How Defibrillator Pacing Works

How a defibrillator sends controlled electrical pulses through therapy pads to stimulate the heart, and why current, impedance, sensing, capture, and cable condition all matter

Many modern defibrillators do more than deliver shocks.

Published August 28, 2026 · Revised September 6, 2026

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In transcutaneous pacing, the defibrillator sends repeated controlled-current pulses through the therapy cable and adhesive pads. The selected rate sets the timing, while the current setting and patient impedance affect the voltage required to deliver each pulse. Electrical output does not by itself prove that the myocardium responded; clinical capture is a separate patient response.

In demand mode, the defibrillator also monitors ECG activity. A detected intrinsic beat should inhibit the scheduled pacing pulse, while the absence of a qualifying beat allows a pulse to be delivered. ECG leads, sensing gain, artifact, cable condition, pads, load impedance, timing, and output must all be considered when pacing behavior appears wrong.

Worked Example: Pacing Pulses Are Missing

Use an approved defibrillator analyzer and the manufacturer's test mode and load. Confirm fixed versus demand mode, selected rate and current, therapy cable, and analyzer connection. If fixed pacing produces correct pulses but demand pacing does not, investigate ECG input, simulated rate, sensing threshold, lead selection, and inappropriate inhibition before blaming the pulse generator.

If neither mode produces output, inspect the complete therapy path and review error logs using the service procedure. Verify pulse rate, current across specified loads, demand inhibition, alarms, and cable condition after repair. High-energy circuits can retain hazardous charge; follow discharge and service-safety requirements.

What Is Transcutaneous Pacing?

Transcutaneous pacing uses external adhesive pads to deliver electrical pulses through the chest.

The goal is to stimulate the myocardium when the patient's own heart rate is inadequate.

It is different from:

even though the same therapy pads may be used.

Defibrillation vs Pacing

Defibrillation delivers:

One large, brief high-energy shock.

Pacing delivers:

Repeated lower-energy pulses.

The electrical output hardware and control method are different.

Pacing Rate

The pacing rate is usually expressed in:

Pulses per minute, often corresponding to beats per minute.

Example:

70 ppm.

The defibrillator should generate approximately:

unless demand pacing suppresses some pulses because intrinsic cardiac activity is detected.

Pacing Current

Pacing output is commonly set in:

milliamps, or mA.

Example:

60 mA.

This controls the strength of the pacing pulse.

Current Is Different From Voltage

The clinician typically selects current.

The device adjusts the voltage needed to drive approximately that current through the patient or test load.

Why Voltage Changes

Ohm's law applies conceptually:

If patient impedance is higher:

The defibrillator may need more voltage to deliver the same pacing current.

Constant-Current Pacing

Many pacing systems are designed to approximate constant-current output.

That means:

Set:

60 mA.

The device attempts to deliver about:

even as patient impedance varies within its supported range.

Current Source

Internally, the defibrillator uses controlled electronics to generate the pacing pulse.

This may involve:

Each pacing pulse has several characteristics.

These may include:

Manufacturer specifications define acceptable limits.

Pulse Width

Pulse width describes how long each pacing pulse lasts.

It may be measured in:

Milliseconds.

The pulse is much longer than many ordinary electronic control signals but far smaller than the interval between paced beats.

Why Pulse Width Matters

Stimulation depends on both:

A pulse with correct current but incorrect width may not represent correct pacing output.

Defibrillator Analyzer

A defibrillator analyzer can independently measure pacing output.

Depending on analyzer capability, it may measure:

Set Current Is Not Proof of Delivered Current

Suppose the defibrillator is set:

60 mA.

That proves:

The commanded pacing current is 60 mA.

It does not prove:

60 mA is actually reaching the load.

Use an analyzer.

Example

Set:

60 mA.

Analyzer:

59 mA.

If within manufacturer tolerance:

Pass.

Low Output

Set:

60 mA.

Analyzer:

35 mA.

Now investigate:

Pacing indicator flashes.

Analyzer measures:

0 mA.

Possible causes include:

Pacing current reaches the patient through adhesive therapy pads.

These may also support:

depending on system.

Therapy Cable

The cable is part of the pacing current path.

Damage can cause:

A therapy cable may pass basic continuity and still have problems under actual output conditions.

Inspect and test using manufacturer-approved procedures.

Connector Damage

Look for:

Repeated high-energy therapy places demands on these interfaces.

Pad Contact

Poor pad contact increases impedance.

The defibrillator may:

The electrical path through the chest presents impedance.

The device may monitor this to determine whether pacing output can be delivered effectively.

Analyzer Load

A defibrillator analyzer provides a known electrical load.

That gives a controlled way to verify pacing current.

Load Matters

If output is specified at a defined resistance:

Use that load.

A pacing circuit may behave differently at:

Some procedures may verify pacing current at more than one load.

That tests the current-regulation system.

Example

Set:

80 mA.

Analyzer at 50 Ω:

79 mA.

Analyzer at 100 Ω:

78 mA.

That suggests good regulation within those test conditions.

Current Drops at High Resistance

If:

the output stage may not be able to produce enough voltage under higher load.

Fixed Pacing

In fixed or asynchronous pacing, the device delivers pacing pulses at the selected rate regardless of intrinsic ECG activity.

Example

Set:

70 ppm.

The device sends approximately:

70 pulses per minute.

It does not wait for intrinsic beats.

Demand Pacing

Demand pacing monitors the patient's ECG.

If the patient's own heart beats faster than or appropriately within the pacing interval:

The device inhibits unnecessary pacing pulses.

Demand Mode Logic

Conceptually:

If another R wave appears:

Reset again.

If no R wave appears before pacing interval expires:

Deliver pacing pulse.

ECG Sensing Is Critical

Demand pacing depends on reliable ECG detection.

If the ECG signal is poor, pacing timing can be wrong even if the pacing-output circuit is perfect.

False R-Wave Detection

Artifact may be mistaken for intrinsic cardiac activity.

The device may inhibit pacing unnecessarily.

Missed R Waves

If real R waves are not detected:

The device may pace even though intrinsic beats are present.

Demand Pacing Problem Can Be an ECG Problem

This is extremely important.

If fixed pacing works correctly but demand pacing behaves incorrectly:

Look at:

before blaming the pacing-output circuit.

ECG Leads vs Therapy Pads

Some defibrillators may obtain ECG through:

depending on configuration.

Know which source is being used for demand pacing.

Lead Selection

Demand pacing may require an ECG lead with clear R waves.

A poor lead selection can create unreliable sensing.

R-Wave Marker

Some devices display markers showing detected intrinsic beats.

Those markers can be useful clues.

If pacing is inhibited unexpectedly but the monitor shows false R-wave markers:

Sensing is the issue.

Pacing Artifact

Each pacing pulse produces a large electrical artifact on the ECG.

The monitor must distinguish:

from:

Actual cardiac response.

Pace Marker

The display may show a marker for each pacing pulse.

This proves the device commanded or detected a pacing event.

It does not necessarily prove current reached the patient.

Electrical Capture

When a pacing pulse actually depolarizes the myocardium, this is called:

Electrical capture.

The ECG may show a QRS complex following the pacing spike.

Mechanical Capture

Mechanical capture means the electrical activation produces an actual cardiac contraction and pulse.

This is a clinical assessment.

Biomed Testing Does Not Prove Clinical Capture

A defibrillator analyzer can verify electrical pacing output.

It cannot prove that a specific patient's heart will respond to that current.

This Distinction Matters

If analyzer confirms:

then the equipment may be functioning correctly even if clinical capture was difficult.

Patient physiology is a separate factor.

Capture Threshold

Clinically, pacing current may be increased until capture occurs.

The required threshold varies by patient.

There is no single mA setting that guarantees capture in everyone.

Service Testing Is Different

During service:

Use defined analyzer loads and manufacturer specifications.

Do not judge device performance by whether a clinical patient happened to capture at a particular setting.

Pacing Rate Accuracy

Analyzer can verify pulse timing.

Example:

Set:

60 ppm.

Expected interval:

About 1 second between pulses.

Rate Error

Set:

60 ppm.

Analyzer:

45 ppm.

That is a timing/control problem.

Pulse Width Accuracy

Set pacing system may have a specified pulse width.

Analyzer verifies whether actual pulse duration falls within required range.

Why Rate and Current Should Be Tested Separately

A device can have:

but:

Wrong rate.

Or:

but:

Low current.

Different functions.

Demand Pacing Test

An analyzer can generate a simulated ECG.

The defibrillator should detect the simulated beats and respond appropriately.

Demand Test Example

Defibrillator set:

70 ppm demand mode.

Analyzer provides ECG:

80 bpm.

Device should generally inhibit pacing because intrinsic simulated rate is faster.

Slow Simulated Rate

Analyzer provides:

40 bpm.

Defibrillator set:

70 ppm.

The device should pace as required between the slower simulated beats according to its timing logic.

Demand Timing

The exact timing behavior is manufacturer-specific.

Use the analyzer's pacing test and service manual requirements.

Refractory Period

After detecting a beat or delivering a pacing pulse, the device may temporarily ignore certain signals.

This helps prevent:

The pacing system must avoid interpreting:

as true intrinsic beats.

ESU Interference

Electrical interference can affect ECG sensing.

The pacing-output system may be fine while demand sensing is disrupted.

Fixed Mode Can Help Isolate

If:

but:

that strongly points toward:

rather than the high-current output path.

No Pacing With Pads-Off Message

The device may intentionally prevent pacing if it does not detect a valid therapy connection.

Possible causes include:

The system may test the electrical path before enabling therapy.

An open circuit may trigger:

Pads Off.

Pacing and Defibrillation Share Accessories

If both:

look for a shared accessory.

Defibrillation Works, Pacing Fails

Now the cable may still be good for the high-energy shock path while another pacing-specific circuit has failed.

Shared accessories do not always mean identical internal electronics.

Pacing Works, Defibrillation Fails

The reverse can also occur.

Pacing output does not prove the defibrillation high-voltage system works.

Pacing Output at Low Settings

Service procedures may test several current settings.

Example:

depending on device.

Multi-Point Testing

One passing current does not prove the entire pacing-output range.

Linear Output Error

Example:

The system consistently delivers about 25% low.

That may suggest calibration or scaling.

Offset Error

Set → Analyzer

20 → 30

60 → 70

100 → 110

A roughly constant offset suggests a different problem.

High Current Only Fails

Example:

20 mA → Pass.

80 mA → Pass.

180 mA → Low.

The output stage may be reaching its voltage/current limit under load.

Battery Condition

Pacing may operate for extended periods on battery.

A weak battery could potentially affect overall therapy performance if the device cannot maintain internal supply rails.

Follow manufacturer battery and pacing-duration tests.

AC vs Battery Testing

If pacing works on AC but fails on battery:

Investigate:

before pacing hardware.

Continuous Load

Unlike a single defibrillation shock, pacing can continue for minutes or hours.

The output circuitry must handle repeated pulses reliably.

Thermal Behavior

A pacing fault may appear only after prolonged output.

If complaint is:

a brief analyzer test may miss it.

Long-Duration Test

Use the manufacturer-defined duration if required.

Observe:

The defibrillator may monitor its own pacing pulses.

If actual output differs from command, it may generate a fault.

Internal Self-Test

Self-test may check portions of:

But external analyzer verification remains important.

Internal Test Does Not Replace External Measurement

The device is checking itself using its own sensors.

Independent test equipment provides separate evidence.

Pacing Current Display

If the screen says:

that is usually the selected setting.

It is not necessarily a measured output value.

Therapy Event Log

The device may record:

This can be useful when investigating a clinical complaint.

Log Shows Setting, Not Necessarily Output

If the log says:

that proves those settings were selected or recorded.

It does not independently prove the therapy cable actually delivered them.

Serious Clinical Events

If a defibrillator is involved in an event where pacing allegedly failed:

Preserve:

according to facility incident procedures.

Do not casually reset or update the device first.

Real-World Example: No Pacing Output

Device enters pacing mode.

Markers appear.

Analyzer:

0 mA.

Known-good therapy cable:

Normal current returns.

Failure follows cable.

Real-World Example: Low Current at High Load

Set:

100 mA.

50 Ω analyzer load:

99 mA.

100 Ω:

65 mA.

Output stage cannot maintain commanded current as required at higher impedance.

Real-World Example: Demand Pacing Erratic

Fixed mode:

Rate and current perfect.

Demand mode:

Pulses inhibited randomly.

ECG waveform shows heavy artifact and false R-wave markers.

Pacing hardware is fine.

Sensing is the problem.

Real-World Example: Paces Too Slowly

Set:

70 ppm.

Analyzer measures:

52 ppm.

Current and pulse width normal.

Timing/control system fails specification.

Real-World Example: “No Capture”

Clinical complaint:

Could not get capture even at high current.

Bench analyzer:

Pacing current, pulse width, and rate all within specification.

Equipment passes.

Clinical capture is patient-dependent and is not itself proof of device failure.

Real-World Example: Intermittent Therapy Connector

Pacing output drops when cable moves.

Defibrillation analyzer testing also shows intermittent therapy connection.

Connector damage found.

Common Mistakes

Treating Selected Current as Proof of Delivered Current

Measure with an analyzer.

Confusing Pacing Output With Clinical Capture

They are different questions.

Replacing the Pacing Circuit When Only Demand Mode Fails

Check ECG sensing first.

Assuming Pace Markers Prove Current Reached the Pads

They may only indicate a commanded pulse.

Ignoring Therapy Cable and Connectors

They are part of the current path.

Testing Only One Current Setting

Range problems may be missed.

Assuming Defibrillation Passing Proves Pacing Passing

They use different output modes.

Assuming Pacing Passing Proves Defibrillation Passing

Same problem in reverse.

A Useful Troubleshooting Framework

For a pacing complaint, ask:

Then:

Then:

Then:

Then:

Then:

Then:

Then:

Does demand mode correctly detect and respond to simulated ECG?

That separates:

Ask:

Is the device failing to generate the pacing pulse, failing to deliver it through the therapy path, or incorrectly deciding when to pace?

Those are three different problems.

What Did You Actually Prove?

If the screen shows:

you proved:

Those pacing settings are selected.

If pacing markers appear:

You proved:

The device is generating or indicating pacing events internally.

You still have not proven:

80 mA is actually reaching the external load.

If a calibrated defibrillator analyzer measures:

within manufacturer specification, you have strong evidence that the pacing-output system is functioning correctly under that test condition.

If demand testing also passes with simulated ECG:

You have additionally verified much of the pacing-sensing and timing path.

Final Thoughts for Biomeds

Defibrillator pacing is easiest to understand as two systems working together:

and:

Electrical Pulse Delivery.

The delivery path is:

Pacing Circuit → Therapy Cable → Pads → Patient/Analyzer.

Demand pacing adds:

ECG → R-Wave Detection → Timing Logic → Pacing Decision.

So when pacing fails, first determine:

Or is the machine deciding to pace at the wrong time?

And do not confuse:

with:

Clinical capture.

A patient not capturing does not automatically prove the defibrillator failed.

Your analyzer tells you what the equipment actually delivered.

And as always:

Pacing waveform shape, current range, pulse width, demand algorithms, ECG sensing requirements, therapy accessories, impedance limits, and verification tolerances vary by defibrillator manufacturer and model. Follow current manufacturer service documentation, use approved therapy cables and calibrated defibrillator analyzers, and complete all required pacing, defibrillation, synchronization, alarm, battery, and safety testing before returning equipment to clinical use.

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