BP monitoring - invasive and non invasive

Question 1

how can BP be measured?

Answer 1

**non- invasive **
- discontinous - mannualy via korotkoff sounds, using oscilonometer, automated occlusive cuff device
- continuous - Penaz technique, doppler USS

** invasive **
* arterial line with piezoresistive strain gauge

Question 2

how is mean BP estimated?

Answer 2

2/3 diastolic + 1/3 systolic
however not very accurate if tachycardia and diastolic time reduced.

Question 3

compare the advantages and disadvantages of invasive and non invasive BP monitoring - include automated vs manual.

Answer 3

automated non invasive devices
* non invasive so safer
* more accurate and reliable than manuale but less than invasive due to being affected by incorrect cuff size, irregular pulse and shivering
* cheap and easy - can be performed by HCA so more practical
* uncomfortable if continually inflating and deflating cuff.

non -invasive manual
* non invasive so safer
* cheap and no electricity required
* less accurate - very subjective/ user dependant - relies on auditory sensitivity
* hard to measure diastolic
* difficult to perform - less practical

invasive
* continuous
* accurate and reliable
* however invasive so risk of infection, bleeding, clotting, ischaemia and accidental arterial injection

Question 4

what size BP cuffs should be used for different individuals

Answer 4

BP cuff size is measured using arm circumference
the bladder within the cuff should encircle at lead 80% of arm
the width of the cuff should be 20% of the arm circumference

as a general rule
3cm - infant
6cm - child
9cm - small adult
12cm - standard
15cm - large adult

Question 5

what improves the accuracy of manual BP measurements?

Answer 5

correct sized cuff, that is funcitoning well with no leaks. making sure no external pressures on the cuff.
trained clinician

Question 6

what happens if the size of BP cuff is wrong?

Answer 6

too small - over estimation of BP - doesnt occlude artery properly so need higher pressures.
too large - under estimation

Question 7

what is the differenc between korotkoff method and riva rocci?

Answer 7

riva rocci - uses pulsation. cuff inflated, pulsation disapears = systolic. cant measure diastolic

korotkoff sounds - classic sounds heard over brachial artery at different phases due to changes in flow can measure systolic and diastolic BP

Question 8

what are the korotkoff sounds ?

Answer 8

The korotkoff sounds are 5 different sounds heard as a BP cuff is deflated. they represent changes to flow within the brachial artery and can help estimate systolic and diastolic BP. due to characteristics of laminar/ turbulent flow

phase 1 = tapping sound when flow first comes back - systolic BP
phase 2 = sound soften, swishing quality or dissapears - auscultatory gap
phase 3 = return of thumping sound
phase 4 = abrupt muffling - diastolic
phase 5 = dissapears - laminar flow

Question 9

why is phase 4 used for diastolic BP in korotkoff sounds?

Answer 9

in dynamic circulation, you may never get phase 5 i.e. laminar flow.

Question 10

other than auscultation, how can korotkoff sounds be measured?

Answer 10

doppler

Question 11

describe the components of manual BP device

Answer 11

inflatable bladder within a cuff that has velcro for attachment
bladder is attached to a inflating bulb for manual inflation
this is connected to a needle valve that allows deflation.
a pressure gauge is attached to the cuff - either mercury manometer or bourdon aneroid gauge to read pressure as the cuff is inflated / deflated.

Question 12

describe the steps of manual BP measurement

Answer 12

1. ensure right size cuff
2. wrap cuff around and attach securely
3. find brachial pulse
4. inflate cuff until this is lost - estimation of systolic.
5. inflate 30mmHg more than this
6. listen with stethoscope whilst deflating
7. when first sound appears = korotkoff 1 = systolic
8. continue to deflate until = korotkoff 4 = diastolic

Question 13

why are mercury manometers rarely used?

Answer 13

large and bulky
risk of mercury poisoning if they break
column needs to be verticle to be accurate

Question 14

how does a Von Recklinghausen oscilonometer work?

Answer 14

oscilonometers are devices that detect vibrations within cuff pressure / measures oscilations

therefore can measure classic oscilations seen at different phases of blood flow equivalent to the korotkoff sounds.

consists of 2 cuffs - larger cuff has normal function inflates and deflates to occlude artery and relase. smaller cuff present inside that detects pressure changes/ oscillations as blood flows through.
so as normal large cuff inflated above systolic and then deflated slowly. As blood starts to flow past cuff it will result in oscilations and these pulsations are transmited to the smaller cuff which is picked up as oscilations on the aneroid gauge.

there is a control level that switches the aneroid gauge between the 2 cuffs - i.e. first to inflate to approapriate pressure and then switch to deflate and read oscilations/ pressure.

the point which osccilations start = systolic
max oscilations = MAP
disappearance of oscilation = diastolic

newer vesions can do single inflation and measurement in one cuff. also automatic inflation device and electronic method of detecting oscilations and sending to microprocessor to displace BP

Question 15

ow

how do automated non-invasive devices work?

Answer 15

use oscilonometers
usually automatic inflation device and deflation device
electronic method to measure oscilations and send to microprocessor
electronic display of BP

Question 16

pros and cons of the von recklinghauson oscilonometer

Answer 16

measures systolic, diastolic AND MAP
improved accuracy and relability - not relying on human auditory system.
simple + cheap
non invasive

however
- inaccurate with varying pulse rate / rhythm
- frequency limited to 1 reading per min

Question 17

what does DINAMAP stand for

Answer 17

device for indirect non invasive automated mean arterial pressure measurement

i.e. automated non invasive BP cuffs - original version

Question 18

how do recent automated BP devices differ from the original DINAMAP ?

Answer 18

one cuff does both inflation and measurement

automatic pump inflation

solenoid valve - electronic deflation - automatic

pressure transducer - records pressure and osccilations within cuff and converts to electrical signal for microprocessor to process

microprocessor - controls inflation, solenoid valve, interprets and displays data

electronic display unit

Question 19

describe the Penaz technique (finapres)

Answer 19

blood volume within the finger will vary with cardiac cycle as blood pulses
a small cuff is placed around the finger and pressure within the cuff changes to keep the volume of blood within the finger constant throughout the cardiac cycle.
infrared sensor is used to measure blood volume (lamberts law) and sends outputs to cuff to maintain constant volume

the amount of pressure required by cuff to keep this volume constant is proportional to the blood volume and pressure.

this can give a continous BP reading.
calibrate with brachial BP

Question 20

what are the pros and cons of the penaz technique?

Answer 20

pros - non invasive, continous, cheap, accurate

cons - tissue fluid relocates causing downward shift in values, painful and ischamic after 20mins.

Question 21

what are the indications for invasive BP monitoring?

Answer 21

more accurate reading may be required in
* critically ill - where rapid changes in BP may be seen and ionotropes/ vasopressors are required
* those with arrhythmias
* in obesity where non-invasive is less reliable

frequent ABG sampling in critical care

CO monitoring using arterial waveform
haemodynamic monitoring - swing
info on contractility and afterload from waveform.

Question 22

when in invasive BP monitoring contraindicated?

Answer 22

PVD in limb
infection in limb
AV fistula in that limb
graft / prosthesis
derranged clotting
patient refuses

Question 23

describe how an invasive arterial line works?

Answer 23

constist of:
* arterial canula - 20G, short and stiff
* connected to fluid filled column (0.9% saline) pressurised at 300mmHg, less than 120cm in length. which inturn is connected to a pressurised bag of saline.
* peizoresistive strain gauge transducer
* microprocesor - amplifies, filters, calibrates
* display - continuous waveform and digital number.

any pulsations in the arterial blood are therefore transmitted via the fluid filled column to the transducer which converts this to electrical signals for processing.

also has a 3 way tap for sampling and flushing

transducer must be zeroed to atmosphere and kept at level of patients right atrium for accuracy.

Question 24

describe the features of the arterial canula?

Answer 24

teflon or polyurethene - reduces thrombus risk
20G- smaller increases thrombosis, larger increases trauma to artery
short and stiff - reduces damping

Question 25

what are the methods of arterial cannulation?

Answer 25

seldinger technique using a venflon
flowswitch - catheter over needle like a peripheral canula

most commonly radial artery is used - as its easy to palpate and there is collateral circulation so less risk of ischaemia
can also use dorsalis pedis, femoral and brachial
(brachial risk of arm ischaemia, femoral high infection rate, dorsalis more technically difficult)

Question 26

how is the equiptment of invasive arterial BP measurement designed to optimise resonance and damping?

Answer 26

**RESONANCE **
All equiptment oscilates at a natural frequency. if energy is applied to this system at the same frequency, the system will oscilate at max amplitude = this is known as resonance.

In relation to invasive BP monitoring. the tubing and equiptment will have a natural frequency. we want the energy applied to this i.e. pulsations of blood pressure to be well away from the same frequency otherwise resonance will occur and the system will oscilate more which will distort the waveform.

usually the natural frequency of the equiptment is 40Hz (HR is 1-2Hz) therefore well away from that of the pulsations of blood.

DAMPING
Damping is the effect of reducing oscilations in a system. if a system is very compliant, it will absorb the energy from vibrations and they will not be transmitted to the transducer. therefore we want to reduce damping for accuracy of the system

Question 27

how is resonance minimised? what is the formula for natural frequency

Answer 27

natural frequency of the system and equiptment should be much higher than that of the pulse rate otherwise resonance will occur and distort the pressure waveform.

natural frequency is detemined by the following formula.
therefore increasing radius and stiffness of tubing will increase natural frequency, whereas increasing length and density will reduce it.

therefore we limit the tube to being 120cm in length, choose non compliant stiff material and a wider bore

Question 28

how is damping optimised in an invasive arterial blood pressure system

Answer 28

by reducing damping, the oscillations are maintained and transmitted to the transducer.

overdamping = energy absorbed, not enough oscilations. may under read BP
under damped = excess oscilations - will not settle and not good for continuous reading as needs to settle and make a reading before the next pulse is transmitted.
therefore should be optimally damped - usually 0.64 damping coeficient

factors increasing damping and are avoided include:
* kinks
* clots / air bubbles
* numerous connection
* narrow long compliant tubing.

hence non compliant tubing free of kinks & clots & airbubbles. the system has a drop rate of 2-4ml/hr to prevent clots

Question 29

what is the square wave test?

Answer 29

tests if damping of the system is optimal or not.
flush the system by applying 300mmHg pressure
results in a square wave
followed by oscilations
should see one or 2 oscilations before it settles - optimal damping - 3rd image.

more = under damped (1st image)
less = over damped (2nd image)

Question 30

what are the different ways error can occur in invasive BP reading?

Answer 30

calibration error - needs zeroing with atmosphere
transducer height - not at level of RA. if low will read high BP
changes to natural frequency and hence resonance
under/ over damping of the system

Question 31

what are the features of an ideal arterial giving set and why?

Answer 31

short canula and tubing - increases natural frequency & damping
large radius - increases natural frequency
no airbubbles, kinks, minimal connections - reduces damping

Question 32

why do air bubbles increase damping?

Answer 32

absorb the energy from pulsations because air is compressable

Question 33

why does BP read higher when placed below patient?

Answer 33

will add the hydrostatic pressure and gravity effect of the column of fluid to the pressure reading.

For every 10 cm the transducer is placed below the heart, the blood pressure will increase by about 7.5 mmHg

Question 34

what different methods of calibration and drift do you know?

Answer 34

offset drift - requires 1 point calibration e.g. zeroing with atmophere - do this by placing line at level of right atrium, opening 3 way tap to atmosphere, pressing zero on machine, then back to patient.

gradient drift - requires 2 point calibration - this needs to be zeroed with atmosphere but also calibrated at a higher known pressure using a manometer.

Question 35

what are the complications of invasive BP monitoring?

Answer 35

infection - local or systemic. minimised by asceptic

haemorrhage - disconnection

thrombosis and ischaemia

accidental arterial injection - should clearly label the line

errors in reading - may be below heart and giving a good reading or above and giving a low reading and accidentaly treated.

Question 36

what factors increase risk of thrombosis of arterial line?

Answer 36

more common with prolonged canulation
polyurethene > teflon
more likely in smaller vessels

Question 37

draw an arterial waveform, describing factors that can be inferred from it

Answer 37

directly measures
* systolic and diastolic BP
* HR
* rhythm

also…
upstroke = contractility
position of dicrotic notch = SVR
area under systolic portion = SV and CO monitoring from contour analysis
up to dicrotic notch = systolic time, then diastolic time.

MAP and pulse pressure - can use systolic and diastolic to measure these.

swing - haemodynamic status

Question 38

which part of arterial waveform gives indication of myocardial perfusion?

Answer 38

area under diastolic proportion - coronaries fill in diastole.

Question 39

what factors of blood vessels determine systolic and diastolic BP

Answer 39

compliance of vessels and ability to recoil

little arterial compliance - high systolic - seen with age
poor abiltiy to recoil - low diastolic

Question 40

draw a graph for hypotension, hypertension and arrhtyhmia ..

Answer 40

hypotension = flatted and loss of dicrotic
hypertension = dicrotic is closer sharper rise and fall
arrhtyhmia = irregular.

Question 41

why does site of arterial cannulation alter the waveform?

Answer 41

the more peripheral the less elastic and more stiff the arteries - therefore higher systolic and lower diastolic. loss of dicrotic notch

Question 42

how can the arterial waveform be used for monitoring haemodynamic status of an individual?

Answer 42

in general a swing with respiratory cycle will indicate hypovolamia

however this is done more accurate via PPV (pulse pressure variation %) - measures the change in SBP-DBP with resipiratory cycle in IPPV. this is only accurate in controlled ventilation when there are no arrythmias.

usually PPV % of 15% means need for fluids.

Stroke volume variation (SVV) % can also be used in a similar way - usually by doppler.

if fluid bolus is given and they respond to this, likely were volume deplete.

Question 43

why does the pulse pressure vary with ventilation in under filled patients?

Answer 43

in inspiration - positive pressure reduces venous return and CO
in expiration the opposite
this is exagerated when fluid deplete.

Question 44

what is plethysmography variability index?

Answer 44

pulse oximetry can be used to measure % of pulse pressure variation and hence fluid depletion
non invasive method.

Question 45

how does the strain gauge work?

Answer 45

low compliance silicone diaphragm stretches in response to pressure transmitted from the column of fluid.
as diaphragm stretches, the resistances in wires changes
within microprocessor is filters, amplifiers and wheatstone bridge.
wheat stone bridge is used to measure the unknown resistance.

Question 46

how can BP be measured in obese?

Answer 46

larger cuff
may need to use leg
or invasive

Question 47

what BP is needed to be able to manually feel a carotid/ peripheral pulse - often used in peri-arrest…

Answer 47

radial pulse is palable above 70mmHg
carotid pulse is palpable above 50mmHg

Question 48

safety features of arterial lines..

Answer 48

Locsip for insertion - ensures asceptic, guidewire removed etc
the tubing has a red strip on it and labelled to help prevent intra arterial cannulation
no port for giving medications - reduces intra-arterial injection

Question 49

draw and explain the concept of a wheatstone bridge

Answer 49

this is an electrical circuit which uses a voltage divider to monitor unknown resistance accurately and quickly.

2 limbs with 2 resistors on each limb in parallel
2 known and 1 variable, 1 unknown

the variable resistor is altered until the voltage division between both limbs is equal - therefore no current flows between them (measured by the galavanometer)
this is a process of null deflection
unknown can now be determined by knowing what the variable was set to to achieve this.

Question 50

what are harmonics?

Answer 50

Harmonics refer to sinusoidal components of a complex wave or signal. if a wave is very complex you need more harmonics to analyse it.

around 10 to 40 harmonics needed for biological signal processing