Flashcards in Integration Deck (18)
Changes in CO can be....
Physiological (exercise/posture) or pathophysiological (HF, haemorrhage)
Why to we not just try increase the preload if someone has heart failure
Because although they need to raise their CO, they have ALREADY got volume overload.
SO what do we focus on in HF patients
-Blood volume (kidneys
What are the differing factors that impact CO?
Cardiac Factors : HR and Myocardial contractility
Coupling Factors: preload and afterload (dependent of vascular function)
CO = SV x HR
CO = change P x TPR
Although HR and SV do impact CO, we need to remember SV is dependent on venous return! Look at the bigger picture.
CO is more becuase of the tissue needs, instead of the
CO must match (within a beat or two) ...
Draw and explain CO curve
Three main determinants of Venous Return
1) Right atrial pressure (back pressure)
2) Degree of filling (blood volume and venous capacitance) ~ mean systemic filling pressure
3) Systemic vascular resistance
ie: the pressure gradient (Ps - Pv) and vascular resistance > "vascular function"
VR= Pa-Pv/ R
Draw Venous function curve (venous return)
-as we change venous return the right atrial pressure (preload) is affected.
As Right atrial pressure increases, venous return decreases, due to pressure gradient driving flow (Pa-Pv)
-So as MRAP falls, flow increases but the curve level off due to the large veins collapsing (inc R), so no further increase in VR
usually Pressure gradient =
PG = 102 (aortic) - 2 (venous)
this is required to ensure venous return is maintained!
What happens when CO= 0L/min (when right atrial P = 7mmHg)
Arterial Pressure drops
Venous Pressure rises due to large diference in compliance (19mmHg drop in arterial P = 1mmHg rise in venous P)
This is because veins are very compliant and already have lots of blood in them, can handle volume change BUT arteries are not and so small volume change > big pressure change.
Meet at 7mmHg "mean systemic filling pressure"
Main determinant of mean systemic filling pressure (Psf)
Blood volume (inc)
venous compliance (dec)
as MSFP increases, venous return increases at any given R atrial pressure
What would an increase in vascular/resistance do to venous return
Drops venous return curve as its harder to get blood back to the heart.
Still plateau at 0/-2 right atrial pressure.
An increase in blood volume leads to...
> increase in mean systemic filling pressure = increase in venous return > increased CO at a high right atrial pressure
What happens in heart failure?
Heart is unable to effective pump. This decrease in peripheral perfusion > synmpathetic activation and increase in fluid retention > increased MSFP. CO remains normal
So the patient will have a higher Right atrial pressure just to compensate /maintain CO
is the increase ing R atrial pressure/ MSFP a good thing
No as you have a higher workload on the heart!! Bad long term