Lecture 13 - Integration of Cardiac and CV Function Flashcards
What is a Frank-Starling curve?
Preload vs measure of cardiac performance aka ventricular output (systolic pressure/stroke volume/stroke work)
What is another name for a Frank-Starling curve?
Cardiac function curve
What 2 factors would cause a left shift of the Frank-Starling curve?
- Increase in inotropy
2. Decrease in afterload
How can cardiac function be assessed?
- Frank-Starling curve
- Force-velocity relationship
- Pressure-volume loop
- Maximum dP/dt
- Ejection fraction
Describe the force-velocity relationship of the heart.
Velocity of shortening of cardiac muscle is inversely related to afterload aka force generated
When is the peak tension of cardiac muscle reached? What is this called?
At a velocity of shortening of 0
=> isometric contraction
How does increased contractility affect the force-velocity curve of the heart?
Shift to the right with a higher Vmax and a higher peak tension
What can we assume when the maximal velocity of shortening of cardiac muscle, Vmax, is increasing?
Increased contractility
How does increased preload affect the force-velocity curve of the heart?
Shift to the right with SAME Vmax and higher peak tension
What is a pressure-volume loop?
- X-axis = volume
- Y-axis = pressure
Reflects the dynamic relationship between LVP and LV volume over the course of the cardiac cycle:
- Diastole: volume and pressure increase
- Isovolumic contraction: volume is constant and pressure rises quickly
- Systole: volume decreases and pressure rises and then dips slightly
- Isometric relaxation: pressure drops quickly and volume is constant
What does a bar spanning the width of the pressure-volume loop represent?
SV
How does increasing preload affect the pressure-volume loop?
Width of the box (aka SV) gets wider by extending on the right side (increased filling, not ejection)
Where is afterload located on a pressure-volume loop?
Top right corner
How does afterload relate to the pressure of the LV during the cardiac cycle?
Afterload = arterial pressure at which the aortic valve opens and ejection begins
How does increasing afterload affect the pressure-volume loop?
- Afterload is higher = higher loop
- Stroke volume decreases because pushing up against a larger afterload = thinner loop by shortening on the left (decreased ejection, not filling)
=> taller, thinner curve
How does increasing contractility affect the pressure-volume loop?
- Afterload is higher = higher loop
2. Stroke volume increases by extending to the left (increased injection, not filling)
What is a maximum dP/dt curve? How does it assess cardiac function?
- X-axis: time
- Y-axis: LVP
=> dP/dt is a measure of contractility of the heart:
Higher contractility => steeper curve because the heart is able to generate pressure really quickly
What is dP/dt max for the LV? When does it occur?
Maximum instantaneous rate of change of LVP = maximum slope of the LV pressure curve during the cardiac cycle
Happens during isometric contraction
How can ejection fraction be measured?
Non-invasively by echocardiography
What is normal ejection fraction?
> 50%
What is the equation for ejection fraction?
SV/EDV
Describe the effect of gravity on venous pressure.
Gravity will help fill up veins as blood pools in your lower extremities (creating a high venous pressure), and the veins will distend as a function of the hydrostatic pressure/column of blood pushing down on them
What is the zero pressure point? When does it happen?
Point at which the venous blood is coming into the RA (below sternal angle) and depending on where the veins of your body are relative to that point, they are either drowning in blood or have none and will collapse at a point determined by the CVP
Happens when standing
How is BP affected by standing? How is this corrected? What is this called?
Upon standing, CO and arterial pressure fall as blood pools in the lower parts of the body (lower venous return aka preload) => baroreceptor reflex acts to maintain normal BP and CO by increasing HR and constricting ALL VEINS
=> Orthostatic hypotension
What can orthostatic hypotension result in under some conditions (e.g. dehydration)?
Vertigo and fainting
What is jugular venous distension? What is it a sign of?
Distended veins around the clavicle and neck
=> usually a sign of right heart failure
What does the compliance of veins refer to?
The ability of veins to stretch as they fill with blood = change in volume associated with a change in pressure
What does the volume vs distending pressure curve of arteries look like?
Very steep curve because their walls are much harder than veins and when you fill them just a little bit, pressure is generated against those walls immediately to push back on the liquid inside
What does the volume vs distending pressure curve of veins look like?
Slow rise in pressure as volume increases because the veins stretch to accommodate the larger fluid pressure
How many mL of blood need to be added to the artery for the pressure to go up 40 mmHg?
70 mL = SV to go from diastolic to systolic pressure
What are 2 mechanisms to keep the blood from pooling in our lower limbs?
- Musculoskeletal pump: venous return is augmented during exercise and movement by contraction of skeletal muscle by the constriction of veins that run longitudinally to skeletal muscle
- Venous valves in veins outside the central venous system maintain uni-directional blood flow back toward the heart
With what pathology is the musculoskeletal pump essential to proper venous return?
Varicose veins
Why is the jugular venous pulse relevant? How can it be visualized? Purpose?
Because it has the same a, v, and c waves as the atrial pressure curve because there is no valve between the veins and RA
It can be visualized by reclining a patient and using a catheter
Purpose: useful in the diagnosis of some forms of heart disease
What is CVP at rest?
2 mmHg
What happens to the pressure when the heart pump is stopped?
Arterial pressure drops and CVP “backs up” => the pressure equilibrates throughout to a “mean circulatory pressure” (MCP) of 7 mmHg
How can the CV system be conceptualized as a simple pump?
- Thorax = pump
- Arterial system
- Resistance point where most of the TPR lies
- Venous system
How does decreasing CO affect arterial pressure and CVP?
- Decrease in arterial pressure
2. Increase in CVP
What is a vascular function curve? What does it show?
- X-axis: CO
- Y-axis: CVP
As CO increases, CVP decreases
How does increasing CVP affect CO? What is this called?
Increases it because increased preload
=> Frank-Starling mechanism
What does the superimposition of the CO vs CVP and CVP vs CO graphs show?
They cross at the resting equilibrium point at CO = 5L/min and CVP = 2 mmHg
Why would straining vigorously during a bowel movement cause vertigo and faintness?
When one strains during a bowel movement or childbirth => raised thoracic pressure => reduced venous return to RA => decrease in CO => decrease in arterial pressure => lightheadedness if cerebral blood flow to the brain is diminished
How do sympathetic drugs affect the cardiac function curves?
Shift to the left and upward
How does heart failure affect the cardiac function curves?
Shift to the right and downward
How does hypervolemia affect vascular function curves?
Shift to the right
How does hypovolemia affect vascular function curves?
Shift to the left
What 2 factors would cause a right shift of the Frank-Starling curve?
- Decrease in inotropy
2. Increase in afterload
What point is important to note on the Frank-Starling curve?
The optimal filling point at which increases in preload will not cause a further increase in ventricular output
What factors decrease heart contractility/inotropy?
- Infarction
- Ischemia
- Heart failure
What could cause a decrease in afterload?
Arterial vasodilation
What is another name for a pressure volume loop?
Compliance curve
What is the central venous system?
Large veins around the heart
How does TPR affect the Frank-Starling and vascular function curves? (with CO and CVP) What about overall?
- Frank-Starling curve: decrease in TPR => shift to the left and up
- Vascular function curve: decrease in TPR => shift to the right
=> Overall: increase in equilibrium point
