Hemodynamic Measurements

and

Dynamics

of

Assisted Circulation

2

that allows accessibility to different parts of the circulation. There are

instances, where a combination of a hyperdermic needle and a pressure

transducer suffices, particularly when the blood vessel is superficial or

under intra-operative conditions. Brachial, radial or femoral arteries are

common superficial sites for

pressure measurements with needle-

transducer systems. Left ventricular chamber pressure measurement with

direct apex insertion of a needle is also common under open chest

conditions.

The performance of a needle-pressure transducer system can be

evaluated through basic mechanical and electrical modeling.

The

simplest representation of the system is an undamped spring-mass

system of natural frequency:

where r is the internal lumen radius of the needle,

is the length of the

needle and

is the fluid density.

Typically the needle and pressure

transducer dome are filled with saline. This provides the required fluid

coupling that is necessary when the needle is inserted into an artery

which is filled with blood. Heparine is often added to prevent blood

clotting in the catheter. Blood pressure pulsation is transmitted via fluid

coupling resulting in the movement of the pressure transducer diaphragm

(stainless steel). The greater the amount of fluid, the greater is the fluid

movement or inertia. Thus, the inertia is represented by

The compliance of the pressure transducer is determined by the

movement of the

stainless steel diaphragm within the fluid-filled

transducer dome. Compliance which is defined as volume displacement

per unit distending pressure, is the inverse of stiffness:

dV

C=-