2 The Electrocardiogram Flashcards
What is an electrocardiogram?
- It is a recording of electrical activity of the heart (potentially other muscles)
- Electrodes are placed on arms, legs, and chest wall to detect electrical activity
- A graph of voltage against time is produced
These are extracellular recordings
Describe the normal path for conduction of electrical activity through the heart:
- SAN undergoes spontaneous discharge
- Electrical activity spreads across into R atrium and across into L atrium
> insulating membrane stops AP going straight down into ventricles - There is a slight pause in the discharge of the Atrioventricular (AV) node
> This is to allow the ventricle to fill completely with blood - When the AV node discharges, this activates the bundle of His, and Purkinje fibre system, ensuring near-simultaneous depolarisation of the muscle mass in both ventricles
How many electrodes are placed around the body?
(and what are they connected to?)
and how many on each place?
There are 10 electrodes placed around the body, which are connected with cables to ECG machine (Galvanometer)
4 are placed on limbs
(2 on legs, 2 on arms)
6 are placed on the chest
These 10 electrodes work together to produce a 12 lead ECG
What are the positions of the electrodes on the limbs?
2 on L+R arm (wrist)
2 on L+R leg (ankle)
but if amputated, place it on the end of the stump, equal distance from the heart
What are the positions of the electrodes on the chest (precordial)?
V1 - 4th Right ICS at Sternal Edge
V2 - 4th Left ICS at Sternal Edge
V3 - equidistant between V2 and V4
V4 - 5th Left ICS at the midclavicular line (apex of the heart)
V5 - horizontally left of V4 at Ant. Axillary line
V6 - more horizontal of V4, V5 at Mid Axillary line
Explain the concept of ECG leads, and how the 12 lead ECG is made up
ECG leads
- In order to record a voltage (i.e. a difference in electrical potential)
- The potential at one electrode has to be compared to that at another electrode (or combination of electrodes)
- There are 3 active limb electrodes
(R leg electrode is an earth electrode)
AND
6 chest electrodes
BUT
- 6 limb leads (I, II, III, aVR, aVL, aVF) and
6 chest leads (V1 - V6)
Name the two different types of limb leads
- Bipolar leads (leads I, II, III)
- Unipolar (augmented leads) - aVR, aVL, aVF
Describe Limb lead I
Lead I
- Left-arm (LA) electrode: +ve
- Right arm (RA) eletrode: -ve
Compares LA electrode to RA
Describe Limb lead II
Lead II
- L Led (LL): +ve
- RA electrode: -ve
Describe Limb lead III
Lead III
- L arm (LA): +ve
- L Leg (LL): -ve
Describe aVR lead
aVR
- Compares RA with an avg. of LA + LL
Describe aVL lead
aVL
- Compares LA with an avg. of RA + LL
Describe aVF lead
aVF
- Compares LL with an avg. of RA + LA
Describe the 6 chest leads
These are simply the electrodes in their positions, measuring in relation to different parts of the heart
Describe what the 12-lead ECG is made up of
Together, the 6 chest leads and 6 limb leads come together to form and produce a 12 lead ECG
Give the structures of a 12 lead ECG
- 4 lines: bottom line is an electrical signal from lead II - known as rhythm strip (10s)
The remaining 3 lines are split into 4 sections, each showing the electrical signals of a different lead
- Each first AP of each lead represents the same heartbeat (same electrical signal = in sync)
Discuss leads and dipoles, and their relation to the ECG
- ECG machines use a galvanometer, as an instrument used to detect, measure + determines the direction of small electric current
- Each ECG lead has a positive and negative pole
- Leads record the electrical current associated with a dipole (i.e. when there is a difference in charge between one bit of a membrane and another)
- Thus, when all of the muscle is polarised (i.e. rest), there is no dipole
- When part of the muscle is depolarised, a dipole exists
- When all of the muscle is depolarised (i.e. contracting), there is no dipole
Explain why isoelectric line presence does not equal inactivity
If an isoelectric line is shown, it’s not indicating inactivity
An isoelectric can come about if a muscle is relaxed, polarised, and hence has no dipole
A partially depolarised muscle (partially contracted) will have a dipole, hence producing a positive deflection
A fully contracted, fully depolarised muscle will have no dipole and hence will still produce an isoelectric line (no p.d. measure/low)
What effects do depolarising events have on the ECG?
Depolarisation moving towards:
- Unipolar electrode
- +ve pole of bipolar lead
> means = positive (upwards) deflection
Depolarisation moving away:
- From a unipolar electrode
- +ve pole of bipolar lead
> means = negative (downward) deflection
What effects do repolarising effects have on the ECG?
Repolarisation moving towards:
- Unipolar electroke
- +ve pole of bipolar lead
> means = negative (downward) deflection
Repolarisation moving away:
- From a unipolar electrode
- +ve pole of bipolar lead
> means = positive (upward) deflection
Describe the typical shape of one AP in lead II of a normal ECG
- P wave - atrial depolarisation
- QRS complex - ventricular depolarisation
> Q wave is septal depolarisation - T wave is ventricular repolarisation
Explain how a T wave can be positive (in most leads) when it is a repolarising event?
- The sub-endocardial myocytes start their AP slightly before those in the sub-epicardium
- In addition, the AP in sub-endocardial myocytes are slightly larger
- Hence, the cells that depolarise first also repolarise last (and vice versa)
In effect, this means ventricular repolarisation travels in the opposite direction to ventricular depolarisation, giving a positive T wave in the ECG
Describe a P wave on a typical ECG
P wave
- Depolarisation of the atria
- Right atrial activation begins first
- Relatively little muscle, therefore small in amplitude
- Normal waves have a slight notch (bifid)
Describe the PR interval on a typical ECG
- Time for conduction through AV node, Bundle of His, Purkinje fibers
- Time from onset of atrial depolarization to the onset of ventricular depolarisation
- It is measured from the start of the P wave to 1st deflection of the QRS complex
< irrespective of whether QRS complex begins with Q wave or R wave
What is the normal duration of the PR interval
The normal duration is 120-200ms
3-5 small squares
Describe what may cause PR interval prolongation
It could be 1st-degree heart block
Describe the QRS complex in a typical ECG
- Ventricular depolarisation
- Large muscle mass of the Left ventricle means QRS predominantly shows the LV signal
Q wave: any initial negative deflection
R wave: any positive deflection
S wave: any negative deflection after R wave
Give the normals waves for the QRS complex
QRS duration, R wave height, S wave depth
Normals values:
- QRS duration: < 120ms
- R wave: can be variable (not always)
- S wave depth: < 30mm
Describe what could be the cause of wide QRS complexes
Frequent, wide QRS complexes:
- Ventricular ectopic beats
Describe what large R waves could signify, and potentially what may cause them
Large R waves - Left ventricular hypertrophy
Causes:
- Aortic stenosis
- Hypertension (compensatory hypertension)
- Athletes (due to large ventricular muscle mass)
Describe Q waves in a typical ECG
- Normal Q waves can be found in leads facing the left ventricles (leads I, II, aVL, V5, V6)
- They occasionally occur in lead III
Give the normal ranges for Q waves
Normal:
- < 2 mm in depth (2 small squares)
- < 40ms in duration (1 small square)
Describe the causes of an abnormally long Q wave
Past infarction of heart muscle
Describe the ST segment in a typical ECG
ST-segment
- QRS complex ends at the J point
- So, ST-segment: J point to start of T wave
It signifies:
- End of ventricular depolarization to the beginning of repolarisation
(muscle is depolarised and is contracting; isoelectric does not mean inactive
It is usually level +/- 1mm from baseline - may slope upwards slightly
Describe what an ST-segment elevation is,
and potential causes:
ST-segment elevation:
- This is when the ST segment is seen to be positioned higher (elevated)
Causes:
- STEMI: ST-elevation myocardial infarction
- Pericarditis: ST elevation would be seen over most of the leads
Describe the QT interval in a typical ECG
It is the total time for depolarisation and repolarisation to occur
Describe what a prolonged QT interval can cause:
A prolonged QT interval or ‘Long QT syndrome can cause:
- Serious arrhythmias when the person is exercising or is stressed
Describe the T wave in a typical ECG
- Represents ventricular repolarisation
- It is asymmetrical - steeper on the ascent
- Rarely exceeds 10mm (height)
Describe the U wave in a typical ECG
- Small deflection after T wave
- Many ECG’s have no discernible U wave
Name and describe the different available anti-arrhythmic drugs
4 classes of drugs:
Class I: Sodium channel blockers (lidocaine)
- Reduce the maximum rate of repolarisation - used to treat ventricular dysrhythmias
Class II: b-adrenoceptor antagonists (atenolol)
- Used to treat tachyarrhythmias. They decrease mortality in post-myocardial infarction
Class III: potassium channel blockers (amiodarone)
- These slow repolarisation and prolong cardiac action potential, thereby increasing the refractory period
Class IV: calcium channel antagonists (verapamil)
- They block L-type calcium channels, slow conduction in the SA node and AV node. They are used to treat supraventricular tachycardias (SVTs)
Others:
- Adenosine (slows AV conduction) - used in SVTs
- Digoxin (increases vagal tone - via CNS) - slows AV conduction, used in SVTs
