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ECG Interpretation: The Basics

This short introduction to ECGs uses a systematic approach to aid pattern recognition. It does not cover cardiac arrest. It outlines a basic framework for recognising some common findings. There is intentionally no reference to the Wolf Parkinson White syndrome. In eleven pages, a simple approach is presented which is inevitably not comprehensive and which assumes some prior knowledge of the ECG (eg orientation of leads and the relationship of normal complexes to the conducting system). After you have read this document, you should be familiar with the rate-rhythm-axis-p wave-qrs-t wave approach to looking at ECGs. Along the way you will have seen examples of:

• Sinus rhythm • Atrial fibrillation • Atrial flutter • Type 2 heart block (Wenckebach type) • Ventricular tachycardia • Left ventricular hypertrophy • Left bundle branch block • Right bundle branch block • Anteroseptal myocardial infarction • Anterior myocardial infarction • Inferior myocardial infarction • Inferolateral myocardial infarction

1) Rate

Divide 300 by the approximate number of big squares between complexes to get the rate. Now test yourself What is the rate in each of the ECG strips which follow? a)

b)

c)

d)

e)

Dr R Clarke www.askdoctorclarke.com 1

Answers Example Number of big squares

between complexes (approximately) Rate (beats per minute) Approximately

a 4.5 60-70b 7 40c 1.5 200d 2 150e 6 50 2) Rhythm

• ? P wave before each QRS- sinus rhythm If long PR interval, this is first degree heart block (see later). If progressively increasing PR interval and then a dropped beat, this is a form of second degree heart block (Wenckebach phenomenon)

• ? evidence of complete heart block with P waves and QRS complexes completely dissociated and very slow rate

• irregular with no clear P wave- atrial fibrillation • saw tooth pattern – atrial flutter (the atria flutter at 300/min, so each saw tooth P

wave occupies approximately one large square) • beware the patient with a heart rate of exactly 150 (2 large squares between

complexes): this is most likely atrial flutter with 2:1 block where every other saw tooth wave is masked by a QRS complex.

• extrasystoles- narrow QRS complex- atrial ectopics; broad QRS complex - ventricular ectopics

• if tachycardia, ?broad complexes or narrow (broad complex tachycardia can be due to ventricular tachycardia or atrial tachycardia with aberrant conduction- bundle branch block pattern)

Now test yourself What is the rhythm in each of the ECG samples which follow? a)

b)


c)

d)

e)

f)

g)

h)


Answers Example Rhythm Comment a) Second degree block

(Wenckebach) Progressive prolongation of P wave leading to a missed beat with a P wave and no subsequent QRS

b) Broad complex tachycardia Most likely ventricular tachycardia; could be atrial tachycardia with bundle branch block

c) Atrial fibrillation with fast ventricular rate

Irregularly irregular complexes

d) Atrial flutter with 9:1 block Saw tooth appearance e) Sinus rhythm Normal pattern f) Paced rhythm Sharp upstroke of

pacemaker potential before each broad complex

g) Atrial flutter with 2:1 block Rate exactly 150

Saw tooth appearance of baseline

h) Probably sinus rhythm with ventricular ectopic

Broad complex ectopic suggests ventricular origin

3) Axis This is the mean direction of the wave of ventricular depolarization (QRS) in the vertical plane. Look at I, II, III and see which has the maximal upward deflection (R wave). Normally either l or ll has the biggest upward deflection. Right axis deviation (RAD) is relatively rare- here III has a bigger R than II. Left axis deviation (LAD) is commoner than RAD. One-up-two-down A quick eyeball test is "one-up, two-down" ie the dominant deflection in I is an upward R wave, bigger than the downward S, and at the same time the dominant deflection in II is downwards ie dominant S wave bigger than the R wave. Think of an odd house advert- one up two down. Now test yourself Which of the following show left axis deviation? Only leads l, ll and lll are shown from each of four ECGs


a) b) c) d)


Answers The axis in a) and c) is normal, but in b) and d) shows the one-up-two down appearance of LAD

4) Now check each wave Check P wave ? normal shape (may be bifid in some patients with mitral valve disease, best seen in V1 and V2) ? flat or absent in hyperkalaemia (associated with tenting of T waves) ? regular in relation to QRS (if not, consider heart block) Check QRS duration Left bundle branch block Wider than 3 small squares with RSR pattern in I, avL and v5-6 This is always abnormal; often due to ischaemic heart disease, hypertension or cardiomyopathy. Its presence means that it is hard to interpret the ST segment and T waves- these are often abnormal as part of LBBB and so cannot be used to diagnose ischaemia, infarction or "strain". Serial ECGs can be very helpful here- if someone has a normal ECG and then develops LBBB as part of an infarct, this carries a worse prognosis. RBBB- wider than 3 small squares with RSR pattern in v1, v2; may well be within normal limits as common finding; 1 in 20 people have RBBB. May be associated with an atrial septal defect and may also be caused by ischaemia in older patients. (Partial RBBB/ LBBB means RSR pattern without prolongation of QRS to 3 small squares- not usually of major significance.) Check R wave size Left ventricular hypertrophy by electrical criteria exists when the R wave height in V6 is added to the S wave depth in V2 and comes to 7 big squares (35mm) (There are other criteria, but this is a reasonable rule of thumb) Q wave Small Q wave normal, but a deep Q may be a sign of full thickness infarct ST segment ST elevation of >2mm is a cardinal sign of myocardial infarction, though there are other rarer causes (pericarditis, ventricular aneurysm etc) ST depression This often indicates ischaemia. May be present with left ventricular hypertrophy; sometimes called LVH with strain; more precisely LVH with repolarisation abnormality T wave inversion Is normal in V1 and sometimes present in lead 111. T wave inversion may be part of left ventricular hypertrophy (LVH)- when it is called LVH with strain or (more accurately) LVH with repolarisation abnormality (as above). T waves often invert after myocardial infarction. When found alone, without other changes, they are a non-specific finding. Very tall tented T waves may suggest hyperkalaemia.

Now test yourself


What are the main abnormalities in these ECGs?

a)

b)

c)


Answers a) LVH with strain pattern LVH: R in V6 plus S in V2= more than 7 large squares “Strain”: inverted T waves and ST depression in l, ll and V4-V6 b) LBBB (broad complexes with M shape of RSR in l, V5 and V6) c) RBBB (with RSR- ie M shape- in V1)

Chest Pain and Myocardial Infarction (MI)

There are three main sources of evidence for an infarct : • History • Serial ECGs • Cardiac enzymes including troponins

It is important to remember that the ECG may be normal initially in about a third of those with MI. In a few people, no clear changes develop. This is sometimes called a non-ST elevation myocardial infarct (NSTEMI). The usual pattern is for ST elevation to occur early (usually within 24 hours) and then Q waves if the infarct is full thickness.

Q wave

ST elevation

Myocardial infarction

• Left main stem Large anterior V1-V6

• Left ant. descending Anteroseptal V1-V4

• Left circumflex Anterolateral 1, aVL, V4-6

• Right coronary Inferior 2, 3, aVF

The affected leads can give a clue to the likely vascular territory of the coronary thrombosis.

Circumflex

Left anterior descendingRight coronary artery

Left main stem


Now test yourself What are the main abnormalities in these ECGs? All these patients presented with chest pain. a)

b)


c)

d)

e)


Answers Example Comments a) Q waves and ST elevation in V2-V4: anteroseptal infarct b) Q waves and ST elevation in V2-V5: anterior infarct c) Q wave in 111 and ST elevation in ll, lll and aVF inferior infarct d) Q waves and ST elevation in ll, lll, aVf with ST elevation in V5 and V6:

inferolateral infarct. The circumflex artery is sometimes dominant in supplying the base of the heart and thrombosis then leads to this pattern, with both inferior and lateral changes.

e) V5 strip showing ST depression; similar changes in V4 and V6 suggested lateral ischaemia

Finally “clever things” ….if you've time on your hands or you're an enthusiast! Check PR interval- should be more than 3 small squares- if shorter, consider pre-excitation - only relevant in a patient with palpitations or funny turns where a tachyarrhythmia might be occurring. That is the only hint of Wolf Parksinson White in this document. Check PR again-? prolonged should be less than or equal to 5 small squares- if longer, ?beta blockers, ?first degree heart block , ? digoxin toxicity, ? hypokalaemia Check QT interval- the "ESR of the heart"- can increase with almost any condition, metabolic abnormality or drug. The upper limit normal at heart rate 60-70 is 420ms ie just over 2 big squares; should be less than half the R-R interval; main risk is when gets very long- over 600ms= 3 big squares, there is a risk of ventricular fibrillation (torsade de pointes). If very high consider hypocalcaemia, drugs such as antidepressants, phenothiazines, quinidine and cardiac disease eg ischaemia, myocarditis, left ventricular hypertrophy etc

Overview Rate: 300 divided by number of large squares between QRS complexes Rhythm: ?SR with P wave before each QRS ? atrial fibrillation with irregularly irregular

complexes ?saw tooth pattern of atrial flutter ?heart block Axis: ? one-up-two down appearance of LAD P wave: flat in hyperkalaemia, long PR is first degree block QRS: broad complexes suggests BBB ST: ? elevation suggesting MI

?depression suggesting repolarisation abnormality if associated with BBB or LVH. Suggests ischaemia without these.

Q: deep Q waves suggest full thickness infarct. T: inverted T normal in V1: elsewhere abnormal

If the only abnormality is a non-specific finding


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