Cardiac reserve mobilization trend during exercise and recovery after exercise ()
1. INTRODUCTION
Exercise capacity is closely associated with survival and mortality [1,2] The study by Myers et al. [2] showed that each 1-MET increase in exercise capacity conferred a 12 percent improvement in survival. Consistently, another study [1] demonstrated that exercise capacity is an independent predictor of all-cause mortality in older men; the relationship is inverse and graded, with most survival benefits achieved in those with an exercise capacity >5 METs; survival improved significantly when unfit individuals became fit. Cardiac reserve is one of the most important physiological bases of exercise capacity. There is a 20-fold difference between the most impaired cardiac function and that of the fittest person [3].
Cardiac contractility reserve (CCR) and heart rate reserve (HRR) are reflected in the dynamic change of the heart status, therefore, the evaluation of CCR and HRR is conducted in various stress tests. However, cardiac function is not only reflected in its up-regulation, but also in the recovery course. Previous studies have mainly focused on the cardiac function up-regulation, having neglected the recovery course. Even if some studies are involved in the recovery course, they are mainly in heart rate recovery [4,5], little in cardiac contractility recovery.
This study tried to explore a method to evaluate the extent and speed of cardiac function up-regulation during exercise and the recovery course of cardiac contractility and heart rate after exercise.
The responses of the heart to exercise include many aspects. Three indicators to evaluate the changes of cardiac function during exercise testing have been selected.
1) Amplitude ratio of the first to second heart sound (S1/S2): One feature of the heart sounds that is a direct manifestation of the left ventricular contractile state and may be used for rapid noninvasive detection of left ventricular systolic function is the first heart sound (S1). Previous studies have shown that there is a very close relationship between the amplitude of the first heart sound and cardiac contractility [6,7]. A study by Hansen et al. [7] showed that changes in the amplitude of the first heart sound are closely related to the maximum rate of rise of left ventricular pressure (r = 0.9551, P < 0.001). Therefore, the change trend of the amplitude of the first heart sound (S1) can be used to evaluate cardiac reserve and cardiac endurance. Since the amplitude of S2 is a reflection of the peripheral resistance [8], S1/S2 ratio reflects the relationship between the cardiac contractility and the peripheral resistance, including the regulatory capacity of the myocardium under stress. The study by Hsieh et al. demonstrated that the relative amplitudes of S1 and S2 (aortic component of second heart sound) represent left ventricular dP/dt and ejection fraction (EF) in humans, and S1, corrected for S2, is decreased in patients with impaired LV systolic function [9];
2) Heart rate (HR): Simultaneous measurement of both cardiac contractility change trend and heart rate change trend after exercise might be beneficial to comprehensive evaluation of cardiac function [10];
3) Power output during exercise: Power output is an important indicator for evaluating exercise performance of both athletes [11,12] and patients [13,14]. Cardiac power output at peak exercise was found to be significantly related to aerobic capacity. One of the best measures of athletic performance is power output during exercise. There is a 20-fold difference between the most impaired cardiac function and that of the fittest subject [3].
In this paper the method for evaluating cardiac reserve mobilization trend during exercise and recovery after exercise is described and the preliminary results of the study are analyzed.
2. SUBJECTS AND METHODS
2.1. Subjects
Ten non-athletes (seven males and three females, aged from 19 to 21 years, Group A) and ten athletes (nine males and one female, aged from 14 to 17 years, Group B) were voluntarily enrolled in this study. The inclusion criteria were: 1) age was less than 25 years old; 2) giving informed consent; 3) free of any cardiovascular diseases and free of any other serious diseases. The exclusion criteria were: 1) on any medication; 2) presence of any contraindications to physical exercise. All subjects signed an informed consent form approved by the Ethics Committee of Institute of Physical Education, Chongqing University.
2.2. Instruments
Cardiac Contractility Monitor (CCM, developed by BoJing Medical Informatics Institute, Chongqing, China) was used for this investigation. The hardware of CCM consists of a phonocardiographic sensor, a heart sound signal preprocessing box, a computer, and a printer. CCM uses a sampling rate of 8 kHz with 8 bits per sample (monochannel). The software includes a fundamental heart sound measurement system, the developing environment of which is Visual Basic 6.0 (Microsoft Inc,). The application’s target operating platform is Windows XP.
2.3. Exercise Testing
Since exercise can simultaneously cause a significant change in the inotropic and chronotropic status of the heart, phonocardiogram exercise test (PCGET) was adopted [15,16] in this study. Examination was started after the subject rested for 15 minutes. A PCG sensor was placed on the subjects’ pericardium. Phonocardiograms were recorded in the sitting position at rest and immediately after a step-climbing exercise. The step was 23 cm high. The subjects completed the designed exercise workload and the signals of cardiac cycle and cardiac contractility were simultaneously collected and recorded by CCM. There are many exercise workload protocols for PCGET. A specific multistage incremental protocol was used, which had 4 stages. Exercise test was performed starting at an initial workload of 1750 J. Each subsequent stage had an increment of 1750 J. The peak workload was 7000 J. There were 1-minute intervals between stages. The subjects repeatedly mounted and descended the step. The required number of times of step-climbing was calculated from the target workload, step height, and the subjects’ body weight. The end point of the exercise test was usually limited by subject symptoms, or by achievement of the designed exercise workload. The time taken for completing the workloads 1750 J, 3500 J, 5250 J, and 7000 J was recorded, respectively. During recovery after exercise, heart sound signals were recorded immediately after exercise, and at 1, 5, 10, 15, 20, 25, and 30 minutes after exercise. The ratio of the amplitude of the first heart sound to the amplitude of the second heart sound (S1/S2) on the same phonocardiogram was calculated from the measured data.
2.4. Basic Method for Signal Measurement and Analysis
Heart sound signal was converted several times during processing, and finally, it was converted to screen coordinates to construct a waveform graph. The basic points concerning heart sound quantitative analysis were: 1) measuring the duration and the amplitude of relevant heart sound components; 2) calculating and analyzing relevant indicators based on the data obtained from the above measurements, including S1/S2 ratio, HR, and power (unit: W). Since the thickness of the chest wall of different subjects is various, the absolute amplitude of S1 cannot be used as an indicator for evaluating cardiac contractility. So, the design of some relative value indicators for evaluating cardiac reserve is very important, such as S1/S2 ratio, D/S ratio, etc, which have dimensionless values. To measure and evaluate heart rate and cardiac contractility change trend, a self-control trial was designed.
2.5. Construction of Cardiac Reserve Mobilization Trend Graph
In order to show the cardiac reserve mobilization trend during exercise, x-y plots were constructed from the measured data (employing the mean values). S1/S2 ratio, HR, and power (vertical coordinates) were plotted against the work done during exercise (marked on the horizontal axis). The points of S1/S2 ratio, HR, and power corresponding 1750 J were connected to the points of their baseline value, producing 3 lines, S1/S2 line, HR line, and power line (W line), which, then connected to their own various points corresponding to 3500 J, 5250 J, and 7000 J, obtaining 3 polygonal lines, respectively. A higher slope value indicates a steeper incline. In order to show the cardiac function recovery course, a graph was constructed to the right of the graph of cardiac reserve mobilization trend, the vertical axes of which were the same as the left panel, and the horizontal axis changed to a time axis marked with the time points of cardiac function recovery.
2.6. Statistical Analysis
Quantitative data are presented as mean ± SD. Statistical comparisons between 2 groups were performed with corresponding t test. A value of P < 0.05 was considered to be statistically significant. Statistical analysis was performed with SPSS software (SPSS Inc).
3. RESULTS
The data of cardiac reserve mobilization trend of two groups are shown in Table 1.
The data of the recovery course of two groups are shown in Table 2.
The basic demographic data of the two groups are shown in Table 3.
Cardiac reserve mobilization trend and the recovery course of athletes are shown in Figure 1.
Cardiac reserve mobilization trend and the recovery course of non-athletes are shown in Figure 2.
4. DISCUSSION
Figure 1 shows the characteristics of the cardiac reserve