Venous FunctionVenous Function

Function of the venous systemFunction of the venous system DefinitionsDefinitions Mean circulatory filling pressureMean circulatory filling pressure Two compartment modelTwo compartment model Dynamic methods of assessing Dynamic methods of assessing

volume statusvolume status

Main PointsMain Points The venous system functions to The venous system functions to

maintain filling of the heart.maintain filling of the heart. The main driving force for venous The main driving force for venous

return is MCFP.return is MCFP. The splanchnic vascular bed is the The splanchnic vascular bed is the

reservoir for venous return.reservoir for venous return. CVP is useless for volume status CVP is useless for volume status

unless it is at the extremes. Dynamic unless it is at the extremes. Dynamic measures for fluid responsiveness is measures for fluid responsiveness is informative.informative.

DefinitionsDefinitions Venous capacityVenous capacity

Blood volume contained in a vein at a Blood volume contained in a vein at a specific distending pressure.specific distending pressure.

Venous capacitanceVenous capacitance The relationship between contained The relationship between contained

volume and distending pressure in a volume and distending pressure in a vein.vein.

Venous complianceVenous compliance Change in volume of blood associated Change in volume of blood associated

with a change in distending pressure.with a change in distending pressure.

Unstressed volumeUnstressed volume A volume of blood in a vein at a A volume of blood in a vein at a

transmural pressure = 0.transmural pressure = 0. Stressed volumeStressed volume

The volume of blood in a vein above a The volume of blood in a vein above a zero transmural pressure.zero transmural pressure.

The sum of stressed and unstressed The sum of stressed and unstressed volume is the total volume of the volume is the total volume of the system.system.

Pressure

Volume

0 P1

V1

Unstressed

Stressed

Vu

Capacity

Stressed volume determines the Stressed volume determines the MCFP and affects venous return and MCFP and affects venous return and cardiac output.cardiac output.

Unstressed volume is a reserve that Unstressed volume is a reserve that can be mobilized when needed.can be mobilized when needed.

It is helpful to think of the volumes It is helpful to think of the volumes as a tub.as a tub.

Stressed volume

Unstressed volume

Arterial flow

Venousresistance

CVP

Function of the Venous Function of the Venous SystemSystem

To return blood to the heart and serve as To return blood to the heart and serve as capacitance to maintain filling.capacitance to maintain filling.

Veins contain 70% of the blood volume and Veins contain 70% of the blood volume and are 30 times more compliant than arteries.are 30 times more compliant than arteries.

Thus they are a reservoir that can easily Thus they are a reservoir that can easily and immediately change volume to maintain and immediately change volume to maintain filling pressure in the right heart.filling pressure in the right heart.

The splanchnic veins contain 20% of the The splanchnic veins contain 20% of the total blood volume. total blood volume.

These are heavily populated with alpha1 These are heavily populated with alpha1 and 2 receptors.and 2 receptors.

Mean Circulatory Filling Mean Circulatory Filling PressurePressure

If you stop the heart, flow through If you stop the heart, flow through the capillaries continues for a brief the capillaries continues for a brief time as the low compliant/high time as the low compliant/high pressure arteries decompress into the pressure arteries decompress into the high compliant/low pressure veins.high compliant/low pressure veins.

Once the pressure equalizes Once the pressure equalizes throughout the entire system, the throughout the entire system, the MCFP can be measured.MCFP can be measured.

Mean Circulatory Filling Mean Circulatory Filling PressurePressure

Flow to the heart is determined by the Flow to the heart is determined by the gradient between the central and peripheral gradient between the central and peripheral venous pressure.venous pressure.

The driving force for venous return (VR) is:The driving force for venous return (VR) is: (MCFP-CVP)/Venous resistance(MCFP-CVP)/Venous resistance

CO is determined entirely by VR as the heart CO is determined entirely by VR as the heart can’t pump more blood than it receives.can’t pump more blood than it receives.

VR can go up by increasing MCFP or VR can go up by increasing MCFP or decreasing CVP (resistance is relatively decreasing CVP (resistance is relatively small).small).

MCFP is determined by stressed volume MCFP is determined by stressed volume and is normally around 7 – 12 mmHg while and is normally around 7 – 12 mmHg while CVP is 2-3 mmHg.CVP is 2-3 mmHg.

So why does an increase in CVP (by bolus) So why does an increase in CVP (by bolus) increase CO in a normal heart?increase CO in a normal heart? The sudden increase in preload would increase The sudden increase in preload would increase

SV temporarily but fall once the volume SV temporarily but fall once the volume redistributes to the venous system.redistributes to the venous system.

The stressed volume increases and increases The stressed volume increases and increases the MCFP greater than CVP.the MCFP greater than CVP.

The pressure gradient is thus increased and so The pressure gradient is thus increased and so VR goes up.VR goes up.

Increased VR = increased CO.Increased VR = increased CO.

Pressure

Volume

0 P1

V1

Unstressed

Stressed

Vu

Effect of Fluid Bolus

While venous return can be increased by While venous return can be increased by a fluid bolus which increases stressed a fluid bolus which increases stressed volume which increases MCFP (think volume which increases MCFP (think increasing the amount of fluid in the tub), increasing the amount of fluid in the tub), it can also be increased by it can also be increased by venoconstriction.venoconstriction.

This decreases venous capacity (not This decreases venous capacity (not compliance) which in turn decreases compliance) which in turn decreases unstressed volume to the benefit of the unstressed volume to the benefit of the stressed volume.stressed volume.

Think moving the outlet hole down.Think moving the outlet hole down.

Stressed volume

Unstressed volume

Arterial flow

Venousresistance

CVP

Pressure

Volume

0 P1

V1

Unstressed

Stressed

Vu

Effect of Venoconstriction

Two Compartment Model of Two Compartment Model of the Venous Systemthe Venous System

It is helpful to think of the venous system It is helpful to think of the venous system as two connected compartments.as two connected compartments.

The splanchnic system is very compliant The splanchnic system is very compliant and slow flow while the non-splanchnic and slow flow while the non-splanchnic system is noncompliant and fast flow.system is noncompliant and fast flow.

An increase in resistance in the arteries An increase in resistance in the arteries feeding the splanchnic veins decreases feeding the splanchnic veins decreases flow and shifts blood into the system flow and shifts blood into the system circulation.circulation.

A decrease in resistance causes blood A decrease in resistance causes blood pooling in the veins.pooling in the veins.

Dynamic methods of Dynamic methods of assessing volume statusassessing volume status

I think it goes without saying that the I think it goes without saying that the CVP is a less useful measure of volume CVP is a less useful measure of volume status (fluid responsiveness) because status (fluid responsiveness) because of the many factors that influence it.of the many factors that influence it. Abdominal pressureAbdominal pressure Pump functionPump function Pericardial pressurePericardial pressure Thoracic pressureThoracic pressure

Dynamic methods are much more Dynamic methods are much more usefuluseful

On PPV, inspiration causes increased LVSV On PPV, inspiration causes increased LVSV because of compression of pulmonary veins, because of compression of pulmonary veins, decreased afterload and decreased RV decreased afterload and decreased RV volume from pulmonary compression.volume from pulmonary compression.

The increased thoracic pressure at end The increased thoracic pressure at end inflation decreases the gradient for venous inflation decreases the gradient for venous return at in a few beats causes a decreased return at in a few beats causes a decreased LVSV.LVSV.

This variation is exacerbated by hypovolemia.This variation is exacerbated by hypovolemia. Variation greater than 12 mmHg better Variation greater than 12 mmHg better

reflects preload inadequacy than CVP.reflects preload inadequacy than CVP.

How does that work?How does that work? Hypovolemia causes a fall in the Hypovolemia causes a fall in the

total volume in the system.total volume in the system. The fall in capacity is partly The fall in capacity is partly

compensated by an immediate reflex compensated by an immediate reflex venoconstriction.venoconstriction.

MCFP initially is preserved to MCFP initially is preserved to maintain venous return.maintain venous return.

Pressure

Volume

0 P1

V1

Unstressed

Stressed

Vu

Venoconstriction in Response to a Fall in Total Volume

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

Pressure

Volume

0 P1

V1

Stressed

Unstressed Volume Exhausted, Further Fall in Volume

P2

V2

Recall that venous return is:Recall that venous return is: (MCFP-CVP)/Venous resistance(MCFP-CVP)/Venous resistance

When the MCFP falls and the venous resistance When the MCFP falls and the venous resistance rises, the normal variation in CVP causes a rises, the normal variation in CVP causes a greater variation in venous return which greater variation in venous return which translate into a greater variation in cardiac translate into a greater variation in cardiac output/blood pressure.output/blood pressure.

Hence why dynamic changes are more reflective Hence why dynamic changes are more reflective of volume status.of volume status. CVP normally varies and subject to external influences.CVP normally varies and subject to external influences. Dynamic changes allows us a look into the status of the Dynamic changes allows us a look into the status of the

stressed and unstressed volumes.stressed and unstressed volumes.

Main PointsMain Points The venous system functions to The venous system functions to

maintain filling of the heart.maintain filling of the heart. The main driving force for venous The main driving force for venous

return is MCFP.return is MCFP. The splanchnic vascular bed is the The splanchnic vascular bed is the

reservoir for venous return.reservoir for venous return. CVP is useless for volume status CVP is useless for volume status

unless it is at the extremes. Dynamic unless it is at the extremes. Dynamic measures for fluid responsiveness is measures for fluid responsiveness is informative.informative.

Function of the venous systemFunction of the venous system DefinitionsDefinitions Mean circulatory filling pressureMean circulatory filling pressure Two compartment modelTwo compartment model Dynamic methods of assessing Dynamic methods of assessing

volume statusvolume status