ECG Flashcards
What does an ECG plot?
Voltage against time
What does an ECG show?
A visual representation of how the electrical potentials of heart muscle cells vary from moment to moment as the signals that stimulate and coordinate contraction of the muscle travel through the heart.
What must the potential difference be in order for cardiac myocytes to contact?
For the myocyte (cardiac cell) to contract, the potential difference across the cellular (semi-permeable) membrane and must change from negative to positive in relation to the inside of the cell.
How are changes in potential difference created?
Changes in potential difference occur through the flow of ions through specialised ion channels in the cellular membrane and also relatively freely through gap junctions.
What does a potential difference between one part of the cell and another (a dipole (lead)) allow?
- that allows the electrical activity of the heart to be ‘viewed’ from a defined position or angle.
- The electric dipole (lead), as shown, consists of two equal and opposite charges,
- +q and –q, separated by a distance d.
- The dipole is a vector
- It has direction as well as magnitude (arrow).
What do the six frontal leads/ limb leads consist of?
What are they designed to do?
Three bipolar leads
The bipolar leads are so designated because each records the difference in electrical potential between two limbs.
Where can these three bipolar leads be placed?
- L – left wrist
- R – right wrist
- F – left ankle
- N – Typically right ankle
How is a 12 leads ECG different?
Leads are placed on arms, legs and across the chest
In a six lead ECG where are the leads placed?
- V1 – 4th IC, right sternal edge
- V2 – 4th IC, left sternal edge
- V3 – equidistant V2-V4
- V4 – 5th IC, midclavicular line
- V5 – left anterior axillary line In horizontal line with V4,
- V6 – mid axillary line, horizontal with V4 and V5,
Why do we prefer to use a 12 lead ECG?
We measure as 12 lead so we can get a whole view round the heart. Each part of the ECG looks at a particular part of the heart.
2,3 and AVF look at the inferior wall
Describe the electrical conduction of the heart
In the heart you have the SA node, AV node, internodal pathways and backmans bundle (takes the wave of depolarisation from the right side to the left side.
The AV node allows the ventricles to fill (up to 200ms).
The his bundle transmits the depolarisation to the apex to facilitate contraction.
What does the P wave represent (hump before QRS complex)?
Atrial depolarisation (of both atria)
What does the PR segment represent on the ECG?
AV node delay
How long is the PR interval?
Typically between 120-200ms
What does the QRS complex wave represent?
Depolarisation of ventricles
When does the QRS complex occur?
Following transversing AV node/ His/ Purkinje system
How long is the QRS complex?
Typically 80-110ms
In the QRS complex, what is the negative deflection and what is the positive deflection?
Q - 1st negative deflection
R - 1st positive deflection
S - 2nd negative deflection
What does the QT represent?
Ventricular depolarisation and repolarisation
How long is the QT interval?
Typically 350-420ms
What does an elevation and depression of the ST segment indicate?
Depression = ischaemia
Elevation = myocardial infarct
What does ST-segment represent
Repolarisation of the ventricles
What is a U wave?
What part of the ECG is the U wave visible?
A broad deflection of low amplitude appearing after the T wave
V3 or V4
What is tachycardia?
Fast heart rate
What is sinus tachycardia?
> 100bpm
What is bradycardia?
Slow heart rate
What is Sinus bradycardia?
<50bpm
What is an arrhythmia?
An arrhythmia is an abnormal cardiac rhythm
What are the two types of arrhythmias?
- Conduction abnormalities, e.g. blocks
* Abnormal impulse initiation e.g. ectopics, VT
What happens in terms of the ECG if the SA fails to initiate impulse?
Results in no P wave/ QRS complex
What can ischaemic heart disease or valve fibrosis cause on an ECG?
3rd degree AV Block / complete AV block
Characterised by a delay or interruption in conduction
Where does ventricular tachycardia originate from?
The ventricles
What can VT cause?
Ventricular tachycardia may impair cardiac output, consequently hypotension, collapse, and acute cardiac failure. This is due to extreme heart rates and lack of coordinated atrial contraction
What are the three basic Arrhythmiagenic mechanisms responsible for initiating tachyarrhythmia?
Altered automaticity; (either normal or abnormal)
Triggered activity’ where normal action potential suddenly swings positive again allowing another depolarisation to occur abnormally. due to – early after depolarisation or Delayed after depolarisations e.g. ectopics
Re-entry
what is SVT?
Supraventricular tachycardia
Which pateints is SVT common in?
May occur in young and healthy patients as well as those suffering chronic heart disease.
Describe AVNRT?
AVNRT is typically paroxysmal and may occur spontaneously or upon provocation i.e. exertion, caffeine, alcohol, beta-agonists i.e. (salbutamol) or amphetamines.
Describe the ECG changes in angina and infarcts Myocardial ischaemia (inadequate O2 supply)
ST depression caused by injury potential difference
Ischaemic myocytes have reduced membrane potentials compared with healthy myocytes.
The difference in potential between the ischaemic region and healthy region displaces the ST segment.
This is called the ‘injury current’ effect.
Describe the changes shown in an ECG when during an MI, Ischemia and injury
Myocardial infarction –MI. (sudden chest pain due to acute arterial obstruction)
A. Ischaemia – manifests ECG with ST depression +/- T wave inversion; result from altered repolarisation. (1)
B. Injury – ST segment elevation +/- loss of R wave; ‘current of injury’ (1)
C. Infarction – deep Q waves (>0.06 sec / > 25% R wave) (1) resulting from absence of depolarisation current from dead tissue (transmural). Manifests receding currents from opposite side of heart as an ‘electrical window’ is formed. (1)
D. Surface ECG on the opposite wall can show reciprocal voltage changes to the infarcted site. (1)
