Basic monitoring Flashcards
Which leads from the 12 lead ECG does the 3-lead ECG monitoring system allow continuous display for?
Leads I, II, III
Which lead is preferably used and why
lead II as it most closely aligns with the axis of the heat
Name 4 sources of artefact which may affect the ECG
Non-standard position
Morphology
Weak bioelectrical signal
Interference
How does non-standard position artefact affect the ECG
Changes in the morphology of ECG trace. E.g. exaggerated P or T wave or very weak bioelectrical signal being generated
How can a change in morphology of the ECG trace affect interpretation
If T or P wave are too large –> machine might interpret these as r-waves and display a rate double that of the true rate
What causes weak bioelectrical potential?
Obesity. Non-standard positioning of electrodes –> if very weak: ‘Asystole” alarm
What is a common cause for electrical interference of the ECG display
Surgical diathermy
How do various ECG machines deal with diathermy related interference?
Some machines have software which recognize the diathermy pattern and interrupt the ECG pattern while diathermy is in use.
Other machines lack this software and might sound a VF alarm
Why is inflated cuff with palpation an inaccurate method for NIBP
Only Systolic BP measured –> may falsely reassure in sepsis when diastolic pressure is lowered to a far greater degree
Why is cuff and auscultation impractical in anaesthetics?
Often the brachial artery is inaccessible to the anaesthetist
Which method of NIBP measurement is routinely used in anaesthetic practice?
Oscillometry - used by automatic non-invasive BP devices
Describe the automated oscillometric technique of NIBP measurement
Single cuff connected to a pump and a pressure transducer.
The pump inflates the cuff above systolic BP - the pressure transducer detects the amplitude of the pulse under the cuff as it is gradually deflated. The signal is then amplified and pocessed by the software within the monitor.
The maximum amplitude of pulsation corresponds to the MAP –> algorithms are used to calculate SBP and DBP
What is the appropriate cuff size for NIBP measurement?
The width of the cuff should be 40% of the mid-arm circumference
What happens if the cuff used for NIBP measurement is too small or too large?
Too small –> BP overestimated
Too large –> BP underestimated
Name 5 situations when NIBP is not accurate
- Inappropriate cuff size
- Cuff not on arm properly
- External pressure on cuff or tubing
- SBP below 60mmHg
- In the presence of cardiac dysrhythmias
What are two complications of NIBP measurement
Ulnar nerve palsy (short time intervals between. measurements)
Petechial haemorrhages
List 4 scenarios where NIBP is inadequate and invasive intra-arterial BP monitoring is indicated
- Unstable patient
- Critical perfusion states (e.g. carotid stenosis)
- Deliberate induced hypotension (e.g. middle ear surgery)
- Movement artefact (e.g. during transfer)
How does IABP monitoring work
20 G cannula into radial or dorsalis pedis artery (NOT brachial artery). Connected via a non-compliant tubing to a pressure transducer which converts the pressure into an electric signal –> fed to a monitor which displays the arterial waveform and numerical values
Why do intra-arterial blood pressure tansducers need to be ‘zeroed’?
BP values represent the pressure above atmospheric pressure
The transducer measures the ABSOLUTE pressure.
The monitor needs to be set to record the atmospheric pressure as zero
When should the monitor be ‘zeroed’
Prior to use and every 8 hours thereafter
How is the IABP monitor zeroed?
By opening the system to room air and pressing the zero button on the monitor
At what level should the IABP transducer be kept at to ensure accurate readings
Level of the heart
What is used to minimize the risk of thrombosis within the catheter and the artery
A pressurized delivery system provides a slow flush (saline)
What is underdamping in the IABP waveform.
How does underdamping affect MAP, SBP and DBP readings
What are the causes?
To produce an accurate signal, the transducer has an element to damp down the waveform slightly. This prevents ‘bounce’ or excessive resonance in the pressure wave.
Waveform - too peaked
Readings:
Accurate MAP
Underestimated SBP
Overestimated DBP
Causes: Intrinsic issues with the transducer/microprocessor
What is overdamping in the IABP waveform.
How does overdamping affect MAP, SBP and DBP readings
What are the causes?
Waveform to flat
Readings:
Accurate MAP
Overestimated SBP
Underestimated DBP
Causes:
- Bubbles
- Clots
- Kinks
- Mal-position near arterial wall
- Arterial spasm
- Loose connections
Why is diagnosis of cyanosis difficult in the operating theatre?
Cyanosis only becomes clinically evident when SaO2 80 - 85%
Surgical drapes cover the skin
Different skin colours
Difficult ambient lighting conditions
What is spectroscopy
A technique used to measure the saturation of haemoglobin with O2
A machine called a co-oximeter is used to shine two or more lights of differing wavelengths through a sample of blood and analyze the absorption of light. The absorption spectra for these different wavelengths differ for oxyHb and deoxyHb as well as COHb and MetHb.
Define a plethysmograph
A plethysmograph is an instrument for measuring changes in volume within an organ or whole body (usually resulting from fluctuations in the amount of blood or air it contains). The word is derived from the Greek “plethysmos” (increasing, enlarging, becoming full), and “graphos” (to write).
What two technological techniques are used by pulse oximetry to enable measurement of the oxygen saturation of the arterial component of blood?
Spectroscopy (Differential light wavelength absorption and analysis)
Pulse plethysmography - the software separates the pulsatile (arterial blood) from the non-pulsatile component (capillary/venous blood).
What enables just the arterial oxygen saturation to be displayed by the pulse oximeter?
Pulse plethysmography in combination with spectroscopy
What are the two wavelengths of light from the electromagnetic spectrum used in spectroscopy?
Red = 660nm Infrared = 940 nm
By comparing the absorptions of these two wavelengths, the relative quantities of deoxyHb and HbO2 can be calculated.
What is the formula used in the algorithm to calculate the SaO2
SpO2/100 = [HbO2] / [HbO2] + [deoxyHb]
What is the point at which wavelength at which the absorption by both deoxyHb and HbO2 is identical called
Isobestic point
Describe the basic mechanical set up of a pulse oximeter
A probe containing two light emitting diodes (LEDs) generating wavelengths of 660nm and 940 nm which flash alternately through the patients finger and onto a photocell. the photocell then transmits the signal to the monitor via a cable which is analyzed using specia software
What is a diode
Electrical component with the fundamental property to conduct electric current in only one direction.
What is a photocell
A photocell is a resistor that changes resistance depending on the amount of light incident on it. A photocell operates on semiconductor photoconductivity: the energy of photons hitting the semiconductor frees electrons to flow, decreasing the resistance
How much does the PaO2 change from 99 to 93% and how is this explained
13kPa to 10kPa - small change
This is explained by the sigmoid shape of OHDC
Set alrams for <92%
List 4 sources of artefact associated with pulse oximetry
- Aberrant pulse plethysmography
- Extraneous light
- Aberrant Hb
How is aberrant pulse plethysmography caused?
- Vasoconstriction (cold patient/hypovolaemic)
- Movement producing vibration
- Diathermy (less of a problem compared to ECG)
What is the effect of extraneous light on the pulse oximeter
Causes the pulse oximeter to under-read - to display erroneously low SaO2 –> occurs when the probe is not properly fitted to the finger
How is Carboxyhaemoglobin (COHb) interpreted by the pulse oximeter?
COHb has almost identical absorption of red (660nm) light. If significant amounts of COHb are present the pulse oximeter will interpret this as HbO2 and over read the true value.
Increased CO: Fire survivors and smokers (5 -13%of Hb is COHb)
Why is it important for the plethysmograph for pulse oximetry to be displayed along with the numerical data?
Used to interpret the displayed SpO2
What is capnography?
Continuous measurement and pictorial display of CO2 concentration.
The capnograph gives numerical information about CO@ levels and displays such information as a waveform
How does a capnometer measure CO2 in inspired and expired gases?
Infrared absorption spectrometry
Describe the capnograph wave form
A. Initial expiration contains no alveolar air and therefore no CO2
B. Gradually the concentration of alveolar air increases in the expired air causing a steep incline of CO2 measured.
C. Once almost all expired air is alveolar air a plateau phase is reached which has a very low gradient increase with time due to different alveolar units emptying at different rates (different time constants).
D. During inspiration there is a sudden decline in etCO2
What is absorption spectrometry?
When white light is viewed through a spectroscope it appears as a continuous band of colour (the spectrum) from red, orange, yellow, Green, Blue, Indigo, Violet.
If light is first past through an absorbing medium, the spectrum no longer appears as a clear continuous band, but shows certain regions that are diminished in intensity or absent altogether. This is known as the absorption spectrum and its character depends on the absorption of light energy by the absorbing medium
With regard to the absorption spectrum, which molecules relevant to anaesthetics absorb light in the infrared region of the spectrum?
Molecules with two or more different atoms in their molecules in the infrared region of the spectrum. Including: CO2 N2O H2O Volatile anaesthetic agent
Molecules that do not absorb light in the infrared region of the spectrum are O2 and N2
Which region of the electromagnetic spectrum does infrared light occupy?
The interval between radio waves and visible light (o.8 - 100 um)
Describe the components of a capnograph
Light source –> interference filter –> Sample chamber containing gas –> multigas filter –> Detector
Which law governs the absorption of infrared light and what does this law state
Beer-Lambert law
Absorption is proportional to the concentration (molar) of the absorbing gas in a sample chamber of known fixed length
What are the two different methods of gas sampling for measuring CO2
Side stream sampling (aspirate from the HMEF)
Main stream sampling (inside breathing circuit)
Explain how a side stream CO2 analyzer works
100 -200 ml aspirated by a fine tube from the patient’s airway –> warm, dried, filtered –> passes into a measuring chamber within the instrument
The chamber:
- volume - a few milliliters
- light filters ensure only infrared light pass through it
- made from sapqhire glass as normal glass absorbes infrared light. An infrared detector measures the light transmitted through the chamber –> the greater the concentration of CO2 the less light will be transmitted
What are the disadvantages of of side stream analyzers
- Transport time delay of a few seconds while the gas is conducted to the measuring chamber
- Low flow circle systems - volume of aspirated gas must be accounted for unless it is returned to the circuit
What are the advantages of side stream analyzers
Robust
Frequent zeroing is performed automatically by switching the aspiration pump to entrain room air
The aspirated sample can be passed through a series of other sensors for the analysis of other gases and vapours
Describe the function of a mainstream CO2 analyzer
Measurement takes place in the main flow
What are the advantages and disadvantages of a mainstream CO2 analyzer
Disadvantages
- No reference chamber –> signal may be prone to drift
- The chamber and analyzer add to the bulk of the circuit
Advantages
- No delay in measurement
- No loss of volume of inspired gas in low flow circle systems
What sources of error may infrared CO2 analysis have
- Interference by N2O as it absorbs infrared light of a similar wavelength
- Pressure changes - Increased pressure in the sampling chamber increases the number of CO2 molecules present and hence erroneously increases the measured CO2 concentration
- COPD –> upward sloping plateau phase due to uneven emptying of alveoli with different V/Q ratios and may not accurately reflect the arterial value of CO2
Why does the capnograph have an upward sloping plateau phase in COPD?
Uneven alveolar emptying with different V:Q ratios. this means that the etCO2 less accurately reflects the PaCO2
Why is capnography essential?
The continuous measurement of expired CO2 demonstrates effective ventilation.
In adult patients with healthy lungs, how much does the etCO2 differ from the PaCO2?
0.5 kPa –> this enables expired gas to be used as a reliable indicator of PaCO2
What is the most commonly used method for measurement of N2O and volatile anaesthetics
Absorption spectrometry - the identical technique to that used for CO2.
N2O and VAA have their own identifying absorption spectra and can be identified by use of the appropriate light filters that provide specific wavelengths of infrared light.
So filters remove all other wavelengths irrelevant to a specific gas. The specific infrared light is emitted through the gas sample and are proportionally absorbed according to their concentration. The amount of absorption is detected by an infrared detector which creates an electric signal for the display of this data onto a monitor.
What is the practical problem in measuring volatile anaesthetic agents? How is this problem resolved
The commonly used halogenated volatile anaesthetic agents absorb infrared at virtually the same wavelengths - but to a different extent.
Each VAA absorbs infrared light at the same wavelengths but to a different extent –> so at five set points within this wavelength range, the machine is exposed to each agent during production (of the machine) and the specific amount that each agent absorbs at each point is stored in the machines memory.
What are the two main methods for measurement of O2 in anaesthetic gas mixtures? Are these different from measuring O2 as part of blood gas analysis?
Distinct from O2 measurement as part of blood gas analysis.
- Paramagnetic analysis
- Fuel cell
What does paramagnetic mean?
It means that a substance is attracted to a magnetic field
Why is oxygen paramagnetic?
O2 is paramagnetic because it has unpaired electrons in its outer shell that are able to generate a magnetic field of their own.
What does diamagnetic mean?
A substance that is repelled by a magnetic field
What is an example of a diamagnetic gas?
Nitrogen (N2)
What is the basis of the action of the paramagnetic O2 analyzer?
O2 is paramagnetic (attracted to a magnet)
N2 is diamagnetic (repelled by a magnet)
When O2 containing gas is drawn through, the O2 molecules are attracted to the magnetic field and displace the N2 gas filled spheres in proportion to the amount of O2 that is present (i.e. partial pressure of O2)
What is the basis of the Fuel Cell O2 analyzer
A current is passed between an anode and a cathode of a fuel cell causing a reaction at the cathode
O2 + 4e- + 2H2O –> 4OH-
The current depends on the partial pressure of O2 at the cathode
What is the limitation of fuel cell O2 Analyzer?
Too slow for breath by breath analysis
Fuel cells life is reduced by the measurement of high PO2 values as the element is consumed in the reaction
What would the etCO2 trace look like in malignant hyperthermia?
Gradual and incremental increase in the amplitude of the capnograph trace
Describe the shape of the capnograph in pulmonary embolism or cardiac arrest
Decreasing amplitude with time
Describe the shape of the capnograph in bronchospasm/COPD
Increasing gradient of the plateau phase due to uneven emptying of alveoli and V:Q mismatching
Describe the shape of the capnograph in upper airway obstruction
Shark fin shaped
Describe the morphology of the capnograph during rebreathing
The base line (inspiratory line) is elevated as CO2 is being inhaled
