questions 7

Cardiac Output and Variables

Topic updated on 10/27/17 9:46am

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)


Qbank (3 Questions)

(M1.CV.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: Topic Review Topic

1. Confusion and irritability
2. Decreased preload
3. Increased stroke volume
4. Decreased sarcomere length in the myocardium
5. Increased thromboxane A2

(M1.CV.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? Topic Review Topic

1. 30 mL/beat
2. 40 mL/beat
3. 50 mL/beat
4. 60 mL/beat
5. 70 mL/beat

(M1.CV.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? Topic Review Topic

1. Increased systemic vascular resistance
2. Decreased heart rate
3. Decreased stroke volume
4. Decreased systemic vascular resistance
5. Decreased blood pressure

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