Atrioventricular synchronization during exercise
Content
General information
During exercise, various parameters need to be programmed and optimized to ensure an appropriate exercise capacity. The main objectives of this differentiated exercise programming are to ensure that atrial systole makes a good contribution to cardiac output at high frequencies, while maintaining atrio-ventricular synchrony, and to enable an adapted acceleration of heart rate, which is the fundamental element in adapting cardiac output to exercise.
A stress test is an important examination for an implanted patient, enabling us to :
- check that the pacemaker is working properly (appropriate rate increase, quality of detection and stimulation, which can vary with breathing, postural changes and movement, preservation of atrio-ventricular synchrony, etc.)
- assess residual symptoms or the appearance of new effort-related symptomatology
- program and optimize specific parameters differentiated according to heart rate (adaptable AV delay and PVARP)
- look for the presence of electrophysiological changes induced by exercise (search for permeable retrograde conduction during exercise, absent at rest)
- evaluate the need for frequency control
Different types of stress test can be performed on an implanted patient, ideally with simultaneous recording of the electrocardiogram and endocardial electrograms using the programmer (facilitated by the wireless connection), so that malfunctions can be detected and corrected in real time:
- traditional stress tests: these assess the overall behavior of the pacemaker and sensor during exercise. It would appear that the frequency adaptation enslaved to exercise is more harmonious and closer to that observed in real life during a test on a treadmill than on a bicycle. This is because the bicycle mobilizes lower limb muscle masses far from the site where the unit is implanted, and the sensor (accelerometer) is activated later. For the same load, frequency acceleration is greater on treadmills than on bicycles for this type of sensor.
- standardized tests for everyday life: a large number of implanted patients are relatively elderly, with limited ability to perform everyday tasks (walking, toileting, cooking, housework, etc.). For these patients, it is essential to ensure good functioning for these everyday tasks. The type of exercise test must therefore be adapted to these specific needs. Various types of exercise can be performed under electrocardiographic supervision: walking on flat ground, stair climbing, flexion of the lower limbs, etc.
Atrio-ventricular synchronization during exercise
When operating in DDDR, DDD or VDD mode, the device can only synchronize atrial rhythms up to a certain frequency. The limits of atrial synchronization include the 2:1 block frequency and the programmed maximum synchronous frequency.
2:1 point
The sum of AV delay and PVARP constitutes the total atrial refractory period (TARP). Any atrial event detected outside the TARP is capable of triggering an AV delay and then ventricular pacing. Conversely, when the intrinsic atrial rate interval is shorter than the TARP, certain atrial events occur within the PVARP and are not monitored. Ventricular tracking occurs only for alternating beats, and a 2/1 block ensues, with every other P wave followed by an AV delay and ventricular pacing. When the pacemaker is operating in DDD and VDD modes, the ventricular pacing rate may drop precipitously (in the absence of a smoothing algorithm), the value of the ventricular pacing rate then being equal to half the current sinus rate.
Let’s take the example of a patient with complete AV block, AV delay programmed at 150 ms, PVARP at 300 ms and maximum rate at 150 beats/minute. The TARP is 450 ms long, corresponding to a frequency of 133 beats/minute.
- as long as the sinus rate remains between the base rate and 133 beats/minute, the AV association is 1/1; each P wave is followed by a stimulated QRS ;
- when the sinus rate exceeds 133 beats/minute, one P wave in 2 is followed by a stimulated QRS, one P wave in 2 falls into the PVARP and is therefore blocked; the ventricular rate drops abruptly to 66.5 beats/minute.
The sudden reduction in cardiac output associated with this sudden drop in heart rate may be associated with disabling symptoms such as blockpnea. The rate drop is sometimes less abrupt, thanks to various rhythm stabilization or smoothing functions.
The 2/1 point should be programmed as high as possible to enable 1/1 monitoring of sinus P-waves over the whole range of frequencies observable in a given patient. It is therefore important to limit the duration of PRAT during exercise (reduction of AV delay + PVARP). To achieve this, it is possible to program automatic shortening of the AV delay and PVARP during exercise.
Maximum synchronous frequency and Wenckebach stimulator operation
In a patient with complete AVB, during effort, when the sinus rate remains below the programmed maximum rate, each P wave is followed by ventricular pacing at the end of the AV delay. When the sinus rate accelerates and exceeds the programmed maximum synchronous rate, ventricular pacing at the end of the programmed AV delay would be associated with exceeding the programmed maximum rate, which is impossible. The ventricular rate can no longer follow the atrial rate in a 1/1 mode, and peaks around this value. To avoid transgressing this limit, the pacemaker lengthens the AV delay. This causes the stimulator to operate in Wenckebach mode. As the sinus rate increases beyond the maximum synchronous rate, the ventricular pacing rate remains at the maximum synchronous rate, and the observed AV delay is lengthened with each pacing cycle. After several pacing cycles, a detected atrial event occurs during PVARP and is not synchronized, resulting in a deficit beat. The P wave that follows is outside any refractory period and again initiates a programmed AV delay. This pattern recurs as long as the sinus frequency remains above the programmed maximum synchronous frequency.
Deficit beating occurs less frequently when the sinus frequency is only slightly above the maximum synchronous frequency, and more frequently when the sinus frequency is well above the maximum synchronous frequency. As soon as the sinus frequency falls below the maximum frequency, the 1:1 AV association is re-established.
Wenckebach behavior can be defined by the frequency at which the deficient beat occurs and by the ratio of detected atrial events to stimulated ventricular events (e.g. 8:7, 7:6, 6:5 or 3:2). If the increase in frequency reaches the 2:1 point, a significant drop in frequency occurs with an atrial/ventricular ratio of 2:1.
In general, the maximum synchronous frequency is programmed to be lower than the 2:1 block frequency during exercise. If this is not the case, the 2:1 block frequency becomes the absolute limit and the maximum synchronous frequency cannot be reached.
Specific programming to maintain atrio-ventricular synchronization during exercise
Adaptable AV delay
Physiologically, in a healthy heart, the PR interval shortens with exercise, averaging 4 ms for every 10 beats of increased frequency. In addition, the duration of the A wave shortens considerably during exercise, as transvalvular flow velocities accelerate due to the catecholamine-induced increase in atrial contractility.
AV delay adaptation, available when the pacemaker is operating in DDDR, DDD, DDIR, DVIR, DOOR and VDD modes, simulates this physiological response. This function enables better detection and synchronization with atrial activity. Stress-shortened detected AV delays increase the window of detection of fast atrial events by shortening the total atrial refractory period and increasing the frequency of occurrence of a 2:1 block. Adaptive AV delay adjusts AV delays linearly (manufacturer-dependent) as heart rate increases.
Adaptable or automatic PVARP
In addition to shortening AV delay, TARP reduction can be achieved by progressively shortening PVARP with frequency acceleration. When automatic PVARP (or adaptable PVARP , depending on the manufacturer) is programmed, the pacemaker determines a value for PVARP based on the mean atrial frequency. Automatic PVARP is designed to increase 2:1 block frequency by shortening PVARP and detected AV delay (if any) at higher follow-up frequencies, and to offer protection against PMT at minimal frequencies with longer PVARP.
The minimum PVARP parameter value (programmable according to manufacturer) defines a limit on the shortest PVARP allowed.
Maximum synchronous frequency
The choice of maximum frequency value depends on age, underlying heart disease, exercise capacity, retrograde conduction times and, consequently, the power of the algorithms for controlling atrial arrhythmias (fallback) and PMTs. It would seem legitimate to ensure 1/1 monitoring for a patient’s entire sinus frequency range. We must therefore avoid programming a maximum synchronous frequency that is too low. The induction of AV dissociation by Wenckebach behavior leads to an increase in myocardial oxygen consumption, a drop in cardiac output and a fall in blood pressure, all of which are often poorly tolerated by the patient.
Atrial sensitivity
1/1 atrio-ventricular monitoring during exercise requires perfect quality of atrial activity detection. Exercise and increased respiratory movements are often associated with impaired atrial detection. This can sometimes lead to an appearance similar to 2/1 block. Markers can be used to diagnose the absence of repetitive AR signals (2/1 block) and the absence of signal detection (no marker). In such patients, it is therefore necessary to increase the margin in relation to the detection threshold.
