Cardiac Performance Flashcards
What is the primary function of the heart?
To eject a sufficient amount of blood into the arterial system to maintain blood pressure that will assure adequate blood flow to all the peripheral tissues
How do you calculate stroke volume?
EDV-ESV
How do you calculate ejection fraction?
SV/EDV x 100
How do you calculate cardiac output?
SV x HR
How do you calculate Cardiac Index?
CO/SA
Where SA = body surface area
How do you calucalate Left Ventricular Stroke Work (LVSW)
SV x left ventricular pulse pressure
Do you need to be able to identify all the steps of heart contraction on a pressure volume loop?
Yes
The area inside the pressure/volume loop represents what?
The work done on that bolus of blood (LVSW)
It is literally SV x LVPP
How do you calculate the Left ventricular pulse pressure?
Systolic pressure- diastolic pressure
It’s a pulse pressure
What is the definition of preload?
Tension (stretch) in ventricular wall at end of diastole (proportional to EDV)
However, we can’t really measure that, so we use End Diastolic Volume in its place.
Preload= EDV
What determines resting fiber length?
Preload
As preload increases, what happens to stroke volume? Why?
Stroke volume increases due to the muscle being stretched to its optimal length and it ejects more blood
What law is this: the strength of the contraction of an isolated cardiac muscle fiber is a function of muscle fiber length (length-tension relationship)
Frank-Starling Law of the Heart
When venous return to the right heart increases, what happens to the output of the left heart?
It will also increase
Right and left ventricular outputs are thereby matched
With increased HR, what happens to ventricular filling time, and therefore, preload?
Ventricular filling time is reduced, so preload will be reduced.
With increased HR, ventricular fill time is reduced, and therefore preload is reduced. What effect does this have on Cardiac output in the ABSENCE of sympathetic stimulation?
Under 150bpm, CO is not affected since the decreased SV is compensated for by the increased HR.
Above 150bpm, CO decreases
Increased HR decreases ventricular filling time, and therefore decreases preload. How does this affect Cardaic Output in the PRESENCE of sympathetic stimulation?
Cardiac output can increase dramatically because SV is preserved due to increased contractility.
CO=SV x HR
What is this: the ability of ventricle to distend under pressure
Ventricular compliance
A highly compliant chamber will undergo a _____ change in volume with a small change in pressure
Large
In hypertrophy and ischemia, what happens to compliance?
Compliance is reduced
Low compliance has what effect on ventricular pressures and ventricular filling?
High ventricular pressures develop with just a small amount of volume added to the chamber
Filling is retarded (no offense intended with the use of that word)
What effect will decreasing intrathoracic pressure have on RV stroke volume?
Increased, which also increase LV preload
What effect does increased CVP (Central venous pressure) have on RV stroke volume?
Increased RV stroke volume= increased LV preload
If you increase the pressure gradient between the inside and outside of the ventricle (delta P), what will happen to RV filling?
(You increase delta P by either increasing CVP or decreasing intrathoracis pressures ex: inspiration0
Increased RV stroke volume = increased LV preload
Atrial systole (kick) has a ____ effect on EDV under normal circumstances?
Small
When does the atrial systole (kick) have a very important effect on preload?
When filling time is limited (ex: high heart rate)
Under normal conditions, what is the job of the pericardium?
Prevent the LV from being overexpanded
Also maintains alignment with major vessels
What happens to preload if the pericardium becomes Over filled with fluid?
The hydrostatic pressure on the external surface of the heart will reduce ventricular compliance which reduces EDV and SV (preload is decreased)
What is a clinical example of the pericardium being filled with fluid?
Cardiac tamponade
What is the definition of Afterload?
Resistance against which the ventricle contracts
(For example, in the LV, the major components are arterial pressure, peripheral vascular resistance, arterial wall compliance, mass of the column of blood in the aorta, and the viscosity of the blood)
What measurement do we use to estimate afterload?
Mean arterial pressure
DP + 1/3 PP
PP in the aorta btw
If you increase afterload, what happens to SV?
Decreased SV
With increased afterload, the aortic valve opens (sooner/later) and closes (sooner/later)
Opens later
Closes sooner
Causes increased ESV (more blood left in the heart at the end of systole) and a reduced SV
How does the heart compensate for increased afterload (which would normally cause increased ESV and decreased SV)
It is compensated for by the Frank starling mechanism. The Right heart doesn’t know that the LV is having trouble ejecting the blood into the aorta, so it will deliver a normal amount of blood to the LV, causing it to stretch, which allows the LV to generate more force. SV is restored to normal.
What effect does aortic stenosis have on afterload
Increased afterload
What effect does elevated arterial pressure (hypertension) have on afterload?
Increased afterload
How does the heart compensate for increased afterload?
It fills more during diastole=more stretch=bigger force of contraction
frank starling mechanism
What is the definition of contractility?
The ability of the heart to do work at any given fiber length
(Independent of preload and afterload)
What types of factors affect contractility?
ANS acitivty (mainly SNS)
Circulating hormones (ie. epinephrine)
Catecholamins increase Ca+ influx and availability = more cross bridges=more force
what is the primary contributor to afterload?
The pressure in the aorta
What does the area in the loop in a pressure/volume loop represent?
Ventricular stroke work
Why does the heart get bigger after a long period of hypertension?
To increase the frank starling method of compensation
-heart is bigger=larger volume of blood in ventricle=more forceful contraction
How does norepinephrine affect contractility?
It stimulate B1 receptors, which increases contractility, and increases the SV at a given EDV. This is an increase in ejection fraction
postive inotropic effect
With SNS stimulation, what happens to the length of systole?
It is shortened, so that time for diastole is not shorter, and the filling time is maintained
What effect will increasing contractility have on ejection fraction and ESV?
Ejection fraction will increase
ESV will decrease (less blood in ventricle at end of systole)
As contractility increases what is the effect on:
Ejection velocity
Stroke volume
ESV
Ejection velocity increases
Stroke volume increases
ESV decreases
TEST QUESTION*
How do we estimate contractility?
By estimating the ejection fraction using ultrasound/echocardiogram.
How do you calculate ejection fraction
SV/EDV x100
What is a normal ejection fraction?
Normal 50-75%
What is Treppe?
As HR increases, Ca++ accumulates, which increases contractility
Helps to maintain cardiac output at very fast HRs
Is maximum HR a function of age or training?
Age
As exercise intensity increases, HR increases linearly. Does SV also increase linearly?
No, it is linear at low intensities, but *plateaus at about 40-60% of VO2 max
Is the heart muscle aerobic or anaerobic?
Highly aerobic
Does the efficiency of heart muscle increase or decrease with exercise
Increases
What type of work increases during exercise: volume work (moving blood-preload) or pressure work (working against afterload)?
Volume work increases.
Volume work is energetically less costly than pressure work
What is Left Ventricular Stroke Work?
How do you calculate it in a lab situation vs in the clinic?
It is the work done by the left ventricle in one stroke
LVSW= SV x LVPP
Only IF you have the values of the pressures in the left ventricle, which would be difficult to do in the clinic.
In the clinic, you would estimate LVPP with MAP
LVSW= SV x MAP
(MAP = diastolic Pressure + 1/3 pulse pressure)
What is left ventricular minute work and how do you calculate it?
It is the amount of work done by the left ventricle in one minute
LVMW=LVSW x HR
BUT you might not always have the LVPP requreied to calculate LVSW, so you would estimate LVPP by using MAP
LVMW= SV x MAP x HR
(MAP= diastolic press + 1/3 Pulse pressure) (LVSW= SV x LVPP)
