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Venous Function

Venous Function. Function of the venous system Definitions Mean circulatory filling pressure Two compartment model Dynamic methods of assessing volume status. Main Points. The venous system functions to maintain filling of the heart. The main driving force for venous return is MCFP.

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Venous Function

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  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

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