Hemodynamic Measurements
Assisted Circulation
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
The performance of a needle-pressure transducer system can be
evaluated through basic mechanical and electrical modeling.
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:
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