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

· Venous return physiology: curve, equation, components

· Factors affecting MSFP

· Factors affecting RAP

· Factors affecting RVR

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 1/3 due to arterial resistance

o 2/3 to venous resistance

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

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)

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

Arterial diameter

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

· Pneumoperitoneum -> ↑SVR -> ↑RVR

Venous diameter

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

· Pneumoperitoneum -> IVC compression -> ↑RVR