There are many methods for measuring and evaluating cardiac function. For example, electrocardiogram examination is the optimal method for monitoring myocardial chronotropism and dromotropism. However, it is unable to be used to monitor inotropism. Cardiac catheterization is objective and quantitative, but it is an invasive procedure, requires a special catheterization area, aseptic manipulation, and can not be used routinely. Echocardiography can measure and evaluate cardiac function noninvasively, structurally, and functionally, but cannot be conveniently used at the bedside, in the field, and for some indicators for which it is not sensitive. Jamal et al. [18] sought to investigate ultrasonic strain rate and strain as new indicators to quantify the contractile reserve of pig stunned myocardium during dobutamine infusion. Cardiac blood-pool developing with radionuclide and nuclear magnetic resonance detection have high sensitivity and specificity, but are expensive and hence not easy to popularize. Maximal oxygen uptake and anaerobic metabolism threshold determination are objective, practical, and quantitative, but are affected by respiratory function, require complex equipment and professional technicians, can only be used in a special laboratory, and hence are not appropriate for everyday use. The method used in this investigation can simultaneously collect and record the signals of cardiac cycle and cardiac contractility, and therefore can measure and evaluate inotropic and chronotropic status of the myocardium [19]. In a previous study [20], the accuracy was 97 %, and the precision was satisfactory: when different examiners measured the S1 amplitude and the cardiac cycle of the same subject, the difference of precision between different examiners were not significant (p > 0.05). At present, recording PCG before and after exercise is sufficient for obtaining relevant data, and the issue of recording PCG during exercise will be studied in real-time monitoring of cardiac reserve.
Until now, the New York Heart Association's Functional Classification (NYHA FC) is still used very widely. Attempts at improvement of NYHA FC by overcoming its subjectivity continue; for example the revision of the 1995 Guidelines for the Evaluation and Management of Heart Failure by ACC/AHA [21], which is intend to complement but not to replace the NYHA FC. Since oxygen consumption is known to be a more accurate and reproducible measure of exercise capacity, a classification method incorporating an oxygen consumption indicator was proposed [7]. However, this technique requires a special setting and is difficult to be popularized. So we tried to study a convenient, yet objective, indicator for complementing cardiac function classification. Summing up the results from this study and previous studies [7, 8] on the relationship between oxygen consumption and exercise capacity, CCCT might be a complementary reference indicator for cardiac function classification. It allows precise timing of diastole and systole (in the order of millisecs), offers the advantages of being noninvasive, convenient, inexpensive, rapid, simple, objective, repeatable, with high sensitivity and specificity [16], and able to be performed at the bedside, in the doctor's office, in the field, or even at home.
Table 1 shows that there is a very significant difference between CCCT(1/4) and CCCT(1) (p < 0.01), which suggests that using indicators CCCT(1/4) and CCCT(1) may be beneficial for evaluating cardiac contractility and cardiac reserve mobilization level. S1/S2 was 1.89 ± 0.94, which is coincident with the fact that when performing a clinical auscultation at the mitral auscultation area, S1 is generally stronger than S2. But we found that in the patient with hypotension, S1 may be decreased, leading to reduced S1/S2. Therefore, S1/S2 might be useful in considering hypotension due to decreased cardiac contractility, since the amplitude of the first heart sound is a standard measure of cardiac contractility [11] and cardiac contractility is one of the important determinants for blood pressure. T1/M1 can be used for evaluating the right heart load, since T1 will compensatively increase when the right heart load increases, for example in pulmonary heart disease. Also, D/S can be used to measure diastolic cardiac blood perfusion time.
There are many factors that can interfere with the intensity of the first heart sound, such as respiration, exercise, psychological activity, drugs, temperature, smoking, disease, etc. Therefore, performing this kind of examination should follow a uniform technical procedure. Only three factors (respiration, exercise, and psychological activity) were considered by us [22]. Other factors influencing cardiac contractility are being studied or will be considered in future studies.
Because the limited number of disease types and corresponding patient examples, we did not perform stratified statistics. But we compared the difference of T1/M1 between 144 subjects without pulmonary heart disease from the group in this study with 32 patients with pulmonary heart disease from the other group; their T1/M1 were 1.44 ± 0.99 and 2.87 ± 0.7, respectively, with the difference was statistically significant(p < 0.01). From the stratified point of view, our sample size should be increased. This will be done in the next survey, in which we will carry out a multiple center study with large sample size, cooperating with colleagues interested in these indicators for evaluating cardiac reserve.
