An electrocardiogram (ECG) is a non-invasive test that records the heart’s electrical activity. Electrodes placed on the skin detect voltage changes as the heart muscle depolarizes and repolarizes with each beat. The resulting tracing is a series of waves, and the direction of these waves is determined by whether the electrical signal travels toward or away from the sensor, or “lead.” When the tracing dips below the baseline, it represents an electrical event. The shape, depth, and timing of this deflection can indicate different heart conditions.
Understanding the Standard ECG Tracing
The electrical cycle of a normal heartbeat is represented by three primary waveforms: the P wave, the QRS complex, and the T wave. The isoelectric line, the flat line between cycles, serves as the baseline for all deflections. The P wave represents the initial electrical activation (depolarization) of the atria, the heart’s upper chambers.
The QRS complex follows the P wave and represents the depolarization of the ventricles, the main pumping chambers. This complex is typically the tallest feature and often contains the first downward deflection, the Q wave. In a healthy heart, the Q wave is usually small and narrow, representing the electrical signal moving through the septum. The T wave is the final wave, representing the repolarization of the ventricles before the next beat.
Deep Q Waves: The Initial Downward Dip
While a small Q wave is normal, an abnormally deep or wide downward dip is a significant finding, often called a pathological Q wave. This deep deflection is typically a sign of prior damage to the heart muscle, most commonly from a past heart attack. The damaged tissue (infarcted myocardium) becomes electrically silent.
Because the damaged area no longer conducts electricity, the overall electrical activity shifts away from that portion of the heart, resulting in a prominent negative deflection. Clinicians often define a pathological Q wave as one that is wider than 0.04 seconds or deeper than one-third the height of the subsequent R wave in certain leads. Their presence indicates the location and extent of previous muscle damage, reflecting an area of the heart wall replaced by non-functional tissue.
ST Segment Depression and T Wave Inversion
Downward shifts also occur later in the cardiac cycle, involving the ST segment and the T wave. The ST segment is the flat line connecting the end of the QRS complex and the beginning of the T wave, representing the period when the entire ventricle is depolarized. When this segment shifts downward below the baseline, it is known as ST segment depression.
ST segment depression is a sign of active myocardial ischemia, a condition where the heart muscle is not receiving enough blood flow and oxygen. This lack of oxygen alters the heart’s electrical recovery process, causing the ST segment to appear lowered. A horizontal or down-sloping ST segment depression of 0.5 millimeters or more in two adjacent leads is suggestive of ischemia.
The T wave normally points upward but may present as a downward deflection, known as T wave inversion. Like ST depression, T wave inversion can signify ischemia, especially if the inversion is deep and symmetrical. These changes reflect an alteration in the repolarization phase. The location of the T wave inversion can provide clues about which coronary artery might be partially blocked.
When to Seek Immediate Medical Attention
An ECG is a diagnostic tool and only one piece of information used to assess heart health. The average person cannot accurately self-diagnose based on a tracing alone, and any concerning symptoms require professional evaluation. If a healthcare provider notes abnormal downward dips, they will correlate the finding with your medical history and current symptoms.
If you are experiencing physical symptoms related to an acute heart problem, seek emergency medical attention immediately. These symptoms include chest discomfort described as pressure, squeezing, or fullness, especially if the discomfort lasts more than a few minutes or spreads to the jaw, neck, or arm. Shortness of breath, cold sweats, light-headedness, or nausea accompanying chest discomfort also warrant an immediate call to emergency services.