Cardiac Function Flashcards
Cardiac output
Litres/min Stroke volume (l/beat) x heart rate (beat/min)
Healthy cardiac output at rest
5 litres/min
Healthy stroke volume at rest
70 ml
Healthy heart rate at rest
70bpm
Factors affecting heart rate
Intrinsic - SA node (pacemaker) and AV node (conduction)
- SNS - activation of beta-adrenoceptors causes increase
- PNS - activation of muscarinic receptors causes decrease
- Hormones
- Extra/intracellular ions
What is stroke volume?
Volume of blood ejected by each ventricle in a single heart beat
Represents the difference between end diastolic volume and end systolic volume
What affects stroke volume
Pre-load
Cardiac contractility
After-load
Pre-load
Filling pressure of the heart at the end of diastole
End diastolic volume dependent upon end-diastolic pressure
What is pre-load influenced by?
Venous return (venous tone, blood volume, posture, intrathoracic pressure)
Atrial contractility
Filling time - increased heart rate reduced filling time and can limit EDV
Describe the length-tension relationship using a curve
Frank-Starling curve - stroke volume y, EDV x
Blood returning to the heart will be pumped out
Force of contraction of cardiac muscle fibres is proportional to degree of stretch Increased stretch (EDV), increased force of contraction and therefore increased contractility (stroke volume increases)
Cardiac contractility
Innate ability for heart to contract
Amount of force generated by cardiac muscle fibres can vary independently from degree of stretch
End diastolic volume
Amount of blood in ventricle at the end of diastole
How can muscle fibres of the heart be made more effective as a contractile pump?
Increased SNS activity
Increased calcium
Positive inotropic drugs eg. Digoxin, changes F-S relationship
How can muscle fibres of the heart be made less effective as a contractile pump?
Decreased SNS activity
Hypoxia
Acidosis (associated with hypoxia)
Heart failure - hypertrophy
After load
Aortic pressure is a force the heart is pushing against
Increased aortic pressure increases after load which reduces stroke volume at a constant pre-load
When is velocity of contraction greatest?
At fixed length
Low loads
Describe LV pressure-volume changes in a single cardiac cycle
- Starting volume (preload) = EDV
- Isovolumic phase of systole - increase in pressure, no change in volume
- When force (left ventricular pressure) exceeds load (aortic pressure) aortic valve opens
- Ejection phase of systole - further slight increase in pressure, reduction in volume
Extent of reduction in volume = stroke volume
Pressure and volume at preload
140ml, 0mmHg
P and V at isovolumic phase of systole
Rise in LVP no change in volume
Isovolumic phase of diastole
Fall in LVP with no change in volume
Ejection phase
Decreases volume, slowing increase of pressure
Ventricular filling
Increased volume
No change in pressure
How does an increased pre-load affect pressure and volume relationship
More forceful ejection - increased pressure
More volume to remove - increased stroke volume
How does increased aortic pressure (afterload) affect pressure-volume relationship?
Same EDV
Have to generate higher pressure in ventricle to exceed aortic
Less time for ejection to take place, smaller ejection of blood
Therefore stroke volume decreases
What would happen in the next heartbeat after a beat with high afterload?
EDV increased, cardiac muscle fibres will contract more to increase stroke volume and return to normal afterload
LT effect of high afterload
Heart failure as heart has to work harder chronically
What is the affect on pressure-volume relationship when cardiac contractility is increased?
Increased stroke volume due to increase contraction
Higher pressure
Less blood at the end of systole (decreased EDV)
Affect on next heartbeat after higher cardiac contractility?
Less contraction needed
