An electrocardiogram, commonly known as an ECG or EKG, is a non-invasive medical test that records the electrical activity of the heart over a period of time. Electrodes placed on the skin detect the tiny electrical changes that occur as the heart muscle depolarizes and repolarizes with each beat. Analyzing the resulting tracing, which is a series of upward and downward spikes and waves, allows clinicians to assess the heart’s overall health. This process is particularly useful for identifying issues with heart rate, rhythm, and the presence of damage to the heart muscle. Understanding the meaning of the downward spikes is fundamental to interpreting the full report.
The Components of a Normal ECG Reading
The standard ECG tracing illustrates the heart’s cardiac cycle as a sequence of three primary waveforms, each reflecting a specific electrical event. The P wave is a small, rounded upward deflection that corresponds to the depolarization (electrical activation) of the atria, the heart’s upper chambers. Following the P wave is the QRS complex, a sharp, multi-pronged structure representing the rapid depolarization of the larger, muscular ventricles. This complex is typically the most prominent feature on the tracing because of the significant muscle mass involved in ventricular contraction.
The QRS complex is then followed by the T wave, a generally smooth, upward-sloping wave that signifies ventricular repolarization (the electrical recovery phase). A general principle of electrophysiology explains the shape of these waves: an electrical impulse traveling toward a positive electrode results in an upward deflection, while one moving away causes a downward deflection. This concept helps explain why the QRS complex features both upward and downward spikes. The entire P-QRS-T sequence represents one complete heartbeat.
Defining the Normal Downward Deflections
The QRS complex contains the two main downward deflections, the Q wave and the S wave, which are expected components of a normal heartbeat. The Q wave is the first negative deflection of the QRS complex, preceding any upward R wave. Physiologically, this small wave represents the initial electrical activation of the interventricular septum, the wall separating the two ventricles. Because this activation moves away from the primary lead placement, it often produces a small negative signal.
In a healthy heart, the Q wave is characteristically narrow and shallow, often measuring less than \(0.04\) seconds in duration and less than \(25\%\) of the height of the subsequent R wave. The S wave is the second main downward deflection, defined as the first negative deflection that follows the R wave. This wave represents the final electrical activation of the ventricles, specifically the depolarization of the basal and posterior portions of the heart.
Pathological Significance of Abnormal Spikes
When these downward deflections exceed their normal parameters, they often signal significant underlying cardiac pathology. A pathological Q wave is a particularly recognizable and serious finding, typically indicating a prior myocardial infarction (MI), or heart attack. This wave becomes abnormal when it is too wide, measuring \(\geq 0.04\) seconds, or too deep, often defined as being \(\geq 25\%\) of the height of the R wave in the same complex. The appearance of a pathological Q wave occurs because the heart muscle that died during the MI becomes electrically inert, or silent. This dead tissue causes the electrical signal to be rerouted, generating a deep, wide downward deflection that serves as a permanent electrical “scar” on the tracing.
Another significant abnormality is an excessively deep S wave, which is often associated with conditions that increase the electrical mass of the heart. For example, left ventricular hypertrophy (LVH), the enlargement of the left ventricle’s muscle wall, can cause deep S waves in the V1 and V2 leads on the chest. The increased muscle mass generates a larger electrical force that travels away from these leads, resulting in a deeper negative deflection.
A third significant downward deflection is the inverted T wave, which is a reversal of the normal upward-pointing T wave. Although not a sharp spike like the Q or S wave, its downward direction is a sign of abnormal ventricular repolarization. An inverted T wave frequently suggests myocardial ischemia (a condition where the heart muscle is not receiving enough blood flow and oxygen). The inversion is a result of the electrical recovery phase being disturbed by the lack of adequate blood supply, and its depth and location can help indicate the severity and location of the underlying cardiac strain.
Clinical Evaluation Following Abnormal Findings
The detection of a significant downward spike, such as a pathological Q wave or an inverted T wave, immediately triggers a comprehensive clinical assessment. The physician must correlate the ECG findings with the patient’s current symptoms, medical history, and physical examination. Blood tests are a next step, specifically measuring cardiac biomarkers like high-sensitivity troponin, a protein released into the bloodstream when the heart muscle is damaged.
A rising and falling pattern of troponin values, combined with abnormal ECG changes, is highly indicative of an acute coronary syndrome, even if the patient’s symptoms are vague. Further imaging is then typically performed, with an echocardiogram being common practice to visualize the heart’s structure, motion, and function. This ultrasound of the heart can assess the pumping efficiency of the ventricles, known as the ejection fraction, and confirm structural abnormalities like ventricular hypertrophy. The treatment approach is entirely dependent on the final diagnosis, ranging from managing chronic conditions with medication to urgent invasive procedures, such as coronary angiography, for acute blockages.