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Quick Reference

Definition: Heart rate is the frequency of ventricular depolarizations, measured as beats per minute (bpm). It is determined by counting QRS complexes over time and reflects the number of times the heart contracts per minute.

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Classification Heart Rate (bpm) Clinical Significance
Severe Bradycardia < 40 High risk, may require pacing
Bradycardia 40-59 Normal in athletes, may be pathologic
Normal 60-100 Standard reference range
Tachycardia 101-150 Physiologic or pathologic
Severe Tachycardia > 150 High risk, requires evaluation
Key Point: While 60-100 bpm is the traditional "normal" range, many cardiologists now consider 50-90 bpm more appropriate for healthy adults. Trained athletes commonly have resting rates of 40-60 bpm without pathology.
ECG waveforms showing intervals for rate calculation
ECG waveforms, segments, and intervals - Understanding these components is essential for accurate heart rate calculation. Source: LITFL
Rate Calculation Methods

Multiple methods exist for calculating heart rate from an ECG, each suited to different situations and rhythm types.

1. The 300 Rule (Large Box Method)

Best for: Regular rhythms, quick estimation

Method: Count the number of large boxes (0.2 second each) between consecutive R waves, then divide into 300.

Formula: Heart Rate = 300 ÷ (number of large boxes between R waves)

Memory Trick: "300-150-100-75-60-50" for 1, 2, 3, 4, 5, and 6 large boxes respectively.

2. The 1500 Rule (Small Box Method)

Best for: Regular rhythms, precise calculation

Method: Count the number of small boxes (0.04 second each) between consecutive R waves, then divide into 1500.

Formula: Heart Rate = 1500 ÷ (number of small boxes between R waves)

Advantage: More precise than the 300 rule, especially for rates between memorized values.

Disadvantage: Requires counting many small boxes, more time-consuming.

3. The 6-Second Method (Rhythm Strip)

Best for: Irregular rhythms (atrial fibrillation, frequent ectopy), bradycardia

Method: Count the number of QRS complexes in a 6-second strip (30 large boxes), then multiply by 10.

Formula: Heart Rate = (number of QRS complexes in 6 seconds) × 10

Why 6 seconds? Standard ECG paper has marks at 3-second intervals. Two marks = 6 seconds = 30 large boxes.

Advantage: Works with irregular rhythms where beat-to-beat intervals vary.

Note: This gives an average heart rate, not instantaneous rate.

4. The 10-Second Method

Best for: Very irregular rhythms, palpation-based assessment

Method: Count QRS complexes over 10 seconds (50 large boxes), then multiply by 6.

Formula: Heart Rate = (number of QRS complexes in 10 seconds) × 6

5. R-R Interval Method (Electronic)

Best for: Digital ECG systems, precise measurement

Method: Measure the R-R interval in milliseconds or seconds, then calculate rate.

Formula: Heart Rate = 60 ÷ (R-R interval in seconds)
Or: Heart Rate = 60,000 ÷ (R-R interval in milliseconds)

Example: R-R interval = 0.8 seconds → Rate = 60 ÷ 0.8 = 75 bpm

Choosing the Right Method

  • Regular rhythm, quick estimate: 300 rule
  • Regular rhythm, precise calculation: 1500 rule
  • Irregular rhythm: 6-second or 10-second method
  • Bradycardia: 6-second method (more accurate at slow rates)
  • Digital ECG: R-R interval method
Tachycardia

Definition: Heart rate greater than 100 bpm at rest.

Physiologic (Appropriate) Tachycardia

Normal compensatory responses to increased metabolic demands:

  • Exercise: Proportional to workload, max HR ≈ 220 - age
  • Fever: Increases ~10 bpm per 1°C rise in body temperature
  • Hypovolemia: Compensatory response to maintain cardiac output
  • Hypoxia: Increased oxygen delivery demand
  • Pain/Anxiety: Sympathetic nervous system activation
  • Pregnancy: Increased heart rate in 2nd/3rd trimester
  • Medications: Sympathomimetics, anticholinergics, caffeine

Pathologic Tachycardia

Inappropriate or disproportionate heart rate elevation:

Sinus Tachycardia

  • Rate: 100-180 bpm (rarely >180 in adults)
  • ECG Features: Normal P waves, normal PR interval, regular rhythm
  • Common Causes: Sepsis, hyperthyroidism, anemia, heart failure, PE
  • Management: Treat underlying cause

Supraventricular Tachycardia (SVT)

  • Rate: 150-250 bpm, often very regular
  • ECG Features: Narrow QRS (<0.12 sec), absent or abnormal P waves
  • Types: AVNRT, AVRT (WPW), atrial tachycardia, atrial flutter
  • Management: Vagal maneuvers, adenosine, rate/rhythm control

Ventricular Tachycardia (VT)

  • Rate: 100-250 bpm, regular or slightly irregular
  • ECG Features: Wide QRS (≥0.12 sec), AV dissociation
  • Significance: Life-threatening, may degenerate to VF
  • Management: Immediate treatment, cardioversion if unstable

Atrial Fibrillation with RVR

  • Rate: Variable, often 110-180 bpm
  • ECG Features: Irregularly irregular, no distinct P waves
  • RVR: Rapid ventricular response (rate >100 bpm)
  • Management: Rate control, anticoagulation, rhythm control

Clinical Assessment of Tachycardia

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Rate Range Clinical Consideration Action
100-120 bpm Mild tachycardia, often physiologic Assess for reversible causes
120-150 bpm Moderate tachycardia, needs evaluation Determine rhythm type, treat cause
150-200 bpm Severe tachycardia, likely arrhythmia ECG, consider pharmacologic treatment
>200 bpm Critical tachycardia, high risk Immediate intervention, cardioversion if unstable

Clinical Pearl: The "rate of 150 rule" – A regular, narrow-complex tachycardia at exactly 150 bpm should raise suspicion for atrial flutter with 2:1 AV block (atrial rate ~300 bpm).

Bradycardia

Definition: Heart rate less than 60 bpm at rest.

Note: Many experts now define pathologic bradycardia as <50 bpm, as rates of 50-60 bpm are common in healthy individuals.

Physiologic Bradycardia

Normal variations not requiring treatment:

  • Athletic Heart: Well-trained athletes commonly have HR 40-60 bpm
  • Sleep: Heart rate naturally decreases during sleep (40-50 bpm common)
  • Vagal Tone: High parasympathetic tone in young, healthy individuals
  • Medications: Beta-blockers, calcium channel blockers (intentional)

Athletic Bradycardia: Elite endurance athletes can have resting rates as low as 28-38 bpm without symptoms. Miguel Indurain (Tour de France winner) had a resting HR of 28 bpm.

Pathologic Bradycardia

Abnormal heart rate requiring evaluation and possible treatment:

Sinus Bradycardia

  • Rate: <60 bpm, regular rhythm
  • ECG Features: Normal P waves, normal PR interval, normal QRS
  • Causes: Hypothyroidism, hypothermia, increased ICP, MI (especially inferior), medications
  • Treatment: Only if symptomatic (dizziness, syncope, hypotension)

AV Blocks

  • First-Degree AV Block: PR >0.20 sec, every P conducted, usually benign
  • Second-Degree Mobitz I (Wenckebach): Progressive PR lengthening until dropped beat, often benign
  • Second-Degree Mobitz II: Sudden dropped beats, risk of complete block, may need pacing
  • Third-Degree (Complete) AV Block: No P-QRS relationship, ventricular rate 20-40 bpm, requires pacing

Sick Sinus Syndrome

  • Features: Alternating bradycardia and tachycardia, sinus pauses
  • Symptoms: Dizziness, syncope, palpitations
  • Treatment: Often requires permanent pacemaker

Symptomatic Bradycardia

Signs and symptoms requiring treatment:

  • Altered mental status, confusion
  • Syncope or near-syncope
  • Chest pain or acute MI
  • Hypotension (SBP <90 mmHg)
  • Signs of shock (cold, clammy skin)
  • Heart failure or pulmonary edema
  • Ventricular arrhythmias related to bradycardia

Emergency Management

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Severity Heart Rate Action
Mild, asymptomatic 50-60 bpm Monitor, no immediate treatment
Moderate, asymptomatic 40-50 bpm Investigate cause, close monitoring
Symptomatic <50 bpm Atropine 0.5-1 mg IV, consider pacing
Severe, unstable <40 bpm Immediate transcutaneous pacing, atropine, dopamine/epinephrine

ACLS Pearl: Atropine is ineffective in third-degree AV block with wide-complex escape rhythm or in cardiac transplant patients (denervated heart). Proceed directly to pacing or chronotropic agents.

Heart Rate in Irregular Rhythms

Irregular rhythms pose unique challenges for rate calculation because beat-to-beat intervals vary. The 6-second or 10-second method provides the most accurate average rate.

Atrial Fibrillation

  • Pattern: Irregularly irregular, no organized atrial activity
  • Rate Calculation: Use 6-second method for average ventricular rate
  • Rate Classifications:
    • Slow: <60 bpm (controlled)
    • Normal: 60-100 bpm (controlled)
    • RVR: >100 bpm (rapid ventricular response)
  • Pitfall: Don't calculate from a single R-R interval – it will be inaccurate

Atrial Flutter with Variable Block

  • Pattern: Regular atrial rate (~300 bpm), variable ventricular response
  • Common Blocks: 2:1 (rate ~150), 3:1 (rate ~100), 4:1 (rate ~75), variable
  • Rate Calculation:
    • Fixed block: Use 300 rule on ventricular rate
    • Variable block: Use 6-second method
  • Identification: Look for flutter waves (sawtooth pattern) in inferior leads

Frequent Ectopy

  • PACs/PVCs: Premature beats create irregular rhythm
  • Rate Calculation: Use 6-second method, count all QRS complexes
  • Clinical Note: Very frequent PVCs may not produce palpable pulses (pulse deficit)
  • Consideration: Report both total rate and underlying rhythm rate

Bigeminy and Trigeminy

  • Bigeminy: Every other beat is premature (PVC-normal-PVC-normal)
  • Trigeminy: Every third beat is premature (normal-normal-PVC)
  • Rate Calculation: 6-second method captures the pattern accurately
  • Pulse Rate: May be half the ECG rate in bigeminy if PVCs don't perfuse

Second-Degree AV Block (Mobitz I)

  • Pattern: Progressive PR lengthening until dropped beat (Wenckebach)
  • Rate Calculation: 6-second method for average ventricular rate
  • Atrial vs Ventricular Rate: Atrial rate is regular and faster than ventricular

Sinus Arrhythmia

  • Pattern: Rate varies with respiration (faster on inspiration)
  • Normal Finding: Common in young, healthy individuals
  • Rate Calculation: Use 6-second method or report range (e.g., "60-75 bpm")
  • Clinical Significance: Benign, indicates good vagal tone

Important Concept: In irregular rhythms, "heart rate" represents an average over time, not instantaneous rate. Always use a longer counting period (6-10 seconds) for accuracy.

Reporting Guidelines

For irregular rhythms, consider reporting:

  • Average rate: "Atrial fibrillation, average rate 85 bpm"
  • Rate range: "Atrial fibrillation, rate 70-110 bpm"
  • Controlled/Uncontrolled: "Atrial fibrillation with RVR, rate 135 bpm"
Rate-Dependent ECG Phenomena

Many ECG parameters and pathologic findings vary with heart rate, making rate assessment crucial for interpretation.

QT Interval and Heart Rate

  • Relationship: QT interval shortens as heart rate increases
  • Correction Required: Must use corrected QT (QTc) to assess for prolongation
  • Bazett's Formula: QTc = QT / √(R-R interval)
  • Normal QTc: <450 ms (men), <460 ms (women)
  • Clinical Importance: Long QT syndrome risk for torsades de pointes

Rate-Related Bundle Branch Block

  • Definition: Bundle branch block that appears only at certain heart rates
  • Mechanism: Rate-dependent refractoriness of bundle branch
  • Critical Rate: Block appears above threshold rate (e.g., >90 bpm)
  • Variants:
    • Tachycardia-dependent: Block at faster rates
    • Bradycardia-dependent: Block at slower rates (rare, paradoxical)

Aberrant Conduction

  • Definition: Abnormal QRS morphology due to rate-related refractoriness
  • Ashman Phenomenon: Wide QRS after long-short R-R sequence in AFib
  • Common Scenario: Long pause followed by early beat → wide QRS (RBBB pattern)
  • Clinical Importance: Distinguish from PVC

U Wave Visibility

  • Best Seen: At slower heart rates (<70 bpm)
  • Mechanism: More time between T wave and next P wave allows U wave visualization
  • Clinical Note: Prominent U waves suggest hypokalemia, drug effects

ST Segment Changes

  • Tachycardia: May cause ST depression (demand ischemia)
  • Bradycardia: May show ST elevation (early repolarization more prominent)
  • Clinical Pearl: "Rate-related ST changes" resolve when rate normalizes

P Wave Morphology

  • Fast Rates: P waves may merge with preceding T wave (hard to identify)
  • Slow Rates: P waves clearly visible, easier to assess morphology
  • Clinical Tip: Use vagal maneuvers to slow rate and reveal P waves

Concealed Conduction

  • Definition: Incomplete penetration of AV node affects subsequent beats
  • Rate Effect: More common at faster rates when AV node is stressed
  • Result: Unexpected variation in PR intervals or dropped beats

Ischemia Detection

  • Exercise-Induced: ST changes appear at higher heart rates
  • Target Heart Rate: Stress test aims for 85% of maximum predicted HR
  • Chronotropic Incompetence: Inability to achieve target HR suggests disease
  • Maximum HR Formula: 220 - age (traditional) or 208 - (0.7 × age) (updated)

Clinical Pearl: When interpreting any ECG, always consider how the heart rate might affect other parameters. A long QT at 60 bpm is different from a long QT at 100 bpm (use QTc).

Pacemaker Rate Assessment

  • Programmed Rate: Compare actual rate to pacemaker settings
  • Rate-Responsive Pacing: Rate varies with activity (sensor-driven)
  • Failure to Capture: Pacer spikes without QRS → lower effective rate
  • Failure to Sense: May cause inappropriate pacing → higher rate
Clinical Pearls
Always Correlate with Patient: A rate of 45 bpm in an asymptomatic athlete is normal; the same rate in a symptomatic elderly patient may require treatment.
Pulse-ECG Discrepancy: If palpated pulse is significantly lower than ECG rate, suspect pulse deficit (common in AFib, frequent PVCs, bigeminy).
The "150 Rule": Regular narrow-complex tachycardia at exactly 150 bpm → think atrial flutter with 2:1 block until proven otherwise.
Fever Effect: Heart rate increases approximately 10 bpm for each 1°C rise in body temperature. A patient with 39°C fever and HR 70 bpm may have underlying bradycardia.
Beta-Blocker Masking: Patients on beta-blockers may not mount appropriate tachycardic response to stress (sepsis, bleeding). A "normal" rate of 90 bpm may indicate significant pathology.
Atropine Dosing: Use adequate doses (0.5-1 mg IV) for symptomatic bradycardia. Doses <0.5 mg may cause paradoxical bradycardia via central vagal effects.
Relative Bradycardia: Heart rate inappropriately low for clinical condition (e.g., HR 80 bpm in severe sepsis). Suggests specific pathogens (Salmonella, Legionella) or conduction disease.
Narrow vs Wide Complex: At rates >150 bpm, always determine if QRS is narrow (<0.12 sec) or wide (≥0.12 sec). Wide complex tachycardia is VT until proven otherwise.
Age-Adjusted Normal: Resting heart rate decreases with age. Newborns average 120-160 bpm, while adults average 60-100 bpm, and elderly may normally run 50-70 bpm.
Unstable = Cardioversion: Any tachycardia with signs of instability (altered mental status, chest pain, hypotension, pulmonary edema) requires immediate synchronized cardioversion, regardless of rate calculation method used.
QTc Matters More Than QT: Always correct QT for heart rate. A QT of 440 ms is normal at 60 bpm but prolonged at 100 bpm.
Document Method Used: When reporting irregular rhythm rates, note method: "AFib, average rate 85 bpm by 6-second method" provides context.
Carotid Massage Caution: Can slow rate to help diagnosis, but contraindicated in elderly (stroke risk), known carotid disease, or recent MI/stroke.
Pacemaker Patients: Know the programmed lower rate limit. If ECG rate is below this, pacemaker malfunction or native rhythm override may be present.
Dig Toxicity: Classic presentation is "regularization of AFib" – atrial fibrillation with a very regular ventricular rate (~80-100 bpm) suggests complete AV block with junctional escape due to digoxin toxicity.
References
  1. Farkas, Josh MD. (2015). Table of Contents - EMCrit Project. EMCrit Project. https://emcrit.org/ibcc/toc/
  2. Khan, M. G. (2007). Rapid ECG Interpretation. Humana.
  3. Sigg, D. C., Iaizzo, P. A., Xiao, Y.-F., Bin He, & Springerlink (Online Service). (2010). Cardiac Electrophysiology Methods and Models. Springer Us.
  4. Wang, K. (2012). Atlas of Electrocardiography. JP Medical Ltd.
  5. ECG Library • LITFL • ECG Library Basics. (2018). Life in the Fast Lane • LITFL • Medical Blog. https://litfl.com/ecg-library/
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