Hemodynamics Flashcards
1. Calculate ejection fraction 2. Describe and explain preload, afterload and contractility 3. Illustrate the changes that occur in preload, afterload and contractility associated with heart failure
Cardiac output formula
HR x stroke volume
CO at rest, CO that can be reached with exercise
5 L/min, 25 L/min
Cardiac index
- clinically a more useful measure than CO -takes into account variations in CO related to size
- CI = CO/BSA
Regulation of CO
1) Heart rate
- sympathetic and parasympathetic NS
2) Stroke volume
- sympathetic NS
- frank-starling law
Frank starling law of the heart (3 principles)
1) heart muscle has intrinsic regulation (ability to regulate contractility independent of sympathetic of parasympathetic input)
2) the strength of contraction is related to the length of the muscle fibers prior to contraction (length-tension relationship)
3) The longer (more stretched) the cardiac muscle fiber is just prior to contraction the stronger the force of contraction (to a point)
What is the length of cardiac muscle a function on
-how filled the heart is at the end of diastole (EDV)
What is EDV a determinant of
EDP
What is the strength of contraction a determinant of
stroke volume ejected from the heart during systole
-total tension developed determines stroke volume
Length-tension relationship (i.e. starling curve)
Stroke volume (tension) increases up to a point with increase in LVEDV (length) -after peak increase sv fall with further increase in length
Heart failure
-by definition characterized by decreased CO and/or volume overload
Primary haemodynamic abnormality in patients with heart failure
A decrease in SV is the primary haemodynamic abnormality (so regulation of SV in heart failure is primary site therapeutic intervention)
Three determinants of stroke volume
1) Preload
2) Contractility
3) Afterload
What represent the preload
LVEDV
Heart failure effect on preload vs. stroke volume
Decreased sv with same preload
How contractiliy effects graph of SV vs. LVEDV
A change in stroke volume at any given EDV
- increased contractiliy = increased SV at any LVEDV
- decreased contractility = decreased SV at any LVEDV
normal ejection fraction
55-75%
Calculating Ejection Fraction
Ejection fraction = (EDV-ESV)/EDV X100
Definition of systolic dysfunction
LVEF < 40%
How to manipulate preload and contractility to help patient in hf
1) decreased preload - give diuretics
2) increased contractility -inotropic therapy
pressure volume loops
A to B : ventricular filling
B to C: isovolumetric contraction
C to D: ventricular ejection
D to A: isovolumetric relaxation
Effect of increasing preload on pressure volume loop, effect of increasing contractility on pressure volume loop
1) Increase distance A (ESV) – B (EDV) (i.e. SV) by increasing EDV (ESV stays the same)
2) Increase SV by decreasing ESV (EDV stays the same)
Afterload
The stress in the ventricular wall necessary to generate the pressure required to eject the stroke volume (approximates the aortic pressure)
Wall stress formula
Pxr/ 2xh
P = ventricular transmural pressure (the LDVEDP in most instances)
r = radius of ventricular chamber
h= wall thickness
Effect of increasing the afterload on pressure volume loop (2)
Decreases A- B distance (decreased SV) and raises LV pressure at which ventricular emptying occurs (C-D)
What determines strength of contraction
-intrinsic property of the myocardium (contractility) + the length-tension relationship (longer a muscle fiber the more tension will generate to a point)
