Home SVCC Area: English  Español  Português
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 B32B1 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 40160 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.
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 0,91,3 s. The main time constant T1 is within the range of 1530 s and T2  1302375 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 0,0050,01 Hz. Besides, it is known that sympathetic regulation circuit is within the range of 0,050,15 Hz and parasympathetic regulation circuit is within 0,150,4 Hz. Consequently, pass band of LFF should be expanded up to 0,40,5 Hz.
FR's of stepbystep, 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 1530 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 PlakketBurman 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 40160 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
III functional classes stable angina pectoris pts were studied. Power in SET
was changed incidentally each 30 seconds within the limits of 40160 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]. Computeraided bicycle ergometer B32B1 (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 2040 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.267273.
2. Chung E. Exercise electrocardiography. Practical Approach Baltimore, WilliamsWilkins Corp., 1979.
3. Eykhoff P. System Identification: parameter estimation.  Mir, Moscow, 1975.
Your questions, contributions and commentaries
will be answered
by the lecturer or experts on the subject in the Ischemic Heart Disease list.
Please fill in the form (in Spanish, Portuguese or English) and press the "Send"
button.
2nd Virtual Congress of Cardiology


Dr. Florencio
Garófalo
Steering Committee President 
Dr.
Raúl Bretal
Scientific Committee President 
Dr.
Armando Pacher
Technical Committee  CETIFAC President 
Copyright© 19992001 Argentine Federation of Cardiology All rights reserved 
This company contributed to the Congress: