2015A14 Describe the determinants of venous return and the effect general anaesthesia would have on these.

 

List:

·         Venous return physiology: curve, equation, components

·         Factors affecting MSFP

·         Factors affecting RAP

·         Factors affecting RVR

 

Venous return physiology:

Curve

Equation

·  Venous return = (mean systemic filling pressure – right atrial pressure) / resistance to venous return

·  Hence factors increasing venous return: ↑MSFP, ↓RAP, ↓RVR

·  Anaesthesia generally decreases venous return due to ↓MSFP

Definitions

·  Mean systemic filling pressure (MSFP):

o Pressure in the systemic circulation when heart is arrested and equilibrium reached

o Dependent upon a) systemic blood volume b) capacitance of the vasculature.

·  Right atrial pressure (RAP):

o Pressure in right atrium at end of diastole, at end expiration

·  Resistance to venous return (RVR):

o 2/3 due to arterial and arteriolar resistance, 1/3 to venous and venular resistance

 

(Note in vivo, venoconstriction increases MSFP more than it increases RVR)

 

Factors affecting MSFP:

Circulating volume

·  Blood loss -> ↓volume -> ↓MSFP

Venous tone

·  ↓SNS output -> venodilatation -> ↓MSFP (e.g. GA, neuraxial)

Posture

·  Trendelenburg -> ↓venous pooling -> ↑MSFP

·  Reverse Trendelenburg -> opposite

Skeletal muscle pump

·  Walking -> soleus/gastrocnemius activity -> compression of venous plexuses

·  Immobilisation, paralysis -> absence of pump

Venous valves

·  Venodilators -> +/- valve incompetence (e.g. GA, neuraxial)

 

Factors affecting RAP:

Cardiac function

·  RV contraction -> RA elongation -> ↓RAP (not a direct determinant)

·  GA -> ↓contractility -> ↑RAP

Intrathoracic pressure

·  Spont vent -> ↓intrathoracic pressure in inspiration -> ↓RAP

·  Positive pressure ventilation -> absence of respiratory pump and ↑RAP

 

Factors affecting RVR:

Arterial diameter

·  Vasodilators -> ↓RVR (e.g. GA, neuraxial)

·  Pneumoperitoneum -> ↑SVR -> ↑RVR

Venous diameter

·  Venodilators -> ↓RVR (e.g. GA, neuraxial)

·  Pneumoperitoneum -> IVC compression -> ↑RVR

 

 

 

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