Optimization during exercise

Device: PM Field: Programming for exercise

1. Basic concepts

Various parameters must be programmed and optimized to allow the adaptation of pacing to exercise. The main objectives of a specific programming dedicated to exercise are to ensure a proper contribution of atrial systole to the cardiac output at high rates while preserving AV synchrony, and allow an adapted increase in heart rate, which represents the main adaptive factor of the cardiac output to exercise.

The performance of an exercise test is an important evaluation in a pacemaker recipient, which allows:

  • the verification of proper pacemaker function, including a) adapted changes in rate, b) reliable sensing and pacing, which might change with breathing, changes in posture and with motion, and c) preservation of AV synchrony
  • the evaluation of residual symptoms or development of new, exercise-induced symptomatology
  • the programming and optimization of specific parameters that vary as a function of the heart rate (AV delays and adaptable PVARP)
  • to find electrophysiologic changes due to exercise (retrograde conduction present during exercise and absent at rest)
  • to evaluate the need for rate responsiveness

Various exercise tests can be used in pacemaker recipients, with ideally the ability to record simultaneously a surface electrocardiogram and intracardiac electrograms, using the programmer to identify dysfunction and fix it in real time.
Standard exercise tests help to evaluate the general functions of the pacemaker and rate-adaptive sensor(s). The heart rate adaptation to treadmill exercise seems more gradual and closer to that observed in real life than that achieved on a bicycle. The bicycle recruits muscle masses in the lower extremities that are distant from the pulse generator, such that the sensor is activated later. At an equal load, the increase in rate is greater on a treadmill than on a bicycle.
Standardized real-life tests may be appropriate given that a large proportion of pacemaker recipients are elderly and limited in their daily activities, such as walking, self-grooming, fixing meals, and housekeeping. The type of exercise test must, therefore, be tailored to these specific activities. Various kinds of exercise can be performed during electrocardiographic surveillance, such as walking on level ground, flexion of the lower extremities, etc.

ATRIOVENTRICULAR SYNCHRONIZATION DURING EXERCISE

When the device operates in DDDR, DDD or VDD mode, it can synchronize atrial rhythms up to the maximum tracking rate. The limits of atrial synchronization are, in particular, the 2:1 block rate and the programmed synchronous upper rate.

The 2:1 block point

The sum of the AV delay and the PVARP represent the total atrial refractory period (TARP). All atrial events sensed outside the TARP is able to initiate an AV delay followed by ventricular pacing.
Conversely, when the cycle of the intrinsic atrial rate is shorter than the TARP, every other atrial event falls in the PVARP and is followed neither by an AV delay nor by ventricular pacing. Ventricular tracking occurs on an alternate basis and 2:1 AV block is installed, with every other P wave followed by an AV delay and a ventricular pacing event. When the pacemaker is in DDD or VDD modes, the ventricular pacing rate falls precipitously, and the ventricular pacing rate is 50% of the ongoing sinus rate.

As an example, a patient with complete AV block and an AV delay set at 150 ms has a PVARP at 300 ms and an upper rate at 150 bpm. The 450 ms TARP duration corresponds to a rate of 133 bpm.

  • as long as the sinus rate remains between the back-up rate and 133 bpm, the AV relationship is 1 :1, with each P wave followed by a paced QRS;
  • f the sinus rate exceeds 133 bpm, every other P wave is followed by a paced QRS, whereas every other P wave falling in the PVARP is blocked and the ventricular rate decreases suddenly to 66.5 bpm.

The sudden decrease in cardiac output associated with this sudden heart rate slowing may be associated with disabling symptoms. However, the decrease in heart rate might be mitigated by various rhythms stabilizing functions or smoothing.
Consequently, the 2:1 point must be set as high as possible to enable the 1:1 tracking of the sinus P waves over the entire range of rates that might be observed in any given patient. The TARP duration (AV delay + PVARP) must, therefore, be shortened during exercise, which can be achieved by programming an automatic shortening of AV delay and PVARP during exercise.

Synchronous upper rate and pseudo-Wenckebach operation of the pacemaker

In the presence of complete AV block and during exercise, when the sinus rate remains below the programmed upper rate, each P wave is followed by ventricular pacing at the end of the AV delay. When the sinus rate accelerates and surpasses the programmed synchronous upper rate, a ventricular pacing event occurring at the end of the programmed AV delay would surpass the programmed upper rate, which is not possible. The ventricular rate can no longer follow the atrial rate on a 1:1 basis and hovers near this value. In order not to violate this limit, the pacemaker lengthens the AV delay and begins to operate in a pseudo-Wenckebach mode. As the sinus rate accelerates beyond the maximum synchronous rate, the ventricular pacing rate remains at the upper synchronous rate and the observed sensed AV delay lengthens with each paced cycle. After several cycles, an atrial event occurs during the PVARP and is not tracked, resulting in a longer V-V cycle. The following P wave is outside all refractory periods and initiates a new programmed AV delay. This sequence repeats itself as long as the sinus rate remains above the programmed maximum synchronous rate.

The longer V-V cycle occurs less often if the sinus rate is only slightly faster than the maximum tracking rate, though and occurs more prominently if the sinus rate significantly surpasses the maximum tracking rate. As soon as the sinus rate falls below the upper rate, the 1:1 association returns.

The pseudo-Wenckebach behavior can be defined by the frequency with which the longer V-V cycle occurs and by the sensed atrial: paced ventricular events ratio (for example, 8:7, 7:6, 6:5 or 3:2). When the increase in sinus rate reaches the 2/1 point, a major decrease in ventricular rate occurs along with a 2:1 atrial/ventricular ratio.

The upper synchronous rate is usually programmed at a rate below the 2:1 rate during exercise. If not, the 2:1 block rate becomes the absolute limit and the maximum synchronous rate cannot be reached.

 SPECIFIC PROGRAMMING TO PRESERVE ATRIOVENTRICULAR SYNCHRONIZATION DURING EXERCISE

Adaptable atrioventricular delay

In the healthy heart, the PR interval shortens physiologically during exercise, on average 4 ms per 10 cycles of rate acceleration. Furthermore, the duration of the A wave shortens considerably during exercise, with acceleration of the speed of the transvalvular flow as a consequence of the increase in atrial contractility caused by catecholamines and cavity filling.

The adaptation of the AV delay that can occur when the pacemaker operates in DDDR, DDD, DDIR, DVIR, DOOR or VDD modes, simulates this physiological response. The same variation is applied to the sensed and paced AV delay. In MVP mode (AAIR<=>DDDR or AAI<=>DDD), the adaptable AV delay functions only when MVP is in DDDR or DDD mode.

This function improves the sensing and synchronization to atrial activity. The sensed AV delays that are shortened during exercise widen the sensing window of rapid atrial events by shortening the TARP and increasing the rate of 2:1 block onset. The adaptable AV delay adapts the latter linearly as the heart rate varies.

With regard to the adaptable AV delay function, the sensed and paced AV delays are set to the values desirable for slow rates. Three additional programmable settings govern the adaptation of the AV delay at faster rates:

  • the start rate: the shortening of the sensed and paced AV delay begins at that rate.
  • the stop rate: the shortest sensed and paced AV delay does not shorten further from there and up to the programmed upper rate.
  • the maximum variation: the greatest shortening (in ms) of the sensed and paced AV delays.


The paced AV delay minus the maximum variation equals the shortest paced AV delay at the stop rate (for example 200 ms - 100 ms = 100 ms). By subtracting the maximum variation from the sensed AV delay, one derives the shortest sensed AV delay (for example 170 ms - 100 ms = 70 ms).

mdt_pace_exerc1.png

PVARP auto

Besides the shortening of the AV delay, the TARP can be decreased by progressively shortening the PVARP as the rate accelerates. When the automatic PVARP is programmed, the pacemaker determines its value as a function of the average atrial rate. The automatic PVARP increases the 2:1 block rate by shortening the PVARP and the sensed AV delay (if pertinent) at higher tracking rates, and protects against PMT at low rates with a longer PVARP.
The automatic PVARP is available when the device is programmed in DDDR, DDD, DDIR, DDI, AAIR<=>DDDR or AAI<=>DDD modes, and adapts the PVARP in response to changes in heart rate. In MVP mode (AAIR<=>DDDR or AAI<=>DDD), the automatic PVARP functions only when the MVP is in DDDR or DDD mode.

mdt_pace_exerc2.png

Maximum synchronous rate

The choice of upper rate depends on the patient’s age, underlying heart disease, exercise capacity, retrograde conduction time and on the performance of the arrhythmia- (fallback) and PMT-control algorithms. It seems appropriate to ensure the 1:1 tracking of the entire range of sinus rate of individual patients, and avoid the programming of an excessively slow synchronous upper rate. The development of AV dissociation due to pseudo-Wenckebach periodicity indeed causes an increase in myocardial oxygen consumption and a fall in cardiac output and arterial pressure that are often poorly tolerated.

Atrial sensitivity

The 1:1 AV tracking during exercise requires a flawless quality of atrial sensing. Exercise and the increase in the amplitude of the respiratory movements are often associated with a degradation of atrial sensing, which, in pacemaker-dependent patients, can cause a fall in the ventricular pacing rate, in which case it is necessary to increase the safety margin by increasing the programmed sensitivity.

 

RATE RESPONSIVE PACING

Some patients suffer from chronotropic insufficiency and inadequate increase in heart rate during physical activity. This inability to increase the heart rate with exercise can be associated with symptoms such as dyspnea, fatigue or limited exercise capacity.

State of the art cardiac pacemakers include sensors capable of monitoring the activity level and accelerate the pacing rate accordingly. A rate responsive pacemaker is a device with a base rate that varies according to the information provided by a specific activity sensor. Its aim, in the presence of chronotropic insufficiency, is to ensure an increase in cardiac output that is as physiologic as possible and corresponding to the instantaneous metabolic needs imposed by exercise.
A rate responsive pacemaker is identified by the 4th letter (R) of the international code: SSIR, DDDR, DDIR, and others.

Each device manufacturer offers its particular sensor and programming of the rate responsiveness. While several types of systems have been developed since the 1970s, three only remain in use in the current devices depending on the models, including the monitoring of physical activity by a piezoelectric quartz or by an accelerometer, and minute-ventilation, using the bio-impedance technique.

The criteria for a physiologically appropriate rate response sensor are:

  • the information provided must be directly related to the activity of the sympathetic nervous system or with the patient’s physical activity;
  • the relationship between the amplitude of the sensor-generated signal and the level of exercise must be linear;
  • the sensitivity of the sensor must be optimal to ensure a rapid reactivity of the system;
  • the range of variation of the parameter that is monitored must be wide enough to meet the patient’s physiologic needs;
  • the information provided must be reproducible;
  • the sensor must be as small as possible to be incorporated in the pacemaker without increasing its size, and consume a minimum amount of energy.

For more informations, refer to chapters specificities by manufacturers.

2. Specificities by company

Medtronic