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Cardiac output is controlled to maintain the proper amount of flow to tissues and to prevent undue stress on the heart.


Generally proportional to body surface area.

Cardiac Index (CI): Approximately 3 liters/min/m 2 of body surface area.

CI varies with age, peaking at around 8 years.


What goes into the heart comes out.

Increased heart volume stretches muscles and causes stronger contraction.
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Stretch increases heart rate as well.
Direct effect on sino-atrial node
Bainbridge reflex (through the brain)


Depends on venous return, which, in turn, depends on the rate of flow to the tissues.

Rate of flow to tissues depends on tissue needs (i.e. it depends on Total Peripheral
Resistance). Therefore, cardiac output is proportional to the energy requirements of the tissues.

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Normal CO – 5 L/min
Plateau – 13 L/min
Hypereffective heart plateau – 20 L/min
Hypoeffective heart plateau – 5 L/min


1.
2.
Effected by:
Nervous excitation.


Cardiac Hypertrophy
Exercise – Marathon runners may get 30 to
40 L/min
Aortic Valve Stenosis

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Valvular disease
Increased output pressure
Congential heart disease
Myocarditis
Cardiac anoxia
Toxicity


Causes increased cardiac output when vessels become dilated (dinitrophenol).

Causes venous constriction during exercise.

Disease States Lowering Total
Peripheral Resistance

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Beriberi: insufficient thiamine – tissues starve because they cannot use nutrients.
AV fistula: e.g. for dialysis.
Hyperthyroidism: Reduced resistance caused by increased metabolism
Anemia (lack of RBCs): effects viscosity and transport of O
2 to the tissues.

Disease States Lowering Cardiac
Output



Heart attack, valvular disease, myocarditis, cardiac tamponade, shock.
Shock: Nutritional deficiency of tissues.
Decreased venous return caused by:



Reduced blood volume
Venous dilitation (increased circulatory volume)
Venous obstruction

Changes in Intrapleural Pressure
Heart
15 L/min
CO
Pericardial
Sac tamponade


Generally shift the cardiac output curve in proportion to pressure change ( breathing
Valsalva maneuver ).
,
Cardiac Tamponade (filling of pericardial sac with fluid) lowers rate of change of CO with right atrial pressure
Rt. Atrial Pressure

Mean systemic filling pressure
Resistance to Flow
Pressure change is slight. Thus, small increase in RA
Pressure causes dramatic reduction in venous return.
(mean systemic filling pressure).
Right
Atrial
Pressure

5 L/min
VR
( CO )
Plateau: collapse of large veins
( => increased resistance)
Working Cardiac Output
Cardiac Output
Curve
Venous return with heart and lung removed.
-4
0
Rt. Atrial Pressure (mm Hg)
Mean systemic filling pressure ~ 7 mm Hg


Mean Circulatory: The pressure within the circulatory system when all flow is stopped
(e.g. by stopping the heart).

Mean Systemic: Pressure when flow is stopped by clamping large veins.

The two are close numerically.

Venous Return & Cardiac Output

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Cardiac output increases with atrial pressure.
Normal atrial pressure is about 0 mm Hg.
Venous return (with heart and lungs removed) decreases with atrial pressure.
Working cardiac output is where venous return curve meets cardiac output curve.

1.
2.
3.
4.
Compensation for Increased Blood
Volume
Increased CO increases capillary pressure, sending more fluid to tissues.
Vein volume increases
Pooling of blood in the liver and spleen
Increased peripheral resistance reduces cardiac output.

Effects of Sympathetic Stimulation

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Increases contractility of the heart.
Decreases volume by contracting the veins.
Increases filling pressure
Increases resistance



Shifts CO to the right
Shifts venous return down and to the left

- Reduced CO
5 L/min
VR
( CO )
Venous return with heart and lung removed.
-4
0
Rt. Atrial Pressure (mm Hg)

1.
2.
3.
4.
5.
6.
7.
Decreased VR resistance.
Slight increased CO because of reduced peripheral resistance.
After restoration of pressure (sympathetic)
Further CO increase.
Increased filling pressure.
Decreased kidney output (leads to higher fluid volume and more increase in CO).
Cardiac hypertrophy (caused by increased workload).

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Electromagnetic/ultrasonic (transit time) flow meter.
Oxygen Fick method:
CO = (Rate of O2 absorbed by lungs)
[O2] la
- [O2] rv
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Indicator dilution method:
Inject cold saline (or dye) into RA, measure temperature (or concentration) in aorta.