Pressure measurement system and artifacts

Question 1

Normal pressure waveforms (fluid filled system)

Answer 1

Sharp w rounded contour
* Correct frequency:
o Visible high frequency oscillation at low pressure
o Rapid upstroke w/o overshoot/hyperoscillation at high pressure
* Membrane in transducer: will have a certain stiffness
o If input signal is close to natural frequency of sensing membrane = optimal amplitude signal

Question 2

Major effective reflection site

Answer 2

Terminal aorta

Question 3

What are reflected waves

Answer 3

• Forward pressure and flow waves → identical shape/timing
  o Reflected pressure waves (Pbackward): summated to forward waves
   Resultant central Ao pressure wave: steady ↑ throughout ejection
  o Reflected flow wave (Fbackward): flow is directional → backward flow reduces magnitude of flow in late ejection

Question 4

Factors influencing magnitude of reflected waves

Answer 4

o ↓: strain phase of Valsalva maneuver, vasodilation (vasodilators, physiologic → fever) hypovolemia, hypotension
o ↑: release of Valsalva maneuver, vasoconstriction, HF, hypertension, Ao/iliac obstruction

Question 5

What is a Wedge pressure

Answer 5

• End hole KT in specific blood vessel → end hole towards capillary bed (no other vessel to exit blood flow)
  o Measured in absence of flow → pressure equilibrates across capillary bed
  o Measured pressure = other side of capillary bed (ie pulmonary venous pressure)
  o In absence of cortriatriatum/PV outflow obstruction: PVP = LAP

Question 6

Sensitivity of the pressure measurement system

Answer 6

• Ratio of amplitude of recorded signal/input signal
  o Rigid sensing membrane → ↓ sensitivity
  o Flaccid membrane → ↑ sensitivity

Question 7

Frequency response of the pressure measurement system

Answer 7

• Ratio of output/input amplitude over a range of frequencies of the input pressure wave
  o Respond over a certain determined frequency range → adequate pressure measurement
  o Must be constant over a broad range of frequencies
  o Range improved by stiff membrane: can go to higher frequencies and move faster

Question 8

Relation of frequency response to sensitivity

Answer 8

↑ frequency response → ↓ sensitivity

Question 9

Desirable frequency response

Answer 9

shortest significant vibration w/I physiologic pressure waves
o 1/10 the period of entire pressure curve
 Essential physiologic information: 1st 10 harmonics of the pressure wav e’s Fourier series
 Physiologic press ure variation: no faster then the 10th harmonic velocity

Question 10

Useful frequency pressure range

Answer 10

<20Hz
To ensure high frequency response range: system set w highest possible natural frequency + optimal damping
Natural frequency α lumen radius of KT system
iα to KT/tubing length + √compliance + density of fluid
Highest: short, wide-bore, stiff KT connected to transducer w/o tubing filled w low density liquid w/o bubbles

Question 11

Frequency response characteristics

Answer 11

o Shock-excitation vibrations method (see fig 7.5): frequency of after vibrations → natural frequency of the system
o Damping coefficient: optimal at 0.64
 Uniform frequency response (±5%) to about 88% of natural frequency

Question 12

What are pressures waves in system

Answer 12

electrical signal

Question 13

Strain gauge

Answer 13

variable resistance transducer
Stretched electrical wire → ↑ resistance to current flow
Wide range where ↑ resistance is α to ↑ length

Question 14

pressure transducer system in cath lab

Answer 14

• Fluid filled KT attached to cockstop connected to small volume displacement strain gauge type pressure transducer.
  o End of fluid filled KT: adjusted to 0 reference level.
  o Fluid filled tube connected to pressurized flush bag with saline
  o System is all filled with saline and flushed

Question 15

pressure transducer system: transducer

Answer 15

pressure sensitive membrane/diaphragm
o Change of P at KT tip → transmitted by small fluid mvts in KT → transducer chamber → membrane → change in electrical resistance α to pressure → voltage change
o Conversion of intravascular pressure change into voltage signal displayed on oscilloscope

Question 16

Calibration of pressure transducer system

Answer 16

mercury manometer at 0 and 100mmHg reference
o Criteria for accurate pressure reading
 Accurate calibration: 0 properly compared w mercury manometer, linear response over the range of normal vascular pressures
 0 should be checked in procedure: compensate baseline drift (T change in transducer dome)
 KT tip and transducer: same vertical level

Question 17

Natural frequency of pressure transducer system

Answer 17

every system has its own
o Resonates and overamplifies the signal
o Should be >20hz to avoid accentuation of higher frequency components of signal and overshoot artifacts
 Damping of higher frequencies often necessary to limit artifact

Question 18

What is damping

Answer 18

• Dissipation of E of oscillations of a pressure measurement system owing to friction
  o Absence of friction: oscillate for an indefinite period at its natural frequency

Question 19

Optimal damping

Answer 19

gradual dissipation of E
 Filtration of high frequencies
 Obtain with: wide bore, short, non compliant/stiff KT, connection directly to transducer, low density liquid, no bubbler
o Determine the frequency response curve (flat line)

Question 20

Damping causes

Answer 20

o Small bore/narrow KT
o Viscous substances in KT
o Long compliant/flexible KT
o Air bubbles: excessive damping → ↓ natural frequency
 High frequency components of pressure waveform → set system in oscillation → pressure overshoot
o Clots at KT tip
o Kinked KT
o Multiple connections btw transducer and KT
o Partially closed cockstop

Question 21

Overdamping

Answer 21

↓ sensitivity → dull or rounded waveform
o Problem in fluid path or calibration

Question 22

Underdamping

Answer 22

↑ sensitivity → narrow spikes or exaggerated overshoot of ventricular pressure
o Small pressure wave can cause big deflection signal

Question 23

Sources of error and artifact

Answer 23

-Deterioration in frequency response
-Whip artifact
-End pressure artifact
-KT impact/entrapment artifact
-Systolic pressure amplification in periphery
-Errors in 0 level, balancing, calibration
-Improper degree of damping
-Hybrid tracing

Question 24

Causes of deterioration in frequency response

Answer 24

• Air bubbles: cause excessive damping, ↓ natural frequency
• Tachycardia: can exceed frequency response

Question 25

causes of whip artifact

Answer 25

• Motion of the tip of KT in heart/vessels → accelerates fluid in KT
  o May produce superimposed waves of +/-10mmHg
  o Common in PAs
• Can cause: overestimation of systolic/underestimation of diastolic pressures
• Difficult to eliminate → eliminating extra loops of Kt or balloon deflation may help

Question 26

causes of end pressure artifact

Answer 26

• Flowing blood has kinetic E
• Sudden stop in flow → kinetic E converted partly into pressure
  o End hole KT pointing upstream: artifactually ↑ pressure recordings

Question 27

Causes of KT impact/entrapment artifact

Answer 27

• Similar to whip artifact
• Fluid filled KT hit a structure (valve, heart wall) → creates pressure transient
  o Common w pigtails in LV hitting opening MV leaflets
  o Bizarre/spiked appearance of ventricular pressure waveform

Question 28

why does Systolic pressure amplification in periphery happen

Answer 28

• Higher peak systolic pressure in peripheral arteries vs Ao
  o MAP will remain the same or slightly ↓

Question 29

Causes of errors in 0 level, balancing, calibration

Answer 29

• Improper 0 level, unbalanced transducer
  o Air bubble, KT kink, blood clot anywhere in system
  o Loose connection/stopcock
  o Defective transducer/poor calibration
• 0 level should be changed if patient is moved

Question 30

Causes of overdamping

Answer 30

air bubbles, kink KT, thrombus, high viscosity contrast agent

Question 31

Characteristics of overdamping curve

Answer 31

 Absent dicrotic notch on PA and Ao waves
 No distinct a and v waves on RA and PCWP waves
 Smooth diastolic ventricular waveform, no A wave

Question 32

What is overdamping

Answer 32

excessive friction absorbing the force of pressure wave
o Loss of frequency response → smooth and rounded tracing
o Falsely lower peak pressures

Question 33

What is underdamping

Answer 33

overshoot or ring artifact

Question 34

Characteristics of underdamped curve

Answer 34

o Narrow spikes overshooting the peak pressure w similar negative spikes
o May lead to overestimation of peak pressure/underestimation of nadir

Question 35

causes of underdamping

Answer 35

air bubbles oscillating back and forth → transmit E to transducer
 Flushing should eliminate artifact

Question 36

cause of hybrid tracing

Answer 36

• KT trapped btwn 2 chambers
• PCWP measurement: if KT not completely occlude PA → waveform will be contamined by PAP