5 - Control of Cardiac Output Flashcards
What is afterload, preload and total peripheral resistance?
Preload - Amount the ventricles are stretched in diastole, related to end diastolic volume or central venous pressure
Afterload - The load the heart must eject against. Roughly aortic pressure
TPR - Resistance to blood flow by all systemic vasculature
Where is the greatest resistance in the systemic circulation?
Arterioles. Constriction of arterioles increases resistance so artery pressure increases and venous pressure decreases
If TPR falls and CO stays the same, what must happen to the vessels b.p, and vice versa?
- Arterial will fall
- Venous will increase
If CO increases and TPR is unchanged what will happen to the pressure of the vessels?
Why would you have pulmonary oedema in left sided heart failure?
Cardiac output decreases so pressure in veins increases, therefore causing high hydrostatic pressure
How does the blood recognise the change in demand for blood supply?
- When blood supply demand increases, vasodilation occurs so total peripheral resistance decreases. This causes the heart to pump more so that the arterial pressure does not fall
- Heart responds to CVP and aBP by intrinsic and extrinsic mechanisms
How do you work out stroke volume?
SV = EDV - ESV
Normally about 2/3 of EDV. Can increase by increasing EDV or decreasing ESV
How does the ventricle stop filling?
Ventricle fills until the walls have stretched enough that they have produced a pressure equal to venous pressure
- Higher venous pressue more heart fills
- More heart fills, higher ventricular pressure
What is the ventricular compliance curve?
Decreased compliance can be caused by hypertrophy of ventricle, causes small volume change to cause large pressure change so ventricle doesn’t fill as much
What is the Frank-Starling Law of the heart?
Therefore, harder heart contracts, higher stroke volume.
Increase EDV, Increases SV
Increase VP, will increase EDV
Draw and explain the Starling curve.
What is the length tension curve?
As the sarcomere length increases the contractile force of the muscle increases
What does Starling’s Law ensure?
That the same volume of blood is being pumped from both sides of the heart. This is the intrinsic control mechanism
What happens to the Starling Curve when you increase the contractility of the heart muscle, and why does this happen?
Extrinsic mechanism that uses noradrenaline from sympathetic system to increase contractility so higher SV at same pressure
What does increasing the arterial pressure do to the stroke volume and the heart over a long period of time?
Higher afterload so heart has to work harder to push against afterload, so decreased stroke volume. Over time this increased afterload causes hypertrophy of left ventricle to overcome afterload
What happens to vessel pressure when you stand up and what happens to combat this?
Venous pressure decreases as blood pools, so CO decreases as less venous pressure. Therefore arterial pressure also decreases. Heart beats faster to raise CO and TPR increases
If reflexes don’t work can get postural hypotension and syncope
What happens to vessel pressure during exercise?
- Initially muscle pumping and vasoconstriction return more blood to the heart
- Later decreased TPR increases venous return
- Increased contractility and heart rate so moves the Starling Curve
What is the JVP and how is it measured?
- Jugular Venous Pressure gives indication of right atria pressure as no valves
- Biphasic pulse being right sternocleidomastoid muscle due to internal jugular vein
- Measure highest visible pulsations above sternal angle + 4cm
- Normally 5 to 8 cm H2O
What does a JVP curve look like with a central line?
(same as atrial pressure on wiggers)
What might increase JVP?
- If right side of heart doesn’t pump blood out properly
- Volume overload with IV infusion
- Something impairing filling of the heart
What are the intrinsic and extrinsic mechanisms of the heart?
