1. Field of the Invention
This invention relates to rate-responsive pacemakers and, more particularly, to pacemakers that employ a minute volume metabolic demand sensor whose output is adjusted in accordance with a rate response dependent on the anaerobic threshold and physical condition of a patient.
2. Description of the Prior Art
Many attempts have been made to control the heart rate of a pacemaker patient so that it will duplicate the intrinsic heart rate of a healthy person both when the patient is at rest and when the patient is involved in various levels of exercise. Metabolic demand related parameters heretofore proposed for controlling the pacing rate include the QT interval, respiration rate, venous oxygen saturation, stroke volume, venous blood temperature, and minute volume, among others. In addition, the use of mechanical and electrical sensors which detect patient motion have also been explored in such attempts at achieving improved rate-responsiveness. Of the various parameters available, it has been found that pacemakers using minute volume as a parameter for controlling pacing rate are particularly advantageous. However, a problem with these types of pacers has been the mapping of minute ventilation to an appropriate metabolic indicated rate in a manner which accurately reflects the intrinsic heart rate.
A further problem is that, in general, even though metabolically-related parameters used for controlling rate-responsive pacemakers, especially with the contribution of the tidal volume component of minute ventilation, react fairly rapidly to reflect changes in the patient's physical activity, the pacemakers' algorithm does not react with the same speed or time constant. This can result in the patient having a hemodynamic deficiency due to the lag time involved between the start of an increased level of exercise and the reaction thereto by the pacemaker. Of course, since linear mapping is easiest to implement, most mapping attempts so far tended to be straight line approximations, as illustrated in FIGS. 1-5. In FIGS. 1-5, both the vertical and the horizontal scales are normalized for ease of comparison. More specifically, in FIG. 1, the minimum and maximum values of the metabolic rate interval are disposed at 0 and 1 respectfully along the vertical axis. Similarly, corresponding minimum and maximum values of the minute ventilation are disposed at 0 and 1 respectively along the horizontal axis. The same convention is used in FIGS. 2-5 except that in the latter Figures the vertical axis indicates a normalized Metabolic Indicated Rate (MIR) as opposed to an interval.
It should be understood that rate responsive systems making use of the minute ventilation as a parameter, first calculate a long term average for the minute ventilation of a patient and then determine the difference between this long term average and an instantaneous minute ventilation obtained as described below, in conjunction with FIGS. 7-11. The resulting differential parameter is referred to as "the minute ventilation" for the sake of brevity. However, in the drawings, the parameter is identified as .DELTA.MV to indicate that, in fact, this parameter corresponds to the variation of the instantaneous minute ventilation from a long term average value.
An early attempt at mapping the minute ventilation was to use a straight linear conversion into a pacing interval, or metabolic rate interval (MRI), as shown in FIG. 1. The mapping of FIG. 1 was found to be undesirable because it led to a hyperbolic relationship between the minute ventilation .DELTA.MV and the metabolic indicated rate (MIR), as shown in FIG. 2. This relationship did not reflect the true correlation between the intrinsic heart rate and the minute volume. See "The Range of Sensors and Algorithms used in Rate Adaptive Cardiac Pacing" Chu-Pak Lau, PACE, Vol 15, pg. 1177, August 1992.
Another type of approach tried in the past was a straight linear correlation between MIR and .DELTA.MV. This method is shown in FIG. 3, and while it conformed more closely to the actual relationship between AMV and the intrinsic heart rate, it was still not satisfactory. More particularly, this straight line approach ignored important factors: first, that initially, at low levels of exercise the relationship between MV and rate is different than at a later stage after the patient has reached anaerobic threshold. Briefly, under anaerobic conditions, a patient's metabolism is operating at a level wherein the oxygen demand exceeds the oxygen intake.
Yet another mapping tried in the past was a two-slope interval-based mapping shown in FIG. 4. This method approximated the real response closely but was still not ideal.
Finally, the mapping, referred to as augmented rate response factor, shown in FIG. 5, was also tried in order to compensate for the slow rate response at low exercise levels. In FIG. 5, line A is the rate response factor (RRF) similar to that of FIG. 3. The rate response factor is a parameter selectable in some pacemakers by a physician based on his evaluation of the patient and his own experience. Profiles B, C and D in this Figure represent three dual slope profiles, each profile consisting of two line segments: an initial, or lower rate segment for the beginning of the exercise having a first slope, and a higher rate segment having a second slope. In order to define the profiles, the physician is given the choice of selecting a low, medium or high initial RRF as the first slope. Each of these initial RRF values is higher than the base RRF. In other words, line segment B1 had a slope equal to a low initial RRF, line segment C1 had a slope equal to a medium initial RRF and line segment O1 had a slope equal to a high initial RRF. In addition to the choice of the initial RRF, the maximum and minimum MIR rates were also left to the physician's discretion or choice based on his experience and physical evaluation of the patient. Next, the break points between the line segments, indicated for each profile as B', C', and D' respectively, was arbitrarily selected at 50% of the maximum MIR, as shown. The second line segment for each profile, i.e., B2, C2, and D2 were then extended from the respective breakpoint B', C', and D' to the upper limit of the base line A, as shown. Even though improved, however, this approach still did not correspond to the actual relationship between the intrinsic heart rate and minute volume, especially at more moderate higher exercise levels. Details of this mapping are described in U.S. Pat. No. 5,292,390 incorporated herein by reference.
In summary, a mapping using a single RRF value as a slope is unsatisfactory because it ignores the need for a faster response during the initial or exercise level, and the need for a decreased response relative to minute ventilation at higher exercise levels. A mapping in which two line segments are used and the initial slope was set to a preselected RRF is still unsatisfactory because it relies on an artificial transition or break point between the line segments.
An ideal solution to the mapping problem would be to set the two slopes independently, as separate variables. However, this approach would be impractical because the transition point between the line segments would be indeterminate and, therefore, may result in responses with either abnormally extended or abnormally contracted initial slopes.