Updated: 3/18/2020

Cardiac Output and Variables

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Cardiac Output
  • Cardiac Output (CO) = Stroke Volume (SV) x Heart Rate (HR)
  • Oxygen delivery = CO x arterial oxygen content (CaO2)
  • Fick principle  
    • VO2 = (Ca-Cv) x CO
      • volume of oxygen consumed = tissue extraction of oxygen times CO
    • applies "conservation of mass" concept to oxygen consumption in body
    • CO = O2 consumption / [ (O2 in pulmonary vein) -   (O2 in pulmonary artery) ]
    • Key
      • the equation above refers to difference of oxygen content between the pulmonary vein (oxygenated blood) and pulmonary artery (deoxygenated blood)
      • when dealing with systemic circulation, one has to substract O2 in systemic veins (deoxygenated blood) from O2 in systemic arteries (oxygenated blood) to avoid negative values
  • Changes in CO
    • ↓ SV in ventricular tachycardia
      • if HR is too high, diastolic filling is incomplete and CO decreases
    • exercise 
      • CO maintained by SV in early stages of exercise
      • CO maintained by HR in late stages of exercise

Stroke Volume
  • Stroke Volume (SV) 
    • SV = CO/HR = End-Diastolic Volume (EDV) - End-Systolic Volume (ESV)
    • volume of blood ejected per heart beat
      • SV ~ 70 mL
  • Variables (assessed by echocardiogram)
    • EDV
      • volume of blood in ventricle before ejection
    • ESV
      • volume of blood in ventricle after ejection
  • Changes in SV 
    • SV in anxiety, exercise, pregnancy
    • ↓ SV in failing heart
  • "SV CAP"
    • Stroke Volume affected by Contractility, Afterload, Preload
      • SV via (1) contractility, (2) ↓ afterload, (3) preload
  • Contractility, ↑ SV via
    • catecholamines
      • activity of Ca2+ pump in SR
    • intracellular Ca2+
    • extracellular Na+
      • activity of Na+/Ca2+ exchanger
    • digitalis
      • intracellular Na+ intracellular Ca2+
  • Contractility, ↓ SV via
    • β1 blockade
      • cAMP
    • heart failure
      • systolic dysfunction
    • acidosis
    • hypoxia/hypercapnea
      • PO2/PCO2
    • non-dihydropyridine Ca2+ channel blockers
  • Myocardial O2 demand via
    • afterload (∝ arterial pressure)
    • contractility
    • HR
    • heart size aka wall tension

Ejection Fraction
  • Ejection Fraction (EF) = SV/EDV
    • fraction of EDV ejected per SV
    • an index of ventricular effectiveness
      • ↓ EF via systolic heart failure
    • an index of cardiac contractility
      • ↑ EF, ↑ contractility
    • EF ~ 0.55 or 55%

Pulse Pressure
  • Pulse Pressure (PP) = Systolic Pressure - Diastolic Pressure
    • PP reflects volume of blood ejected from left ventricle on a single beat
      • PP ∝ SV

Mean Arterial Pressure
  • Mean Arterial Pressure (MAP)
    • average pressure in a complete cardiac cycle
  • Equations
    • MAP = CO x Total Peripheral Resistance (TPR)
    • MAP = 2/3 Diastolic Pressure + 1/3 Systolic Pressure
    • MAP = 1/3 PP + Diastolic Pressure
    • as HR increases, diastole decreases and systole increases in the percentage of time spent, thus influencing the MAP

Preload and Afterload
  • Preload
    • aka ventricular EDV
      • preload "pumps up the heart"
    • preload by
      • exercise (slightly)
      • ↑ blood volume
        • e.g., over-transfusion
      • excitement
        • sympathetics
    • Pathology
      • right wall myocardial infarction (inferior wall)
        • ST elevation seen in leads II, III, and avF
        • preload dependent for cardiac function, thus do not diurese
  • Afterload
    • aka MAP
      • ∝ TPR
  • Pharmacology
    • venodilators (e.g., nitroglycerin) ↓ preload 
    • vasodilators (e.g., hydrAlAzine) ↓ Afterload (Arterial)
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(M1.CV.13.126) An 83-year-old male presents with dyspnea, orthopnea, and a chest radiograph demonstrating pulmonary edema. A diagnosis of congestive heart failure is considered. The following clinical measurements are obtained: 100 bpm heart rate, 0.2 mL O2/mL systemic blood arterial oxygen content, 0.1 mL O2/mL pulmonary arterial oxygen content, and 400 mL O2/min oxygen consumption. Using the above information, which of the following values represents this patient's cardiac stroke volume? Tested Concept

QID: 100642
1

30 mL/beat

0%

(0/28)

2

40 mL/beat

82%

(23/28)

3

50 mL/beat

11%

(3/28)

4

60 mL/beat

4%

(1/28)

5

70 mL/beat

4%

(1/28)

M 1 E

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(M1.CV.13.146) A 27-year-old man is running on the treadmill at his gym. His blood pressure prior to beginning his workout was 110/72. Which of the following changes in his cardiovascular system may be seen in this man now that he is exercising? Tested Concept

QID: 100662
1

Increased systemic vascular resistance

32%

(9/28)

2

Decreased heart rate

0%

(0/28)

3

Decreased stroke volume

0%

(0/28)

4

Decreased systemic vascular resistance

68%

(19/28)

5

Decreased blood pressure

0%

(0/28)

M 1 E

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(M1.CV.12.94) A 40-year-old Caucasian male presents to the emergency room after being shot in the arm in a hunting accident. His shirt is soaked through with blood. He has a blood pressure of 65/40, a heart rate of 122, and his skin is pale, cool to the touch, and moist. This patient is most likely experiencing all of the following EXCEPT: Tested Concept

QID: 100610
1

Confusion and irritability

7%

(2/30)

2

Decreased preload

0%

(0/30)

3

Increased stroke volume

73%

(22/30)

4

Decreased sarcomere length in the myocardium

3%

(1/30)

5

Increased thromboxane A2

13%

(4/30)

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