Cardiac Output

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Cardiac Output
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Learning Outcomes

• By the end of this session you should be able to
• Define Heart Rate
• Define Stroke Volume
• Explain the factors that influence stroke volume
• Define Cardiac Output
• Explain how cardiac output is regulated

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Heart Rate (HR)

• Number of times the heart beats in one minute
• Average is between 60 and 80 bpm (beats per
  minute)
• Dependent upon age
• When we exercise, HR increases in proportion to
  the increase in intensity until near exhaustion
• At this point (maximum heart rate) the HR levels
  off

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Maximum Heart Rate (MHR)

• MHR decreases with age
• MHR 220 age (in years)

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Stroke Volume (SV)

• Amount of blood ejected by the ventricles in one
  beat (ml per beat)
• Determined by
• Volume of venous return to the heart
• Amount the ventricles can stretch when filling
  (elasticity)
• Amount the ventricles can contract when emptying
  (contractility)
• Amount of pressure in aorta and pulmonary
  arteries that the ventricles must contract
  against (resistance)

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Cardiac Output (Q)

• Amount of blood pumped from the heart in one
  minute
• Q HR x SV

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At Rest

• Q approx 5 litres of blood per minute

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During Exercise

• Q increases proportionately with an increase in
  intensity (i.e. if work rate increases then so
  does the amount of blood pumped around the body)
• This increase is brought about by an increase in
  HR, SV or both

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During Exercise (cont)

• Early stages of exercise increase in Q caused
  by both HR and SV increasing
• At higher levels of intensity (approx 40-60 of
  capacity), SV plateaus (reaches a level and does
  not rise any more)
• Due to the fact that there is only a set amount
  of blood the heart can eject at any time

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During Exercise (cont)

• If intensity continues to increase, Q will only
  increase if HR increases
• A rapid rise in heart rate occurs to compensate

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Learning Outcomes

• By the end of this session you should be able to
• Define Heart Rate
• Define Stroke Volume
• Explain the factors that influence stroke volume
• Define Cardiac Output
• Explain how cardiac output is regulated