ECG criteria
- Assess the patient's pulse and consciousness — an ECG pattern without clinical assessment is not diagnostic.
- Ventricular fibrillation (VF): chaotic waves without discernible QRS, wave frequency is variable, amplitude is variable (coarse and fine ventricular fibrillation)
- Ventricular flutter: regular sinusoidal waves usually around 200–300/min, QRS and T are indistinguishable
- Pulseless ventricular tachycardia: wide QRS (>120 ms), rate usually >100/min, monomorphic or polymorphic, but without a palpable pulse
- Asystole: absence of electrical activity, isoelectric line (check calibration and leads, rule out artifacts)
- Pulseless electrical activity (PEA): organized rhythm (may resemble sinus rhythm, narrow or wide QRS) without a pulse
- Life-threatening bradyarrhythmias: circulatory arrest, syncope, or hemodynamic instability associated with bradyarrhythmia — particularly complete AV block without an adequate escape rhythm and prolonged sinus pause/sinus arrest without an escape rhythm
- Polymorphic VT of the torsades de pointes type in the setting of prolonged QT — independently assess QTc before the episode
What it looks like on ECG
This is not a single pattern, but a group of rhythms united by clinical danger.
| Rhythm | HR | QRS | Regularity |
|---|---|---|---|
| Ventricular fibrillation | wave frequency cannot be reliably assessed | undetectable | chaotic |
| Ventricular flutter | usually 200–300/min | sine wave, QRS/T fused | regular |
| pulseless VT | >100/min | wide, >120 ms | regular or irregular |
| Torsades de pointes | 150–250/min | wide, "spindle" around isoelectric line | irregular amplitude |
| Asystole | 0 | absent | — |
| PEA | any | narrow or wide | organized |
| Complete AV block | atrial and ventricular rhythms are independent | depends on the level of the block | AV dissociation |
In complete (third-degree) AV block, atrial and ventricular rhythms are independent of each other, and there is no fixed P–QRS relationship: PR intervals vary, P waves "overlap" different parts of the ventricular cycle. The atrial rate is usually higher than the escape ventricular rhythm (junctional, about 40–60/min, or a slower ventricular rhythm). AV dissociation on its own cannot be considered a sufficient sign of complete block — it also occurs in ventricular tachycardia and accelerated junctional rhythm with retrograde block; therefore, it is important to confirm a regular independent atrial rhythm (which, however, does not have to be regular — for example, in concurrent atrial fibrillation) and the complete absence of conduction to the ventricles.
During monitor observation, evaluation is performed using a single lead (usually lead II), but to verify VT, differentiate it from SVT with aberrancy, and search for the cause (ACS, electrolytes, QT), a full 12-lead ECG is needed if the situation allows for it to be recorded.
Mechanism
The underlying mechanism is either re-entry in the setting of structural myocardial damage (post-infarction scar, cardiomyopathy), a severe disturbance of ionic currents (hyper-/hypokalemia, QT prolongation, ischemia), or disturbances in impulse formation and conduction in the sinus node and AV junction. In bradyarrhythmias, depression of sinus node automaticity, impaired impulse exit, or AV conduction block with subsequent inadequate escape rhythm may occur; in sinus arrest or sinus node dysfunction, subsidiary atrial, junctional, or ventricular escape rhythms may persist, while in complete AV block, atrial and ventricular activities are dissociated. VF is maintained by chaotic electrical activity, usually with multiple re-entry waves and/or focal triggers, resulting in the absence of coordinated ventricular contraction — the pumping function of the heart ceases completely. In asystole, electrical activity is completely absent. PEA is a paradoxical situation: an electrical impulse is formed and conducted, but mechanical contraction does not produce an effective output (massive PEA, tamponade, severe hypovolemia, severe acidosis).
Causes
- Acute coronary syndrome, especially STEMI and occlusion myocardial infarction (OMI)
- Structural cardiomyopathy (dilated, hypertrophic, ARVC) with scar-related re-entry
- Electrolyte disturbances: severe hyperkalemia, hypokalemia, hypomagnesemia
- Channelopathies: long QT syndrome, Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia
- Cardiotoxic drugs and poisonings (sodium channel blockers, QT-prolonging drugs)
- Severe hypothermia, hypoxia, severe acidosis
- Massive pulmonary embolism, cardiac tamponade, tension pneumothorax (reversible causes of PEA and asystole)
- Sinus node dysfunction, complete AV block without an adequate escape rhythm
Differential diagnosis
- Coarse VF vs. artifact (tremor, poor electrode contact) — with artifacts, the pulse is usually preserved and periods with a normal rhythm are visible
- Asystole vs. electrode disconnection or fully minimized gain — check calibration, another lead, and the clinical picture
- Pulseless VT vs. VT with a pulse — decided not by the ECG, but by pulse palpation and assessment of consciousness/breathing
- Fine VF vs. asystole — the distinction affects the algorithm (defibrillation is indicated for VF/pulseless VT, not for asystole)
- PEA vs. profound shock with a barely palpable pulse — check the pulse for no more than 10 seconds; if the pulse cannot be reliably determined, immediately initiate CPR
Clinical significance and management
Recognition of a life-threatening rhythm is an indication for the immediate initiation of resuscitation measures according to the cardiac arrest algorithm. For VF and pulseless VT, CPR and defibrillation are performed. For asystole and PEA, defibrillation is not indicated: high-quality CPR is performed, airway patency and ventilation are provided as indicated, epinephrine is administered according to the algorithm, rhythm is assessed every 2 minutes, and reversible causes are addressed (hypoxia, hypovolemia, hypo-/hyperkalemia and other electrolyte disturbances, hypothermia, tamponade, tension pneumothorax, coronary and pulmonary thrombosis, toxins). After restoration of spontaneous circulation (ROSC) — search for the cause: 12-lead ECG for STEMI/OMI, assessment of QTc, electrolytes, echocardiography. If STEMI or occlusion equivalents are identified, emergency revascularization is performed. In unstable complete AV block or recurrent Stokes-Adams attacks, urgent temporary pacing is required (transvenous if necessary); after excluding reversible causes (ischemia, electrolyte disturbances, drugs), indications for permanent pacing are evaluated.
Common interpretation errors
- Assessing rhythm without checking the patient's pulse and consciousness — the ECG picture alone does not diagnose cardiac arrest
- Confusing fine VF with asystole due to insufficient signal gain or recording in a single lead
- Attempting to precisely classify a wide complex rhythm (VT vs. aberrant SVT) instead of immediately initiating resuscitation in the absence of a pulse
- Ignoring reversible causes (4Hs and 4Ts) during repeated episodes of asystole/PEA
- Underestimating the significance of a prolonged QT interval before an episode of polymorphic VT (torsades de pointes)
- Diagnosing complete AV block based solely on independent atrial and ventricular rhythms without ruling out VT and other forms of AV dissociation
"4Hs and 4Ts" of reversible causes of cardiac arrest: Hypoxia, Hypovolemia, Hypo-/hyperkalemia and other metabolic disorders, Hypothermia — Tamponade, Thrombosis (coronary/pulmonary), Toxins, Tension pneumothorax
1. Which of the following rhythms indicates immediate defibrillation?
Defibrillation is indicated for VF and pulseless VT; in asystole and PEA, it is ineffective and not indicated.
2. What distinguishes pulseless electrical activity (PEA) from standard cardiac arrest with asystole?
PEA is the presence of organized electrical activity on the ECG (narrow or wide QRS) without effective mechanical contraction and pulse.
3. What mnemonic is used for the reversible causes of cardiac arrest?
4 Hs and 4 Ts (hypoxia, hypovolemia, hypo-/hyperkalemia and metabolic disorders, hypothermia — tamponade, thrombosis, toxins, tension pneumothorax) is the standard checklist of reversible causes that must be ruled out during resuscitation.
Frequently asked questions
Which rhythms are considered life-threatening on the ECG?
These include ventricular fibrillation and flutter, pulseless ventricular tachycardia, asystole, pulseless electrical activity, as well as severe bradyarrhythmias with cardiac arrest, syncope, or unstable hemodynamics — primarily complete AV block without an adequate escape rhythm and prolonged sinus pause without an escape rhythm.
How to distinguish VF from a monitor artifact?
In the case of an artifact (tremor, poor electrode contact), the pulse is usually preserved, and normal rhythm segments are periodically visible. If VF is suspected, check the pulse, electrodes, and multiple leads, as the amplitude and morphology of the waves depend on the lead and electrode contact. True VF is generally accompanied by the absence of an effective pulse and does not show organized QRS complexes.
Is asystole as dangerous as ventricular fibrillation?
Yes, both situations indicate an arrest of effective circulation, but the management differs: for VF and pulseless VT, defibrillation is indicated; for asystole and PEA, defibrillation is not indicated, and the emphasis is placed on chest compressions, adrenaline, and searching for a reversible cause.
Can a life-threatening rhythm be diagnosed based solely on the ECG, without examining the patient?
No. The exact same electrical pattern (e.g., an organized rhythm on the monitor) can be accompanied by either a preserved pulse or its complete absence (PEA). The diagnosis is always confirmed clinically by checking the pulse and consciousness.
What is the difference between VT with a pulse and pulseless VT on the ECG?
Electrocardiographically, they may be identical — wide QRS, high rate. The distinction is made exclusively based on the presence or absence of a palpable pulse and clinical signs of circulation in the patient.
ECG Course
Interpretation from basics to myocardial infarction — learn to read any ECG using a systematic algorithm.
- Video lessons and theory for every topic
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- Study guide: "All ECGs in 25 minutes"
ECG Strip Simulator
ECG, radiology, histology, pathomorphology, pharmacology — thousands of flashcards with detailed explanations.
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Related terms
More in the section "Ventricular Arrhythmias"
Sources
- Goldberger A.L. «Clinical Electrocardiography: A Simplified Approach»
- Hampton J.R. «The ECG Made Easy»
- Surawicz B., Knilans T. «Chou's Electrocardiography in Clinical Practice»
- Clinical guidelines for cardiopulmonary resuscitation (Russian Ministry of Health / European Resuscitation Council, current versions)
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.