A terminal R wave is the last upward deflection at the end of the QRS complex on an electrocardiogram (ECG). It appears as a small or large positive bump just before the QRS complex finishes, representing the final phase of electrical activation spreading through the ventricles. Depending on which ECG lead it shows up in and how tall it is, a terminal R wave can be a normal finding, a sign of a conduction delay, or a red flag for drug toxicity.
How the QRS Complex Produces a Terminal R Wave
Every heartbeat generates electrical signals that travel through the ventricles in a specific sequence. The QRS complex on an ECG captures this entire process: the initial deflection reflects the first areas of the ventricle to activate, and the terminal portion reflects the last. When some part of the ventricle depolarizes later than usual, that delayed electrical activity registers as an upward deflection at the tail end of the QRS, creating what’s called a terminal R wave (often written as R’ or “R prime” in reports).
The reasons for this delayed activation vary. It could be a normal quirk of how the right ventricle finishes its electrical cycle, or it could mean that electrical signals are taking a detour because of a blocked conduction pathway. It might also indicate that a drug or toxin is slowing the sodium channels that cardiac cells rely on to conduct impulses. The lead where the terminal R wave appears, its size, and the overall width of the QRS complex all determine what it means.
Terminal R Wave in Leads V1 and V2
The most common clinical context for a terminal R wave is in the right-sided chest leads, V1 and V2. Here, a prominent R’ wave is the hallmark of right bundle branch block (RBBB), a condition where the electrical signal to the right ventricle is delayed because the right bundle branch isn’t conducting normally. The left ventricle fires on time, but the right ventricle activates late, producing that characteristic extra upward bump at the end of the QRS.
A complete RBBB is diagnosed when the QRS duration is 120 milliseconds or longer, with an rSR’, rsR’, or rsr’ pattern in V1 and V2. The rSR’ pattern, where a small initial r wave is followed by a deep S wave and then a tall R’ wave, is the most common. The rsR’ variant is sometimes called the “bunny ear” pattern because of its two peaked humps. When the QRS duration falls between 100 and 119 milliseconds with a similar shape, it’s classified as an incomplete RBBB, which is often a benign finding in younger, healthy individuals.
A terminal R wave in V1 and V2 also plays a role in evaluating Brugada syndrome, a genetic condition that increases the risk of dangerous heart rhythms. In Brugada syndrome, the r’ wave in these leads tends to be larger than normal. Two competing theories explain this: one points to abnormal differences in how the inner and outer walls of the right ventricular outflow tract repolarize, while the other attributes it to delayed activation of the same region. Cardiologists use the specific angles formed by the r’ wave’s shape to help distinguish a suspicious but harmless ECG pattern from one that confirms Brugada syndrome.
Terminal R Wave in Lead aVR
Lead aVR is often overlooked during routine ECG interpretation, but a terminal R wave here carries serious implications, particularly in poisoning and drug toxicity. When drugs that block sodium channels reach toxic levels, they slow electrical conduction through the ventricles. This widens the QRS complex and produces a prominent terminal R wave (R’) in lead aVR.
The classic scenario is tricyclic antidepressant overdose, but any drug that blocks cardiac sodium channels can cause the same pattern. Antiepileptic medications like carbamazepine and lacosamide, certain antiarrhythmics, and even cocaine can do it. In one reported case, a patient on a combination of carbamazepine and lacosamide developed a QRS width of 240 milliseconds with a dominant terminal R wave in aVR, a pattern consistent with significant sodium channel blockade.
Specific size thresholds in lead aVR help predict how dangerous the toxicity is. A terminal R wave amplitude of 3 mm or more is the most reliable predictor of seizures or cardiac arrhythmias in sodium channel blocker poisoning, with 81% sensitivity. A terminal R wave of 5 mm or greater is less sensitive (50%) but highly specific (97%), meaning that when you see one that tall, the risk of a serious cardiac event is very real. Emergency physicians use these measurements alongside QRS width to guide treatment decisions in overdose situations.
Role in Identifying Heart Rhythm Problems
Terminal R waves also help clinicians figure out whether a fast, wide-complex heart rhythm is ventricular tachycardia (VT) or a supraventricular tachycardia (SVT) with abnormal conduction. This distinction matters because the two conditions require different treatments. One approach compares the speed of the initial ventricular activation to the speed of the terminal activation. When the terminal portion of the QRS is slower than the initial portion (a ratio of initial to terminal velocity of 1 or less), it favors VT. A faster initial activation with a sluggish terminal R wave suggests the impulse originated in the ventricle rather than traveling down from above.
In heart attacks, terminal R wave changes can also appear. Ischemia and oxygen deprivation slow the affected heart muscle’s ability to conduct electricity, which can prolong the QRS complex and alter its terminal portion. These changes reflect the damaged muscle’s struggle to depolarize at a normal speed, as energy-starved cells produce lactic acid and lose the fuel they need for proper electrical function.
When a Terminal R Wave Is Normal
Not every terminal R wave signals a problem. A small r’ in V1 with a normal QRS duration (under 100 milliseconds) is a common, harmless variant, especially in younger people and those with slender builds. An incomplete RBBB pattern in an otherwise healthy person with no symptoms is typically considered benign and doesn’t require treatment or further workup on its own.
Context is everything. A terminal R wave’s meaning depends on which lead it appears in, how tall it is, how wide the overall QRS complex is, what medications you’re taking, and whether you have symptoms. The same squiggle that’s completely harmless in one setting can be a medical emergency in another.