Cardiac Physiology – Control of Cardiac Output

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Cardiac Physiology Control of Cardiac Output
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Factors Controlling CO

• Four factors control CO heart rate, myocardial
  contractility, preload, and afterload.
• Heart rate and contractility are intrinsic
  factors, characteristics of cardiac tissues,
  influenced by neural and humoral mechanisms.
• Preload and afterload depend on the
  characteristics of both the heart and the
  vascular system.

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• Preload and CO relationships can be described in
  two curves cardiac function curve and vascular
  function curve.
• The cardiac function curve is a characteristic of
  the heart and is an expression of the
  Frank-Starling relationship
• The vascular function curve defines the
  dependence of the central venous pressure on the
  CO.

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The Vascular Function Curve Relates Central
Venous Pressure to CO

• The vascular function curve defines the changes
  in central venous pressure evoked by changes in
  CO (CVP is the dependant variable and CO is the
  independent variable).
• In contrast, with the cardiac function curve the
  CVP is the independent variable and the CO is the
  dependent variable.

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How to Derive the Vascular Function Curve

• Assume that the entire heart is a single pump.
• The high resistance microcirculation is the
  peripheral resistance (20 mmHg/L/min).
• Systen compliance is subdivided into the arterial
  compliance (Ca) and the venous compliance (Cv).
• Assume that the venous compliance is about 19
  times greater than the arterial compliance.
• Assume Pa 102 and Pv 2 mmHg

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• Induce cardiac arrest.
• Arteriovenous pressure gradient of 100 mmHg will
  force a flow of 5 L/min through the peripheral
  resistance of 20 mm Hg/L/min. (IV/R)
• Although CO is 0 L/min, the flow through the
  microcirculation transiently is 5 L/min.
• Gradually, the blood volume in the arteries
  progressively declines and the blood volume in
  the veins progressively increases until the
  pressure gradient is 0.

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• When the pressure gradient is zero, flow ceases
  through the microcirculatrion.
• At zero flow, arterial and venous pressure
  equalizes and the final pressure depends on the
  relative compliance of these vessels.
• Had the arterial and venous compliance been
  equal, the decline in Pa would have been equal to
  the rise in Pv.
• However, the veins are much more compliant than
  the arteries and the transfer of blood from
  arteries to veins at equilibrium would induce a
  fall in arterial pressure that is 19 X as great
  as the concomitant rise in venous pressure.

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• The final pressure in the circulatory system in
  the absence of flow is the mean circulatory
  pressure.
• From this, two important points on the vascular
  function curve have been derived.
• Figure A represents the normal operating system
  (CO 5 L/min, Pv 2 mmHg).
• Then when flow was stopped (CO0), Pv became 7
  mmHg.

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• Next, the arrested heart is suddenly restarted,
  and begins pumping at 1 L/min.
• Blood is being moved from the veins at a rate of
  1 L/min and the arterial volume is increasing at
  the same rate.
• Hence Pv begins to fall and Pa begins to rise.
• Because of the difference in compliance, Pa will
  rise 19 times faster than Pv will fall.
• This will continue until the pressure gradient
  becomes 20 mmHg. This gradient will force a flow
  of 1 L/min through a resistance of 20 mmHg/L/min.
  (VIR)

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• You may have noticed the straight line on the
  curve.
• At some critical maximal value of CO, sufficient
  fluid will be translocated from the venous to the
  arterial side of the circuit to reduce Pv below
  the ambient pressure.
• The vessels will collapse when the intravascular
  pressure falls below the extravascular pressure
  and obstruct venous return.
• Hence, in this case, there is a limit on the
  maximal value of CO to 7 L/min.

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Effect of Blood Volume on the Vascular Function
Curve

• The vascular function curve shifts to the right
  and left (no change in slope) with increases and
  decreases in blood volume.
• The maximal value of CO becomes progressively
  more limited as the total blood volume is
  reduced.
• This is why you cant drive up CO in a dry
  patient with pressors.

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Effect of Peripheral Resistance on the Vascular
Function Curve

• Increases or decreases in arteriolar tone do not
  significantly alter the mean circulatory
  pressure.
• Increased resistance moves more blood from the
  venous to arterial side and decreases the CVP for
  the same CO.
• The opposite occurs in vasodilation.

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The Heart and Vasculature are Coupled Functionally

• The intersection bewteen the vascular and cardiac
  function curve is homeostasis.
• It is the point that the system will return to
  after any perturbations.
• Consider a rise in Pv from the equilibrium point
  to point A.

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• The elevated Pv would increase CO (A to B) during
  the next systole (Frank Starling).
• The increased CO results in the transfer of blood
  from the venous to the arterial side of the
  circuit, with a consequent reduction in Pv (B to
  C).
• Because of this reduction in Pv, the CO during
  the next beat diminishes (C to D) by an amount
  dictated by the function curve.
• Because point D is still above the intersection
  point, the heart will pump blood from the veins
  to the arteries at a rate greater than that at
  which the blood will flow across the peripheral
  resistance from arteries to vein.
• This process will continue, in diminishing steps
  with each heartbeat, until the point of
  intersection is reached.

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• Myocardial Contractility
• Consider the equilibrium values for CO and Pv are
  designated by point A.
• Cardiac sympathetic nerve stimulation would
  abruptly raise CO to point B before Pv would
  change appreciably.
• However, this high CO would increase the net
  transfer of blood from the venous to the arterial
  side of the circuit.
• Consequently, Pv would begin to fall (point C).
• CO would continue to fall until a new equilibrium
  point (D) was reached.

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• Peripheral Resistance
• Predictions concerning the effects of changes in
  peripheral resistance are complex because both
  the cardiac and vascular function curves shift.
• With increased peripheral resistance, the
  vascular function curve is moves
  counterclockwise.
• The cardiac function curve is also shifted
  downward because (1) as peripheral resistance
  increases, arterial pressure tends to rise and
  (2) at any given Pv, the heart is able to pump
  less blood against a greater afterload.
• Whether point B will fall directly below point A
  or will lie to the right or left of point A
  depends on the magnitude of the shift in each
  curve.

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