1
Q
The Electrocardiogram (ECG)
A
- The electrical events in the heart are conducted throughout the entire body because of the conducting properties of our cells
- These electrical signals can be recorded by placing electrodes on our skin.
- non-invasive. Does not hurt. - Because of the regularity of the cardiac electrical events, other electrical events can be electrically filtered out.
- The ECG can be used clinically to diagnose cardiac conduction problems and myocardial infarction.
2
Q
Major components of an ECG
A
- P-wave: (50-80 ms): atrial depolarisation
- QRS complex: (80-120 ms)
- ventricular depolarisation
- atria repolarising simultaneously - T wave:
- ventricular repolarisation - PR interval (120-200 ms)
- PR segment = AV nodal delay
- Whatever the duration in time from the beginning of the atrial depolarisation to the beginning of the ventricular depolarisation = shows how good then signal is at getting through the AV node
- ST segment (80-120 ms)
- ventricular contraction (emptying) - TP interval: (70-500 ms)
- ventricular relaxation (filling)
- varies greatly with heart rate
3
Q
Cardiac conduction problems: cardiac arrhythmias
A
- Heart block: problems with conductance through the AV node
- The PR interval represents atrial depolarisation and contraction.
- Incorporates the period of AV nodal delay (PR segment)
- Max duration of the PR interval 200 ms
- If > 200 ms, suggests damage to AV node.
4
Q
Formula for heart rate on ECG
A
FORMULAS:
- HR = 60/RRI (sec)
- RRI = approx ~0.96s
- 60/0.96 = 62.5 beats/min
- Time interval for 1 large square (5 small squares) is 0.2 sec (200 ms)
5
Q
Second and third degree heart block
A
2nd-degree heart block:
- Missed QRS
- AV node problem
- experience “missed beats”
3rd-degree heart block:
- normal P wave
- no link to QRS
- Ventricles generate own signal (such as from bundle branch)
- no link b/w the QRS and P wave
Why the QRS is coming much slower even though SA node is firing off at its normal rate:
- If you are not getting propagation through the AV node, then the SA node is not causing depolarisation in the ventricles, therefore you get pacemakers in the ventricles starting to take over
6
Q
Atrial Fibrillation (AF)
A
- No P waves = the atria are not depolarising in the way that they are meant to depolarise
- Atrial fibrillation is the most common rhythm disorder.
- The most common symptom is a fast and irregular heart rate, although it may be asymptomatic.
- The main problem with AF is that it increases the risk of blood clot formation in atria → increases the risk of stroke
- Cause: disorganised electrical impulses usually originating in the roots of the pulmonary veins, overwhelm the normal electrical impulses from the sinoatrial node leading to irregular conduction of impulses across the atria.
7
Q
Long QT syndrome and sudden arrhythmic death
A
- Sudden arrhythmic death syndrome = a genetic condition affecting younger people creating abnormal heart rates which can stop blood pumping to the brain
- Can be helped by an implanted heart starter
- The QT interval is the time required for the ventricles to undergo a single cycle of depolarisation and repolarisation
- HERG channel = most commonly affected in Long QT syndrome
- Normal values are between 0.3-0.45s
- A prolonged QT interval can be caused by a malfunctioning K+ channel
- Long QT syndrome can be genetic or acquired
- Genetic defect estimated at 1:7000 of population
- Acquired - very common drug side effect e.g. antiarrhythmics, antipsychotics, antibiotics, antihistamines, low blood K+, Mg2+
- A premature heart beat (QRS) occurring during the downslope of the T wave can trigger a ventricular tachycardia (medical term for a heart rate over 100 bpm)
- The R on T is describing what is happening in the ECG = there is an early QRS occurring before the T is finished
- This is described as an ”R on T”.
- And what happens here is that this can trigger a ventricular tachycardia, where the ventricles just fire off action potentials very rapidly, causing very rapid heart beat (in the order of 200 beats/min).
- The problem here is that the heart is beating so fast that there is not enough time for the ventricles to fill up with blood again, and so little blood is actually being ejected from the ventricles.
- This is a fatal arrhythmia, unless the person is defibrillated.
- One large square = 0.2 sec
- QT interval = ~10 small squares (400ms)
- Long QT interval = ~16 squares = 640 ms
8
Q
ECG changes with cardiac ischaemia
A
- Normally the ST segment is at the iso-electric point.
- Under conditions of cardiac ischemia or myocardial infarct, the ST segment may be elevated or depressed, depending on the site of the ischaemia.
