2Cardiac 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.
3Cardiac 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.
4Frank-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 nodeBainbridge reflex (through the brain)
5Cardiac OutputDepends 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.
6Limit of Cardiac Output Normal CO – 5 L/minPlateau – 13 L/minHypereffective heart plateau – 20 L/minHypoeffective heart plateau – 5 L/min
7Hypereffective Heart Effected by: Nervous excitation. Cardiac HypertrophyExercise – Marathon runners may get 30 to 40 L/minAortic Valve Stenosis
9Autonomic Nervous System Causes increased cardiac output when vessels become dilated (dinitrophenol).Causes venous constriction during exercise.
10Disease 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 metabolismAnemia (lack of RBCs): effects viscosity and transport of O2 to the tissues.
12Changes in Intrapleural Pressure Pericardial SacGenerally 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 pressureHeartRt. Atrial PressureCOtamponade15 L/min
13Determinants of Venous Return Mean systemic filling pressureRight Atrial PressureResistance to FlowPressure change is slight. Thus, small increase in RA Pressure causes dramatic reduction in venous return. (mean systemic filling pressure).
14Normal Venous Return Curve Plateau: collapse of large veins ( => increased resistance)Working Cardiac Output5 L/minVenous return with heart and lung removed.Cardiac Output CurveVR (CO)-4Mean systemic filling pressure ~ 7 mm HgRt. Atrial Pressure (mm Hg)
15Filling PressureMean 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.
16Venous 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.
17Compensation for Increased Blood Volume Increased CO increases capillary pressure, sending more fluid to tissues.Vein volume increasesPooling of blood in the liver and spleenIncreased peripheral resistance reduces cardiac output.
18Effects of Sympathetic Stimulation Increases contractility of the heart.Decreases volume by contracting the veins.Increases filling pressureIncreases resistance
19Effects of Sympathetic Inhibition Shifts CO to the rightShifts venous return down and to the left- Reduced COVenous return with heart and lung removed.5 L/minVR (CO)-4Rt. Atrial Pressure (mm Hg)
20Effects 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).
21Measurement of COElectromagnetic/ultrasonic (transit time) flow meter.Oxygen Fick method:CO = (Rate of O2 absorbed by lungs)[O2]la - [O2]rvIndicator dilution method:Inject cold saline (or dye) into RA, measure temperature (or concentration) in aorta.