Fluid-filled Monitoring Systems

Pascal’s Principle

Pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and to the walls of the containing vessel.

Poiseuille’s Law

Q = pi x pressure gradient x r^4 / 8 x viscosity x length. Flow is directly proportional to the pressure difference and r^4, and inversely proportional to the fluid’s viscosity and the tube’s length

Resistance in series

Cumulative. Eg. Adding a cannula to a line

Resistance in parallel

Reduces total resistance by providing more pathways. Eg. Adding a Y connector to split a line

Reynolds Number (Re)

Density x Velocity x Diameter / Viscosity. Dimensionless ratio predicting whether flow is laminar (<2000) or turbulent (>4000). with transition flow in between.

steps to zero a pressure transducer

Block off primed line by turning the stopcock off to the circuit and remove cap to expose to atmosphere

Zeroing

Exposing all arms of the Wheatstone bridge to atmosphere to establish room pressure as the 0 mmHg baseline reference point.

Phlebostatic axis

Positioning transducer at the level of the right atrium: 4th intercostal space, mid-chest to eliminate hydrostatic pressure errors when monitoring patient hemodynamics

Transducer placement 1in above pressure line placement

Displayed pressure reads 2 mmHg too low (hydrostatic)

Transducer placement 1 in below pressure line placement

Displayed pressure reads 2 mmHg too high (gravity)

Fidelity

Ability to reproduce the patient’s actual pressure waveform, preserving amplitude, timing, and shape

Natural frequency (Fn)

The frequency at which a mechanical system prefers to oscillate naturally. Lower in longer + more compliant systems and higher in shorter + stiffer systems

Dynamic response

How accurately the system follows rapidly changing pressure

Resonance (underdamping)

Signal is amplified when the incoming pressure frequency approaches the natural frequency of the monitoring system. Causes overshoot & exaggerated waveform amplitude

Damping

Loss of energy from an oscillating system. Too little = excessive oscillation, waveform amplitude, Appropriate = faithful waveform, Too much = loss of waveform detail

Underdamped readings

High SBP, low DBP, wide pulse pressure. Artifact, overshoot, ringing. Relatively preserved MAP due to opposing systolic and diastolic error

Overdamped readings

Low SBP, high DBP, narrow pulse pressure. No dicrotic notch

Square-Wave (or Fast-Flush or snap) test

Brief pigtail/plunger pulling to expose the transducer to 300 mmHg pressure bag fluid to evaluate dampening of the waveform following squaring

causes of overdamping

Air bubbles (compressible break in fluid), blood or clots, excessive tubing length or compliance, kinks, small-lumen catheter or one up against the vessel wall

causes of underdamping/resonance

Hyperdynamic signals (more frequent striking of catheter), tachycardia, sepsis, tubing vibration

catheter whip

Motion artifact from catheter movement

Calibration

Verification or reproduction of the pressure signal

Hydrodynamic pressure

Pressure produced by fluid in motion

Hydrostatic pressure

Pressure exerted by a stationary fluid column.

viscosity

A fluid’s internal friction or resistance to flow between adjacent layers. Promotes decreased Re, laminar flow

density

Mass per unit volume of a fluid, representing its inertia. Increased inertia increases Re making turbulent flow more likely

Laminar flow

Smooth streamlined flow in parallel layers. Re < 2000. Provides predictable flow profiles, lower resistance to movement, lower pressure drops across vessels or tubing

Turbulent flow

Disorganized chaotic fluid movement with swirls. Re > 4000. Significantly increases fluid resistance and places hydrodynamic stress on vessel walls (jet-blasting, endothelial dysfunction, plaque disruption)

Resistance-type transducer

Converts physical pressure against a diaphragm into a proportional change in electrical resistance and voltage output. Most common type using a wheatstone bridge containing a bonded strain gauge.

bonded strain gauge

Microscopic resistor chip on the back of the transducer diaphragm. Alters electrical resistance in direct proportion to the applied force

wheatstone bridge

Electrical circuit configuration of resistors (including strain gauges) that converts tiny pressure-induced resistance changes into a measurable voltage output signal