ST-Segment Evaluation: Use Either TP or PR Segment as Reference
Both the TP segment and PR segment are acceptable reference points for measuring ST-segment elevation, as they represent the isoelectric baseline on the ECG 1, 2.
The Guideline Recommendation
The AHA/ACCF/HRS scientific statement explicitly states that "ST- and T-wave amplitudes are referenced against the TP or PR segments of the ECG" 1, 2. This guideline does not prioritize one over the other, recognizing both as valid isoelectric references.
Practical Considerations
When to Use Each Segment:
TP segment (preferred when visible): The interval between the end of the T wave and the beginning of the P wave represents true electrical silence and is theoretically the most accurate isoelectric baseline
PR segment (more practical in many situations): The interval from the end of the P wave to the beginning of the QRS complex is often easier to identify, especially at faster heart rates when the TP segment may be shortened or absent
Critical Technical Point:
The choice between TP and PR segments becomes less relevant when low-frequency filtering is applied during ECG acquisition 1. Modern ECG machines use filtering to remove baseline drift, which means the actual DC voltage levels cannot be determined. In this context, what appears as ST elevation may actually represent PR/TP depression, true ST elevation, or both 1, 2.
Measurement Technique
Regardless of which reference segment you choose:
- Measure ST deviation at the J point (junction of QRS complex and ST segment)
- In specific contexts like exercise testing, measure 40-80 ms after the J point 1
- Apply age-, sex-, and race-specific thresholds for normal ST elevation, particularly in leads V2-V3 1, 2
Common Pitfall to Avoid
Do not switch between reference points when serially comparing ECGs in the same patient. Consistency in your measurement technique is essential for tracking changes over time. The research evidence shows that different measurement methods produce significantly different results 3, so maintaining the same approach is crucial for accurate clinical assessment.