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L11 Cardiac Output and Venous Return
Tuesday, April 21, 2015
5:00 PM
Cardiac output generally is the quantity of blood pumped into Aorta each minute
CO = SV x HR
MAP - Central Venous Pressure = CO x SVR
Venous return = quantity of blood flowing from veins into right atrium -> Venous return MUST equal CO
- With increased venous return, Right Atrial Pressure (RAP) increases
- Increased RAP -> CO increases
○ This is similar to Frank-Starling curve where RAP is Preload and CO is SV
○ This relationship can be mapped on a cardiac function curve
 Factors that IMPROVE pump function (aka higher CO curve)
□ Increase in Sympathetic and Decrease in Parasympathetic Activity
 Autonomic control of CO changes CO based on body requirements
◊ Sympathetic increases CO by increasing Contractility, HR,
Conductance, and vasoconstriction to increase venous return
◊ Parasympathetic decreases CO by decreasing conductance, HR
□ Physiologic hypertrophy
 Factors that IMPAIR pump function (lower CO curve)
□ Abnormal rate/rhythm
□ Coronary artery disease
□ Congenital heart disease
- Cardiac output in the Left heart = Cardiac output in Right heart (into pulmonary trunk)
○ Left ventricular EDV is important determinant of CO
○ LV EDV depends on venous return
○ Venous return determines right atrial pressure
○ THUS, CO is dependent on right atrial pressure -> Venous return supports CO while CO
creates Venous return
- Factors that influence CO/Venous return
Physio Page 1
Central Venous Pressure (CVP) = Blood pressure at Atrial-Vena caval junction
Many factors effect central venous pressure and venous return
- Venous pressure is much LOWER than Arterial pressure
○ Arterial to venous pressure gradient drives blood flow in body -> small increases in venous
pressure cause veins to swell but do not increase in resistance due to the high compliance of
 The High compliance of veins makes them an optimal choice for Coronary Artery
Bypass Grafting (CABG). For example, Great saphenous vein is put into place
backwards (so that the one way valves don’t stop flow) in order to bypass the blocked
coronary vessel.
- Sympathetic nerve activity can decrease compliance of veins
○ Sympathetic nerve activity increases tone/decreases diameter of veins => decreases
compliance => increases CVP
○ Also helps increase venous return => increased EDV => increases SV
- Body Position can affect blood pressure
○ When you are reclining/laying down, pressure on your heart is low and blood is flowing
"equally" everywhere
○ Due to gravity, when you stand, blood pools in inferior regions and Venous pressure goes
up, with lower venous regions of the body having a greater increase in pressure than more
superior regions
 Orthostatic hypertension occurs when standing in those who have impaired
○ Venous return is increased with activity because the skeletal muscles pump blood forward
while the one way valves prevent black flow
- Respiratory Function
○ As you inspire, a "vacuum" of air OPENS the vena cava more, thus decreasing Pressure and
increasing venous return
 This is important when you think about exercising as the increase in breathing rate
works cooperatively with the contraction of muscles to increase venous return
○ Valsalva Maneuver
 Is a method to check integrity of baroreceptor reflex
 Forcefully exhaling against closed glottis => increases intrathoracic pressure =>
decreases venous return => decreases CO and arterial pressure
 If baroreflex is intact, sympathetic response would activate and test would show
increase in HR
 Stopping the test would have a rebound increase in venous return, CO, and pressure
=> baroreflex directs a decrease in HR
- Summary of Factors impacting CVP
Physio Page 2
Physio Page 3