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Presentation on theme: "CARDIAC OUTPUT."— Presentation transcript:


2 Cardiac Output, Venous Return and their Regulation
Cardiac output is controlled to maintain the proper amount of flow to tissues and to prevent undue stress on the heart.

3 Cardiac Output Generally proportional to body surface area.
Cardiac Index (CI): Approximately 3 liters/min/m2 of body surface area. CI varies with age, peaking at around 8 years.

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

5 Cardiac Output 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.

6 Limit of Cardiac Output
Normal CO – 5 L/min Plateau – 13 L/min Hypereffective heart plateau – 20 L/min Hypoeffective heart plateau – 5 L/min

7 Hypereffective Heart Effected by: Nervous excitation.
Cardiac Hypertrophy Exercise – Marathon runners may get 30 to 40 L/min Aortic Valve Stenosis

8 Hypoeffective Heart Valvular disease Increased output pressure
Congential heart disease Myocarditis Cardiac anoxia Toxicity

9 Autonomic Nervous System
Causes increased cardiac output when vessels become dilated (dinitrophenol). Causes venous constriction during exercise.

10 Disease States Lowering Total Peripheral Resistance
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 O2 to the tissues.

11 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

12 Changes in Intrapleural Pressure
Pericardial Sac 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 Heart Rt. Atrial Pressure CO tamponade 15 L/min

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

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

15 Filling Pressure 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.

16 Venous Return & Cardiac Output
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.

17 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.

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

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

20 Effects of AV Fistula 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).

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


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