Blood Pumps Pressure/Flow/Resistance

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

Blood Pumps Pressure/Flow/Resistance