Cardiac Function Flashcards
What are the typical volumes/pressures of each of the following:
- ESV
- SV
- EDV
- EDP
ESV = 75ml SV = 75ml EDV = 150ml EDP = 50mmHg
What is afterload?
The load encountered by the ventricle as it commences contraction
- imposed by arterial hypertension and LV outflow tract obstruction
What is preload?
The amount of blood the heart has to pump (EDV is a measure) –> the stretch on the myocyte fibres before they commence contraction
- imposed by increased venous return
What is the approximate distribution of blood in the CV system?
65% in systemic veins 13% in systemic arteries 5% in systemic capillaries 10% in lungs 7% in the heart
What is mean circulatory filling pressure?
The mean pressure that exists in the circulatory system when the blood has had a chance to redistribute evenly to all vessels and organs. It is approximately 7mmHg
What influences mean circulatory filling pressure?
The volume of circulating blood and the tone in the walls of the venous system (which determines the capacity of the system)
What happens to mean circulatory filling pressure when blood volume is increased or there is venoconstriction?
MCFP is higher
What happens to MCFP when there is decreased TPR?
Nothing, as TPR has no effect on the compliance of vessels
What is central venous pressure and how is it assessed?
Pressure in the great veins just outside the heart, assessed clinically by the JVP
- it is the filling pressure for the heart, and needs to be adequate to maintain CO
- rises as a result of a failing heart and falls when venous return is poor
What is the short term control of BP?
Predominantly neural, the baroreflex
- have baroreceptors in the carotid sinus and aortic arch
- these are thin walled and very compliant, pick up stretch via sodium channels
- the more they are stretched the more they fire
- respond quickly (within 1 cardiac cycle)
How does the sympathetic NS act in the regulation of BP?
Increases HR, decreases AV conduction time, increases cardiac contractility (by increasing intracellular calcium), increase TPR, increase venous return
When is the baroreflex reset?
Threshold for baroreflex firing resets to new pressure levels within 1 - 2 days
What other mechanisms are there for regulating BP when the baroreceptors may not work?
Chemoreceptors - they respond to very low O2, high CO2, low pH
- if BP drops below 60 there is silence from the baroreceptors
What is the diurnal variation of BP?
Lower at night by around 20mmHg due to less sympathetic activity (renin-angiotensin system looks after it)
- early morning burst of BP
What is the seasonal variation of BP?
Summer BP is about 3mmHg lower than winter
- potentially due to body weight/sweat/vasodilation
