ECGs Flashcards
How do ECGs record electrical activity of the heart? What do the squares on the graph show?
ECG machines record electrical activity of the heart in a standardised way on a graph
The graph is drawn on 1mm squared paper that passes through at a rate of 25mm per second.
This means each small square represents 0.04 seconds, each large square is 0.2 seconds and 5 large squares equate to 1 second.
Vertically, one large square is 0.5mV
What do the lines of an ECG represent?
+ charge moving toward the + electrode gives an upward deflection
+ charge moving away from the + electrode gives a downward deflection.
- charge moving toward the - electrode also gives an upward deflection
isoelectric line = when electrical activity of the heart is moving perpendicular to the axis of the lead, OR no electrical activity
The diff position of electrodes on diff leads mean the same wave can point in opp directions in different leads
Describe the leads
12 universally standardised leads look at electrical activity from different angles. Standardised so that abnormalities are not due to lead placement
We break the leads up into 3 groups
6 chest leads (V1-6)
3 unipolar limb leads (aVR, aVL, aVF, )
3 bipolar leads (I, II, III)
The only lead that produces an opp pattern is the AVR
What are the chest/precordial leads and where are they placed?
V1 – 4th R IC space
V2 –4th L IC space
V3 – equidistant between V2 and V4
V4 – 5th IC space L midclavicular line
V5 – L anterior axillary line
V6 – L mid axillary line in line with V5
These leads place a + electrode on the chest.
What are the Standard limb/ bipolar leads?
The Standard Limb Leads record pd between 2 limbs at a time, so they are bipolar. In these leads, 1 limb carries a + electrode and the other a - one.
The limb electrodes, I, II and III form Einthoven’s Equilateral Triangle at the right arm (RA), left arm (LA) and left leg (LL).
What are the Augmented/unipolar limb leads?
The Augmented Limb Leads record from 1 limb at a time using a null point w a relative zero potential. Therefore son unipolar.
These leads= aVR, aVL and aVF
How does sinus rhythm appear on an ECG trace?
On an ECG trace, 3 distinctive waves represent sinus rhythm: P wave, QRS complex and T wave
We can use this as a rough guide to check for sinus rhythm:
Is there a P wave and is it rounded?
Are all P waves the same?
Does a QRS complex follow every P wave?
Is the PR interval 3-5 small squares (120-200ms)?
Draw and label an ECG
What do the P wave and PR segment on an ECG mean?
The P wave occurs when the SAN depolarises the atria. Positive P wave= sinus
If the positive p wave is very fast= sinus tachycardia. No p wave= SAN has failed to fire, giving a flatter line. Could be a problem w t or p cells which means contraction has not happened- sinus arrest in a significant pause
Once depolarisation reaches the AVN it is held there for a short time before it sends a.potentials down to the ventricles.
Therefore + charge is not moving to any electrodes. This forms the isoelectric PR segment.
What does the Q wave mean?
Depolarisation moves from the AVN to the Bundle of His and then thru the L and R bundle branches.
The left bundle depolarises the IV septum towards the right (and a bit upwards).
+ charge moves to the - electrode, causing a downward deflection. This septal depolarisation causes the Q wave
What does the R wave mean?
Electrical activity moves down the bundle branches and spreads thru the purkinje system, out towards the apex.
The LV generates a stronger depolarisation than the RV, so hay depolarisation wave slightly to the left, toward the + electrode.
This produces an upward R wave.
What do the S, ST segment and T waves represent?
Depolarisation then spreads upwards to the BASE of the heart. + charge moves to the neg electrode, causing a downward S wave.
ST segment :depolarisation of the entire ventricles, no hay net movement of charge.
Hay repolarisation of the L and R ventricles. Hay - charge spreading up to the - electrode. This causes the upward T wave
What happens to the R and S wave from V1 to V6?
Describe the location of the heart that each lead focuses on
Describe sinus tachycardia
Sinus tachycardia is a normal sinus rhythm with a HR in excess of 100BPM
Common in exercise and stress, usually asymptomatic
Medical causes include hypovolaemia (e.g. dehydration or haemorrhage) and stimulant abuse
Describe sinus bradycardia
Sinus bradycardia is a normal sinus rhythm with a HR below 60BPM
It is often asymptomatic and doesn’t require treatment
Causes include being fit or asleep, as well as iatrogenic causes (e.g. as an effect of CCBs and beta-1 blockers
What does sino atrial block/sinus arrest look like on an ECG and why?
No p wave= SAN has failed to fire, giving a flatter line. Could be a problem w t or p cells which means contraction has not happened- sinus arrest is a significant pause
What is the difference between 1st and 2nd degree AV blocks?
1st: PR interval is longer than 0.2s
2nd: more than 1 P wave to a QRS complex. Initial P waves cause nothing to happen- only the 2nd or 3rd P wave will stimulate the AVN (mobid type 1). In periods of intense exercise, AVN conducts slower and slower- wenckebach phenomenon
What is 3rd degree AV Block / complete AV block?
3rd: Usually life threatening. The atria are contracting and no a.potentials are going through. Ventricles fire without any input from the atria- AV dissociation, complete AV block
Describe a Left BBB
Describe a right BBB
What is the difference between DAD and EAD?
Early - occur during late phase 2 or 3. Can lead to a salvo of several rapid a.potentials or a prolonged series of action potentials e.g. VT. This form of triggered activity is more likely to occur when the action potential duration is increased.
Delayed - occur in late phase 3 or early phase 4 when the action potential is nearly or fully repolarised.
Describe Atrial extrasystole
What does this show?
Ventricular ectopic: extra abnormal ventricular wave. You know its ventricular bc its wide, so its coming from the purkinje fibres
What is the difference between VT and VF?
Ventricular Tachycardia above (VT) is a broad complex tachycardia originating in the ventricles.There are several different varieties of VT — the most being Monomorphic VT. Ventricular tachycardia (VT) may impair cardiac output, consequently hypotension, collapse, and acute cardiac failure.
Ventricular Fibrillation below (VF) This rapid and irregular electrical activity renders the ventricles unable to contract in a synchronised manner, resulting in immediate loss of cardiac output The heart is no longer an effective pump and is reduced to a quivering mess
Describe AF
Atrial fibrillation (AF) is the most common sustained arrhythmia. It is characterised by disorganised atrial electrical activity and contraction.
P wave – No distinct P waves - fibrillation waves (best seen in V1). R-R – Varying
Fibrillatory waves may be present and can be either fine (amplitude < 0.5mm) or coarse (amplitude > 0.5mm
AF: Lots of P waves but no contraction= fibrillation. Bc the atria are not contracting you can’t have active filling in the cardiac cycle. R-R is also irre
What does this show?
Atrial flutter is a form of supraventricular tachycardia caused by a re-entry circuit within the right atrium.
R-R – Irregular Ventricular rate depends on AV conduction ratio
What does this show?
•The commonest - may occur in young and healthy patients as well as those suffering chronic heart disease. SVT: No/retrograde P wave. Adenosine works on the AVN to slow impulses down
