Invasive monitoring Flashcards
What is dynamic response?
The speed at which it is able to settle on a new value following a stimulus
What 3 factors interact to affect dynamic response?
- Damping
- Input frequency
- Natural (resonant) frequency
What is natural frequency?
Frequency at which system oscillates when set in motion
What is input frequency?
The frequency of energy input into the system
What is resonance?
When there is an energy input at the same frequency as the natural frequency
What is damping?
Loss of energy
What is critical damping?
- Damping coefficient of 1
- No overshoot & long time for amplitude to settle at 0
What does a higher damping coefficient mean?
Smaller amplitude response to resonance
When does maximum resonance occur?
When input frequency is equal to natural frequency (1)
How is resonance avoided in an arterial line?
Natural frequency should be at least 8 times greater than the max anticipated input frequency
How is natural frequency calculated?
Natural freq = speed / wavelength
What is the optimum damping coefficient for a system?
0.64
What is the role of an arterial transducer?
Convert pressure energy into electrical energy
What are the 3 types of arterial transducers?
- Wire strain gauge
- Bonded strain gauge
- Capacitive transducer
What is resistivity?
The degree to which a material opposes the flow of electrical current
How do wire strain & bonded arterial transducers work?
- Contain wires that vary their resistance as arterial pressure is altered
- Fluctuating in accordance with geometry & resistivity
- WIRE: Inc art pressure decreases tension of resistance wire = inc CSA & reduced length
- BONDED: Coil of resistance wire, as art pressure wires stretched - tension & resistance inc
What is the equation for resistance of a wire?
R = p x l/CSA
p = resistivity
l = length
CSA = Cross-sectional area
How does temperature affect resistivity?
Inc temp: semiconductors = decrease, metal = increase
How does a capacitive transducer work?
- 2 capacitor plates, 1 attached to diaphragm
- Inc in art pressure reduces distance between 2 plates
- Inc in capacitance of the capacitor so reduces resistance
- Change in resistance produces waveform
What is a Wheatstone bridge circuit?
- 4 resistors with different properties arranged in a quarter/full-bridge circuit
- R1/R2 = R3/R4. (R4 = unknown resistance of strain gauge)
- R4 is calculated by altering R3 resistor until voltmeter = 0
- R4 can be plotted against time and converted into arterial pressure
How does adding a 3rd Sin wave into Fourier analysis affect an arterial trace?
Wave more likely to equal resonant frequency of the system
Resonance more likely to occur - amplitude increases x10
How is pulse contour analysis calculated?
Area under sin wave of arterial trace
Calculates SV & CO
What does the dicrotic notch on an arterial trace represent?
End of systole & AV valve closure
How does hypovolaemia effect the dicrotic notch?
Shifts to the right due to delayed AV closure
How does early inspiration effect swing on an arterial line?
- Inc intrathroacic pressure compresses pulmonary vessels
- Inc LV preload
- Inc SV
How does late inspiration effect swing on an arterial line?
- Prolonged inc in intrathoracic pressure compresses now empty pulmonary vessels
- Reduces LV preload
- Reduces SV
What 2 parameters quantify degree of swing on art line?
- SV Variation: Variation in SV over resp cyle (30secs)
- Pulse pressure variation: Variation in PP over resp cycle (30secs)
How can an arterial line be amplified?
- Simple: Increases amplitude of signal by adjusting gain
- Differential: Reduces electrical interference by bandwidth frequency/common-mode rejection
What are the calibration issues with an art line?
- Zero: Actual reading at 0 displays inaccurately (high)
- Gain: Actual reading displays as lower - needs amplifier
