What to do when two electrocardiograms (ECGs) report different heart rates for the same patient?

Approach to Discrepant Heart Rates on Two ECGs

When two ECGs report different heart rates for the same patient, first verify the accuracy of both recordings by checking for technical artifacts, lead misplacement, and proper electrode positioning, then manually calculate the heart rate from both tracings to confirm which measurement is correct. 1

Immediate Verification Steps

Check for Technical Artifacts and Recording Quality

• Examine both ECGs for lead misplacement, which can alter computer-generated measurements and produce spurious heart rate calculations 1
• Verify proper electrode positioning, as precordial lead misplacement occurs in more than one-third of routine recordings and can affect automated measurements 1
• Look for baseline wander, electrical interference, or poor electrode contact that may compromise automated heart rate detection 1
• Check for pacemaker artifacts that may be missed by automated systems, particularly with modern low-amplitude bipolar pacing, leading to incorrect heart rate calculations 1

Manually Calculate Heart Rate from Both Tracings

• Count the number of QRS complexes in a 6-second strip and multiply by 10, or use the 300-divided-by-large-boxes method between consecutive R waves 2
• Measure heart rate in multiple leads to ensure consistency, as automated systems may select different leads for analysis 1
• If rhythm is irregular (e.g., atrial fibrillation), recognize that instantaneous heart rate varies beat-to-beat, and a single automated measurement may not represent the true average rate 1

Assess for Computer Algorithm Differences

Recognize Manufacturer-Specific Variations

• Different ECG machines use proprietary algorithms that can produce substantially different measurements, even from the same cardiac electrical activity 1
• Newer electrocardiographs typically use global measurements from simultaneous multilead acquisition, while older machines use single-lead measurements, resulting in systematic differences 1
• When comparing serial ECGs, ideally use the same electrocardiograph instrument to eliminate algorithm-related discrepancies 1

Verify Computer Interpretation

• Never accept automated heart rate measurements without physician verification, as computer interpretation errors remain common 3, 2
• Computer-generated ECGs are not recognized as properly interpreted without qualified physician review 2

Determine Clinical Significance

Compare with Previous ECGs

• Review prior tracings with correct technique to establish the patient's baseline heart rate pattern 1
• A heart rate change of ≥20 bpm crossing thresholds of 50 or 100 bpm constitutes a significant change requiring clinical correlation 1

Correlate with Clinical Context

• Assess whether the patient's symptoms, medications, or clinical status could explain heart rate variation (e.g., beta-blockers causing bradycardia, fever causing tachycardia) 3
• Consider physiologic variations: heart rate normally fluctuates with respiration, position changes, and autonomic tone 4
• Sinus bradycardia (<60 bpm) may be a normal variant, especially in athletes 3

Common Pitfalls to Avoid

• Do not assume the computer-generated heart rate is accurate without manual verification, particularly when two measurements differ significantly 1, 2
• Avoid comparing ECGs from different manufacturers without recognizing potential algorithm-related measurement differences 1
• Do not overlook lead misplacement, which occurs frequently and can produce artifactual changes in automated measurements 1
• Recognize that blocked atrial bigeminy or other rhythm disturbances can simulate bradycardia if P waves are not carefully examined 1

Resolution Strategy

• If manual calculation confirms one ECG is accurate, document which measurement is correct and note the reason for discrepancy (technical artifact, lead misplacement, algorithm difference) 1
• If both measurements are technically accurate but differ due to physiologic variation, document both values with timestamps and clinical context 1
• When uncertainty persists, obtain a third ECG with meticulous attention to proper technique and use manual calculation as the gold standard 1