The white line on a heart monitor is almost always a pacemaker spike, a thin vertical line that appears when an implanted pacemaker sends an electrical pulse to the heart. These spikes are very short in duration, typically lasting about 2 milliseconds, and show up as sharp, narrow lines that stand out from the normal rolling waveform of a heartbeat. If you’re watching a loved one’s monitor or looking at your own, these lines indicate the pacemaker is firing to keep the heart beating in rhythm.
What a Pacemaker Spike Looks Like
On a heart monitor, the normal heartbeat trace has rounded humps and larger wave patterns. A pacemaker spike looks different: it’s a very thin, nearly vertical line that shoots up (or down) sharply before the heart’s electrical wave follows. The spike represents the moment the pacemaker delivers a tiny electrical stimulus to the heart muscle, triggering it to contract.
The size of the spike varies depending on the type of pacemaker lead. Unipolar leads produce a larger, more visible spike, while bipolar leads create a much smaller one that can be hard to spot on certain monitor views. Leads placed on the outside surface of the heart also tend to produce smaller spikes than those threaded through a vein into the heart’s interior.
Where the Spike Appears Matters
The position of the white line relative to the heart’s normal waveform tells medical staff which part of the heart is being paced.
- Before the small initial bump (P wave): The pacemaker is pacing the upper chambers of the heart (atrial pacing). The spike appears, then the small rounded wave follows as the upper chambers contract.
- Before the large spike-and-dip complex (QRS): The pacemaker is pacing the lower chambers (ventricular pacing). The heart’s main pumping contraction follows the spike, and the resulting waveform looks wider than a normal heartbeat.
- Two spikes per heartbeat: A dual-chamber pacemaker is pacing both the upper and lower chambers. You’ll see one spike before the small bump and another before the large complex.
Some advanced pacemakers used in cardiac resynchronization therapy pace both sides of the lower chambers. In these cases, two spikes may appear close together before the main heartbeat wave, one for each side. These can fire simultaneously or in a timed sequence, with the left side often firing slightly before the right.
A Normal Sign in Pacemaker Patients
If someone has a pacemaker, seeing these white lines on their monitor is expected and usually reassuring. It means the device is delivering electrical pulses and the heart is responding. The key thing nurses and doctors look for is whether a normal heartbeat wave follows each spike. When the spike fires and the heart muscle contracts in response, that’s called “capture,” and it means the pacemaker is working correctly.
In patients with dual-chamber pacemakers, the monitor may not always show two spikes per beat. If the heart’s upper chambers are firing on their own, the pacemaker senses that natural activity and only paces the lower chambers. So you might see stretches where spikes appear before every beat, then periods where fewer spikes show up. This is normal and means the pacemaker is adapting to what the heart needs moment to moment.
When the Spike Appears Without a Heartbeat
Occasionally, a pacemaker spike shows up on the monitor without a normal heartbeat wave following it. This is called failure to capture, and it means the electrical pulse wasn’t strong enough to trigger a contraction, or something else prevented the heart from responding. In one documented case, a patient experienced dizziness and near-fainting episodes caused by pacing spikes that weren’t consistently producing heartbeats. The issue was resolved by adjusting the pacemaker’s settings.
A related problem is sensing failure, where the pacemaker doesn’t properly detect the heart’s own electrical activity. This can cause spikes to fire at the wrong time, sometimes appearing right after the heart has already beaten on its own. Both issues are typically fixable through reprogramming the device, which is done externally without surgery.
Other Possible Explanations
If the person being monitored doesn’t have a pacemaker, a sharp white line on the monitor is most likely electrical artifact. This happens when something interferes with the signal: muscle movement, a loose electrode on the skin, nearby electrical equipment, or even shivering. Artifact lines tend to look irregular and don’t have the consistent, precise appearance of true pacemaker spikes. They also won’t appear in a predictable relationship to the heartbeat pattern.
Some older bedside monitors use a scanning refresh line, a bright bar that sweeps across the screen erasing the old tracing as the new one is drawn. This moving vertical line is part of the display itself, not part of the heart rhythm data. You can tell the difference because it moves steadily across the screen at a constant speed regardless of what the heart is doing.