Pacing & Sensing

Device: PM Field: Pacing & Sensing

1. Basic concepts

PACING

Definition of pacing threshold

The pacing threshold is the lowest electrical pulse, delivered outside the natural refractory periods, that consistently elicits the propagation of a depolarizing wavefront. It is measured as voltage amplitude (V) or pulse width (ms).

Chronaxie and rheobase

Lapicques voltage - duration (or chronaxie - rheobase) relationship, expresses the non-linear relation between the threshold voltage and pulse duration. The pacing threshold voltage increases significantly as the pulse duration decreases (in clinical practice below 0.2 ms). All points that are above the curve defined by the threshold voltage - pulse duration relationship are associated with effective capture, whereas all points that are below the curve are associated with absence of capture.

Rheobase is defined as the lowest effective pulse amplitude at an infinite pulse duration (in clinical practice above 2.0 ms).

Chronaxie is defined as the shortest effective pulse duration at a pulse amplitude equal to twice the rheobase. The energy consumed by a pulse of which the duration is equal to the chronaxy is minimal.

Chronaxie and rheobase define the electrical properties of a pacing lead. The chronaxie associated with all current lead models range between 0.3 and 0.4 ms, a value which corresponds to the usual nominal pulse width of pacemakers. It is often longer with left ventricular pacing leads.

The pacing threshold is usually lower when measured by gradually lowering instead of increasing the pulse strength, a phenomenon known as the Wedensky effect.

In clinical practice, the measurement of the pacing threshold is of major importance, since the programming of pulse amplitude and duration determines 1) the pacing safety margin and 2) the energy consumption of the pulse generator, hence the battery longevity. It is generally recommended to program a 100% safety margin, which corresponds, by convention, to programming a voltage at least twice threshold. This safety margin accounts for the circadian variations in pacing threshold, which, among patients is variably influenced by sleep, food consumption, physical activity, fever, and other factors.
 

Factors that influence the safety margin

Activity
Acceleration of the heart rate lowers the pacing threshold. During exercise, it is decreased by catecholamines, while it is decreased by sleep and postprandial state.

Drugs and metabolic disorders
Glucocorticoids, epinephrine, ephedrine and isoproterenol lower the threshold, as opposed to propranolol, verapamil, spironolactone, quinidine, and amiodarone. The class IC antiarrhythmic drugs flecainide and propafenone have the greatest potential to considerably increase the pacing threshold. Consequently, changes in drug therapy should, theoretically, prompt threshold reassessment.

Other factors that increase the pacing threshold include hyperkalemia, hypoxia, hypercapnia, hyperglycemia, acidosis and metabolic alkalosis.

Degree of fibrosis
At the time of implantation, the direct trauma of the electrode on the endocardium creates a lesion current and a transient rise in threshold lasting a few minutes. Before the introduction of steroid-eluting leads, a rise in capture threshold was often observed within the first 6 weeks after implantation due the inflammation caused by the electrode-induced trauma. During that period, a high pacing voltage amplitude was programed to guarantee an adequate safety margin. Since the introduction of steroid-eluting leads, the pacing threshold remains stable from the time of implantation, allowing the programing of a nominal 2.5 V amplitude.

Longer term, as a result of a chronic foreign body-type inflammatory reaction, fibrotic tissue develops around the electrode at its contact point with the endocardium, separating the electrode from the excitable myocardial cells. Consequently, the capture threshold tends to rise over time, despite the increased stability offered by state-of-the-art lead systems. This rise requires the programming of higher pulse strengths and, consequently, a shortening of the battery longevity. In this perspective, it is recommended to use an automatic adaptive capture algorithm allowing capture with a pulse strength just above threshold, while constantly verifying its reliability.
 

Influence of the electrode configuration on capture threshold and lead impedance

The size, shape and constituent of the electrode influence the capture threshold. The current density delivered by the distal electrode should be as high as possible, with a view to lower the threshold. Since the current density is highest at its edges, a spherical electrode is associated with higher capture thresholds than an annular electrode.

From the equation E = U2 x t /Z, the higher the pacing impedance, the lower the current consumption. The pacing impedance, which reflects the sum of forces impeding the flux of current in an electrical circuit, is determined by 3 ohmic resistances:

  • Resistance of the conductor, which must be as low as possible, since the current expended to overcome this resistance is lost and dissipated in heat;
  • Resistance of the electrode, which must be as high as possible, with a view to lower the current consumption and prolong the battery life. The smaller the radius of the electrode, the higher the electrode resistance, increasing the current density and lowering the capture threshold;
  • The polarization impedance, which must be the lowest possible.

A porous and broad microscopic surface covers the electrode in order to 1) maintain a small radius of curvature and thereby increase its resistance, and 2) lower the polarization impedance.
 

Influence of polarity

A cathodal pulse is associated with a lower threshold than an anodal pulse. Cathodal stimulation narrows the transmembrane potential difference among cardiomyocytes, whereas anodal stimulation causes hyperpolarization, followed by depolarization, with an increase in the quantity of energy needed. Furthermore, the refractory periods are shorter with anodal than with cathodal stimulation, which is associated with a higher theoretical risk of arrhythmogenesis, particularly in vulnerable circumstances, such as ischemia and hypoxia.

The anode of a pacing lead is theoretically floating, thus associated with a very low risk of anodal stimulation, although this is observed when the anode touches the wall and the pacing amplitude is high. Thus, in the majority of cases, bipolar stimulation corresponds to cathodal stimulation.
 

SENSING

Sensing, or sensitivity, expressed in millivolts (mV) defines the ability of the cardiac pacemaker to correctly detect spontaneous cardiac events.

The devices are equipped with entrance filters to allow the specific sensing of P waves in the atrium and R waves in the ventricle, based on the analysis of 3 characteristics of these electrical signals, including the frequency spectrum, slope and amplitude.

The proper programming of the sensing level should allow the detection of all spontaneous cardiac events occurring in the chamber containing the lead, and should reject events of other origins, such as crosstalk from another chamber, myopotentials, or distant interference.

Compared with unipolar sensing, the programming of bipolar sensing increases the specificity by limiting the likelihood of extracardiac signal sensing or crosstalk, and allows the programming of a high sensitivity (0.3 – 0.5 mV in the atrium, 2 to 3 mV in the ventricle). On the other hand, in unipolar configuration, the risk of crosstalk or sensing of extracardiac signals mandates the programming of lower sensing levels (1 to 1.5 mV in the atrium and 4 à 5 mV in the ventricle) with a higher risk of undersensing.

The frenquency spectum

The frequency of a signal is expressed in hertz (Hz), and is the inverse of its periodicity. Pacemakers amplify incoming signals corresponding to the myocardial depolarization signals in a range of frequencies between 10 and 70 Hz. Signals with frequencies above and below that range are filtered and are not sensed or are faintly sensed by the system.

Example: in the ventricular channel, the frequency spectrum of the amplified R wave is between 10 and 30 Hz. On the other hand, T waves, the frequency spectrum of which is <5 Hz, are filtered. Likewise, the frequency of signals originating from the atria that are sensed in the ventricle is usually very low and nearly always filtered.

The frequency spectrum of myopotentials, such as those originating from the pectoral muscle, is superimposed over the P and R wave frequency. In unipolar configuration, the sensing field ranges from the distal electrode of the pacing lead to the pulse generator implanted under the pectoral muscle, with a heightened risk of interference from the sensing of signals of muscular origin during efforts.
 

Slope

This parameter, expressed in mV/ms, describes the change in cardiac signal amplitude over time. The pacemaker senses the fastest component of the signal, which corresponds to the travel of the depolarization front near the electrode.

If the signal is fragmented, as is sometimes the case with extrasystoles, the slope of its various components is often shallower, increasing the risk of undersensing.

At the time of implantation, the pacing lead(s) should ideally be implanted at a site where the depolarization slope is ≥1 mV/ms in the ventricle and ≥0.5 mV/ms in the atrium. The measurement of the signal slope depends on its conditioning and, in particular, on its filters, which vary depending on the system, whether an external device or the pulse generator that will ultimately be connected. The differences observed can be prominent in both directions. The direct recording of the signal at the time of implantation can nevertheless be useful when searching for the site that yields the largest intrinsic deflection.

The intrinsic deflection of an endocardial signal nearly never coincides with the onset of the surface electrocardiographic signal. For example, the sensing of a ventricular depolarization signal in a patient with complete right bundle branch block is very late. Likewise, in the atrium, the atrial electrogram may be sensed at the end of the surface electrocardiographic P wave.
 

Signal amplitude

The amplitude of the signal measured by the pacemaker, expressed in mV, corresponds to the amplitude that remains detected by the pacing system after its frequency has been analyzed and slope measured. This parameter is used at the end of the signal conditioning process to determine the level of sensitivity of the system.

The programing of a cardiac pacemaker to a sensitivity of 4 mV means that signals with amplitudes >4 mV can be sensed, after the frequency and slope of the signal have been properly conditioned. All signals with lower amplitudes are rejected.

Increasing the programmed value of sensitivity (e.g. to 12 mV) lowers the sensitivity of the system, since an amplitude >12 mV will be needed for the signal to be detected.

Ultimately, when implanting a pacing lead, attempts must be made to obtain a signal corresponding to the pacemaker’s bandwidth, with the fastest intrinsic deflection and the highest amplitude, i.e. ≥5 mV in the ventricle and ≥2 mV in the atrium.
 

Automatic sensing

The amplitude of the cardiac depolarization signals are not stable and might vary as a function of activity, use of medications, or metabolic changes. In addition, the amplitude of signals associated with atrial or ventricular arrhythmias might differ markedly from normal signals, often considerably smaller, as in the case of atrial fibrillation. Thus, a fixed sensitivity does not assure the detection of all events.

In presence of automatic sensing, the sensitivity level applied at the onset of a sensed event is a percentage (usually 50 to 75%) of the amplitude of the sensed signal. Thereafter, and for a 120 ms duration, the sensitivity level remains stable, before increasing progressively during diastole, until a maximum programmable sensitivity has been reached.
 

Automatic sensitivity

When the automatic sensitivity function is activated, the pacemaker monitors the amplitude of the sensed signals. Depending on the results, the sensitivity is automatically increased or decreased in order to preserve a sufficient sensing margin, commensurate with the patient’s sensed P and R waves. When activated, automatic sensitivity continuously adjusts the sensitivity, as a function of the measured amplitudes, to avoid under- or oversensing.
Depending on the age of the pacemaker series, from Kappa, EnPulse, Sensia, up to the Adapta model, a fixed sensitivity value might be programmable for the entire duration of the cardiac cycle. Alternatively, in Ensura, Advisa and later pacemaker series, as exists in the pulse generator of defibrillators, automatic sensing is adapted to the amplitude of the previous signal, and changes throughout the cardiac cycle.

2. Specificities by company