The ST segment is a specific part of an electrocardiogram (ECG), a test that records the heart’s electrical activity. This segment provides important insights into heart health, representing a crucial phase of its electrical cycle. An ECG helps assess heart function.
The Heart’s Electrical Journey
The heart’s ability to pump blood relies on a precise electrical conduction system. This system begins with the sinoatrial (SA) node, the natural pacemaker, located in the right upper chamber of the heart. The SA node generates electrical impulses that spread through the atria, causing them to contract.
The electrical signal then travels to the atrioventricular (AV) node, which delays the impulse before sending it down through specialized pathways like the bundle of His and Purkinje fibers. These pathways distribute the electrical signal to the ventricles, leading to their contraction and blood pumping. An ECG records these electrical signals as a series of waves and segments.
On an ECG tracing, electrical events are depicted as distinct waves: P, Q, R, S, and T. The P wave represents atrial activity, while the QRS complex indicates ventricular activation. The ST segment appears after the QRS complex, connecting the end of the S wave to the beginning of the T wave, which represents ventricular recovery.
The Normal ST Segment
The normal ST segment represents the period between ventricular electrical activation and recovery. This phase, known as the plateau phase of the ventricular action potential, occurs when ventricular muscle cells are uniformly depolarized.
On a healthy ECG, the ST segment typically appears as a flat, or “isoelectric,” line. This flatness indicates minimal net electrical activity, as the entire ventricular muscle is in a similar electrical state. The ST segment should generally be at the same level as the PR segment, which serves as a baseline reference.
A flat ST segment suggests the heart muscle is functioning as expected, with a balanced electrical state. Small variations, up to 0.5 mm, can be normal. The typical duration of the ST segment is around 0.08 seconds.
What Abnormal ST Segment Changes Mean
Deviations from the normal flat ST segment can signal underlying heart conditions. These changes are categorized as either ST elevation or ST depression. Their magnitude, measured from the J point (where the QRS complex ends and the ST segment begins) to the baseline, provides important diagnostic information.
ST elevation occurs when the ST segment appears raised above the baseline. This finding is often associated with an ST-elevation myocardial infarction (STEMI), commonly known as a heart attack. A STEMI indicates a complete blockage of a coronary artery, leading to heart muscle damage. Other causes of ST elevation include pericarditis or benign early repolarization.
Conversely, ST depression occurs when the ST segment appears lowered below the baseline. This change often suggests myocardial ischemia, a condition with reduced blood flow to the heart muscle. Myocardial ischemia can manifest as angina, a type of chest pain, or occur without symptoms. ST depression can also be caused by low potassium levels (hypokalemia), certain medications like digitalis, or left ventricular hypertrophy.
The Role of Professional Interpretation
Interpreting ECGs and understanding ST segment changes requires specialized knowledge and training. While ST segment abnormalities can indicate heart conditions, a definitive diagnosis is not made solely based on an ECG reading. Medical professionals consider ECG findings alongside a comprehensive patient evaluation.
A thorough medical evaluation includes assessing symptoms, medical history, and performing a physical examination. Other diagnostic tests, such as blood tests for cardiac enzymes, are often conducted to confirm or rule out specific conditions. Relying on self-interpretation of ECG results is not advisable due to their complexity and potential for misinterpretation.