Venous Function

1. Venous Function

2. Function of the venous system • Definitions • Mean circulatory filling pressure • Two compartment model • Dynamic methods of assessing volume status

3. Main Points • The venous system functions to maintain filling of the heart. • The main driving force for venous return is MCFP. • The splanchnic vascular bed is the reservoir for venous return. • CVP is useless for volume status unless it is at the extremes. Dynamic measures for fluid responsiveness is informative.

4. Definitions • Venous capacity • Blood volume contained in a vein at a specific distending pressure. • Venous capacitance • The relationship between contained volume and distending pressure in a vein. • Venous compliance • Change in volume of blood associated with a change in distending pressure.

5. Unstressed volume • A volume of blood in a vein at a transmural pressure = 0. • Stressed volume • The volume of blood in a vein above a zero transmural pressure. • The sum of stressed and unstressed volume is the total volume of the system.

6. Volume Capacity V1 Stressed Vu Unstressed 0 P1 Pressure

7. Stressed volume determines the MCFP and affects venous return and cardiac output. • Unstressed volume is a reserve that can be mobilized when needed. • It is helpful to think of the volumes as a tub.

8. Arterial flow Stressed volume Venous resistance CVP Unstressed volume

9. Function of the Venous System • To return blood to the heart and serve as capacitance to maintain filling. • Veins contain 70% of the blood volume and are 30 times more compliant than arteries. • Thus they are a reservoir that can easily and immediately change volume to maintain filling pressure in the right heart. • The splanchnic veins contain 20% of the total blood volume. • These are heavily populated with alpha1 and 2 receptors.

10. Mean Circulatory Filling Pressure • If you stop the heart, flow through the capillaries continues for a brief time as the low compliant/high pressure arteries decompress into the high compliant/low pressure veins. • Once the pressure equalizes throughout the entire system, the MCFP can be measured.

11. Mean Circulatory Filling Pressure • Flow to the heart is determined by the gradient between the central and peripheral venous pressure. • The driving force for venous return (VR) is: • (MCFP-CVP)/Venous resistance • CO is determined entirely by VR as the heart can’t pump more blood than it receives. • VR can go up by increasing MCFP or decreasing CVP (resistance is relatively small).

12. MCFP is determined by stressed volume and is normally around 7 – 12 mmHg while CVP is 2-3 mmHg. • So why does an increase in CVP (by bolus) increase CO in a normal heart? • The sudden increase in preload would increase SV temporarily but fall once the volume redistributes to the venous system. • The stressed volume increases and increases the MCFP greater than CVP. • The pressure gradient is thus increased and so VR goes up. • Increased VR = increased CO.

13. Volume Effect of Fluid Bolus V1 Stressed Vu Unstressed 0 P1 Pressure

14. While venous return can be increased by a fluid bolus which increases stressed volume which increases MCFP (think increasing the amount of fluid in the tub), it can also be increased by venoconstriction. • This decreases venous capacity (not compliance) which in turn decreases unstressed volume to the benefit of the stressed volume. • Think moving the outlet hole down.

15. Arterial flow Stressed volume Venous resistance CVP Unstressed volume

16. Volume Effect of Venoconstriction V1 Stressed Vu Unstressed 0 P1 Pressure

17. Two Compartment Model of the Venous System • It is helpful to think of the venous system as two connected compartments. • The splanchnic system is very compliant and slow flow while the non-splanchnic system is noncompliant and fast flow. • An increase in resistance in the arteries feeding the splanchnic veins decreases flow and shifts blood into the system circulation. • A decrease in resistance causes blood pooling in the veins.

18. Dynamic methods of assessing volume status • I think it goes without saying that the CVP is a less useful measure of volume status (fluid responsiveness) because of the many factors that influence it. • Abdominal pressure • Pump function • Pericardial pressure • Thoracic pressure • Dynamic methods are much more useful

19. On PPV, inspiration causes increased LVSV because of compression of pulmonary veins, decreased afterload and decreased RV volume from pulmonary compression. • The increased thoracic pressure at end inflation decreases the gradient for venous return at in a few beats causes a decreased LVSV. • This variation is exacerbated by hypovolemia. • Variation greater than 12 mmHg better reflects preload inadequacy than CVP.

20. How does that work? • Hypovolemia causes a fall in the total volume in the system. • The fall in capacity is partly compensated by an immediate reflex venoconstriction. • MCFP initially is preserved to maintain venous return.

21. Volume Venoconstriction in Response to a Fall in Total Volume V1 Stressed Vu Unstressed 0 P1 Pressure

22. Once the unstressed volume is completely mobilized into the stressed volume, further fall in the total body volume results in a fall in MCFP and therefore, venous return.

23. Volume Unstressed Volume Exhausted, Further Fall in Volume V1 V2 Stressed 0 P1 P2 Pressure

24. Recall that venous return is: • (MCFP-CVP)/Venous resistance • When the MCFP falls and the venous resistance rises, the normal variation in CVP causes a greater variation in venous return which translate into a greater variation in cardiac output/blood pressure. • Hence why dynamic changes are more reflective of volume status. • CVP normally varies and subject to external influences. • Dynamic changes allows us a look into the status of the stressed and unstressed volumes.

25. Main Points • The venous system functions to maintain filling of the heart. • The main driving force for venous return is MCFP. • The splanchnic vascular bed is the reservoir for venous return. • CVP is useless for volume status unless it is at the extremes. Dynamic measures for fluid responsiveness is informative.

26. Function of the venous system • Definitions • Mean circulatory filling pressure • Two compartment model • Dynamic methods of assessing volume status