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Exercise Stochastic Test and Perspectives
for its Use in Ischemic Heart
Disease Diagnostics

Sidorenko, Georgy; Frolov, Alexander;
Kotova, Oksana; Stankevich, Vladimir

Scientific and Clinical Centre of Cardiology, Minsk, Belarus

SUMMARY
Background: Development of more perfect methods of stress electrocardiography for ischemic heart disease (IHD) diagnostics is an actual problem. The exercise stress frequency response of random nature is more adequate to frequency response of cardiac activity then traditional step upright test (SUT).
Objectives: The conventional step upright load was compared to the stochastic type of exercise test (SET), commensurable by power and total volume. The frequency response of cardiovascular system and informative capability of stress test were studied.
Material and Methods: SET was performed with usage B32-B1 programmed cycle ergometer with a continuous ECG monitoring. Parameters of 8 healthy persons and 7 IHD II functional class stable angina pectoris pts were studied. The power was changed incidentally through each 30 seconds within the limits of 40-160 Watt. The SUT was performed with 50,100,150 Watt during 3 minutes for each step as the control method. It had the same duration, total work and mean power.
Results: In the SET the full volume of received information was 2,6 bits, that is 1,6 times as large as in SUT. In the healthy persons during the SET the submaximal HR was reached 1,5 times as fast with approximate 2 times as small total volume of executed activity as in SUT. In the IHD pts during the SET ischemic events emerged 2 times as fast with 2 times as small total volume work as in SUT, also.
Discussion: Usage of the SET, due to not predictable sequence of power, allows to make more precise estimation of heart frequency response and corresponds to real everyday and labors loads.
Conclusion: The SET is adequate to frequency response of a cardiovascular system, its increases informative capability of stress ECG and can be used for diagnostics and rehabilitation of the IHD pts.

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BACKGROUND
   Exercise electrocardiography due to its informativeness and noninvasiveness is recognized as standard and is included into unified schemes for diagnostics and rehabilitation of ischemic heart disease (IHD) patients. Transient reactions of cardiovascular system can signalize about myocardial infarction, provide the estimation of the circulation dynamic reserves. They are necessary in rehabilitation of myocardial infarction patients (V. Alkhimovich et al, 1994; D. Aronov et al, 1992; A. Vorobiev et al, l998, E. Chang, 1979). Development of exercise tests mainly takes the path of search for new reliable criteria of ischemia, organism dynamic reserves exhaustion, improvement in ECG leads, equipment and software. At the same time, the major property of the exercise tests remains out of the limits of the cardiologists attention. The point is that during exercises not only myocardial status and working capacity are estimated, but simultaneously, behavior of the cardiovascular system regulation is tested. To analyse heart regulatory properties, it is necessary to resort to automatic control methods which application is more widely developed in technics. The possibility of using both time and frequency methods of investigation is opened up in analyzing regulation systems. Thus the information about cardiovascular system will be more comprehensive.

   The theoretical substantiation and the study of possibilities of new type of exercises- stochastic ones in cardiological practice is the aim of the current investigation.

   The step upright ergometry test (SUT) with power 50, 100, 150 Watt and with step duration of 3 minutes is traditionally used in functional diagnostics. In complicated cases, exercise levels decrease twice according to scheme 25, 50, 75 Watt. However, one can propose a number of critical remarks concerning the use of traditional determinate SUT. First, disturbing action should meet the complexity level of the object being studied, should not bring harm, and should not lead the object into essential nonlinearity area (P. Eykhoff, 1978). On this account, impulse and step upright signals cannot be considered as the optimal ones for the analysis of complex cardiovascular system. Second, time and frequency response of test signal should be agreed with time constant and frequency response of the object under investigation.

    We will compare frequency response (FR) of cardiac activity with frequency features of main test signals. Linear model of heart rate (HR) response to physical exercise was obtained by N.Tidt (1981). This model corresponds to two parallel aperiodic circuits (Figure 1).

Figure 1

   Time constants of delay constitutes 0,9-1,3 s. The main time constant T1 is within the range of 15-30 s and T2 - 130-2375 s. For simplification we will neglect the delay and oscillatory component of transient process. Then, FR of cardiovascular system with respect to HR can be represented by simple Batterworth low frequency filter (LFF) with cutoff frequency of 0,005-0,01 Hz. Besides, it is known that sympathetic regulation circuit is within the range of 0,05-0,15 Hz and parasympathetic regulation circuit is within 0,15-0,4 Hz. Consequently, pass band of LFF should be expanded up to 0,4-0,5 Hz.

   FR's of step-by-step, linear and SUT tests have the features not similar to LFF, i.e. do not correspond to FR of cardiovascular system. It can imply that not all the components of cardiovascular regulation will respond to a similar extent in such tests. We will also remind that regulation system of cardiac rhythm belongs to complex hierarchic systems with vegetative, endocrine and central regulation circuits. Each of these control circuits is characterized by the own oscillation frequency. Thus, to obtain authentic information about the quality of HR control system, it is desirable to synthesize such test signal, which spectrum corresponds to FR of cardiovascular system as close as possible.

   The priority in problem definition and arranging the tasks for optimal exercise tests belongs to papers by N. Tidt (1981), V. Vopnyarsky, G. Sidorenko, A. Shirokov et al (1983). The authors of these papers proposed a novel concept on the usage of stochastic exercises. The considerations are as follows:
     - stochastic exercises are most adequate to real life;
     - separate load steps with duration of 15-30 s are adequate to main time constant of HR and allow to trace HR transition to a new level.

   N. Tidt used two levels of stochastic exercise: 55 and 110 Watt alternating after 20 s in a random way according to plan by Plakket-Burman with 8 reference points. V. Vopnyarsky et al proposed the exercise with pseudonormal distribution law. We formed the stochastic type of exercise test (SET) with greater approach to Gauss normal distribution. FR of pseudonormal load, SET and FR of cardiovascular system are shown in Figure 2. As seen, FR of Gauss distribution is more close to FR of cardiac activity.

   Information approach can serve as an additional criterion in comparing determined and stochastic loads. Let each power level represents information about corresponding state of cardiovascular system. We will consider that successively switched over load levels are independent. Then, full information about the object can be determined as Figure 3.

Figure 3

   In traditional determined load with equal probability of variants 50, 100 and 150 Watt full information constitutes Id=1,6 bits. The designed SET was with 18 switches of power levels within the range of 40-160 Watt with average power of 100 Watt. Full information will be Is=2,6 bits. Thus, according to information theory, the transition from determined load to stochastic one increases information volume about the object being studied by 1,6 times.

MATERIAL AND INVESTIGATION METHODS
   SET and SUT were performed with programmed cycle ergometer with a continuous ECG monitoring. Parameters of 8 healthy persons and 7 IHD I-II functional classes stable angina pectoris pts were studied. Power in SET was changed incidentally each 30 seconds within the limits of 40-160 Watt. SUT was performed with 50, 100, 150 Watt in healthy subjects and with 25, 50, 75 Watt in IHD pts during 3 minutes for each step as the reference method. It had the same duration, total work and mean power. General volume of work performed constituted 54 kJ or 27 kJ, average power 100 or 50 Watt. Clinical and ECG criteria for load termination were applied in accordance with international recommendations [1,2]. Computer-aided bicycle ergometer B32-B1 (Pramen, Belarus) and HR radio sensor (Kettler, Germany), placed on chest of the subject have been used.

RESULTS AND DISCUSSION
Some typical examples are shown.
   The subject L., m. 43 years old, healthy, performed determined 50, 100, 150 Watt SUT. Submaximal HR as cause for terminating the test was achieved at 7 min 50 s. HR was reached 150 beats/min unlike initial one 76, arterial pressure was increased from 120/80 to 160/80 mm Hg. Likewise submaximal HR at 6 minute was achieved in SET.

    Patient Z., m., 47 years old, IHD II FC. SUT according to scheme 25, 50, 75, 100 Watt continued till ischemia onset at 10 minute. Work of 33 kJ was perfomed by the patient. Ischemia reaction established due to ST segment depression was achieved in performing SET also. Ischemia is detected at 4 min 20 s. At the same time, work of 14,1 kJ was performed. Similar results were obtained in patient B., m.52 years old, IHD, I FC. Ischemia reaction appeared at determined load at 13 minute. Total work of 54,6 kJ was performed. Stochastic test also confirmed ischemic reaction at 7 minute. Curves of HR changes in determined and stochastic load in patient B. are shown in Figure 4.

    In the healthy persons during SET submaximal HR was reached 1,5 times as fast with approximate 2 times as small total volume of executed activity as in SUT. In the IHD pts during SET ischemic events emerged 2 times as fast with 2 times as small total volume work as in SUT, also. In the SET the full volume of received information was 2,6 bits, that is 1,6 times as large as in SUT.

CONCLUSIONS
   We consider that usage of SET, due to not predictable sequence of power, allows to make more precise estimation of heart frequency response and corresponds to real everyday and labour loads. One can already state that stochastic loads like determined ones are sensitive to detection of ischemic reactions. But ischemia onset in SET occurs considerably earlier as the test time.

   Our opinion is that stochastic loads due to its nature will be very useful in cardiological diagnostics and rehabilitation. For diagnostic purposes, it is desirable to select the one step time within 20-40 seconds to control transient processes. To augment training effect in rehabilitation one must probably increase step time and allow the cardiovascular system to reach stable state. It is also important that stochastic load is tolerated by organism better.

   The methods of random and nonlinear processes in cardiology have good perspectives.

   Thus, SET is adequate to frequency response of a cardiovascular system, its increases informative capability of stress ECG and can be used for diagnostics and rehabilitation of the IHD patients.

REFERENCES

1. Bruse R. Prognosis of coronary heart pts evaluated by data obtained by noninvasive methods/- Berlin, Springer Verlag, 1983.-p.267-273.

2. Chung E. Exercise electrocardiography. Practical Approach- Baltimore, Williams-Wilkins Corp., 1979.

3. Eykhoff P. System Identification: parameter estimation. - Mir, Moscow, 1975.

 

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2nd Virtual Congress of Cardiology

Dr. Florencio Garófalo
Steering Committee
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Dr. Raúl Bretal
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