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Tuesday, 23 September 2014

Autumn Class 1 - ECG Methodical Analysis. The Art of Bluffing

Firstly, The most important thing to know, is normal values.

Providing the paper speed is standard at 25mm/second, each small square = 0.04seconds.
So the only other thing you need to know, in order to correctly identify ECG abnormalities, is your 4x table! Simples.

One small square on a ECG trace (at 25mm/sec speed) = 0.04sec.

The P wave    0.08 - 0.11 seconds (2-3 small squares)

PR Interval    0.11- 0.20 seconds (3-5 small squares)

QRS complex 0.06 - 0.11 seconds (1.5 to 2.5 small squares)

QT Interval  0.36 - 0.44  (9-11 small squares)

The Basics of ECG Analysis

Call me sad, but when I see an ECG,  I like to play that ‘KEEP TALKING' Quizz game:

“Talk away for 3mins without hesitation,  interruption or repetition”

It’s a great exam technique for students – don’t let the examiner get a word in edgeways.  
Fill the time with FACTS, and don’t make guesses (for 3minutes, at least) and you can’t go wrong!

For every ECG, all you need to do is describe what you see and you’re half way there.
Always consider the following aspects, learn them by rote, and just talk away.

First ask yourself the following three questions, and then just talk away:

1. Is there electrical Activity seen?
2. Is this a 12 lead, or single lead, analysis?
3. Is the paper speed (25mm/s) and gain (1mv=1cm) standard?

Next comment on:

Atrial rhythm – regular or irregular
Ventricular rhythm – regular or irregular
Overall regularity – regular or irregular

Atrial rate (P waves)
Ventricular rate (QRS complexes)

Appearance (morphology),
Size  (<3mm tall)
Consistency (in appearance)
Relationship to QRS

P-R Interval
Consistency (does it alter between complexes?)

QRS Complex
Appearance – Morphology (narrow, duration < 3 small squares)
Consistency – same in all leads?
QT interval  

Morphology (rounded? tented? axis/orientation? - should point in the same direction as the associated R wave, but be of smaller amplitude)

ST Segment
Isoelectric (normal) or Depression? / Elevation?

Axis  (if 12 lead)

All of this is just factual stuff. You can talk away for 3minutes without even attempting to analyse, let alone have a stab at a diagnosis!

If you methodically consider the above whenever faced with an ECG, you’ll always end up with some sort of sensible diagnosis.  Even if you guess wrong, the intelligent banter that you've just displayed will as least make people think it was a very educated guess!

The main things to remember and to know by heart, are:

1. The lead appearances and orientation of a normal 12 lead ECG.
2. The normal values for size, and duration, of the waves forms.

So, for starters, here's a NORMAL 12 lead ECG:

Always bear in mind the direction of a normal electrical impulse through the myocardium, and the position of the electrodes on the external chest wall/limbs. That way, you should be able to remember, which lead complexes/waveforms should always be positive. 

For this reason the QRS wave aVR should always be negative.

Remember - ECG interpretation is an 'art form'. Factual descriptions, based on the above, can never be truly 'wrong'. 

Reassuring for us dummies  :-)

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