Monday, March 10, 2008

Cardiac Output made EZ

A guaranteed subject that you will be tested on during your NCLEX or USMLE examination is basic physiology of the heart. It is so important that we spent 2 straight weeks on this subject at my medical school during our cardiology module. So before diving into those NCLEX and USMLE review books, check this out...

Cardiac Output

  • The cardiac output (CO) is the result of the stroke volume x HR/min
  • Stroke volume (SV) is never 100% of the volume in the ventricle at the end of diastole, but is usually an ejection fraction of 60-80%
  • The stroke volume and subsequent CO depends upon four parameters
  1. Preload (PL)• Amount of blood in the left ventricle at the end of diastole
    • It is determined by the compliance of the ventricle and the amount delivered by the venous system
    • Constriction of the venous system causes blood to be delivered faster, increasing preload
    • When preload is higher, the ventricles get stretched more: Think of a bow and arrow; the more you stretch the string, the more forceful the ejection of the arrow
    • Thus with more stretching (preload), you get a more forceful contraction, up until a critical point, when the heart is overstretched. Again think if you pull the bow and arrow string too far and it breaks; no ejection of the arrow


  2. Contractility
    • The ability of the heart to contract and the force at which it does so
    • The force of contraction is determined by how much Ca is stored in the SR
    • Contractility can be increased by flooding the cell with more Ca (beta agonist) or by keeping more Ca in the SR and not letting it escape outside of the cell (Digoxin)
    • Contractility can be decreased by the opposite effect on Ca or by damage to the myocytes (myocardial infarction)

  3. Afterload
    • Essentially equivalent to aortic back pressure that the heart must pump against
    • Afterload is actually controlled by the resistance in the capillaries
    • As afterload increases it takes longer for the pressure in the ventricle to become higher than aortic pressure, and only when this happens does the aortic valve open and allow blood to flow out (remember things always go from high to low pressure)
    • Therefore there is less time to expel blood during systole, so CO decreases with increased afterload

  4. Heart Rate
    • Remember CO = HR x SV
    • Do the math and see that an increase in HR also increases CO
    • Increased HR also increases amount of blood in the vasculature, which stretches the arteries resulting in an increased BP
    • This increase in CO continues to a point at about 140 bpm, when diastolic filling time gets too short and SV begins to decrease, causing an eventual decrease in CO
Summary:
• Think of a bucket with a faucet filling and a pump and hose draining
• The faucet = Preload or amount filling heart
• Pump = Heart contractility
• Hose = Diameter of hose is Afterload

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