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Overview
  • Cardiac output (CO) curve 
    • describes state of cardiac function
    • a plot of relationship between CO and right atrial pressure 
      • ↑ venous return → ↑ right atrial pressure → ↑ end diastolic volume (EDV), end-diastolic fiber length → ↑ CO
      • ↓ venous return → ↓ right atrial pressure → ↓ EDV, end-diastolic fiber length → ↓ CO
  • Vascular return curve
    • describes state of vascular function
    • a plot of inverse relationship between venous return and right atrial pressure
      • ↓ right atrial pressure → ↑ ΔP (systemic arteries, right atrium) → ↑ venous return
      • ↑ right atrial pressure → ↓ ΔP (systemic arteries, right atrium) → ↓ venous return
  • Mean systemic pressure
    • also known as mean circulatory pressure
    • x-intercept of vascular function curve
    • shows pressure that would be measured throughout cardiovascular system if heart were stopped
      • stopping the heart guarantees that pressure is equal throughout vasculature
Inotropic Effects
 
  • Inotropy describes the state of cardiac contractility
  • Positive inotropy (↑ contractility)
    • positive inotropic agent → ↑ cardiac contractility, ↑ stroke volume, and ↑ CO
      • e.g., sympathetic nervous system activity, and digitalis
    • CO curve shifts upward
    • new steady state or equilibrium → ↓ right atrial pressure (EDV)
      • ↓ right atrial pressure → more blood is ejected from heart on each beat as a consequence of increased contractility and increased stroke volume
  • Negative Inotropy (↓ contractility
    • negative inotropic agent → ↓ cardiac contractility, ↓ stroke volume, ↓ CO
      • e.g., congestive heart failure and narcotic overdose
    • CO curve shifts downward  
    • new steady state, or equilibrium → ↓ CO, ↑ right atrial pressure (EDV)
      • ↑ right atrial pressure → less blood is ejected from heart on each beat as a consequence of decreased contractility and decreased stroke volume
  • Effects of changes in blood volume
    • increases in blood volume
      • ↑ blood volume (e.g., transfusion) → ↓ venous compliance → ↑ mean systemic pressure
        • ↓ venous compliance → blood is shifted from veins to arteries
      • venous return curve shifts to right
      • new steady state or equilibrium → ↑ CO and ↑ right atrial pressure (EDV)
    • decreases in blood volume
      • ↓ blood volume (e.g., hemorrhage) → ↑ venous compliance → ↓ mean systemic pressure
        • ↑ venous compliance → blood is shifted from arteries to veins 
      • venous return curve shifts to left
      • new steady state or equilibrium → ↓ CO, ↓ right atrial pressure (EDV)
  • Effects of changes in total peripheral resistance (TPR) 
    • increase in TPR
      • vasoconstriction of arterioles → ↑ TPR
        • e.g., hemorrhage
      • ↑ TPR → ↑ mean arterial pressure (MAP) → ↑ afterload → ↓ CO
        • cardiac function curve shifts downward
      • ↑ TPR → ↓ venous return
        • venous return curve rotates counterclockwise
    • decrease in TPR
      • vasodilation of arterioles → ↓ TPR
        • e.g., exercise
      • ↓ TPR → ↓ MAP → ↓ afterload → ↑ CO
        • cardiac function curve shifts upward
      • ↓ TPR → ↑ venous return
        • venous return curve rotates clockwise
 

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