Cardiac pacer which compensates for effects of abrupt changes in heart rate

Abstract

Clinical studies of heart patients have demonstrated that ventricular tachyarrhythmia often is preceded by a foreshortened cardiac cycle length followed by a relatively long compensatory pause, thus producing in an abrupt short-to-long cycle length change. An implantable apparatus for preventing tachyarrhythmia measures the cardiac cycle length and detects the occurrence of a foreshortened cardiac cycle length more than a predefined amount between consecutive cycles. When a normal heart beat does not occur within a predefined period of time after such an abrupt change in cycle length, the resulting compensatory pause is eliminated by a cardiac pacer applying an appropriately timed electrical pulse to produce a contraction of the heart. The apparatus also includes a defibrillator to shock the heart in the event that the preventive pacing is not effective.

I claim:  
     
       1. A method of preventing tachycardia comprising the steps of: 
       selecting threshold value for a change in heart cycle length from one heart cycle to a subsequent heart cycle;  
       selecting a time period, which is longer than the subsequent heart cycle;  
       measuring heart cycle lengths of an animal;  
       deriving a change in the heart cycle length from one heart cycle to a subsequent heart cycle;  
       comparing the change in the heart cycle length to the threshold value to detect when the heart cycle length decreases;  
       detecting when a new heart beat fails to occur within the time period which commences after the subsequent heart cycle;  
       if a new heart beat fails to occur within the time period after the change in heart cycle length exceeds the threshold value, commence treating the animal with a cardiac pacing technique to prevent tachycardia from occurring.  
     
     
       2. The method as recited in claim  1  wherein the step of deriving derives a change in heart cycle length between two consecutive heart cycles. 
     
     
       3. The method as recited in claim  1  wherein the cardiac pacing technique comprises applying an electrical pulse to a heart of the animal. 
     
     
       4. The method as recited in claim  1  wherein the cardiac pacing technique comprises: 
       applying an electrical pulse to a heart of the animal; and  
       applying subsequent electrical pulses to the heart of the animal whenever a heart beat fails to occur within the time period following a preceding electrical pulse.  
     
     
       5. The method as recited in claim  1  wherein the cardiac pacing technique comprises: 
       applying an electrical pulse to a heart of the animal; and thereafter  
       a) increasing the time period to an increased time period;  
       b) applying a subsequent electrical pulse to the heart of the animal if a heart beat fails to occur within the increased time period.  
     
     
       6. The method recited in claim  5  wherein steps (a) and (b) are repeated until at least one spontaneous heart beat occurs. 
     
     
       7. The method as recited in claim  1  further comprising applying cardioversion/defibrillation to the animal, if after the a cardiac pacing technique, the animal fails have a normal heart rate. 
     
     
       8. The method as recited in claim  2  wherein a determination is made that the change in heart rate cycle length exceeds the threshold value when the subsequent heart cycle has a length which is 80 percent or less of a length of the one heart cycle. 
     
     
       9. The method as recited in claim  1  wherein comparing the change in the heart cycle length determines when the subsequent heart cycle length is shorter than the one heart cycle. 
     
     
       10. The method as recited in claim  1  wherein the time period is at least 1.2 times a length of the subsequent heart cycle. 
     
     
       11. The method as recited in claim  1  wherein the step of selecting a time period comprises multiplying a length of the subsequent heart cycle by a value that is greater than one. 
     
     
       12. The method as recited in claim  11  wherein the value is at least 1.20. 
     
     
       13. An apparatus for preventing tachycardia in a heart of an animal, said apparatus comprising: 
       a cardiac pacer which responds to a  control signal  signals by applying an electrical pulse to the animal to contract the heart;  
       a mechanism that measures the cardiac cycle length;  
       a detector coupled to said mechanism to produce a first indication when the cardiac cycle length decreases more than a predefined amount from one cycle to a subsequent cycle;  
       a processor which responds to the first indication by producing a second indication when a new heart beat fails to occur within a predefined time period after the subsequent cycle, where the predefined time period is longer than the subsequent cycle; and  
       an evaluator that responds to the second indication by sending the control signals to the cardiac pacer to perform a cardiac pacing technique to prevent tachycardia from occurring.  
     
     
       14. The apparatus as recited in claim  13  further comprising a telemetry circuit for altering the predefined amount. 
     
     
       15. The apparatus as recited in claim  13  further comprising a telemetry circuit for altering the predefined period of time. 
     
     
       16. The apparatus as recited in claim  13  further comprising a defibrillator which is responsive to said evaluator for applying a shock to the heart if a normal heart rhythm fails to occur after a given number of electrical pulses are applied by said cardiac pacer. 
     
     
       17. A method of preventing tachycardia comprising: 
       measuring a first length of a first heart cycle;  
       measuring a second length of a second heart cycle which occurs after the first heart cycle;  
       determining when the second length is shorter than the first length by at least a predefined amount;  
       deriving a time period as a function of the second length, where the time period is longer than the second length;  
       detecting when a new heart beat fails to occur within the time period following the second heart cycle and in response thereto producing an indication of such occurrence; and  
       responding to the indication by commencing treatment of the animal with a cardiac pacing technique to prevent tachycardia from occurring.  
     
     
       18. The method as recited in claim  17  wherein the second heart cycle consecutively follows the first heart cycle. 
     
     
       19. The method as recited in claim  17  wherein the cardiac pacing technique comprises: 
       (a) applying an electrical pulse to a heart of the animal; and thereafter  
       (b) increasing the time period to an increased time period;  
       (c) applying a subsequent electrical pulse to the heart of the animal if a heart beat fails to occur within the increased time period; and  
       (d) repeating steps (b) and (c) until at least one spontaneous heart beat occurs.  
     
     
       20. A method of preventing tachycardia comprising the steps of: 
       
         selecting threshold value for a change in heart cycle length from one heart cycle to a subsequent heart cycle;  
       
       
         detecting a patient's heartbeats;  
       
       
         measuring heart cycle length of the patient's heart;  
       
       
         deriving a change in the heart cycle length from a first measured heart cycle to a subsequent measured heart cycle;  
       
       
         comparing the change in the heart cycle length to the threshold value;  
       
       
         if the change in heart cycle length exceeds the threshold value, treating the patient with a tachycardia preventing cardiac pacing technique.  
       
     
     
       21. The method as recited in claim  20  wherein the step of deriving derives a change in heart cycle length between two consecutive heart cycles.  
     
     
       22. The method as recited in claim  20  wherein the step of treating the patient with a tachycardia preventing cardiac pacing technique comprises: 
       
         defining a time period following the subsequent measured heart cycle which is longer than the subsequent measured heart cycle;  
       
       
         delivering a pacing pulse to the patients on expiration of the defined time interval in an absence of a sensed heartbeat during the defined interval.  
       
     
     
       23. The method as recited in claim  20  or claim  21  or claim  22  wherein the step of treating the patient with a tachycardia preventing cardiac pacing technique comprises: 
       
         defining gradually increasing pause intervals and delivering pacing pulses to the patient's heart on expirations of the pause intervals absent sensed heartbeats during the pause intervals.  
       
     
     
       24. The method as recited in claim  20  or claim  21  or claim  22  the comparing step comprises determining that the change in heart rate cycle length exceeds the threshold only when the subsequent heart cycle length is shorter than the one heart cycle.  
     
     
       25. The method as recited in claim  20  or claim  21  or claim  22  wherein the comparing step comprises determining that the change in heart rate cycle length exceeds the threshold value when the subsequent measured heart cycle has a length which is  80  percent or less of the length of the first measured heart cycle.  
     
     
       26. The method as recited in claim  20  or claim  21  or claim  22  wherein the selecting step comprises selecting a threshold which is at least a  20  percent decrease in cycle length.  
     
     
       27. A method of preventing tachycardia comprising: 
       
         measuring a first length of a first heart cycle of a patient's heart;  
       
       
         measuring a second length of a second heart cycle the patient's heart which occurs after the first heart cycle;  
       
       
         determining whether the second length is shorter than the first length by at least a predefined amount; and  
       
       
         in response to the second length being shorter than the first length by at least the predefined amount; commencing treatment of the patient's heart with a cardiac pacing technique, comprising:  
       
       ( a )  defining a time period commencing after the second heart cycle which is greater than the second length;    
       ( b )  applying an electrical pacing pulse to the patient's heart on expiration of the defined time period; and    
       ( c )  thereafter applying pacing pulses to the patient's heart at gradually increasing intervals.    
     
     
       28. The method as recited in claim  27  wherein the step of measuring the length of the second heart cycle comprises measuring the length of the heart cycle which consecutively follows the first heart cycle.  
     
     
       29. An apparatus for tachycardia, comprising: 
       
         means for selecting threshold value for a change in heart cycle length from one heart cycle to a subsequent heart cycle;  
       
       
         means for detecting a patient's heartbeats;  
       
       
         means for measuring heart cycle length of the patient's heart;  
       
       
         means for deriving a change in the heart cycle length from a first measured heart cycle to a subsequent measured heart cycle;  
       
       
         means for comparing the change in the heart cycle length to the threshold value;  
       
       
         means responsive to the change in heart cycle length exceeding the threshold value for treating the patient with a tachycardia preventing cardiac pacing technique.  
       
     
     
       30. The apparatus as recited in claim  29  wherein the deriving means comprises means for deriving a change in heart cycle length between two consecutive heart cycles.  
     
     
       31. The apparatus as recited in claim  29  wherein the treating means comprises: 
       
         means for defining a time period following the subsequent measured heart cycle which is longer than the subsequent measured heart cycle;  
       
       
         means for delivering a pacing pulse to the patient on expiration of the defined time interval in an absence of a sensed heartbeat during the defined interval.  
       
     
     
       32. The apparatus as recited in claim  29  or claim  30  or claim  31  wherein the treating means comprises: 
       
         means for defining gradually increasing pause intervals; and  
       
       
         means for delivering pacing pulses to the patient's heart on expirations of the pause intervals absent sensed heartbeats during the pause intervals.  
       
     
     
       33. The apparatus as recited in claim  29  or claim  30  or claim  31  wherein the comparing means comprises means for determining that the change in heart rate cycle length exceeds the threshold only when the subsequent heart cycle length is shorter than the first heart cycle.  
     
     
       34. The apparatus as recited in claim  29  or claim  30  or claim  31  wherein the comparing means comprises means for determining that the change in heart rate cycle length exceeds the threshold value when the subsequent measured heart cycle has a length which is  80  percent or less of the length of the first measured heart cycle.  
     
     
       35. The apparatus as recited in claim  29  or claim  30  or claim  31  wherein the selecting means comprises means for selecting a threshold which is at least a  20  percent decrease in cycle length.  
     
     
       36. An apparatus for preventing tachycardia comprising: 
       
         means for measuring a first length of a first heart cycle of a patient's heart;  
       
       
         means for measuring a second length of a second heart cycle of the patient's heart which occurs after the first heart cycle;  
       
       
         means for determining whether the second length is shorter than the first length by at least a predefined amount;  
       
       
         means responsive to the second length being shorter than the first length by at least the predefined amount for treating the patient's heart with a tachycardia preventing cardiac pacing technique to prevent tachycardia from occurring.  
       
     
     
       37. The apparatus as recited in claim  36  wherein the means for measuring the length of the second heart cycle measures the length of the heart cycle which consecutively follows the first heart cycle.