Heart as a pump Flashcards
Define Cardiac output, including equation and average CO at rest
volume of blood pumped per minute by left ventricle
CO= HR x Stroke volume
CO= arterial pressure/ total peripheral resistance
Avg CO = 4-6L/min
Define stroke volume
volume of blood pumped per beat
What is Cardiac index?
CO normalized to body size, measured as surface area in m2. Normal range is 2.6 to 4.2 L/min/m2
High HR alone leads to what?
Less time for filling = decreased stroke volume (in absence of other regulation)
What determines stroke volume?
strength of contraction of the heart, venous return (“preload”), and vascular resistance (“afterload”)
How is strength of contraction of heart controlled?
- Length-dependent intrinsic mechanism = Frank-Starling Law of the Heart
- Length-independent mechanism = Inotropy (or “contractility”), regulated via sympathetic nervous system stimulation
Can you draw the pressure volume changes during cardiac cycle? (Wiggers diagram)
See notes
What is the “a wave”?
In diastole: A bump in the atrial and ventricular pressure that occurs as the atrium begins to contract (atrial systole) and blood flows into the ventricle
What is the isovolumetric contraction phase of cardiac cycle?
Ventricle begins contracting but both the mitral valve and aortic valve are closed, but the blood has no where to go yet
What is the ejection phase?
Ventricle continues to contract, ventricular pressure exceeds that in the aorta, thus the aortic valve is pushed open and blood begins to flow
What is the isovolumetric relaxation phase?
The ventricle continues to relax with both valves closed, so the pressure falls rapidly
Know what the pressure volume curves look like for systolic and end diastolic
See notes
End diastolic volume is approximated by what?
preload (for left ventricle)
Explain the end diastolic pressure volume relationship
Pressure-volume relationship during filling, before contraction. Corresponds to passive elastic properties of ventricle (slope is inverse of compliance).
Explain the systolic pressure volume relationship
Pressure-volume relationship at peak of isometric contraction. Depends on the afterload (~aortic pressure)
How is EDPVR affected by decreased compliance?
EDPVR becomes steeper
What is active tension?
difference in force between peak systolic pressure and end diastolic pressure curves, i.e., tension developed by the contraction itself, independent of the preload.
What is a starling curve?
- aka ventricular function curve: Curve that represents active tension. -Y-axis: pressure, tension, cardiac output, or stroke volume
- X-axis: volume or length
- Has ascending and descending limb
What is Frank-Starling law of the heart?
INTRINSIC mechanism by which the heart adapts to changes in preload (in the normal physiological range).
a) Heart responds to an increase in EDV by increasing the force of contraction
b) Healthy heart always functions on the ascending limb of the ventricular function curve
.c) Cardiac output MUST equal venous return and cardiac output from left and right ventricles MUST match
What is the Bainbridge reflex?
Increased venous return stretches sinus node which increases heart rate
Be able to draw a PV loop diagram
See notes
Which points on the PV loop represent filling phase?
A-c
Which points on the PV loop represent isovolumetric contraction? What is the volume called?
C-D
End diastolic volume
Which points on the PV loop represent Ejection phase?
D-F
Which points on the PV loop represent Isovolumetric relaxation phase?
F-A
How Do you measure blood pressure on PV loop?
o End diastolic pressure at point D (~80 mm Hg)
o Peak systolic pressure at point E (~130 mm Hg)
o Difference = pulse pressure (~ 50 mm Hg)
What is stroke volume on PV loop?
SV = EDV - ESV SV= C-A
What is ejection fraction on PV loop?
o The fraction of the EDV ejected during systole.
o EF = SV/EDV = (EDV – ESV) / EDV
-EF= (C-A)/ C
normal= 50-70%
What is stroke work on PV loop?
o Stroke work = energy per beat (in Joules), corresponds to the area inside the PV loop diagram
o NOT the same for left & right sides of heart
What factors affect preload?
- blood volume
- filling pressure & time
- resistance to filling (right atrial pressure, AV valve stenosis ,Ventricular compliance)
What factors affect ventricular compliance?
Hypertrophy reduces compliance, Dilation increases compliance
how does increased EDV affect stroke volume?
increases it on the next beat
Factors that affect afterload?
Aortic pressure is the major determinant of afterload – hypertension increases afterload
aortic stenosis
how does increased afterload affect stroke volume?
decreases it on the next beat
How does ionotropy affect ventricular function?
Changes in inotropy describe new ventricular function (Starling) curves.
What affect does increased ionotropy have on stroke volume?
increases stroke volume on the next AND SUBSEQUENT beats
