PHYS: Cardiac Function Flashcards
What is cardiac output?
volume of blood pumped per minute. CO = HR x SV
What is cardiac work?
the amount of pressure the heart has to overcome to pump the blood into the system cardiac work = change in pressure x change in volume
Why are the systolic and diastolic function curves different?
Because the ventricle can stretch quite generously in diastole, the increase in volume doesn’t really increase the pressure much as the ventricle simply stretches. When it can stretch no more (around 150ml) you start to see the pressure increase. The tension of contraction in systole opposes the cardiac muscles ability to stretch, so volume of the ventricle doesn’t change with increased blood, the pressure just increases.
How do you calculate stroke volume?
SV = end diastolic volume – end systolic volume
How do you calculate ejection fraction?
EF = Stroke Volume /End Diastolic Volume
What is pulse pressure? How do you calculate it?
Pulse pressure = force with which blood is ejected into the aorta PP = systolic pressure (SBP) – diastolic pressure (DBP)
What are the 3 most important factors affecting stroke volume?
preload, contractility, afterload
What are the factors influencing preload
♣ Peripheral venous tone ♣ Gravity ♣ The muscle pump ♣ Respiration (another pump) ♣ Blood volume
What is preload? How does it affect stroke volume
Called preload because it represents the load placed on the myofibres before they contract. Higher venus pressure = more blood into heart = higher load on myofibres. More blood into the heart will stretch the heart causing stronger contraction (Frank-Starling law) and hence a greater SV.
What is contractility? How does it affect stroke volume?
Contractility = inotropy (force of contraction) o Contractility is changed by changing levels of calcium in the myocytes. o More calcium = stronger contraction = higher SV
What are the inotropic agents?
+ agents = NA and adrenaline - agents = ACh
How does the sympathetic system increase contractility?
Releases NA and adrenaline which bind to B1 receptors on the heart. These open Calcium channels to increase levels in the myocytes (stronger contraction) They also increase the activity of the Ca pumps to increase amount of Ca in the sarcoplasmic reticulum (greater release from SR = more calcium in cell = stronger contraction). They also increase the activity of the pumps removing Ca from the cytosol so that the myocytes can contract again sooner (allows for higher HR).
What is afterload? How does it affect stroke volume?
Afterload = tension that the heart needs to overcome to pump blood into the aorta o The heart is essentially pumping blood against pressure supplied by the aorta. o When we have higher aortic pressure it takes more work from the heart to open the aortic valves, they stay closed for longer and you reduce stroke volume and ejection fraction, as more of systole is spent in isometric contraction and less in ejection.
What can increase afterload?
higher blood pressure and aortic valve stenosis will both increase aortic pressure and hence afterload.
How does exercise change the pressure volume relationship?
- Increases preload via muscular and respiratory pumps.
An increase in preload puts more volume into the heart before it enters systole, causing a right shift of the end diastolic volume.
- Increase contractility through sympathetic stimulation.
Increasing contractility will mean that at the same volume of blood the pressure will be higher, so the systolic function curve will rise.
- Increase afterload through BP rising.
An increase in afterload will mean the aortic pressure is higher and hence the pressure at which the aortic valve opens and shuts (A and C) will increase. Note C won’t go straight up as it has to sit on the systolic curve.
Label the important events/measurements
