2.05 - Cardiac Output & Starling's Law Flashcards
Define Cardiac Output (CO)
The ejected volume of blood per time
CO = SV•HR
What is the Cardiac Index?
Cardiac index is the Cardiac output normalised per unit body surface area
CI = (SV•HR)/BSA
What is Ejection Fraction?
EF is the ratio of SV to end-diastolic volume in %
EF = (SV/EDV)•100 = (0.08/0.12)•100 = 66.67%
What factors make up Stroke Volume?
Force on contraction: muscular properties. End-diastolic fibre lenght (preload). Contractility (force generation of cardiac fibre).
Afterload: circulatory properties. The pressure against which the heart has to pump (aortic pressure). Load in the heart is increased if the pressure it has to pump against is great.
What is the relationship between SV and HR
SV and HR rate are linearly related. As heart rate increases the time for ventricular filling decreases, this SV decreases linearly with increasing heart rate
At which heart rate is CO maximised?
CO is maximised at ~130bpm. Drops when higher. Above the optimal frequency, the increasing heart rate is insufficient to compensate for the decreasing stroke volume.
Thus, increasing heart rate too much is inefficient
What is preload? And why is it important?
Preload is the pressure (or volume) at the end of diastole. It is important because it sets the end-diastolic ventricular fibre length.
Describe the association between preload and stroke volume
EDV expands the ventricle to a particular volume and sets the cardiac fibre length. Put simply: **as preload increases, stroke volume increases. **
The stretching of the muscle fibers augments cardiacmuscle contraction by increasing the calcium sensitivity of the myofibrils, causing a greater number of actin-myosin cross-bridges to form within the muscle fibers. The force that any single cardiac muscle fiber generates is proportional to the initial sarcomere length (known aspreload), and the stretch on the individual fibers is related to the end-diastolic volume of the left and right ventricles.
What is a determinant of preload?
Compliance. If ventricular filling causes a small change in ventricular pressure, then the ventricle is comliant. Otherwise it is stiff and causes a much larger increase in pressure
How can contractility be modulated?
Caclium levels & Drugs can affect he magnitude of contractility
Decribe how calcium affect contractility
Contractility depends on intracellular calcium concentration for EC-coupling.
A higher intracellular calcium results in a larger isometric force
What is afterload?
Afterload is the pressure (or volume) at the end of systole (systole ends at the closure of the aortic valve)
It is essentialy the pressure against which the ventrile must contract to eject blood in to the aorta
How does contractility relate to fibre thickness?
Force increases with hypertrophy.
More contractile proteins (myofilaments) per myocyte produce a bigger force.
MCQ
Which of the following statements best describes the increased cardiac output that occurs with increased sympathetic stimulation of the heart?
Decreased heart rate and increased contractility
Decreased diastolic filling time and increased heart rate
Increased contractility and increased heart rate
Decreased ventricular relaxation and increased ejection fraction
Increased ventricular relaxation and decreased ejection fraction
C. Increased contractility and increased heart rate
