Blood Pressure Measurement Flashcards
The equation for pressure is P = F/A. Why is this concept less clear with regards to the pressure generated in the ventricles of the heart.
It is less clear because the contraction of the muscle is action tangentially at the surface of the heart chamber. This force gives rise to tension. Tension is the force applied per unit length.
What is La Places law for a sphere
P = 2T/R
P - Transmural pressure
T - Wall tension
R - radius of the chamber
A chronically ill and distending heart has an increasing radius. Thus the pressure produced will fall unless the force of contraction increases the wall tension –> increasing myocardial oxygen demand:supply.
As opposed to Frank Starling Law in the health heart, wherein increased preload stretches the myocardial cells to obtain a more efficacious arrangement of the actin-myosin filaments resulting in an increased force of contraction with increasing preload.
Describe how the measured blood pressure varies depending on site of measurement in a standing 2 m tall man. Explain why this occurs
Head - 53 mmHg
Upper arm - 90 mmHg
Feet - 202 mmHg
Hydrostatic forces increase with gravity increases BP reading as measurement proceeds more inferiorly in a standing patient.
What are the formulae for mean arterial blood pressure
MAP = SVR x CO
MAP = DP + 1/3 (SBP - DBP)
What is the formula for cardiac index and how is TBSA calculated
CI = CO (L/min) / TBSA (m^2)
Normal: 2.5 - 4.0 L/min.m^2
TBSA
= Square root of [(Height (cm) x Weight (kg) / 3600]
Classify the methods used to measure blood pressure
- MANUAL, INTERMITTENT, NON-INVASIVE BP
- AUTOMATIC, INTERMITTENT, NON-INVASIVE BP
- CONTINUOUS, NON-INVASIVE BP
- CONTINUOUS, INVASIVE BP
What is an appropriate sized blood pressure cuff
- Bladder width 40% of limb circumference
- Bladder length 80 - 100% of limb circumference
- Bladder length : width ratio at least 2:1
How will NIBP readings be affected by cuffs that are too big and too small
Too small –> BP falsely high
Too large –> BP falsely low
What are limitations of palpating the BP
- Underestimate SBP by 25%
2. Cannot calculate DBP or MAP
When is doppler used for SBP determination
Reserved for anticipated very low measurements, i.e. in vascular insufficiency.
This is because the doppler method is very sensitive
What is the mechanism of the Korotkoff sounds?
Not known for certain but probably some combination of the following theories
- Cavitation theory (formation air bubbles)
- Arterial wall theory (sudden stretching of a. wall)
- Turbulence theory (Turbulence)
Which sound should be used to determine diastolic BP
The 5th korotkoff sound unless the pulsation continues to be audible on complete deflation of the cuff in which case the 4th Korotkoff sound should be used.
Describe the tone and intensity of the 5 Korotkoff sounds
NO sound
Korotkoff 1 - snapping tone low intensity
Korotkoff 2 - murmurs reduced intensity
Korotkoff 3 - Thumping tone increased intensity
Korotkoff 4 - Muffled tone decreasing intensity
Korotkoff 5 - sounds disappear
Compare initial oscillometry to the Von Recklinghausen Oscillotonometer
The intiital oscillometer: needle oscillations over an aneroid gauge maximum at MAP then decrease at a less precise point around diastolic BP
The Von Recklinghausen Oscillotonometer improved on this simple observation by using electronic oscillometric equipment which improved accuracy and allowed for automatic measurement
Describe the function of a Von Recklinghausen Oscillotonometer
Two cuffs
1. Occluding cuff
2. Sensing cuff
Connected to two bellows
Lever between the two cuffs acts as a switch
With lever in sensing position, occluding cuff inflated above estimated SBP and then gradually deflated using a bleed valve. When the needle starts moving in the sensing cuff the lever switches back to the occlusion cuff pressure and this pressure recorded –> SBP.
Lever then switches back to sensing position deflation continues and then switches back to occlusion cuff when the oscillations are at their maximum, this is the mean arterial pressure.
Lever switched back to sensing cuff until oscillations decrease dramatically –> back to occlusion cuff reading which reads this cuff pressure as DBP. (least accurate of all readings)
Give an example of a liquid manometer
Mercury Sphygmomanometer measures the height of a mercury (mmHg) column i.e. the gauge pressure (pressure above atmospheric pressure).
The cuff is connected to this mercury column and a deflation accomplished by a bleed valve. This is an open manometer
Why was the mercury column replaced by an aneroid gauge?
More robust
Avoids problems associated with mercury toxicity
How does an aneroid gauge work? What are the limitations
Cuff expands bellows which moves a pointer over a scale.
Loses accuracy over time so requires regular calibration
List the techniques used for automated, intermittent, non-invasive BP measurement
- DINAMAP (Automated oscillotonometer) - two cuffs
2. Modern oscillotonometers - use one cuff that fulfils occlusion and sensing function
What does DINAMAP stand for
Device forIntermittent Non-invasive MAP measurement
What are the pitfalls of cuff systems for BP measurement
- Accuracy requires regular rate and rhythm
- Cuff size NB
- Accidental movement
- Limb level with heart
- External cuff compression
- Artery compression altered in diseased artery’s
- Compression: can damage tissue (nerves)
- Often painful in awake patient (esp high P’s)
- Cannot detect BP’s below 50 mmHg
What are the pitfalls of invasive BP systems
- Must be zeroed
- Transducer must be level with heart
- Complications of arterial cannulation
- Infection
- Nerve injury
- Thrombosis
- Air embolus
- Infarction distal limb
- Haemorrhage
- Inadvertent arterial injection of drugs
What is the Penaz technique used for. What does the Penaz principle state
This is the technique used to give continuous measurement of finger blood pressure
Penaz principle: ‘ a force exerted by a body can be determined by measuring an opposing force that prevents physical disruption’
What type of equipment uses the Penaz technique
The Finapres (Finapres Medical Systems)
Describe how the Penaz technique measures continuous finger blood pressure
Small cuff around finger
LED shine light through finger with detector on opposite side
–> amount of light absorbed by tissues is proportional to the volume of tissue through which it passes.
–> Cardiac cycle: variable blood in finger –> variable light absorbed
–> The cuff attempts to keep light absorption (and hence volume of blood in finger) constant by applying pressure to the finger throughout cardiac cycle.
The applied pressure waveform correlates to the pressure waveform of the arterial supply of the finger.
When is the Penaz technique (Finapres) accurate and when is it inaccurate
Accurate in vasodilated patients and in patients with normal circulation
Inaccurate in hypotensive patients or in vascular insufficiency
What is the gold standard of blood pressure measurement
Continuous invasive BP monitoring gives accurate beat-to-beat information
How do SBP and DBP measurements by NIBP compare with IABP
In IABP –> SBP slightly higher and DBP lower (larger PP)
So NIBP underestimates PP
When is invasive BP monitoring indicated
CVS instability
- Labile BP expected
- Large fluid shifts
- Pharmacological effects (e.g. vasopressors/ inotropes)
NIBP will be inaccurate
- Obesity
- Arrhythmias (e.g. AF)
- Non-pulsatile blood flow from cardio-pulmonary bypass)
Long term measurements and sampling
- ICU patients
What is hydraulic coupling and when is it used
It is the basis for invasive arterial blood pressure monitoring
Provide a solid column of fluid which connects arterial blood to a pressure transducer
List the components required for continuous invasive BP monitoring
- Intra-arterial cannula
- Tubing (incorporating an infusion system)
- Transducer
- Microprocessor and display screen
- Mechanism for zeroing and calibration
Describe the type and location of insertion of the intra-arterial cannula
Type
- Teflon or polyurethane
- 20 G adults, 22G and 25G for children and neonates
- short and parallel sided
Location
- Best: Radial / dorsalis pedis
- Not great (end arteries): Femoral, brachial, axillary
- Never: carotid
- Allan’s test to check collateral supply to the hand not 100% accurate
Explain how the transducer works in IABP monitoring
The fluid within the intra-arterial cannula and tubing lumen is in direct contact with the diaphragm within the transducer. The pressure waveform from the artery therefore moves the diaphragm.
The transducer is most commonly a strain gauge:
‘Resistance of a wire increases with increasing length and visa versa’
–> If a wire is stretched it becomes longer and thinner —> increased resistance to flow
–> The diaphragm is connected to a wire with a strain gauge. The strain gauge will shorten and lengthen with movement of the diaphragm and hence the length will change altering the resistance. A Wheatstone bridge circuit can be used to measure these changes in resistance. This can then be amplified and displayed on a monitor
Why is teflon or polyurethane used for the cannula
Why is it short with parallel sides?
Teflon Minimizes risk of thrombosis and resonance frequency > 40Hz
These are stiff materials, the cannula sides are parallel and it is short to increase the resonance frequency > 40Hz. If the resonance frequency is < 40Hz, these oscillations will be combined with the arterial waveform by Fast Fourier Transform (microprocessor) and produce increased amplitude pressures and pressure waveforms. If the resonance frequency of the cannula, tubing and transducer system is kept above 40 Hz, filters will remove this frequency and Fast Fourier Transform will not incorporate this resonance into the display.
What is the difference between a Galvanometer and an Ammeter
Galvanometer - shows direction and magnitude of the current
Ammeter - shows magnitude of current only
How can the effects of temperature on resistance be cancelled out in the transducer
The diaphragm is connected to two strain gauges on one side and two on the other side. With any movement, two are stretched and two are shortened –> the effect of temperature is cancelled out as all of the strain gauges are the same temperature
All the strain gauges are part of the same wheatstone bridge circuit which increases the sensitivity four fold
By how much will raising or lowering the transducer relative to the patient’s heart change the pressure reading
Raise probe 10 cm above right atrium –> Hydrostatic forces within the tubing will increase (with the relative effect of gravity) retarding movement of the diaphragm attached to the strain gauge within the transducer. This erroneously lower pressures will be recorded: For ver 10 cm raised –> 7.5 mmHg lower than actual BP.
And visa versa for lowering the transducer
What effect does the cannula height relative to the transducer have
Minimal
What is resonance
Diaphragm movement in the transducer is associated with small amount of movement to and fro with arterial pressure pulsations. Just as a weight on the end of a spring will oscillate at a particular frequency (known as the resonant frequency), so the pressure measuring system of consisting of the cannula, tubing and transducer, saline column possess a resonant frequency at which oscillations can occur. If less than 40Hz –> Fast Fourier Transform (microprocessor) will superimpose these sine waves onto the pressure waveform, giving distortion from resonance (jagged saw tooth graphic).
How is the resonance frequency of the pressure transducing system increased
- Shorter cannula
- Stiffer cannula (material: teflon/polyurethane)
- Wider cannula
the problem is generally worse with longer cannulae (i.e. 8cm femoral a lines)
What is damping and what causes it
Any restriction to the transmission of the blood pressure from the artery to the transducer diaphragm can cause damping. Damping is the smoothed out and lower amplitude appearance of the pressure waveform graphic.
Importantly bubbles within the transducing system will absorb some of the transduced pressure –> damping.
Blood clots within the cannula are another common cause.
Occlusion or obstruction of the system by a bent patient wrist is another.
What is the effect of resonance and damping on the SBP, DBP and MAP
Affects SBP and DBP more than MBP
How does the pressure wave form change in the peripheral arteries versus more proximal ones:
- Aortic root
- Radial artery
- Dorsalis pedis
As sensing of IABP becomes more distal from the heart:
- Amplitude increases (smaller diameter of vessel)
- Peak amplitude is delayed (time taken to get distal)
What is the frequency range of the arterial pressure wave
0 - 40 Hz
How does the fibreoptic transducer tipped pressure monitoring catheter work?
Mirror coated moving diaphragm which reflects light carried to the tip by an optical fibre. the position of the diaphragm, which changes in response to changes in pressure, determines the fraction of incident light that is reflected back down a second fibre. The second fibre connects to a OPTOELECTRIC module which converts the light into electrical signal. Associated electronics interpret the reflected light intensity in terms of pressure.
How is a CVP pressure transducing system zeroed
- Patient horizontal. Get three way tap.
- Spirit level on a rod attached to manometer is set to the level of the midaxillary line, which is taken as representative of the level of the RA. Zero here. Can also use pressure transducer (continuous)
- Stop saline drip for measurements
Why is some damping useful in IABP monitoring
It reduces resonance
