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
