Ventilatory and Blood Gas
Response to Exercise
Rest to work Transitions
• Change in pulmonary ventilation
• Observed below lactate threshold
Rest to work Transitions
• Transition from rest  constant-load-submaximal
exercise
• Arterial tensions of PCO2 and PO2 are relatively
unchanged during transition during submaximal
exercise
• BUT: arterial PO2 decreases & PCO2 increases slight in
transition from rest  steady-state exercise
• Therefore: at the start of exercise:
Increase in ventilation is NOT as fast as increase in
metabolism
Rest to work transitions
• At the onset of constant-load submaximal
exercise:
– Initially, ventilation increases rapidly
• Then, a slower rise toward steady-state
– PO2 and PCO2 are maintained
• Slight decrease in PO2 and increase in PCO2
Changes in Ventilation, PO2 and
PCO2 in transition from rest to
steady state submaximal exercise
Prolonged Exercise in a hot
Environment
• Cool, low humidity environment = 19°C, 45%
humidity
• Hot, high humidity = from 28°C, 75% humidity
• Hot environments hamper heat loss from the
body
• Increase in body temperature which directly
affects respiratory control centre
= Minute ventilation increases (with increased
breathing frequency)
Prolonged Exercise in a hot
Environment
During prolonged
sub maximal
exercise:
• Ventilation tends
to drift upwards
• Little change in
PCO2
• Higher ventilation
not due to
increased PCO2
Incremental Exercise
• Linear increase in ventilation
- Up to 50 – 75% VO2max
• Exponential increase beyond this point
• Ventilatory threshold
- Point where minute ventilation increases
exponentially
Ventilatory Threshold
• Reflects aerobic fitness without the need to
directly measure maximal oxygen uptake
• Point during exercise training at which pulmonary
ventilation becomes disproportionately high with
respect to oxygen consumption during an
incremental exercise test
• Used as a guide to determine exercise intensity
• Therefore: Ventilatory threshold = intensity of exercise that
shows a larger ventilation than required to do work.
• At this point, the contribution of anaerobic metabolism
becomes significant to produce larger concentrations of lactic
acid.
• Lactic acid accumulates, reducing pH & increasing metabolic
acidosis.
• Because one of the functions of the respiratory system is
acid-base balance, respiration must increase to compensate
for the increased acidosis.
• The point were ventilation deviates from linearity is termed
the ventilatory threshold (TVENT).
Ventilatory response to exercise
Trained vs. Untrained:
•
-
Trained:
Decrease in arterial PO2 near exhaustion
pH maintained at a higher work rate
Ventilatory threshold occurs at higher work
rate
Effect of Training on Ventilation
• Untrained: able to maintain PO2 in arteries within 10–
12 mmHg of resting value
• Trained: PO2 decreases by 30- 40 mmHg during heavy
exercise
*low arterial PO2 vales during exercise = exercise induced
hypoxemia
* Low arterial Po2 values are also seen in patients with
severe lung disease
Males Vs. Female athletes
• 50% of highly trained male endurance athletes
develop exercise induced hypoxemia
• Females are suggested to have experience
exercise induced hypoxemia more often than
males
Causes
• Failure of pulmonary system?
• Ventilation perfusion mismatch?
Indicates matching of blood flow to ventilation
– Ideal: ~1.0
– Light exercise improves
– Heavy exercise = inequality
• Diffusion limitations during exercise ?
Reduced amount of time that red blood cells spend in the
pulmonary capillaries...caused by high cardiac outputs from
athletes  less time for gas equilibrium to be achieved
Ventilatory Response to exercise: Trained Vs. Untrained
Revision questions
1.
2.
3.
4.
5.
6.
7.
8.
Describe the changes in PO2 and PCO2 with
different exercises.
What happens during prolonged sub maximal
exercise? Why does this occur?
What happens during incremental exercise?
Explain the Ventilatory Threshold
How do trained and untrained athlete’s ventilatory
responses differ?
How do male and female ventilatory responses
differ?
What is hypoxemia and how does it relate to
exercise?
What are the suggested causes to hypoxemia
in athletes?
(6)
(6)
(3)
(8)
(6)
(2)
(4)
(6)