*20 Exercise Physiology

Question 1

Normal physiologic response to exercise

• Skeletal muscle contraction
• ATP
• If O2 supplies are inadequate,…

Answer 1

• Skeletal muscle contraction
  
  • Uses up large amounts of ATP, which must be replaced if exercise is to continue
• ATP
  
  • Generated primarily through the oxidation of glucose (glycolysis), which is normally followed by the reactions of the citric acid or Krebs cycle and then by oxidative phosphorylation
• If O2 supplies are inadequate,…
  
  • Glycolysis converts from an aerobic to an anaerobic process
  • This produces two important physiologic effects
    
    • (1) it markedly reduces ATP synthesis, which ultimately limits exercise
    • (2) anaerobic metabolism produces a metabolic acidosis through the production of lactic acid

Question 2

Normal physiologic response to exercise

• CO2
• During exercise, muscle cells…
• Exercise-induced lactic acidosis also causes…
• Related equation

Answer 2

• CO2
  
  • Generated as a byproduct of the citric acid cycle
• During exercise, muscle cells…
  
  • Produce a large quantity of CO2, which diffuses into the tissue capillaries and is transported to the lungs
• Exercise-induced lactic acidosis also causes…
  
  • Increased CO2 production through the carbonic acid - bicarbonate buffer system
• Related equation
  
  • H2O + CO2(↑) <–> H2CO3 <–> H+(↑) + HCO3-

Question 3

The role of the cardiovascular and respiratory systems during exercise

• The cardiovascular system
• The respiratory system
• PCO2 equation
• Lactic acidosis equation

Answer 3

• The cardiovascular system
  
  • Must supply sufficient O2 to the muscles to maintain adequate ATP production
  • It does this by increasing CO through an increase in both HR and SV
  • O2 delivery is also determined by the hemoglobin concentration and the hemoglobin O2 saturation
• The respiratory system
  
  • Must excrete the excess CO2 produced during exercise
• PCO2 equation
  
  • PCO2 = k x VCO2 / VE – VD
  • Alveolar ventilation must increase in proportion to CO2 production in order to maintain a normal arterial PCO2
• Lactic acidosis equation
  
  • H2O + CO2(↓) <–> H2CO3 <–> H+(↓) + HCO3-
  • Ventilation must also increase to compensate for lactic acidosis, if it occurs

Question 4

The physiology of exercise:
Incremental cardio-pulmonary exercise test

• Incremental cardio-pulmonary exercise test
• VO2 and VCO2
• CO2 production and O2 consumption

Answer 4

• Incremental cardio-pulmonary exercise test
  
  • A subject exercises on a stationary bike as the resistance to pedaling is progressively increased
  • During exercise, HR, minute ventilation, oxygen consumption (VO2), and CO2 production (VCO2) are continuously measured
• VO2 and VCO2
  
  • Initially increase in proportion to the level of exercise
• CO2 production and O2 consumption
  
  • Note that the amount of CO2 produced is initially less than the amount of O2 consumed (remember R ≈ 0.8)

Question 5

The physiology of exercise:
Incremental cardio-pulmonary exercise test

• Anaerobic threshold
• Minute ventilation
  
  • Initially
  • Once anaerobic threshold is reached
  • Later

Answer 5

• Anaerobic threshold
  
  • At some point during progressive, incremental exercise, CO will reach a maximum level
  • The cardiovascular system will no longer be able to meet the O2 demands of the exercising muscles
  • At the onset of anaerobic metabolism, the rate of CO2 production increases and it will eventually exceed oxygen consumption (i.e. R > 1)
• Minute ventilation
  
  • Initially increases in proportion to VO2 and VCO2
  • Once anaerobic threshold is reached, however, minute ventilation increases at the same rate as CO2 production
  • Later, minute ventilation increases out of proportion to CO2 production as the respiratory system compensates for increasing lactic acidosis

Question 6

The physiology of exercise:
Incremental cardio-pulmonary exercise test

• The onset of anaerobic metabolism (anaerobic threshold) is determined by…
• In the absence of cardiovascular or pulmonary disease, maximum exercise is determined solely by…
• Exercise is typically terminated shortly after…
• In other words, exercise is normally limited by…
• Exercise is not normally limited by…

Answer 6

• The onset of anaerobic metabolism (anaerobic threshold) is determined by…
  
  • Measuring the VO2 at which the rate of rise in CO2 production and minute ventilation increases
• In the absence of cardiovascular or pulmonary disease, maximum exercise is determined solely by…
  
  • The amount of oxygen that can be delivered to the skeletal muscles
• Exercise is typically terminated shortly after…
  
  • The onset of anaerobic metabolism
• In other words, exercise is normally limited by…
  
  • The cardiovascular system
• Exercise is not normally limited by…
  
  • Minute ventilation, which can be increased far above that required during maximal exertion

Question 7

Exercise limitation

• In the presence of disease, exercise capacity can be limited either by…
• Factors that can significantly reduce exercise capacity

Answer 7

• In the presence of disease, exercise capacity can be limited either by…
  
  • The inability of the cardiovascular system to deliver sufficient oxygen
  • The inability of the respiratory system to excrete the increased CO2 load
• Factors that can significantly reduce exercise capacity
  
  • Factors that impair tissue oxygen delivery
    
    • Primarily low cardiac output
    • Also anemia and hemoglobin desaturation)
  • Factors that impair alveolar ventilation (e.g.)
    
    • Abnormal respiratory mechanics
    • Increased dead space
    • Rspiratory muscle weakness

Question 8

Exercise limitation

• In clinical medicine, incremental cardio-pulmonary exercise tests are used to determine…
• Equations that factor in age, sex, and height are used to predict…
• Prior to exercise, the maximum voluntary ventilation (MVV) is measured by…

Answer 8

• In clinical medicine, incremental cardio-pulmonary exercise tests are used to determine…
  
  • The cause of a patient’s reduced exercise capacity
• Equations that factor in age, sex, and height are used to predict…
  
  • Normal maximum O2 consumption, which reflects exercise capacity
  • Normal maximum HR, which reflects cardiovascular function
• Prior to exercise, the maximum voluntary ventilation (MVV) is measured by…
  
  • Having the patient breathe in and out as quickly and forcefully as possible for 12 seconds
  • The measured exhaled volume is then multiplied by 5 to predict the maximum minute ventilation that the patient is capable of achieving

Question 9

Exercise limitation

• Cardio-pulmonary exercise tests are interpreted by…
• Abnormal exercise capacity is indicated by…
• If exercise capacity is abnormal, we then determine…

Answer 9

• Cardio-pulmonary exercise tests are interpreted by…
  
  • First comparing the actual with the predicted VO2 at peak exercise
• Abnormal exercise capacity is indicated by…
  
  • A maximum oxygen consumption that is less than 90% of the predicted value
• If exercise capacity is abnormal, we then determine…
  
  • Whether exercise is limited by the cardiovascular or the respiratory system

Question 10

Exercise limitation

• Patients with a cardiovascular limitation to exercise
  
  • Minute ventilation
  • Heart rate
  • Anaerobic threshold
• Patients with a respiratory limitation to exercise
  
  • Minute ventilation
  • Heart rate
  • Anaerobic threshold

Answer 10

• Patients with a cardiovascular limitation to exercise
  
  • Have a maximum minute ventilation that is far below their MVV
  • Have a peak heart rate that exceeds 90% of their predicted value
  • Reach anaerobic threshold early during exercise since tissue oxygen delivery is impaired
• Patients with a respiratory limitation to exercise
  
  • Quickly reach their maximum attainable minute ventilation, which will be very close to or even greater than their calculated MVV
  • Since exercise is terminated early, they will not reach their predicted maximum heart rate and often don’t reach anaerobic threshold

Question 11

Characteristics of an exercise study

• (Predicted) VO2 max
  
  • Normal
  • Cardiovascular limitation
  • Respiratory limitation
• (Predicted) HR max
  
  • Normal
  • Cardiovascular limitation
  • Respiratory limitation

Answer 11

• (Predicted) VO2 max
  
  • Normal > 0.9
  • Cardiovascular limitation < 0.9
  • Respiratory limitation < 0.9
• (Predicted) HR max
  
  • Normal > 0.9
  • Cardiovascular limitation > 0.9
  • Respiratory limitation < 0.9

Question 12

Characteristics of an exercise study

• (Predicted) VO2 max
  
  • Normal
  • Cardiovascular limitation
  • Respiratory limitation
• (Predicted) VE max
  
  • Normal
  • Cardiovascular limitation
  • Respiratory limitation

Answer 12

• (Predicted) VO2 max
  
  • Normal > 0.4
  • Cardiovascular limitation < 0.4
  • Respiratory limitation > 0.4 (if reached)
• (Predicted) VE max
  
  • Normal < 0.7
  • Cardiovascular limitation < 0.7
  • Respiratory limitation > 0.7