Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-06-07
2004-05-04
Jastrzab, Jeffrey R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S020000
Reexamination Certificate
active
06731984
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to medical devices, and, more particularly, to implantable medical devices for providing various types of therapies to patients.
2. Description of the Related Art
A cardiac pacemaker (i.e., pacemaker) is an implantable medical device that delivers electrical stimulation (i.e., “pacing”) pulses to cardiac tissue. Pacemakers are typically used to relieve symptoms associated with bradycardia, a condition in which patients cannot normally maintain physiologically acceptable heart rates. A wide variety of pacemakers are known and commercially available.
Early pacemakers delivered pacing pulses at regular intervals (i.e., constant rates) to maintain preselected heart rates. The preselected heart rate was typically deemed appropriate when the patient was at rest. Such pacemakers were known as “asynchronous” pacemakers because they did not synchronize pacing pulses with natural cardiac activity.
In contrast, the heart rate of a typical healthy person with a properly functioning heart increases during periods of elevated physical activity, and decreases during periods of reduced physical activity, to meet changing metabolic and physiologic needs. Accordingly, the metabolic and physiologic requirements of a patient receiving therapy via a pacemaker producing pacing pulses at a constant rate are typically not met when the patient is engaged in physical activity. During periods of elevated physical activity, the patient may experience adverse physiological consequences, including lightheadedness and/or episodes of fainting.
To reduce the adverse effects of constant rate pacing, “rate responsive” pacemakers have been developed that automatically adjust patients' heart rates to meet changing metabolic and physiologic demands. In a typical rate responsive pacemaker, the rate at which pacing pulses are produced (i.e., the “pacing rate”) is variable between predetermined minimum and maximum rates. The minimum and maximum rates may be, for example, selected and programmed into the pacemaker by a physician. A “target” pacing rate of a rate responsive pacemaker may be expressed as:
Target Pacing Rate=Minimum Rate+ƒ(sensor output)
where ƒ is a linear or monotonic function of an output of a single sensor, or the combined or “blended” outputs of multiple sensors.
Some known rate responsive pacemakers include only a single “activity” sensor (e.g., a piezoelectric crystal). In this situation, the rate response function ƒ is function of the activity sensor output. When the output of the activity sensor indicates that the patient's activity level has increased, the pacing rate is increased from the minimum rate by an incremental amount, which is determined as a function of the output of the activity sensor. As long as the activity sensor output indicates patient activity, the target pacing rate is periodically increased by incremental amounts calculated according to the above formula, until the maximum rate is reached. When patient activity ceases, the target pacing rate is gradually reduced, until the minimum rate is reached.
For any rate responsive pacemaker, it is desirable that the activity sensor output correlate to as high a degree as possible with the metabolic and physiologic needs of the patient, such that the pacing rate determined by the activity sensor output meets the metabolic and physiologic needs of the patient. It is noted that activity sensor output only indirectly represents a level of metabolic need. In addition, physical activity sensed by an activity sensor can be influenced by upper body motion. For example, an exercise involving arm motion may result in an activity sensor output corresponding to a relatively high level of metabolic need, while the actual level of metabolic need is much lower. Conversely, an exercise that stimulates the lower body only, such as bicycle riding, may result in an activity sensor output corresponding to a relatively low level of metabolic need, while the actual level of metabolic need is much higher.
Other known types of rate responsive pacemakers include multiple sensors, and the rate response function ƒ may be a function of an output of one or more of the multiple sensors at any given time. For example, a rate responsive pacemaker may include an activity sensor and a “minute ventilation sensor.” Minute ventilation (V
c
) is a parameter that has been demonstrated clinically to correlate directly to the actual metabolic and physiologic needs of a patient. Minute ventilation may be defined by the equation:
V
c
=RR×VT
where RR is a “respiration rate” in breaths per minute, and VT is a “tidal volume” of each breath in liters. Clinically, the measurement of V
c
is performed by having the patient breathe directly into a device that measures the exchange of air and computes the total volume per minute.
While it is not possible for an implanted device, such as a pacemaker, to directly measure minute ventilation, it is possible for such an implanted device to measure impedance changes in the thoracic cavity. It is well known that a change in thoracic impedance corresponds to a change in tidal volume (VT), and a frequency of such changes over time corresponds to respiration rate (RR). (See, for example, U.S. Pat. No. 4,702,253 issued to Nappholz et al. on Oct. 27, 1987.) In a rate responsive pacemaker, circuitry configured to measure thoracic impedance, to extract respiratory rate (RR) and tidal volume (VT) values from thoracic impedance measurements, and to produce an output that represents a product of the respiratory rate (RR) and tidal volume (VT) values may be considered a “minute ventilation sensor.”
Both respiration rate (RR) and tidal volume (VT) have inherent physiologic time delays due to the response of CO
2
receptors and the autonomic nervous system. As a result, an increase in minute ventilation (V
c
) occurs after the onset of exercise and lags behind a need for increased cardiac output.
In rate responsive pacemakers having multiple sensors, rate response function ƒ may be selected such that the pacing rate is based on the combined or “blended” outputs of the multiple sensors. For example, known rate responsive pacemakers include an activity sensor and a “minute ventilation sensor” as described above. In such rate responsive pacemakers, the rate response function ƒ may be selected such that the pacing rate is based substantially (or even solely) on the activity sensor output when the patient is relatively inactive, and based substantially on the output of the “minute ventilation sensor” when the patient is relatively active.
Human sleep-wake cycles are examples of biological rhythms called “circadian rhythms”—internally originating cycles of behavior or biological activity with a period of about 24 hours. It is believed that human sleep-wake cycles are generated by an internal clock that is synchronized to light-dark cycles in the environment and other daily cues.
While the typical healthy person with a properly functioning heart is awake but relatively inactive, the person's heart rate is usually at a “resting rate.” When the person is sleeping, the person's heart rate typically drops to a “sleeping rate” that is less than the resting rate. On the other hand, the heart rate of a patient receiving therapy via a typical rate responsive pacemaker is maintained at the above described minimum rate when the patient is both awake but relatively inactive and sleeping. While the difference between the “resting rate” and the “sleeping rate” may be relatively small (e.g., about 5 beats per minute), the inability of the typical pacemaker to reduce the patient's heart rate when the patient is sleeping may cause the patient to have difficulty falling asleep and/or sleeping well. In addition, since it is likely that the patient could tolerate, and even benefit from, a lower heart rate while sleeping, the pacemaker may be viewed as wasting limited energy reserves by maintaining the unnecessar
Cho Yong Kyun
Jensen Donald N.
Mongeon Luc R.
Jastrzab Jeffrey R.
Medtronic Inc.
Oropeza Frances P.
Seldner Michael C.
Wolde-Michael Girma
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