IBP Flashcards
Wheatstone Bridge
- Electrical circuit with one unknown resistor, one variable resistor, two unknown resistor
- Pulse into transducer causes mechanical deformation of strain gauge
- Pressure then sent to unknown resistor –> pressure gauge
Measuring Device
aneroid manometer, commercial transducer/physiograph
Transducer Level with Newer Monitors
internally compensate for vertical differences btw patient, transducer with ‘offset pressure’ – once zeroed, cannot change height of transducer without rezeroing
Transducer Level with Older Monitors
Older monitors: no offset feature, transducer/zeroing stopcock must be at level of RA
Frequency Response of Measuring System Determined By:
- Resonant Frequency
- Damping Coefficient
Resonant Frequency
rate of system oscillation IRT change in pressure
duration of one complete cycle (peak to peak, trough to trough)
Damping Coefficient
rate at which oscillations rest after change in pressure
Basics of IBP
–each cardiac contraction exerts pressure = mechanical motion of flow within catheter, transmitted to transducer via rigid fluid-filled tubing
–Transducer converts motion to electrical signials
–Monitor displays beat to beat arterial waveform as well as numerical pressures
Challenges with IBP in Cats
–Arteries = small, expertise required
–Constrict with cut down manipulation
–Collateral circulation spare: femoral, MT, coccygeal placement can lead to distal ischemia
Pressure Wave Analysis
Allows for better understanding of patient’s heart function as correlates to cardiac cycle
Systolic Phase
Rapid rise in pressure to peak followed by rapid decline
Opening of poetic valve, corresponds to LV ejection
Dicrotic Notch
Closure of aortic valves
Beginning of diastole
Turns into the dicrotic wave distally DT delay
Diastolic Phase
Run off of blood into peripheral circulation
Distal Systolic Pulse Amplification
Occurs further from aorta that measurement is occurring
SAP increases, MAP/DAP decreases
Dicrotic notch displaces R DT delay - transforms into dicrotic wave
Small, Weak Pulses
Pulse wave diminished, pulse feels weak/small
Upstroke may feel slowed, peak is prolonged
Causes:
–decreased SV
–Increased PVR: exposure to cold, severe heart failure
Normal Pulses
Pulse pressure ~30-40mm Hg
Pulse contour smooth, rounded - notch not palpable
Large, Bounding Pulses
Pulse pressure increased
Rise, fall may feel rapid - peak is brief
Causes:
–Increased SV: slow HR
–Decreased peripheral resistance: can occur with increased SV in fever, anemia, hyperthyroidism, aortic regurgitation, AV fistula, PDA
–Decreased aortic wall compliance: aging or athlerosclerosis in people, primates, birds
Bisferiens Pulse
Increased arterial pulse with double systolic peak
Causes:
–Pure aortic regurgitation
–Combined aortic stenosis, regurgitation
0-HCM
Pulses Alternans
Pulse alternates in amplitude from beat to beat even though rhythm basically regular
Can be caused by left ventricular failure or decreased ventricular filling
Bigeminal Pulses
May mimic pulses alternans
Normal beat alternating with premature contraction
–SV of premature beat diminished IRT that of normal beat - pulse varies in amplitude accordingly
Paradoxical Pulses
Palpate decrease in pulse’s amplitude on quiet inspiration, decrease by >10mm Hg
Pericardial tamponade, exacerbations of asthma, COPD
Similar to pulse pressure variation
Contraindications for arterial line placement
Infection
–Can be catastrophic in critically ill patients/multiple comorbidities
–Location of catheter, duration of placement affect rates
–Remove as long as no longer required
Lack of collateral circulation resulting in vascular insufficiency
Hematoma formation
Venous stenosis
Blood Loss
Embolism
Excessive heparinization
Inadvertent Nerve Damage
Column Manometer
Long fluid administration set suspicion from high point
MAP 140mm Hg = 186cm column of water
Aneroid Manometer
–Expel all air from air/fluid tubing via air from syringe 2 out through stopcock 1
–Close stopcock 2 to syringe 2
–Inject sterile saline from syringe 1 into tubing toward manometer until pressure»_space;> MAP
–Close stopcock to syringe 1
–Allow high pressured saline to flow from air/fluid mixed tubing toward patient –> equilibrated pressure = MAP
Wheatstone bridge Circuit
–4 resistances: 2 known, 1 unknown
–Silicone diaphragm of transducer moves small plate connected to 4 strain gauges
With any one movement, two gauges stretched, two compressed
–Mechanical stretch causes change in resistance
–Potential difference generated proportional to pressure applied
Law Associated with Wheatstone Bridge
Ohm’s Law: V = IR
(potential difference [voltage] = current * resistance)
Atmospheric Pressure and Wheatstone bridge
Atmospheric pressure must be discounted from pressure measurement: transducer exposed to atmospheric pressure, calibrated to 0
Level of RA, midaxillary line
Once zeroed, relative height of transducer cannot be changed without re-zeroing
Pressure in System and Transducer Height
Weight of column of fluid within tubing exerts hydrostatic pressure on transducer: proper leveling minimizes effect
For every 2.5cm above/below catheter (transducer?) level, pressure in system changes 1.87mm Hg
If the system is too low…
Transducer exerts greater pressure and produces abnormally high values
7.5mm Hg subtracted for every 10cm above heart
If the system is too high…
Abnormally low readings
7.5mm Hg added for every 10cm above heart
Fidelity of Reproduction of PP Waveform
Result of interactions btw frequency response of catheter and measuring system; patient factors
Frequency response = 2 parameters
1. Resonant frequency
2. Damping Coefficient
Natural Frequency
Frequency at which system tends to oscillate in absence of any driving or damping forces
Every system has own natural oscillatory frequency (resonant frequency)
Arterial pressure waveform: many different sine waves (Fournier analysis) with each sine wave having own frequency
Problem with Natural Frequencies of Arterial Waveform vs Transduction System
If resonant frequency to transduction system coincides with one of frequencies making up arterial waveform, resonance and distortion of signal will occur
Resonant Frequency of ABP
Fn waveforms = 30 Fn - goal is 6-10x HR
LJ: typically 10-25Hz
Optimal frequency depends on HR, systolic vigor
Resonant Frequency
rate of system oscillation IRT change in pressure
Resonant Frequency of Catheters, Infusion Tubing
Fn >40 Hz
Resonant frequency of Transducers
50-100Hz - 1.5-5x HIGHER than frequency of waveforms monitor
Optimally damped transducer system with resonant frequency >36Hz accurately reproduce shape of arterial waveform up to HR of 180bpm
Damping
Anything that reduces energy in oscillating system, reduces amplitude of oscillations
Some degree of damping required in all systems
–If excessive (overdamping) or insufficient (underdamping), output effected
Where does most damping come from?
Friction of fluid pathway
Damping Coefficient
rate at which oscillations rest after change in pressure
Following system flush, amplitude ratio of two consecutive resonant waves calculated by dividing smaller ratio by larger
Dynamic Pressure Response Test
Sudden release of pressure on measurement system, ie flushing
Determines resonant frequency, amplitude reduction ratio
Observe oscillation pattern as pressure returns to baseline
Registered pressure >300mm Hg, returns to baseline with 1-2 pos/neg oscillations if dampened appropriately
Amplitude Reduction Ratio
height of any half complete cycle (peak to trough or trough to peak) divided by height of previous half cycle
o Typical: 0.2-0.6
Amplitude Reduction Ratio
height of any half complete cycle (peak to trough or trough to peak) divided by height of previous half cycle
o Typical: 0.2-0.6
Overdamped Signals
Coefficient >1
System will not oscillate freely, details lost (eg dicrotic notch)
Underestimate SAP, overestimate DAP
Causes of Overdamped Signals
- Small gauge catheter, often limited by patient size
- Blood clots, air bubbles
- Longer, more compliant tubing
Underdamped Signals
Coefficient <0.7
Tendency to overshoot, oscillate around resting point - exaggerated, spikey appearance
Overestimate SAP, underestimate DAP
How respond to underdamped signals
- Change something in measurement system: remove kinks/clots, remove or add air bubble, change to different length or different compliant tubing
Wilson et al 2018 (VAA)
comparison of IBP in three pairs of peripheral arteries – facial vs transverse facial, metatarsal vs facial, metatarsal vs transverse facial
o Poor agreement btw sites: largest = SAP btw F/TF, smallest MAP btw TF/MT
o BP measured in peripheral arteries cannot be used interchangeably
