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Anaesthetic FRCA Primary - eLFH Equipment > eLFH - Invasive monitoring > Flashcards

eLFH - Invasive monitoring Flashcards

1
Q

Components of invasive arterial blood pressure monitoring

A

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

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2
Q

Hagan-Poiseuille equation

A
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3
Q

How does arterial cannula determine flow

A

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

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4
Q

Mistake which could alter viscosity in invasive BP measurement system

A

Inadvertent use of 5% dextrose instead of normal saline

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5
Q

Dynamic response definition

A

Speed at which it is able to settle on a new value following a stimulus

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6
Q

Three factors which affect dynamic response of an arterial line system

A

Natural (resonant) frequency

Input frequency

Damping

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7
Q

Natural (resonant) frequency definition

A

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

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8
Q

Input frequency definition

A

Frequency of energy input into the system

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9
Q

Resonance definition

A

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

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10
Q

Damping definition

A

Energy loss of a swinging or oscillating body through friction / resistance

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11
Q

Damping representation

A

Damping represented by the Damping Coefficient (D)

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12
Q

Under damping definition

A

Takes very long time for system to settle on a new value (zero on x axis following stimulus)

Damping coefficient < 0.64

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13
Q

Critical damping definition

A

Damping coefficient = 1.0

Characterised by no overshoot and long time for amplitude to settle at zero

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14
Q

Optimal damping definition

A

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)

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15
Q

Graphical representation of dynamic response and relationship between input frequency, natural frequency and damping

A

Maximum response when input frequency : Natural frequency ratio = 1 (i.e. input frequency = natural frequency)

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16
Q

When does resonance occur in arterial line system

A

When input frequency (heart rate) = the natural frequency of the system

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17
Q

Effect of resonance on arterial line trace

A

Significantly reduces quality of arterial line trace

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18
Q

How to avoid resonance in arterial line (or other) system

A

Natural frequency of a system should be at least 8x greater than the maximum anticipated input frequency

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19
Q

Minimum natural frequency of a system in clinical use to avoid resonance

A

20 Hz

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20
Q

Method used to determine natural frequency and level of damping in an arterial line system

A

Flush test

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21
Q

Calculation of natural frequency of a system using flush test

A
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22
Q

Calculation of damping level of a system using flush test

A
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23
Q

Optimal damping coefficient for all clinical systems to provide best dynamic response

A

0.64

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24
Q

Transducer definition

A

Device that converts one form of energy into another

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25
Arterial pressure transducer definition
Converts pressure energy into electrical energy
26
Three types of transducer commonly used in arterial pressure measurement
Wire strain gauge Bonded strain gauge Capacitive transducer
27
Generic mechanism of Wire strain and Bonded strain gauges
Both contain wires which very their resistance as arterial pressure is altered
28
Resistance of a wire equation
Geometry and Resistivity of a wire are important
29
Resistivity definition
Degree to which a material opposes the flow of electrical current It is constant for a given material at a given temperature
30
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
31
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
32
Most commonly metal used in transducer wires and why
Constantan (copper-nickel alloy) Resistivity does not significantly change with temperature
33
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
34
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
35
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
36
Capacitance equation
37
Reactance of capacitor equation
Reactance of capacitor is same as resistance of capacitor
38
How are the small changes in resistance created by diaphragm movements measured accurately in transducers
Wheatstone bridge
39
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
40
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
41
Wheatstone bridge Full-bridge mechanism
Contains 4 strain gauges More complicated maths - don't worry about that
42
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
43
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
44
Pulse contour analysis definition
Further mathematical processing of arterial pressure waveform Allows derivation of other useful values including stroke volume and cardiac output
45
Pulse contour analysis - information obtained from arterial waveform
46
Two main systems of pulse contour analysis in clinical use to determine SV and CO
PiCCO LiDCO / PulseCO
47
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
48
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
49
Physiological mechanism resulting in arterial trace respiratory swing
Swing is more pronounced in hypovolaemia
50
Parameters determined from pulse contour analysis to quantify degree of respiratory swing of arterial line trace
Stroke volume variation (SVV) Pulse pressure variation (PPV)
51
Stroke volume variation definition and calculation
Variation in stroke volume over respiratory cycle, measured over 30 second time period
52
Pulse pressure variation definition and calculation
Variation in pulse pressure over respiratory cycle, measured over 30 second time period
53
Requirements for SVV and PPV to be accurate to therefore guide fluid management
Ventilated patient Sinus rhythm
54
Why is amplification used
Biopotentials usually have small amplitudes Amplification increases signal amplitude to improve clarity of display
55
Types of amplification used in arterial transducers
Simple amplification - more commonly used Differential amplification
56
Simple amplification mechanism
Increases amplitude of signal by adjusting Gain
57
Gain definition
Ratio of input signal to output signal Can be manipulated during calibration of a system
58
Differential amplification mechanism
Reduces electrical interference by using: - Common mode rejection - Bandwidth frequency
59
Calibration - features which are adjusted
Zero offset (bias) Gain
60
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
61
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
62
Stewart Hamilton equation use
Calculates area under temperature change-time curve Used to calculate cardiac output during thermodilution
63
Effect of hypovolaemia on location of dicrotic notch
Shifts dicrotic notch to right as changes aortic pressure for aortic valve closure
64
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
65
Does the transducer need to be at level of heart before Zeroing for atmospheric pressure?
No

Anaesthetic FRCA Primary - eLFH Equipment (4 decks)

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