A “bad” EKG shows patterns that fall outside normal ranges for heart rhythm, timing, or electrical activity. These abnormalities can range from completely harmless variations to signs of a heart attack in progress. The most common abnormal findings are changes in the ST segment and T wave, which appear in roughly 35% of abnormal EKGs, followed by signs of chamber enlargement and evidence of prior heart damage.
Understanding what doctors look for on an EKG helps you make sense of your results and know which findings are urgent versus routine. Here’s what the major categories of abnormal patterns actually mean.
What a Normal EKG Looks Like
A normal EKG produces a repeating pattern of waves and intervals that fall within specific timing windows. The PR interval, which represents the time it takes an electrical signal to travel from the upper chambers to the lower chambers, normally runs between 120 and 200 milliseconds. The QRS complex, the sharp spike that represents the lower chambers contracting, lasts between 80 and 120 milliseconds. The corrected QT interval, measuring the full cycle of contraction and recovery, stays below about 460 milliseconds. A normal heart rate falls between 54 and 96 beats per minute.
When any of these intervals stretch too long, shorten too much, or when the shape of the waves changes, that’s what makes an EKG “bad.” The specific way it deviates from normal tells doctors what’s going wrong.
ST Segment and T Wave Changes
The ST segment is the flat stretch between the main spike (QRS complex) and the following rounded bump (T wave). In a normal EKG, this segment sits right along the baseline. When it rises or drops, it signals that part of the heart muscle isn’t getting enough blood or is actively being damaged.
ST elevation is the hallmark of a heart attack in progress. The thresholds are surprisingly small: a rise of just 0.2 millivolts (about 2 millimeters on the paper) in certain leads is significant for men over 40, while the threshold for women is 0.15 millivolts. In men under 40, the cutoff is slightly higher at 0.25 millivolts. ST depression, where the segment dips below the baseline, can indicate reduced blood flow to the heart without full blockage.
T wave changes are the single most common EKG abnormality. Normally, T waves are gently rounded and upright. Flattened, inverted, or unusually tall (“peaked”) T waves all suggest different problems. Deep, symmetric T wave inversions in the chest leads can indicate Wellens syndrome, a pattern that warns of an impending heart attack even when blood tests look nearly normal. This pattern is critical to catch because stress testing, which might seem like a logical next step, is actually dangerous in these patients.
Dangerous Rhythm Abnormalities
Some of the most alarming EKG patterns involve the heart’s rhythm going haywire. In atrial fibrillation, the regular repeating pattern disappears. Instead of neat, evenly spaced beats, the rhythm becomes completely irregular with no identifiable P waves (the small bumps that represent the upper chambers firing).
Ventricular tachycardia looks dramatically different from a normal tracing. The QRS complexes become wide and bizarre-looking, the heart rate jumps above 150 to 200 beats per minute, and the normal wave pattern is replaced by broad, fast, repetitive shapes. This is a medical emergency. Any wide, fast, regular rhythm without normal activity from the upper chambers should be treated as ventricular tachycardia until proven otherwise, because misidentifying it can cause the heart to stop.
Heart Block Patterns
Heart blocks happen when the electrical signal between the upper and lower chambers gets delayed or completely interrupted. They come in degrees, and each looks different on the EKG.
- First-degree block: Every beat looks normal in shape, but the PR interval stretches beyond 200 milliseconds. The signal gets through every time, just slowly. This is often harmless.
- Second-degree block (Mobitz type I): The PR interval gets progressively longer with each beat until one beat is completely dropped. Then the cycle resets. This creates a distinctive repeating pattern of gradually stretching intervals followed by a missing beat.
- Second-degree block (Mobitz type II): Beats are randomly dropped without any warning pattern. The PR interval stays the same on conducted beats, then a QRS complex simply doesn’t appear. This is more concerning because it can progress to complete block.
- Third-degree (complete) block: The upper and lower chambers beat independently of each other. On the EKG, you can see P waves “marching through” the QRS complexes at their own faster rate while the ventricles fire slowly and separately. There is no relationship between the two. This typically requires a pacemaker.
Bundle Branch Blocks
The heart’s electrical system splits into two branches to activate the left and right lower chambers simultaneously. When one branch is blocked, the signal has to take a detour, which widens the QRS complex beyond 120 milliseconds and changes its shape.
A right bundle branch block produces a characteristic “bunny ear” pattern in certain leads, with an extra upward deflection at the end of the QRS complex. The T waves in those leads flip in the opposite direction of this extra deflection. An incomplete right bundle branch block looks similar but the QRS width stays between 100 and 119 milliseconds.
A left bundle branch block is generally considered more clinically significant because it can mask the EKG signs of a heart attack. When a left bundle branch block is new, it’s treated with the same urgency as ST elevation.
Signs of Electrolyte Problems
Potassium imbalances leave distinctive fingerprints on an EKG, and these changes can signal a life-threatening situation even before symptoms appear.
High potassium (hyperkalemia) produces tall, narrow, peaked T waves as the earliest sign. As levels climb higher, the P waves flatten and eventually vanish, the QRS complex widens, and the PR interval stretches. In severe cases, the T wave and QRS complex merge into a smooth, undulating “sine wave” pattern. This is a pre-cardiac arrest rhythm.
Low potassium (hypokalemia) creates the opposite picture: T waves become shallow and flattened, and a new wave called the U wave appears after the T wave. In severe cases (potassium below 3 mEq/L), the U wave can grow so large it swallows the T wave entirely, making the overall interval look dangerously prolonged.
When “Abnormal” Is Actually Normal
Not every unusual-looking EKG is dangerous. Early repolarization, a pattern where the ST segment is slightly elevated with a concave (scooped upward) shape, is common in young, healthy people. It can look alarmingly similar to a heart attack on paper, but several features help distinguish the two.
In benign early repolarization, the ST elevation is typically modest (around 1 millimeter), the shape is concave rather than straight or convex, and the ratio of ST elevation to T wave height stays below 0.25. Reciprocal ST depression, where leads on the opposite side of the heart show matching drops, appears only in one specific lead. In a true heart attack, reciprocal depression shows up across multiple leads, the ST changes evolve over time as symptoms fluctuate, and the R waves (the tall central spikes) tend to be smaller.
Other benign findings that can make an EKG look abnormal include sinus arrhythmia (heart rate that speeds up when you breathe in and slows when you breathe out, completely normal in young people), athletic heart changes from endurance training, and minor nonspecific T wave changes that have no clinical significance. Your doctor may note these as “abnormal” on the report while reassuring you that no treatment is needed.