Activity

Acceleration of heart rate can lower threshold. During exercise, the threshold may be lowered slightly by catecholamines. Sleep and postprandial phases increase it.

Drugs and metabolic disorders

Glucocorticoids, epinephrine, ephedrine and isoproterenol lower the threshold, while propranolol, verapamil, spirinolactone, quinidine and amiodarone do not. Class IC antiarrhythmics (flecainide and propafenone) can increase the threshold considerably. Any change in medical therapy should, in theory, lead to verification of the pacing threshold.

Hyperkalemia, hypoxia, hypercapnia, hyperglycemia, acidosis or metabolic alkalosis all increase the stimulation threshold.

Degree of fibrosis

On implantation, the direct trauma of the electrode on the endocardium creates a lesion current and a threshold rise that lasts only a few minutes. Before the use of steroid-eluting leads, a rise in pacing thresholds in the first 6 weeks after implantation was frequently observed, due to the inflammatory phenomenon induced by electrode trauma. During this period, high stimulation amplitude values were programmed to guarantee an adequate safety margin. Since the advent of steroid-eluting leads, stimulation thresholds have remained stable from the post-implantation phase, enabling a nominal stimulation voltage of 2.5 Volts to be programmed.

In the longer term, and due to a chronic inflammatory reaction to foreign bodies, fibrosis tissue develops in the vicinity of the electrode in contact with the endocardium, moving the electrode away from excitable myocardial cells. The stimulation threshold therefore tends to increase over time, even though modern probes offer much more stable thresholds than in the past. This possible increase in thresholds then leads to the programming of higher stimulation energies, and hence a reduction in battery life. In this context, it is possible to use an algorithm for automatic adaptation of delivered energy, enabling myocardial stimulation just above the stimulation threshold, with continuous cycle-by-cycle or daily monitoring of effectiveness.

Influence of electrode configuration on threshold and impedance

Electrode size, shape and material influence stimulation threshold. The current density applied to the distal electrode should be as high as possible to reduce threshold. This current density is highest at the edges of the electrode. Thus, a spherical electrode is associated with a higher threshold than an annular one.

Given the formula E = U2 x t /Z, the higher the stimulation impedance, the lower the current consumption. Stimulation impedance represents the sum of the forces opposing current flow in an electrical circuit.

It is made up of 3 ohmic resistances:

1. resistance of the conductor (which must be as low as possible, as the current spent overcoming this resistance is lost as pure waste and heat);
2. resistance of the electrode (which must be as high as possible, to reduce current consumption and extend battery life). The smaller the electrode radius, the higher the electrode resistance, which increases current density and reduces the stimulation threshold;
3. polarization impedance, which should be as low as possible.

A porous surface with a large microscopic area covers the electrode to 1) maintain a small radius of curvature and thus increase its resistance 2) reduce polarization impedance.

Influence of polarity

Cathodic stimulation generates a lower threshold than anodic stimulation.

In fact, cathodic stimulation creates a decrease in the transmembrane potential difference of cardiomyocytes, whereas anodic stimulation creates hyperpolarization, followed by depolarization, with an increase in the amount of energy required.

Moreover, natural refractory periods are shorter with anodic stimulation, which is associated with a higher theoretical arrhythmogenic risk, especially in high-risk situations such as ischemia or hypoxia.

The anode of a pacing lead is theoretically floating; the risk of anodic stimulation is therefore very low, but is observed when the anode is in contact with the wall and the stimulation amplitude is high. In the majority of cases, bipolar stimulation therefore corresponds to cathodic stimulation.

The threshold for unipolar stimulation is generally lower than for bipolar stimulation.

Influence of the stimulation waveform

Antibradycardic stimulation is performed using monophasic stimuli. A monophasic stimulation threshold is lower than a biphasic stimulation threshold.

The concept of Auto Threshold and Auto Capture

Determining the stimulation threshold is of major importance, as programming the voltage and pulse duration affects the safety margin and determines the energy consumption of the prosthesis and therefore the rate of battery wear. It is generally recommended to program a safety margin of 100%, which corresponds to twice the threshold voltage. This safety margin is intended to take into account circadian variations in the stimulation threshold, which varies from one subject to another depending on sleep, meals, physical activity, fever, etc.

All modern pacemakers can now be programmed with a function for automatic measurement of the ventricular threshold, which may or may not be combined with automatic adjustment of the stimulation amplitude with cycle-by-cycle verification of capture efficiency (Autocapture model allowing amplitudes very close to the threshold to be delivered with high-amplitude safety stimulation in the event of loss of capture) or adaptation for prolonged periods after planned threshold checks but without cycle-by-cycle verification (Autothreshold model requiring larger margins).