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1. RELATIONSHIPS BETWEEN CARDIAC
OUTPUT AND VENOUS RETURN
Intended to be studies alongside companion notes of the same title.
Prepared and presented by
Marc Imhotep Cray, M.D.
Basic Medical Sciences and
Clinical Knowledge (CK) Teacher
From:
USMLE Step 1 CV Review Tools Cloud Folder

2. Topics Discussed
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Cardiac Function Curve (CO)
Vascular Function Curve(VR)
Right Atrial Pressure (RAP)
Mean Systemic Pressure (MSP)
Slope of the Vascular Function Curve
Combining Cardiac and Vascular Function Curves
Cardiovascular parameters that can alter CO, VR
and RAP
• Inotropy
• Total Peripheral Resistance (TPR)
• Blood Volume (BV)
2

3. Cardiac and vascular function curves
Source: Tao Le T and Bhushan V, Cardiovascular, IN First Aid for the USMLE
Step 1 2013; McGraw·Hill 2013:255
1- Operating point of heart
(cardiac output and venous
return are equal)
2-↑TPR, e.g., exercise, AV
shunt
3-↓ TPR, e.g., hemorrhage
before compensation can
occur
4- As in heart failure, narcotic
overdose
5- X-intercept of venous
return curve = mean systemic
filling pressure
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4. Cardiac cycle
The mechanical and electrical events that
occur during one cycle are shown.
 Atrial systole (A);
 isovolumetric ventricular contraction (B);
 rapid ventricular ejection (C);
 reduced ventricular ejection (D);
 isovolumetric ventricular relaxation (E);
 rapid ventricular filling (F);
 reduced ventricular filling (diastasis) (G)
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5. Cardiac function curve
(or Cardiac output curve)
Is based on the Frank-Starling relationship for the left
ventricle
Increase Inotropy
SV/CO/VR
Normal | Reference Point
Decrease Inotropy
LVEDV / RAP/Preload
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6. Cardiac and vascular function curves
o The cardiac function curve is
cardiac output as a function of
right atrial pressure.
o The vascular function curve is
venous return as a function of
right atrial pressure.
o The curves intersect at the
steady state operating point
(filled circle) where cardiac
output and venous return are
equal
o In steady state, volume of blood left ventricle ejects as cardiac
output equals or matches volume it receives in venous return
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7. Vascular Function Curves
(or Venous return curve)
o Plots show effect of central venous
pressure on cardiac output
• Central venous pressure equivalent
to pressure at right atria
• Flow greatest at right atrial pressure
of 0 mm Hg, so greatest cardiac
output
• If right atrial pressure increases,
return is compromised and see less
cardiac output
o Mean systemic pressure is right atrial
pressure or central venous pressure at
which cardiac output is zero

8. Changes in Vascular Function Curves
o Total Peripheral Resistance
• ↑ TPR causes counter-clockwise
shift in curve
• ↓TPR causes clockwise shift in
curve
o Blood Volume or shift in flow to
organs and tissues
• ↑ blood volume or ↑ restriction
of flow to organs or tissues shifts
curve upward
• ↓ blood volume or ↑ flow to
organs or tissues previously
restricted shifts curve downward

9. Combined Cardiac and
Vascular Function Curves
o Cardiac output or venous
return on y-axis
o Right atrial pressure or enddiastolic pressure on x-axis
o Intersection of two curves is
equilibrium or steady-state
point:
• point at which system operates
o Changes in either cardiac
output curve and/or vascular
function curve will cause a
shift in the equilibrium point

10. Inotropic Agents
o Positive inotropic agent
specific for the heart (e.g.
digitalis) will shift cardiac
output curve counterclockwise
o Negative inotropic agent (e.g.
metoprolol) specific for heart
will shift cardiac output curve
clockwise.
• similar shift for heart failure

11. Effects of inotropic agents on the cardiac
and vascular function curves
o Effects of positive inotropic agents (A)
o Effect of negative inotropic agents (B)
• The solid lines show the normal relationships, and
the dashed lines show changes
• The circle intersecting the dashed line shows the
new steady state operating point
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12. Total Peripheral Resistance
• Increase in TPR shifts cardiac
output curve clockwise and
vascular function curve
counter-clockwise
• Decrease in TPR shifts cardiac
output curve
counterclockwise and
vascular function curve
clockwise

13. Effects of increased total peripheral resistance
(TPR) on the cardiac and vascular function curves
o Effects of increased (TPR) (A)
o Effects of decreased TPR (B) on the cardiac and vascular
function curves
• The solid lines show the normal relationships, and
the dashed lines show the changes
• The circle intersecting the dashed lines shows the
new steady state operating point
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14. Effect of changes in stressed volume
on mean systemic pressure (MSP)
o Total blood volume is the
sum of unstressed volume
(in the veins) and stressed
volume (in the arteries)
o Increases in stressed
volume produce increases
in mean systemic pressure
So, two factors influence the value for MSP:
(1) blood volume and
(2) The distribution of blood between the unstressed
volume and the stressed volume.
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15. Effects of blood volume on the cardiac
and vascular function curves
o Effects of increased blood volume (A)
o Effects decreased blood volume (B)
on the cardiac and vascular function curves
• The solid lines show the normal relationships, and the
dashed lines show changes
• The circle intersecting the dashed line shows the new
steady state operating point
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16. The End,
Thank you for your attention!!!
Also see companion PowerPoint: Cray MI, Physiologic and
Pathophysiologic Function of the Heart: Cardiac Cycle Graphs,
Curves, Loops and CO Calculations. Last updated-11-13
References and suggested reading :
Costanzo LS, Cardiovascular Physiology. In Physiology: with STUDENT
CONSULT Online Access, 5e; Saunders 2013:189-95
Klabunde RE, Ch. 4-Cardiac Function and Ch. 5-Vascular Function. In
Cardiovascular Physiology Concepts.2e; LLW 2011:60-120
Tao Le T and Bhushan V, Cardiovascular, In First Aid for the USMLE Step 1
2013; McGraw·Hill 2013:254-57
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