7. Pressure & measurement Flashcards
Damping
Causes x 5
Reduced -
Reduction in oscillation in resonant system caused by dissipation stored energy
- Drag of fluid & connection
- Compression air bubbles
- Defective flush - clots
- Distensible / complaiant connection
- Kinking
Reduced silicone transducer
Methods of measuring CO x 6
Oesophageal Doppler Transthoracic impedance PiCCO NICO Thermodilution Lithium dilution.
PA Catheter
Path:
Respsecitve normal pressures along route
Path: Ra-RV-PA-Occlusion (indirect LAP) 0-12 2-25 12-25 8-12
A line complications x8
Haematoma formation Distal ischaemia Infection Embolisation Pseudoaneurysm Arteriovenous fistula Compartment syndrome Blood loss.
Testing Transducer catheter system What wave can be applied What is frequency response x fundamental Damping Optimal Tubing
Square wave signal calibrate the system.
Ffrequency response x 10 fundamental frequency.
Damping and resonance alter systolic and diastolic pressures leaving the mean pressure unaffected.
An optimal damping factor is between 0.6 and 0.7. The arterial waveform is sufficiently damped to allow slight overshoot.
The tubing used to conduct the pulsatile fluid movement from the tip of the cannula to the transducer system ideally should be stiff, non-pliant, less than 120 cm in length and of a ‘large’ diameter.
Physical properties - Methods of measuring pressure
Change of electrical resistance in a wire
Oscillotonometry
Torricellian vacuum
Variable inductance
wire strain gauge
what will vary a wire elec ressitance
in what relationshio
what does it act as in what
Stretching or compression of a wire will change its electrical resistance.
Varying resistance of a wire when stretched
ecomes longer and thinner and consequently its resistance increases
A strain gauge acts as a resistor, which are used in pressure transducers. In a pressure transducer the diaphragm moves as the pressure changes and this alters the tension in the resistance wire, thus changing its resistance.
The changes in current flow through the resistor are then amplified and displayed as a measure of pressure change.
However, to measure or monitor these changes in resistance, a Wheatstone bridge is frequently used.
Torricellian vacuum
Exists above a column of mercury in a manometer: t
his is the principle of the common barometer for measuring atmospheric pressure.
Penaz tecnhnique
Continually measure bp in finger
eg finapress
Cuff - pressure tducer - servo airpump
photocell + IR diode - plethysmo
air pump feedback, inflate + deflate -
maintain constant output plethy
some perfusion occurs during inflation but risk ischaemia
Finger cuff claibrated measure mean
led and phoycell - calc IR absorb at mean pressure
Servo control - contin inflating + deflating keep IR absorp constant
Natural frequency related to:
:
directly related to the catheter diameter
inversely related to the square root of system compliance
inversely related to the square root of the length of tubing
inversely related to the square root of the density of fluid in the system
What is the natural frequency
system is resonant.
natural frequency should be at least 10 times the fundamental frequency.
What is the fundamental frequency
Fundamental frequency is the lowest frequency at which the system resonates. A harmonic is a multiple of this frequency
Bourdon gauge / aneorid gague
meausre
shape
connected
face
meaure cylinder + pipeline pressure
Coil - oval shaped tubing
outer end connect to gass supply
sealed inner tube connected to needel pointer
face - reinfroce heavy glass if tube rupturess
Bernoulli effect
describe it
venturi principle
what causes jet entrainment
whats entrainement ratio
fall in pressure at a constriction
at constriction - kinetic energy increase
tot energy same
pressure / pot energy decease
Total entrain fluid - thru side arm into low pressure caused by constriction = venturi princple
jet entrain = frictional forces increase amt fluid drawn in
Entranment ratio
entrained flow to driving flow
Osmometer calculates
what principle
what does one mole of a sub do when added to kg water
osmolality - based
coligtaive
depression freezing point solutin direct proport to osmolality
1 mole sub added to 1kg water depress freeze point by 1.86
