Measuring and assessing cardiac electrical stability

• US 5,713,367 A
• Filed: 01/26/1995
• Issued: 02/03/1998
• Est. Priority Date: 01/26/1994
• Status: Expired due to Term

• Current Assignee: Spacelabs Healthcare, Inc. (OSI Systems Incorporated)
• Original Assignee: Cambridge Heart Inc.
• Inventors: Librett, Kevin S., Cohen, Richard J., Albrecht, Paul, Arnold, Jeffrey M.
• Primary Examiner(s): Jastrzab, Jeffrey R.
• Application Number:
  US08/379,375
• Time in Patent Office: 1,104 Days
• Field of Search: 128/696, 128/702-705, 128/707, 128/920, 128/923, 128/924, 364/413.06
• US Class Current: 600/517
• CPC Class Codes: A61B 5/0535 Impedance plethysmography for measuring blood flow A61B5/0295
• CPC Class Codes: A61B 5/222 combined with detection or measurement of physiological parameters, e.g. heart rate
• CPC Class Codes: A61B 5/25 Bioelectric electrodes therefor
• CPC Class Codes: A61B 5/257 using adhesive means, e.g. adhesive pads or tapes
• CPC Class Codes: A61B 5/28 for electrocardiography [ECG]
• CPC Class Codes: A61B 5/282 Holders for multiple electrodes for introduction into the body A61B5/287
• CPC Class Codes: A61B 5/349 Detecting specific parameters of the electrocardiograph cycle
• CPC Class Codes: A61B 5/355 Detecting T-waves
• CPC Class Codes: A61B 5/4884 inducing physiological or psychological stress, e.g. applications for stress testing
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61B 5/7207 of noise induced by motion artifacts
• CPC Class Codes: A61B 5/721 using a separate sensor to detect motion or using motion information derived from signals other than the physiological signal to be measured
• CPC Class Codes: Y10S 128/901 Suppression of noise in electric signal

Abstract

The alternans pattern of cycle-to-cycle variability in physiologic waveforms is assessed by applying transducers to a subject, recording physiologic signals, and analyzing the alternans pattern of variation in waveform morphology. Preferred embodiments include methods and apparatus for measuring and assessing alternans in the presence of physiologic stress, such as exercise, in order to induce or increase the amplitude of the alternans, improvements in signal processing to improve the sensitivity and specificity of the analysis, improvements to provide real-time analysis and feedback to the operator conducting the recording of physiologic signals, reduction in the effect of intercycle interval variability on waveform variability, reduction in the effects of respiration on the alternans measurement, improved means for determining the statistical significance of the alternans measurement, handling of abnormal beats such as atrial and ventricular premature beats, choosing of data epochs over which to perform the alternans measurement, using multiple electrodes to improve the accuracy of the alternans measurement, and combining measurement of alternans with other cardiovascular diagnostic tests to facilitate and improve the combined diagnostic capability.

Claims

1. A method for measuring alternans in at least one physiologic signal representative of activity of the heart of a patient, the physiologic signal including a sequence of substantially repeating physiologic waveforms, the method comprising the steps of:

altering a physiologic condition of a patient to stress the heart of the patient without applying pacing stimuli directly to the heart;

receiving a physiologic signal representative of activity of the stressed heart; and

digitally processing the physiologic signal to determine a level of alternans in the signal.

2. The method of claim 1 wherein:

the physiologic condition of the patient is altered by naturally occurring stress during an extended period, and

the step of receiving the physiologic signal comprises receiving the physiologic signal from a portable recording device.

3. The method of claim 1 wherein the physiologic condition of the patient is altered so that the patient'"'"'s heart rate is above a predetermined rate.

4. The method of claim 3 wherein the physiologic condition of the patient is altered so that the patient'"'"'s heart rate is between about 90 and about 150 beats per minute.

5. The method of claim 1 wherein said step of altering the physiologic condition of the patient comprises exercising the patient.

6. The method of claim 1 wherein the patient'"'"'s heart is stressed by exercise.

7. The method of claim 6 wherein the patient exercises by pedalling a bicycle.

8. The method of claim 6 further comprising the step of producing a signal for controlling the patient'"'"'s exercise intensity.

9. The method of claim 8 wherein the step of producing a signal for controlling the patient'"'"'s exercise intensity includes adjusting the signal to control the patient'"'"'s exercise intensity to achieve a desired heart rate in the patient.

10. The method of claim 6 further comprising the steps of receiving signals representative of the patient'"'"'s rate of exercise and using the signals representative of the patient'"'"'s rate of exercise in analyzing the physiologic signal.

11. The method of claim 6 further comprising the steps of producing a signal for controlling the patient'"'"'s exercise and using the signal to control the patient'"'"'s exercise.

12. The method of claim 11 wherein the signal for controlling the patient'"'"'s exercise controls the patient'"'"'s rate of exercise so as to reduce interference of noise related to the rate of exercise with determining the level of alternans.

13. The method of claim 11 wherein the controlling signal is adjusted to reduce noise that interferes with determining the level of alternans.

14. The method of claim 13 wherein the controlling signal is adjusted to reduce noise having a substantially repeating component that repeats at a frequency of about one half the patient'"'"'s heart rate and noise having a substantially repeating component that repeats at a frequency of a submultiple of one half of the patient'"'"'s heart rate.

15. The method of claim 11 wherein the controlling signal is produced to reduce noise generated by at least one source other than the patient'"'"'s heart.

16. The method of claim 15 wherein the controlling signal is produced to reduce noise artifacts generated as a result of respiration of the patient.

17. The method of claim 11 wherein said controlling signal is produced based on a predetermined exercise protocol specifying target exercise rates.

18. The method of claim 6 wherein the patient exercises by walking on a treadmill.

19. The method of claim 6 wherein the patient exercises by stair stepping.

20. The method of claim 1 further comprising the steps of determining the patient'"'"'s heart rate and using the determined heart rate in analzye the level of alternans in the physiologic signal.

21. The method of claim 20 further comprising the step of producing an output signal representative of the patient'"'"'s heart rate.

22. The method of claim 1 further comprising the steps of assessing the patient'"'"'s respiratory activity, producing an output signal representative of the patient'"'"'s respiratory activity, and displaying the output signal to permit visual assessment of the level of alternans in the physiologic signal.

23. The method of claim 1 further comprising determining a frequency of a rhythmic artifact in the received physiologic signal and wherein the determination of the level of alternans is based on portions of the received physiologic signal corresponding to periods when the rhythmic artifact is at frequencies that do not interfere with the determination of the level of alternans.

24. The method of claim 23 wherein the patient'"'"'s heart is stressed by exercise and the rhythmic artifact corresponds to the patient'"'"'s rate of exercise.

25. The method of claim 1, further comprising the steps of determining the patient'"'"'s respiratory rate and determining the patient'"'"'s heart rate, wherein the determination of a level of alternans is based on portions of the received physiologic signal corresponding to periods when the patient'"'"'s respiratory rate is not an even sub-multiple of the patient'"'"'s heart rate.

26. The method of claim 1, further comprising the step of detecting abnormal beats in portions of the physiologic signal, wherein the level of alternans is determined by selecting portions of the received physiologic signal based on the presence of abnormal beats.

27. The method of claim 26 further comprising displaying a signal indicative of the presence of abnormal beats.

28. The method of claim 1 further comprising the steps of determining the patient'"'"'s heart rate and simultaneously a signal representative of the determined level of alternans and producing a signal representative of the determined heart rate.

29. The method of claim 1 wherein the step of digitally processing the physiologic signal to determine a level of alternans comprises reducing the effect of noise by compensating for interfering variability in the morphology of the substantially repeating waveforms in the received signal.

30. The method of claim 29 wherein the step of compensating for interfering variability comprises compensating for intercycle interval variability.

31. The method of claim 29 wherein the step of compensating for interfering variability comprises compensating for variability generated by the patient'"'"'s respiratory activity.

32. The method of claim 1 wherein said step of altering the physiologic condition of the patient comprises administering to the patient a pharmacological agent that stresses the heart of the patient.

33. The method of claim 32 wherein the pharmacological agent that is administered is a beta-sympathetic agent.

34. The method of claim 32 wherein the pharmacological agent that is administered is a parasympathetic blocking agent.

35. The method of claim 32 wherein the pharmacological agent that is administered is a vasodilator.

36. The method of claim 1 wherein said step of altering the physiologic condition of the patient comprises applying negative body pressure to the lower body of the patient.

37. The method of claim 1 wherein said step of altering the physiologic condition of the patient comprises altering the position of the patient'"'"'s body to be different from a supine position.

38. The method of claim 37 further comprising the step of producing a signal representative of heart rate.

39. The method of claim 1 wherein the physiologic condition of the patient is altered spontaneously.

40. The method of claim 39, wherein the spontaneous alteration of the physiologic condition is a result of daily activity.

41. The method of claim 40, further comprising recording physiologic signals using a portable recording device, wherein the step of receiving a physiologic signal comprises receiving a physiologic signal recorded using the portable recording device.

42. The method of claim 41, further comprising determining the presence of spontaneous alteration of the physiologic condition by reference to a recorded physiologic signal.

43. The method of claim 42, further comprising determining a heart rate and determining the presence of spontaneous alteration of the physiologic condition by reference to the heart rate.

44. The method of claim 1 further comprising the step of processing the physiologic signal to determine whether the patient has coronary artery disease.

45. The method of claim 44 wherein coronary artery disease is determined by determining physiologic indices of ischemia in the patient.

46. The method of claim 22, wherein the signal related to the patient'"'"'s respiratory rate indicates whether the respiratory rate is an even sub-multiple of the patient'"'"'s heart rate.

47. The method of claim 1 further comprising the steps of:

obtaining a second signal from the patient;

determining a relationship between variability in the first and second received signals; and

using the relationship in the determination of a level of alternans.

48. The method of claim 47 wherein the step of determining a relationship comprises relating desired features of the first and second signals.

49. The method of claim 47 wherein the step of determining a relationship comprises relating noise components of the first and second signals.

50. The method of claim 47 wherein the step of using comprises combining the first and second signals to noise.

51. The method of claim 47 wherein the second obtained signal represents interbeat interval variability.

52. The method of claim 47 wherein said first and second signals are ECG signals measured from respective ECG leads.

53. The method of claim 47 further comprising the step of processing the first and second signals to determine a distortion metric, and wherein the first and second received signals are combined in a manner so that the distortion metric is minimized.

54. The method of claim 47 wherein the second signal is representative of noise.

55. The method of claim 54 wherein the second signal is representative of impedance.

56. The method of claim 54 wherein the second signal represents a measure of respiration.

57. The method of claim 1 further comprising the steps of:

receiving a second physiologic signal representative of activity of the patient'"'"'s heart;

determining a relationship between desired features of the first and second received physiologic signals;

determining a relationship between noise components of the first and second received physiologic signals; and

combining the first and second received physiologic signals to reduce noise in the determination of a level of alternans.

58. The method of claim 57 wherein said first and second received signals are ECG signals measured from respective ECG leads, said first and second signals being combined in a manner preserving the orientation of the ECG leads in space.

59. The method of claim 58 further comprising the step of processing the first and second signals to determine a distortion metric, and wherein the first and second received signals are combined in a manner so that the distortion metric is below a prescribed level.

60. The method of claim 1 wherein the processing step further comprises basing the determination of the level of alternans on portions of the received physiologic signal that are less affected by interfering processes than other portions of the received physiologic signal.

61. The method of claim 60 wherein the processing step comprises basing the determination of the level of alternans on portions of the received physiologic signal that are less affected by abnormal beats than other portions of the received physiologic signal.

62. The method of claim 60 wherein the processing step further comprises basing the determination of level of alternans on portions of the received physiologic signal that are less affected by noise due to respiration than other portions of the received signal.

63. The method of claim 1 further comprising the steps of detecting abnormal waveforms in the physiologic signal;

independently estimating the level of alternans based on sequences of data between abnormal waveforms; and

combining the independent estimations to determine the measure of the level of alternans.

64. The method of claim 1 wherein the step of digitally processing the physiologic signal to determine a level of alternans comprises relying on a predetermined assumption that a particular abnormal waveform is followed by a normal waveform of a given phase.

65. The method of claim 64 further comprising using the predetermined assumption to adjust a phase relationship between normal waveforms that precede the abnormal waveform and normal waveforms that follow the normal waveform.

66. The method or claim 65 further comprising using the predetermined assumption to determine whether to delete the abnormal waveform or replace the abnormal waveform with a computed waveform in the determination of the level of alternans.

67. The method of claim 1 further comprising measuring ECG signals from one or more transducers placed on the patient, determining a respiratory rate of the patient by analysis of the measured ECG signals, and displaying the respiratory rate to permit visual assessment of the level of alternans in the signal.

68. The method of claim 67 wherein analysis of said ECG signals comprises analyzing the amplitude or vectorcardiographic angle of the features of the measured ECG signals.

69. The method of claim 1, further comprising:

displaying the determined level of alternans as an alternans measure;

generating a reference signal that corresponds to reliability of the alternans measure; and

displaying the reference signal with the alternans measure in a way that permits visual evaluation of the alternans measure in view of characteristics of the reference signal.

70. The method of claim 69, wherein the step of displaying the reference signal further comprises graphically displaying the reference signal with the alternans measure.

71. The method of claim 70, wherein the step of displaying the reference signal comprises displaying the reference signal and the alternans measure using a common time axis.

72. The method of claim 69, wherein the reference signal is indicative of a parameter that affects reliability of the alternans measure, the method further comprising:

generating a second reference signal that corresponds to reliability of the alternans measure and is indicative of a second parameter that affects reliability of the alternans measure; and

displaying the alternans measure and the first and second reference signals in a way that permits visual evaluation of the alternans measure in view of characteristics of the reference signals.

73. The method of claim 69, wherein the reference signal comprises a measure of the patient'"'"'s heart rate.

74. The method of claim 69, wherein the reference signal comprises a measure of defects in the physiologic signal.

75. The method of claim 74, wherein the physiologic signal comprises a sequence of ECG beats and wherein the reference signal comprises a measure of a number of ectopic beats in the sequence.

76. The method of claim 74, wherein the physiologic signal comprises a sequence of ECG beats and wherein the reference signal comprises a measure of a rate of ectopic beats in the sequence.

77. The method of claim 69, wherein the reference signal comprises a measure of noise in the physiologic signal.

78. The method of claim 77, wherein the reference signal comprises a measure of noise in a frequency band of the physiologic signal.

79. The method of claim 78, wherein the frequency band is related to the patient'"'"'s heart rate at different times.

80. The method of claim 77, wherein the reference signal comprises a measure of relative amplitudes of a level of noise in the physiologic signal and of the physiologic signal.

81. The method of claim 69, wherein the reference signal comprises a measure of the patient'"'"'s respiratory activity.

82. The method of claim 69, wherein the reference signal comprises a measure of exercise by the patient.

83. The method of claim 69, wherein the step of displaying the alternans measure comprises displaying the alternans measure on a video monitor.

84. The method of claim 69, wherein the step of displaying the alternans measure comprises printing the alternans measure.

85. The method of claim 84, wherein the step of displaying the alternans measure comprises printing the alternans measure using a chart recorder.

86. The method of claim 1 further comprising producing an output signal representative of a rhythmic artifact in the received physiologic signal.

87. The method of claim 86 wherein the determination of the level of alternans is based on portions of the received physiologic signal corresponding to portions of the output signal that meet predetermined criteria.

88. The method of claim 1 further comprising displaying a signal indicative of the presence of abnormal beats.

89. The method of claim 1, wherein the step of receiving a physiologic signal representative of activity of the heart of the patient comprises receiving the physiologic signal using a multi-segment electrode having multiple electrode segments defined on a common basepad.

90. A method for measuring alternans in at least one physiologic signal representative of activity of the heart of a patient, the physiologic signal including a sequence of substantially repeating physiologic waveforms, the method comprising the steps of:

receiving a physiologic signal representative of activity of the heart of the patient;

digitally processing the physiologic signal to determine a level of alternans in the physiologic signal; and

producing a control signal for controlling a physiologic condition of the patient in a manner such that the effects on the physiologic signal of interfering noise sources is reduced.

91. The method of claim 90 wherein the control signal is produced to control the respiratory rate of the patient.

92. The method of claim 90, wherein the step of receiving a physiologic signal representative of activity of the heart of the patient comprises receiving the physiologic signal using a multi-segment electrode having multiple electrode segments defined on a common basepad.

93. A method for measuring alternans in at least one physiologic signal representative of activity of the heart of a patient, the physiologic signal including a sequence of substantially repeating physiologic waveforms, the method comprising the steps of:

altering a physiologic condition of the patient to stress the heart of the patient without applying pacing stimuli directly to the heart to achieve a heart rate above a predetermined rate;

receiving a physiologic signal representative of activity of the stressed heart of the patient; and

digitally processing the physiologic signal to determine a level of alternans in the signal.

94. The method of claim 93 wherein the physiologic condition of the patient is altered so that the patient'"'"'s heart rate is between about 90 and about 150 beats per minute.

95. The method of claim 93 further comprising the steps of determining the patient'"'"'s heart rate, and simultaneously producing a signal representative of the determined level of alternans and producing a signal representative of the determined heart rate.

96. The method of claim 93, wherein the step of receiving a physiologic signal representative of activity of the heart of the patient comprises receiving the physiologic signal using a multi-segment electrode having multiple electrode segments defined on a common basepad.

97. A method for measuring alternans in at least one physiologic signal representative of activity of the heart of a patient, the physiologic signal including a sequence of substantially repeating physiologic waveforms, the method comprising the steps of:

receiving a physiologic signal representative of activity of the heart of the patient; and

digitally processing the physiologic signal to determine a level of alternans in the signal, wherein the processing step comprises processing the signal to reduce the effect of noise signals having a frequency at about half of the patient'"'"'s heart rate and noise signals having a frequency at about a submultiple of half of the patient'"'"'s heart rate.

98. The method of claim 97 wherein the processing step comprises adaptively removing noise signals before a level of alternans is determined.

99. The method of claim 97 wherein the step of processing the signal to reduce the effect of noise signals comprises basing the determination of a level of alternans on portions of the received signal corresponding to periods when the patient'"'"'s heart rate is above a predetermined level.

100. The method of claim 97 wherein the step of processing the signal to reduce the effect of noise signals comprises basing the determination of a level of alternans on portions of the received signal corresponding to periods when the patient'"'"'s respiratory rate is different from the patient'"'"'s heart rate and is different from sub-multiples of the patient'"'"'s heart rate.

101. The method of claim 97 wherein the processing step further comprises basing the determination of a level of alternans on portions of the received physiologic signal that are less affected by interfering processes than other portions of the received physiologic signal.

102. The method of claim 97 wherein the processing step comprises compensating for interfering variability in the morphology of the substantially repeating waveforms in the received signal.

103. The method of claim 102 wherein the interfering variability is generated by the patient'"'"'s respiratory activity.

104. The method of claim 102 wherein the interfering variability that is compensated is intercycle interval variability.

105. The method of claim 104 wherein the step of reducing the effect of intercycle interval variability comprises determining a relationship between the variation in intercycle intervals and changes in waveform morphology.

106. The method of claim 105 wherein the step of determining a relationship is performed by a filter relating measurements of a given waveform to the sequence of preceding intercycle intervals.

107. The method of claim 97 wherein the processing step comprises relating the variability in a noise signal to the variability in the morphology of the substantially repeating waveforms.

108. The method of claim 107 wherein the noise signal represents a measure of the respiratory activity of the patient.

109. The method of claim 97, wherein the step of receiving a physiologic signal representative of activity of the heart of the patient comprises receiving the physiologic signal using a multi-segment electrode having multiple electrode segments defined on a common basepad.

110. A method for measuring alternans in at least one physiologic signal representative of activity of the heart of a patient, the physiologic signal including a sequence of substantially repeating physiologic waveforms, the method comprising the steps of:

receiving a physiologic signal representative of activity of the heart of the patient;

digitally processing the physiologic signal to determine a level of alternans in the signal, wherein the processing step comprises characterizing at least one portion of the received signal based on at least one preselected criterion selected from a group consisting of the patient'"'"'s heart rate, the patient'"'"'s respiration rate, or the presence of abnormal beats in the received signal, and using the characterization of the at least one portion of the received signal to determine a level of alternans in the signal.

111. The method of claim 110 wherein the characterizing step comprises selecting portions of the received signal to include in the determination of alternans step.

112. The method of claim 111, further comprising the step of determining the patient'"'"'s heart rate, wherein the selecting step comprises selecting portions of the received signal corresponding to periods when the patient'"'"'s heart rate satisfies a predetermined criterion.

113. The method of claim 111, further comprising determining the patient'"'"'s respiratory activity, wherein the selecting step comprises selecting portions of the received signal corresponding to periods when the patient'"'"'s respiratory activity meets a predetermined criterion.

114. The method of claim 111 wherein the selecting step comprises selecting portions of the received signal corresponding to periods when the presence of abnormal beats meet a predetermined criterion.

115. The method of claim 110 wherein the at least one preselected criterion comprises the patient'"'"'s heart rate.

116. The method of claim 115 wherein measured signals are removed from consideration when corresponding to periods when an interfering noise signal varies at a multiple or submultiple of the patient'"'"'s heart rate.

117. The method of claim 110 wherein said at least one preselected criterion comprises the patient'"'"'s respiration rate.

118. The method of claim 110 wherein said at least one preselected criterion comprises the presence of abnormal beats in the received signal.

119. The method of claim 110, wherein the step of receiving a physiologic signal representative of activity of the heart of the patient comprises receiving the physiologic signal using a multi-segment electrode having multiple electrode segments defined on a common basepad.

120. The method of claim 110 wherein the characterizing step comprises selecting portions of the received signal that are less affected by interfering processes than other portions of the received physiologic signal.

121. A real-time method for measuring alternans in at least one physiologic signal representative of activity of the heart of a patient, the physiologic signal including a sequence of substantially repeating physiologic waveforms, the method comprising the steps of:

receiving a physiologic signal representative of activity of the heart of the patient;

digitally processing the physiologic signal to determine a level of alternans in the signal; and

concurrently with the processing step, providing an output representative of the level of alternans as the level of alternans is determined.

122. The method of claim 121 wherein, after a predetermined number of the substantially repeating waveforms are received to form a segment of data, the data segment is processed to determine a level of alternans, the determined level of alternans is provided, and the steps of receiving, processing, and providing are repeated.

123. The method of claim 122 further comprising the steps of:

using a preselected criterion for assessment of the condition of the patient'"'"'s heart to determine when the receiving, processing and providing steps have been performed a sufficient number of times to permit accurate assessment of the condition of the patient'"'"'s heart; and

terminating the steps of receiving, processing, providing, and repeating when the receiving, processing and providing steps have been determined to have been performed for the sufficient number of times.

124. The method of claim 121, wherein the step of receiving a physiologic signal representative of activity of the heart of the patient comprises receiving the physiologic signal using a multi-segment electrode having multiple electrode segments defined on a common basepad.

125. A method for measuring alternans in at least one physiologic signal representative of activity of the heart of a patient, the physiologic signal including a sequence of substantially repeating physiologic waveforms, the method comprising the steps of:

receiving a physiologic signal representative of activity of the heart of the patient;

digitally processing the received physiologic signal to determine a level of alternans and to determine a level of noise; and

using the determined level of noise to determine with a predetermined level of statistical certainty whether the alternans level is above a threshold.

126. The method of claim 125 wherein the using step comprises comparing the level of alternans against a first predetermined threshold and comparing the ratio of the alternans level to the noise level against a second predetermined threshold.

127. The method of claim 126 wherein the level of alternans is determined from a first portion of a measured substantially repeating waveform and the level of noise is determined from a different portion of the same waveform.

128. The method of claim 127 wherein the received signal is an ECG signal and the level of noise is determined from the PQ segment.

129. The method of claim 125 wherein said step of comparing comprises comparing the level of alternans in one portion of a measured substantially repeating waveform to a measure of noise in a different frequency band of the same waveform.

130. The method of claim 125 further comprising the steps of:

determining the level in an alternating component in the received signal and determining the standard deviation of the energy of noise in the signal; and

determining that the patient is at risk for cardiac instability if the level in the alternating component is greater than a predetermined threshold by one or more standard deviations of the noise level.

131. The method of claim 125 further comprising the steps of comparing the determined noise level against a predetermined threshold, and determining that the assessment of the patient'"'"'s risk for cardiac instability is indeterminate if said noise level is greater than predetermined threshold.

132. The method of claim 125, wherein the step of receiving a physiologic signal representative of activity of the heart of the patient comprises receiving the physiologic signal using a multi-segment electrode having multiple electrode segments defined on a common basepad.

133. An apparatus for measuring an alternans pattern of cycle-to-cycle morphology variations in a sequence of substantially repeating physiologic waveforms in at least one signal measured from a patient, said apparatus comprising:

one or more transducers for respectively measuring one or more signals from a patient, at least one of said one or more transducers being configured to measure a signal from the heart of the patient;

a processor configured to obtain, from said one or more measured signals, a measure of the level of alternans in said signal from the heart of the patient for assessing the condition of the patient'"'"'s heart; and

a controller for producing signals to control a stressing device configured to alter a physiologic condition of the patient to stress the heart of the patient and thereby affect the signal from the heart of the patient, the controller producing the signals in response to a signal from the processor.

134. The apparatus of claim 133 wherein said processor determines indices of ischemia from said one or measured signals.

135. The apparatus of claim 133, further comprising an exerciser for controllably exercising the patient under control of the signals produced by said controller.

136. The apparatus of claim 133 wherein said stressing device comprises a device configured to administer a pharmacological agent that controllably alters the heart rate of the patient.

137. The apparatus of claim 133 wherein at least one of said one or more transducers comprises a multi-segment electrode having multiple separate surfaces for achieving electrical contact with a region on the surface of the patient.

138. The apparatus of claim 137 further comprising circuitry coupled to said one or more transducers to measure a signal representative of the respiratory pattern of the patient.

139. The apparatus of claim 137 further comprising circuitry coupled to said multi-segment electrode to measure a signal representative of the electrical impedance between said multi-segment electrode and the patient.

140. The apparatus of claim 133, wherein at least one of said one or more transducers comprises a multi-segment electrode having multiple electrode segments defined on a common basepad.

141. An apparatus for measuring an alternans pattern of cycle-to-cycle morphology variations in a sequence of substantially repeating physiologic waveforms in at least one signal measured from a patient, said apparatus comprising:

one or more transducers for respectively measuring one or more signals from a patient, at least one of said one or more transducers being configured to be coupled to the patient to measure a cardiac signal comprising a sequence of substantially repeating waveforms representative of one or more signals from the heart of the patient, said one or more transducers being configured to remain coupled to the patient for a period in excess of about one hour;

a recorder coupled to said one or more transducers and configured to store signals, including the cardiac signal, measured by said one or more transducers for a period in excess of one hour, the recorder comprising a portable recorder sized and configured to be carried by the patient as the patient engages in normal activities; and

a processor configured to characterize portions of said stored signals corresponding to one or more periods according to one or more preselected criteria, and further configured to obtain from said characterized portions of said stored signals a measure of the level of alternans in the cardiac signal for assessing the condition of the patient'"'"'s heart.

142. The apparatus of claim 141, wherein the processor is configured to generate output signals comprising a signal representative of noise and a signal representative of the determined level of alternans.

143. The apparatus of claim 142 wherein the processor is configured to generate an output signal representative of the patient'"'"'s heart rate.

144. The apparatus of claim 141, wherein at least one of said one or more transducers comprises a multi-segment electrode having multiple electrode segments defined on a common basepad.

145. An apparatus for measuring an alternans pattern of cycle-to-cycle morphology variations in a sequence of substantially repeating physiologic waveforms in at least one signal measured from a patient, said apparatus comprising:

one or more transducers for respectively measuring one or more signals from a patient, at least one of said one or more transducers being configured to be coupled to the patient to measure a cardiac signal comprising a sequence of substantially repeating waveforms representative of one or more signals from the heart of the patient;

a heart rate monitor coupled to at least one of said one or more transducers to measure the heart rate of the patient from signals measured by said at least one transducer; and

a processor configured to obtain, from said one or more measured signals and from said measured heart rate, a measure of the level of alternans in said cardiac signal for assessing the condition of the patient'"'"'s heart.

146. The apparatus of claim 145, wherein at least one of said one or more transducers comprises a multi-segment electrode having multiple electrode segments defined on a common basepad.

147. A method for measuring a pattern of cycle-to-cycle morphology variations in a sequence of substantially repeating physiologic waveforms in at least one signal measured from a patient, said method comprising the steps of:

measuring one or more signals from a patient, at least one of said one or more signals comprising a cardiac signal including a sequence of substantially repeating waveforms representative of one or more signals from the heart of the patient;

processing said one or more measured signals to obtain a measure of the level of alternans in said one or more measured signals for assessing the condition of a patient'"'"'s heart; and

enhancing the measurement of the pattern of cycle-to-cycle morphology variations by reducing the effect of variability in the interval between adjacent waveforms in said cardiac signal.

148. The method of claim 147, wherein the step of measuring one or more signals comprises measuring at least one signal using a multi-segment electrode having multiple electrode segments defined on a common basepad.

149. A method for measuring an alternans pattern of cycle-to-cycle morphology variations in a sequence of substantially repeating waveforms in at least one signal measured from a patient, comprising the steps of:

measuring one or more signals from a patient, at least one of said one or more signals comprising a sequence of substantially repeating waveforms representative of one or more signals from the heart of the patient;

processing the one or more signals to compute a measure of alternans in the repeating waveforms;

additionally processing the one or more signals to produce an additional diagnostic from the group consisting of a signal averaged electrocardiogram and an analysis of QRS complex variability.

150. The method of claim 149, wherein the step of measuring one or more signals comprises measuring at least one signal using a multi-segment electrode having multiple electrode segments defined on a common basepad.

151. An apparatus for measuring an alternans pattern of cycle-to-cycle morphology variations in a sequence of substantially repeating waveforms in at least one signal measured from a patient, said apparatus comprising:

one or more transducers for respectively measuring one or more signals from a patient, at least one of said one or more transducers being coupled to the patient to measure at least one signal comprising a sequence of substantially repeating waveforms representative of one or more signals form the heart of the patient;

a processor configured to process the signals to produce a diagnostic corresponding to alternans and an additional diagnostic from the group consisting of a signal averaged electrocardiogram and an analysis of QRS complex variability.

152. The apparatus of claim 151, wherein at least one of said one or more transducers comprises a multi-segment electrode having multiple electrode segments defined on a common basepad.