EKG interpretation cheat sheet: a complete guide for nursing students

EKG interpretation is one of the most critical clinical skills for nurses working in telemetry, critical care, step-down, and emergency settings. This reference covers everything you need to recognize common rhythms, understand key intervals, and know when to act and when to escalate.

Bookmark it, print it, or share it. This is the reference page you will return to throughout nursing school and your early clinical years.

Quick reference: normal EKG intervals

Before interpreting any rhythm strip, anchor yourself to these normal values. Every deviation you identify will be measured against them.

Small squares on standard EKG paper: each small square = 40 ms (0.04 s) at standard paper speed of 25 mm/s. Each large square = 200 ms (0.20 s).

How to read an EKG: a reliable step-by-step approach

Use this sequence every time. Working through the same steps in the same order prevents you from jumping to a diagnosis before gathering all the data.

Step 1: Calculate the rate

For regular rhythms — the 300 method: Count the number of large squares between two consecutive R waves. Divide 300 by that number.

• 1 large square = 300 bpm
• 2 large squares = 150 bpm
• 3 large squares = 100 bpm
• 4 large squares = 75 bpm
• 5 large squares = 60 bpm
• 6 large squares = 50 bpm

For irregular rhythms — the 6-second method: Count the number of QRS complexes in a 6-second strip and multiply by 10. (Most rhythm strips have tick marks at 3-second intervals.) This gives a reliable rate range for atrial fibrillation and other irregular rhythms.

Alternate method: Count small squares between R waves and divide 1,500 by that number. More precise, but slower.

Step 2: Assess the rhythm

Is the rhythm regular or irregular?

Place a caliper (or the edge of a piece of paper) at two consecutive R waves. Slide across the strip. If the spacing is consistent throughout, the rhythm is regular. If it varies:

• Regularly irregular — a pattern exists (e.g., grouped beats in Wenckebach)
• Irregularly irregular — no discernible pattern (classic for atrial fibrillation)

Step 3: Evaluate P waves

Ask three questions:

1. Are P waves present?
2. Do they look the same (uniform morphology)?
3. Is there one P wave before every QRS complex?

Normal sinus P waves are upright and rounded in leads II and aVF, biphasic in V1. If P waves are absent, flutter waves are present, or P waves and QRS complexes have no relationship, you have a significant finding.

Step 4: Measure the PR interval

Count the small squares from the beginning of the P wave to the beginning of the QRS complex. Multiply by 0.04 s.

• Normal: 120–200 ms (3–5 small squares)
• Prolonged (>200 ms): AV conduction delay or heart block
• Short (<120 ms): accessory pathway (WPW), junctional rhythm

For a detailed breakdown of what a prolonged or shortened PR interval means clinically, see the PR interval reference guide.

Step 5: Examine the QRS complex

• Width: Normal <120 ms (less than 3 small squares). Wide QRS suggests bundle branch block, ventricular origin, hyperkalemia, or accessory pathway conduction.
• Morphology: Look for Q waves, R wave progression across precordial leads, and symmetry.
• Height: Very tall QRS in precordial leads may suggest ventricular hypertrophy.

Pathological Q waves (>40 ms wide or >2 mm deep in non-septal leads) suggest prior myocardial infarction.

Step 6: Examine the ST segment

The ST segment connects the QRS complex to the T wave and should sit at the isoelectric line (baseline).

• ST elevation: ≥1 mm in two or more contiguous limb leads, or ≥2 mm in two or more contiguous precordial leads = STEMI until proven otherwise
• ST depression: suggests myocardial ischemia or NSTEMI
• Downsloping ST depression: classic pattern of digoxin effect

Evaluate for reciprocal changes — ST depression in leads opposite to the territory of ST elevation strongly supports acute MI.

Step 7: Assess T waves

Normal T waves are slightly asymmetric with a gradual upstroke and faster downstroke, upright in most leads.

• T wave inversion: ischemia, ventricular hypertrophy, bundle branch block, digitalis effect
• Peaked (tall, narrow, symmetric) T waves: hyperkalemia — one of the earliest and most dangerous EKG signs of elevated potassium
• Flat T waves: hypokalemia, hypomagnesemia

Understanding T wave changes in the context of electrolyte status is covered in the electrolyte imbalances nursing reference.

Interval reference: what abnormal values mean

PR interval (normal: 120–200 ms)

For a full breakdown of heart block patterns and a clinical mnemonic to remember them, see the heart block poem.

QRS duration (normal: <120 ms)

QT interval (normal: <440 ms; QTc corrected for rate)

A prolonged QTc (>440 ms in men, >460 ms in women) increases the risk of torsades de pointes — a life-threatening polymorphic ventricular tachycardia.

Common causes of prolonged QT:

• Medications: amiodarone, sotalol, quinidine, haloperidol, many antibiotics (fluoroquinolones, macrolides)
• Electrolyte imbalances: hypokalemia, hypomagnesemia, hypocalcemia
• Hypothyroidism, myocarditis, acute MI
• Congenital long QT syndrome

Nursing action: Flag any patient with QTc >500 ms for urgent provider review. Review the medication list for QT-prolonging agents.

Common rhythms: quick reference table

Sinus rhythms

Clinical notes:

• Sinus bradycardia: Common in athletes, vagal tone, beta-blocker use, inferior MI, hypothyroidism. Treat only if symptomatic (hypotension, altered mental status, syncope).
• Sinus tachycardia: Always has a cause — pain, fever, hypovolemia, anxiety, sepsis, hyperthyroidism, PE. Treat the underlying cause, not the rate.
• Sinus arrhythmia: Rate increases with inspiration, decreases with expiration. Normal variant, especially in children and young adults.

Atrial arrhythmias

Clinical notes:

• AFib: The hallmark is “irregularly irregular” — no two R-R intervals are the same, and P waves are replaced by a chaotic fibrillatory baseline. Major risks: stroke (clot formation in left atrial appendage) and heart failure. Management depends on hemodynamic stability, duration, and rate/rhythm strategy.
• Atrial flutter: The sawtooth flutter waves at ~300/min are the diagnostic clue. Most commonly conducts at 2:1, giving a ventricular rate of approximately 150 bpm. Always investigate a heart rate around 150 bpm for flutter.
• SVT: A sudden-onset narrow-complex tachycardia at 150–250 bpm. Vagal maneuvers are first-line. If hemodynamically stable, adenosine 6 mg rapid IV push followed by 20 mL saline flush is the standard pharmacologic treatment.
• PACs: Isolated early beats, usually benign. Increased frequency can trigger sustained atrial arrhythmias in susceptible patients.

Ventricular arrhythmias

Clinical notes:

• PVCs: Characterized by wide, bizarre QRS complexes followed by a full compensatory pause. Bigeminy = every other beat is a PVC. Trigeminy = every third beat. Three or more consecutive PVCs = a run of V-tach. PVCs on a T wave (R-on-T phenomenon) can trigger V-fib.
• Ventricular tachycardia (V-tach): A wide-complex tachycardia at ≥100 bpm with AV dissociation. Sustained V-tach (≥30 seconds) is a medical emergency. If the patient has a pulse and is hemodynamically stable, synchronized cardioversion or antiarrhythmic therapy (amiodarone). If pulseless: defibrillation per ACLS protocol.
• Ventricular fibrillation (V-fib): Chaotic, disorganized ventricular activity — no effective cardiac output. This is cardiac arrest. Begin CPR immediately and defibrillate as soon as a defibrillator is available. V-fib is the most common initial rhythm in sudden cardiac arrest.

Heart blocks

For the clinical mnemonic that maps these four patterns to unforgettable poem lines, see heart block poem.

Junctional rhythms

Junctional rhythms originate from the AV node or His bundle rather than the SA node. The SA node has failed as the dominant pacemaker, and the junction has taken over.

Clinical notes: Inverted P waves in leads II, III, and aVF (or P waves immediately after the QRS) are the hallmark of junctional conduction. Causes include digitalis toxicity, inferior MI, sick sinus syndrome, and increased vagal tone. The rhythm itself is usually not emergent — find and treat the underlying cause.

Axis deviation: a quick guide

The QRS axis reflects the dominant direction of ventricular depolarization. Check leads I and aVF:

Left axis deviation causes: left anterior fascicular block, left bundle branch block, left ventricular hypertrophy, inferior MI, Wolff-Parkinson-White, paced rhythm.

Right axis deviation causes: right ventricular hypertrophy, right bundle branch block, left posterior fascicular block, pulmonary embolism, lateral MI, normal variant in thin young adults and children.

Why it matters: Axis deviation narrows your differential. New RAD in a patient with sudden dyspnea raises the suspicion for pulmonary embolism. New LAD alongside a wide QRS suggests left anterior fascicular block.

STEMI vs. NSTEMI: the critical distinction

Both represent acute myocardial infarction, but they look different on EKG and require different initial management.

STEMI (ST-elevation MI)

• ST elevation ≥1 mm in two or more contiguous limb leads, OR
• ST elevation ≥2 mm in two or more contiguous precordial leads
• New left bundle branch block (LBBB) may also represent STEMI equivalent
• Reciprocal ST depression in opposite territory strengthens the diagnosis

Territory localization:

Nursing action for suspected STEMI: This is a time-critical emergency. Immediately notify the provider. Activate the cath lab if your institution has that protocol. Obtain IV access, continuous telemetry, oxygen (if SpO2 <90%), and 12-lead confirmation. Do not delay for pain management or additional workup.

NSTEMI and unstable angina

• ST depression in two or more contiguous leads, OR
• T wave inversions in the distribution of a coronary artery, OR
• Dynamic EKG changes correlating with chest pain
• No ST elevation; diagnosis confirmed by elevated troponin (NSTEMI) or negative troponin (unstable angina)

Key point: A normal EKG does not rule out acute coronary syndrome. Serial EKGs and serial troponins are required in any patient with ongoing chest pain or equivalent symptoms.

Clinical applications: when nurses act vs. escalate

Recognize and act immediately (call rapid response or code)

• Ventricular fibrillation → CPR + defibrillation
• Pulseless ventricular tachycardia → CPR + defibrillation
• Third-degree heart block with hemodynamic instability → transcutaneous pacing, ACLS drugs
• STEMI pattern → activate cath lab protocol, notify MD stat

Recognize and escalate urgently (notify provider within minutes)

• Symptomatic second-degree Mobitz II (high risk of progression to complete block)
• Sustained V-tach with pulse → notify, prepare for cardioversion
• Rapid ventricular response AFib with hypotension
• New ST elevation or dynamic ST/T changes
• QTc >500 ms — review medications, notify provider
• Narrow complex tachycardia at 150 bpm (rule out flutter)

Monitor and document (notify at next opportunity or per policy)

• Asymptomatic sinus bradycardia (rate >40)
• First-degree AV block (isolated)
• Mobitz I (Wenckebach) with stable hemodynamics
• Occasional PVCs (no R-on-T, hemodynamically stable)
• Asymptomatic sinus tachycardia with clear cause (fever, pain)

The nurse’s core principle

Nurses do not treat EKG findings in isolation — they treat the patient. Whenever you see an abnormal rhythm, immediately correlate with the clinical picture: Is the patient conscious? Are they perfusing? What is the blood pressure? A slow rhythm in a sleeping athlete is very different from the same rate in a postoperative patient with altered mental status.

Common confusions: artifacts vs. true dysrhythmias

Artifact mimicking V-fib

A loose electrode or patient movement can produce a chaotic, high-amplitude waveform that looks like ventricular fibrillation. Before calling a code:

1. Is the patient responsive? V-fib patients are unconscious and pulseless.
2. Check other leads — artifact rarely appears identically in all leads.
3. Check electrode contact and the patient’s baseline motion.

Never defibrillate a conscious patient. If in doubt, check the pulse.

Artifact mimicking V-tach

Regular, wide-complex tachycardia can be artifact from a shivering patient, muscle tremor, or a 60-Hz AC interference pattern (regular, 60/second noise). Again — check the patient first.

SVT vs. sinus tachycardia

Both produce narrow-complex regular tachycardias. Key differences:

• Sinus tachycardia rarely exceeds 150–160 bpm and has a gradual onset; it has a clear P wave before each QRS
• SVT typically has abrupt onset (“turned on like a switch”), may exceed 200 bpm, and often has no visible P waves or retrograde P waves

Mobitz II vs. nonconducted PAC

Both produce a P wave without a following QRS. The distinction:

• In Mobitz II, the non-conducted P wave has the same morphology as all other sinus P waves, and the PR interval before the dropped beat is constant
• A nonconducted PAC has an early, differently-shaped P wave that lands during the refractory period

This distinction matters enormously — Mobitz II requires urgent escalation; nonconducted PACs generally do not.

Wide complex tachycardia: V-tach vs. SVT with aberrancy

Wide complex tachycardia should be treated as V-tach until proven otherwise. Do not assume SVT with aberrant conduction without electrophysiology confirmation. In clinical practice, the hemodynamically unstable patient with a wide complex tachycardia gets cardioverted — the rhythm diagnosis is secondary to stabilization.

Putting it together: a fast rhythm interpretation framework

When you pick up a rhythm strip on a busy unit, work through this mental checklist in 30 seconds:

1. Rate — bradycardia, normal, or tachycardia?
2. Regular or irregular? — if irregular, is there a pattern?
3. P waves? — present, absent, or disorganized?
4. One P per QRS? Fixed PR? — or are they dissociated?
5. QRS width — narrow (<120 ms) or wide?
6. ST segment — isoelectric, elevated, or depressed?
7. Clinical picture — does the patient match what the strip shows?

Most rhythms in clinical practice are sinus. Deviations from sinus rhythm follow recognizable patterns. The more rhythm strips you read, the faster these patterns become automatic — and the faster you will catch the ones that matter.

Lindsay Smith, AGPCNP, is a practicing advanced practice nurse with clinical experience in critical care and telemetry nursing. All content is reviewed against current AHA guidelines and ACLS reference materials.