Exercise and ventilation

Write brief notes on breathlessness in exercise (1993)

 

Time course

Exercise

• At, just immediately just before, the start of exercise, ventilation increases instantly (phase I).
• At moderate levels of exercise, ventilation then increases further with time (phase II), and an equilibrium level of ventilation (phase III) is reached within 3 minutes.
• With heavy exercise, ventilation increases further and reaches equilibrium at a higher level of ventilation
• With severe exercise, ventilation continues to increase

Recovery

• After exercise, ventilation falls to resting levels in a few minutes.
• Where exercise is more intense, the fall will take longer as oxygen debt is repaid and lactate level returns to normal.

 

[See diagram 20050306(3) - "Exercise and ventilation"]

 

Oxygen consumption and ventilation

Relationship between minute volume and oxygen consumption is approximately linear up to about oxygen consumption of 2 L/min in an untrained person.

Slope of the linear part = ventilation equivalent for oxygen

=> about minute volume of 20-30 L/min per 1 L/min of oxygen consumption

=> doesn't alter with training

NB: Resting level of oxygen consumption = 200-250 mL/min

NB: Resting level of minute volume = ?????

NB: Oxygen consumption for exercise = 12mL of O2 per minute for each watt (watt = joule/sec)

Owles point

Above a critical level of oxygen consumption (Owles point), ventilation increases more in proportion to oxygen consumption, probably due to lactic acidosis.

In an untrained person, Owles point is at about 2L/min of oxygen consumption.

In a trained person, Owles point occurs at higher levels of oxygen consumption.

[See diagram 20050306(3) - "Oxygen consumtion and ventilation"]

Limitation to exercise

Limitation to exercise is usually determined by the breathlessness

Breathlessness occurs when exercise ventilation uses a high proportion of the maximal breathing capacity (MBC).

=> Rising levels of lactate increass ventilation, leading to breathlessness

There is a close correlation between MBC and maximal oxygen uptake.

NB. Diffusion capacity normally doesn't limit exercise in normal person at sea level.

Maximal breathing capacity (MBC)

(aka maximal voluntary ventilation)

=> maximal minute volume of ventilation that a subject can maintain for 15 seconds.

• Average young fit male - 170L/min
• Male: 47-253 L/min
• Female: 55-139 L/min

Dyspnoea occurs at about 1/3 of MBC

Ventilation is about 60% of MBC at maximal oxygen uptake. This can be changed by training.

Maximal oxygen uptake

=> about 3 L/min for young fit adult male

Can be increased by exercise, and decreased by sedentary lifestyle.

Control of ventilation

Control of ventilation during exercise is still largely unknown.

• Phase I can be in part due to a learned response. Can also be due to joint/muscle receptor.
• Arterial blood gases are probably not the main factor in the increase of ventilation during exercise.
  => PaO2 is normal, PaCO2 is reduced.
  NB: However, 100% O2 does reduce minute volume for a particular level of oxygen consumption.
• Hyperthermia may contribute a little too.
• Metabolic acidosis causes excess ventilation during heavy and severe exercise

Role of training

Training affects

• Owles point
• Cardiac fitness, thus oxygen delivery, thus the aerobic exercise capacity
• Ability of muscles to remove lactate
• (In animal study) ability of liver to remove lactate
• Tolerance to lactate
• Fraction of MBC that can be sustained, via stronger and more enduring respiratory muscles

Training does not affect

• Linear relationship between power generated and oxygen consumed
• Maximal expiratory flow
• Slope of ventilation vs oxygen consumption curve (but Owles point is affected)

Thus, training improves performance of skeletal muscles and cardiovascular system, rather than changes in respiratory function.

 

Additional notes

Levels of exercise

Moderate exercise - below the subject's anaerobic threshold and the arterial blood lactate is not raised. Considered as steady state.

Heavy exercise - above the anaerobic threshold. The arterial blood lactate is elevated but remains constant. Considered as steady state.

Severe exercise - well above the anaerobic threshold. The arterial blood lactate continues to rise. This is an unsteady state.