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7/21/2019 Relationships Between Cardiac Output And Venous Return
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RELATIONSHIPS BETWEEN CARDIACOUTPUT AND VENOUS RETURN
Prepared and presented byMarc Imhotep Cray, M.D.
Basic Medical Sciences andClinical Knowledge (CK) Teacher
Intended to be studies alongside companion notes of the same title .
From:
USMLE Step 1 CV Review Tools Cloud Folder
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Topics Discussed
2
o Cardiac Function Curve (CO)o Vascular Function Curve (VR)o Right Atrial Pressure (RAP)o Mean Systemic Pressure (MSP)o Slope of the Vascular Function Curve o Combining Cardiac and Vascular Function Curveso Cardiovascular parameters that can alter CO, VR
and RAP• Inotropy• Total Peripheral Resistance (TPR)• Blood Volume (BV)
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Cardiac and vascular function curves
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Source: Tao Le T and Bhushan V, Cardiovascular, IN First Aid for the USMLEStep 1 2013; McGraw·Hill 2013:255
1- Operating point of heart(cardiac output and venousreturn are equal)2-↑TPR , e.g., exercise, AVshunt
3-↓ TPR, e.g., hemorrhagebefore compensation canoccur4- As in heart failure, narcotic
overdose5- X-intercept of venousreturn curve = mean systemicfilling pressure
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Cardiac cycle
4
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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Cardiac function curve(or Cardiac output curve)
5
S V / C O / V R
LVEDV / RAP/Preload
Increase Inotropy
Decrease Inotropy
Normal | Reference Point
Is based on the Frank-Starling relationship for the leftventricle
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Cardiac and vascular function curves
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o The cardiac function curve iscardiac output as a function ofright atrial pressure.
o The vascular function curve isvenous return as a function ofright atrial pressure.
o The curves intersect at thesteady state operating point(filled circle) where cardiac
output and venous return areequal
o In steady state, volume of blood left ventricle ejects as cardiacoutput equals or matches volume it receives in venous return
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Vascular Function Curves(or Venous return curve)
o Plots show effect of central venouspressure on cardiac output• Central venous pressure equivalent
to pressure at right atria• Flow greatest at right atrial pressure
of 0 mm Hg, so greatest cardiacoutput
• If right atrial pressure increases,return is compromised and see less
cardiac outputo Mean systemic pressure is right atrial
pressure or central venous pressure atwhich cardiac output is zero
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Changes in Vascular Function Curves
o Total Peripheral Resistance• ↑ TPR causes counter-clockwise
shift in curve• ↓TPR causes clockwise shift in
curveo Blood Volume or shift in flow to
organs and tissues• ↑ blood volume or ↑ restriction
of flow to organs or tissues shiftscurve upward
• ↓ blood volume or ↑ flow toorgans or tissues previouslyrestricted shifts curve downward
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Combined Cardiac andVascular Function Curves
o Cardiac output or venousreturn on y-axis
o Right atrial pressure or end-diastolic pressure on x-axis
o Intersection of two curves isequilibrium or steady-statepoint:
• point at which system operates
o Changes in either cardiacoutput curve and/or vascularfunction curve will cause ashift in the equilibrium point
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Inotropic Agents
o
Positive inotropic agentspecific for the heart (e.g.digitalis) will shift cardiacoutput curve counterclockwise
o Negative inotropic agent (e.g.metoprolol) specific for heartwill shift cardiac output curve
clockwise.• similar shift for heart failure
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Effects of inotropic agents on the cardiacand vascular function curves
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o Effects of positive inotropic agents (A)o
Effect of negative inotropic agents (B)• The solid lines show the normal relationships, andthe dashed lines show changes
• The circle intersecting the dashed line shows thenew steady state operating point
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Total Peripheral Resistance• Increase in TPR shifts cardiac
output curve clockwise andvascular function curvecounter-clockwise
• Decrease in TPR shifts cardiacoutput curvecounterclockwise andvascular function curveclockwise
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Effects of increased total peripheral resistance(TPR) on the cardiac and vascular function curves
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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, andthe dashed lines show the changes
• The circle intersecting the dashed lines shows thenew steady state operating point
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Effect of changes in stressed volumeon mean systemic pressure (MSP)
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o Total blood volume is thesum of unstressed volume(in the veins) and stressedvolume (in the arteries)
o Increases in stressedvolume produce increasesin 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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Effects of blood volume on the cardiacand vascular function curves
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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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References and suggested reading :
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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. InCardiovascular 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
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
