Pacing Fundamentals

A pacing system has two components. The pulse generator sits in a subcutaneous pocket, usually below the left clavicle. It contains a lithium-iodide battery and a microprocessor that controls timing, output, and sensing.

The leads are insulated wires threaded through the subclavian vein, through the superior vena cava, and into the right atrium and/or right ventricle. At the tip of each lead, a small electrode makes direct contact with the endocardium.

Current flows from the cathode (the electrode tip) through the myocardium to the anode and back to the pulse generator, completing the circuit. In a bipolar configuration, the anode is a ring electrode a few centimeters proximal to the tip. In a unipolar configuration, the anode is the metallic casing of the pulse generator itself (the "can").

Bipolar pacing is standard in modern devices. The short distance between cathode and anode produces a tighter electrical field, reducing the risk of skeletal muscle stimulation and making the system less susceptible to electromagnetic interference.

Capture means the pacing stimulus successfully depolarizes the myocardium at the electrode-tissue interface. The minimum energy required to achieve this consistently is the capture threshold.

Pacing energy is the product of two programmable variables: amplitude (in volts) and pulse width (in milliseconds). A higher amplitude or a longer pulse width delivers more energy to the tissue. The strength-duration curve describes the tradeoff: at very short pulse widths, high voltage is needed; as pulse width increases, the voltage required to capture falls, until it plateaus at a floor called rheobase.

At implant, the operator measures the threshold by gradually decreasing the output until capture is lost. In the weeks that follow, threshold rises. Inflammation at the electrode-tissue interface creates a layer of edematous tissue that increases the distance between the cathode and excitable cells.

Modern leads address this with a steroid-eluting tip. A small reservoir of dexamethasone at the electrode surface suppresses local inflammation and limits fibrosis. After the acute rise, the threshold typically settles to a chronic value lower than the peak.

The standard safety margin is to program the output at twice the threshold voltage. If the threshold is 0.75V at 0.4ms, the device is programmed to 1.5V at 0.4ms. This ensures reliable capture even with day-to-day fluctuations in threshold, while preserving battery longevity.

A pacemaker must know when the heart is beating on its own. Sensing is the ability to detect intrinsic cardiac depolarizations so the device can withhold its stimulus when one is unnecessary.

The device measures the intracardiac electrogram (EGM) at the lead tip. The amplitude of this signal, in millivolts, determines whether the device "sees" a native beat. Atrial signals are smaller, typically 1.5 to 5 mV. Ventricular signals are larger, usually 5 to 15 mV.

Sensitivity is programmed as a voltage threshold. Any signal larger than this threshold is counted as a sensed event. Any signal smaller is ignored. The terminology can be counterintuitive: a lower sensitivity number (e.g., 1.0 mV) means the device is more sensitive, because it will detect smaller signals. A higher number (e.g., 5.0 mV) means the device is less sensitive, ignoring everything below that amplitude.

Undersensing occurs when the device fails to detect an intrinsic beat. The pacing spike fires into a heart that has already depolarized. Oversensing occurs when the device mistakes noise (T waves, myopotentials, electromagnetic interference) for a real beat and inappropriately withholds pacing.

Impedance measures the total resistance to current flow in the pacing circuit. It includes the lead conductor, the electrode-tissue interface, and the myocardium itself. Normal values range from 400 to 1200 ohms.

A sudden rise in impedance suggests a lead fracture. The conductor is broken, and current cannot flow through the circuit. This can cause failure to capture and failure to sense.

A sudden drop in impedance suggests an insulation breach. Current is leaking through a crack in the lead's outer coating, taking a shortcut back to the can instead of traversing the myocardium. This can also impair capture and cause oversensing of electrical noise generated at the breach site.

Impedance is trended over time at every device check. A stable impedance is reassuring. A gradual drift or an abrupt change prompts further investigation with chest X-ray and lead testing.

The pacing rate is expressed as a programmed interval. A rate of 60 beats per minute corresponds to a 1000 ms interval between consecutive beats. At 70 bpm, the interval is approximately 857 ms.

After each paced or sensed event, the device starts a timer equal to this interval. If no intrinsic beat appears before the timer expires, the device delivers a pacing stimulus. If the device senses a native depolarization before the timer runs out, it resets the clock and waits again.

This behavior is called demand pacing. The device fires only on demand, when the heart fails to produce a beat in time. It is the foundation of every modern pacing mode. A pacemaker programmed to VVI at 60 bpm will pace the ventricle at 1000 ms intervals, but only when the intrinsic ventricular rate falls below 60.

Several refractory and blanking periods prevent the device from being confused by its own stimulus or by electrical events that follow immediately after a beat. The ventricular blanking period (typically 15 to 30 ms after a paced event) makes the amplifier deaf so it does not count the pacing artifact as a sensed event. The ventricular refractory period (200 to 350 ms) prevents the device from sensing the T wave and misinterpreting it as a new QRS.