Everything You Need to Know About Normal ECG intervals for Step 1

Electrocardiograms love to show up on Step 1/2—often in the form of “is this interval normal?” or “which electrolyte abnormality explains this?” If you lock down normal ECG intervals, you’ll catch easy points, recognize dangerous patterns fast, and avoid being baited by common distractors (like mistaking benign sinus arrhythmia for pathology).

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The Big Picture: What “Intervals” Actually Represent

An ECG is just a surface recording of cardiac electrical activity over time. Intervals measure time between key electrical events, and they map onto:

• Depolarization (electrical activation)
• Conduction through nodes/His-Purkinje
• Repolarization (recovery)

Step-style questions often ask you to connect an abnormal interval to:

• Where conduction is slowed (AV node vs His-Purkinje vs ventricular myocardium)
• What physiology/electrolyte/drug caused it
• What the most dangerous complication is (e.g., torsades)

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Normal ECG Values (High-Yield Table)

Core “Must-Memorize” Normals

Speed trick: At 25 mm/s, one small box = 40 ms, one large box (5 small) = 200 ms.

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How Each Interval Works (and How Step Tests It)

PR Interval (120–200 ms): AV Nodal Delay

Definition

PR interval = start of P wave → start of QRS.
It includes atrial depolarization + physiologic delay in AV node.

Pathophysiology (Why the AV node delays at all)

That AV nodal delay allows:

• Ventricular filling
• Prevention of excessively rapid ventricular rates during atrial tachyarrhythmias (to a degree)

AV nodal conduction is heavily influenced by:

• Vagal tone (slows)
• Calcium-dependent conduction (L-type Ca channels)

High-yield abnormal patterns

• Prolonged PR (>200 ms) = 1st-degree AV block
  • Often incidental/benign, but can be due to:
    • Increased vagal tone (athletes)
    • AV nodal blockers (β-blockers, non-DHP CCBs, digoxin)
    • Inferior MI (AV nodal ischemia)
• Short PR (<120 ms):
  • WPW (accessory pathway: Bundle of Kent)
    • Often also has delta wave (slurred upstroke of QRS)
    • Risk: AVRT, and AF with rapid conduction → can degenerate into VF (dangerous if treated incorrectly)

Clinical presentation

• 1° AV block is usually asymptomatic
• Preexcitation syndromes may present with palpitations, syncope, or sudden cardiac death risk (rare but tested)

Diagnosis (Step approach)

• Measure PR with box counting
• If PR prolonged: ask is every P followed by QRS?
  • If yes → 1° AV block
  • If no → consider 2°/3° blocks (beyond “normal intervals,” but commonly adjacent concepts)

Treatment (high yield)

• 1° AV block: usually no treatment
• WPW with tachyarrhythmia:
  • AVRT (narrow) often: vagal maneuvers, adenosine (with caution/clinical context)
  • AF + WPW: avoid AV nodal blockers (adenosine, β-blockers, non-DHP CCBs, digoxin)
    Use procainamide or ibutilide; definitive ablation

First Aid cross-reference

• Arrhythmias / AV blocks
• WPW (preexcitation, delta wave; management caveat)

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QRS Duration (<120 ms): Ventricular Depolarization

Definition

QRS = ventricular depolarization time (His-Purkinje + ventricular myocardium).

Pathophysiology (why it widens)

A wide QRS typically means depolarization is not traveling through the fast His-Purkinje system:

• Bundle branch block (RBBB/LBBB)
• Ventricular origin rhythm (PVCs, VT)
• Electrolyte/drug effects that slow conduction:
  • Severe hyperkalemia (sine wave progression)
  • Sodium channel blockade (e.g., TCAs, class I antiarrhythmics)

High-yield patterns when abnormal

• RBBB: “RSR’” in V1 (“rabbit ears”)
• LBBB: broad/notched R in lateral leads; can obscure MI diagnosis
• HyperK progression: peaked T → PR prolongation → QRS widening → sine wave → asystole

Clinical presentation

Wide QRS itself is a finding, not a symptom, but it may accompany:

• Palpitations, syncope (VT)
• Post-MI conduction disease
• Drug overdose

Diagnosis

• QRS ≥ 120 ms = wide
• Combine with rhythm and axis findings for localization (Step 2 often expects basic pattern recognition)

Treatment (high yield associations)

• Hyperkalemia with ECG changes: IV calcium first (membrane stabilization), then shift K (insulin + glucose, albuterol), remove K (diuretics, dialysis, resin)
• TCA overdose (wide QRS): IV sodium bicarbonate

First Aid cross-reference

• Hyperkalemia ECG changes
• TCA poisoning treatment
• Bundle branch blocks (often in cardiology/arrhythmia sections)

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QT Interval (Rate-Corrected): Repolarization Risk Zone

Definition

QT = start of QRS → end of T wave.
Because QT changes with HR, Step questions often care about QTc.

A common correction formula:

• Bazett: $QT_c = \frac{QT}{\sqrt{RR}}$
  (QT and RR in seconds)

Why QT matters (pathophysiology)

The QT interval reflects the duration of the ventricular action potential—especially phases influenced by:

• K+ efflux (repolarization)
• Ca2+ influx (plateau)

Prolonged QT increases risk of early afterdepolarizations → torsades de pointes (polymorphic VT).

What prolongs QT (super high yield)

Think: “anything that delays repolarization”

• Electrolytes
  • ↓K (hypokalemia)
  • ↓Mg (hypomagnesemia)
  • ↓Ca (hypocalcemia)
• Drugs
  • Class IA (quinidine, procainamide, disopyramide)
  • Class III (amiodarone, sotalol, dofetilide, ibutilide)
  • Many antipsychotics (e.g., haloperidol, ziprasidone)
  • Macrolides, fluoroquinolones
  • Methadone, ondansetron
• Congenital long QT
  • Channelopathies (often tested with syncope during exercise/startle)

What shortens QT

• Hypercalcemia (short QT; “stones, bones…” scenario + ECG clue)

Clinical presentation

• Often asymptomatic until:
  • Syncope
  • Palpitations
  • Sudden cardiac arrest (torsades/VF)

Diagnosis

• Identify QT prolongation and associate with culprit
• Watch for torsades: polymorphic VT that “twists” around baseline

Treatment (torsades)

• IV magnesium sulfate (even if Mg level is normal)
• Stop offending drug, correct electrolytes
• Unstable: defibrillation
• Recurrent/bradycardia-associated torsades: overdrive pacing or isoproterenol (context-dependent)

First Aid cross-reference

• Antiarrhythmic drug classes (IA and III prolong QT)
• Electrolyte abnormalities and ECG changes
• Torsades de pointes treatment

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RR Interval and Heart Rate: The “Time Between Beats”

Definition

RR interval = time between successive R waves.

Key formulas (testable)

• HR (regular rhythm):
  • 300 rule: HR ≈ 300 / (# large boxes between R-R)
  • 1500 rule: HR ≈ 1500 / (# small boxes between R-R)

High-yield rhythm tie-ins

• Irregularly irregular RR: atrial fibrillation
• Respiratory sinus arrhythmia (normal variant): HR increases with inspiration, decreases with expiration (especially in kids/young adults)

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Quick “Normal Interval” Differential: If They Give You an ECG Stem

If PR is prolonged…

• Is it benign 1° block?
• Is the patient on AV nodal blockers?
• Inferior MI (AV node supplied by RCA often)?

If QRS is wide…

• Think BBB vs VT vs hyperK vs Na channel blocker toxicity
• Look at context: overdose, renal failure, post-MI, syncope

If QT is prolonged…

• Check electrolytes and meds
• Think torsades risk and management (Mg)

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High-Yield Association Table (Classic Step Pairings)

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Common USMLE Traps (Don’t Fall For These)

• Using QT (uncorrected) without considering HR: tachycardia shortens QT normally; bradycardia lengthens it.
• Treating AF + WPW like typical AF: AV nodal blockers can preferentially route conduction down the accessory pathway → VF risk.
• Missing hyperkalemia early signs: peaked T waves can be subtle; progression is a favorite testing pattern.
• Assuming all wide QRS rhythms are BBB: if unstable or very fast, assume VT until proven otherwise clinically.

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Rapid Review (What to Memorize Tonight)

• PR: 120–200 ms
• QRS: <120 ms
• QTc: <440 ms (men), <460 ms (women) (rule-of-thumb)
• Small box: 40 ms; large box: 200 ms (at 25 mm/s)
• Prolonged QT → torsades → IV magnesium
• Short PR + delta wave → WPW; AF + WPW → procainamide, avoid AV nodal blockers
• HyperK → peaked T → PR ↑ → QRS widens → sine wave