What is the isoelectric line on an ECG used as the reference for measuring the ST segment?

Isoelectric Reference for ST Segment Measurement

The P-Q junction (end of the PR segment) is the standard isoelectric baseline used to measure ST segment deviation on ECG. 1

Primary Reference Point

• The P-Q junction serves as the isoelectric reference line because the T-P segment (which would be the true isoelectric baseline) becomes unusable during tachycardia when the T wave merges with the P wave at heart rates above 130 beats/min. 1

• The ST segment level is measured relative to the P-Q junction, with measurements taken at the J point (junction of QRS end and ST segment beginning) and at 60-80 ms after the J point. 1

• At faster heart rates, the end of the PR segment is used as a compromise isoelectric baseline by automated algorithms, as the standard T-P segment is invariably merged with the P wave in patients with sinus rhythm. 1

Key Measurement Points

The three critical measurements for ST segment assessment are:

• Identification of the P-Q junction (the isoelectric reference line) 1
• The J point (QRS end/ST segment beginning) 1
• 60 or 80 ms after the J point for determining the extent of ST segment displacement 1

Clinical Application Algorithm

For baseline ST depression:

• Measure the net difference from the J junction to determine the magnitude of exercise-induced displacement. 1

For baseline ST elevation (early repolarization):

• When the J junction becomes progressively depressed during exercise, measure ST-segment displacement from the P-Q junction, NOT from the resting elevated J junction. 1

For standard ST deviation measurement:

• Use the isoelectric line (P-Q junction) as reference, measuring at J + 60-80 ms. 2

Common Pitfalls to Avoid

• Avoid using the T-P segment during exercise or tachycardia as it becomes unreliable when heart rate exceeds 130 bpm due to T-P wave fusion. 1

• Errors in determining the end of the PR segment can occur when the measurement point is actually on the descending limb of a P wave with shortened PR interval during exercise, leading to incorrect automated ST-segment measurements. 1

• Do not confuse positional ST changes with ischemia - observe for associated QRS changes, which indicate position-related artifact rather than true ischemia. 1

• Ensure consistent electrode placement as precordial leads are especially prone to waveform changes when electrodes are relocated even 1 cm from their original location. 1