Operation Manual

MEDRAD
®
Salient Operation Manual 23
4.2. Understanding pressure and flow, pressure limiting & adaptive flow
4.2.1. Pressure Basics
In all hydraulic systems, pressure is required to cause fluid to flow through the conduit. The pressure in the
reservoir (syringe) must exceed the resistance of the conduit (tubing and catheter) and fluid. The conduit offers
resistance by its diameter and length. The fluid offers resistance by its thickness, or viscosity.
All hydraulic systems require pressure to cause the fluid to flow through the conduit. But here is an often
misunderstood basic fact of pressure: pressure will be dissipated in the conduit. Pressure serves only to propel
the fluid through the conduit. Because there is no opposition (resistance) to the fluid at the open end of the
conduit, the fluid will be at zero pressure at the open end. The pressure is highest at the connection between the
reservoir and the conduit (syringe and connector tubing). At the open end of the conduit (catheter distal end
within the patient), the fluid will be at zero pressure (or rather the patient’s systemic pressure), because there is
no opposition to the fluid at the open end. The illustration below shows how pressure varies in a conduit. To
simplify the following discussion, neglect the patient’s systemic pressure and assume that there is zero pressure
at the open end of the conduit. Pressure is greatest in the fluid reservoir and at the connection of the conduit to
the reservoir. Halfway down the conduit, the pressure will be half of the reservoir pressure. And at the end of the
conduit, pressure will be zero.
4.2.2. Summary
1. The reservoir (syringe) pressure must exceed resistance in the conduit (tubing and catheter) if fluid is to
flow through the conduit
2. Higher pressure is required the smaller the diameter of the conduit (tubing, catheter), the longer the
conduit, and the more viscous the media.
3. The diameter of the conduit is the factor with the highest impact on pressure (and achievable flow rates)
4. Pressure will dissipated (decrease) in the conduit so that the pressure at the open end will be zero as
long as there is no obstruction at the outflow end.
5. If the pressure limit is set lower than the pressure required to propel media at the desired flow rate, the
flow rate through the catheter will be reduced.
6. If the pressure limit is set higher than the pressure required to propel media at the desired flow rate, the
flow rate through the catheter will not be affected. This pressure limit will protect the catheter only if
the catheter becomes blocked.
Highest pressure at
output of reservoir
(syringe)
Median pressure
half-way down pipe
Zero pressure at
open end of pipe
Fluid
200psi
100psi
0psi
Force
Fluid scatters
(no restrictions)
Pipe (catheter)
Pressurised reservoir –
piston and barrel of
syringe