Blood Pumps Pressure/Flow/Resistance Brian Schwartz, CCP Perfusion I September 16, 2003.

Blood PumpsPressure/Flow/Resistance

Brian Schwartz, CCP

Perfusion I

September 16, 2003

Blood Pumps

Purpose of Blood Pumps

Ideal Blood Pump

Types of Blood Pumps

Most Commonly Used Pumps

Types of Blood Flow

Other Blood Pumps Used

Development of Blood Pumps

To replace the beating heart during heart surgery

They propel blood and other physiologic fluids throughout the extracorporeal circuit; which includes the patient’s natural circulation as well as the artificial one

The Ideal Blood Pump

Move volumes of blood up to 5.0 L/Min Must be able to pump blood at low

velocities of flow All parts in contact with blood should

have smooth surface Must be possible to dismantle, clean and

sterilize the pump with ease, and the blood handling components must be disposable

The Ideal Blood Pump(continued)

Calibration should be easy, reliable, and reproducible

Pump should be automatically controlled; however, option for manual operation in case of power failure

Must have adjustable stroke volume and pulse rate

FYI

The average human heart can pump up to 30 liters of blood per minute under extreme conditions.

In the operating room setting this is not necessary due to may reasons:– patient is asleep– patient is given muscle relaxants– patient metabolic rate is greatly reduced– patient is cooled during CPB

Types of Blood Pumps

Kinetic Pumps–Centrifugal pumps

Positive Displacement Pumps:–Rotary Pumps–Reciprocating Pumps

Centrifugal Pumps

The pumping action is performed by the addition of kinetic energy to the fluid through the forced rotation of an impeller

Centrifugal Pumps

Designed with impellers arranged with vanes or cones

Centrifugal pumps are magnetically driven and produce a pressure differential as they rotate

It is the pressure differential between the inlet and outlet that causes blood to be propelled

Positive Displacement Pumps

This type of pump moves blood forward by displacing the liquid progressively, from the suction, to the discharge opening of the unit

Positive Displacement Pumps (continued)

Rotary Pumps–Roller Pumps–Screw Pumps

Reciprocating Pumps–Pistons–Bar Compression–Diaphragm

Rotary Pumps

Rotary Pumps–use rollers along flexible tubing to provide

the pumping stroke and give direction to the flow

Archimedean Screw Pumps–a solid helical rotor revolving within a stator

with different pitches so the blood is drawn along the threads

Rotary Pumps (continued)

Multiple Fingers–the direction of flow is produced by a series

of keys that press in sequence against the tubing

Reciprocating Pumps

Pistons–this pump uses motor driven syringes that are

equipped with suitable valves, delivering pulsatile flow

–limited to low output capacity Bar Compression

–blood moves from the alternate compression and expansion of the tube or bulb between a moving bar and a solid back-plate

Reciprocating Pumps (continued)

Diaphragm Pumps–with a flat diaphragm or finger shaped

membrane made of rubber, plastic, or metal, blood is propelled forward

Ventricle Pumps–a compressible chamber mounted in a

casing and are activated by displacement of liquid or gas in the casing

Two Most Common Pumps Today

Roller Pump–Advantages

Occlusive, therefore if power goes out the arterial line won’t act as a venous line

Out put is accurate because it is not dependent of the circuits resistance (including the patients resistance)

–Disadvantages Can cause large amounts of damage to blood

(hemolysis) if over-occluded

Two Most Common Pumps Today (continued)

Centrifugal Pump–Advantages

Reduced hemolysis No cavitation No dangerous inflow/outflow pressures Air gets trapped in pump No need to calibrate

Two Most Common Pumps Today (continued)

Centrifugal Pump–Disadvantages

Causes over-heating Over heating promotes clotting Difficult to de-air If power goes out, arterial line acts like a

venous line

Roller Pump

Two Types of Perfusion

Pulsatile Flow (simulates the human heart)–Decreases peripheral resistance–Increases urinary flow–Better lymph formation–Increases myocardial blood flow–Need 2.3 times more energy to deliver

blood in a pulsatile manner than with non-pulsatile flow

Two Types of Perfusion (continued)

Non-Pulsatile Flow –Simply means continuous flow

Various Opinions on Pulsatile Flow

Advocates–It simulates the beating heart, aiding in

preserving capillary perfusion and cell function

–With the extra energy produced with pulsatile flow, we can avoid the closing down of the capillary beds.

Various Opinions on Pulsatile Flow (continued)

Opponents–Pulsatile flow is a more complex procedure

for minimal benefits–Capillary Critical Closing Pressure: (although

never seen under microscope) The belief that when the pressure in the capillary system goes below a certain point the capillaries will close…reducing the gas exchange between the blood and the tissues

Flow, Pressure and Resistance

Blood Flow: defined as the movement of blood flow through the body, or in our case, the extracorporeal circuit

Pressure: defined as the force vector that is exerted at a 90 degree to that of blood flow

Resistance: the force vector opposite to that of pressure

The Relationship Between Pressure, Flow and Resistance

Flow = Pressure / Resistance

Resistance = Pressure / Flow

Pressure = Flow X Resistance

Laminar Flow

Definition: Referring to blood flow, where all the layers run parallel to the walls of the blood vessels or tube

Reynold’s Number

An equation that enables us to determine whether blood flow is laminar or turbulent

R.N = 2 (fluid density)( average velocity)(r) (fluid viscosity)

If R.N. < 2000 flow is laminar If R.N. > 3000 flow is turbulent If R.N. between 2000 and 3000 flow

unstable

Reynold’s Number (continued)

Blood acts as a Newtonian fluid, one that has a constant viscosity at all velocities

A thixotropic fluid : the viscosity is altered by changing velocities

Viscosity

Another important factor that effects the flow of blood

Viscosity = Shear Stress / Shear Rate

Poiseuille’s Law

Expresses how different variables effect flow. The most notable variable is radius of the vessel or tube.

Flow = (Pressure gradient)(3.14)(radius 4) 8 (viscosity)(length)

Resistance

The main source of resistance is the arterioles. This resistance comes after the pressure source (the heart) giving up peripheral resistance

TPR = MAP/F TPR= Sum of all factors effecting the

resistance to flow

Resistance (continued)

• SVR= PA - PV / Q• PA= MAP• PV= RAP• Q= Flow Rate

• SVR= (MAP-CVP/C.O.) X 80

Pressure

• When the heart contracts and the pressure rises, the highest point is called systolic pressure

• When the heart relaxes and the aortic pressure reaches the lowest point.. this is called diastolic pressure

• Mean arterial pressure = SP/DP

Pressure (continued)

• Because vessels aren’t normally rigid, rather they are flexible, you will see a nice rise in the arterial wave form.

• If the aorta, the most flexible vessel, is rigid, the systolic pressure would rise sharply. (A good diagnostic indicator)

Resistance

The main source of resistance is the arterioles

Viscosity = Shear Stress / Shear Rate

F= (P1-P2) X 3.14 X r4/8L X Viscosity