The maximum percentage that the cardiac output can increase above normal is called the cardiac reserve. Thus, in the healthy young adult, the cardiac reserve is 300 to 400 per cent. In athletically trained persons, it is occasionally 500 to 600 per cent. But in heart failure, there is no cardiac reserve. As an example of normal reserve, during severe exercise the cardiac output of a healthy young adult can rise to about five times normal; this is an increase above normal of 400 per cent—that is, a cardiac reserve of 400 per cent.
Any factor that prevents the heart from pumping blood satisfactorily will decrease the cardiac reserve. This can result from ischemic heart disease, primary myocardial disease, vitamin deficiency that affects cardiac muscle, physical damage to the myocardium, valvular heart disease, and many other factors, some of which are shown in Figure 22–4.
Diagnosis of Low Cardiac Reserve—Exercise Test. As long aspersons with low cardiac reserve remain in a state of rest, they usually will not know that they have heart disease. However, a diagnosis of low cardiac reserve usually can be easily made by requiring the person to exercise either on a treadmill or by walking up and down steps, either of which requires greatly increased cardiac output. The increased load on the heart rapidly uses up the small amount of reserve that is available, and the cardiac output soon fails to rise high enough to sustain the body’s new level of activity. The acute effects are as follows:
1. Immediate and sometimes extreme shortness of breath (dyspnea) resulting from failure of the heart to pump sufficient blood to the tissues, thereby causing tissue ischemia and creating a sensation of air hunger
2. Extreme muscle fatigue resulting from muscle ischemia, thus limiting the person’s ability to continue with the exercise
3. Excessive increase in heart rate because the nervous reflexes to the heart overreact in an attempt to overcome the inadequate cardiac output
Exercise tests are part of the armamentarium of the cardiologist. These tests take the place of cardiac output measurements that cannot be made with ease in most clinical settings.
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