Method and apparatus for managing and monitoring cardiac rhythm using active time as the controlling parameter

• US 5,235,976 A
• Filed: 12/13/1991
• Issued: 08/17/1993
• Est. Priority Date: 12/13/1991
• Status: Expired due to Term

• Current Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Original Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Inventors: Spinelli, Julio C.
• Primary Examiner(s): Kamm, William E.
• Application Number:
  US07/806,626
• Time in Patent Office: 613 Days
• Field of Search: 128/419 D, 128/419 PG
• US Class Current: 607/25
• CPC Class Codes: A61N 1/36521 the parameter being derived from measurement of an electrical impedance

Abstract

A cardiac rhythm management device is described in which the rate controlling parameter of a rate adaptive pacemaker is the heart'"'"'s total active time. The active time is evaluated using the intraventricular impedance technique, the active time being the length of the interval between the onset of contraction and the point where a line passing through two points on the fast filling segment of the impedance wave form reaches the impedance level corresponding to the end-diastole impedance of the preceding beat. The approach results in a upper rate limit corresponding to the maximum heart rate that does not compromise circulatory function, e.g., result in a decrease in cardiac output, or an increase in cardiac workload without an associated increase in cardiac output. The application also teaches the use of the heart'"'"'s total active time as an indication of hemodynamic instability for triggering a defibrillation.

Claims

1. A cardiac rhythm management device having a variable rate pulse generator and further comprising:

(a) means for measuring intracardiac impedance variations due to changes in blood volume of the heart;

(b) means for detecting the occurrence of a natural or stimulated cardiac depolarization signal;

(c) means for computing from said impedance variations the total active time of a cardiac cycle, said total active time being the interval starting with the occurrence of a detected natural or stimulated cardiac depolarization signal and ending with the completion of the filling phase of the heart with the ventricles assumed to be refilled at their fast filling rate;

(d) means for producing a control signal related to said total active time; and

(e) means for applying said control signal to said variable rate pulse generator.

2. In a cardiac rhythm management device of the type including means for sensing cardiac activity including the heart'"'"'s R-to-R interval and for stimulating cardiac tissue in the event of hemodynamic instability, the improvement comprising:

(a) means for measuring intracardiac impedance variations due to changes in blood volume in the heart;

(b) means for computing from said impedance variations the total active time of a cardiac cycle;

(c) hemodynamic instability determining means for determining whether said R-to-R interval is lower than a predetermined percentage of the computed total active time; and

(d) means responsive to said hemodynamic instability determining means for activating said means for stimulating cardiac tissue.

3. The device as in claim 2 wherein said means for stimulating cardiac tissue is a defibrillator.

4. In a demand cardiac pacer of the type including a variable rate pulse generator for generating cardiac stimulating pulses in the absence to intrinsic cardiac activity, pacing lead means for coupling said pulse generator to the heart, said pacing lead means having means for sensing said intrinsic cardiac activity and means for applying said stimulating pulses to the heart, the improvement comprising:

(a) means for measuring the total active time of the cardiac cycle, the total active time being the total time elapsing between the start of a cardiac contraction and the end of the filling phase of the ventricles with the ventricles being filled at their fast filling rate;

(b) means for producing a control signal proportional to said total active time; and

(c) means for applying said control signal to said variable rate pulse generator for controlling the rate at which said cardiac stimulating pulses are produced.

5. The demand cardiac pacer as in claim 4 wherein said means for measuring total active time includes means for sensing the intracardiac impedance variations due to the influx and outflow of blood from one ventricular chamber;

and means for determining from said intracardiac impedance variations the maximum filling rate following end-systole.

6. The demand cardiac pacer as in claim 4 wherein said means for measuring the total active time includes means for measuring the time interval between one of said cardiac stimulating pulses and said intrinsic cardiac activity and the time where a linear regression of said maximum filling rate reaches the minimum impedance associated with the same beat.

7. In a demand cardiac pacer of the type including a variable rate pulse generator for generating cardiac stimulating pulses at a rate falling between lower and upper rate limits in the absence of intrinsic cardiac activity, pacing lead means for coupling said pulse generator to the heart, said pacing lead means having means for sensing said intrinsic cardiac activity and means for applying said stimulating pulses to the heart and means for sensing a change in physiologic demand coupled to said pulse generator for varying the rate at which said stimulating pulses are produced between said lower and an upper rate limits, the improvement comprising:

(a) first sensing means for sensing the start of a cardiac contraction;

(b) second sensing means for sensing the end of the filling phase of the ventricles with the ventricles being filled at their fast filling rate;

(c) means responsive to said first and second sensing means for measuring the total time elapsing between the start of a cardiac contraction and the end of said filling phase;

(d) means for producing a control signal proportional to said total active time; and

(e) means responsive to said control signal for setting said upper rate limit.

8. A cardiac monitor having means for measuring intracardiac impedance changes due to the in-flux and outflow of blood from the heart as its beats, comprising:

(a) first and second electrical leads, each having a distal tip electrode and a surface electrode mounted proximal of said tip electrode, said first lead having its distal tip electrode and its surface electrode adapted for positioning in an atrial chamber of the heart and said second lead having its distal tip electrode and its surface electrode adapted for positioning in a ventricular chamber;

(b) means for coupling a constant current source of a predetermined frequency between only one of said pair of tip electrodes and said pair of surface electrodes;

(c) means for sensing voltage variations across the other one of said pair of tip electrodes and pair of surface electrodes; and

(d) microprocessor means coupled to said first and second electrical leads for measuring the impedance between said other of said pair of tip electrodes and pair of surface electrodes; and

means including said microprocessor means for computing from said impedance measurements the heart'"'"'s total active time.

9. A method for controlling an implantable cardiac rhythm management device, comprising the steps of:

(a) measuring the instantaneous intracardiac impedance changes with time due to the influx and outflow of blood from the heart of a patient;

(b) determining from the impedance vs. time measurement the total active time of the cardiac cycle;

(c) producing a control signal proportional to said total active time; and

(d) applying said control signal to said cardiac rhythm management device for modifying an operating condition of said device.

10. The method as in claim 9 wherein the step of determining from the impedance vs. time measurement the total active time of the cardiac cycle comprises the steps of:

(a) detecting one of a ventricular sensed R-wave and a ventricular pacing pulse;

(b) determining the volume of blood contained in the heart at end-diastole in a preceding beat;

(c) measuring the fast-filling phase rate from said impedance vs. time measurement; and

(d) computing the time interval from a detected one of said ventricular sensed R-wave and a ventricular pacing pulse to the point at which the heart has filled to the volume which it contained at end-diastole in the previous beat.