Exam 3 - Faithful Reproduction Flashcards
1
Q
Fluid version of Ohm’s Law
A
Pressure = Flow (Q) x Resistance (R)
R = (8)(n)(L) / (pi)(r^4)
2
Q
What Patient Pressures do we measure
A
- Arterial P
Those measured by SWAN:
- Central Venous P
- Pulmonary Artery P
- Pulmonary capillary wedge pressure
- Reflection of L atrial P (pre-load)
3
Q
System pressures that we measure
A
- Arterial line P
- Cardioplegia line P
- Cardiac chamber pressures
4
Q
Pressure
A
P = Force / Area
N/m^2 = Pascal
5
Q
Hydrostatic Pressure
A
- Force is constant
6
Q
Hydrodynamic Pressure
A
- Force is varying
- Pulsatile nature of arterial blood pressure
7
Q
Closed system
A
- Patient and ECC for our purposes
- Flow is relatively laminar and vessel lumen is small
8
Q
Static fluid pressure key determinant
A
- Height (or difference in)
- Gravity and density will stay the same
9
Q
Artifact
A
- False signals superimposed on true signal
10
Q
Calibration
A
- Making sure system is accurate and can read known values
11
Q
Catheter Whip
A
- Movement of catheter tip in vessel due to pulsatile flow, etc.
12
Q
Damping
A
- Loss of energy and vibrations within monitoring system
- Think of as resistance
13
Q
Dynamic response
A
- System’s ability to measure physiologic pressure changes
14
Q
Fidelity
A
- System’s capability to faithfully reproduce physiologic event
- System accuracy
- can use snap test to check
15
Q
Natural frequency
A
- Frequency at which monitoring system vibrates when stimulated by pulsatile signals
- Resonant frequency
16
Q
Overshoot / Undershoot
A
- When patient’s pressure wave has a component of the system’s natural frequency causing some kind of artifact
17
Q
Ringing
A
- Multiple small spikes on waveform
18
Q
Zeroing
A
- Removing affect of atmospheric and hydrostatic pressures
19
Q
What types of pressures does our monitoring system measure
A
- Hemodynamic P (energy in pressure wave form originating from LV)
- Kinetic energy P (energy associated with motion of fluid)
- Hydrostatic P (Difference in height from patient to pump)
20
Q
Summary of Invasive monitoring
A
- Catheter (picks up pressure waves)
- fluid column in tubing carries pressure wave to diaphragm of transducer
- transducer is link between fluid and electronics by converting pressure signal into electrical (change in voltage)
- electrical signal (change in voltage) is transmitted to monitor, amplified, modified, then displayed as # or pictures
21
Q
Components of Fluid-filled monitoring system
A
- Catheter
- Low compliance pressure tubing
- Pressure transducer
- Amplifier / Display
- Flush system (3-5 ml/hour, prevent clot and backflow)
22
Q
Fourier Analysis
A
- Think of electronics as black box
- Takes complex wave form and breaks down into individual SIN and COS waves without changing them
- Puts waves back together and then displays on monitor
- Measures first six harmonics
23
Q
Fundamental frequency
A
- 1st harmonic
- Convert HR to freq (Hz)
- usually just divide by 60…use dimensional analysis - Subsequent harmonics are just multiples of fundamental
- Amplitude also keeps getting smaller
24
Q
Frequency response
A
- ability of monitoring system to pass harmonics with no distortion in amplitude
- response needed is related to degree of fidelity desired
- usually a distortion leads to increase in amplitude
25
Characteristics needed by black box
- Amplitude linearity (amplitude same out as in regardless of frequency)
- Adequate bandwidth (can cover all harmonics or range of freq's)
- Phase linearity (fundamental lag or lead)
- can't fix this....caused by black box....may/may not get amp distortion
26
6 physical principles of monitoring system
- Hydrodynamic pressure
- Effect of Intertia
- Effect of freq. response and natural freq.
- Snap Test (dynamic response test)
- Orientation of catheter
- Zeroing, leveling, calibration of system
27
Hydrodynamic pressure
- Pressure exerted by fluid motion
- Pulsatile in nature because of systole/diastole
- creates range of pressures
28
Effect of Inertia
- Energy or pressure caused by MOVEMENT of blood
- Monitoring system cannot respond in tandem with results
- Which is why long tubing causes greater lag
29
Effects of frequency response and natural frequency
- Natural frequency = fundamental = resonant frequency
(Natural freq decreases w/ decrease in stiff or increase in mass)
- We want to MAXIMIZE resonant frequency to avoid overshoot
- At least bigger than 6th harmonic....20 Hz at minimum
- Catheter radius and stiffness affect natural frequency
-WANT: short, stiff, wide bore cath (18 gauge)
30
Underdamped
- Waveform with overshoot or over-exaggeration
- Natural frequency may also be too low
- Looks narrow and peaked
- Overestimates SBP and underestimates DBP
- MAP unchanged
- Causes: long tubing, increased vascular resistance (patient effect)
31
Overdamped
- Waveform looks blunted
- Widened and slurred
- Underestimates SBP and overestimates DBP
- MAP does not change
- Causes: air bubbles, over-compliant tubing, kinks, clots, small / half open stopcocks, injection ports, no flush fluid, low flush pressure
32
Snap Test
- Can test ability of system to faithfully reproduce pressure
- aka fast flush test aka dynamic response test
- pull valve....quickly let go....look at waveforms on monitor
- can test the dampening of the system
- Good dampening will return 1 or 2 oscillations
- If not....check system
33
William A. Gardner
- Looked into effects of frequency response / dampening on monitoring equipment
34
Increase dampening if....
- Increase fluid viscosity
- Decrease catheter radius
- decrease density of fluid
35
Optimal damping
- 1 or 2 oscillations
- 0.64 with a high resonant frequency
- If increase natural freq....then we can lower dampening
- If decrease natural freq....then we increase the dampening
36
Catheter placement
- Point upstream for most accurate (ABP)
- Venous pressure may be downstream
- Best is perpendicular but will not see with catheters
37
Zeroing, leveling, calibrating
- Once all set....want to Zero to room conditions
- Make sure equipment is level with mid chest line of patient
- otherwise might see overestimation/underestimation
38
Patient factors that affect monitoring
- Hypertension
- Shock
- Hyper-dynamic circulation
- Sepsis, aortic regurg., atherosclerosis
- Tachycardia (increases fundamental frequency)
39
Reflected waves
- Inherent in system
- can augment true waves
- vasoconstriction, heart failure, hypertension, aortic obstruct
- can diminish true waves
- vasodilation, hypovolemia, hypotension
40
Factors of monitoring system we have control over
- Tube length (3-4 feet)
- No fluid on diaphragm of reusable transducers)
- Air in system
- Small bubbles can lower natural frequency
- Large bubbles can overdamp
- Tubing compliance (want stiff)
- Luminal size of Cath (want large bore)
