Cardiopulmonary Response to Exercise in Health and Disease

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Chapter 23

Cardiopulmonary Response to Exercise in Health and Disease

Physiology of Exercise

Strenuous exercise greatly increases the body’s requirements for oxygen delivery and carbon dioxide elimination. Close coupling between increased metabolic activity and cardiopulmonary function is necessary to supply adequate oxygen to exercising muscles and maintain a normal blood acid-base balance. This coupling is schematically represented in Figure 23-1.

Normal exchange of O2 and CO2 between muscle cells and the air requires the following:1 (1) efficient alveolar ventilation, (2) pulmonary blood flow matched to ventilation, (3) adequate blood hemoglobin content, (4) adequate cardiac pump function, (5) systemic blood flow matched to tissue requirements, and (6) ventilatory control mechanisms sensitive to arterial blood gas changes. These factors normally interact in a finely coordinated fashion, responding precisely to the metabolic needs of exercising muscle.

Normally, respiratory and cardiac reserve capacities are so large that only great physiological impairment reduces daily functioning ability. Exercise testing is clinically important because it stresses all physiological systems, revealing their capacity to respond. Respiratory and cardiac impairments produce characteristically different patterns of response to exercise, allowing cardiovascular and pulmonary limitations to be differentiated. The purpose of this chapter is to provide a physiological basis for evaluating exercise performance, not to provide a definitive guide for interpreting exercise test results.

Metabolism during Exercise

The source of energy for skeletal muscle contraction is adenosine triphosphate (ATP). The structure of the ATP molecule is symbolized as follows:2

AdenosinePO3PO3PO3

image

The bonds between the PO3image radicals, designated by the symbol ~, are high-energy phosphate bonds. The process of metabolism breaks these bonds, releasing the energy needed to form and break cross-bridges between the muscle’s actin and myosin myofibrils, as described in Chapter 17; in this way, muscle fibers shorten and generate force. During exercise conditions, each high-energy bond stores about 7300 calories (7.3 kcal) per mole of ATP molecules.2 The hydrolysis of the first PO3image bond produces 7300 calories, generating adenosine diphosphate (ADP) in the process. Hydrolysis of the remaining PO3image bond yields yet another 7300 calories and creates adenosine monophosphate (AMP) for a total energy release of about 14.6 kcal/mol.2

ATP stores in skeletal muscle are extremely small and can sustain strenuous muscular activity for only about 3 seconds.2 Therefore ATP molecules must be reconstituted continuously during exercise through aerobic (with oxygen) and anaerobic (without oxygen) metabolic processes.

The energy required for sustained exercise is derived from internal and external mechanisms.3 Internal mechanisms include anaerobic production of ATP, O2 release from hemoglobin, and dissolved O2 uptake from plasma and tissue fluids. External mechanisms involve the delivery of atmospheric O2 molecules to the tissues. Atmospheric O2 is relatively unimportant for power surges of a few seconds and the first few minutes of exercise. Internal mechanisms are crucial for initiating exercise because of the delay involved in transporting atmospheric O2 to the tissues. As exercise continues, energy production becomes primarily dependent on the adequate delivery of atmospheric O2 to the tissues. As exercise intensity reaches maximum limits, internal anaerobic sources of energy become increasingly important.

Aerobic Metabolism

Aerobic metabolism of carbohydrates (glucose), fats (fatty acids), and proteins (amino acids) provides the major energy source for ATP synthesis.2 During exercise, stored glycogen in the muscles breaks down into glucose. Glucose breaks down into pyruvic acid, which reacts with O2 in the cell’s mitochondria, forming ATP, CO2, and water; thus the aerobic or oxidative process of ATP synthesis consumes O2 and generates CO2.

Aerobic metabolism of fatty acids also produces ATP molecules. For a given rate of ATP production, fat metabolism requires more O2 than carbohydrate metabolism (Figure 23-2). This means carbohydrate metabolism uses O2 more efficiently and requires less cardiovascular work per mole of ATP synthesis. However, fat metabolism generates less CO2 per mole of ATP produced than carbohydrate metabolism (see Figure 23-2); in other words, fat metabolism requires less ventilatory work than carbohydrate metabolism in generating a given amount of energy.2

Anaerobic Metabolism and Lactic Acid Production

Anaerobic metabolic processes occur in the absence of O2. An example of anaerobic metabolism is the breakdown of creatine phosphate (CP), which, similar to the ATP molecule, possesses a high-energy phosphate bond:

CreatinePO3

image

The breakdown of this bond supplies the immediate energy for ATP resynthesis from ADP, as shown:

CreatinePO3+ADPcreatine+ATP

image

However, this process produces only enough ATP to fuel muscle contraction for 8 to 10 seconds during heavy exercise.2

If the cardiovascular system does not supply enough O2 for aerobic metabolism, glucose from muscle glycogen is metabolized in a process called anaerobic glycolysis. In this process, pyruvic acid is metabolized anaerobically into lactic acid and ATP (see Figure 23-2). The lactic acid subsequently diffuses into the interstitial fluid and blood, creating lactic acidosis, a form of metabolic acidosis. Increased blood lactic acid concentration indicates the presence of anaerobic metabolism, implying inadequate O2 delivery to the tissues.

Anaerobic Threshold

As exercise intensity increases, energy requirements gradually increase until aerobic metabolism alone can no longer meet the demand for ATP resynthesis. Consequently, the body must rely on anaerobic metabolism to generate additional ATP. The onset of anaerobic metabolism, the anaerobic threshold, normally occurs at about 50% to 65% of the body’s maximum oxygen consumption (V˙imageO2max).3 Because fat cannot be metabolized anaerobically, less fat is used as exercise intensity increases.2 The major energy sources at maximal exercise are aerobic and anaerobic glucose metabolism. For this reason, the body’s glucose stores limit exercise endurance. Sustained exercise such as marathon running may totally deplete glucose stores, explaining the reason some marathon runners suddenly fatigue or “hit the wall” after about 20 miles.

Figure 23-2 shows that aerobic metabolism produces ATP far more efficiently than anaerobic metabolism. In aerobic metabolism, one molecule of glucose produces 36 molecules of ATP compared with only two molecules of ATP in anaerobic glucose metabolism. In addition, anaerobic glucose metabolism generates two lactic acid molecules. Blood bicarbonate immediately buffers newly formed lactic acid, generating more CO2, adding to the CO2 already produced by aerobic metabolism (see Figure 23-2). This increased CO2 production stimulates a proportional increase in ventilation.3

Respiratory Quotient and Respiratory Exchange Ratio

The respiratory quotient (RQ) is the ratio of CO2 molecules produced to O2 molecules consumed by the tissues (V˙CO2/V˙O2image), as shown in Figure 23-1.2 (In Figure 23-2, the RQ for glucose is about 1.0, whereas for fat, it is 0.71.) The fat RQ is lower because fat metabolism generates less CO2 per ATP molecule than carbohydrate metabolism. At rest, the body metabolizes nearly equal amounts of fats and carbohydrates, yielding an RQ of about 0.85.2 As exercise intensity increases, the RQ rises toward 1.0 as the metabolic substrate changes to glucose.

The respiratory exchange ratio (R) is the ratio between CO2 elimination and O2 uptake, measured from inhaled and exhaled gases at the mouth (see Figure 23-1).2 In steady-state conditions, R equals RQ. The steady state is characterized by a CO2 elimination rate that matches the CO2 production rate; likewise, the lung’s O2 uptake rate equals the tissue’s O2 consumption rate in steady-state conditions. During short bursts of maximal exercise such as the 100-yard dash, RQ and R are different because tissue O2 consumption and CO2 production momentarily exceed the lung’s O2 uptake and CO2 elimination. Like RQ, R increases as exercise intensity increases; the onset of anaerobic metabolism further increases R as bicarbonate reacts with lactic acid and generates more CO2

Exercise Testing Methods

Exercise tests in the laboratory involve treadmills or bicycle ergometers that provide the subject with graded exercise over a period of time. In a graded exercise test, the workload is progressively increased in a series of stages by increasing the treadmill speed (and its angle from the floor) or by increasing the

bicycle’s pedal resistance. For example, the subject may exercise at a certain treadmill speed and angle for 3 minutes, after which the speed and angle are increased for the next 3 minutes, and so on. The subject exercises through a well-defined series of stages, each more difficult than the last, until maximum capacity is reached. During this time, a machine called a metabolic cart measures the inhaled and exhaled O2 and CO2 concentrations, tidal volume (VT), respiratory rate, and minute ventilation.

To obtain these measurements, the subject breathes through a specially designed mouthpiece attached to the metabolic cart by large-bore tubes. Nose clips prevent breathing through the nose. The lips must be sealed tightly around the mouthpiece to prevent mixing of respiratory gases with atmospheric air. An electrocardiogram (ECG) machine measures the heart rate (HR) and ECG pattern during the exercise test. Depending on the data desired, various other physiological parameters may be measured. Table 23-1 lists physiological measurements grouped according to the invasiveness of the test and the monitoring equipment needed.

TABLE 23-1

Measurements during Graded Exercise Testing

Level of Rest Measurements
Group 1 No mouthpiece
  Patient response and symptoms
  Heart rate
  Blood pressure
  ECG
  SaO2 by oximetry
Group 2 Mouthpiece, oxygen, and carbon dioxide analyzer
  Respiratory rate
  Tidal volume
  Minute ventilation
  End-tidal gas measurements
  V˙imageO2 and O2 pulse
  V˙imageCO2
  Respiratory exchange ratio
Group 3 Arterial line
  Blood gas values: PaO2, PaCO2, and pH
  HCO3image and lactate
  VD/VT
Group 4 Right-sided heart catheter
  Pulmonary artery pressures
  Cardiac output
  Mixed venous PO2 and SO2

image

ECG, Electrocardiogram; SaO2, arterial oxygen saturation; V˙imageO2, oxygen consumption per minute; O2 pulse, oxygen consumption per heartbeat; V˙imageCO2, carbon dioxide production per minute; PaO2, arterial oxygen pressure; PaCO2, arterial carbon dioxide pressure; pH, measure of blood H+ concentration; HCO3, blood bicarbonate; VD/VT, dead space-to-tidal volume ratio; PO2, oxygen pressure; SO2, oxygen saturation.

Modified from Martin L: Pulmonary physiology in clinical practice: the essentials for patient care and evaluation, St Louis, 1987, Mosby.

Physiological Changes during Exercise

Oxygen Consumption, Cardiac Output, and Blood Pressure

Figure 23-3 represents the exercise performance of a normal person. V˙imageO2 increases linearly with the work rate from rest to maximal exercise. The slope of V˙imageO2 increase is about the same for all normal people; it is unaffected by training, age, or gender.1 Because V˙imageO2 depends primarily on the amount of work done by exercising muscle, V˙imageO2 can be predicted from the work rate.2,3

As exercise intensifies, contracting muscles consume ever greater amounts of ATP, which means aerobic ATP regeneration requires an increasingly greater O2 supply from the blood flow. Cardiac output rises to meet this demand by increasing stroke volume (SV) and heart rate. SV increases to its maximum value within the first half of the tolerable work rate range (at about 45% of the maximum V˙imageO2).3 From that point on, SV remains fairly constant; the only way the heart can increase its output further is to increase its rate of contraction. The early increase in SV is the result of an increased ejection fraction rather than greater diastolic filling; sympathetic nervous stimulation and circulating epinephrine increase systolic ejection.3 Heart rate increases linearly with O2 requirements until the maximum heart rate (HRmax) is achieved (Figure 23-4).

As cardiac output increases with exercise, systolic blood pressure rises significantly (often ≥200 mm Hg). Diastolic pressure changes little, if at all, in normal, healthy people because the peripheral vasculature dilates significantly in response to increased O2 demand, which greatly reduces vascular resistance.2,3 Thus, the pulse pressure (systolic – diastolic) increases significantly, forcing open all of the capillaries in working skeletal muscle. In heavy exercise, up to 80% of the cardiac output is diverted to skeletal muscle, making possible greatly increased O2 delivery and V˙imageO2 rates.

HRmax is age related and does not change with fitness. The predicted HRmax for adults is as follows:4

HRmax=220Age(years)

image

HRmax, and thus, cardiac output are reduced in older persons because their hearts are less responsive to beta-adrenergic stimulation.4 An unconditioned person and a conditioned athlete of the same age have the same HRmax, but the conditioned athlete has a much greater SV and therefore a much higher cardiac output at HRmax. For a given amount of submaximal exercise, a conditioned person’s HR is much lower than the HR of a sedentary person. This explains why a sedentary person reaches the HRmax sooner and at much lower exercise intensity than an athlete. Because the unconditioned person’s cardiac output is lower at the HRmax, maximum O2 delivery and V˙imageO2 are also lower.

Carbon Dioxide Production, Respiratory Exchange Ratio, and Minute Ventilation

The increased metabolic activity of exercising muscles raises not only the V˙imageO2 but also the V˙imageCO2. Before the anaerobic threshold is reached, V˙imageCO2 increases linearly and parallel with V˙imageO2. Thus, R remains constant. The ventilation rate and depth rise to accommodate the increase in V˙imageCO2, producing a linear increase in V˙Eimage parallel with V˙imageCO2 and V˙imageO2 (see Figure 23-3, before anaerobic threshold).

Although it is generally accepted that increased V˙imageCO2 during exercise is the stimulus for the higher V˙Eimage, some evidence suggests that a neurally mediated, muscle-derived signal plays a role. Apparently, peripheral muscle receptors send signals to the central nervous system, increasing blood pressure, HR, and ventilation. These signals are enhanced by muscle ischemia but are not present in ischemia alone without exercise.5 Thus, muscle movement appears to be partly responsible for the increased V˙Eimage of exercise.

The ventilatory response to exercise was discussed in Chapter 11. The immediate hyperpnea of exercise has long been a controversial subject and remains incompletely understood.3 The abrupt increase in V˙Eimage at the onset of exercise (phase I) occurs long before any chemical or humoral change can occur in the body. Rather than being geared to a chemoreceptor reflex mechanism, the abrupt increase in ventilation appears to be a response to anticipated future metabolic demand. Evidence points to a sophisticated internal respiratory controller that integrates multiple afferent and efferent signals to predict the level of metabolic activity that will occur and then adjusts ventilation before the fact.6 A study of children with congenital central hypoventilation syndrome revealed that passive leg motion induced by a motor-driven ergocycle increased V˙Eimage, although these children had no V˙Eimage response to inhaled CO2.7 Increased V˙Eimage occurred immediately with the first breath after the onset of pedaling motion in children with congenital central hypoventilation syndrome and normal controls. This study supports the idea that muscle or joint receptors play a role in exercise hyperpnea.

Normal people increase V˙Eimage by increasing both VT and respiratory rate. Early in exercise, a higher VT produces most of the increase in V˙Eimage. After about 60% to 70% of the vital capacity is reached, VT plateaus, and increased respiratory rate is responsible for additional V˙Eimage increases.8

Figure 23-5 shows that dead space-to-tidal volume ratio (VD/VT) decreases as exercise progresses. The initial steep reduction in VD/VT reflects early increases in VT with respect to the fixed anatomical dead space. VD/VT decreases from normal values of 0.3 to 0.4 at rest to 0.15 to 0.2 in exercise, in part due to a larger VT but also because of significant blood flow diversion to previously underperfused upper lung zones.3,8 As the respiratory rate begins to account for more of the increase in V˙Eimage, the decline in VD/VT is less steep.

Events Occurring after Anaerobic Threshold

As shown in Figure 23-3, the anaerobic threshold is the point at which the V˙imageCO2 slope becomes steeper than the V˙imageO2 slope, indicating an increase in CO2 generation rate. This increase in CO2 generation is due to the bicarbonate buffering of anaerobically produced lactic acid.3,4 The increased generation of CO2 stimulates ventilation, and the V˙Eimage slope rises in parallel with the V˙imageCO2 slope, maintaining a constant PaCO2. In the end, the amount of CO2 exhaled per minute increases with respect to the amount of O2 taken up by the lungs, and R increases. The new V˙Eimage slope has two phases. For some time, immediately after an aerobic threshold (AT) appears, the V˙Eimage slope rises to match the increased V˙imageCO2 slope. This short phase is referred to as isocapnic buffering, in which ventilation increases in concert with CO2, momentarily compensating for the evolving lactic acidosis (see Figure 23-3).

As bicarbonate stores diminish and arterial pH falls, ventilation is stimulated more intensely, causing a second-phase increase in V˙Eimage, out of proportion with the V˙imageCO2 increase. (See the steeper V˙Eimage slope after the second dotted vertical line in Figure 23-3

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