Are both the PR (P-R) interval and the ST (S-T) segment supposed to be flat in a normal electrocardiogram (EKG)?

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Normal EKG Baseline Segments

Yes, both the PR interval and ST segment should be flat (isoelectric) in a normal EKG, serving as the baseline reference against which other waveform deviations are measured. 1

PR Interval Characteristics

The PR interval represents the time from atrial depolarization through AV nodal conduction and should appear as a flat, isoelectric segment on a normal EKG 1. This segment serves as one of the primary baseline references for measuring ST-segment deviations 1.

Normal PR Interval Parameters:

  • Duration: 120-200 milliseconds from the beginning of the P wave to the start of the QRS complex 2
  • Appearance: Flat baseline between the end of the P wave and beginning of the QRS complex 1

Important Clinical Caveat - PR Depression:

While the PR segment should be flat in normal individuals, PR segment depression is a pathological finding that can help differentiate acute myopericarditis from ST-elevation myocardial infarction 3. PR depression in both precordial and limb leads has 96.7% positive predictive power for myopericarditis versus STEMI 3. This finding is common in acute myopericarditis (88.2% sensitivity) but rare in STEMI (78.3% specificity) 3.

ST Segment Characteristics

The ST segment should also be flat and isoelectric in most leads, though some normal elevation can occur in specific precordial leads 1.

Baseline Reference Standard:

  • ST-segment amplitudes are measured against the PR or TP segments as the baseline reference 1
  • The electrophysiological basis for the flat ST segment is that the plateau phase of the cardiac action potential maintains approximately the same voltage (+10 to -10 mV) across all ventricular myocardial cells simultaneously 4
  • This uniform voltage during the plateau phase produces an isoelectric ST segment on the surface EKG 4

Normal ST-Segment Elevation Exceptions:

The ST segment is NOT always completely flat in normal individuals, particularly in precordial leads V1-V3 1. Age, sex, and race significantly affect normal J-point elevation thresholds:

Men:

  • White men <40 years: Up to 0.25-0.33 mV elevation at J-point in V2 is normal 1
  • White men ≥40 years: Up to 0.25 mV elevation at J-point in V2 is normal 1
  • Black men ≥40 years: Up to 0.20 mV elevation at J-point in V2 is normal 1

Women:

  • White women (all ages): Up to 0.15 mV elevation at J-point in V2 is normal 1
  • Black women ≥40 years: Up to 0.15 mV elevation at J-point in V2 is normal 1

Key Distinguishing Feature:

Normal ST elevation (early repolarization) typically has a steeply downsloping or rapidly upsloping ST segment, NOT a horizontal ST segment 1. The horizontal ST-segment pattern is more characteristic of myocardial ischemia 1.

Clinical Implications

Measurement Technique:

  • ST-segment displacement is measured at the J-point (junction of QRS complex and ST segment) and sometimes 40-80 ms after the J-point during exercise testing 1
  • When low-frequency filtering removes baseline drift during EKG acquisition, ST elevation may actually reflect PR/TP depression, true ST elevation, or both 1

Pathological Deviations:

Any significant deviation from the flat baseline in the PR segment or inappropriate ST-segment changes should prompt evaluation for:

  • PR depression: Myopericarditis 3
  • ST elevation: Acute myocardial infarction, pericarditis, or early repolarization variant 1
  • ST depression: Myocardial ischemia, hypokalemia, drug effects, or secondary repolarization abnormalities 1

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

2
Guideline

Normal PR Interval

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

3
Research

PR depression is useful in the differential diagnosis of myopericarditis and ST elevation myocardial infarction.

Annals of noninvasive electrocardiology : the official journal of the International Society for Holter and Noninvasive Electrocardiology, Inc, 2012

4
Guideline

Cardiac Action Potential Plateau Phase

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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