Supraventricular tachycardia (SVT) has a distinctive appearance both on a heart monitor and in the person experiencing it. On an EKG, it shows up as a fast, regular rhythm with narrow electrical spikes, typically between 150 and 220 beats per minute. In person, someone having an episode may look pale or anxious, with a visibly pulsating neck, and they’ll usually describe a sudden pounding sensation in their chest.
What SVT Looks Like on an EKG
The hallmark of SVT on a heart tracing is a narrow QRS complex, the tall spike that represents each heartbeat. In SVT, that spike measures less than 120 milliseconds wide, which tells clinicians the electrical signal is traveling through the heart’s normal wiring rather than taking a slow, abnormal path through the muscle itself. The rhythm is almost always regular, with evenly spaced beats clicking along at 150 to 220 beats per minute, though rates can reach as high as 300.
What’s often missing from the tracing is just as telling. In the most common form of SVT, the P waves (small bumps that normally appear before each spike and represent the upper chambers firing) are either invisible or buried inside the main spike. This happens because the upper and lower chambers of the heart are activating almost simultaneously. When P waves do peek through, they can show up as a tiny extra notch at the top of the spike in one view, or a small downward dip in another. These subtle findings are easy to miss at a glance, which is one reason doctors sometimes need to compare tracings taken during and after an episode.
How SVT Differs From Dangerous Fast Rhythms
The critical visual distinction on a monitor is the width of that main spike. SVT keeps its QRS complex narrow (under 120 ms) because the signal uses the heart’s built-in fast-track wiring. Ventricular tachycardia (VT), a more dangerous rhythm originating in the lower chambers, produces wide, bulky complexes, typically over 140 ms. The initial slope of the spike also differs: SVT produces a sharp, steep rise, while VT often starts with a slower, more sluggish upstroke because the signal is crawling through muscle tissue instead of specialized pathways.
This distinction matters because the two rhythms require very different responses. A fast, narrow, regular rhythm on a monitor is reassuring in the sense that it points toward SVT rather than VT, even though it still needs attention.
What It Feels Like During an Episode
SVT episodes tend to start and stop abruptly, like flipping a switch. One moment your heart is beating normally, and the next it’s racing. People commonly describe palpitations (a forceful, rapid thumping in the chest or throat), dizziness, nausea, sweating, and a wave of anxiety that can feel like a panic attack. Some people experience atypical chest pain or a sense of pressure. In more intense episodes, lightheadedness can progress to near-fainting or full fainting.
From the outside, the most visible sign is rapid pulsation in the neck, where the jugular veins can be seen bouncing with each fast beat. One patient described friends becoming nervous when they saw their neck pulsating visibly. Skin may appear flushed or pale, and the person often looks distressed or restless. Episodes can last anywhere from a few minutes to, in rare cases, several days.
Common Triggers
SVT affects roughly 2.29 per 1,000 people, with about 89,000 new cases diagnosed each year in the United States. Known triggers include emotional stress, high caffeine intake, excessive alcohol use (more than 14 drinks per week for men or 7 for women), smoking, and stimulant drugs like cocaine or methamphetamine. Some over-the-counter medications contain stimulants that can also set off an episode. Many people find it helpful to keep a log of what they were doing, eating, or drinking before each episode to identify personal patterns.
Stopping an Episode With Vagal Maneuvers
Because SVT involves an electrical loop running through or near a specific junction in the heart, you can sometimes break the circuit by stimulating the vagus nerve, which slows conduction through that junction. The most effective technique is the modified Valsalva maneuver: sit semi-reclined, take a deep breath, and blow hard into a 10 mL syringe (or as if you’re inflating a stiff balloon) for about 15 seconds. Immediately after, lie flat and have someone raise your legs to a 45-degree angle for 15 seconds, then sit back up.
A large meta-analysis found this modified technique converts SVT back to a normal rhythm in roughly 43 to 58 percent of attempts, more than double the success rate of the standard sitting-upright version (which works only about 17 to 20 percent of the time). Carotid sinus massage, where firm pressure is applied just below the jaw angle, is another option but has the lowest conversion rate of the commonly used maneuvers at under 1 percent probability of being the most effective single-attempt technique.
When an Episode Becomes an Emergency
Most SVT episodes are uncomfortable but not dangerous. They become a medical emergency when they’re accompanied by fainting, severe dizziness that doesn’t let up, chest pain that persists, increasing shortness of breath, low blood pressure, or signs of heart failure such as swelling or fluid in the lungs. A heart rate that stays above 120 beats per minute and won’t respond to vagal maneuvers also warrants emergency evaluation, where medications or a brief, controlled electrical reset can restore normal rhythm quickly.
The Two Main Subtypes on a Monitor
Most SVT falls into one of two electrical patterns. The more common type, AVNRT, involves a short circuit looping within the heart’s central junction. On a tracing, P waves are usually invisible or hidden inside the QRS spike. The less common type, AVRT, uses an extra electrical pathway that runs between the upper and lower chambers. Because the signal has to travel a longer loop, P waves are visible after the main spike in virtually 100 percent of cases, appearing as a distinct bump between beats. In one study comparing the two, AVNRT showed no visible P waves in 27 percent of cases and disguised P waves (the pseudo-R’ or pseudo-S pattern) in 57 percent, while AVRT showed clearly visible P waves in all 26 cases examined.
For most people experiencing SVT, the subtype doesn’t change what the episode feels like. But it does matter for long-term treatment planning, since the location of the short circuit determines whether and how a catheter procedure can permanently fix the problem.