Preload, afterload and contractility

Question 1

Contractility

Answer 1

the pressure the heart places on the ventricle to contract during systole.

Question 2

Effect of sympathetic stimulation on contractility

Answer 2

Increases contractility

Question 3

What is the impact of increased contractility on the heart?

Answer 3

Increase of contractility enables the heart to:

1. Handle a greater pre-load (i.e. filling volume)
2. Empty more completely
   
   1. Normally after contraction about 40% blood remains in heart
   2. We can reduce ESV
   3. This is done against increased after-load (increased aortic pressure)
3. Deliver an increased stroke volume (even when an increased HR reduces time for ventricular filling)

Question 4

What effect does parasympathetic stimulation have on cardiac output?

Answer 4

• Decreased force of contraction
• Inhibition of noradrenaline release from sympathetic nervous system
• Activation of M2 muscarinic receptors
• Reduces cAMP production
• Cardiac slowing – inhibition of AV conduction

Question 5

What is cardiac output?

Answer 5

the volume of blood pumped by one ventricle of the heart (usually taken to be the left) in one minute.

Question 6

How do you calculate cardiac output?

Answer 6

Cardiac output = heart rate (bpm) x stroke volume (ml/beat)

Question 7

Preload

Answer 7

a.k.a end diastolic volume (EDV)

Blood that is coming back from the venous system; this is dependent on venous return.

Question 8

What factors affect preload?

Answer 8

• Filling time of the heart
  
  • Lower heart rate = longer period for ventricular filling = greater distension of the ventricle
• Venous return

Question 9

What is venous return?

Answer 9

Rate of blood flow returning to the heart (Central venous pressure)

Question 10

What does this image show and what is its relevance to preload?

Answer 10

• Image shows the skeletal muscle pump
• Contraction of skeletal muscle causes compression of veins, forcing blood back to the heart and increasing preload

Question 11

How does sympathetic nervous system stimulation increase preload?

Answer 11

• SNS stimulation causes venous vasoconstriction
• This increases central venous pressure and thus preload

Question 12

What is the respiratory pump and how does it affect preload?

Answer 12

• Respiratory pump = describes how the movement of the diaphragm for inspiration affects blood return from the abdomen
• On inspiration, the diaphragm moves caudally, reducing thorax pressure but increasing abdominal pressure
• This increased abdominal pressure causes increased return of blood from the abdomen

Question 13

Afterload

Answer 13

resistance to ventricular ejection (pressure that the ventricles have to pump against)

Question 14

What effect would increased afterload (e.g. caused by hypertension) have on stroke volume and the cardiac cycle as a whole?

Answer 14

• Increased afterload would mean reduced stroke volume, because the ventricle can no longer pump out the same volume of blood against this greater pressure
• The heart will beat faster, having to work harder to pump the same volume of blood

Question 15

What factors affect afterload?

Answer 15

• Arterial vascular tone and compliance
• Arterial blood volume
• Cardiac output

Question 16

Blood pressure

Answer 16

the pressure in the arterial system, influenced by resistance and cardiac output.

Question 17

How do you calculate blood pressure?

Answer 17

Blood pressure = cardiac output x total peripheral resistance

Question 18

How do you calculate pulse pressure?

Answer 18

Pulse pressure = P_systole - P_diastole

Question 19

True/False:

Pulse pressure and mean arterial pressure are the same thing?

Answer 19

FALSE

• Pulse pressure is the difference between P_systole and P_diastole
• Mean arterial pressure is the average blood pressure over a period of time, calculated:
  
  • MAP = DAP + 1/3 (SAP-DAP)

Question 20

What factors influence pulse pressure?

Answer 20

• Arterial compliance
• Stroke volume
• Ejection rate

Question 21

How does arterial compliance affect pulse pressure?

Answer 21

• Arterial compliance determines the ability of the arterial system (esp aorta) to accommodate the increase in pulse pressure after ventricular systole
• Compliance of arteries reduces with age, so the pressure of blood exiting the heart creates a larger afterload than before
• This means the heart must work harder