Cardiac Function Flashcards
How to calculate cardiac output (and units)
Cardiac output = Stroke volume (litres/beat) x heart rate (beat/min)
Litres/min
Cardiac output, stroke volume, heart rate at rest
5 L/min; 70ml; 70bpm
Effect of sympathetic nervous system on heart rate
Activation of B-adrenoceptors causes an increase in heart rate
Effect of parasympathetic nervous system on heart rate
Activation of muscarinic receptors causes a decrease in heart rate
Effect of hormones on heart rate
Adrenaline acting on beta-adrenoceptors causing an increase in heart rate
Effect of intra/extracellular ions on heart rate
Alterations in membrane potential - e.g. K+
Stroke Volume
Volume of blood ejected by Left ventricle in a single heart beat
End Diastolic Volume vs End Systolic Volume
The volume of blood in the heart during the respective phases of contraction
Stroke volume is 70ml; difference
**note that ESV is not 0
Factors affecting stroke volume
Pre-load (If more blood enters, more will leave)
Cardiac Contractility (ability of heart to contract)
After-load (pressure against which blood is pushed)
Pre-load and factors influencing it
Amount of blood is returned to the heart during a cardiac cycle
Primarily influenced by venous return
Also affected by filling time & atrial contractility
Why does an increase in LV volume cause an increase in stroke volume (mechanically)
If there is more stretch on cardiac muscle fibres, their contraction will have more force and can push out more blood (length-tension relationship)
Cardiac contractility
The amount of force generated by cardiac muscle fibers. It can vary independently from the degree of stretch i.e. ventricular contractility can vary at any given end-diastolic volume depending upon other influences
Factors cause increases in stroke volume by affecting cardiac contractility
Increased SNS activity
Calcium
Positive inotropic drugs
e.g. digoxin
Factors cause decreases in stroke volume by affecting cardiac contractility
Reduced SNS activity
Hypoxia
Acidosis
Heart failure!!
Afterload
Amount of pressure that the heart needs to exert to eject the blood during ventricular contraction
An increase in aortic pressure will increase afterload as more pressure needs to build up in the LV to allow valve opening