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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
Q

Arterial pressure transducer definition

A

Converts pressure energy into electrical energy

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26
Q

Three types of transducer commonly used in arterial pressure measurement

A

Wire strain gauge

Bonded strain gauge

Capacitive transducer

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27
Q

Generic mechanism of Wire strain and Bonded strain gauges

A

Both contain wires which very their resistance as arterial pressure is altered

28
Q

Resistance of a wire equation

A

Geometry and Resistivity of a wire are important

29
Q

Resistivity definition

A

Degree to which a material opposes the flow of electrical current

It is constant for a given material at a given temperature

30
Q

Effect of temperature changes on resistivity of semiconductors

A

Increase in temperature results in decrease in resistivity of semiconductors

I.e. Increased electrical current at higher temperatures

31
Q

Effect of temperature changes on resistivity of metals

A

Increase in temperature results in increase in resistivity of metals

I.e. Decreased electrical current at higher temperatures

32
Q

Most commonly metal used in transducer wires and why

A

Constantan (copper-nickel alloy)

Resistivity does not significantly change with temperature

33
Q

Wire strain gauge mechanism

A

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
Q

Bonded strain gauge mechanism

A

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
Q

Capacitive transducer mechanism

A

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
Q

Capacitance equation

A
37
Q

Reactance of capacitor equation

A

Reactance of capacitor is same as resistance of capacitor

38
Q

How are the small changes in resistance created by diaphragm movements measured accurately in transducers

A

Wheatstone bridge

39
Q

Wheatstone bridge (Quarter-bridge) mechanism

A

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
Q

Wheatstone bridge Full-bridge use

A

Used by most modern arterial pressure transducers

More complicated maths but greatly increases sensitivity and allows compensation for temperature changes

41
Q

Wheatstone bridge Full-bridge mechanism

A

Contains 4 strain gauges

More complicated maths - don’t worry about that

42
Q

Fourier analysis overview

A

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
Q

Why is Fourier analysis used for arterial pressure waveforms

A

Allows for further mathematical processing of the wave - e.g. integration, area under the curve, etc

This is called pulse contour analysis

44
Q

Pulse contour analysis definition

A

Further mathematical processing of arterial pressure waveform

Allows derivation of other useful values including stroke volume and cardiac output

45
Q

Pulse contour analysis - information obtained from arterial waveform

A
46
Q

Two main systems of pulse contour analysis in clinical use to determine SV and CO

A

PiCCO

LiDCO / PulseCO

47
Q

PiCCO summary

A

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
Q

LiDCO / PulseCO summary

A

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
Q

Physiological mechanism resulting in arterial trace respiratory swing

A

Swing is more pronounced in hypovolaemia

50
Q

Parameters determined from pulse contour analysis to quantify degree of respiratory swing of arterial line trace

A

Stroke volume variation (SVV)

Pulse pressure variation (PPV)

51
Q

Stroke volume variation definition and calculation

A

Variation in stroke volume over respiratory cycle, measured over 30 second time period

52
Q

Pulse pressure variation definition and calculation

A

Variation in pulse pressure over respiratory cycle, measured over 30 second time period

53
Q

Requirements for SVV and PPV to be accurate to therefore guide fluid management

A

Ventilated patient

Sinus rhythm

54
Q

Why is amplification used

A

Biopotentials usually have small amplitudes

Amplification increases signal amplitude to improve clarity of display

55
Q

Types of amplification used in arterial transducers

A

Simple amplification - more commonly used

Differential amplification

56
Q

Simple amplification mechanism

A

Increases amplitude of signal by adjusting Gain

57
Q

Gain definition

A

Ratio of input signal to output signal

Can be manipulated during calibration of a system

58
Q

Differential amplification mechanism

A

Reduces electrical interference by using:
- Common mode rejection
- Bandwidth frequency

59
Q

Calibration - features which are adjusted

A

Zero offset (bias)

Gain

60
Q

Zero offset definition and correction

A

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
Q

Gain calibration definition and correction

A

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
Q

Stewart Hamilton equation use

A

Calculates area under temperature change-time curve

Used to calculate cardiac output during thermodilution

63
Q

Effect of hypovolaemia on location of dicrotic notch

A

Shifts dicrotic notch to right as changes aortic pressure for aortic valve closure

64
Q

Why must transducer be at level of heart for accurate arterial pressure measurement

A

Due to effects of gravity on column of fluid

Vertical offset by 10 cm results in pressure change of 8.5 mmHg

65
Q

Does the transducer need to be at level of heart before Zeroing for atmospheric pressure?

A

No

Anaesthetic FRCA Primary - eLFH Equipment (4 decks)

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