1. Blood Pressure and Sound (2)
Dept. of Biomedical Engineering
YOUNHO HONG

2. IBP ( Invasive BP ) measurement
catheter
Blood
vessel Pi
pill up with
some liquid
(ex.saline)
diaphragm
strain gages
cable Po t Pi
If we choose sticky and dense liquid,
We can’t get the signal of Pi just like the graph.
To get the signal, We should concern with
distance, diameter of a catheter and liquid, air-
bubble inside a catheter. t Po

3. IBP ( Invasive BP ) measurement
# Equivalent Circuit Model Of Catheter-Sensor System
(1) Resistance L A
resistivity P1 P2 R
viscosity

4. IBP ( Invasive BP ) measurement
(2) Capacitance or Compliance
(3) Inductance or Inertance

5. IBP ( Invasive BP ) measurement
multi
physics
Electric Circuit
Fluid Mechanics
voltage
current
charge
pressure
flow
volume

6. Equivalent Circuit Model of IBPVi
Compliance of
diaphragm + Vo - i
catheter Pi
liquid
diaphragm
strain gages
cable Po
damping
ratio
natural
frequency

7. Equivalent Circuit Model of IBP
(1) Frequency Transfer Function

8. Equivalent Circuit Model of IBPw
|H| Wn w -π
-π/2 ∠H

9. Equivalent Circuit Model of IBP
Methods to solve 2nd order ODE

10. Steady State Freq. Response
-4/π
-1.8π ∠H
|H| f f1 f2 K
0.5

11. Unit Step Response
In reality, We need a unit step function for a starting point.
For example, should be
input signal
underdamping
critical damping
overdamping

12. Transient Step Response
bulb
balloon
saline
underdamping
critical damping
overdamping

13. Example
(7.1) A 5mm-long air bubble has formed in the rigid-walled catheter to
a Statham P23Dd sensor. The catheter is 1m long, 6 French diameter,
and filled with water at 20 ℃. Plot the frequency-response curve of the
system with and without the bubble.
log f
1.34
1.95

14. Example
(7.2) By changing only the radius of the catheter, redesign the (no-bubble)
catheter of Figure 7.9 to achieve the damping ratio ζ=1.
Calculate the resulting natiral frequency fn.
1.46
log f

15. Thank you.