Timing Cycles of Implantable Devices

Single-Chamber Asynchronous Pacing (AOO, VOO)

Single-chamber asynchronous pacing (generically designated SOO) is the simplest of all pacing modes. Depending on the relevant chamber, it can be AOO (atrium) or VOO (ventricle). No intrinsic events are sensed, and pacing occurs in the chamber independent of the intrinsic rhythm at the lower rate interval (LRI) or lower rate limit (LRL), which is the only timing cycle in this mode ( Figs. 36-1 and 36-2 ). The VOO mode can be temporarily enabled to prevent ventricular pacing inhibition from inappropriate oversensing of electromagnetic interference (EMI) in a pacing-dependent patient. Asynchronous pacing has a potential risk of arrhythmia induction if the pacing stimulus falls in the vulnerable period (relative refractory period) of the myocardium; in the absence of contributing factors such as myocardial ischemia, metabolic or electrolyte abnormalities, or autonomic imbalance, this risk is quite small.

Single-Chamber Inhibited Pacing (AAI, VVI)

In single-chamber inhibited pacing (generically termed SSI), when an intrinsic event is sensed, it results in inhibition of pacing (see Figs. 36-1 and 36-2 ). The AAI mode is indicated in patients with sinus node dysfunction but intact atrioventricular (AV) conduction. The VVI mode is most useful for patients with atrial fibrillation (AF) and a slow ventricular rate, and is also used for backup pacing in patients with an infrequent need for pacing.

Single-Chamber Triggered Pacing (AAT, VVT)

Single-chamber triggered pacing mode (SST) delivers a pacing output when a native event is sensed. These modes have been used diagnostically to mark sensed events (in evaluating undersensing or oversensing) before the availability of intracardiac electrograms (EGMs) and marker channels. These modes are rarely used in current practice. However, a modified VVT mode is now being utilized in some CRT devices to trigger biventricular pacing when intrinsic ventricular activity is sensed in one ventricle.

Dual-Chamber Asynchronous Pacing (DOO)

In dual-chamber asynchronous pacing, a pacing pulse is delivered in the atrium, followed by a pacing pulse in the ventricle after completion of the atrioventricular interval (AVI). This sequential AV pacing occurs at the LRI regardless of intrinsic events in the atria or the ventricles. Like the VOO mode, this mode may also be transiently used in pacing-dependent patients to avoid inappropriate pacing inhibition from EMI during surgeries or other interventions.

Dual-Chamber Tracking Modes (DDD, VDD)

Tracking modes have the capability to pace the ventricle in response to atrial events (thereby tracking the atrial sensed events). This is termed P-synchronous pacing. The tracking modes are identified by “D” in the 3rd position of the pacing code, which denotes dual response to atrial sensing: an atrial sensed event inhibits pacing in the atrium, but triggers pacing in the ventricle.

The DDD mode is the most commonly used mode in dual-chamber devices and in biventricular devices. Depending on the intrinsic rate and AV conduction, AV sequential pacing (AP, VP), atrial pacing with spontaneous ventricular conduction (AP, VS), atrial tracking (AS, VP), or complete inhibition of pacing (AS,VS) can occur in the DDD mode ( Fig. 36-3 ).

The VDD mode is similar to the VDD mode but lacks atrial pacing (see Fig. 36-3 ). When the atrial rate is faster than the LRL, atrial tracking is observed; when the atrial rate is slower than the LRL, VVI pacing occurs at the LRL. The most common use of this mode is with the use of a single-lead system that integrates atrial sensing electrodes with a ventricular pace/sense electrode.

Dual-Chamber Modes Without Tracking (DDI, VDI, DVI)

These modes lack P-synchronous pacing.

The DDI mode is similar to the DDD mode but lacks P-synchronous pacing (see Fig. 36-3 ). The ventricular paced rate is always at the lower rate regardless of the atrial rate. AV sequential pacing will only occur at the LRL if no ventricular event is sensed after the atrial paced event. This mode is commonly programmed as a mode switch to prevent tracking of atrial tachyarrhythmias.

The DVI mode lacks atrial sensing (see Fig. 36-3 ). It can be used in patients with marked sinus bradycardia or sinus arrest with atrial lead malfunction (oversensing) to provide AV synchrony. If used in patients with normal sinus node function, asynchronous atrial pacing may precipitate atrial fibrillation.

The VDI mode lacks atrial pacing. Because there is no atrial pacing or tracking, AV dissociation will occur with any pacing in this mode regardless of the atrial rate. It essentially is the VVI mode with atrial sensing. It is available as a mode switch feature in some devices.

Dual-Chamber Triggered Modes

The DDT mode deserves special mention. The DDT mode can potentially involve ventricular triggering, atrial triggering, or both. The timing with respect to atrium and ventricle can be DDI (nontracking mode) or DDD (tracking mode). Most devices that offer the DDT mode trigger pacing in the ventricle after sensed ventricular events (ventricular triggering) to force CRT ( Fig. 36-4 ).

Lower Rate Interval

The lower rate interval (LRI) or lower rate limit (LRL) determines the base rate of pacing (LRI describes the interval, whereas LRL describes the rate). It is an essential interval of all pacing modes. In single-chamber modes, it is the interval between successive pacing stimuli in the relevant chamber; a pacing stimulus is delivered when the LRI is completed, which starts another LRI cycle. In single-chamber asynchronous modes, it is the only timing interval and is not reset by intrinsic events (see Figs. 36-1 and 36-2 ). In single-chamber inhibited modes, it can be reset by sensed events outside the refractory period ( Figs. 36-5 and 36-6 ).

Refractory Period

Refractory periods are an essential component of all pacing modes that involve sensing of intrinsic cardiac events. In single-chamber modes with sensing, there is a ventricular refractory period (VRP) or an atrial refractory period (ARP), depending on the relevant chamber. It is initiated by paced or sensed events; after a sensed event, the refractory period prevents double counting the same event, whereas after a paced event, it prevents sensing the pacing stimulus, its after-potential, or the evoked response. Events within the refractory period do not reset the LRI. The remaining portion of the LRI following the refractory period is termed the open interval or alert interval (Open interval = LRI − Refractory period), during which sensed events reset the LRI. The refractory period is typically composed of an initial blanking period, followed by an unblanked portion of the refractory period. No events are sensed in the blanking period; signals sensed in the unblanked portion of the refractory period do not reset the LRI, but may drive other pacemaker functions (see “ Blanking and Refractory Periods ” in the section on “ Dual-Chamber Pacing ”).

Sensing an event that does not represent a distinct depolarization from the same chamber is termed oversensing. For instance, in the VVI mode, portions of the QRS complex, the T wave, afterdepolarization following a pacing stimulus, atrial activity, noise from lead abnormalities, myopotentials, and EMI are possible causes of ventricular oversensing. The VRP should be long enough to include the T wave, which may be inappropriately sensed ( Fig. 36-7 ). The ARP should be long enough to prevent inadvertent sensing of ventricular activity (far-field R wave) ( Fig. 36-8 ).

Single-Chamber Asynchronous Pacing (AOO, VOO)

The only timing cycle in these modes is the LRI (A-A interval or V-V interval) ( Table 36-4 ). Pacing occurs in the relevant chamber at the LRL, which cannot be reset by intrinsic events (see Figs. 36-1 and 36-2 ). Asynchronous pacing may be described either as lacking refractory and alert periods, or as the refractory period extending throughout the LRI.

Single-Chamber Inhibited Pacing (AAI, VVI)

In addition to the LRI (A-A interval or V-V interval), the timing cycles in single-chamber inhibited pacing include a refractory period (ARP or VRP), the initial portion of which is a blanking period (ABP or VBP) (see Table 36-4 and Figs. 36-5 and 36-6 ). Blanking and refractory periods are triggered by any sensed or paced events.

Lower Rate Interval

The LRI in dual-chamber modes can be defined in terms of ventricular events or atrial events (see section on “Lower Rate Behavior”). In ventricular-based timing, it is the interval between a ventricular event and the next paced ventricular event.

Atrioventricular Interval and Ventriculoatrial Interval

The LRI in dual-chamber pacing modes is divided into the atrioventricular interval (AVI) and the ventriculoatrial interval (VAI). The AVI is an essential interval of all dual-chamber pacing modes. It is the interval between an atrial event and the next scheduled ventricular paced event. The VAI (also called the atrial escape interval, AEI) is the interval between a ventricular event and the next scheduled atrial paced event. It can be derived from the LRI and AVI (AEI = LRI − AVI).

A ventricular pacing (VP) stimulus is delivered at the end of the AVI, and starts the VAI. An atrial pacing (AP) stimulus occurs at the end of the VAI and starts the AVI. In the dual-chamber asynchronous pacing mode (DOO), the LRI and the AVI (and therefore the VAI) are not reset by intrinsic cardiac events; atrial and ventricular pacing occur sequentially at the same rate. In the DDD mode, the VAI is reset by ventricular sensed (VS) events outside the ventricular refractory period and is terminated by atrial sensed (AS) events outside the atrial refractory period; the AVI is usually terminated by ventricular sensed (VS) events (see “ Atrioventricular Interval ”).

Refractory Periods

Dual-chamber sensing mandates refractory periods in the atria and the ventricles. The VRP is initiated by a ventricular event. The atrial refractory period in dual-chamber modes is initiated by an atrial event. It extends throughout the AVI but also includes a period following the ventricular event, called the postventricular atrial refractory period (PVARP), which is designed to prevent sensing the far-field R wave or the retrograde P wave. The total atrial refractory period (TARP), therefore, is the sum of the AVI and the PVARP. The PVARP may be considered a fundamental timing interval, and the TARP can be derived from the AVI and the PVARP. Refractory periods are typically composed of an initial blanking period, followed by an unblanked portion (see section on “ Blanking and Refractory Periods ”).

Upper Rate Interval

Present in all dual-chamber tracking modes, the upper rate interval (URI) or upper rate limit (URL) determines the maximum paced ventricular rate in response to sensed atrial activity (also termed maximum tracking rate, MTR). The TARP determines the functional limit for tracking atrial activity, but a separate URI longer than this may be programmed (see “ Upper Rate Behavior ”). If a separate URI is not present or programmed, the TARP serves as the URI.

The basic dual-chamber timing cycles can be best understood by considering the timing cycles in the DDD mode, which contains all the timing cycles pertinent to all other modes. The refractory and blanking periods and the AVI are discussed below, whereas the lower and upper rate behavior are discussed later.

Confusing Terminology Regarding Blanking and Refractory Periods

The concept of blanking and refractory periods as described above applies to traditional pacemakers with fixed sensitivity. In this traditional concept, the sense amplifier is disabled during the blanking period and no events are sensed; events sensed in the unblanked portion of the refractory period are used for noise response, tachyarrhythmia detection, and other features but do not affect the LRI or the AVI. The advent of ICDs and sophisticated device functions that allow sensing within the “blanking period” has blurred the distinction between blanking and refractory periods and has led to confusing terminology used by different manufacturers regarding them. Although there is no consensus, certain points are worth noting: (a) All refractory periods should be considered to start with a blanking period. (b) In most current devices, the sense amplifier is not turned off during the “blanking period.” Rather, following initial sensing of an intrinsic event, there is repetitive sensing for a defined period to determine the maximal amplitude of the sensed event (automatic sensing algorithms). (c) If sensing can occur within the blanking period, one practical way to distinguish this from the refractory period would be to designate the blanking period as the period when sensed events are not counte d for tachyarrhythmia detection. With this definition, the ventricular sensing channel of ICDs is considered as having blanking periods alone, even though some manufacturers use the term “refractory period” (see “ Blanking and Refractory Periods in Implantable Cardioverter-Defibrillators ” in the section on “ Timing Cycles of Implantable Cardioverter-Defibrillators ”). This definition would also satisfy the blanking and refractory periods in the atrial channel (with some exceptions related to PVAB in some devices). (d) Automatic ventricular capture algorithms identify the evoked potential following a pacing stimulus to determine capture; however, the standard sense amplifier is blanked during this period and a separate sensing circuit (and a dedicated evoked response sense amplifier) is utilized for this purpose. (e) In traditional pacemakers, only sensed events in the unblanked portion of the refractory period are used for noise response. In many current devices, noise detection windows may be part of the “blanking periods” (see “ Noise Response ” in the section on “ Response to External Influences ”).

Atrioventricular Crosstalk and Ventricular Safety Pacing

As discussed earlier, crosstalk refers to inappropriate sensing of far-field signals from the opposite cardiac chamber that can potentially result in pacing inhibition. Cross-chamber blanking and refractory periods are designed to prevent crosstalk. One of the most serious consequences of crosstalk in a dual-chamber pacing system is oversensing of far-field atrial stimuli on the ventricular channel, resulting in ventricular pacing inhibition and asystole in a pacing-dependent patient (see Case Study 36-1 ).

Case Study 36-1

Syncope in a Pacemaker-Dependent Patient

A 78-year-old male with ischemic cardiomyopathy (left ventricular ejection fraction 30%) initially had a single-chamber implantable cardioverter-defibrillator (ICD) placed, which was subsequently upgraded to a biventricular ICD. Later on, due to recurrent atrial tachyarrhythmias with rapid ventricular rates, he underwent ablation of the atrioventricular (AV) junction. He underwent a routine pulse generator replacement when the existing device reached elective replacement indicator. All leads were tested at that time and found to be fine. A few weeks following this, he presented with several episodes of dizziness and a few episodes of frank syncope. Telemetry strips during an episode of dizziness are shown ( Fig. E36-1 ). Device interrogation revealed normal device function with normal pacing impedances and acceptable pacing thresholds. The programmed parameters are shown (see Fig. E36-1 ). Chest x-ray was obtained that revealed leads in expected position. Device diagnostics revealed several mode switch episodes. Sample tracings from these episodes are shown ( Fig. E36-2 ).

Clinical Question

What is the cause for the patient's symptomatic episodes? What can be done to prevent these episodes?

Discussion

Several findings can be made from the device tracings. Intermittent signals are seen in the ventricular channel that correlate temporally with the atrial pacing stimulus, suggesting that they are due to the atrial pacing stimuli and/or afterpotentials ( Fig. E36-3 , arrows ). These signals are occasionally sensed by the device (VS, red ovals ) causing pacing inhibition. Therefore the patient's symptoms are due to intermittent AV crosstalk—sensing of the atrial pacing stimulus/afterpotential in the ventricular channel, causing pacing-inhibition.

The postatrial ventricular blanking period (PAVB) is designed to prevent crosstalk. Some signals can be inferred to fall within this period, since no “VS” markers are seen in the ventricular channel (green ovals). Signals falling within the PAVB are not seen by the device, and pacing occurs at the programmed atrioventricular delay (AVD).

Some potentials fall within the crosstalk detection window (CDW) (blue ovals). This is evident from the “VSP” marker; signals falling within the CDW do not inhibit, but rather trigger, ventricular pacing at a shortened AVD (ventricular safety pacing).

Whereas the PAVB is designed to prevent crosstalk, the CDW is designed to prevent the consequences of crosstalk. In this patient, obvious programming considerations should include prolonging the PAVB and/or the CDW. Review of programmed parameters revealed that the PAVB is programmed to the maximum duration (52 msec) for the device, and the CDW is nonprogrammable (12 msec) in this device. However, crosstalk is occurring beyond this 64-msec duration in this patient.

Other than manipulating the PAVB and the CDW, options for minimizing crosstalk or its consequences include: (i) using bipolar leads, (ii) decreasing the atrial pacing output, or (iii) decreasing ventricular sensitivity.

Bipolar leads are already used here and the atrial pacing output has been programmed to a low value. Can the ventricular sensitivity be decreased? Decreasing the ventricular sensitivity can potentially resolve this problem. However, if the ventricular sensitivity is decreased in an ICD, ventricular fibrillation (VF) may be not be appropriately sensed. Is it possible to have the best of both worlds?

St. Jude Medical devices offer the option of programming different sensitivities for pacing and tachycardia detection. The sensing threshold for tachycardia detection can be kept low to allow appropriate sensing of VF, whereas the sensing threshold for pacing can be set higher to prevent inappropriate inhibition of pacing. Sensed events between the two threshold settings will not inhibit pacing but will be counted for tachyarrhythmia detection; sensed events higher than both thresholds will inhibit pacing and will be counted for tachyarrhythmia detection.

The maximum sensitivity for pacing (brady) was changed from “same as defib” to 2 mV and the maximum sensitivity for tachyarrhythmia detection (defib) was left at 0.3 mV ( Fig. E36-4 ). Testing for AV crosstalk was performed at maximal atrial output and no inhibition of pacing was noted. The patient had no further episodes of dizziness or syncope in follow-up.

Testing for crosstalk can be done by forcing atrial and ventricular pacing, by increasing the lower rate, and shortening the AVD (not required if the patient has complete atrioventricular block). Testing should be performed at maximal atrial output and maximal ventricular sensitivity (worst-case scenario).

To counteract this, an atrial pacing output triggers two periods on the ventricular channel within the AVI ( Fig. 36-12 ). The first is a PAVB, during which the ventricular sense amplifier is turned off. Immediately after this, there is a ventricular safety pacing (VSP) window (CDW), during which the ventricular sense amplifier is active. Signals sensed during the crosstalk sensing window are considered nonphysiologic because of the close coupling interval to the atrial stimulus, and are likely to be due to oversensing of atrial pacing afterpotentials, spontaneous premature ventricular depolarizations, or noise. Signals sensed during this window do not cause pacing inhibition, but rather trigger delivery of a ventricular pacing output at an abbreviated AV interval, usually around 110 msec (VSP). The premise is that this will prevent pacing inhibition if the sensed event is crosstalk or noise, while at the same time preventing pacing in the ventricular vulnerable period if the sensed event is due to a true ventricular depolarization. The shortened AVI makes the identification of VSP apparent ( Fig. 36-13 ). VSP may also be seen when atrial undersensing occurs and the conducted ventricular beat is sensed in the CDW (see Fig. 36-13 ).

Differential Atrioventricular Interval

The AVI initiated after a sensed atrial event (SAV) can be programmed different (typically shorter) from the AVI initiated after a paced atrial event (PAV). This is called a differential AVI or sensed AV offset. With atrial pacing, the atrial depolarization and the AVI start with the pacing artifact. With atrial sensing, the impulse typically starts in the sinus node and is on its way to the AV node as it reaches the atrial lead. The event is therefore sensed on the atrial lead about 20 to 50 msec after the onset of the P wave, the actual delay being determined by the distance between the site of impulse origin and the atrial lead, and the conductive properties of the atrium. It follows that the device-initiated AVI (SAV), which starts when the atrial event is sensed, is 20 to 50 msec shorter than the true AVI, which starts at the onset of the P wave. The SAV is therefore programmed shorter than the PAV to account for this and provide similar physiologic AVIs in both situations. The SAV may be expressed as a percentage of the PAV, or as an absolute difference between the two.

Dynamic or Rate-Adaptive Atrioventricular Interval

Dynamic or rate-adaptive AVI is a programmable parameter that permits modulation of the sensed or paced AVI based on the heart rate, either intrinsic or sensor driven. The shortening of AVI with exercise mimics the normal physiologic shortening of the PR interval to provide optimal AV synchrony. In addition to this hemodynamic benefit, rate-related shortening of the AVI decreases the TARP, allowing a correspondingly higher 1 : 1 atrial tracking rate.

Dynamic AVI is programmed from a baseline AVI to a minimum AVI, and different parameters may be programmed for PAV and SAV. The range of heart rates (based on the sensed atrial rate or the sensor-driven rate) over which this applies is programmable, and modulation over this range may be linear or nonlinear ( Fig. 36-14 ).

Atrioventricular Interval Hysteresis

As previously discussed, hysteresis refers to adaptation of a timing interval in response to a sensed intrinsic event. AVI hysteresis refers to adjustment of the AVI in response to sensed events. Positive AV/PV hysteresis is prolongation of the AVI in response to a ventricular sensed event. This permits intrinsic AV conduction and minimizes unnecessary ventricular pacing. It is most beneficial in patients who have variable AV conduction. During ventricular pacing, a search function extends the AVI periodically from the baseline value to a longer programmed AVI hysteresis interval. If intrinsic AV conduction occurs (a spontaneous ventricular event is sensed), the AVI is left at the longer hysteresis interval; if intrinsic AV conduction is longer than the hysteresis interval, no spontaneous R waves occur and the AVI is shortened to the original programmed value (see “ Algorithms to Minimize Ventricular Pacing ”).

Negative AV/PV hysteresis is designed to promote and maintain ventricular pacing. After an atrial event, if a ventricular event is sensed, the next AVI will be shortened to promote ventricular pacing. This feature is useful to promote pacing for hypertrophic cardiomyopathy or biventricular pacing (see “ Timing Cycles of Biventricular Pacing ”).

Ventricular-Based Timing

All dual-chamber devices originally used ventricular based timing, in which ventricular events start or reset the LRI (and the VAI). The interval from a ventricular event to the subsequent ventricular paced event defines the LRI. The hallmark of ventricular based timing is that the VAI (AEI) always remains constant (the A-A interval varies). A ventricular paced event starts the LRI and the VAI. A ventricular sensed event in the VAI will reset the LRI and the VAI, whereas a ventricular sensed event in the AVI will terminate the AVI and start the LRI and the VAI. A consequence of this is that in the presence of intrinsic AV conduction, the resultant atrial pacing interval (A-A interval) will be shorter than the programmed LRI ( Fig. 36-15 ). A PVC will reset or start the LRI and VAI, depending on whether it falls in the VAI or in the AVI; the PVC-VP interval will therefore be equal to the LRI ( Fig. 36-16 ).

Atrial-Based Timing

In atrial-based timing, atrial events start or reset the LRI. The LRI is the interval between an atrial event and the subsequent atrial paced event. Atrial-based timing is characterized by a fixed A-A interval (the VAI varies to maintain a constant A-A interval). The A-A interval is started or reset by any atrial event. In the presence of intrinsic AV conduction, the ventricular sensed event, which occurs in the AVI, will not affect the AA interval; therefore the atrial pacing rate will remain at the programmed LRL (see Fig. 36-15 ). Following a PVC, devices use one of two responses : (i) Common type (also termed modified atrial-based behavior). The PVC-VP interval equals the LRI, similar to ventricular based timing. (ii) Uncommon type or hysteresis response (also termed “pure” atrial-based behavior). The PVC resets the A-A interval, so that the PVC-AP interval equals the LRI (see Fig. 36-16 ).

Early sensing of PVCs on the atrial channel (VA crosstalk) in a device with atrial-based timing (and common type PVC response) may result in prolonged AEIs. When atrial sensing of the far-field R wave of the PVC occurs before detection of the near-field R wave by the ventricular channel, the A-A interval is reset by the atrial sensed event, resulting in a prolonged AEI following the PVC ( Fig. 36-17 ). This behavior is promoted by a high atrial sensitivity and/or low ventricular sensitivity and may be corrected by reducing atrial sensitivity and/or increasing ventricular sensitivity.

Comparison of Atrial-Based and Ventricular-Based Timing

Ventricular-based timing is characterized by a fixed VAI, whereas a fixed AA interval defines atrial-based timing. For the most part, during constant ventricular pacing, the two systems are similar: with constant AV sequential pacing or constant atrial tracking, there is no difference between the two; however, if a differential AV delay is programmed, and AS-VP is followed by AP-VP, the AS-AP interval is shorter with ventricular-based timing (see Fig. 36-15 ). The main difference between the two systems is in the presence of spontaneous AV conduction. In the presence of spontaneous AV conduction, the atrial pacing rate increases in ventricular-based pacing, whereas it remains constant in atrial-based pacing. The increase in atrial pacing rate in ventricular-based systems in the presence of spontaneous AV conduction is exaggerated during sensor-driven pacing, when AV conduction may be rather brisk. The response to spontaneous AV conduction is also the basis for slightly different patterns seen with both systems with intermittent AV conduction (e.g., 2 : 1 AV block). Table 36-5 summarizes the differences between ventricular-based and atrial-based lower rate timing.

In the DOO mode, both ventricular- and atrial-based systems function exactly the same way. Atrial-based timing is not applicable to the VDD and VDI modes, as there is no atrial pacing. True atrial-based timing is incompatible with the DDI mode.

Hybrid Timing

Most current devices combine features of both atrial-based and ventricular-based timing to avoid heart rate variations associated with either system. This has been termed modified atrial-based timing or hybrid timing. Current devices primarily use atrial-based timing to avoid heart rate acceleration during intrinsic AV conduction (AP-VS sequences) that is seen with ventricular-based timing. Following PVCs, most devices demonstrate modified atrial-based behavior (common type response, similar to ventricular-based timing) to avoid the pause seen with hysteresis response. Different modifications (manufacturer- and device-specific) may be applied for other situations (DDI mode, differential AVI, etc.)

Dual-Chamber Rate Hysteresis

Dual-chamber devices can also have rate hysteresis analogous to ventricular or atrial hysteresis. With dual-chamber pacing, whether atrial-sensed or ventricular-sensed events trigger rate hysteresis depends on the pacing mode and the manufacturer; in almost all devices, atrial-sensed events trigger hysteresis in the DDD mode (see “ Algorithms for Rate Adjustment ”).