Dynamic exercise tests. Regulation of Serum Lipids by Physical Exercise
01.01.1982, CRC-Press (toim. E. Hietanen), 65-70, (& R. Rauramaa, K. Kukkonen)
Liikunnan merkitys rasva-aineenvaihdunnassa.
Dynamic exercise tests. Regulation of Serum Lipids by Physical Exercise
Exercise stress tests have several indications of which the most common is the diagnosis of coronary artery disease. In clinical practice traditional loading tests, with clinical observation, auscultation, and electrocardiographic monitoring can nowadays be completed by such modern methods as echocardiography and isotope scintigraphies. Since physical inactivity is related to increased risk of coronary heart disease(1-4), physical exercise during leisure is suggested to promote health. On the other hand, because sedentary middle-aged persons may have asymptomatic heart disease, all efforts should be taken to reveal such pathological cardiovascular processes which might lead to acute heart attack during exercise(5-8). Therefore, before engaging in physical activity it is recommended that a thorough medical examination and laboratory tests including exercise tests are performed(9).
The purpose of this review is to focus on some cardiovascular and metabolic aspects relevant to exercise stress testing in healthy persons as well as in some common patient groups before initiating physical training. The use of exercise stress testing in the diagnosis of myocardial ischemia and prognosis of coronary heart disease will not be dealt with in-depth here. In this respect the reader is referred to recent reviews by Faris et al(10). and McHenry and Morris(11). 11.
II.Characteristics and optionsin exercise test
The objective of an exercise test is to examine the capacity of the oxygen delivery system and its limitations. Obviously, in established coronary heart disease the defect is central and myocardial oxygenation is hampered while in most noncoronary persons the disorder is peripheral, which means disturbances in oxygen utilization in the skeletal muscle.
In connection with exercise stress test ECG, heart rate, and blood pressure are the main cardiovascular functions registered. Myocardial oxygen consumption can he estimated using the so-called double product which means systolic blood pressure multiplied by heart rate(12,13). Measurement of oxygen consumption (V02) gives an estimate of the rate of aerobic energy metabolism in skeletal muscle. V02 can be measured either directly (using, e.g., paramagnetic oxygen analysis, 14) or can be estimated from, heart rate responses during increased loading on a bicycle (15). For estimation of maximal VO2 on treadmill there are various mathematical formulas(16), but such an indirect approach is less precise. In connection with direct measurement of oxygen consumption the METs (metabolic multiples) principle can be applied. One MET equals to the subject's oxygen consumption at rest (3.5 ml/kg x min). This is useful when training prescription is based on data from exercise test(17).
If only heart rate is used as an indicator of the intensity of exercise, it must be taken into consideration that mean maximal heart rate decreases with age and that the interindividual variation is also considerable(15). In addition, in coronary heart disease, the increase in heart rate during exercise may be deficient and expected maximal target heart rate (related to age) will not always be achieved. Therefore performing a so-called submaximal exercise test with a prefixed target heart rate (75 to 90% of maximal) may he fallacious if only heart rate is followed(18).
The question of submaximal vs. maximal exercise test has been debated(19). Submaximal test has been suggested to be safer but the criteria of submaximality on the basis of heart rate are not clear as discussed above. It is not known to what proportion symptoms indicative of myocardial ischemia occur after the heart rate level of 85% of maximal heart rate has been reached if none have appeared before that(20) In addition, the practical significance of ischemic myocardial changes occurring only in maximal exertion has been questioned(21).
In Scandinavian countries, bicycle ergometer is most often used whereas in North America treadmill ergometer is the most popular. Testing protocols for bicycle ergometer include either work conducted, graded (each step lasting for 1 to 6 min, 15), or pulse conducted (22) exercise tests. For the various testing protocols on treadmill the reader is referred to several reviews(19,23-25).
The advantages of bicycle ergometer include determination of work intensity without direct measurement of oxygen consumption and recording of ECG and blood. Pressure without great technical difficulties. However, riding a bicycle is a less physiological and less familiar type of physical activity for sedentary persons than walking on a treadmill. VO2 is greater on treadmill because of recruitment of larger muscle mass and less local fatigue than when bicycling. Performance on a motordriven treadmill can also be better controlled externally while bicycling is more patient-controlled, especially when using a mechanically braked bicycle(19). For a more detailed discussion on conducting exercise tests in practice see, e.g., Faris et al.(10), Mead(16), and Sheffield and Roitman(19).
III. Cardiorespiratory responses to exercise in healthyan diseased subjects
In healthy persons cardiorespiratory responses to graded exercise can be evaluated on the basis of several physiological and metabolic parameters. Increased heart rate as such or in relation to progressively increasing work load has most widely been used when assessing physical performance capacity. There is a nearly linear correlation between heart rate and work load. Heart rate is the most important factor in determining increased cardiac output-during exercise since stroke volume increases only up to the level corresponding to 50% of maximal work load(26). The rationale for using only heart rate in the assessment of physical working capacity is acceptable especially when serial testing is used but more pertinent information can be obtained by the direct measurement of oxygen consumption and carbon dioxide production. At present there are automated systems easy to operate and reliable in results. Respiratory quotient (the ratio of carbon dioxide produced and oxygen used) may also give valuable information of performance capacity during submaximal exercise levels. Ventilation and respiration rate increase linearily with oxygen consumption up to the submaximal level.
In obese persons oxygen consumption during submaximal exercise levels tends to be higher both on treadmill and bicycle as compared to normal weight subjects (27, 28) al though only near or at the maximal level is the difference most distinct(27). The higher oxygen consumption is simply clue to greater body mass. In addition, blood pressure during submaximal exercise has been suggested to be higher in the obese(28). When the obese are engaged in physical training programs, treadmill testing seems to be more sensitive in detecting functional improvement(29), especially when combined with direct measurement of oxygen consumption (30). Pulmonary function is usually hampered only in massive obesity. Even if there are no differences in respiratory capacity between the lean and the obese after training, increased aerobic power in the obese may be partially explained by the increased maximal ventilation (31).
In coronary heart disease the typical features during exercise testing are symptoms (angina pectoris) and ST-T changes in ECG. Sometimes heart rate and blood pressure do not increase proportionally with increased loading compared to healthy individuals. Such a nonresponsive state may suggest insufficient myocardial function(18). Choosing the criteria for significant ST-T changes in ECG either during exercise or immediately after it influences the sensitivity and specificity of the exercise test. A careful analysis of 33 studies comparing exercise test and coronary angiography showed a wide variation in these two parameters(32). The little variation in specificity of a 0.2 mV S-T depression was offset by very low sensitivity. Such discrepancies were suggested to result from different methodological standards. Generally a negative exercise test is most reliable in asymptomatic subjects and in subjects with atypical chest pain, while a positive test is most valuable in patients with typical chest pain or after myocardial infarction(33).
The prognostic value of exercise test in asymptomatic men proved useful only when combined with other physical findings such as calcification of coronary arteries(34) or conventional risk factors(35) If one or more risk factors were present and two or more exertional risk predictors (chest pain during exercise, short duration of exercise, poor heart rate increase and ischemic S-T-depression) were present, the 18 times greater risk ratio in about 1% of healthy men was not altered by age(35).
The significance of exercise testing soon after myocardial infarction has been evaluated in large series of patients recently in several studies(36-41). In all these studies the group with increased mortality during 1 to 2 years after infarction could be described using either ST-segment depression or arrhythmias as the criteria of increased. risk. No complications during the tests were reported. The subjects with low risk can be recommended for unsupervised exercise training at home(42).
There are few reports concerning cardiorespiratory responses of diabetics during exercise tests. According to Hagan et al. (43) there are no significant differences in submaximal and maximal responses in insulin-dependent diabetic patients with a short duration of the disease as compared with normal subjects. Poor physical performance capacity has been reported in insulin-dependent diabetes of long duration(44) and in non insulin dependent diabetes(45). Direct determination of oxygen consumption in addition to the measurement of heart rate during exercise test is accentuated by the fact that heart responses may be variable in the diabetics due to autonomic neuropathy(44,46,47).
Exercise tests are performed either for healthy, asymptomatic, or diseased persons. Traditional indication has been the diagnosis of coronary heart disease. Later the test has been applied for screening of healthy persons with or without risk factors before physical training. According to Chung(48) the indications in this respect apply for all men above 40 years of age and for all women above 50. In addition, if risk factors for coronary heart disease or symptoms exist, testing should be undertaken even at a younger age, always under a medical doctor's supervision.
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