An Electrocardiogram (EKG or ECG) is a non-invasive tool that records the electrical activity of the heart. Electrodes placed on the skin detect the tiny electrical currents generated by the heart muscle as it contracts and relaxes. The resulting tracing displays a repeating pattern of waves and complexes, known as the PQRST complex, which represents the electrical cycle of a single heartbeat. The Q wave specifically focuses on the onset of the heart’s main pumping action.
Understanding the QRS Complex
The Q wave is the initial component of the larger QRS complex, which represents ventricular depolarization. This is the rapid spread of an electrical impulse through the ventricles, the heart’s main pumping chambers, triggering their contraction. Consequently, the QRS complex is typically the largest and most prominent feature on the EKG tracing. The Q wave is defined as the very first downward, or negative, deflection of the QRS complex, immediately preceding the tall, positive R wave.
Characteristics of a Normal Q Wave
In a healthy heart, the Q wave is often extremely small or may not appear at all in many EKG leads. When present, it is termed a “septal Q wave” and reflects the electrical activation of the interventricular septum. This septal depolarization moves away from the recording electrodes, creating the small, initial negative deflection. A normal Q wave is very narrow, lasting less than 0.04 seconds (one small box on the EKG paper). Its depth is also shallow, usually measuring less than 25% of the height of the subsequent R wave in that same lead.
When the Q Wave Indicates Heart Damage
A Q wave becomes “pathological” when it exceeds normal measurements for width and depth, indicating a permanent electrical change in the heart muscle. The presence of a pathological Q wave is a key sign of a prior myocardial infarction (heart attack). A heart attack causes the death of muscle tissue, leading to necrosis, which is replaced by scar tissue. This scar tissue is electrically silent and cannot generate or conduct an impulse.
When the electrical impulse spreads through the ventricles, the scar tissue acts as an electrical “hole.” The electrical signal moves away from the damaged area, causing a shift in the overall direction of the current. This shift results in the Q wave becoming unusually wide and deep on the EKG. Pathological Q waves are typically defined by a duration greater than 0.04 seconds or a depth greater than one-third of the following R wave’s height in the same lead.
Unlike changes that signal an ongoing heart attack, such as ST-segment elevation, the pathological Q wave is a long-lasting marker. It serves as a permanent electrical footprint confirming an infarction occurred, even if the event was in the distant past. The specific EKG leads where the deep Q wave appears can also help localize the region of the heart that sustained the damage.