In the majority of healthy human subjects the change of the body posture from the supine to the upright position results in a decrease in the stroke volume by 40-50%, and in the reduction of the cardiac output by 30%. At the same time the heart rate and the increases by 30%. The diastolic and mean arterial blood pressure as well as the total peripheral resistance increase while the changes in the systolic blood pressure are not consistent. However, there are also reports in literature showing that the cardiac output may be not altered or even may increase after transition from the horizontal to the vertical position (1 - 3).

The atypical adaptation of the cardiovascular system to the gravitational forces frequently occurs in patients suffering from hypertension (4, 5), heart failure 6 - 9), and diabetes mellitus (5) as well as the in the pregnant women (10).

In 1980 Darcmelia and Bekania divided the cardiovascular responses to gravitational forces into three types. The subjects responding with a significant fall in cardiac output in response to the postural change from the supine to the standing position were classified as the type I (hypodynamic), those in whom the cardiac output did not change in response to the change of the body position were classified as type II (eukinetic), whereas the subjects responding with the increase of the cardiac output were included into the type III (hyperkinetic) (11).

The purpose of the present study was to find out whether the subjects manifesting type I and type III of the cardiovascular adaptation to the gravitational forces at rest show also different type of cardiovascular adaptation to the dynamic exercise.


MATERIAL AND METHODS
The study was performed on the group of 249 healthy men 20 to 60 years old, without complaints in medical history, showing no abnormalities during physical examination, and in ECG tracing (at rest and during exercise), and with normal blood and urine analysis, chest X ray and spirometry.

Each subject gave an informed consent for participation in exercise tests. All tests were performed between 9 and 12h in the morning, in constant ambient temperature of 20-22°C. The subjects had small breakfast at least 90 minutes before the test. They did not perform any intense physical exercises and did not drink coffee or alcohol at least 24h before the exercise tests.

Determination of the type of cardiovascular responses to gravitational forces at rest

The subjects were divided into three groups depending on the direction and magnitude of changes in CO after transition from the upright to the supine position. The subjects with the cardiac output in the upright position below 95% of the cardiac output found in the supine position were classified as the type I. The type II included the participants in whom the cardiac output in the standing position was above 95% and below 105% of that found in the supine position. The participants with cardiac output in the standing position amounting more than 105% of the cardiac output found the in supine position were classified as the type III of cardiovascular regulation (11).

Exercise tests

The dynamic exercise tests with increasing workload were performed on cycloergometer in the sitting and the supine position. Four workloads, each lasting 4 minutes were applied. The intensity of the first workload and the increases in workloads during subsequent periods of exercise depended on age, fitness and the body mass of the individual participant.

The hemodynamic cardiac parameters were recorded using the impedance method evaluating the amplitude of the rheographic signal. Before the exercise the hemodynamic parameters were recorded ether in the sitting or in the supine position depending on the position in which the exercise test was to be performed. During the exercise ECG and reographic curve were recorded continuously. Blood pressure was measured every minute. During the exercise the hemodynamic parameters were evaluated under steady state conditions, four min after the onset of each of the subsequent workloads.

The following hemodynamic parameters were evaluated by means of the rheography: stroke volume (SV), cardiac output (CO) and systolic heart function (SF). Heart rate (HR) was assessed by means of ECG and systolic (SP), diastolic (DP) and mean arterial blood pressure (MABP) by means of the Korotkoff method. Stroke volume was calculated with the formula of Kubiczek:

SV = • L2 • Z0-2 • T • Z/t

where - constant equal to 150 corresponding to electric resistance of the blood and technical conditions of recording of the reographic signal; L - distance between the electrodes (cm); Z0 - basic impedance (ohms) according to the reograph digital indicator; T - mean stroke time measured on the basis of 15 rheography signal cycles (CSR, sec) determined from the first derivative of reographic curve, Z/t - mean amplitude estimated on the basis of 15 CSR signal cycles (ohm/sec) and determined from CSR reographic curve.

Depending on heart rate attained during exercise under steady state conditions the participants were divided into 4 groups performing exercise: with low workload (76 - 100 beats/minute, group 1), moderate workload (101 - 125 beats/minute, group 2), high workload (126 - 150 beats/minute, group 3), and very high workload (151 - 175 beats/minute, group 4).

Statistical analysis

The statistical analysis was performed on the data obtained in subjects with Type I and Type III of the cardiovascular regulation. Results of the exercise test were compared: 1) within each type between the supine and the sitting position, and 2) between the Type I and Type III performing exercise in the supine and the sitting position.

Depending on the distribution of data (distribution analysis - Shapiro-Wilk test) the parametric (univariate and multivariate ANOVA, Pearson correlations) and non-parametric (ANOVA rang Kruskal-Wallis, Mann-Whitney test, Wilcoxon test, chi square test, Spearman correlation) methods of statistical analysis were used.


RESULTS
In the whole population of subjects who entered the study the correlation coefficients between the percent changes in CO occurring during transition from the standing to the supine position, and the percent changes in SV, SF and CO during the dynamic physical exercise in the sitting position were -0.468 (P<0.001), -0.419 (P<0.001) and -0.427 (P<0.001), respectively. On the other hand, the analysis of data obtained in the subjects performing exercise in the supine position revealed that the corresponding correlation coefficients between the above parameters assumed the positive values and amounted to 0.551 (P < 0.001), 0.515 (P < 0.001) and 0.532 (P < 0.001), respectively.

The type I of cardiovascular regulation was found in 174 out of 249 participants (70%), while the type III was present in 44 out of 249 subjects (18%). The resting hemodynamic parameters found before exercise test in the groups with type I and type III of the cardiovascular regulation are presented in Table 1. The subjects with the type I and type III of the cardiovascular regulation performing exercise in the standing or in the supine position did not differ significantly from each other with regard to the age, height, body mass and BMI. Neither significant differences were found in the resting hemodynamic parameters between the groups performing exercise of different intensity and classified as the type I or the type III of the orthostatic cardiovascular adaptation.

Table 1. Parameters characterising subjects representing type 1 and type 3 of cardiovascular adaptation to the gravitational forces.

Means ± SD are show. P (S vs S) - significance of differences between the sitting and the supine position. P (I vs III) - statistical significance of difference between the groups with type I and type III of cardiovascular adaptation to the gravitational forces.

The individuals included into the type I of the cardiovascular orthostatic adaptation were able to perform significantly greater exercise, at the same heart rate in the sitting position than in the supine position (Table 2). In other words, they responded with significantly greater (P<0.01) increase in HR, during exercise with the same workload in the supine position than in the sitting position. The subjects classified as the type III responded with similar increases in HR to the workloads performed in the sitting and in the horizontal position. In the supine position the subjects manifesting type III of cardiovascular regulation were able to perform significantly greater workload with the same HR than the subjects belonging to the type I.

Table 2. Magnitude of the workload (J/kg) during dynamic physical exercise performed in the sitting and the supine position by the subjects with the type I and III of the cardiovascular adaptation to the gravitational forces. 1-4 levels of the workload (see text for explanation).

P I vs III - significance of differences between the sitting and the supine position

In both types of orthostatic cardiovascular adaptation the systolic and mean arterial blood pressure increased progressively with the increase in the workload in the sitting and the supine position (P<0.001 for each of these parameters). Small but significant increase of the diastolic pressure was also observed (P<0.05). Both the absolute values and the percent changes in the arterial blood pressure in subjects representing type I and type III were not different either in the sitting or in the supine position.

Tables 3 and 4 present values of SV, CO and SF at rest and during physical exercise performed in the sitting and the supine position in subjects classified as type I or type III of the cardiovascular orthostatic adaptation. Comparison of cardiovascular hemodynamic parameters in both types of subjects revealed that the absolute values of SV, CO and SF during exercise performed in the sitting position were significantly higher in the subjects with type I of cardiovascular adaptation than in the type III (P<0.05 for each parameter; Fig. 1). No significant differences in the above parameters were found during exercise performed in the supine position between the subjects included into the type I and type III of cardiovascular adaptation.

Table 3. Hemodynamic parameters during the dynamic exercise performed in the sitting position by the subjects representing type I of cardiovascular adaptation to the gravitational forces. The percentage changes in comparison to the resting level in the sitting position are also shown. 1,2,3,4, - exercise workloads estimated basing on the changes of the heart rate.

P(K-W) - statistical significance of differences between the groups performing work with different workloads (Kruskal-Wallis test); italics - exercise values significantly different from the resting values; underlined values - rest parameters significantly different from the other values. Medians with the centile ranges (in the brackets) are presented. The centiles were adjusted adequately to the number of the participants in the group.

Table 4. Rest and exercise values of hemodynamic parameters and change of the exercise parameters in comparison to their rest values at different workload in the sitting and the supine position in the group of parameters with the type III of cardiovascular adaptation to the gravitational forces. For other explanations - see Table 3.

Fig. 1. Comparison of relative changes (%) of cardiac parameters during exercise performed in the sitting and the supine position by subjects with type I and III of cardiovascular adaptation to gravitational forces. Black lines: subjects representing type I; gray lines - subjects representing type III of cardiovascular adaptation. P - significance of differences in the exercise workloads between the supine and the sitting position.

DISCUSSION
The present study was focused on elucidation whether the cardiovascular adaptation to the gravitational forces and to the increased energy demands during the dynamic exercise is determined by the type of the cardiovascular adaptation to the gravitational forces. The results reveal that indeed the mode of adaptation of the cardiac hemodynamic parameters to changes in the body position correlates well with adjustment of the cardiac parameters to the workload. Thus, the main parameters determining the pumping capacity of the heart i.e. SF, SV and CO were significantly influenced by changes in the body position and the type of cardiovascular regulation, as opposed to HR and blood pressures that that are not affected by the gravitational forces.

Previous studies have shown clear relationship between resting values of SV and its changes during exercise (12 - 14). In the present study the same relationship was found in the whole population of subjects and in the subjects belonging into the type I when the exercise was performed in the sitting position, however this was not the case when the subjects classified as type 1 were exercising in the supine position. In the latter position the increase of SV was small or even absent and the increase of CO was achieved mainly by the significant increase in HR. It should be emphasized that in the subjects manifesting the type I of orthostatic cardiovascular adaptation who were performing exercise in the sitting position the significant increase in the SV (even 2.0 - 2.5 times) started from significantly lower baseline resting level than in the subjects manifesting the type III because the type I responded with a significant fall in SV during transition from the horizontal to the vertical position. During dynamic exercise the subjects classified as the type III responded with significantly greater increases of SV when the exercise was performed in the horizontal position as compared to the sitting position. Thus far, the studies devoted to the regulation of the cardiovascular system during exercise performed under different gravitational conditions were carried out on heterogenic populations. The present results reveal that the more uniform is the group of exercising subjects with the type I of the cardiovascular adaptation to the gravitational forces, the greater is the increase in SV during increasing exercise load. Thus far, the majority of studies devoted to cardiovascular adaptation to dynamic exercise were performed on young and healthy individuals in whom the type I of the cardiovascular regulation was probably prevailing. However, the analysis of data presented in literature reveals significant discrepancy of results with regard to changes in SV during exercise performed in the same body position, particularly when the exercise tests are carried out on patients suffering from the dysfunction of the cardiovascular system and those with diabetes mellitus (3, 7, 15 - 20) in whom presumably the type III of the cardiovascular adaptation to the gravitational forces may be more frequent; in some studies no differences in CO changes were found when the exercise was performed in the sitting and in the supine position (3, 20). In contrast to the present study which was performed on large group of subjects the previous investigations were conducted on smaller number of subjects and the diverging results were usually regarded as the atypical changes or as artifacts (1 - 3, 7, 21). It can be approximated that if the number of subjects with the type III of the cardiovascular adaptation would amount to 50% in the present study, the differences between the cardiac output and the stroke volume during exercise performed in the supine and in the sitting positions would not differ.

In summary, the results of the present study provide evidence that the cardiac output and the other parameters determining the pumping efficacy of the heart clearly depend on the type of the cardiovascular adaptation to the gravitational conditions. It appears that an assessment of the type of adaptation of the cardiovascular system to the gravitational forces at rest may be an helpful prognostic index evaluating the heart reserve during dynamic exercise performed in the upright and the horizontal position by healthy subjects and by patients suffering from the cardiovascular diseases.

Acknowledgements: The study was partly supported by the Medical University of Warsaw, grant MA/N/06.


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