Recently, I was reminded of the importance of understanding the settings of a cardiac pacemaker when I had the opportunity to work with a patient who had a new pacemaker implanted after her old device's battery power began to fail. During the first week of her cardiac rehab program it became apparent that her heart rate response to exercise was unusual and warranted closer attention. In order to understand why such responses were occurring, I needed to understand how her pacemaker was programmed. This reminded me that a review might be beneficial for anyone prescribing exercise in a population that might include individuals with cardiac pacemakers. In the first of this two part series, I hope to describe how pacemakers are categorized according to a standard coding system and briefly discuss which pacemakers are best suited to treat certain cardiac rhythms. Later, I will describe the physiologic considerations for rate responsive pacemakers and provide suggestions on how to ensure that appropriate exercise modalities are selected based on the pacemaker settings.
Although fixed-rate pacemakers have been used in the past, demand pacemakers are the predominant devices seen in the clinical setting. These pacemakers have the ability to respond or not respond to the signals they sense from the heart's electrical system and they can be programmed to increase their impulse delivery rate for activities such as exercise3 . There is an international coding system that is used to describe the settings of pacemakers. The system was developed in 1987 by the North American Society of Electrophysiology (NASPE) and the British Pacing and Electrophysiology Group (BPEG) and it is referred to as the NBG Code. In 2001, because of advances in pacemaker technology, the system was revised and simplified2 . The table below summarizes the revised NBG code system that is used to describe the options for pacemaker settings.
A VVI pacemaker is one that paces the ventricle, senses the ventricle, is inhibited by incoming impulses from the heart's electrical system, and does not have the ability to adjust heart rate with activity. By comparison, a DDDR pacer will sense and pace both chambers, be both inhibited and triggered by native electrical impulses, and adjust heart rate with activity. Consider the individual who has a problem with the sinoatrial (SA) node and with an inability to consistently generate the stimulating action potential. The pacemaker best suited for this condition may be an AAI or AAIR pacemaker in which the chamber paced and sensed will be the atrium and the response to sensing will be inhibited so that if a signal is generated by the SA node, the pacemaker will not send an impulse to the atria1 . Or, consider the individual who has atrial fibrillation and requires a pacemaker due to a low ejection fraction; the best pacemaker for this individual would be one that paces and senses the right ventricle. Either a VVI or VVIR pacemaker would be selected, as there is no use for atrial stimulation. Finally, consider an individual with an inability to consistently conduct an electrical impulse from the atria to the ventricles due to an advanced atrioventricular block. The best pacemaker in this case would be a DDD or DDDR device that has the ability to provide coordinated stimulation of the atria and ventricles in order to maintain atrioventricular synchrony.
Knowledge of the programmed pacemaker settings allows the clinician to assess the appropriateness of the pacing response from an electrocardiographic standpoint. Additional information, such as heart rate limits and rate modulation settings will be needed to further assess the appropriateness of the pacemaker's ability to allow for normal hemodynamic responses during exercise. In our next issue, I will discuss how pacemakers increase heart rate during activity and provide a clinical case that illustrates what may happen if the exercise modalities and pacemaker settings are incompatible.
Key 1 References
1. Belardinelli, R.. Pacemakers. In: LeMura L, von Duvillard S, editors. Clinical Exercise Physiology: Application and Physiologic Principles. Philadelphia, PA: Lippincott, Williams, and Wilkins; 2004 pp 113-128.
2. Bernstein A, Daubert J, Fletcher RD,et al. The revised NASPE/BPEG generic code for antibradycardia, adaptive-rate, and multisite pacing. PACE 2002;25:260-264.
3. Goldberger, A. Pacemakers and implantable cardioverter-defibrillators. In:Clinical Electrocardiography: a Simplified Approach. 7th ed. Philadelphia, PA: Mosby; 2006. pp 251-265.
Key Two: Pacemaker Rate Modulation
As mentioned previously, my patient had demonstrated unusual heart rate responses to exercise that were inconsistent with her apparent effort level. The heart rates that were recorded at rest were within a couple of beats of those that were recorded on the arm ergometer, stationary cycle, combination arm/leg ergometer, and while walking on the track. In addition, blood pressure measurements that were recorded during the various exercise modalities were only slightly higher (~10 mm Hg, systolic) than those recorded at rest. When asked to rate her perceived exertion she provided responses of 5 to 7 (strong to very strong on the Borg Category Ratio Scale). Considering her perceived effort, these hemodynamic responses did not seem to represent normal physiologic changes. As a result, a decision was made to investigate her pacemaker settings to see if they might be responsible for the attenuated heart rate and blood pressure responses during exercise.
Before I discuss what was found with her pacemaker, let me provide a simplified description of how heart rate is modulated in pacemakers; but first, a disclaimer. Cardiac pacemakers are very complex devices that often require specialists to manage them. So (not being a pacemaker specialist), I may make some oversimplifications. The goal for rate modulation in a pacemaker is to match the physiologic changes that result from increased muscular activity with an increase in heart rate so that cardiac output may meet the demand of the metabolizing tissue. While there are several types of sensors that measure various physiologic parameters such as the QT interval, catecholamine concentrations, minute ventilation, respiratory rate, and blood pH and temperature, the most commonly used sensor for rate modulation is the activity sensor 1. The major advantage of the activity sensor over the physiologic sensor is its shorter response delay time. In some pacemakers both activity and physiologic sensors (usually QT interval or minute ventilation) are used in combination. In my experience, the activity sensor is the most common. The information that is acquired by the sensor is then used in the algorithm to adjust the activity of the pulse generator in order to elicit the appropriate heart rate response.
There are two types of activity sensors: a piezoelectric crystal and an accelerometer 2. Because the piezoelectric crystal can be attached to the casing of the pacemaker rather than to the internal circuitry, and because it senses vibratory changes that are transmitted through the chest wall, it is prone to sensing vibrations that may be unrelated to exercise, such as driving down a bumpy road. The accelerometer is usually part of the internal pacemaker circuit board and it senses the body's positional changes that occur in the horizontal and vertical planes. Because exercise may be performed in a stationary manner, there may be instances when an accelerometer may be unable to sense movement on certain exercise modalities. This usually occurs when there is very little upper body movement and/or when the sensing thresholds are not set to levels that will respond to the activity.
Once again considering the patient, after acquiring the pacemaker card and looking up the technical details on the manufacturer's website, we were able to determine that an accelerometer was used for rate modulation. This was not surprising, but the fact that she did not show typical chronotropic increases during track walking was. Therefore, some other factor seemed to be interfering with the pacemaker's ability to adjust heart rate during exercise. We then contacted technical support and discovered that the default setting for rate modulation for this pacemaker was set to "passive," suggesting that the feature may be turned off if it was not changed during implantation. As a result we contacted the patient's cardiologist and provided a review of the exercise and heart rate data. The patient was then seen by the pacemaker specialist and the settings were revised to allow for appropriate rate modulation. Upon return to cardiac rehab the patient was able to perform all modalities with proper heart rate, blood pressure, and perceived exertion responses as a result of the changes in the pacemaker settings.
Here are some suggestions when working with pacemaker patients.
- Secure a copy of the pacemaker card, which should include the model and serial numbers.
- Determine what type of rate modulation sensor the pacemaker is using.
- Perform an assessment of hemodynamic and rating of perceived exertion responses on a variety of machines that use different combinations of arm and leg movement.
- Contact the patient's cardiologist if the hemodynamic exercise responses are inappropriate. Hopefully, the pacemaker will be interrogated and possibly its settings will be reprogrammed prior to the patient's return.
Author's Note: For a more in-depth review of pacemaker settings and responses during exercise, the reader is referred to a review by Freedman et al. 2001 3.
Key 2 References
1. Belardinelli R. Pacemakers. In: Clinical Exercise Physiology: Application and Physiologic Principles, LeMura L, von Duvillard S. editors. Philadelphia, PA: Lippincott, Williams, and Wilkins; 2004. pp 113-128.
2. Lau C-P, Tai Y-T, Fong P-C, et al. Clinical experience with an activity sensing DDDR pacemaker using an accelerometer sensor. PACE 1992;15, 7-19: .
3. Freedman RA, Hopper DL, Mah J, Hummel J, Wildoff BL. Assessment of pacemaker chronotropic response: implementation of the Wilkoff Mathematical Model. PACE. 2001;24, 27
Author: Jeff Soukup, Ph.D. ACSM-CEP