The Cardiac Pump

1
The Cardiac Pump
2
Outline

• The Cardiac Cycle
• Work Output of the Heart
• Preload, Afterload and Contractility
• Regulation of Heart Function The Frank Starling
  Mechanism
• Measurement of Cardiac Output

3
The Cardiac Cycle
4
Ventricular Filling

• During systole, blood accumulates in the atria.
• At end systole, the higher pressure forces open
  the AV valves causing rapid ventricular filling.
• This lasts about 1/3.
• In the middle 1/3, there is minimal flow.
• In the last 1/3, the atria contracts to deliver
  up to 20 of the total ventricular volume.

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Isovolumic Contraction

• At the start of systole, the intraventricular
  pressure rises which closes the AV valves.
• For approximately 0.02 to 0.03 seconds, the
  pressure continues to rise but is less than that
  required to open the semilunar valves.
• This is called isovolumic contraction because the
  ventricular volume does not change.

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Ejection Period

• Once the semilunar valves open the ejection phase
  begins.
• About 70 of the total blood ejected occurs in
  the first 1/3.
• This is called the rapid ejection period
• The final 30 empties in the next 2/3 and is
  called the slow ejection period.

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Isovolumic Relaxation

• At end-systole, ventricular relaxation begins
  suddenly and causes intraventricular pressure to
  fall rapidly.
• The semilunar valves close once its pressure is
  greater than intraventricular pressure.
• For 0.03 0.06 seconds the muscle continues to
  relax, pressure continues to fall but no filling
  occurs because the AV valves are still closed.
• This is the period of isovolumic relaxation.

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Aortic Pressure Curve

• After the aortic valve opens, blood enters the
  aorta, stretching it and causes the pressure to
  rise to 120 mmHg.
• An incisura occurs just before the aortic valve
  closes from a short backward flow of blood.
• During diastole, the aortic pressure slowly falls
  as blood flows out to the venous side.

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Work Output of the Heart

• The stroke work output of the heart is the amount
  of energy converted to work per beat.
• Two forms of work output
• Volume pressure (external) work moving blood
  from the low pressure veins to high pressure
  arteries.
• Kinetic energy of blood flow accelerate the
  blood to its velocity of ejection.
• RV external work is 1/6 of the LV because of the
  six fold difference in systolic pressure.

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Work Output of the Heart

• Understand how the systolic and diastolic
  pressure curves are derived.
• By combining the end diastolic and systolic
  curves, the volume-pressure diagram can be
  defined.
• The area inside the VP diagram is the EW.

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Preload and Afterload

•  

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Preload

• Preload can be described as the stress
  experienced at end-diastole
• Preload(EDP x EDR)/2w
• Thus, preload represents all the factors that
  contribute to passive ventricular wall stress
  (or tension) at end diastole.
• This means that EDP (P) or EDV (R) contribute to,
  be should not be equated to preload.

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Afterload

• Laplaces Law can be used to describe afterload
  as ventricular stress during systolic ejection.
• Therefore, stressTP x R/2w
• Afterload represents all the factors that
  contribute to total myocardial wall stress (or
  tension) during systolic ejection.
• Arterial pressure and TPR contribute to afterload
  but should not be equated with afterload.

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Preload and Afterload

• Focusing on wall stress is important
• Metabolic cost is related to the wall tension
• The greater the tension, the greater the oxygen
  demand.
• Physiological and therapeutic regimens reduce
  wall stress and restore oxygen supply and demand.
• The relationship among P, R and w provides a
  clear physiological explanation for the different
  patterns of hypertrophy and remodelling.

15
Contractility

• Contractility is the peak isometric force
  generated at a given preload and afterload.
• A increase in contractility causes incremental
  increases in developed force and velocity of
  contraction.
• Results from different degrees of binding between
  myosin and actin filaments.
• This is dependant on the intracellular calcium
  concentration.

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Frank-Starling Mechanism

• The amount of blood pumped by the heart is
  determined by the rate of blood flow from the
  veins (venous return).
• The intrinsic ability of the heart to adapt to
  increasing volumes of blood is the Frank-Starling
  mechanism.
• With the extra delivery of blood, the cardiac
  muscle contracts with greater force because of
  improved actin/myosin interaction.

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Frank-Starling Mechanism

• The ventricular function curve is a way of
  expressing the Frank-Starling mechanism.
• Increases in atrial pressure causes an increase
  volume and strength of contraction which causes
  an increase in cardiac output.

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Indicator dilution techniques

• Suppose blood flow is Q (ml/s) and q mg of dye is
  injected.
• If the concentration of dye is continually
  measured farther downstream, a curve of the dye
  concentration, c, is recorded as a function of
  time, t.
• The amount of dye at point B between the time t1
  and t2 will be q cQ(t2-t1).
• Therefore, Q q/(t2-t1)c

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Indicator dilution techniques

• c is properly defined as an integral with limits
  of t1 to t2.
• Clinically, we use the temperature as the
  indicator instead of a dye.
• Therefore, we can adjust the equation to
• What would the curve look like in a high cardiac
  output state? Low? What is the effect of
  tricuspid regurgitation?

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To Review

• The Cardiac Cycle
• Work Output of the Heart
• Preload and Afterload and Contractility
• Regulation of Heart Function The Frank Starling
  Mechanism
• Measurement of Cardiac Output

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Questions?
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• Wall Stress
• An increase in wall stress achieved by either
  increasesd LV size or intraventricular pressure
  will increase myocardial oxygen uptake.
• This is because a greater rate of ATP use is
  required as the myofibrils develop greater
  tension.
• Wall Stress, Preload and Afterload
• Preload can now be defined as the wall stress at
  the end of diastole and therefore at the resting
  maximal resting length of the sarcomere.
• Afterload, being the load on the contracting
  myocardium, is also the wall stress during LV
  ejection.

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• Peak systolic wall stress reflects the three
  major components of the afterload-peripheral
  resistance, arterial compliance, and peak
  intraventricular pressure.
• Preload
• The stretch of the individual sarcomere regulates
  the performance of the heart.
• Afterload
• This is the force against which muscle contracts.
• Contractility
• This is the intrinsic ability of the heart muscle
  to generate force and to shorten. It is manifest
  as the rate of pressure development and
  shortening from any preload.

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• Ventricular Function Curve
• The dependancy of stroke volume on preload was
  described more than 100 years ago by Otto Frank
  and E.H. Starling and since then has been called
  the Frank-Starling mechanism. Using this
  relationship between preload and stroke volume or
  stroke work, a ventricular function curve can be
  consructed by plotting stroke work at various
  levels of preload.