eLFH - Invasive monitoring Flashcards
Components of invasive arterial blood pressure monitoring
Arterial cannula - Hagan-Poiseuille
Fluid filled tubing - Damping and resonance
Transducer - Wheatstone bridge circuit
Signal processor - Fourier and pulse contour analysis
Amplifier - Amplification
Display - Calibration
Hagan-Poiseuille equation
How does arterial cannula determine flow
Hagan-Poiseuille equation
For arterial BP measurement:
- Pressure difference maintained by using pressure bag set to 300 mmHg
- Cannula radius is fixed (but clot in lumen may change this)
- Length is fixed
- Fluid viscosity is assumed to be constant
Mistake which could alter viscosity in invasive BP measurement system
Inadvertent use of 5% dextrose instead of normal saline
Dynamic response definition
Speed at which it is able to settle on a new value following a stimulus
Three factors which affect dynamic response of an arterial line system
Natural (resonant) frequency
Input frequency
Damping
Natural (resonant) frequency definition
Frequency at which a system oscillates when set in motion
Unique for each system
Represented by sine wave
E.g. tuning fork vibrates at its natural frequency
Input frequency definition
Frequency of energy input into the system
Resonance definition
Effect observed when input frequency is the same as natural frequency
If energy is input at same frequency as natural frequency, then amplitude of swing increases exponentially
If they don’t match, then amplitude decreases overall
E.g. playground swing increases amplitude if energy is input at same frequency as natural frequency
Damping definition
Energy loss of a swinging or oscillating body through friction / resistance
Damping representation
Damping represented by the Damping Coefficient (D)
Under damping definition
Takes very long time for system to settle on a new value (zero on x axis following stimulus)
Damping coefficient < 0.64
Critical damping definition
Damping coefficient = 1.0
Characterised by no overshoot and long time for amplitude to settle at zero
Optimal damping definition
Damping coefficient = 0.64
Takes shortest time for amplitude to settle at zero
Typically has 1x overshoot and then settles (i.e. over reads once, under reads once and then settled)
Graphical representation of dynamic response and relationship between input frequency, natural frequency and damping
Maximum response when input frequency : Natural frequency ratio = 1 (i.e. input frequency = natural frequency)
When does resonance occur in arterial line system
When input frequency (heart rate) = the natural frequency of the system
Effect of resonance on arterial line trace
Significantly reduces quality of arterial line trace
How to avoid resonance in arterial line (or other) system
Natural frequency of a system should be at least 8x greater than the maximum anticipated input frequency
Minimum natural frequency of a system in clinical use to avoid resonance
20 Hz
Method used to determine natural frequency and level of damping in an arterial line system
Flush test
Calculation of natural frequency of a system using flush test
Calculation of damping level of a system using flush test
Optimal damping coefficient for all clinical systems to provide best dynamic response
0.64
Transducer definition
Device that converts one form of energy into another
Arterial pressure transducer definition
Converts pressure energy into electrical energy
Three types of transducer commonly used in arterial pressure measurement
Wire strain gauge
Bonded strain gauge
Capacitive transducer
Generic mechanism of Wire strain and Bonded strain gauges
Both contain wires which very their resistance as arterial pressure is altered
Resistance of a wire equation
Geometry and Resistivity of a wire are important
Resistivity definition
Degree to which a material opposes the flow of electrical current
It is constant for a given material at a given temperature
Effect of temperature changes on resistivity of semiconductors
Increase in temperature results in decrease in resistivity of semiconductors
I.e. Increased electrical current at higher temperatures
Effect of temperature changes on resistivity of metals
Increase in temperature results in increase in resistivity of metals
I.e. Decreased electrical current at higher temperatures
Most commonly metal used in transducer wires and why
Constantan (copper-nickel alloy)
Resistivity does not significantly change with temperature
Wire strain gauge mechanism
Increased arterial pressure decreases the tension of the resistance wire
I.e increases cross sectional area and reduces length - thus reduces resistance
Change in resistance is plotted against time and calibrated to a known pressure (atmospheric) when transducer is Zeroed
Pressure-time arterial trace is displayed
Bonded strain gauge mechanism
Coil of resistance wire bonded to the diaphragm
As arterial pressure increases and moves the diaphragm, the wire coil is stretched
Coil tension increases and therefore resistance increases
Capacitive transducer mechanism
Diaphragm forms one plate of the capacitor
Increase in arterial pressure reduces the distance between the two plates
Capacitance is inversely related to the plate separation distance
Waveform plotted is still resistance (resistance is also called reactance referring to capacitors)
Reactance is inversely proportional to capacitance
Capacitance equation
Reactance of capacitor equation
Reactance of capacitor is same as resistance of capacitor
How are the small changes in resistance created by diaphragm movements measured accurately in transducers
Wheatstone bridge
Wheatstone bridge (Quarter-bridge) mechanism
R1 / R2 = R3 / R4
R1, R2 and R3 are known so R4 can be calculated
Value of R4 is plotted against time - converted into arterial pressure waveform by calibration
Wheatstone bridge Full-bridge use
Used by most modern arterial pressure transducers
More complicated maths but greatly increases sensitivity and allows compensation for temperature changes
Wheatstone bridge Full-bridge mechanism
Contains 4 strain gauges
More complicated maths - don’t worry about that
Fourier analysis overview
Combining multiple sine waves can recreate an accurate representation of the pressure-time arterial waveform
Fourier analysis performs these steps in reverse to break down arterial pressure waveform into sine waves
Why is Fourier analysis used for arterial pressure waveforms
Allows for further mathematical processing of the wave - e.g. integration, area under the curve, etc
This is called pulse contour analysis
Pulse contour analysis definition
Further mathematical processing of arterial pressure waveform
Allows derivation of other useful values including stroke volume and cardiac output
Pulse contour analysis - information obtained from arterial waveform
Two main systems of pulse contour analysis in clinical use to determine SV and CO
PiCCO
LiDCO / PulseCO
PiCCO summary
Integrate the find area under systolic part of pressure time arterial trace
Divide by SVR
Add contractility x Aortic compliance
Multiply by HR
Multiply calibration factor
Result is Cardiac Output
LiDCO / PulseCO summary
Pressure time arterial trace
Convert to Volume time arterial trace
Provides nominal SV and heart beat duration
Nominal CO can be calculated
Calibration by lithium dilution
Result is Cardiac Output
Physiological mechanism resulting in arterial trace respiratory swing
Swing is more pronounced in hypovolaemia
Parameters determined from pulse contour analysis to quantify degree of respiratory swing of arterial line trace
Stroke volume variation (SVV)
Pulse pressure variation (PPV)
Stroke volume variation definition and calculation
Variation in stroke volume over respiratory cycle, measured over 30 second time period
Pulse pressure variation definition and calculation
Variation in pulse pressure over respiratory cycle, measured over 30 second time period
Requirements for SVV and PPV to be accurate to therefore guide fluid management
Ventilated patient
Sinus rhythm
Why is amplification used
Biopotentials usually have small amplitudes
Amplification increases signal amplitude to improve clarity of display
Types of amplification used in arterial transducers
Simple amplification - more commonly used
Differential amplification
Simple amplification mechanism
Increases amplitude of signal by adjusting Gain
Gain definition
Ratio of input signal to output signal
Can be manipulated during calibration of a system
Differential amplification mechanism
Reduces electrical interference by using:
- Common mode rejection
- Bandwidth frequency
Calibration - features which are adjusted
Zero offset (bias)
Gain
Zero offset definition and correction
Occurs when actual pressure reading of zero does not correspond to a reading of zero on the display
This is corrected when pressure transducer is zeroed to atmospheric pressure
Gain calibration definition and correction
Gradient error where actual pressure to display pressure graph angle is wrong
Corrected by adjusting gain of system with amplifier - usually set my manufacturer of transducer
Stewart Hamilton equation use
Calculates area under temperature change-time curve
Used to calculate cardiac output during thermodilution
Effect of hypovolaemia on location of dicrotic notch
Shifts dicrotic notch to right as changes aortic pressure for aortic valve closure
Why must transducer be at level of heart for accurate arterial pressure measurement
Due to effects of gravity on column of fluid
Vertical offset by 10 cm results in pressure change of 8.5 mmHg
Does the transducer need to be at level of heart before Zeroing for atmospheric pressure?
No
