control of cardiac output Flashcards
Frank-Starling law of heart allows heart to increase / decrease stroke volume as
volume of VR changes
Increased VR produces increased myocardial stretch which increases SV and HR
As metabolism changes –
SVR has to change
Increased metabolism – Decreased arteriole constriction - Increased tissue flow
Only way this works is if the body maintains a constant MAP
With nervous control, baroreceptors etc. kept
the MAP constant thus allowing CO to change appropriately with the change in SVR and metabolism
without nervous control
MAP decreases as SVR deceases with a small change in CO
Highest rate of metabolism from birth to about
10 years of age
≈ 4.0 L/min/m2
Decrease CI with age due to
decreasing muscle mass and overall decrease in activity
≈ 2.4 L/min/m2 @ 80 years
Normal Values
Healthy young male: CO = 5.6 L/minute Healthy young female: CO = 4.9 L/minute
Average for CO for resting adult: 5.0 L/min Average weight: 70 kg Average surface area: 1.7 m2 Average cardiac index: 3 L/min/m2
Levels of Cardiac Effectiveness
Normal Normal levels of nervous tone
Hypereffective Increased levels of CO
Caused by: Increased levels of nervous tone (sympathetic) Hypertrophy
Increased levels CO result from changes in contractility and HR
Hypoeffective Decreased levels of CO Variety of problems (see list pg 231 of Guyton)
max co normal
Normal conditions (normal ANS tone) RESTING VALUES: RAP = 0 mmHg CO = 5 L/min MAX VALUES: RAP ≥ 4 mmHg CO = 13 L/min
max co hypereffective heart (nervous)
MAX VALUES: RAP ≥ 4 mmHg CO = 25 L/min
max co Hypereffective heart (hypertrophy)
Increased muscle mass 50 to 75%
Increase max CO 60 to 100%
MAX VALUES: RAP ≥ 4 mmHg CO = 30-40 L/min
Max value represents summation of nervous response and hypertrophy
Factors That Alter External Pressure
Cyclic changes in respiration Breathing against negative pressure
Shifts curve to left Positive pressure breathing
Shifts curve to right Opening thoracic cage
Intrapleural pressure = 0 mmHg. Shifts curve to right 4 mmHg
Cardiac tamponade
increased cvp
Cardiac output
Venous return
More volume leaves the central venous compartment than enters so CVP begins to. Will continue until back at equilibrium
CVP driven to value where CO and VR are equal
CO & VR stabilize where two function curves intersect
Cardiac function curve can shift
Change in cardiac effectiveness
Change in external pressure
Venous function curve can shift
Change in mean systemic filling pressure
Change in venous resistance