Ultrasound methods and implantable medical devices using same

• US 20030204140A1
• Filed: 04/25/2002
• Published: 10/30/2003
• Est. Priority Date: 04/25/2002
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Ferek-Patric, Bozidar, Breyer, Branko
• Granted Patent: US 7,037,266 B2
• Time in Patent Office: Days
• US Class Current: 600/439
• CPC Class Codes: A61N 1/36528 the parameter being measured by means of ultrasound
• CPC Class Codes: A61N 1/36571 controlled by blood flow rate, e.g. blood velocity or cardiac output

Abstract

An ultrasound method and medical devices using same provide for various techniques of sampling blood flow velocity, e.g., at several sampling rates. To minimize the energy required for ultrasound monitoring, pulsed Doppler signal packages provided by a pulsed ultrasound circuit are switched in such a way that the repetition rate is the lowest possible and yet sufficiently high to be able to record the blood flow velocity within the heart. For example, an ultrasound circuit may be activated only within a part of the cardiac cycle designated as the Doppler Measurement Interval (DMI); the ultrasound circuit may be switched between an on state and an off state during the DMI; and/or the ultrasound circuit may also be switched on and off in different sampling modes: detection mode and measurement mode (e.g., using different sampling rates).

Claims

1. An ultrasound method to monitor blood flow velocity for use in an implantable medical device, the method comprising:

activating an ultrasound circuit during a first portion of at least one cardiac cycle, wherein the ultrasound circuit is operable to provide pulsed Doppler signal packages during the first portion of the at least one cardiac cycle;

deactivating the ultrasound circuit during a second portion of the at least one cardiac cycle, wherein the ultrasound circuit is inoperable to provide pulsed Doppler signal packages during the second portion of the at least one cardiac cycle; and

switching the ultrasound circuit between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided during the first portion of the at least one cardiac cycle at a first sampling rate.

2. The method of claim 1, wherein the method further comprises:

switching the ultrasound circuit between the on state and off state at the first rate during at least a first time period of the first portion of the at least one cardiac cycle; and

switching the ultrasound circuit between the on state and the off state at a second rate during at least a second time period of the first portion of the at least one cardiac cycle.

3. The method of claim 1, wherein the pulsed Doppler signal packages are provided at a region proximate a tricuspid valve.

4. The method of claim 1, wherein the method further comprises:

switching the ultrasound circuit between the on state and off state at the first rate during at least the first portion of a first cardiac cycle; and

switching the ultrasound circuit between the on state and the off state during a first portion of at least another cardiac cycle at a second rate.

5. The method of claim 4, wherein the first rate is associated with a detection mode, and further wherein the second rate is associated with a measurement mode.

6. The method of claim 5, wherein the detection mode is configured to detect an atrial blood flow velocity filling wave, and further wherein the measurement mode is configured to determine an acceleration of a diastolic blood flow velocity filling wave.

7. The method of claim 4, wherein the method further comprises:

detecting a rate switching event; and

switching from the first rate to the second rate based on the detection of the rate switching event, wherein the second rate is different from the first rate.

8. The method of claim 7, wherein detecting the rate switching event further comprises:

monitoring a physiological parameter of a patient; and

detecting the rate switching event based on the monitored physiological parameter.

9. The method of claim 8, wherein the physiological parameter comprises patient activity.

10. The method of claim 8, wherein the physiological parameter comprises a heart rate.

11. The method of claim 7, wherein the method further comprises switching from the second rate to the first rate based on the detection of an additional rate switching event.

12. An ultrasound method, comprising:

activating an ultrasound circuit during at least a first time interval, wherein the ultrasound circuit is operable to provide pulsed Doppler signal packages during the first time interval;

deactivating the ultrasound circuit during at least a second time interval, wherein the ultrasound circuit is inoperable to provide pulsed Doppler signal packages during the second time interval; and

switching the ultrasound circuit between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided during at least a portion of the first time interval at a first rate.

13. The method of claim 12, wherein the first time interval and the second time interval fall within a cardiac cycle.

14. The method of claim 12, wherein the method further comprises tuning a frequency of the provided pulsed Doppler signal packages based on a predetermined maximum blood flow velocity.

15. The method of claim 12, wherein pulsed Doppler signal packages are provided at a region proximate a tricuspid valve.

16. The method of claim 12, wherein the method further comprises switching between the on state and the off state during at least another portion of the first time interval at a second rate.

17. The method of claim 16, wherein switching between the on state and the off state at the first rate occurs during one or more cardiac cycles, and further wherein switching between the on state and the off state at the second rate occurs during one or more different cardiac cycles.

18. The method of claim 16, wherein the first rate is associated with a detection mode, wherein the second rate is associated with a measurement mode, and further wherein the first rate is slower than the second rate.

19. The method of claim 18, wherein the detection mode is configured to detect an atrial blood flow velocity filling wave, and further wherein the measurement mode is configured to determine an acceleration of a diastolic blood flow velocity filling wave.

20. The method of claim 16, wherein the method further comprises:

detecting a rate switching event; and

switching from the first rate to the second rate based on the detection of the rate switching event.

21. The method of claim 20, wherein the method further comprises switching from the second rate to the first rate based on the detection of an additional rate switching event.

22. The method of claim 20, wherein detecting the rate switching event further comprises:

monitoring a physiological parameter of a patient; and

detecting the rate switching event based on the monitored physiological parameter.

23. The method of claim 22, wherein the physiological parameter comprises patient activity.

24. The method of claim 22, wherein the physiological parameter comprises a heart rate.

25. An ultrasound method to monitor blood flow velocity for use in an implantable medical device, comprising:

activating an ultrasound circuit during at least a first time interval, wherein the ultrasound circuit is operable to provide pulsed Doppler signal packages during the first time interval;

deactivating the ultrasound circuit during at least a second time interval, wherein the ultrasound circuit is inoperable to provide pulsed Doppler signal packages during the second time interval;

switching the ultrasound circuit between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided during at least a portion of the first time interval at a first rate; and

switching the ultrasound circuit between the on state and the off state during at least another portion of the first time interval at a second rate.

26. The method of claim 25, wherein the method further comprises tuning a frequency of the provided pulsed Doppler signal packages based on a predetermined maximum blood flow velocity.

27. The method of claim 25, wherein pulsed Doppler signal packages are provided at a region proximate a tricuspid valve.

28. An ultrasound method to monitor blood flow velocity for use in an implantable medical device, comprising:

activating an ultrasound circuit during a first portion of at least one cardiac cycle, wherein the ultrasound circuit is operable to provide pulsed Doppler signal packages during the first portion of the at least one cardiac cycle;

sampling blood flow velocity with the ultrasound circuit at a first sampling rate during the first portion of the at least one cardiac cycle; and

deactivating the ultrasound circuit during a second portion of the at least one cardiac cycle, wherein the ultrasound circuit is inoperable to provide pulsed Doppler signal packages during the second portion of the at least one cardiac cycle.

29. The method of claim 28, wherein sampling blood flow velocity with the ultrasound circuit at the first sampling rate comprises switching the ultrasound circuit between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided over at least a first period of time at the first sampling rate.

30. The method of claim 29, wherein the method further comprises sampling a blood flow velocity with the ultrasound circuit at a second sampling rate during the first portion of the at least one cardiac cycle, wherein sampling blood flow velocity with the ultrasound circuit at the second sampling rate comprises switching the ultrasound circuit between the on state and the off state over at least a second period of time at the second sampling rate.

31. The method of claim 29, wherein the method further comprises sampling blood flow velocity with the ultrasound circuit at a second sampling rate during a first portion of at least another cardiac cycle, wherein sampling blood flow velocity with the ultrasound circuit at the second sampling rate comprises switching the ultrasound circuit between the on state and the off state over at least a second period of time at the second sampling rate.

32. The method of claim 31, wherein the first sampling rate is associated with a detection mode, and further wherein the second sampling rate is associated with a measurement mode.

33. The method of claim 32, wherein the detection mode is configured to detect an atrial blood flow velocity filling wave, and further wherein the measurement mode is configured to determine an acceleration of a diastolic blood flow velocity filling wave.

34. The method of claim 31, wherein the method further comprises:

detecting a rate switching event; and

switching from the first sampling rate to the second sampling rate based on the detection of the rate switching event, wherein the second sampling rate is different from the first sampling rate.

35. The method of claim 34, wherein detecting the rate switching event further comprises:

monitoring a physiological parameter of a patient; and

detecting the rate switching event based on the monitored physiological parameter.

36. The method of claim 35, wherein the physiological parameter comprises patient activity.

37. The method of claim 35, wherein the physiological parameter comprises a heart rate.

38. The method of claim 34, wherein the method further comprises switching from the second sampling rate to the first sampling rate based on the detection of an additional rate switching event.

39. The method of claim 28, wherein the pulsed Doppler signal packages are provided at a region proximate a tricuspid valve.

40. An ultrasound method to monitor blood flow velocity for use in an implantable medical device, comprising:

providing pulsed Doppler signal packages for use in sampling blood flow velocity; and

selecting a frequency of the provided pulsed Doppler signal packages based on a predetermined maximum blood flow velocity.

41. The method of claim 40, wherein the method further comprises selecting the frequency of the provided pulsed Doppler signal packages based on an angle between the provided pulsed Doppler signal packages the monitored blood flow velocity.

42. The method of claim 40, wherein the pulsed Doppler signal packages are provided during only a portion of a cardiac cycle.

43. An ultrasound method to monitor blood flow velocity for use in an implantable medical device, comprising:

sampling blood flow velocity at a first sampling rate;

detecting a rate switching event;

switching from the first sampling rate to a second sampling rate of blood flow velocity based on the detection of the rate switching event, wherein the second sampling rate is different from the first sampling rate; and

sampling blood flow velocity at the second sampling rate.

44. The method of claim 43, wherein sampling blood flow velocity at the first sampling rate comprises switching an ultrasound circuit between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided over at least a first period of time at the first sampling rate, and further wherein sampling blood flow velocity at the second sampling rate comprises switching the ultrasound circuit between the on state and the off state over at least a second period of time at the second sampling rate.

45. The method of claim 44, wherein the at least a first period of time and the at least a second period of time fall within each cardiac cycle of a plurality of cardiac cycles.

46. The method of claim 43, wherein sampling blood flow velocity at the first sampling rate occurs during one or more cardiac cycles, and further wherein sampling blood flow velocity at the second sampling rate occurs during one or more different cardiac cycles.

47. The method of claim 43, wherein the first sampling rate is associated with a detection mode, wherein the second sampling rate is associated with a measurement mode, and further wherein the first sampling rate is slower than the second sampling rate.

48. The method of claim 47, wherein the detection mode is configured to detect an atrial blood flow velocity filling wave, and further wherein the measurement mode is configured to determine an acceleration of a diastolic blood flow velocity filling wave.

49. The method of claim 43, wherein the method further comprises sampling blood flow velocity in a region proximate a tricuspid valve.

50. The method of claim 43, wherein the method further comprises switching from the second sampling rate to the first sampling rate based on the detection of an additional rate switching event.

51. The method of claim 43, wherein detecting the rate switching event further comprises:

monitoring a physiological parameter of a patient; and

detecting the rate switching event based on the monitored physiological parameter.

52. The method of claim 51, wherein the physiological parameter comprises patient activity.

53. The method of claim 51, wherein the physiological parameter comprises a heart rate.

54. A method for detecting a cardiac event and delivering a therapy in response thereto, the method comprising:

sampling blood flow velocity at a first sampling rate;

pacing a heart based at least on the sampled blood flow velocity;

monitoring a heart rate;

detecting a rate switching event based on the monitored heart rate;

switching from the first sampling rate to a second sampling rate of blood flow velocity based on the detection of the rate switching event, wherein the second sampling rate is different from the first sampling rate;

sampling a diastolic blood flow velocity filling wave at the second sampling rate;

determining an acceleration of the diastolic blood flow velocity filling wave;

detecting tachycardia if the acceleration of the diastolic blood flow velocity filling wave satisfies a diastolic blood flow velocity filling wave acceleration threshold;

delivering a therapy upon detecting tachycardia; and

switching from the second sampling rate to the first sampling rate based on the detection of another rate switching event.

55. An implantable medical apparatus, comprising:

an ultrasound circuit operable to provide pulsed Doppler signal packages; and

controller circuitry in communication with the ultrasound circuit, the controller circuitry operable to;

activate the ultrasound circuit during at least a first time interval, wherein the ultrasound circuit is operable to provide pulsed Doppler signal packages during the first time interval;

deactivate the ultrasound circuit during at least a second time interval, wherein the ultrasound circuit is inoperable to provide pulsed Doppler signal packages; and

switch the ultrasound circuit between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided during at least a portion of the first time interval at a first rate.

56. The apparatus of claim 55, wherein the controller circuitry is further operable to switch the ultrasound circuit between the on state and the off state during at least another portion of the first time interval at a second rate.

57. The apparatus of claim 56, wherein the apparatus further comprises a detection sensor in communication with the controller circuitry, wherein the detection sensor is operable to detect a rate switching event, wherein the controller circuitry is further operable to switch from the first rate to the second rate based on the detection of the rate switching event.

58. The apparatus of claim 56, wherein the controller circuitry is further operable to:

switch between an on state and an off state at the first rate during one or more cardiac cycles; and

switch between an on state and an off state at the second rate during one or more different cardiac cycles.

59. The apparatus of claim 56, wherein the first time interval and the second time interval fall within a cardiac cycle.

60. The apparatus of claim 56, wherein the first rate is associated with a detection mode, wherein the second rate is associated with a measurement mode, and further wherein the first rate is slower than the second rate.

61. The apparatus of claim 60, wherein the controller circuitry while in detection mode is operable to detect an atrial blood flow velocity filling wave, and further wherein the controller circuitry while in measurement mode is operable to determine an acceleration of a diastolic blood flow velocity filling wave.

62. The apparatus of claim 57, wherein the controller circuitry is further operable to switch from the second rate to the first rate based on the detection of an additional rate switching event.

63. The apparatus of claim 57, wherein the detection sensor is further operable to:

detect a physiological parameter of a patient; and

detect the rate switching event based on the detected physiological parameter.

64. The apparatus of claim 63, wherein the detection sensor comprises an accelerometer, and further wherein the physiological parameter comprises patient activity.

65. The apparatus of claim 63, wherein the detection sensor comprises a heart rate sensor, and further wherein the physiological parameter comprises a heart rate.

66. The apparatus of claim 55, wherein the ultrasound circuit is operable to provide pulsed Doppler signal packages at a frequency based on a predetermined maximum blood flow velocity.

67. The apparatus of claim 55, wherein the apparatus further comprises an implantable lead in communication with the ultrasound circuit, wherein the implantable lead is operable to deliver pulsed Doppler signal packages to a region proximate a tricuspid valve.

68. An implantable cardiac apparatus, comprising:

sensing circuitry for sensing cardiac activity, wherein cardiac activity comprises a plurality of cardiac cycles;

an ultrasound circuit operable to provide pulsed Doppler signal packages; and

controller circuitry in communication with the ultrasound circuit and the sensing circuitry, the controller circuitry operable to;

activate the ultrasound circuit during a first portion of at least one cardiac cycle of the plurality of cardiac cycles, wherein the ultrasound circuit is operable to provide pulsed Doppler signal packages during the first portion of the at least one cardiac cycle;

deactivate the ultrasound circuit during a second portion of the at least one cardiac cycle of the plurality of cardiac cycles, wherein the ultrasound circuit is inoperable to provide pulsed Doppler signal packages during the second portion of the at least one cardiac cycle of the plurality of cycles; and

sample blood flow velocity with the ultrasound circuit at a first sampling rate during the first portion of the at least one cardiac cycle of the plurality of cardiac cycles.

69. The apparatus of claim 68, wherein the controller circuitry is further operable to:

switch the ultrasound circuit between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided at the first sampling rate during at least a first time period of the first portion of the at least one cardiac cycle; and

switch between the on state and the off state at a second sampling rate during at least a second time period of the first portion of the at least one cardiac cycle.

70. The apparatus of claim 68, wherein the controller circuitry is further operable to:

switch the ultrasound circuit between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided at the first sampling rate during at least a first time period of the first portion of at least a first cardiac cycle; and

during the first portion of the at least another cardiac cycle, switch the ultrasound circuit between the on state and the off state at a second sampling rate.

71. The apparatus of claim 70, wherein the apparatus further comprises a detection sensor in communication with the controller circuitry, wherein the detection sensor is operable to detect a rate switching event, wherein the controller circuitry is further operable to switch from the first sampling rate to the second sampling rate based on the detection of the rate switching event.

72. The apparatus of claim 71, wherein the controller circuitry is further operable to switch from the second sampling rate to the first sampling rate based on the detection of an additional rate switching event.

73. The apparatus of claim 71, wherein the detection sensor is further operable to sense a physiological parameter of a patient for use in providing the rate switching event.

74. The apparatus of claim 73, wherein the detection sensor comprises an accelerometer, and further wherein the sensed physiological parameter comprises patient activity.

75. The apparatus of claim 73, wherein the detection sensor comprises a heart rate sensor, and further wherein the sensed physiological parameter comprises a heart rate.

76. The apparatus of claim 70, wherein the first sampling rate is associated with a detection mode, wherein the second sampling rate is associated with a measurement mode, and further wherein the first sampling rate is slower than the second sampling rate.

77. The apparatus of claim 76, wherein the controller circuitry while in detection mode is operable to detect an atrial blood flow velocity filling wave, and further wherein the controller circuitry while in measurement mode is operable to determine an acceleration of a diastolic blood flow velocity filling wave.

78. The apparatus of claim 68, wherein the ultrasound circuit is operable to provide pulsed Doppler signal packages at a frequency based on a predetermined maximum blood flow velocity.

79. The apparatus of claim 68, wherein the apparatus further comprises an implantable lead in communication with the ultrasound circuit, wherein the implantable lead is operable to provide pulsed Doppler signal packages at a region proximate a tricuspid valve.

80. An implantable cardiac apparatus, comprising:

sensing circuitry for sensing cardiac activity, wherein cardiac activity comprises a plurality of cardiac cycles;

an ultrasound circuit operable to provide pulsed Doppler signal packages; and

controller circuitry in communication with the ultrasound circuit and the sensing circuitry, the controller circuitry operable to;

activate the ultrasound circuit during a first portion of at least one cardiac cycle of the plurality of cardiac cycles, wherein the ultrasound circuit is operable to provide pulsed Doppler signal packages during the first portion of the at least one cardiac cycle of the plurality of cardiac cycles;

sample blood flow velocity with the ultrasound circuit at a first sampling rate during the first portion of the at least one cardiac cycle; and

deactivate the ultrasound circuit during a second portion of the at least one cardiac cycle, wherein the ultrasound circuit is inoperable to provide pulsed Doppler signal packages during the second portion of the at least one cardiac cycle.

81. The apparatus of claim 80, wherein the controller circuitry is further operable to switch between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided over at least a first period of time during the first portion of the at least one cardiac cycle at the first sampling rate.

82. The apparatus of claim 81, wherein the controller circuitry is further operable to sample blood flow velocity with the ultrasound circuit at a second sampling rate over at least a second period of time during the first portion of the at least one cardiac cycle, wherein the controller circuitry switches the ultrasound circuit between the on state and the off state at the second sampling rate.

83. The apparatus of claim 81, wherein the controller circuitry is further operable to sample blood flow velocity with the ultrasound circuit at a second sampling rate over at least a second period of time during a first portion of at least another cardiac cycle of the plurality of cardiac cycles, wherein the controller circuitry switches the ultrasound circuit between the on state and the off state at the second sampling rate.

84. The apparatus of claim 83, wherein the apparatus further comprises a detection sensor in communication with the controller circuitry, wherein the detection sensor is operable to detect a rate switching event, wherein the controller circuitry is further operable to switch from the first sampling rate to the second sampling rate based on the detection of the rate switching event.

85. The apparatus of claim 83, wherein the first sampling rate is associated with a detection mode, wherein the second sampling rate is associated with a measurement mode, and further wherein the first sampling rate is slower than the second sampling rate.

86. The apparatus of claim 85, wherein the controller circuitry while in detection mode is operable to detect an atrial blood flow velocity filling wave, and further wherein the controller circuitry while in measurement mode is operable to determine an acceleration of a diastolic blood flow velocity filling wave.

87. The apparatus of claim 80, wherein the apparatus further comprises an implantable lead in communication with the ultrasound circuit, wherein the detection sensor is operable to provide pulsed Doppler signal packages at a region proximate a tricuspid valve.

88. The apparatus of claim 80, wherein the ultrasound circuit is operable to provide pulsed Doppler signal packages at a frequency based on a predetermined maximum blood flow velocity.

89. An implantable cardiac apparatus, comprising:

an ultrasound circuit operable to provide pulsed Doppler signal packages;

a detection sensor; and

controller circuitry in communication with the ultrasound circuit and the detection sensor, wherein the controller circuitry is operable to;

sample blood flow velocity at a first sampling rate;

detect a rate switching event using the detection sensor;

switch from the first sampling rate to a second sampling rate of blood flow velocity based on the detection of the rate switching event, wherein the second sampling rate is different from the first sampling rate; and

sample blood flow velocity at the second sampling rate.

90. The apparatus of claim 89, wherein the ultrasound circuit is further operable to provide pulsed Doppler signal packages at a frequency based on a predetermined maximum blood flow velocity.

91. The apparatus of claim 89, wherein the first sampling rate is associated with a detection mode, wherein the second sampling rate is associated with a measurement mode, and further wherein the first sampling rate is slower than the second sampling rate.

92. The apparatus of claim 91, wherein the controller circuitry while in detection mode is operable to detect an atrial blood flow velocity filling wave, and further wherein the controller circuitry while in measurement mode is operable to determine an acceleration of a diastolic blood flow velocity filling wave.

93. The apparatus of claim 89, wherein the apparatus further comprises an implantable lead in communication with the ultrasound circuit, wherein the implantable lead is operable to provide pulsed Doppler signal packages at a region proximate a tricuspid valve.

94. The apparatus of claim 89, wherein the controller circuitry is further operable to switch from the second sampling rate to the first sampling rate based on the detection of an additional rate switching event.

95. The apparatus of claim 89, wherein the detection sensor is further operable to sense a physiological parameter of a patient, wherein the sensed physiological parameter comprises patient activity or heart rate.

96. A pacing apparatus, comprising:

sensing and pacing circuitry for sensing cardiac activity and generating pacing pulses;

an ultrasound circuit operable to provide pulsed Doppler signal packages; and

controller circuitry in communication with the ultrasound circuit and the sensing and pacing circuitry, the controller circuitry operable to;

sample blood flow velocity using the ultrasound circuit at a first sampling rate;

sense a heart rate using the sensing and pacing circuitry;

compare the sensed heart rate to a heart rate threshold value;

switch from the first sampling rate to a second sampling rate if the heart rate is greater than the heart rate threshold value;

sample a diastolic blood flow velocity filling wave at the second sampling rate using the ultrasound circuit;

detect an acceleration of the diastolic blood flow velocity filling wave; and

detect tachycardia if the acceleration of the diastolic blood flow velocity filling wave is less than a diastolic blood flow velocity filling wave threshold.

97. The pacing apparatus of claim 96, wherein the apparatus further comprises an output circuit in communication with the controller circuitry, wherein the output circuit is operable to deliver a first therapy based on detection of tachycardia.

98. The pacing apparatus of claim 97, wherein the controller circuitry is further operable to, after delivery of the first therapy, switch from the second sampling rate to the first sampling rate if the sensed heart rate is below the heart rate threshold value.

99. An ultrasound method to monitor blood flow velocity for use in an implantable medical device, the ultrasound method comprising sampling blood flow velocity during a time interval of each cardiac cycle of a plurality of cardiac cycles, wherein the time interval of the cardiac cycle sampled varies within each subsequent cardiac cycle of the plurality of cardiac cycles.

100. The method of claim 99, wherein sampling blood flow velocity comprises switching an ultrasound circuit between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided over the time interval at a first sampling rate.

101. The method of claim 99, wherein the method further comprises:

detecting a QRS complex of each cardiac cycle of the plurality of cardiac cycles;

triggering a sampling delay interval upon detection of the QRS complex; and

sampling blood flow velocity at the first sampling rate during the time interval after the sampling delay interval, wherein each subsequent sampling delay interval for subsequent cardiac cycles increases in time by a predetermined value.

102. The method of claim 99, wherein the pulsed Doppler signal packages are provided at a region proximate a tricuspid valve.

103. The method of claim 99, wherein the method further comprises tuning a frequency of the provided pulsed Doppler signal packages based on a predetermined maximum blood flow velocity.

104. An implantable cardiac device, comprising:

an ultrasound circuit operable to provide pulsed Doppler signal packages; and

controller circuitry in communication with the ultrasound circuit, the controller circuitry operable to sample blood flow velocity during a time interval of each cardiac cycle of a plurality of cardiac cycles, wherein the time interval of the cardiac cycle sampled varies within each subsequent cardiac cycle of the plurality of cardiac cycles.

105. The apparatus of claim 104, wherein the controller circuitry is further operable to switch the ultrasound circuit between an on state where pulsed Doppler signal packages are provided and an off state where pulsed Doppler signal packages are not provided over the time interval at a sampling rate.

106. The apparatus of claim 104, wherein the apparatus further comprises sensing circuitry in communication with the controller circuitry, wherein the sensing circuitry is operable to detect a QRS complex of each cardiac cycle of the plurality of cardiac cycles, wherein the controller circuitry is further operable to:

trigger a sampling delay interval upon detection of the QRS complex; and

sample blood flow velocity during the time interval after the sampling delay interval, wherein each subsequent sampling delay interval for subsequent cardiac cycles increases in time by a predetermined value.

107. The apparatus of claim 104, wherein the ultrasound circuit is further operable to provide pulsed Doppler signal packages at a region proximate a tricuspid valve.

108. The apparatus of claim 104, wherein the ultrasound circuit is further operable to preselect a frequency of the provided pulsed Doppler signal packages based on a predetermined maximum blood flow velocity.