Free ECG Course — Kraemer Academy
> Spent money and still haven't learned ECG? Does every ECG course feel the same to you? Have you given up? Tired of unfulfilled promises on this topic?
Well, you've never seen a course like this one from the Kraemer Academy with Professor Dr. Alessandro Kraemer, Cardiologist and Electrophysiologist specializing in ECG for 30 years teaching his students in Curitiba, Brazil. This course is now available to you for free!
It is part of a larger course delivered at Universidade Positivo, using the exact same slides presented to in-person students. Enjoy! It's free. But watch it from the beginning.
Are we really starting with the "worst" topic — the electrical axis? Yes, and it's actually the BEST way to start learning in Kraemer Academy's "disruptive mode." You'll be surprised. Just watch it to the end. To the very end, OK?
Afterwards, don't forget to leave your LIKE and a comment. We'd love to know you enjoyed it! The video below gives access to the full playlist:
▶ Watch the video — Full Playlist
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About this Document
Free ECG Course for Electrocardiogram Interpretation (Updated 2026) — A complete and definitive guide for students, beginners, and "Nubs," totally free and top-quality!
This document consolidates the fundamental ECG lessons, organizing them in a structured, didactic, and progressive way, with a focus on practical clinical interpretation — the same content presented to 6th-year Medical students at Universidade Positivo in Curitiba, Brazil.
You can watch everything described in this document on the TEMECG! by Kraemer Academy YouTube channels.
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1. ECG Fundamentals
The ECG machine measures voltage from the heart's electrical activity.
| Phenomenon | Definition | Mechanical result |
|---|---|---|
| Depolarization | Electrical activation of cardiac cells | Systole |
| Repolarization | Return to the baseline electrical state | Diastole |
Our free ECG course starts with the electrical axis on purpose — it's exactly where students get lost from the very beginning of medical school. Don't worry: you will learn this topic now in a disruptive way that will impress you from the very first video.
> 💡 Tip: By watching the electrical axis series, you'll be able to interpret an ECG like a cardiologist by the seventh video — in just a few seconds of analysis!
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2. ECG Waves and Complexes
P Wave
Atrial depolarization. Although it appears as a single wave, it actually contains two P waves: one for the right atrium and one for the left atrium.
QRS Complex
Ventricular depolarization. Used as a synonym for "ventricle." The term "QRS" is used regardless of whether all three typical waves are present — a "QRS" can be only an "R," in which case we say the "QRS has an R pattern."
Ventricular T Wave
Ventricular repolarization. Represents ventricular diastole.
Atrial T Wave
Atrial repolarization. Not visible on the standard ECG because it occurs simultaneously with the QRS complex. It is a wave opposite to the P wave and adjacent to it — it becomes visible only in complete AV blocks, when the QRS separates from the P wave.
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Why is the T wave not the mirror image of the QRS?
In the ventricles, depolarization travels through the His-Purkinje system, while repolarization uses the ventricular syncytium (which is slower). Repolarization travels "in reverse" from epicardium to endocardium — producing a vector identical to depolarization — causing the T wave to have the same polarity as the dominant QRS, but broader.
This has a fundamental consequence for ischemia analysis:
1. The non-ischemic T wave and its narrow QRS will always be concordant in polarity.
2. The non-ischemic T wave and its wide QRS will always be discordant in polarity.
3. Exception: V1, V2, and V3 may follow any pattern — always interpret in the clinical context.
> More on this in the Coronary Syndrome section.
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3. QRS Complex Morphologies
The QRS complex can assume different morphologies:
| Morphology | Notes |
|---|---|
| qRs | Small q, dominant R, small s |
| Rs | Dominant R with small s |
| R | R wave only |
| qR | Small q with dominant R |
| Qr | Pathological Q with small r |
| QS | No R wave — semiological value of a pathological Q wave |
| Rsr' | "Rabbit ear" pattern — characteristic of RBBB in V1 |
| Rsr's' | "Fragmented" QRS — small foci of necrosis or electrical tissue loss (storage diseases, Chagas disease, etc.) |
QRS Width
- Narrow QRS: < 3 small squares — normal pattern.
- Wide QRS: > 3 small squares — defines Bundle Branch Block (if sinus/supraventricular rhythm) or ventricular arrhythmia (usually Ventricular Tachycardia).
q and Q Waves
| Wave | Characteristics | Meaning |
|---|---|---|
| q (lowercase) | < 1 mm in width and/or height | Always physiological (narrow QRS) |
| Q (uppercase) | > 1 mm in width and/or height | Always pathological (narrow QRS) |
> When a Q wave is pathological but still smaller than R, the QRS is classified as qR with pathological Q.
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4. Intervals and Segments
PR Interval
Normal values: 3 to 5 small squares.
| Value | Meaning |
|---|---|
| < 3 small squares | Pre-excitation syndromes (e.g., Wolff-Parkinson-White) |
| 3 to 5 small squares | Normal |
| > 5 small squares | Atrioventricular blocks (1st, 2nd, or 3rd degree) |
PR Segment
The segment between the end of the P wave and the beginning of the QRS (a flat line). It marks the ECG baseline used to determine whether the ST segment is elevated or depressed.
QT Interval
Encompasses both ventricular depolarization and repolarization — represents the complete cardiac cycle. It is the most difficult interval to measure on the ECG.
Mandatory measurement in:
- Patients with syncope
- Poly-medicated patients (risk of sudden death from Torsades de Pointes)
> 💡 Use the qtc.mocha.app application to safely and quickly measure QTc by photographing your ECG.
▶ How to use the QTc app (narrow QRS)
!QT Nomogram using Bazett's formula in millimeters
Kraemer Academy simplified QTc calculation by eliminating formulas from the workflow. Just count the small squares for the QT and RR intervals and plot them on the nomogram above — or use the app:
▶ How to use the QTc app (narrow QRS — second tutorial)
#### QTc with Wide QRS (Bundle Branch Block)
When the QRS is wide, the QTc must first be corrected using the Bogossian formula before applying heart rate correction formulas. The qtc.mocha.app automates this entire process: measure the QT as usual and request the Bogossian correction at the end.
▶ QTc with wide QRS — full tutorial
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5. Atrioventricular Blocks (AVB)
Types of AVB
1st-degree AVB
PR persistently prolonged (> 5 small squares). All P waves conduct.
2nd-degree AVB — Mobitz I (Wenckebach)
Progressive PR prolongation until a P wave fails to conduct. The next P wave restarts the cycle with the shortest PR in the sequence. This decremental behavior is typical of the AV node — a less severe conduction defect. Can be caused by negative chronotropic drugs; withdrawing the medication usually resolves it.
2nd-degree AVB — Mobitz II
P wave is blocked without progressive PR prolongation. This "all-or-nothing" behavior is characteristic of the His bundle — a more severe defect. Negative chronotropic drugs do not affect the His bundle.
2nd-degree AVB — 2:1
Two P waves for each QRS. Cannot distinguish Mobitz I from Mobitz II without two preceding PR intervals before the blocked P wave.
2nd-degree AVB — 3:1 or higher
Three or more P waves per QRS — called advanced atrioventricular block (not to be confused with complete heart block).
3rd-degree AVB (Complete Heart Block)
AV dissociation with a possible escape rhythm. With escape beats present, differentiate from other degrees by searching for false short PR intervals (PR < 3 small squares in a tracing with ventricular bradycardia).
> ⚠️ In AV blocks, the P wave always has an equal or higher heart rate than the QRS. If the P rate is lower than the QRS rate, the problem lies in the sinus node's ability to command the ventricles (sick sinus syndrome or sinus arrest).
Pacemaker Indication — General Guidance
| Degree of AVB | Pacemaker indication |
|---|---|
| 1st-degree AVB | ❌ No |
| 2nd-degree Mobitz I | ❌ No |
| 2nd-degree Mobitz II | ✅ Yes |
| Advanced AVB | ✅ Yes |
| 3rd-degree AVB | ✅ Yes |
> ⚠️ Warning: This table is a general mnemonic that may lead to incorrect decisions. Always consult the guidelines (AHA, SOBRAC, SBC). Do not rely on mnemonics as a sole reference.
▶ Video summary — ECG from Zero (TEMECG! channel)
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7. Calculating Heart Rate on the ECG
One of the most practical ways to calculate heart rate (HR) on the ECG is through rule-of-three methods using preset time references.
7.1 Using the Full ECG Strip (10 seconds)
A standard ECG strip = 10 seconds = 50 large squares of 200 ms each.
Method: count the QRS complexes on the strip and multiply by 6.
> Example: 10 QRS complexes in 10 seconds → 10 × 6 = 60 bpm
This method is especially useful for irregular rhythms.
▶ Heart rate calculation — explanatory video
7.2 Using the Interval Between Two Beats
With just two consecutive beats, apply one of the formulas below:
```
1500 ÷ (number of small squares between beats) = HR in bpm
300 ÷ (number of large squares between beats) = HR in bpm
60,000 ÷ (milliseconds between beats) = HR in bpm
```
Mnemonic sequence for large square intervals:
> 300 → 150 → 100 → 75 → 60 → 50 → 40 bpm
▶ Full heart rate series on the ECG
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8. Heart Rate Calculation in Very Slow Rhythms
In extremely slow rhythms, it is more appropriate to express pauses in seconds rather than bpm.
8.1 Measuring the Pause
> Example: interval between QRS complexes spanning 18 large squares → 18 × 200 ms = 3,600 ms = 3.6 seconds
>
> Report as: "pauses of up to 3.6 seconds"
This approach is particularly relevant in advanced blocks, escape rhythms, and pacemaker implantation assessment.
▶ Slow rhythms — explanatory video
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9. Conceptual Integration: Applied Electrical Fundamentals
The ECG records cardiac electrical activity captured at the body surface — it measures differences in electrical potential generated by myocardial depolarization and repolarization, allowing graphical representation of the resulting vectors.
▶ How the ECG works — why Einthoven won the Nobel Prize
9.1 Depolarization and QRS Complex Formation
Ventricular depolarization occurs in an organized fashion through the His-Purkinje system. The vector sequence determines QRS morphology in each lead.
- Narrow QRS (< 3 small squares) → preserved intraventricular conduction.
- Wide QRS → bundle branch block or ventricular origin (non-supraventricular rhythm).
▶ QRS formation — ECG from Zero video
9.2 Ventricular Repolarization and the T Wave
Ventricular repolarization generates a T wave that is typically concordant with the QRS — because the spatial sequence of repolarization is opposite to depolarization, producing a similar final electrical vector.
Fundamental practical rules:
| Situation | T / QRS relationship | Interpretation |
|---|---|---|
| Narrow QRS + concordant T | ✅ Normal | Expected repolarization |
| Narrow QRS + discordant T | ⚠️ Suspect ischemia | Evaluate context |
| Wide QRS + discordant T | ✅ Normal | Appropriate discordance |
| Wide QRS + concordant T | ⚠️ Suspect ischemia | Evaluate context |
> ⚠️ Attention to V1, V2, and V3: the rule above has exceptions in these leads — always interpret in the clinical context.
Extension of the T wave rule to the ST segment (valid for LBBB):
| ST vs. wide QRS (LBBB) | Interpretation |
|---|---|
| Discordant with wide QRS (LBBB) | Normal (expected discordance) |
| Concordant with wide QRS (LBBB) | Suspect Acute Coronary Occlusion |
> ⚠️ In RBBB, ST elevation is always equivalent to an acute coronary occlusion; ST depression does not necessarily represent an occlusion.
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10. QRS Electrical Axis in 1 Second — Visual Cartesian Method
Calculating the electrical axis does not require precise determination in degrees. In clinical practice, simply identify the dominant vector quadrant.
!The axis is a boy. The lead is a girl. Girls register positivity when boys approach
10.1 Cartesian System Applied to the ECG
The positive pole of each lead ("girl") is always positive. The vector ("boy") either approaches or moves away from it. Einthoven strategically positioned the electrodes so that the normal vector (directed downward and to the left) would register as positive.
| Lead | Axis represented | Electrode position |
|---|---|---|
| I (D1) | X axis | Left arm (left positive, right negative) |
| aVF | Y axis | Foot (inferior positive, superior negative) |
| V1 | Anteroposterior dimension | Anterior positive, posterior negative |
10.2 Rapid Quadrant Determination
| Lead I | aVF | Quadrant | Axis |
|---|---|---|---|
| ➕ | ➕ | Inferior-left | Normal |
| ➕ | ➖ | Superior-left | Left axis deviation |
| ➖ | ➕ | Inferior-right | Right axis deviation |
| ➖ | ➖ | Superior-right | Extreme / indeterminate axis |
> The normal axis is between +90° and −30° in the frontal plane.
▶ Electrical axis — video 3 of the TEMECG! series
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11. The −30° Inconvenience and the Practical Solution
The lower boundary of the normal axis (−30°) can be confusing: a negative QRS in aVF does not always indicate pathological left axis deviation.
11.1 Strategy Using Lead II (D2)
Observe Lead II in addition to Lead I:
- Lead I and Lead II both positive (or isoelectric) → axis is necessarily between +90° and −30° → normal.
- Lead II negative → true left axis deviation.
- Lead I negative → consider right or extreme axis deviation.
- Lead III may occasionally be negative even with a normal axis.
> 💡 The combined observation of Lead I + Lead II + aVF resolves the axis calculation in approximately 1 second, with no angular computation needed.
▶ The −30° inconvenience — video 4 of the TEMECG! series
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12. Rapid Diagnosis in the Emergency Department Using the Electrical Axis
Most abnormal sinus ECGs can be initially stratified using just two pieces of information: axis + QRS width.
12.1 Core Premises
1. The normal heart is rotated to the left and anteriorly.
2. Narrow QRS (< 120 ms) → suggests abnormality due to hypertrophy.
3. Wide QRS (> 120 ms) → suggests bundle branch block.
12.2 Diagnosis by Axis and Width
| QRS | Axis | Likely diagnosis |
|---|---|---|
| Narrow | Left / inferior / posterior + high amplitude | Left Ventricular Overload |
| Narrow | Right / anterior | Right Ventricular Overload |
| Wide | Left / posterior | Left Bundle Branch Block |
| Wide (terminal portion rightward/anterior) | Right / anterior | Right Bundle Branch Block |
12.3 Left Fascicular Blocks
| Block | Lead I | aVF |
|---|---|---|
| Left Anterior Fascicular Block | ➕ | ➖ |
| Left Posterior Fascicular Block | ➖ | ➕ |
> ⚠️ Other fascicular combinations must be interpreted in the clinical context.
12.4 Is the P Wave Always Sinus?
The P wave is not synonymous with sinus rhythm. For it to be sinus, the P wave's electrical axis must point downward, to the left, and posteriorly (small in V1). In V1, the normal P wave is biphasic (plus-minus):
- Positive component (forward, toward the right atrium): if > 1 mm → right atrial overload.
- Negative component (backward, toward the left atrium): if > 1 mm → left atrial overload (Morris index).
If the P wave axis does not point inferior-left-posterior → ectopic atrial rhythm (non-sinus). If above 100 bpm → ectopic atrial tachycardia.
▶ P wave and sinus rhythm — TEMECG! video
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Conclusion: What Do I Do Now That I Can Interpret the Electrical Axis?
You have just learned to interpret ~80% of emergency department and office ECGs. In practice:
- Normal ECG → conclude the basic analysis.
- Abnormal ECG → probable cardiomyopathy → refer to a cardiologist.
- Syncope + bundle branch block → possible intermittent complete heart block → call a cardiologist; consider electrophysiology study to measure the HV interval.
- Syncope + AVB present → possible more advanced intermittent AVB → order Holter monitoring and refer to cardiology.
- Chest pain + bundle branch block → apply the T wave and ST concordance rules (LBBB/RBBB) to identify acute myocardial infarction with coronary occlusion.
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17. Coronary Syndrome on the ECG
Interpreting coronary syndrome requires an integrated understanding of the relationship between QRS complex, ST segment, and T wave, taking QRS width and clinical context into account.
17.1 T Wave Rule — General Summary
| QRS | T wave vs. QRS | Interpretation |
|---|---|---|
| Narrow | Concordant | ✅ Normal |
| Narrow | Discordant | ⚠️ Suspect ischemia |
| Wide (LBBB) | Discordant | ✅ Normal (appropriate discordance) |
| Wide (LBBB) | Concordant | ⚠️ Suspect ischemia |
| Wide (RBBB) | Discordant (terminal portion) | ✅ Normal |
| Wide (RBBB) | Concordant (terminal portion) | ⚠️ Suspect ischemia |
> ⚠️ Exception: V1, V2, and V3 may show physiological variations — always interpret in the clinical context.
In LBBB, the T wave rule also applies to the ST segment: discordant ST elevation or depression is expected; concordance suggests Acute Coronary Occlusion (ACO).
17.2 International Definition of STEMI
ST elevation ≥ 1 mm in at least two contiguous leads of the same wall (always with narrow QRS).
Exception in V2–V3:
| Group | Criterion |
|---|---|
| Women | ≥ 1.5 mm |
| Men > 40 years | ≥ 2.0 mm |
| Men < 40 years | ≥ 2.5 mm |
> The presence of ST depression in other leads reinforces that the ST elevation is a true positive for ACO.
17.3 ACO and SACO — New Concept
Acute myocardial infarction is now understood as a spectrum of coronary occlusion:
- ACO — Acute Coronary Occlusion (100%)
- SACO — Subacute Coronary Occlusion (~99%)
Patterns such as Wellens and De Winter frequently represent severe sub-occlusion states.
> 💡 The hyperacute T wave ("shark fin") is the earliest pattern of ACO — it precedes ST elevation and is very difficult to diagnose. Artificial intelligence applications outperform human diagnosis for this pattern. Kraemer Academy is developing an AI app trained to detect hyperacute T waves — stay tuned for its launch.
17.4 Classic High-Risk Patterns
- "Happy" ST elevation → does not rule out ACO; repeat ECG in 10 minutes if in doubt.
- Wellens (type A and B) → critical LAD sub-occlusion (severe SACO from spontaneous recanalization of an ACO).
- Pardee pattern → typical transmural ST elevation.
- De Winter → equivalent of very severe SACO (possible left main coronary or equivalent — frequently associated with ST elevation in aVR).
- Hyperacute T wave → initial phase of STEMI — difficult to diagnose for humans.
- Disproportionate ST elevation in hypertrophy.
17.5 AMI versus Pericarditis
Features favoring AMI with ST elevation:
- ST depression (except in V1 or aVR)
- ST elevation in Lead III greater than in Lead II
- Horizontal or convex ST elevation
- Absence of PR depression
- Absence of Spodick's sign
> 🧠 Kraemer Academy mnemonic: "Pericarditis is afraid of the rabbit accelerating in V1 and listening to its neighbor aVR — pericarditis is always happy but hates the rabbit revving its engine."
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18. Tachyarrhythmias on the ECG
Tachyarrhythmias are rhythms with HR above 100 bpm (RR interval < 600 ms). They require systematic analysis of:
- QRS width
- RR interval regularity
- Relationship between P waves and QRS complexes
18.1 Initial Classification by QRS
| QRS | Likely origin |
|---|---|
| Narrow | Supraventricular (uses His-Purkinje system) |
| Wide monomorphic | Ventricular or supraventricular with aberrancy |
| Wide polymorphic | Always ventricular (polymorphic VT, VF, or Torsades) |
Differentiating polymorphic wide QRS by context:
- Prolonged QT → Torsades de Pointes
- ST elevation present → Ventricular Fibrillation
- Structurally normal heart → Catecholaminergic VT
> 💡 Two-Rhythm Concept: always consider an atrial rhythm (P waves) and a ventricular rhythm (QRS). AV dissociation strongly suggests Ventricular Tachycardia.
18.2 Atrial Fibrillation and Flutter — The "Magic Number" 200 ms
On ECGs with irregular RR, use the FF interval (between flutter/fibrillation waves):
| FF Interval | Diagnosis |
|---|---|
| < 200 ms | Atrial Fibrillation |
| 200–250 ms | Atrial Flutter |
| > 250 ms | Atrial Tachycardia |
Atrial fibrillation is characterized by chaotic f waves and an irregularly irregular RR interval.
18.3 Atrial Premature Beats and Atrial Fibrillation
Very early atrial premature beats can trigger atrial fibrillation. They may be conducted with a narrow or wide QRS (aberrancy) and, when non-conducted, can mimic 2:1 AV block.
18.4 PSVT — Paroxysmal Supraventricular Tachycardia
Mainly includes:
- AVNRT — AV Nodal Reentrant Tachycardia
- Accessory pathway tachycardia (concealed or manifest — WPW)
- Focal atrial tachycardias
| RP' interval | Most likely diagnosis |
|---|---|
| RP' < 70 ms | AVNRT |
| RP' > 70 ms | Accessory pathway |
> Most PSVTs revert with adenosine. Treatment failure suggests an alternative diagnosis.
18.5 Wolff-Parkinson-White Syndrome (WPW)
Characterized by a delta wave and a short PR interval. During orthodromic tachycardia, the delta wave may disappear. Catheter ablation is the curative treatment.
18.6 Ventricular Tachycardias
- Wide monomorphic QRS with AV dissociation → VT.
- Fusion beats or sinus capture beats confirm ventricular origin.
- Structural heart disease significantly increases the probability of VT.
After polymorphic VT or VF, evaluate QTc in sinus rhythm to investigate long QT syndrome.
> ⚠️ Important: We no longer recommend using Brugada criteria (or derivatives) for the definitive diagnosis of wide QRS tachycardias. Pre-test probability and clinical context are more relevant. Electrophysiology study is essential for both diagnosis and curative treatment.
For invasive diagnosis, Kraemer Academy and the Electrophysiology Laboratory of Curitiba created an exclusive application for use in the EP lab:
> lec.mocha.app — 100% diagnostic accuracy for inducible arrhythmias during invasive electrophysiology study. A complete literature review of the past 10 years (through 2026), developed by Alessandro Kraemer without the use of artificial intelligence.
Conclusion on Tachyarrhythmias
Interpreting tachyarrhythmias should prioritize:
1. QRS width
2. Rhythm regularity
3. Presence or absence of AV dissociation
Clinical reasoning outperforms complex algorithms — pre-test probability and clinical context are the determinants of a correct diagnosis.
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About the Author
Alessandro Kraemer is a clinical cardiologist and invasive electrophysiologist. Professor of ECG at Universidade Positivo (6th year of Medical School, top score on ENAMED). He works at the LEC — Electrophysiology Laboratory of Curitiba.
He is the author of "The Art of Electrocardiography" (ISBN 9792471000061), a collection of 8 ECG books teaching all levels simultaneously.
🗓️ Schedule a medical appointment
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ECG IN 24H HOLTER MONITORING
KRAEMER, A. (2017). Identification of the site of origin of ventricular premature beats by 24-hour Holter monitoring.
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ATYPICAL ACCESSORY PATHWAYS
Arq. Bras. Cardiol. 80 (1) • Jan 2003
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LONG QT INTERVAL
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PACEMAKER ON THE ECG
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