Fluid-Filled Monitoring Systems

Exam 2

Hemodynamic monitoring

• Describes the relationship between pressure, flow and resistance

• Assess cardiac function, vascular function, perfusion, and response to therapy

Pascal’s Principle

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

• Monitoring: patient → catheter → fluid → column → transducer

• Incompressible fluid, rigid tubing, and no air

Poiseuille’s law

• Flow through a cylindrical tube is

  • directly proportional to the pressure difference and the fourth power of the radius

  • inversely proportional to the fluid’s viscosity and the tube’s length

• Flow is dependent on radius, length, and viscosity/resistance of fluid

Cross Sectional Areas of vessel

i. Aorta – 2.5 cm²

ii. Small Arteries – 20cm²

iii. Arterioles 40 cm²

iv. Capillaries – 2500 cm²

v. Venules – 250 cm²

vi. Small veins cm²

vii. Venae cava – 8 cm²

Capillaries have the largest cross-sectional area due to have billions in the body

Cross-sectional Area and velocity

• The same volume of blood must pass through all vessels

• Velocity is inversely proportional to vascular cross-sectional area

  • velocity/speed increases as the cross-sectional area decreases

• Capillaries have the slowest velocity due to the largest cross-sectional area → more time for nutrient exchange

Flow vs velocity

Flow = how much blood is being moved

Velocity = speed/how fast the blood is traveling

Reynold’s Number

a.     Compares inertial forces with viscous forces to predict flow patterns

• Inertia = fluid's desire to keep moving

• Viscosity = fluids resistance/friction

b.     Predicts whether blood flow will be smooth and orderly (Laminar)  or chaotic and swirling (Turbulent)

• Velocity and diameter are highly influential

  • in aorta: large diameter and high velocity = high Re - may be turbulent

  • In capillaries: small diameter, low velocity = low Re - laminar

Reynold’s Number Equation

Re = (p*v*d)/u

p = density of the blood (mass/volume; inertia)

v = velocity of blood flow (how fast)

d = diameter of the blood vessel (resistance)

u = viscosity of blood (how thick/sticky)

Reynold’s Number predictions

i. If Re < 2000 = Laminar

ii. If Re > 4000 = Turbulent

iii. If 2000 < Re < 4000 = Transition flow

Catheter + noncompliant tubing

• Carries pressure from the patient to the transducer

• Catheter design

  • Biocompatible, smooth, nonthrombogenic, as short as practical, low-compliance walls, small outer diameter

Pressure transducers

• Converts mechanical pressure to an electrical signal when pressure pushes on a pressure-sensitive diaphragm

• Transducer senses movement produced by pressure transmitted through the fluid column

Capacitance transducer

Uses two plates → pressure pushes on the diaphragm causes the plates to push against each other → oscillating voltage → increase in capacitance

Inductance Transducer

Uses a coil → pressure causes diaphragm to move and change position of coil → changes inductance of system

Resistance Type Transducer

• Uses a Strain gauge bonded to the back of the diaphragm

• Pressure pushes on the diaphragm, causing it to bend/move → the strain gauge is stretched or compressed → resistance changes → electrical signal changes

• Wheatstone bridge: changes in resistance are easily converted to a change in voltage

  • Makes small changes easy to detect

Diaphragm moves → strain gauge resistance changes → Wheatstone bridge detects it → voltage changes → pressure displayed

Amplifier

strengthens the electrical signal coming from the transducer

Flush system + pressure bag + stopcocks

• Flush: continuously provides a small amount of fluid through the pressure tubing

  • Prevents clotting; keeps catheter open

• Pressure bag: applies pressure on the flush solution so that it is higher than patient pressure (prevents backflow)

• Stopcocks: controls flow direction; used to connect the transducer to the patient, collect samples, zero the transducer

Monitor/signal processor

• Processes the electrical signal and converts it to pressure measurements

Important functions

1. Zero: establishes atmospheric pressure as 0 mmHg → balancing the Wheatstone bridge

2. Scale: changes how the waveform is displayed

3. Alarms: identify pressure outside selected limits

Natural Frequency

• Frequency at which any structure prefers to oscillate/vibrate naturally after it as been disturbed

  • Longer and more compliant = lower natural frequency

  • Shorter and stiffer = higher natural frequency

    • Monitoring system should have much higher natural frequency contained in the patient’s waveform

  
  

What can disturb pressure wave forms

• Catheter diameter and length

  • Longer tubing = diminished pressure

  • High compliance = absorbs changes in pressure

  • Connection/stockcocks = distortion

• Air bubbles

Resonance

• Resonance happens when an outside force is applied at or very close to a system’s natural frequency

• Can amplify the waveform signal

Damping

The loss of energy from an oscillating system

• some is needed

Overdamped

• low systolic BP and High diastolic BP

• narrow pulse pressure

• Causes: air bubbles, kinks/poor connection, small-lumen catheter, blood backed into the pressure tubing

Underdamped

• High Systolic BP and Low Diastolic BP

• Wide pulse pressure

• Causes: resonance, hyperdynamic signals, tachycardia, and tubing vibration

Optimal Transducer Waveform

Square-Wave/Fast Flush Test

• Evaluates the dynamic response of the transducer monitoring system

• Process

  • Activate the fast-flush device → release → observe waveform response

• diagnose damping from transducer

Square-Wave/Fast flush test

adequate damping

Square-Wave/Fast flush test

Underdamping

Square-Wave/Fast flush test

Overdamping

Patient factors that influence damping

• higher-frequency signals occur with tachycardia or sepsis

• Aortic regurgitation (blood leaks backward into the LV) or hyperdynamic circulation

Zeroing transducer

• calibrate the system to atmospheric pressure to program it as 0 mmHg

• Process: close transducer off to the system, open the Wheatstone bridge to air, press zero on the monitor system

Leveling Transducer

• location of transducer when zeroing + monitoring pressure to prevent hydrostatic error

  • If too low, the transducer diaphragm is pushed on by the weight of the fluid

• Phlebostatic axis: in line with the patient’s heart/RA

2 mmHg for every 1 unit of vertical difference from the phlebostatic axis

Lag time

longer tubing the longer the lag time, because of inertia, fluid-filled column within system should be transmitted simultaneously and with equal force

Oscillations

numbers of cycles per second

Fidelity

• Ability to reproduce the patient’s actual pressure waveform

• How faithfully the system reproduces the patient’s pressure waveform

  • High fidelity = preserves timing, shape, and amplitude

Dynamic response

• How accurately the system follows rapidly changing pressure

• Response must be quick without exaggerating or blunting the waveform

Ringing

repeated oscillations caused by inadequate damping

Catheter whip

motion artifact from catheter movement

Hydrodynamic pressure

Pressure produced by fluid in motion

Hydrostatic Pressure

Pressure exerted by a stationary fluid column