Pacing a patient means using an external or internal device to deliver electrical impulses that stimulate the heart to beat at an adequate rate. It’s indicated when a dangerously slow heart rhythm (bradycardia) causes low blood pressure, altered mental status, signs of shock, chest pain, or acute heart failure, and medication like atropine has failed to correct it. The two main approaches are transcutaneous pacing (external, through the skin) and transvenous pacing (a wire threaded into the heart through a vein). Transcutaneous pacing is faster to set up and serves as the first-line bridge, while transvenous pacing provides more reliable, longer-term capture.
When Pacing Is Indicated
The AHA bradycardia algorithm calls for pacing when a patient has a persistent slow heart rhythm that is causing hemodynamic instability. The key signs are hypotension, acutely altered mental status, signs of shock, ischemic chest discomfort, or acute heart failure. Atropine is typically tried first, but if it fails to restore an adequate heart rate and blood pressure, the next step is transcutaneous pacing, sometimes alongside a vasopressor infusion. Pacing may also be used prophylactically in patients at high risk for progressing to complete heart block, such as after cardiac surgery or during certain types of myocardial infarction.
Transcutaneous Pacing: Step by Step
Preparing the Patient and Pads
Start by preparing the skin and attaching standard limb leads for cardiac monitoring. Place the therapy pads on the torso, avoiding any implanted devices, medication patches, or broken skin. Full adhesion is essential for effective current delivery.
Two pad configurations are used. In anteroposterior placement, one pad goes on the front of the chest at the cardiac apex (roughly the V3 electrode position) and the other on the back between the left shoulder blade and the spine. In anterolateral placement, one pad goes on the left side of the chest (V5 position) and the other on the right upper chest near the sternum. Evidence suggests anteroposterior placement may produce better capture rates. If the pads are marked positive and negative, the negative pad goes anterior (or to the left in anterolateral placement).
Setting the Mode and Rate
Power on the defibrillator and switch it to pacing mode. Select demand (or VVI) mode, which delivers a pacing impulse only when the device does not detect a native heartbeat. This prevents the pacer from firing on top of the patient’s own rhythm. Confirm that the device is sensing the patient’s intrinsic rhythm by looking for arrows or triangles on the monitor. Adjust the gain if sensing markers are absent.
Set the pacing rate between 60 and 80 beats per minute, high enough above the patient’s native rate to improve cardiac output and produce a palpable pulse increase.
Finding the Capture Threshold
The approach to setting the current output depends on whether the patient is conscious or unconscious.
- Conscious or prophylactic patients: Begin at minimal output (5 to 10 mA) and increase in 5 to 10 mA increments until every pacer spike on the monitor is followed by a wide QRS complex. This is electrical capture. Stop increasing once capture is achieved or the patient reports intolerable discomfort.
- Unconscious or pulseless patients: Start at maximum output (200 mA) to capture the heart as quickly as possible. Once capture is confirmed, gradually decrease the current until capture is lost. That lowest effective level is the pacing threshold.
Most patients achieve capture somewhere between 20 and 140 mA. Relatively stable patients tend to fall in the 40 to 80 mA range. Once you identify the threshold, set the final output 5 to 10 mA above it to maintain a reliable safety margin.
Confirming Mechanical Capture
Electrical capture on the monitor (a QRS complex after every pacer spike) does not guarantee the heart is actually pumping effectively. Mechanical capture must be confirmed separately. Palpate a central pulse, check for a pulse oximetry waveform that matches the set pacing rate, or use bedside ultrasound to visualize ventricular contraction. An arterial line reading that matches the pacer rate is another reliable confirmation method. If there’s electrical capture without a pulse, the current output may need to be increased or underlying causes of pulselessness addressed.
Transvenous Pacing
Transvenous pacing involves threading a pacing wire through a central vein and positioning the tip inside the right ventricle. It delivers current directly to the heart muscle, requiring far less energy than external pacing and providing more stable, sustained capture. It is typically the next step when transcutaneous pacing is needed for more than a short bridge period.
The preferred access sites are the right internal jugular vein and the left subclavian vein. Ultrasound guidance improves accuracy and safety during both vein access and lead positioning. A subxiphoid or modified subxiphoid ultrasound view can help confirm that the wire tip is in the right ventricle. On the monitor, ST-segment elevation appears when the wire tip contacts the inner wall of the ventricle, signaling correct placement.
Transvenous pacing carries higher procedural risks than external pacing. In a study of 244 patients, right ventricular perforation occurred in about 3.3% of cases, cardiac tamponade in 1.6%, and vein thrombosis in 0.8%. Lead dislocation requiring repositioning, pneumothorax, and infection are also recognized complications.
Managing Patient Comfort
Transcutaneous pacing is painful for conscious patients. The repeated electrical stimulation causes chest wall muscle contraction that ranges from uncomfortable to severe. Sedation and pain management should be initiated as soon as hemodynamics allow. The goal is to keep the patient comfortable enough to tolerate pacing while maintaining a safe blood pressure and airway. In practice, this often means short-acting sedatives and analgesics titrated carefully, since many of these patients already have compromised circulation.
Transvenous pacing, once placed, is generally well tolerated. The primary discomfort comes from the insertion procedure itself, which is managed with local anesthesia at the access site and systemic sedation when needed.
Reading the Monitor: Normal vs. Abnormal Patterns
Understanding what the cardiac monitor shows during pacing is critical for catching problems early.
Normal ventricular pacing looks like a sharp vertical spike immediately followed by a wide QRS complex on every beat. If the device is pacing both chambers (dual-chamber pacing), you’ll see a spike before each P wave and another spike before each QRS. Fusion beats, where the pacer fires at the same moment the heart generates its own impulse, produce a QRS that looks like a hybrid between the paced and native morphology. These are normal and indicate the patient’s own rhythm is intermittently competing with the pacer.
Troubleshooting Common Problems
Failure to Capture
You see pacer spikes on the monitor, but no QRS complex follows them. The device is firing, but the heart isn’t responding. Common causes include lead displacement, insufficient output current, electrolyte abnormalities, or poor pad contact in transcutaneous pacing. The immediate fix is to increase the output. If that fails, check pad adhesion and position, or suspect lead dislocation in transvenous systems.
Undersensing
The pacemaker fails to detect the patient’s native heartbeat, so it fires pacing spikes even when the heart is beating on its own. This shows up on the monitor as too many pacing spikes appearing without regard to the patient’s underlying rhythm. The danger is that a pacing impulse could land on the vulnerable repolarization phase of the cardiac cycle (the T wave), potentially triggering ventricular fibrillation. Causes include a sensitivity setting that is too high (in millivolts), lead displacement, myocardial fibrosis at the lead tip, or electrolyte disturbances. Adjusting the sensitivity threshold lower (making the device more sensitive to smaller signals) often corrects this.
Oversensing
The opposite problem. The pacemaker detects signals it shouldn’t, such as T waves, skeletal muscle activity, or electromagnetic interference, and interprets them as native heartbeats. It then withholds pacing when the patient actually needs it. The monitor shows fewer pacing spikes than expected, and the patient’s heart rate may drop. Increasing the sensitivity threshold (making the device less responsive to small signals) or identifying and removing the source of interference addresses oversensing.
Output Failure
No pacing spikes appear on the monitor at all. The device simply isn’t firing. This can result from a lead fracture, generator malfunction, battery depletion, or oversensing severe enough to completely inhibit output. Check all connections, replace the battery or generator if needed, and verify lead integrity.
Ongoing Monitoring
Once pacing is established, continuous cardiac monitoring is essential. Regularly verify that every pacer spike produces a QRS complex and that the patient maintains a palpable pulse. For transvenous systems, monitor the insertion site for signs of infection, secure the lead to prevent displacement, and perform periodic threshold checks by briefly lowering the output to confirm the minimum current needed for capture hasn’t changed. A rising threshold can indicate lead displacement or developing fibrosis at the tip.
Sensitivity should also be rechecked. A pacemaker set in AAI mode (atrial pacing only) requires particularly close attention because it provides no backup if the patient develops a rhythm problem below the atria, such as complete heart block. DDD and VVI modes offer more inherent safety by monitoring and pacing the ventricles directly.
Transcutaneous pacing is a temporary bridge. The chest wall muscle contractions, skin irritation, and patient discomfort make it unsuitable for extended use. The clinical goal is always to transition to transvenous pacing or a permanent pacemaker, or to resolve the underlying cause of the bradycardia, as soon as conditions allow.