed/couch, if possible.
Prepare skin on electrode sites.
Attach electrodes and ECG leads.
Record Auto 12 lead.
Adjust, if necessary.
Inform patient of next step.
Label ECG and dispatch appropriately.
Recording Procedure
ECG
Specifications
Speed: 25mm/sec
Sensitivity: 10mm/mV
Frequency Response: 0.05 – 150Hz
Filter Off!
Measurement
ECG
Leads
Leads
LIMB LEADS CHEST LEADS
ECGWeb Resources
www.scst.org.uk/resourceswww.bhf.org.ukwww.bcs.comwww.ems12lead.comwww.bem.fi/book/06/06www.invasivecardiology.comwww.medicine.mcgill.ca/physio/vlab/cardio/ecgbasicswww.ecglibrary.comwww.ivline.orgwww.my.clevelandclinic.orgwww.texheartsurgeons.comwww.rpw.chem.ox.ac.ukwww.nottingham.ac.ukwww.frca.co.ukwww.drmcdougall.com
Web Resources