Simple approaches to difficult
topics
Measurement and Monitoring
Dr Alan McLintic
Middlemore Hospital
Q: How do you measure Cardiac output using
thermodilution?
Q: How do you measure Cardiac output using
thermodilution?
• Summary
– Thermodilution principle is a modification of the Fick principle
Amt.substance takenup by organ  AVconcentrationdifference blood flow
Q: How do you measure Cardiac output using
thermodilution?
• Summary
– Thermodilution principle is a modification of the Fick principle
Bloodflow throughorgan 
Amount taken up by organ
AVconcentrationdifference
250ml.min1
Q
10
20  15 ml.dl-1
.
.
Q  5 l.min1
Q: How do you measure Cardiac output using
thermodilution?
• Summary
– Pulmonary artery catheter (‘Swan-Ganz’ catheter)
Proximal
lumen
Thermistor
Balloon
Connection
for thermistor
Distal lumen
Q: How do you measure Cardiac output using
thermodilution?
• Summary
– Inserted through large neck vein
Q: How do you measure Cardiac output using
thermodilution?
• Summary
– Floated through heart until the tip is in the pulmonary artery
Q: How do you measure Cardiac output using
thermodilution?
10 ml dextrose (21ºC)
Dilution of ‘coldness’
measured here
Colder 
Q: How do you measure Cardiac output using
thermodilution?
Recirculation
Body temperature
Time 
Q: How do you measure Cardiac output using
thermodilution?
High
cardiac
output
Time 
Lower
cardiac
output
Colder 
Colder 
The greater the cardiac output, faster the
dilution, the smaller the Area Under the Curve
(AUC)
Time 
Q: How do you measure Cardiac output using
thermodilution?
Dye dilution:
Mass of dye (g)
Volume 
Mass dye added (g)
Mean concentration (g/l)
Mean concentration dye (g)
Q: How do you measure Cardiac output using
thermodilution?
Concentration dye (g/l) 
Dyes:
.
Q
Mass dye added (g)
AUC (g/l.min)
Time 
Q: How do you measure Cardiac output using
thermodilution?
Thermodilution
.
Mass cold added
AUC
Colder 
Q
Body temperature
Time 
Q: How do you measure Cardiac output using
thermodilution?
Thermodilution
Mass cold added Volume.TBody  TInjectate.k
Q

AUC
AUC
Colder 
.
Body temperature
Time 
Q: How do you measure Cardiac output using
thermodilution?
Thermodilution
.
Colder 
Q
Mass cold added Volume.TBody  TInjectate.k


AUC
 TB (t )dt
0
Modified Stewart-Hamilton
equation
Body temperature
Time 
Q: How do you measure Cardiac output using
thermodilution?
Thermodilution
.
Mass cold added
AUC
Colder 
Q
Body temperature
Time 
Q: How do you measure FRC using a Body
Plethysmograph?
Q: How do you measure FRC using a Body
Plethysmograph?
• The Body Plethysmograph is
a method to measure lung
volumes by the application of
Boyle’s Law
Q: How do you measure FRC using a Body
Plethysmograph?
Box pressure
Mouth pressure
Shutter
Calibrating syringe
Q: How do you measure FRC using a Body
Plethysmograph?
• Step1.
– Calibrate changes in
box pressure as
changes in volume of
air in the box
Box
volume
Q: How do you measure FRC using a Body
Plethysmograph?
• Step2.
– Apply Boyle’s Law to
lung air….
– …while panting against
closed shutter
Box
volume
Q: How do you measure FRC using a Body
Plethysmograph?
• Step2.
– Apply Boyle’s Law to
lung air….
PBar. VFRC = (PBar- P). (VFRC + V)
Box
volume
Q: How do you measure FRC using a Body
Plethysmograph?
• Step2.
Atmospheric
pressure: 100 kPa
Box
volume
FRC?
 Box volume
PBar. VFRC = (PBar- P). (VFRC + V)
FRC?
Mouth pressure when
shutter closed
Q: How do you measure FRC using a Body
Plethysmograph?
•Summary:
Summary
– Method of measuring lung volumes by the application of Boyle’s
law
– Briefly explain set up and calibration of box pressure for box air
volume
– Write equation
Atmospheric
pressure: 100 kPa
FRC?
 Box volume
PBar. VFRC = (PBar- P). (VFRC + V)
FRC?
Mouth pressure when
shutter closed
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
• Full answer regarding accuracy
– Practical aspects
• Prevention clot, kinking, choice of artery, cannulae
• Zeroing
– Static accuracy
– Dynamic accuracy
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Natural
frequency (FN)
Damping
Frequency at which a
system oscillates most
freely
Tendency for a system to
resist oscillation through
friction
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Natural
frequency (FN)
Frequency at which a
system oscillates most
freely
The FN is the same frequency as the
upstroke of trace  resonance and
overshoot
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Natural
frequency
High as
possible
Prevents resonance from
biological signals
 HR 
F 

 4 
N
Short, stiff, short, wide tubing
Small stiff transducer
Low density fluid
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Natural
frequency
High as
possible
Prevents resonance from
biological signals
Undamped
D=0
Damping
Underdamped
D = ~ 0.3
Optimal
D = 0.64
Critical
D = 1.0
Optimal
In a totally undamped
system the system
would oscillate at the
undamped natural
frequency without any
decrease in amplitude
In an underdamped
system, overshoot is
common and the
system oscillates at the
Fn with progressively
diminishing amplitude.
This system would
result in distortion and
over-reading from
overshoot
In an optimally
damped system,
there is only 7 %
overshoot. Ideal
compromise
between minimal
overshoot and
response speed
In a critically
damped system,
damping has
increased to the
point where
overshoot is just
avoided. The
response speed of
this system would
be too slow
7% overshoot in fast flush test
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Natural
frequency
High as
possible
Damping
Optimal
Prevents resonance from
biological signals
Short, stiff, short, wide tubing
Small stiff transducer
D = 0.64
High density fluid
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Natural
frequency
High as
possible
Prevents resonance from
biological signals
To produce flat
frequency response
Damping
Optimal
Prevents amplitude distortion of
high frequency waveforms
Prevent phase distortion
All elements of the waveform are
delayed by the same time interval
To produce flat
frequency response
Arterial waveforms are made
up of several different sine
waves of different frequencies
Fourier analysis
Too big
Very under-damped
(0.1)
1.0
Too small
Amplitude relative to correct amplitude
To produce flat
frequency response
Ideal
Flat frequency response to 2/3 FN
Optimal damping (0.64)
FN
Frequency of sine waves
All but the very
fastest
waveforms will be
reproduced
without amplitude
distortion
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Natural
frequency
High as
possible
Prevents resonance from
biological signals
To produce flat
frequency response
Damping
Optimal
Prevents amplitude distortion of
high frequency waveforms
Prevent phase distortion
All elements of the waveform are
delayed by the same time interval
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Prevent phase distortion
90
180
Time
delay
All elements of the waveform are
delayed by the same time interval
If the same time delay is applied
to both component waveforms, the
1 Hz waveform will be delayed by
90 and the 2 Hz waveform by
180. Thus the phase delay would
be proportional to the frequency.
This will only occur if damping is
optimal.
Q: What are the important physical principles in
the design of an invasive pressure monitoring
system?
Natural
frequency
High as
possible
 HR 
F 

 4 
N
Damping
Prevents resonance from
biological signals
To produce flat
frequency response
Optimal
D = 0.64
Prevent phase distortion
Q: How does BIS analyse EEG?
Q: How does BIS analyse EEG?
How the algorithm was
determined
Step 1
EEG analysed
from 2000
healthy adults
undergoing
different levels
of anaesthesia
Step 2
Clinical levels
of anaesthesia
scored
Step 3
Statistically
determine
EEG patterns
commonest at
each level of
anaesthesia?
Q: How does BIS analyse EEG?
How the real time analysis
works on patients
EEG patterns
compared with
algorithm
Patient’s EEG
analysed
Score
determined
from 0 – 100
Anaesthesia
recommended
40-60
Q: How does BIS analyse EEG?
Bispectral:
Degree of
Phase
EEGcoupling
synchronisation
Power spectral analysis
Burst suppression
Q: How does BIS analyse EEG?
Q: How does BIS analyse EEG?
Q: How does BIS analyse EEG?