cardiac output

Question 1

Describe the equation for cardiac output

Answer 1

Question 2

define Frank-Starling Law

Answer 2

• Force of ventricular contraction is a function of ventricular end-diastolic muscle length (related to Ca++ sensitivity)
• More volume returns to the ehart during diastole, the more can get expelled during systole

–> increase end diastolic volume, the heart must generate a larger stroke volume to accommodate the larger volume

–> Larger preload stretches the individual fibers, generating a larger tension (larger preload = larger end diastolic volume)

Question 3

describe the effects of Afterload

Answer 3

• Larger afterload requires less shortening for the same (pre)load
• In the ehart a larger afterload is related to a LARGER END SYSTOLIC VOLUME
• since stroke volume is the difference between end diastolic and end systolic volumes, STROKE VOLUME DECREASES WITH INCREASES IN AFTERLOAD

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Question 4

What are the effects of afterload and stroke volume with the introduction of Epinephrine

Answer 4

• Epinephrine (sympathetic activity) increases contractile copacity, WHICH CHANGES STARLING CURVE
• Increasing contratile efficiency (force generated) will INCREASE the STROKE VOLUME for a similar load (end diastolic volume)

–> Increase in contractility –> increase in stroke volume and NO CHANGE IN AFTERLOAD

Question 5

describe the regulatory mechanisms for cardiac output

Answer 5

• Cardiac parasympehtetic nerve activty (negative chronotropic)

–> DECREASE HR –> DECRESE CARDIAC OUTPUT

• Cardiac sympathetic nerve activity (positive chronotropic)

–> INCREASE HR –> INCREASE CARDIAC OUTPUT

–> INcrease contractility –> INCREASE CARDIAC OUTPUT

• Arterial Pressure (increase in arterial pressure/AFTERLOAD) –> DECREASE IN STROKE VOLUME –> INCREASE CARDIAC OUTPUT
• Increase in FILLING PRESSURE (increase preload) –> INCREASE STROKE VOLUME –> INCREASES CARDIAC OUTPUT

Question 6

describe the influence of blood volume on Preload

Answer 6

• Mean systolic filling pressure (MSFP) = to the x-intercept on the venous function curve and the equilibrium point between central venous pressure and peripheral venous pressure

–> point at which venous return will cease

–> incluence ONLY by changes in blood volume

• Increase in blood volume causes increase in preload causes incrase in stroke volume

Question 7

describe the influence of arteriolar tone on preload

Answer 7

• The Slope of the curve can be influenced by both volume (more blood will require larger rate of return than a smaller volume) AND ARTERIOLAR TONE (level of constriction/dilation in arteriole)

–> increased arteriolar tone (vasoconstriction) will SLOW THE RATE OF RETURN

–> Arteriolar dilation will allow MORE BLOOD TO FLOW into the venous circulation faster, thus INCREASEING THE RATE it will come back to Heart (increasing PRELOAD)

Question 8

describe the fick method

Answer 8

• Cardiac output (L/min) = (oxygen consumption)/(arterial - venous oxygen difference)

–> CO = 250 / (200-150) = 5L/min

• Requires a catheter into the heart to assess the differences

Question 9

Describe Indicator dilution

Answer 9

• An indicator injected upstream and thoroughly mixed within blood and measure continuously downstream –> area under curve is inversely proportional to flow (thermal dilution = indicator is cold saline)