Contractility And Cardiac Output Flashcards
What are the differences b/w skeletal and cardiac muscle?
In cardiac:
More influence of adrenergic receptor input
&
Relies more heavily on SR Ca levels and release
What are cardiac glycosides used to treat?
Mechanism of Action?
Tx heart failure
- inhibits Na/K ATPase binding K binding site
- increases Na concentration
- decreases Ca efflux thru Ca/Na exchanger
- increases Ca intracellular
What is the formula to calculate
Cardiac Output?
CO = HR x SV
What is stroke volume?
Relates partially to myocardial contractility but also to coupling factors (preload & afterload)
How do you exact changes in CO?
Related to length tension relationship
-generally proportional to amount of Ca that is available to Troponin on actin filaments of contractile apparatus
What is Preload?
Amount of blood ready to be pumped, diastole
Left Ventricle- End Diastolic VOlume
What is LV EDV?
Wall tension in LV just before contraction is initiated
Aka fiber length at end of diastole
What does more blood returning to the RV mean?
Greater preload
What will volume at EDV be related to>?
Related to venous return
CO(Q) = Venous REturn (Steady State)
What is the Frank Starling Relationship?
More blood that comes back to LV, the more that is going to get pumped
—> greater EDV = i am going to get pumped harder
(Volume of ejected blood depends on volume present in ventricle at end of diastole)
What is afterload?
Pressure required to eject blood (open aortic valve)
Essentially equal to aortic/pulmonary artery pressure
How is velocity of shortening affected by afterload?
Decreases as afterload increases
Greatest if afterload = 0
When is preload measured?
At end of isovolumetric contraction period
Remember = end diastolic fiber length
How does preload affect contractility?
Increase Preload
—> increases CO and contractility
How does afterload affect contractilikty?
Increase afterload
—> decreases CO
—> increased HR
-heart must increase contractility to overcome decreased CO
How do you calculate Stroke volume?
Normal value?
SV = EDV - ESV
~70 mL
What is the normal value for ESV?
~50 mL
What is stroke volume?
Volume of blood ejected by ventricle w/ each beat
How do you calculate Ejection fraction?
Normal value?
EF% = SV / EDV
55%
What is ejection fraction (EF%)?
Fraction of the EDV ejected in each stroke volume
-measure of efficiency and contractility
What is cardiac output?
Total volume of blood ejected by ventricle per minute
What is the normal value of CO?
5 L per minute
How does HR affect contractility?
Increased HR —> increases contractility (SV)
What is the Positive Staircase Effect (bowditch staircase)?
Auto regulation method by which myocardial tension increases w/ an increase in heart rate
-more stimuli sent to muscle
—> contractions show successive increase in amplitude
What is the positive staircase effect caused by?
More Ca enters cells and is taken up into SR
What is the sympathetic influence on CO?
Positive inotropic (SV) effect
-Beta AR activation
- Phosphorylates sarcolemma Ca channels
- Phosphorylates phospholamban (stimul.)
- Phosphorylates Troponin I (inhib.)
What is the parasympathetic influence on CO?
Negative inotropic effect IN ATRIA ONLY - affecting PHASE 4 (RMP)
(No influence on ventricular myocytes)
Muscarinic receptor activation
- Decreases inward ca current during plateau
- ACh increases outward k current
in a ventricular pressure-volume loop,
What does the right handed vertical line from point 1 —> point 2 represent?
Isovolumetric contraction
What is point 1 on a Ventricular Pressure-Volume Loop?
End of diastole = EDV = preload
- so pressure is low
- but volume would be high
What is Point 2 on a Ventricular Pressure-Volume loop?
Point where ejection of blood begins = when aortic valve opens = Afterload
So pressure is high (around ~70-80)
And volume is same as preload
What is Point 3 in a Ventricular Pressure VOlume Loop?
End of systole - closing of Aortic valve
ESV too
What does the line between points 2-3 represent?
Ventricular ejection
-pressure will reach a max. B/w these two points
How do you calculate SV using the Ventricular Pressure Volume loop?
SV = EDV - ESV (volume)
SV = Point 1/2 - point 3/4
What is point 4 in a Ventricular PRessure-VOlume Loop?
Represents the opening of the Mitral/tricuspid valve
What does the line b/w Points 3 and 4 represent?
Isovolumetric relaxation
-ventricular pressure falls quickly but volume (ESV) remains constant
What does the line b/w Points 4 and 1 represent?
Ventricular filling
How will the Ventricular Pressure-volume loop change w/ increased preload?
-more venous return, more blood volume
Points 1 and 2 move laterally
=greater EDV = GREATER Stroke volume
-afterload and contractility remain constant
How will the Ventricular Pressure-volume loop change w/ increased Afterload?
Point 2 moves up, points 3 and 4 move closer (graph becomes taller)
- greater pressure needed to open aortic valve
- reduced Stroke volume
- reduced ejection fraction
How will the Ventricular Pressure-volume loop change w/ increased contractility?
-points 3 and 4 will move laterally, point 2 up just a bit
- increased SV
- increased EF%
- less blood left in heart
What can cause increased afterload?
Due to aortic stenosis or hypertension
What can cause Increased Contractility
Adrenergic stimulation
What is volume work?
Cardiac ouput
What is pressure work?
Aortic pressure
What is Minute work re: Cardiac work?
CO x Aortic pressure
Or
Volume work X Pressure work
What is Stroke work?
SV x Aortic pressure
= area w/in the pressure volume loop
What performs stroke work?
Left ventricle
Is pressure work or volume work more costly?
Pressure work uses the largest percent of O2 consumption
Why does the LV work proportionally harder than the RV despite similar COs?
Bc systemic pressure is greater than pulmonary pressure and can be further accentuated by conditions that increase LV pressure work
I.e. aortic stenosis or systemic hypertension
What is the fick principle?
Measure of CO
O2 consumption / [(o2-pulm. V.) - (o2-pulm. A.)]
What is the relationship b/w CO adn Venous return?
At steady state, the volume of blood as cardiac output ejected by the LV equals the volume it receives in venous return
CO = Venous return
What are the parameters needed for CO to equal VR?
At equilibrium CO = VR
RA pressure = ~+2 mmHg
CO/VR = 5 L/min.
What does the vascular function curve tell us?
Can figure out the Mean Systemic Pressure (MSP) by where it crosses x-axis
(Bc there is no CO there and it completely depends on vascular compliance and blood volume)
How does Cardiac failure affect cardiac and vascular function?
Decreased CO and SV
-veins are normal- just heart is failing
-increased blood volume
But systemic vascular function changes to help restore CO
—> Decreased vascular compliance
—> Increased SVR/TRP
How does exercise affect cardiac function?
Decreased TPR
—> increased CO
—> increased VR
How does hemorrhage affect cardiac function?
Increased TPR
—> decreased CO
—> decreased VR
How does increased Blood volume affect cardiac function?
Increased BV = more for veins to return but heart still pumps out same amt.
= no change in CO
=increased VR
