How to Read an ECG


Interpreting ECG’s is trickier than you think. One must have a system, preferably one that will stand up to the pummel of ED situations and environmental influences: time pressures, incomplete information, typhoons, that sort of thing.


The technique I employ has been honed over years of having a handful of seconds to come up with a germane and reasonably firmish electrocardiographic diagnosis. Mostly ECGs can be read by skidding; skimming the page with one of the currently unused quadrants of your vision (a kind of reverse homonymous quadrantinopia). This double-jointed vision is very useful for an Emergency Physician, and allows the remaining three free to scan 1) the patient, 2) the intern trying to cannulate a flailing arm in the next bed, 3) the ruckus swarming around a trolley being wheeled in from the ambulance bay, and 4) the monitor squealing the Very Important Alarm across the corridor. Realising that these don’t add up you discover you have not been looking at the ECG at all, and return to it, asking ‘so what did the patient come in with?’


Once you establish that the patient does not have VF or asystole, both of which are suboptimal to see on a 12 lead, you are free to enjoy the puzzle of ECG Interpretation. Mostly people rely on either Type 1 (heuristics, rule of thumb, pattern recognition – the chicken sexing sort) thinking, or Type 2 thinking, which involves the laborious but ultimately rewarding system of trawling through each element of the ECG (rate, rhythm, axis, intervals, ST’s yada yada). I recommend the use of Type 3 thinking* which involves a combination of panic (usually about something else, such as you not remembering the important piece of information imparted to you by a nurse which requires acting on Right Now), visual memory of the thousands of ECGs which have been flashed in front of you during your career, cross-referencing the ECG with the clinical vignette provided, obscure fascination with the history of ECGs leading to an unwavering joy that these random squiggly lines can tell you an accurate story about the heart, a mental search of Life in the Fast Lane’s rare ECG library, and glancing at the computer diagnosis to reassure you that your diagnosis is the compete opposite of theirs. This has a diagnostic success rate in the high 90s, so who could argue with it?


There is an opulence of texts and sites around to lead you through the specifics of each of the moieties of the ECG panorama. I recommend you avail yourself of one of these for the quotidian details.


To really understand something, though, one should pay homage to its history. It’s hard not to get excited by the scintillating backstory of the ECG with its 3 lead string galvanometric Einthovian beginning, which then evolved, barely, by the addition of several leads and a bit of Descartian mathematical wizardry.  What is really fascinating, however, is how prodigiously little the ECG has changed over a century and a half. The paper result looks essentially the same. This is an unusual occurrence in human history. Change is the hallmark of we bipedal beasts. It is thought that what split Homo erectus, and then the fancy new hominid, sapiens sapiens, from its slightly embarrassing Neanderthal relatives, was the desire and ability to constantly change, particularly when it came to tools. Tools did not change one iota for a million years, then along came these upstart creatures wanting to get into the sophisticates’ club, and we can see their tools improve generation by generation. We are hardwired to want to change things. This goes quite some way towards explaining the behaviour of government departments, who prize rebranding, renaming, rezoning, and generally shuffling about of names and roles. Although perhaps is this, this transition to the illusion of change, where evolution, indeed, stops? Could we blame the end of evolution on administrators?


A peculiar element of ECG interpretation is the appearance of completely ‘new’ diseases within the last decade or so – syndromes that were allegedly not around when those more wizened of us trained. Wellen’s syndrome, ST elevation in aVR, Brugada’s syndrome, Spodicks sign etc, as though these pathophysiological conditions have recently materialised out of nowhere. Where were they before? What did clinicians make of those distinctive ECG patterns? Did they just fob them off as non-specific changes, or simply ignore them as being wiggly annoyances?



Do check the leads are on the right way. Many a ponderously rare and fascinating diagnosis has been made by neglecting to recognise that pattern. If it’s all ass over elbow in the augmented unipolar leads then be suspicious.


Applying numerical criteria to ECG assessment should be done at your own risk. The sense of failure at STILL not being able to remember Brugada’s criteria holds a particularly heavy weight of shame. Other fanciful mathematical criteria can also be worked around by a robust system of alternatives. And a phone. Always carry a phone.


Make some leads your favourites. aVR is a good one for unveiling Very Bad Things. aVL is also rather nice, as even a sniff of sagginess in its ST’s is incredibly useful in the search for ischaemia. Not wishing to insult the others, but they really don’t pull as much weight, in my irregular opinion. If you are feeling adventurous, you can add more of them. Right sided, seven to nine, Lewis, all of them contributing more and more information.


Do not worry if you can’t tell the difference between some of the uncommon variants of heart block. Nobody can, although they may argue vociferously down the phone to the contrary.


Beware of the ECG shysters. Tremour artefact (do try to avoid shocking this), miniscule pacing spikes (no wonder you couldn’t apply the bloody Sgarbossa criteria), LVADs and heart transplants and machine malfunctions; all of these have a particular pattern, and are hilarious in ECG quizzes, but are not so amusing on the shop floor if unrecognised. Once you see a few though, your lovely brain will assist by storing them away in your hippocampal ECG drawer for retrieval, just when you need to look clever.


Let’s face it. The ECG is a formidable tool. To be able to translate the action of that muscle buried deep in the chest into a pulsing set of lines. It is the pounding, insistent song-sheet of the living. The heart as troubadour. One could argue that the ECG is an epic poem, Homeric perhaps, written in dactylic hexameter**, letting us read the thump of the heart, see it, diagnose the blocks and the beats and the hypertrophy and the strain and the weird things in the head and the strange things in the vessels, and probably new things in the future we haven’t even thought of. Next time you read one, fast, or slow, perhaps take a moment to consider its wonder.


*From the yet unpublished “Emergency Medicine: Thinking Fast and Weird”


** Now you may eye-rollingly dismiss this as just another of my faintly ridiculous metaphors, but bear with me. We thrive on metres. We comprehend things best in song. Dactylic refers to the fingers (long, short, short), and Homer wrote his magnificent epics (see, he too could be EPIC) with six of them per line. Very different from Shakespeare’s iambic pentameter (tee-TUM, five times over). Additionally, Homer was a master at creating visual-aural patterns to enable people to remember his poetry – consider the epithet ‘rosy-fingered dawn’. Dawn is only ever rosy-fingered, and is mentioned 20 times in the Odyssey. See? Actually, when it comes to it, perhaps you are correct after all.

About the author

Dr Michelle Johnston is a consultant Emergency Physician who works at an inner city hospital. Mostly her days consist of trauma and mess. Also, she writes.

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