Summary of ECG Abnormalities

Original article by Tom Leach | Last updated on 10/9/2014
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ECG sign
Seen in
Sinus rhythm
regular p waves, and each p wave is followed by a QRS. 60-100bpm
All leads (best to look at the rhythm strip)
Sinus Tachycardia
Same as above, except >100bpm
All leads (best to look at the rhythm strip)
Does not represent cardiac patholoy. May be a sign of anxiety, dehydration, recent exercise, or general illness (e.g. sepsis, pneumonia, respiratory pathology, other illness)
Sinus bradycardia
Same as above except <60bpm
All leads (best to look at the rhythm strip)
This is normal in young fit people
Right ventricular hypertrophy
Negative QRS
Lead I
Because the cardiac axis has shifted from 11-5 o’clock to 1-7 o’clock, thus lead I which measures laterally from right to left now gets a negative signal because the signal is going from left to right. This axis shift is called right axis deviation.
Right ventricular hypertrophy
Taller QRS
Lead III – becomes taller than lead II
Because lead III measures vertically but also slightly left to right, and this is pretty much the exact direction of the new shifted axis. Lead II, measuring from right arm to left leg is no longer lined up as well. This axis shift is called right axis deviation.
Transition point moved to the left – equal sized R and S (normally seen in V3/V4)
Equally sized R and S now seen in V5/V6
Left Ventricular Hypertrophy
Small lead I QRS, negative leads II and lead III QRS
Leads I-III
Left axis deviation – this is often the results of a conduction defect, and not an increased bulk of left ventricular tissue.
Absent P waves – just an irregular baseline.
As well as no p waves, the rhythm will be irregularly irregular. There will be a fibrillating baseline due to uncoordinated activity.
The causes of atrial fibrillation are:
  1. Ischaemic heart disease
  2.  Thyrotoxicosis (hyperthyroidism)
  3. Sepsis
  4. Valvular heart disease
  5. Alcohol excess
  6. PE
Note that AF can also co-exist with complete heart block, in which case the QRS will be regular!
Irregularly Irregular, irregular QRS (but QRS is normal shape)
Rhythm strip
Might look messy! E.g.
Atrial Flutter
Rhythm strip
There will be saw tooth p waves that occur at 300bpm, but the QRS complexes will only be at 150, 100 or 75 bpm due to various blocks. The QRS can be regular or irregular.
It can be very difficult to see t waves – what looks like a T wave will probably just be a p wave. The p waves occur at very regular intervals.
Can’t tell if T/P waves are present – rhythm is too fast (250bpm). Often associated block; i.e. there are QRS complexes at a lower rate than the p waves
Lead where p waves are most easily visible – you should use drugs to slow down the heart rate to see what is going on
Atrial tachycardia
>150bpm, p waves superimposed over t waves of preceding beat, normal QRS
Any where p waves are best seen
Caused by a foci of the atria (outside of the SA node) depolarising quickly
Junctional tachycardia
P waves very close to QRS, or no QRS visible. QRS is normal
Due to a ‘re-entry’ loop; there is an area of depolarisation near the AV node; this not only transmits a signal throughout the rest of the ventricles to depolarise them 
1st degree heart block
PR interval >0.2s (one big square)
Allover – best in I or V1
This is an AV node block
Can be caused by CAD, acute rheumatic carditis, digoxin toxicity, or electrolyte disturbance
It is NOT an medical emergency
2nd degree heart block
Mobitz type 1 - Wencebach
Mobitz type 2
2:1 and 3:1 conduction
Progressive lengthening of the PR interval followed by absent QRS, then cycle repeats. Cycles are variable in length. R-R interval shortens with lengthening of PR interval
This can be an AV node block (nearly always), or an SA node block. usually benign and generally doesn’t require specific treatment. can be caused by CHD or acute MI.
It is usually symptomless, but can present with:
-          Dizziness / light-headedness / syncope
Absent QRS every now and again
This can be an SA node block, or far more commonly infra-Hisian block (distal block). It can progress to complete heart block, from which there is often no escape rhythm; and thus this needs treatment! the definitive treatment is an implanted pacemaker.
Can be caused by CHD or MI
This is the ratio of P:QRS
May require a pacemaker, particularly if the rate is slow
Complete (third degree) heart block
90 P waves/min, only about 38 QRS/min, and not relationship between the P waves and the QRS complexes. QRS will often have an abnormal shape, and be broad (>120ms). However, the P-P intervals will be regular, as will the R-R intervals – they are just not in time with each other. The rhythm of the ventricles is the escape rhythm.
Best in II and V1
This is an AV node block. Atrial activity will be completely normal, but this conductivity does not pass into the ventricles.
This always indicates underlying disease – the disease is often fibrosis rather than ischaemia, but it can occur in MI.
RBBB – right bundle branch block
ECG may appear normal. In some people there may be 2 R waves. This creates a distinctive pattern:
V1 – there is an M shaped QRS – this is sometimes called an RSR pattern
V6 – there is a W shaped QRS
Wide QRS (120ms)
These are infra-Hisian blocks. In bundle branch blockages, the wave of depolarisation can still reach the IV septum, then the PR interval will be normal – and it is. However, the time taken for the depolarisation to spread throughout the ventricles is longer – thus QRS complex duration is lengthened.
In the acute setting it may be caused by MI
RBBB – may indicate right sided disease. The two R waves indicate the depolarisation of the right and left sides of the heart at different times (the right depolarises after the left).
You can remember the pattern with the word MarroWthere is M in V1, and W in v6, and the ‘rr’ tells you it is on the right!
There is NOT specific treatment, and it is often caused by an atrial septal defect.
In the acute setting it may be caused by MI
LBBB – often indicates left sided heart disease. Remember the pattern with WillaM.
Aortic stenosis, dilated cardiomyopathy, acute MI, CAD
Syncope, and in more severe cases; heart failure. Those with syncope and / or heart failure will usually be treated with a pacemaker.
LBBB – left bundle branch block
V1 – there is an W shaped QRS
V6 – there is a M shaped QRS
Wide QRS (>120ms)
The axis can be deviated either way in BBB’s, but it is most commonly normal
Sinus bradycardia
Normal rhythm <60bpm
Associated with; athletic training, fainting, hypothermia, myxedema (hypothyroidism), seen immediately after MI
Sinus Tachycardia
Normal rhythm >100bpm
Associated with; exercise, fear, pain, haemorrhage, thyrotoxicosis
Supraventricular rhythms
This is any rhythm that originates outside the ventricle
Examples include:
-          Sinus rhythms
-          LBBB
-          RBBB
Ventricular rhythms
(aka escape rhythms)
Atrial escape
Junctional escape
Ventricular escape
Accelerated idioventricular rhythm
Wide QRS complexes
Abnormal p wave (e.g. inverted)
Normal QRS
Some normal beats after the abnormal one
This occurs when the SA node fails to depolarise. Instead, some other part of the atrium depolarises and sends the signal to the ventricles.
No p waves
Normal QRS
Slightly slow rate (max 75bpm)
The escape occurs somewhere at the AV junction. It occurs when the rate of depolarisation of the SA node falls below the rate of the AV node, thus the AV node starts the beat instead. The resulting bradycardia reduces cardiac output and can cause symptoms similar to other bradycardias such as:
-          Dizziness
-          Light-headedness
-          Syncope
-          Hypotension
Usually the bradycardia can be tolerated as long as it is above 50bpm
Two types:
-          Many p waves per QRS (complete heart block)
-          Occasional missing p wave, followed by long gap, and then a ventricular QRS, then normal rhythm
Somewhere along the line the p waves isn’t getting conducted to the ventricles, and thus the ventricles depolarise at their normal escape rate.
Wide QRS
Rhythm of about 75bpm
No p waves
Abnormal T waves
Don’t confuse this with ventricular tachycardia – which requires a HR of >125pbm. Otherwise it looks very similar.
Usually benign and does not need to be treated. Also associated with MI
(aka ectopics)
These are easy – they are the same as ventricular escapes, except that where in escapes the escape beat comes after a pause in the rhythm, in extrasystole, there is an abnormal beat earlier than expected.
The QRS complexes are the same as those of sinus rhythm, but there are usually abnormal p waves that tend to come immediately before or immediately after the QRS.
Inferior MI
(probably the right coronary artery)
ST elevation
II, III, aVF (the inferior leads)
The ST elevation in these leads is often accompanied by ST depression in the antero-lateral leads – V1-V6, and possibly in lead I and aVL
Anterior MI
(probably the left anterior descending)
ST elevation
V2-5 – the anterior leads
This will also cause deep q waves. The presence of Q waves implies a full thickness infarction.
Posterior MI
ST depression, tall R waves
Posterior MI is unusual! The changes that occur are opposite to the changes of other type of MI. thus the tall R waves are the opposite of Q waves (remember Q waves are negative), and ST depression occurs in place of ST elevation
ST elevation MI
ST elevation >2mm in 2+ chest leads OR >1mm in 2+ limb leads,
T-wave inversion (after several hours)
Pathological Q waves (24 hours +)
T wave inversion occurs within a few hours of MI, pathological Q waves occur several days after initial MI
Both factors, if they occur, are usually permanent. In a full thickness infarction then there are pathological Q waves, and T wave inversion, but in a non-full thickness MI then there is only T wave inversion. The differentiation between full /thickness and non full thickness is pretty much the same as ST elevation / non-ST elevation
Pathological Q waves only
Ventricular tachycardia
Wide QRS, no p waves, T waves difficult to identify, rate >200bpm
Can be difficult to differentiate from BBB. BBB has p waves, and a QRS generally 120-160ms. VT is more likely scenario after MI, and has QRS >160ms
Supraventricular tachycardia
Narrow QRS
Ventricular fibrillation
No discernable pattern, no QRS, no P, no T
Patient is very likely to lose consciousness – thus the diagnosis is easy!
Delta waves present, right axis deviation, short PR interval, short QRS
Accessory pathway, usually from the left atria to the left ventricle allows direct transition of the signal, bypassing the AV node, hence the shortened PR interval. It has a risk of mortality as it can cause re-entry tachycardia; however, most patients are symptomless and live with no problems.
The digoxin effect
Depression of ST, inverted T waves
This causes a sloping ST segment that has a ‘reversed tick’ look. This occurs because digoxin blocks the na/K pump, which increases intracellular Ca2+ concentrations. (similarly, ischaemia causes reduced production of ATP, and thus reduced pump activity)
T wave inversion (rare: also ST elevation)
If ST elevation does occur, then the ST waves will appear ‘saddle shaped’ thus helping you to differentiate it from MI. also, the elevation in MI tends to be confined to a certain area, but in pericarditis, it is widespread
P pulmonale
Tall ,peaked T waves, p wave height >2mm in lead II
Lead II
Seen in cor pulmonale, or pretty much anything that causes right atrial enlargement (or hypertrophy)such as tricuspid stenosis or pulmonary hypertension
Bifid P waves (‘P-Mitrale’)
P waves with two peaks, broad – looks like an ‘M’; hence the name ‘Mitrale’
Left ventricular hypertrophy
Bi-phasic T waves
T waves with t peaks
Can occur as a result of MI
Prolonged QT interval
Prolonged QT
The corrected QT, is the QT interval as it would be at 60bpm. if this is long, then there is a risk of sudden cardiac death. It can be congenital, but also caused by drugs
Wide, tall, ‘tented’ T waves, shortened/absent ST segment, small or absent p waves, wide QRS
Can lead to VF and AF
Left ventricular hypertrophy
S wave in V1 or V2 >35mm AND R wave in V5 or V6 >35mm                                             R in aVF >20mm
R in aVL >11mm                                                                                                                     Any chest lead >45mm
R in lead I >12mm                                                                                                                 
Occasional P waves, not related to QRS, QRS precede by large spike, QRS complexes broad
The large spike is pacemaker stimulus. The QRS’s are wide because the stimulus originates in the ventricles


Axis deviation
Lead I
Lead II
aVR should always be negative!
If it is positive,it is called north-west axis. it could be due to incorrect limb lead placement, dextrocardia, or artificial pacing, due to the pacemaker wire - this enters the heart at the apex.
Carotid sinus pressure
By applying pressure to the carotid sinus you can stimulate the AV and SA nodes via vagal stimulation. This will reduce the frequency of discharge of the SA node, and increase the time of conduction across the AV node.
Thus, by applying pressure to the carotid sinus you can:
  • Reduce the rate of some arrhythmias
  • Completely stop some arrhythmias
  • It will have NO EFFECT ON VENTRICULAR TACHYCARDIAS – thus is can help you differentiate.
Applying the pressure basically reduces the frequency of QRS complexes, and allows the underlying atrial arrhythmia to become more visible.