The typical adult sinus rhythm range is between 65 and 85 heart beats per minute (bpm). Generally, rates between 60 and 100 bpm are not a cause for concern. This range is called the sinus rate range. Rates falling outside the sinus rate range are known as arrhythmias. An arrhythmia in which the sinus rate is above 100 bpm is called tachycardia. An arrhythmia in which the sinus rate is below 60 bpm is called bradycardia.
Pacing devices are used to provide artificial cardiac pacing to patients exhibiting bradycardia. It is increasingly more common to combine pacing devices with cardioverter/defibrillator devices. This allows a physician to prescribe a single cardiac stimulating device that is capable of administering treatment for bradycardia, tachycardia and fibrillation.
Because these devices are implanted into a patient's body, they necessarily are powered by an internal energy source. The lack of a suitable energy source was a major impediment to the development of modern implantable cardiac devices. Initially, mercury zinc cells were used, but were unable to provide the peak current requirements of cardioverters which typically are in the range of one to two amperes for approximately 10 seconds. Lithium vanadium pentoxide batteries were able to meet this requirement, but had a characteristic precipitous voltage decline towards the end of their usable life. These batteries were abandoned because of the difficulty in accurately determining the remaining life of the battery.
Currently, lithium silver vanadium pentoxide cells are favored for powering high voltage circuitry in cardioverter/defibrillators and lithium iodine cells are favored for powering low voltage circuitry. These batteries are favored for two reasons: (1) they have greater energy density than their predecessors, more charge stored per unit of battery volume combined with a lower internal impedance; and (2) they have a gradual voltage decline over their usable life. The latter characteristic allows a more accurate prediction of when the battery's usable life is near its end.
The newer Lithium/Carbon Monoflouride (Li/CFx) cells, manufactured by Wilson Greatbach, LTD, Clarence, N.Y., provide a medium current capability with a very low internal impedance characteristic.
Because time and usage both degrade the abilities of all batteries to supply power, physicians need to be able to predict when surgical replacement of the battery (or the entire device) will be needed. For patients who are dependent on the device, timely replacement of the battery is critical. For that reason, the physician needs accurate data on remaining battery life in order to properly schedule replacement surgery.
Present systems do not adequately enable physicians to forecast the time when replacement surgery should be scheduled. Present systems, known in the art, typically base the recommended replacement time (RRT) on the assumption that the device could operate at (worst case) 100% pacing at the current programmed settings and at the current current drain. Prior systems do not take into consideration such factors as elevated/lowered rates due to activity, metabolic demand circadian rhythms (sleep modes), P-wave tracking and/or R-wave inhibition, or lowered current drain modes due to automatic features (e.g., autocapture).
Nor do the prior known systems take into account the reduction in current drain which varies according to the declining battery voltage as a result of the ohmic circuits and the starvation of the charge pump circuitry (which results in voltage/current limited stimulation pulses). It is well known that a battery's internal impedance increases with time and usage. As a battery's internal impedance increases, the terminal voltage of the battery decreases because more voltage is lost across the battery's internal impedance which tends to act like a voltage divider circuit. The decrease in terminal voltage eventually reaches a point where the battery cannot supply the voltage needed to operate an implantable cardiac stimulating device. That voltage corresponds to the battery's end of life. Typically, batteries have a recommended replacement time ("RRT") which corresponds to a voltage slightly higher than the battery's end of life voltage. The difference between the two creates a margin of error.
The discharge characteristics of batteries can be expressed by curves (or equations) of internal battery impedance as a function of expended battery capacity (in terms of charge). Internal battery impedance can be measured directly or may be derived by, for example, measuring the terminal voltage of the battery and the load resistance and applying Ohm's law to find a normalized current flow as expressed in the equation EQU I=V/R.sub.t =V.sub.oc /(R.sub.L +R.sub.b)
where V is the measured battery voltage, R.sub.t is total resistance, R.sub.b is internal battery impedance, V.sub.oc is the battery open circuit voltage, I is normalized current, and R.sub.L is the load resistance. It follows that: EQU R.sub.b =(.DELTA.V/V)R.sub.L; EQU where .DELTA.V=V.sub.oc -V.
The relationship between charge depleted (Q) and internal battery impedance may be depicted in a look-up table format or a graphical format which represents corresponding values of R.sub.b and Q for each resistive load. By using variations of the equation T=Q/I where T is time, Q is charge, and I is the battery current drain, the time it takes to deplete a selected amount of battery charge may be determined.
Previously known programming devices use variations of the foregoing expressions to display some indication of the recommended replacement time of a battery. For example, previously known devices may, upon interrogation of an implanted device, display that the recommended replacement time has been reached, or may display the percent of battery depletion. Previously known devices do not allow a patient or a physician to easily and accurately determine the time remaining to the recommended replacement time.
What is needed therefore is an automatic and amenable system and method of forecasting and displaying the time remaining to the recommended replacement time of the battery. Furthermore, the system and method should be able to be practiced with existing implantable pacemakers, cardioverters, defibrillators or any combination thereof.