EXERCISE STRESS TESTING

Transcription

1 EXERCISE STRESS TESTING Șuș Ioana, Gliga Mihai I. THEORETICAL BACKGROUND 1. Strength, power and endurance of muscles The parameters that characterize muscles performance are strength, power and endurance. The strength a muscle can develop is expressed in its contractile force and has a maximum value of 3-4 kg/cm 2 of muscle cross-sectional area. Thus, muscle strength is determined mainly by the muscle s size, which in turn is dependent mainly on heredity and secondary on testosterone and training levels. The holding strength of muscles is about 40% greater than their contractile strength. The power a muscle can achieve is the total amount of work that the muscle can perform in a given period of time. Thus, muscle power is determined not only by the strength of muscle contraction, but also by the distance it moves the kilogram weight during its contraction and the number of times it contracts each minute. Muscle power is thus measured in kg-force x m/min (kgm/min) or in watts (1 kgm/second = watts). Endurance is how long muscles can continue in their activity. This depends to a great extent on the nutritive support for the muscles, especially on the amount of glycogen already stored in the muscle. A high-carbohydrate diet provides an enhanced endurance in comparison to a high-fat diet. 2. Energy supply by cellular metabolic systems Power and endurance of a muscle depend on how much energy is available within muscles cells and for how long. The intracellular systems that provide energy for muscle contraction are basically the same as in all other cells of the body but the particularity of muscle cells is the much higher rate of energy consumption. For example, the amount of ATP present in the muscles, even in a well-trained athlete, is sufficient to sustain maximal muscle power for only about 3 seconds. The additional energy, that is practically also instantaneously available from transformation of the muscle cell phosphocreatine into adenosine triphosphate (ATP), holds for another 5 to 7 seconds of maximum muscle power. Together, ATP and phosphocreatine are called the phosphagen energy system and, in a highly trained athlete, they can sustain up to 7,000 kgm/min muscle power of all muscles in the body, but only for about 8 to 10 seconds. In contrast, the aerobic system, which consists in oxidation of molecules obtained from food inside the cells mitochondria, can sustain, in the case of the same highly trained athlete, only about 1,700 kgm/min, but for half an hour or so. In between these two systems we have the glycogen-lactic acid system, which usually acts at the beginning of a heavy physical exercise with sudden onset. It splits the intracellular stored glycogen into glucose and further, during glycolysis, each glucose molecule splits into two pyruvic acid molecules and a couple of ATP molecules. As the demand of oxygen is much greater than the oxygen availability at the abrupt onset of a heavy physical exercise, much of the pyruvic acid will not enter the mitochondria of the muscle cells as a fuel molecule for the aerobic system, but will rather be converted into lactic acid (Figure 1). In doing so, the glycogen-lactic acid system requires no oxygen and provides energy 102

2 approximately twice faster than the aerobic system, but two times slower than the phosphagen system. The glycogen-lactic acid system can sustain about 4,000 kgm/min power developed by the body muscles during the first 1 to 2 minutes of heavy physical exercise. Figure 1. The glycogen-lactic acid energy system. 3. Oxygen debt, energy equilibrium and recovery of muscle metabolic systems For physical exercises lasting more than 2 minutes and for a given power that muscles have to develop, equilibrium can be established between the energy required for contraction and the energy produced by the aerobic system. The highest level of this equilibrium, i.e. the maximum power that can be sustained by body muscles, is dictated mainly by the rate of O 2 delivery to the muscle cells by the cardiovascular and respiratory systems and, to a lesser extent, by the rate of nutrients delivery. For a rectangular type of effort (sudden onset of an effort of constant intensity which lasts for a given period of time and then ends abruptly), as shown in Figure 2, it is clear that even during the early stages of exercise, the energy capabilities of the phosphagen system and then of the glycogen-lactic acid system are depleted and, if the physical exercise is heavy enough for a given person, even a portion of that person s aerobic energy capability is depleted. Figure 2. Excess post-exercise oxygen consumption (EPOC). VO2 = oxygen consumption. While the muscles metabolic systems are depleted and O 2 stores are in the course of being used, O 2 at increasing rates is provided to the body by the response to physical stress of the respiratory and cardiovascular systems. Eventually, the energy equilibrium is reached inside muscle cells by the tremendous increase of the rate of O 2 uptake but, at the end of a maximal power exercise, the organism still has an oxygen debt to muscle cells. This is why, even after the exercise is over, the person breathes heavily and the heart is beating faster and much stronger than at rest in order to pay this debt. Overall, the rate of O 2 uptake by 103

3 the body and delivered to muscle cells still remains above normal resting state after the end of the exercise. The extra amount of O 2 the muscle cells utilize within the first few minutes, about 3.5 liters, is called alactacid oxygen debt and is needed for rebuilding the portion of stored O 2 that was used and for reconstituting the phosphagen system. The extra amount of O 2 the muscle cells utilize in the next hour, about 8 liters, is called lactic acid oxygen debt and it is used for removal of lactic acid in two ways: a small portion is reconverted into pyruvic acid and then metabolized oxidatively by all cells of the body, but the largest portion is reconverted into glucose mainly by the liver, and then glucose is used to replenish glycogen stores of the muscle. 4. Physical stress capacity. Role of the respiratory and cardiovascular systems. Based on the information provided so far, it becomes obvious that the most important factor that dictates the maximal combination of power and endurance the body muscles can achieve is O 2 availability. Tissue oxygen consumption at rest is approximately 3.5 ml/kgbw/min, also known as metabolic equivalent (MET). During effort, tissue O 2 consumption is significantly increased, and is expressed in ml/kgbw/min or as multiples of MET. The rate of O 2 usage by the entire body under maximal aerobic metabolism is denominated maximal aerobic capacity or maximum O 2 consumption (VO 2 max). This parameter is used to evaluate physical stress capacity of an individual. The theoretical VO 2 max is in average 3,600 ml/min for an untrained adult male, but the individual genetics plays here an important role, as there are persons, such as marathon runners, who are able of a maximal O 2 consumption of up to 5,100 ml/min. On the other hand, the rate of O 2 delivery to the body cells, and so to the muscle cells, is a function of the cardiovascular and respiratory systems. Therefore, the maximal rate of O 2 delivery by these systems during maximal exercise represents the real VO 2 max. The most important mechanisms that ensure the adaptation of the respiratory and cardiovascular systems to the increased requirements of the body during heavy physical activity are: hyperventilation (increased amplitude and rate of ventilations) and increased diffusion of respiratory gases through the alveolo-capillary membrane, leading to an increased amount of circulating O 2 increased cardiac output (CO) due to an increase in both heart rate and cardiac stroke volume (CO = heart rate x stroke volume). The CO can increase as much as 4 to 6-fold above basal levels during maximal exercise, depending on genetic endowment and on the level of training. redistribution of blood flow - As O 2 has to be delivered to tissues with increased metabolism during effort, there will be arterial vasodilation at three main sites: skeletal muscle involved in the physical activity, myocardium and brain, with varying degrees of vasoconstriction in all other territories. During maximal exercise, only about 65% of the maximum pulmonary ventilation is actually needed for the uptake of the equivalent of VO 2 max. Thus, the respiratory system is not normally the most limiting factor in the delivery of O 2 to the muscles during maximal muscle aerobic metabolism. On the other hand, both the heart rate and the stroke volume can increase during maximal exercise to about 95% of their maximal levels (the heart rate can increase from 75 bpm to 195 bpm, the stroke volume from 75 ml to 110 ml per beat). Therefore, the cardiac output during maximal exercise can reach about 90% of the 104

4 maximum that a person can achieve and thus, the cardiovascular system is normally much more limiting on the real VO 2 max than is the respiratory system. The real VO 2 max can be determined either directly, by measuring blood gases or by using a closed-circuit respiratory apparatus, or indirectly, by using nomograms (such as the modified Astrand-Ryhming nomogram), based on the maximal heart rate and workloads. The theoretical VO 2 max can be calculated using the following equation: VO2 max 45.8 (0.17 age) The difference between the theoretical VO 2 max and the real (measured or calculated) VO 2 max represents the functional aerobic deficit (FAD). Based on the value of the FAD, the effort capacity of a given individual can be evaluated: FAD Effort capacity < 25% no reduction 25-50% minor reduction 50-75% moderate reduction >75% major reduction 5. Exercise Testing Exercise testing is a useful tool in detecting impairment in the functional capacity of the cardiopulmonary system and may be used for diagnostic, prognostic, therapeutic, physical activity counseling and exercise prescription purposes. Based on the purpose of the test, specific methodology and end points will be selected on a case-to-case basis. The World Health Organization classified physical exercise in four grades based on oxygen consumption and heart rate (Table 1). Table 1. World Health Organization classification of physical exercise. Grade Level Heart rate (bpm) O 2 consumption (L/min) Relative load index (% of VO 2 max consumption) METs I Light < < 25 < 3 II Moderate III Heavy IV Severe >150 >2.4 >75 >7 Depending on the intensity of the effort, exercise stress tests can be classified as maximal or submaximal tests. For maximal tests, the effort is performed until the subject reaches the maximum agepredicted heart rate (MPHR), representing the heart rate at which a person reaches his/her VO 2 max. The MPHR is calculated using the following equation: MPHR = 220-age (years) 105

5 More frequently, submaximal tests are used, when the subject has to reach a target heart rate (THR) representing 85% of MPHR. In practice, several testing modes can be used: field tests (walking or running on a standard distance), treadmill tests, cycle ergometer tests, or step tests. When performing an exercise stress test, careful preparation of the subject needs to be ensured, including: detailed explanation on testing procedure and purpose of the test no eating or smoking for at least 3 hours before the test no alcohol, caffeine or energy drinks before the test no strenuous exercise 24h prior to the test a brief history and physical examination should be performed. II. EXPERIMENTAL OBJECTIVES AND PROCEDURES Objectives to understand the effect of isotonic exercise on the cardiovascular and respiratory systems to illustrate the changes in heart rate and blood pressure during physical activity to understand different types of testing protocols: maximal, submaximal, graded exercise 1. The Schellong II test Schellong II is a submaximal test that evaluates cardiovascular recovery after effort in nonathletes Materials sphygmomanometer stopwatch 1.2. Protocol The resting heart rate and blood pressure are measured while standing. Then, the subject performs 30 squats, and then lies down. The heart rate and blood pressure are measured in the first 15 seconds of every minute for at least 4 minutes or until recovery. Stop immediately if subject complains of chest pain, (occipital) headache, or faintness during the exercise stress test! 1.3. Interpretation of results After performing the Schellong II test: heart rate should increase slightly, to not more than 120 bpm systolic blood pressure should increase by mmhg diastolic blood pressure should not change recovery should occur within 2 to 3 minutes. In untrained persons, the increase in heart rate and blood pressure is more important, requiring a longer time to return to baseline values. 106

6 2. The three-minute step test The premise behind this test is that submaximal steady state will be reached within 3 minutes of step testing. Fit individuals will reach a lower heart rate both during exercise and recovery compared to unfit individuals Materials 30 cm high step stopwatch 2.2. Protocol The subject will step at a stepping rate of 24 steps/minute for three minutes. Immediately after, the heart rate is measured for 1 minute. The measured heart rate is used to evaluate qualitatively the level of fitness by using Table 2. Stop immediately if subject complains of chest pain, (occipital) headache, or faintness during the exercise stress test! Table 2. Rating of fitness for the three-minute step test. Age Rating >65 Excellent Men Women Good Men Women Above average Men Women Average Men Women Below average Men Women Poor Men Women Very poor Men Women

7 VO 2 max can be calculated by using the heart rate (HR) measured in the first 15 seconds after effort, based on the following equations: Men: VO 2 max = (0.42 x HR) Women: VO 2 max = ( x HR) 3. The Astrand-Ryhming test The Astrand-Ryhming test is a 6 minute single-stage submaximal protocol that allows indirect calculation of VO 2 max. The goal is to reach a heart rate between 110 bpm and 85% of MPHR. The workload is set based on gender and individual fitness status, as follows: men, unconditioned: W; - women, unconditioned: W men, conditioned: W; - women, conditioned: W 3.1. Materials cycle ergometer stopwatch 3.2. Protocol The subject pedals on the cycle ergometer at 50 rotations per minute (rpm) for 6 minutes at a workload chosen as described above. Immediately after finishing the test, heart rate is measured during the first 10 second and then calculated for one minute. VO 2 max is read from the modified Astrand-Ryhming nomogram (Figure 3) and maximum aerobic capacity (MAC) is calculated by dividing the calculated VO 2 max by the ideal weight calculated using Lorentz s formula: Effort capacity is then evaluated function of MAC, gender and age (Table 3). Stop immediately if subject complains of chest pain, (occipital) headache, or faintness during the exercise stress test! Table 3. Evaluation of effort capacity based on the Astrand-Ryhming test. Age (years) Effort capacity Very low Low Average High men women < 38 < >52 > men women < 34 < >48 > men women < 30 < >44 > men < >40 108

8 50-65 women < > men < >36 4. The YMCA test The YMCA test is a graded submaximal test that uses two to four 3-minute stages of continuous exercise in order to reach a heart rate between 110 bpm and 85% of MPHR, as for the Astrand-Ryhming test Materials cycle ergometer stopwatch 4.2. Protocol In the first stage of effort, the subject pedals on the cycle ergometer at 50 rpm for 3 minutes at 25 watts workload. The heart rate is measured during the final 15 seconds of the 3 rd minute of effort. For the second, third and fourth stages of effort, workloads are set based on the measured heart rate, according to Table 4. The heart rate is measured during the last 15 seconds of each stage. The test is terminated when the subject reaches 85% of MPHR. After terminating the test, the heart rate is measured each minute until recovery. Stop immediately if subject complains of chest pain, (occipital) headache, or faintness during the exercise stress test! Table 4. Workload setting for the 2 nd, 3 rd and 4 th stages of effort according to the YMCA protocol. 1 st stage 25 W (150 kg-meter/min) Heart rate < >100 (bpm) 120 W 100 W 75 W 50 W 2 nd stage (750 kgm/min) (600 kgm/min) (450 kgm/min) (300 kgm/min) 150 W 120 W 100 W 75 W 3 rd stage 4 th stage (900 kgm/min) 170 W (1050 kgm/min) (750 kgm/min) 150 W (900 kgm/min) (600 kgm/min) 120 W (750 kgm/min) (450 kgm/min) 100 W (600 kgm/min) For each stage of effort, heart rate values are plotted against the workload and extrapolated to MPHR to estimate the workload that would have been achieved if the subject had performed a maximal test. The predicted maximum workload is used to estimate VO 2 max using the equation: Then, the results are compared with the standard values (Table 5). 109

9 Table 5. Standard values for VO 2 max (men/women). % ranking Effort Age capacity Over /95 95/95 90/75 83/72 65/58 53/55 95 Excellent 75/69 66/65 61/56 55/51 50/44 42/ /59 60/58 55/50 49/45 43/40 38/ /56 55/53 49/46 45/41 40/36 34/31 80 Good 56/52 52/51 47/44 43/39 38/35 33/ /50 50/48 45/42 40/36 37/33 32/ /47 49/45 43/41 39/35 35/32 31/28 Above 65 49/45 45/44 41/38 38/34 34/31 30/27 average 60 48/44 44/43 40/37 36/32 33/30 29/ /42 42/41 38/36 35/31 32/28 28/25 50 Average 44/40 40/40 37/34 33/30 31/27 27/ /39 39/37 36/33 32/29 30/26 26/ /38 38/36 35/32 31/28 28/25 25/22 Below 35 39/37 37/35 33/30 30/27 27/24 24/21 average 30 38/35 34/34 31/29 29/26 26/23 23/ /33 33/32 30/28 27/25 25/22 22/19 20 Poor 35/32 32/30 29/26 26/23 23/20 21/ /20 30/28 27/25 25/22 22/19 20/ /27 27/25 24/24 24/20 21/18 18/16 5 Very poor 26/24 24/22 21/20 20/18 18/15 16/ /15 15/14 14/12 13/11 12/10 10/10 5. Cardiac stress testing At rest, the O 2 extraction by the myocardium is equal to the maximum O 2 extraction by skeletal muscles during heavy exercise. Since this extraction ratio can no longer be increased, during exercise, when there is an increased myocardial oxygen demand, the supply can only be met by an increase in coronary blood flow. In the presence of normal coronary arteries, coronary blood flow adapts perfectly to myocardial oxygen demands during exercise. Contrarily, significant stenosis (narrowing that causes a significant reduction in maximal flow capacity in the distal vascular beds) or obstruction of one or several of the coronary arteries will prevent the coronary blood flow to adapt to myocardial oxygen demands during exercise and will result in myocardial ischemia (insufficient coronary blood flow for the metabolic requirements of the myocardium). Since myocardial ischemia creates disturbances in myocardial repolarization and these disturbances can be assessed on a surface ECG, exercise stress testing with simultaneous recording of the ECG is a useful tool to detect coronary artery disease. Exercise stress testing can be used to assess: the presence of coronary artery disease in patients with symptoms of angina (retrosternal pain, irradiating in the jaw, shoulder or arm, described as pressure or constriction, lasting for not more than 10 minutes, related to exercise or emotional stress) in the absence of resting ECG abnormalities 110

10 chronotropic competence (heart rate adaptation to effort) and arrhythmias (exercise can trigger arrhythmias in patients with ischemic heart disease) the patient s response to medical interventions (anti-ischemic drugs, coronary angioplasty see the Cardiac catheterization chapter) physical capacity and effort tolerance. Several protocols can be used for exercise stress testing, such as the 25 Watts/2-minutes staged protocol and the Bruce protocol. The choice of the protocol is influenced by the patient s estimated functional capacity, underlying conditions, age, and physical fitness The 25 Watts/2-minutes staged protocol Materials cycle ergometer electrocardiograph sphygmomanometer stopwatch Protocol Before starting the test, the patient s target heart rate (THR) is calculated (see above: Theoretical background Exercise testing ); the resting heart rate and blood pressure are measured and a resting 12-lead ECG is recorded. Continuous ECG monitoring is performed throughout the test. In the first stage of effort, the subject pedals on the cycle ergometer at 25 watts workload for 2 minutes. Every 2 minutes the workload is increased by 25 watts. At the end of each stage of effort the heart rate and blood pressure are measured. The test ends when the patient s heart rate reaches the THR or if one of the criteria for terminating the test is met (see below). After completing the test, the heart rate and blood pressure are measured every 3 minutes and ECG monitoring is continued for at least 10 minutes or until complete recovery of any ECG changes The Bruce protocol Stop immediately if subject complains of chest pain, (occipital) headache, or faintness during the exercise stress test! Materials treadmill electrocardiograph sphygmomanometer stopwatch Protocol Before starting the test, the patient s THR is calculated; the resting heart rate and blood pressure are measured and a resting 12-lead ECG is recorded. Continuous ECG monitoring is performed throughout the test. The test consists of 3-minute stages during which the speed and slope of the treadmill are increased progressively, as indicated in Table

11 Table 6. Treadmill speed and slope for each effort stage according to the Bruce protocol. Stage Time Speed (mph) Slope (%) METs 1 3 min min min min min min min The test ends when the patient s heart rate reaches the THR or if one of the criteria for terminating the test is met (see below). After completing the test, the heart rate and blood pressure are measured every 3 minutes and ECG monitoring is continued for at least 10 minutes or until complete recovery of any ECG changes. Stop immediately if subject complains of chest pain, (occipital) headache, or faintness during the exercise stress test! Both the 25 Watts/2-minutes staged protocol and the Bruce protocol end when the patient s heart rate reaches the THR or if one of the following criteria for terminating the test is met: patient s request to stop clinical signs or symptoms: angina, dizziness, fatigue, dyspnea, cyanosis, pallor muscle cramps, joint pain drop in systolic blood pressure >10 mmhg exaggerated hypertensive response: systolic blood pressure >250 mmhg and/or diastolic blood pressure >115 mmhg sustained ventricular arrhythmia that interferes with normal maintenance of cardiac output during exercise ECG changes typical for ischemia. Interpretation of results (for both the 25 Watts/2-minutes staged protocol and the Bruce protocol) The classic ECG criteria for myocardial ischemia during cardiac stress testing are: ST segment elevation of 1 mm or higher, persisting for at least msec after the J-point (the point located at the junction between the termination of the QRS complex and the beginning of the ST segment), in one or more ECG leads horizontal ST segment depression of 1 mm or greater, persisting for at least msec after the J-point, in one or more ECG leads down-sloping ST segment depression of 1 mm or greater, persisting for at least msec after the J-point, in one or more ECG leads. Other non-specific ECG changes such as T-wave flattening or inversion, or ST segment depression of less than 1 mm, can be seen in patients with myocardial ischemia, but they are not considered as diagnostic. 112

12 The test is considered: positive for myocardial ischemia if angina and/or classic ECG criteria for myocardial ischemia are present negative for myocardial ischemia if there are no exercise-induced abnormalities (angina or classic ECG criteria) at the THR non-diagnostic / equivocal for myocardial ischemia if the test was stopped before reaching the THR or if there are only non-specific changes on the ECG. Exercise stress tests also allow assessing hemodynamic changes: heart rate changes early heart rate acceleration appears mainly in untrained subjects, without prognostic significance chronotropic incompetence, characterized by inadequate increase in heart rate (HR), is defined as a chronotropic index of less than 80% heart rate recovery - in healthy subjects, the heart rate decreases steeply in the first 30 sec after ending the effort, with a slower decrease afterwards; abnormal heart rate recovery is defined as a decrease of less than 18 bpm 1 minute after ending the effort or less than 42 bpm 2 minutes after ending the effort. blood pressure changes exaggerated systolic blood pressure response is defined as an increase in the systolic blood pressure during exercise to values >210 mmhg in men or >190 mmhg in women exercise-induced hypotension is defined as a decrease of >20 mmhg in the systolic blood pressure during exercise after an initial rise and is usually the result of severe coronary artery disease low systolic pressure peak is defined as a rise in systolic blood pressure during exercise to less than 140 mmhg or an overall rise in systolic blood pressure lower than 10 mmhg during effort systolic pressure recovery normally occurs within 2 to 3 minutes upon termination of the effort test. The pressure-time index (PTI) is a marker of the heart s oxygen consumption and is calculated by multiplying the maximum systolic blood pressure with the maximum heart rate reached during the test. The calculated value is then compared with the theoretical value, calculated as follows: The result is expressed as myocardial aerobic deficit (MAD), calculated as follows: MAD is interpreted as follows: 0-20% = no deficit 20-40% = minor deficit 40-60% = moderate deficit >60% = major deficit. 113

13 Figure 3. The modified Astrand-Ryhming nomogram. 114

14 TEST YOUR KNOWLEDGE 1. The adaptation of the cardiovascular system to physical exercise consists in: a. increased oxygen consumption by the heart muscle b. increased overall blood volume of the organism c. decreased length of the cardiac cycle d. decreased mean arterial pressure due to an increase in arterial elasticity e. decreased resistance to blood flow in all body tissues 2. In the case of the YMCA test: a. the heart rate is measured during the final 15 seconds of each stage b. the O 2 extraction by the myocardium increases c. the test always ends when the subject reaches 85% of MPHR d. the four stages workloads are settled according to the individual s heart rate e. interpolation is necessary in order to interpret the individual s performance 3. A 67-year-old woman with multiple cardiovascular risk factors (age >65 years, smoker, diabetic) comes to your office for an exercise stress testing. At rest, her heart rate is 75 bpm and her blood pressure is 130/70 mmhg. You choose the 25 Watts/2-minutes cycle ergometer protocol that has to be stopped at stage 4 for severe muscle tiredness. The peak blood pressure is 200/80 mmhg. By the end of the test she has no angina and she has the following ECG: 115

15 Describe the ST segment changes: How would you interpret this exercise stress testing? Justify your answer. a. positive for ischemia, because b. negative for ischemia, because c. equivocal for ischemia, because Interpret the hemodynamic results: 4. A 45-year-old man, 185 cm height, 87 kg weight, who exercises regularly 30 minutes a day, is performing the Astrand-Ryhming test in order to determine his effort capacity before increasing the intensity of his training. At the end of the test his heart rate is 124 bpm. What workload would you choose for this subject? Justify your answer. How would you evaluate his effort capacity? 116