Cardiac Function Curve:

A cardiac function curve is a graph, which shows the relationship between the cardiac output (the Y-axis) and the right atrial pressure (the X-axis).

Shape of Cardiac Function Curve:

The shape of the cardiac function curve shows a steep relationship at relatively low filling pressures and a plateau, where further stretch is not possible and so increases in pressure have little effect on output. The pressures where there is a steep relationship lie within the normal range of right atrial pressure (RAP) found in the healthy human during life. This range is about -1 to +2 mmHg. The higher pressures normally occur only in disease, in conditions such as heart failure, where the heart is unable to pump forward all the blood returning to it and so the pressure builds up in the right atrium and the great veins. Swollen neck veins are often an indicator of this type of heart failure.
At low right atrial pressures this graph serves as a graphic demonstration of the Frank–Starling mechanism, that is as more blood is returned to the heart, more blood is pumped from it without extrinsic signals.

Changes in the cardiac function curve:

The extrinsic factors such as an increase in activity of the sympathetic nerves, and a decrease in vagal tone cause the heart to beat more frequently and more forcefully. This alters the cardiac function curve, shifting it upwards. This allows the heart to cope with the required cardiac output at a relatively low right atrial pressure. We get what is known as a family of cardiac function curves, as the heart rate increases before the plateau is reached, and without the RAP having to rise dramatically to stretch the heart more and get the Starling effect.
In vivo sympathetic outflow within the myocardium is probably best described by the time honored description of the sinoatrial tree branching out to Purkinges fibers. Parasympathetic inflow within the myocardium is probably best described by influence of the vagus nerve and spinal accessory ganglia.

Overview of Cardiac Function Curve:

Cardiac Function Curve

•  Cardiac Output Curve
  • describes state of cardiac function
  • a plot of relationship betweenCO and right atrial pressure
    • ↑ venous return → ↑ right atrial pressure → ↑ 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 betweenvenous 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 on Cardiac Function Curve:

• Inotropy describes the state of cardiac contractility
• Positive Inotropy (↑ Contractility)
  • positive inotropic agent → ↑ cardiac contractility, ↑ stroke volume, ↑ cardiac output
    • e.g., sympathetic nervous system activity, digitalis
  • cardiac output curve shifts upward
  • new steady state, or equilibrium → CO, ↓ 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, ↓ cardiac output
    • e.g., congestive heart failure, narcotic overdose
  • cardiac output 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, ↑ 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 → ↓ mean arterial pressure (MAP) → ↓ afterload → ↑ CO
      • cardiac function curve shifts upward
    • ↓ TPR → ↑ venous return
      • venous return curve rotates clockwise

–http://upload.medbullets.com/topic/8009/images/cv-functional-curve.jpg
–http://www.medbullets.com/step1-cardiovascular/8009/cardiac-vascular-function-curves
–http://en.wikipedia.org/wiki/Cardiac_function_curve

1. McConaghy JR, Oza RS. Outpatient diagnosis of acute chest pain in adults. Am Fam Physician. 2013 Feb 1;87(3):177-82. Review.PMID:23418761 (Link to Abstract)
2. Boateng S, Sanborn T. Acute myocardial infarction. Dis Mon. 2013 Mar;59(3):83-96. doi: 10.1016/j.disamonth.2012.12.004. Review. PMID:23410669 (Link to Abstract)

1. First Aid for the USMLE STEP 1. Le, Tao. New York: McGraw-Hill Medical, 2013.
2. Rapid Review Pathology Revised Reprint. Goljan, Edward. New York: Mosby, 2011.
3. Clinical Microbiology Made Ridiculously Simple 5th Ed. Gladwin, Mark. Miami: MedMaster Inc, 2011.
4. medEssentials for the USMLE Step 1. Manley, Michael. New York: Kaplan Publishing, 2012.
5. Katzung & Trevnor’s Pharmacology Examination and Board Review. Trevor, Anthony. New York: Lange Medical Books, 2012.