Write brief notes on breathlessness in exercise (1993)
[See diagram 20050306(3) - "Exercise 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)
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 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.
(aka maximal voluntary ventilation)
=> maximal minute volume of ventilation that a subject can maintain for 15 seconds.
Dyspnoea occurs at about 1/3 of MBC
Ventilation is about 60% of MBC at maximal oxygen uptake. This can be changed by training.
=> about 3 L/min for young fit adult male
Can be increased by exercise, and decreased by sedentary lifestyle.
Control of ventilation during exercise is still largely unknown.
Thus, training improves performance of skeletal muscles and cardiovascular system, rather than changes in respiratory function.
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.
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Created | 20050306 |
Updated | 20050306 |