Lecture 16: Ventricular function Flashcards
What happens to ventricular volume at the VE/IVR time period and why? (wiggers diagram)
Ventricular volume becomes negative as their is negative flow (backwards) because E aorta is less than E ventricle. (some back flow)
How can the phases filling and ejection be divided in the wiggers diagram?
Into rapid and reduced time periods.
What is the general difference between left and right ventricles in terms of pressures and valve sequences?
Pressure is lower in the right ventricle therefore timing is slightly different.
Describe the timing of the mitral and tricuspid valves
The tricuspid valve opens before the mitral
The tricuspid valve closes after the mitral
i.e LV has less filling time
Describe the timing of the aortic and pulmonary valve
The pulmonary valve opens before the aortic valve
The pulmonary valve closes after the aortic
i.e LV has less ejection time
Why do the AV and semilunar valves have different timings between the left and right sides of the heart?
RV valves open sooner and close later because:
- Differences in electrical activation and pressures (takes less time to generate pressure in the RV.)
Describe the pressure of the RA
~3mmHg
Describe RV pressures
(s) 18mmHg
d) 0mmHg (i.v.r
Describe PA pressures
(s) 18mmHg
(d) 12mmHg
Describe PA wedge pressures (cap)
8mmHg average
Describe LA pressures
8 mmHg average
Describe LV pressures
(s) 130 mmHg
(d) 0 mmHg
Describe the systemic aortic pressure
(s) 130 mmHg
(d) 75 mmHg
Whats the components of CO?
CO = HR x SV
What are the determinants of SV?
- Preload
- Afterload
- Inotropy
How does HR affect CO?
HR directly influences CO and also indirectly acts via the determinants of SV particularly preload and inotropic state.
Describe the main factors of CO;
CO = SV x HR
SV = EDV - ESV
EDV influenced by Preload
ESV influenced by Afterload and Inotropic state
HR influences preload and Inotropic state
How does HR influence preload?
Reduced preload by reducing the time of ventricular filling therefore redoing SV
How does HR influence inotropic state?
HR relates to the force frequency relationship that plays a minor role in ionotropy.
What are the relavant graphs for ventricular performance?
PV loop Ejection Fraction Peak dP/dT Ventricular function curve CO/VR MRAP curve a.k.a guyton VP graph
What does the linear line on the P-V graph represent?
Potential pressure of the chamber
This is never achieved as the valves open once threshold pressure is reached
Potential pressure increases linearly with and represents what relationship?
EDV
This clearly reflects the force-sarcomere length relationship.
When is the length tension relationship for sarcomeres exhibited?
During isometric or fixed length muscle contraction
Why can sarcomeres generate varying levels of tension at a fixed resting length?
Inotropic state accounts for this and means that sarcomeres can generate more force from an unchanged resting length
What can be obtained from a P-V loop?
The ventricular work performed each cardiac cycle (stroke work)
SW = SV x TPR (P xV)
What are the four determinants of cardiac performance?
Preload
Afterload
Inotropic state
Chronotropic state
What is preload?
The tension produced by the degree of stretch of myocyte filaments
- L-T relationship
- Length dependant activation
(a) - Increased stretch increases troponin C sensitivity to Ca
(b) - Increased stretch increases the activation of stretch sensitive Ca channels
What is Afterload?
The pressure of which the ventricle must pump against
Increased in hypertension.
What is inotropic state?
The degree of activation of contractile proteins by Ca in the myocyte sarcomere.
What influences inotropy?
- AP plateau phase (excitation contraction coupling)
- External ion gradient (NCX and Na/K ATP ase transporters)
- Force frequency relationship
- ANS
- Drugs
\ Caffiene, inhibits PDE prevents cAMP breakdown
\ Digoxin, cardiac glycoside inhibits Na/K ATPase, NCX reverses
\ Verapamil, Ca channel inhibitor - Heart failure
How does chonrotropic state influence CO?
- HR increases, CO increases
- Force frequency relationship
- High HR reduces SV, decreasing CO
What does the equilibrium between CO and VR represent on the venous return/ CO curve?
This is the steady state equilibrium at which the heart works with some transient deviations.
The MRAP at this point = mean systemic filling pressure
What are normal values for Ejection Fraction?
Rest = ~50% Exercise = ~85%
What are the two types of ventricular function curves?
Stroke volume vs preload
Stroke Work vs preload
Why is stroke volume vs preload not used?
Stroke volume is difficult to measure and not independent of afterload
Why is stroke work vs preload good?
Not susceptible to changes in after load
LV EDP is used instead of EDV as it is easier to measure
If MAP increases, SV decreases keeping SW constant.
What does a ventricular function curve represent?
Summery of the P-V and frank-starling relations
A family of curves on the ventricular function curve represents?
Different inotropic states
What cardiac factors does a ventricular function curve account for?
Preload
After load
Inotropic state
Describe how LA pressure is obtained?
A fluid filled catheter with a ballon tip is passed through the femoral vein via the heart into the PA. Ballon inflates and measures the pre-capillary pressure which is representative of the LA pressure mostly…
This can measure both pulmonary artery pressure and pulmonary wedge pressure.
Describe ventricular diastole:
- Begins with closure of the semilunar valves
- Followed by isovolumetric relaxation (ends when atrial pressure exceeds that in the ventricle)
- AV valves open and rapid ventricular filling occurs
- This is followed by a phase of slow filling (Diastasis)
- Atrial systole terminates ventricular diastole and the cycle begins again
Generically describe the phases of the cardiac cycle:
1) Passive ventricular filling (diastole)
2) Isovolumic contraction (systole)
3) Ejection (Systole)
4) Isovolumic relaxation (diastole)
What can be used as a measure of preload?
EDP
Whats important regarding the heart when it comes to afterload?
Fibre tension and chamber geometry (law of laplace)
Describe the law of laplace:
Wall stress (tension, T) = P*r / 2 x wall thickness
Regarding the law of laplace what happens with increased preload?
- Increased preload = Inc. vol
= Decreased wall thickness
= Increased ventricular radius
= Increased wall tension to generate the same pressure
Regardin the law of laplace what happens with increased afterload?
Increased pressure needed thus the wall stress needs to be produced.
What does increased inotropy do to the PV loop?
Increases the max potential pressure that can be generated thus the steepness increases.
Check lecture PV loops
Now and add to this
Whats the implication of the frank starling mechanism with two sides of the heart:
System is closed
- Both sides must pump same volume over time
- Hence changes must effect both L and R
- Increased afterload leads to increased preload
Arrhythmias impact balance
Describe the the P-V relationship of the heart in response to increased afterload over one cardiac cycle;
Increased afterload in Aorta =
- Decreased LV SV
- The RV has continued pumping into pulmonary circuit, increased stretch of capacitence circuit increases pressure of flow into LV
- LV EDV increases (preload)
- SV increases
Refer to diagram if need be
note if EDV increases, the ventricle will not contract down to the same volume even if SV is increased
How does dP/dT assess ventricular performance?
Max dP/dT used as indicator of contracility
*Negative during relaxation
What is stroke work? and its formula
The work done by the heart to eject blood
SW = P x dV/dt
Estimated by MAPxSV
Stroke work is the area inside the PV loop, bottom wedge is the passive filling + Atrial contraction + inertia of ventricular filling (but is negligible)
