Oxygen consumption and ventilation

Liner relationship below Owles point

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 = 250 mL/min
• Resting level of minute volume = 6-8L/min
• 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 ~ 2L/min of oxygen consumption
• In a trained person
  --> Owles point is higher

##See diagram 20050306(03) - "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

Significance

• Dyspnoea occurs at about 1/3 of MBC
• Ventilation is about 60% of MBC at maximal oxygen uptake
  * Can be changed by training

Maximal oxygen uptake

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

• Can be increased by exercise
• Can be decreased by sedentary lifestyle