19. Blood Pressure Measurement Flashcards
How can you measure blood
pressure?
1 Non Invasive
- Discontinous
a) Manual Occlusive method - riva rocci or korotkoff
b) Von Recklinghausen oscillonometer
c) Automated occlusive cuff method
- Continuous
a) Penaz
b) Doppler u/s
2. Invasive
Arterial line
Piezoresistive strain gauge
Describe the manual occlusive cuff method.
This uses a cuff most commonly
placed around the upper limb.
The cuff must cover two-thirds of the
length of the limb
(or it must be 20% greater that the diameter of the limb).
The width of a standard adult cuff is 14 cm
and the cuff length should be twice the width.
If the cuff is too small it will over-read and if too large it will under-read.
> Riva-Rocci method:
Blood pressure was measured by palpation of
the brachial or radial artery as the cuff was inflated.
The loss of the pulse represented systolic blood pressure. Riva-Rocci (an Italian physician) was
credited with developing the first conventional sphygmomanometer.
> Korotkoff method: Blood pressure was measured by auscultation over the brachial artery.
In 1905, Korotkoff (a Russian army surgeon) described
a series of noises that could be heard over the brachial artery as the cuff deflated:
• Phase I: Tapping sound appears (systolic blood pressure)
• Phase II: Sounds muffle or disappear (auscultatory gap)
• Phase III: Sounds reappear with a tapping quality
• Phase IV: Sounds muffle again
(diastolic blood pressure in the UK)
• Phase V: Sounds disappear
(diastolic blood pressure in the USA).
In a hyperdynamic circulation,
the sounds may never disappear and this is
why we use phase IV to denote diastolic pressure.
What are the advantages and
disadvantages of using Manual Occlusive method
Advantages:
> Simple
> No electricity required
> Doctor–patient contact
Disadvantages:
> Operator dependent
> Correct cuff size required
> Artefacts with arrhythmias and movement
> Tourniquet effect can cause nerve damage
> Underestimates hypertension and overestimates hypotension
How does a von Recklinghausen’s oscillotonometer work?
The old-fashioned versions of these machines
work using two cuffs.
The first is an occluding cuff,
which sits proximally,
and the second a sensing cuff
against which the blood pulses
once the occluding cuff has been deflated
sufficiently to allow blood to flow under it.
Each cuff is attached to two
bellows and these bellows
are attached to a single needle.
A lever enables the operator to select which bellow’s pressure is displayed by the needle
(i.e. it can either make the needle ‘sensing’ or ‘reading’ depending on which position it is in).
> At systolic blood pressure – there is a sudden increase in needle oscillations.
> At mean arterial pressure – there is maximal amplitude in needle oscillations.
> At diastolic blood pressure – there is a sudden decrease in needle oscillations.
How does a ‘DINAMAP’ work?
DINAMAP® is the trade name for
one of the original automated occlusive BP
cuff measuring devices.
It is based on the principle of an oscillotonometer,
but now the two cuffs have been
merged into a single cuff, which performs
both occluding and sensing functions.
There is also a pneumatic pump, bleed valve, transducer, processor and display monitor.
> As the cuff deflates,
the transducer detects the
flow of blood under the cuff.
> The processor, with a built-in algorithm,
then relates the rate of change of
pressure transients to
systolic, diastolic and mean blood pressures.
> These readings are then displayed on the monitor.
Describe the Penaz technique (the ‘Finapres’).
Penaz principle states that
‘the force exerted on a body
can be determined by measuring an opposing force that prevents physical disruption’.
It consists of an infrared plethysmograph
within a pneumatic cuff.
> The Finapres cuff is wrapped around the distal phalanx of a finger (over a digital artery).
It shines an infrared light through the finger,
which is detected on the opposite side.
The amount of light absorbed is directly
proportional to the volume of the finger
(this volume changes during systole and diastole).
> A pneumatic pump is controlled by the infrared signal.
It continuously adjusts the cuff pressure in
order to maintain a constant infrared signal
(it aims to keep the volume of the finger constant, which represents the mean arterial pressure).
> The pressure inside the cuff required to
achieve this is measured and
gives a continuous arterial BP reading.
What are the advantages and disadvantages of this method?
Advantages:
> Continuous reading
> Accurate
Disadvantages:
> Downward drift due to tissue fluid relocation
> Painful after 20–30 min
> Ischaemia of digit
How can Doppler ultrasound be used to measure BP?
Arterial pressure waveform
This is based on the principle of the Doppler shift.
> Transducer crystals within a probe
transmit and receive ultrasound waves.
> Probe positioned over artery with a coupling medium (e.g. gel).
> As the arterial wall moves during systole and diastole there is a Doppler shift in frequency of the ultrasound waves
