Cardiac Output

# Cardiac Output

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## Cardiac Output

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1. Cardiac Output

2. 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

3. 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

4. Maximum Heart Rate (MHR) • MHR decreases with age • MHR = 220 – age (in years)

5. 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)

6. Cardiac Output (Q) • Amount of blood pumped from the heart in one minute • Q = HR x SV

7. At Rest • Q = approx 5 litres of blood per minute

8. 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

9. 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

10. During Exercise (cont) • If intensity continues to increase, Q will only increase if HR increases • A rapid rise in heart rate occurs to compensate

11. 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