The ST interval represents the period between the end of the S wave and the beginning of the T wave on an electrocardiogram (ECG), signifying the interval between ventricular depolarization and repolarization.
Understanding the ST Interval
The ST interval is a critical component of the ECG, providing insights into the heart's electrical activity, specifically the time between the end of ventricular depolarization (the ventricles contracting) and the beginning of ventricular repolarization (the ventricles relaxing). It's usually isoelectric, meaning it should be flat relative to the baseline.
Key Features of the ST Interval:
- Location: It starts at the J-point (the end of the QRS complex) and ends at the beginning of the T wave.
- Significance: It reflects the early phase of ventricular repolarization, when the ventricles are relatively refractory to further stimulation.
- Baseline Reference: The PQ and TP segments are typically used as baselines for evaluating ST segment elevation or depression. These segments represent diastolic potentials.
Clinical Importance
Changes in the ST interval, such as elevation or depression, can indicate various cardiac conditions, including:
- ST-Elevation Myocardial Infarction (STEMI): ST elevation is a hallmark sign of a heart attack where an artery is completely blocked.
- Non-ST-Elevation Myocardial Infarction (NSTEMI): ST depression or T-wave inversion can indicate a heart attack where an artery is partially blocked.
- Angina: ST depression can also be a sign of myocardial ischemia (reduced blood flow to the heart) without complete infarction.
- Pericarditis: ST elevation can be a sign of inflammation of the sac surrounding the heart.
- Other Conditions: Electrolyte imbalances, medication effects, and other non-cardiac conditions can also affect the ST segment.
ST Segment vs ST Interval
It's important to note the difference between the ST segment and the ST interval. The ST segment is just the flat, isoelectric portion between the end of the S wave (J-point) and the beginning of the T wave. The ST interval encompasses the ST segment plus the J-point itself. In clinical practice, the terms are often used interchangeably, but understanding the distinction can be helpful.
In conclusion, the ST interval is a vital part of the ECG, reflecting the early phase of ventricular repolarization and providing crucial information about the heart's health and possible ischemic events.
