Coupling of cardiac and vascular funtions Flashcards
How is the cardiac output related to the venous return?
1) The heart can pump what it receives
2) The heart generates pressure by ejecting the stroke volume against resistance
3) Venous return requires a pressure gradient between the venous blood and the right atrium
4) Over time the venous system can only pump the volume of blood it gets
5) At a steady state the cardiac output ejected by the left ventricle is equal to the venous return by the right atrium
- Very strong relationship between CO and VR
What is the frank-starling relationship?
It states that an increase in the venous return will increase the atrial pressure and end-diastolic volume which will result in an increased fiber length and thus cardiac output
- Shown by the cardiac function curve
What are the parameters expressed in a cardiac/vascular function curve?
1) Cardiac output
2) Right atrial pressure
3) Venous return
What does the vascular function curve (venous return) indicate?
- An inverse relationship between the venous return and right atrial pressure
- The greater the pressure in the right atrium the smaller the pressure gradient between he right atrium and the vein, indicating a lower venous return
- The flat portion of the curve indicates that the arterial pressure is negative, which means that large veins collapse and the VR levels
- The end of the line where the VR raches the X axis (right atrial pressure) indicates the mean systemic pressure
What is the mean systemic pressure?
It is the right atrial pressure when the venous return comes back to zero
What increases the mean systemic pressure?
1) Increased blood volume
2) Decrease in venous compliance
- If they occur they will shift the vascular function to the right
- However, if a decrease in blood volume and an increase in vein compliance occurs the venous curve will shift to the left
How to identify the CO and VR using the graph?
It is the point where they intersect and the number on the X-axis below their intersection represents the right atrial pressure
What is the effect of positive and negative ionotrops on the cardiac function curve?
- Substances like digoxin a positive inotrope will shift the curve upwards and to the left, increasing the cardiac output and venous return, decreasing the right atrial pressure (less blood volume due to increased pumping), a negative inotrope will shift the curve downwards, and to the right decreasing the CO and VR while increasing the right atrial pressure (ore blood volume = more stretch = more pressure)
What is the effect of positive and negative ionotrops on the vascular function curve?
No effect, with no effect on the mean systemic pressure
Which function is affected by the blood volume?
Only the vascular function
What is the effect of changes in blood volume on the vascular curve?
1) Increased blood volume or Decreased compliance:
- The curve will shift upwards and to the right, increasing the mean systemic pressure
- Both the CO and right atrial pressure will increase
2) Decreased blood volume or increased compliance:
- The vascular function will shift toward the left and downwards, decreasing the mean systemic pressure, CO, and right atrial pressure
What is the effect of the total peripheral resistance on the vascular and cardiac function curves?
- Changes in the TPR changes both curves
1) Increased TPR
- Decreases the CO due to increased afterload, shifting the cardiac function curve downwards
- Less VR shifting the vascular function curve downwards, with no change in the mean systemic pressure
- The total effect of increased TPR is shifting the equilibrium point downwards
2) Decreased TPR
- CO will increase due to the decrease in the afterload shifting the cardiac function curve upwards
- Increased VR shifting the vascular function curve upwards without a change in the mean systemic pressure
- The end effect of a decreased TPR is the upward shift of the equilibrium point
What is the response of the CVS to the exercise?
1) Central Command
- Increased sympathetic and decreased parasympathetic
- Increased systolic arterial pressure (due to increased heart rate, contractility, and CO)
- Constriction of the splanchnic and renal arterioles
- Constriction of the veins, decreasing the unstressed volume, and increasing the VR
2) Local response
- Dilation of the arterioles of the skeletal muscles, decreasing the TPR
What are the changes that occurs in the CO and VR during exercise?
- Exercise will significantly increase the CO, due to the sympathetic stimulation which will lead to increased contractility and heart rate
- Exercise will also increase the venous return
What are the changes that occur in the CO and right arterial pressure in heart failure?
In a failing heart there are three outcomes:
The first thing that happens is that the cardiac function curve will go down and the CO is about 3L
1) Great increase in sympathetic activity, which will increase the contractility of the heart shifting the cardiac function curve upwards, it will also increase the VR which will shift the vascular function upwards to compensate
2) The greatly increased sympathetic activity will reduce however the body wants to maintain the CO which it will try to do so by retaining fluid shifting the cardiac function downwards but the vascular function towards the left and upwards
3) The sympathetic activity will come back to normal shifting the cardiac function downwards, but fluid retention will increase further, shifting the vascular function upwards and toward the right
- At the end of these compensatory mechanisms the CO will return to 5L but with an increase in arterial/venous pressure (mean systemic pressure)
- Edema might appear which might lead to pulmonary congestion “difficulty in breathing”, the patient might receive digoxin to maintain the CO and a diuretic for fluid retention
