G.Sidorenko, A.Frolov, O.Kotova, V.Stankevich, A.Vorobjev
National Research and Practical Centre of Cardiology

The present state of cardiologic diagnostic methods frequently requires obtaining dynamic characteristics able to provide enough information as to the dynamics of different systems functioning. In this connection, the demand for such functional tests as stress  tests has become even more pressing. Dependent on the nature of the stress and the controlled criteria, it becomes possible to assess the condition and the reserves of the coronary blood circulation, as well as the myocard contractility function [1]. At the same time such tests are important not only for diagnostic purposes, they also reflect the adaptation range. They help to recognize the hierarchical sequence (continuum): from the healthy state to the prenosological state, from early stages of a disease right up to the heaviest conditions [2]. In some cases it can be diagnostically important to conduct an acute pharmacological test or a hemodynamic unloading, which would demonstrate a possible therapeutic correction [3].

The application of functional stress tests is exceptionally important in the rehabilitation practice. It is the adaptation reserve identification in the dynamics that enables to quantitatively assess the efficiency of the rehabilitation process and to manage it. ” To measure really means to control ”.

When assessing the whole range of functional stress tests it is worth considering that in order to judge their efficiency it is necessary not only to consider the convenience of their fulfillment and result obtaining but to see their relevance to the real situation for people at work or at home. Such situations resulted in the introduction of a new term – surrogate or non-natural stress tests, which underlines that the traditional bicycle or treadmill tests within the framework of the rigid protocols do not provide enough correlation with real life conditions [4,5]. Another evidence of the same kind is the inadequate correlation of the surrogate tests results with the hemodynamic non-invasive and invasive indices and life quality assessment [6].

It is not accidental that recently a new requirement has been put forward, the one concerning a search for a new variety of functional tests that would be similar to natural loads of individuals. Taking exactly that into account brings about the following ground of the stochastic stress test.

Previously we made an amplitude and frequency analysis of the heart rate system regulation in various stress tests, and compared it with stress properties (figure 1).

Fig.1. Амплитудно-частотные характеристики различных типов физических нагрузок и системы регуляции сердечного ритма. 1 - модель регуляции сердечного ритма, 2 - нагрузка

Fig.1.1. Protocol of the stochastic load for healthy individuals (in 2 times smaller for ICD patients)

At the same time we made a mathematical analysis of the proposed original stochastic stress [7,8,9]. One can easily see that it is only in case of a stochastic, i.e. randomly changing stress, when a correlation of the stress properties and cardiac activities can be observed. Besides, the “signal theory” (Franks, 1974) also formed a demand for the parameters coordination of the stress and the system under study. This served as a basis for the further research and for the development of the programmed veloergometer.

Material and methods

The research involved two groups of patients with the ischemic cardiac disease. Group 1 included patients with stress angina pectoris of functional class I and II, according to the Canadian classification – 45 persons (30 males and 15 females) aged 5110. Group 2 included patients with myocardial infarction in the anamnesis – 45 persons (30 males and 15 females) aged 538. With all the patients the clinical diagnoses were verified during the test with a physical load on the veloergometer. A group of healthy subjects – 45 persons (30 males and 15 females) aged 4612 was used as a control group.

The physical load was fulfilled by means of a programmed veloergometer V3-21 that made it possible to obtain the standard and the stochastic protocols. The latter can also be fulfilled by means of other industrially manufactured veloergometers in the “free protocol” mode. The standard stress test protocol with stepwise increments of workload in groups of patients was conducted with the workloads of 25, 50 and 75 w within 3 minutes each. In the control group the workloads were 50, 100 and 150 w.

The scholastic test was conducted with the workload of the steps along the pseudo-normal distribution in the range of 40 to 80 w, 30 seconds each step. This time was given preference due to the heart rate regulation constant [10]. The stochastic stress tests in the group of healthy subjects were conducted in the range of 40-80 w.

During the stress tests the state of the patients was checked by means of ECG with pulse rate and ST segment displacement indication every 10 seconds and blood pressure count every 3 minutes. All the stress tests were fulfilled till the first signs of the acknowledged criteria of test abortion. The interval between the two types of stress amounted to 1.5-2.0 hours. Heart rate variability is carried out before and after stress tests. The sensitivity and specificity parameters were defined with consideration of the number of pseudo-positive, pseudo-negative, truly-positive and truly-negative cases in the groups under study.  The relative numbers of positive and negative test results are used as assessment evaluation of sensitivity and specificity.

Results

The averaged reactions of healthy subjects to both types of stress tests are shown in Figure 2. Pay attention to the submaximum pulse rate value under the standard stress to be achieved within 7,0+ 1.3 min at the capacity of 50.0 w and the same value at 44.5 w within 5.1+1.0 min under stochastic stress (p<0,05).

Fig.2. Double product (HRxSAP) and heart rate increase during  standard and stochastic load in healthy individuals

The ICD patients’ reaction to the two stress tests is shown in Figures 3 and 4.

Fig.3. Double product and heart rate increase during  standard and stochastic load in stress angina pectoris FC I-II pts

F ig.4. Double product and heart rate increase during  standard and stochastic  load in myocardial infarction pts

The given data show that the cessation of testing due to the ECG and clinical  criteria  during the stochastic stress takes place earlier (Table 1).

Randomness activates the attention and the vegetative and central nervous system regulation. Apart from that the frequential property of the stochastic stress leads to 1.7 shorter times of the achievement of the load cessation criteria.

It is of interest to compare the times of the load cessation criteria achievement for the standard test with stepwise increments and the stochastic test. The difference between these values is likely to show the reserves of the blood circulation system of regulation.

Modern industrial and household stresses are caused not so much by their intensity but rather by the rate of their change and unpredictable nature. This is also important for sporting cardiology. It is not accidental that I.Aulik in his time proposed to assess the neuro-muscular coordination in sports. This value can be used to characterize the level of the body adaptation, which can also help to assess the rehabilitation dynamic process.

The above data speak in favour of the stochastic tests increased sensitivity at the 25% rate in males and 35% in females. We know that these estimations of the sensitivity should be checked by means of coronarography.

A desire has been pronounced to look for stress tests whose nature could be similar to industrial and household conditions. Such an approach has resulted in the introduction into the cardiologic practice of such tests as a 6-minute walk [13,14]. Despite the simple and easy application of this method, its mechanics need some correction, as we reported earlier [15].

To deviate from the rigid protocols of the stress tests [16,17] it has been proposed to measure physical load daily or fortnightly by means of stepmeters [18]. Although in this case the seismological sensors count mainly the vertical shifts of the centre of gravity and the procedure itself takes a longer time.

The above presented stochastic test can be conducted with the help of industrial equipment.  Observations of such workloads have shown episodes of “dynamic rest”, when a decreased load is perceived as rest, although the work goes on. It is noteworthy to add that some armed forces use a “broken” rhythm of workloads in the training process of servicemen. The American sporting medicine college (1981) for fitness sessions of healthy individuals recommended to include short maximum loads (98-100 %) against the background of medium-intensive loads, which is quite close to our algorithm.

Continuous changeability and lack of linearity is characteristic of living nature. According to N.Bernstein, the body constantly finds itself in dynamically changing situations. This should be taken into consideration when stimulating stress tests. Man actually inhabits a stochastic world. As N.Winer put it, it is not the world where all the events are predetermined by the once and for all established harmony [19].

Human life is indivisibly united with the constantly changing environment. The regulation systems of the body should remain in the state of readiness for action, to immediately react to stochastic situations.

That is why it is necessary to study the laws of stochastic processes. By the way, the literal translation of the word “stochastics” is “surmise, conjecture” (from Greek). It is undoubted that on the path of the changing human body study we are to come across numerous puzzles and conjectures.

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