Chapter 20 CO, Venous return Flashcards
Define cardiac output
the quantity of blood pumped into the aorta each minute by the heart
Define venous return
The quantity of blood flowing from the veins into the right atrium each minute
List 4 factors that can affect cardiac output
- basic level of body metabolism
- whether the person is exercising
- the persons age
- the size of the body
Define cardiac index
the cardiac output per square meter of body surface area
What is the primary controller of cardiac output? the heart or venous return?
venous return
Frank-Starling law of the heart
when increased quantities of blood flow into the heart, the increased blood stretches the walls of the heart chambers, contracting with increased force
T/F: Stretching of the heart causes the heart to pump faster
true; by as much as 10-15 %
When PVR increases, CO ______
decreases
What are two factors that cause a hypereffective heart?
- Nervous stimulation
- Hypertrophy of the heart muscle
How much can nervous stimulation increase effectiveness of the heart?
Heart rate can increase from 72 bpm to 180-200 and the contractility can increase to twice its normal strength
Name some factors that can cause hypoeffectiveness of the heart
- increased atrial pressure
- inhibition of nervous excitation of the heart
- pathological factors
- coronary block
- Valvular heart disease
- congenital heart disease
- myocarditis
- cardiac hypoxia
How does exercise affect the arterioles?
the increased metabolism in active skeletal muscles acts on muscle arterioles to relax them to allow adequate oxygen and other nutrients needed to sustain muscle contraction
How does the nervous system compensate for dilated arterioles during exercise?
The autonomic nervous system excites circulatory activity , causing large vein constriction, increased HR, and increased contractility
What is the normal interpleural pressure?
-4 mmHg
What are 4 factors that can alter the external pressure on the heart?
- Cyclical changes of interpleural pressure during respiration
- Breathing against a negative pressure
- Positive pressure breathing
- Cardiac tamponaden.
What are the 3 principal factors that affect venous return to the heart?
- Right atrial pressure
- Mean systemic filling pressure
- Resistance to blood flow
What is the mean systemic filling pressure?
7 mmHg
What is the reason for the plateau in the normal venous return curve
Its caused by the collapse of the large veins entering the chest when the right atrial pressure falls below atmospheric pressure
Define mean systemic filling pressure (Psf) and what is the normal level?
the pressure measured everywhere in the systemic circulation after blood flow has been stopped by clamping the large blood vessels at the heart so pressures in the systemic circulation can be measured independently from those in the pulmonary circulation; nearly equal to mean circulatory filling pressure (7mmHg)
Formula to calculate venous return
Psf-PRA/RVR
PRA- right atrial pressure
RVR- resistance to venous return
Normal Venous return level?
5 mmHg
What are the 3 compensatory mechanisms for greatly increased cardiac output
- Capillary pressure increases so fluid begins to transude out of the capillaries into the tissues
- The veins distended gradually causing venous resevoirs
- Autoregulation occurs increasing PVR
What are the effects on cardiac output with complete spinal anesthesia?
Psf falls to about 4 mmhg and the effectivness of the heart as a pump decreases to 80% of normal. CO decreases to 60% of normal
What are the two ways to measure cardiac output?
- Ficks method
- indicator dilution method
