ECG Interpretation - Tachy or Brady, Rhythm, Axis, Intervals, ST Segment, T/U Waves

Oct 13, 2026

ECG Interpretation cheat sheet covering key ECG components and how to categorize abnormalities. It guides readers through rhythm classification (regular, irregularly irregular), rate assessment (tachycardia vs bradycardia), cardiac axis deviation using S greater than R rules, and interval/wave analysis including P-QRS relationships for AF, flutter, tachycardia with block, junctional tachycardia, and conduction blocks. It also summarizes Q and QRS wave significance, ST segment depression/elevation patterns for demand vs supply ischemia, and T and U wave interpretations for infarct timing, hyperkalemia, hypokalemia, and related patterns such as Wolff-Parkinson-White.

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ECG Interpretation

Rhythm and Rate

Cardiac Axis and Intervals

P Wave and Conduction Blocks

Q Wave and QRS Complex

ST Segment

T Wave and U Wave

put together by Alex Yartsev: Sorry if i used your images or data and forgot to reference you. Tell me who you are. [aleksei.igorevich@gmail.com](aleksei.igorevich@gmail.com) ECG Interpretation 1) RHYTHM: regular, regularly irregular, irregularly irregular 2) RATE: tachy or brady 4) CARDIAC AXIS DEVIATION: S greater than R in lead I = RIGHT AXIS S greater than R in lead II = LEFT AXIS Max QRS = 3 small squares PR = 1 big square 3) P wave =atria depolarising should be 1 P for every QRS: How many Ps per QRS? How long is the PQ interval? irregular P with irregular rhythm QRS = AF absent P with wide QRS = Ventricular Tachy absent P with narrow QRS = Junctional Tachy continuos undulating sawtooth baseline P = Atrial Flutter continuos with 2P per 1 QRS = Atrial Tachy with block bifid Long P waves = LA enlargement peaked tall P waves = RA enlargement normal rate, 2Ps per QRS = second degree block Progressive PQ lengthening = second degree block Long PQ interval = first degree block Ps don’t match to QRS, very brady = complete block No P wave but a solitary QRS = ventricular extrasystole 4) Q wave =septum depolarising or hole in conduction pattern HOW BIG? Normal unless large, Big Q wave = Infarct in the direction of THAT LEAD 5) QRS =ventricles depolarising; HOW BIG? Normal under 25mm, HOW WIDE? Hyperkalemia, BBB DEFORMED QRS? Huge tall QRS = LV hypertrophy Weak little QRS = old infarcted muscle RSR pattern (“M”) in V1 = Right Bundle Branch Block SRS pattern (“W”) in V1= Left Bundle Branch Block A “Delta”wave (gently up-sloping R) = = Wolff-Parkinson-White Syndrome 6) ST SEGMENT: DEPRESSED OR ELEVATED? Biggest ST points to the lesion Depressed = demand ischaemia, elevated = supply ischaemia V1 V6 P is the HEART BLOCK WAVE P is also the ENLARGED ATRIUM WAVE Q is the INFARCT WAVE QRS is the CARDIAC AXIS COMPASS ST is the ISCHAEMIA SEGMENT T is the HYPERKALEMIA WAVE U wave is the HYPOKALEMIA WAVE Long P = LAH; RSR = RBBB; ST Depression = Demand ischaemia RBBB LBBB Lead II looks from the NORMAL DIRECTION II, III, aVF 􀀁 inf. view V1, V2 = Rt Heart V3, V4 = Septum V5, V6 = Lt Heart Evolution of an infarct: ST􀀁 Q wave 12hrs later 􀀁 T inversion QRS in lead I is smaller and in lead II is bigger on inspiration Down-sloping ST = Digoxin therapy CONCAVE ST elevation in all leads, with elevated PR in aVR 􀀁 pericarditis 7) T wave =ventricles repolarising TALL? INVERTED?? WITH “U” WAVE??? inverted = infarct in last 24-48 hrs; without Q waves = Subendocardial infarct continuously painlessly inverted = LV hypertrophy with U wave = HYPOKALEMIA Tall T waves, Wide QRS, no ST segment = HYPERKALEMIA 9) U wave just a little bump on the end of the T wave = HYPOKALEMIA 􀀁 Wolff-Parkinson-White syndrome

FAQs

How do you interpret ECG rhythm and rate from this sheet?

Rhythm is classified as regular, regularly irregular, or irregularly irregular. Rate is assessed as tachy or brady.

How do you determine cardiac axis deviation using lead comparisons?

If S is greater than R in lead I, it indicates RIGHT axis. If S is greater than R in lead II, it indicates LEFT axis.

What does ST segment depression vs elevation indicate?

ST depressed corresponds to demand ischemia, while ST elevated corresponds to supply ischemia.

Oct 13, 2026

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