Pressure measurement system and artifacts Flashcards
Normal pressure waveforms (fluid filled system)
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
Major effective reflection site
Terminal aorta
What are reflected waves
- 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
Factors influencing magnitude of reflected waves
o ↓: strain phase of Valsalva maneuver, vasodilation (vasodilators, physiologic → fever) hypovolemia, hypotension
o ↑: release of Valsalva maneuver, vasoconstriction, HF, hypertension, Ao/iliac obstruction
What is a Wedge pressure
- 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
Sensitivity of the pressure measurement system
- Ratio of amplitude of recorded signal/input signal
o Rigid sensing membrane → ↓ sensitivity
o Flaccid membrane → ↑ sensitivity
Frequency response of the pressure measurement system
- 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
Relation of frequency response to sensitivity
↑ frequency response → ↓ sensitivity
Desirable frequency response
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
Useful frequency pressure range
<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
Frequency response characteristics
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
What are pressures waves in system
electrical signal
Strain gauge
variable resistance transducer
Stretched electrical wire → ↑ resistance to current flow
Wide range where ↑ resistance is α to ↑ length
pressure transducer system in cath lab
- 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
pressure transducer system: transducer
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
Calibration of pressure transducer system
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
Natural frequency of pressure transducer system
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
What is damping
- 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
Optimal damping
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)
Damping causes
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
Overdamping
↓ sensitivity → dull or rounded waveform
o Problem in fluid path or calibration
Underdamping
↑ sensitivity → narrow spikes or exaggerated overshoot of ventricular pressure
o Small pressure wave can cause big deflection signal
Sources of error and artifact
-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
Causes of deterioration in frequency response
- Air bubbles: cause excessive damping, ↓ natural frequency
- Tachycardia: can exceed frequency response
causes of whip artifact
- 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
causes of end pressure artifact
- Flowing blood has kinetic E
- Sudden stop in flow → kinetic E converted partly into pressure
o End hole KT pointing upstream: artifactually ↑ pressure recordings
Causes of KT impact/entrapment artifact
- 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
why does Systolic pressure amplification in periphery happen
- Higher peak systolic pressure in peripheral arteries vs Ao
o MAP will remain the same or slightly ↓
Causes of errors in 0 level, balancing, calibration
- 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
Causes of overdamping
air bubbles, kink KT, thrombus, high viscosity contrast agent
Characteristics of overdamping curve
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
What is overdamping
excessive friction absorbing the force of pressure wave
o Loss of frequency response → smooth and rounded tracing
o Falsely lower peak pressures
What is underdamping
overshoot or ring artifact
Characteristics of underdamped curve
o Narrow spikes overshooting the peak pressure w similar negative spikes
o May lead to overestimation of peak pressure/underestimation of nadir
causes of underdamping
air bubbles oscillating back and forth → transmit E to transducer
Flushing should eliminate artifact
cause of hybrid tracing
- KT trapped btwn 2 chambers
- PCWP measurement: if KT not completely occlude PA → waveform will be contamined by PAP
