1
Direct Blood Pressure Measurement
Raquel Lopez, Candidate for M.A.Sc.- Clinical Engineering, Institute for Biomaterials and
Biomedical Engineering
Abstract— Direct blood pressure measurement is critical for
the treatment, diagnosis, and monitoring of critically ill patients.
The system investigated herein to measure direct blood pressure,
is a catheter-transducer system comprised of: a catheter (fluidfilled sensor), pressure transducer, amplifier, analog-to-digital
converter, and a processor with display. The dynamic response
components (natural frequency and damping coefficient) of this
damped, second-order system define the pressure waveform
accuracy. Studies conducted, namely by Gardner and Van
Genderingen, determined the relationship between dynamic
response components and accuracy. It was found that for
patients with a normal resting heart rate, the absolute minimum
natural frequency of the system leading to an accurate waveform
was 7Hz, occurring at a damping coefficient of 0.5. For patients
with high resting heart rates, such as neonates, the absolute
minimum natural frequency of the system is 4Hz, also occurring
at a damping coefficient of 0.5. To ensure that the dynamic
response components do not fall out of the accurate damping
range, the following procedural steps need to be addressed during
the setup and operation of the catheter-transducer system:
priming and monitoring of system components, zeroing, leveling,
and dynamic response testing.
Index Terms— invasive blood pressure measurement, arterial
pressure waveform, dynamic response components
I. INTRODUCTION
T
HE most accurate method of measuring blood pressure
is by its direct measurement via an intravascular sensor
device. For routine physical exams, this method is not
practical due to its invasive nature however, for treatment,
diagnosis, and monitoring of critically ill patients, invasive
blood pressure measurement is inherent. In particular, arterial
pressure (AP) is monitored as it correlates to system perfusion
and consequently, it is an excellent indicator of the response to
a treatment or surgical procedure. Further, an AP waveform
provides continuous information on the pumping action of the
heart, such as valve closure and systolic and diastolic arterial
pressures. Since the accuracy of this AP waveform is more so
limited by the dynamic response of the hydraulic system
(tubing, stopcocks, and catheter) than by the electronic
equipment of the system (amplifier, processor), the need to
control the hydraulic system is more prominent. The purpose
of this report is to present the optimal dynamic response
components (natural frequency and damping coefficient) of the
hydraulic system that ensure waveform accuracy. It further
aims to discuss factors influencing these components and
clinical methods to address such factors.
II. BACKGROUND
A. Catheter-Transducer System
Direct blood pressure is typically measured by means of a
catheter-transducer system (Figure 1). The catheter serves to
provide access to the artery and also detects the pressure
waves generated therein by cardiac contraction. It may be a
fluid-filled or solid pressure probe. Solid probes, although
less subject to error, are more difficult to implant and more
expensive. Consequently, fluid-filled catheters are more
commonly used.
Within these, the pressure wave is
transmitted via fluid-filled tubing and stopcocks to the sensing
diaphragm of the pressure transducer. The transducer converts
this mechanical signal into an electrical one. In blood pressure
monitoring, the transducer has a silicon wafer diaphragm
which when deformed, produces an alteration in electrical
resistance.
The altered resistance is measured by a
Wheatstone bridge and propagated by a transmitter to an
amplifier. Filters are usually found within the amplifier to
minimize waveform noise. The signal is then converted at an
analog-to-digital converter. It is transmitted to a processor that
calculates other hemodynamic parameters, such as mean
arterial pressure, based on the measured variables. Finally, the
signal is displayed on a monitor in its analog form along with
its digital output. To produce minimal error in the arterial
pressure waveform, this catheter-transducer system needs to be
accurate, sensitive, linear, and have minimal drift. This report
focuses on the major contributor to inaccuracy, the hydraulic
system. It does not discuss inaccuracy attributed to the
electronic system (signal conditioning unit) for the reason that
(i) it is not as significant a contributor and (ii) in terms of
clinical use of the catheter-transducer system, it is the
hydraulic system that can be most affected by user error. [1]
Pressure Transducer
Pressure
Sensor
(Fluid-filled
Catheter)
Transmitter
Signal Conditioning Unit
Amplifier
Analog-toDigital
Converter
Processor
&
Monitor
Figure 1: Schematic of Catheter-Transducer System Process
B. The Typical Arterial Pressure Waveform
The typical arterial pressure waveform for the cardiac cycle
of a healthy individual is depicted in Figure 2. The following
cardiac parameters are some of which can be interpreted
2
directly from the waveform: systolic pressure, diastolic
pressure, pulse pressure, dicrotic notch pressure, heart rate,
and ejection time. Systolic arterial pressure is that pressure
exerted on the artery walls due to contraction of the left
ventricle of the heart. Diastolic pressure is pressure exerted on
the walls due to relaxation of the left ventricle. The normal
range of systolic pressure is 110-140mmHg and that of
diastolic pressure is 60-90mmHg. [2] Pulse pressure is the
difference between the peak systolic and diastolic pressures.
The dicrotic notch pressure is the pressure reflecting the
closure of the aortic valve which leads to some backflow of
blood in the artery downstream the valve. Heart rate is the
frequency of contraction, measured in beats per minute.
Ejection time is the time of left ventricle blood ejection.
Examples of when these parameters are significant include:
diagnosis of such diseases as hypertension, drug therapy for
atherosclerosis patients, and monitoring success of implantable
medical devices.
P2 ( f )

P1 ( f )
1
2
f
f
 2 j
1
2
Fn
 Fn
(1)
D. Factors Affecting Pressure Waveform Dynamic
Response Components
The dynamic response waveform components, natural
frequency and damping coefficient, can be affected by three
mechanical factors, leading to an error in pressure waveform
of as much as 50 percent. [4] These factors include:
compliance (C), fluid inertia (I), and fluid resistance (R).
Compliance refers to the flexibility of the pressure transducer
diaphragm and the system tubing. Fluid inertia refers to the
coefficient related to the pressure needed to accelerate the
liquid through the tubing. Fluid resistance refers to the
viscosity of the fluid in the tubing and the friction at the walls
of the tubing. [1],[3],[5] Given the values for C, I, and R,
Equation (2) can be used to determine
Fn 
Fn and  . [1]
1
2 IC
(2)
R C

2 I
Figure 2: Typical Arterial Pressure Waveform [2]
C. Pressure Waveform Dynamic Response Components
The catheter-transducer system can be modeled as a secondorder dynamic system. [1],[3] This means that the pressure
waveform transmitted to the diaphragm of the transmitter via
fluid-filled tubing dampens, such that with each successive
pulse, its amplitude decreases. There are two dynamic
response components of a waveform: natural frequency and
damping coefficient. The natural frequency of a system is its
number of oscillations per unit time occurring without any
damping. The damping coefficient is related to the time taken
to dampen the waveform. The transfer function that allows the
calculation of the output transducer signal (in mV) from an
input transducer signal (mmHg) is of second order. Equation
(1) shows the second-order transfer function, where
P1 , P2 are
These values when substituted back into the transfer
function, determine the arterial pressure waveform for any
frequency.
The resulting damping coefficient deems if the system is
underdamped, overdamped, or optimally damped. In an
underdamped system, the pressure waves tend to reverberate
within the catheter and tubing leading to formation of wave
reflections or harmonics. These harmonics are additive and
lead to the overestimation of systolic pressure and
underestimation of diastolic pressure. In an overdamped
system, frictional forces impede the arterial waveform such
that it loses energy. Figure 3 depicts a schematic of an arterial
waveform in an (a) underdamped and (b) overdamped system.
Notice that underdamping leads to a narrow, peaked tracing
which overestimates systolic pressure and underestimates
diastolic pressure while overdamping leads to a broad,
flattened waveform which underestimates systolic blood
pressure and overestimates diastolic blood pressure.
(a)
(b)
output and input signals of the pressure transmitter,
Fn is the natural
frequency,  is the damping coefficient, and j is the complex
respectively, f is an arbitrary frequency,
number. [1]
Figure 3: Damping Affect on Arterial Pressure Waveform: (a)
Underdamped Waveform and (b) Overdamped Waveform (SBP –
systolic blood pressure; DBP – diastolic blood pressure) [6]
3
1.2
1.1
1.0
.8
.7
Unacceptable
.6
.5
.4
Optimal
. .2
.3
Damping Coefficient
.9
Overdamped
Adequate
Underdamped
.1
A catheter-transducer system accurately measures arterial
pressure waveform if its dynamic waveform components
(natural frequency and damping coefficient) are accurate. In
other words, if the system is accurate, a pressure waveform can
return to its baseline waveform after being perturbed. Studies
by Gardner and Van Genderingen determined the effect of
dynamic waveform components on system accuracy. [1],[7]
Van Genderingen investigated these effects for systems
requiring a quick response, such as systems for measuring
neonatal arterial pressure. These systems are required to
detect waveforms resulting from high resting heart rates (150230 beats per minute). [5] Gardner investigated the effects of
dynamic waveform components on accuracy for systems
requiring a response to normal resting heart rates (50-100
beats per minute). Both studies used pressure simulators to
generate waveforms of different natural frequency and
damping coefficients and recorded the extent to which the
perturbed waveform differed from the original one. The
method of perturbation used is known at the square wave test
or the fast-flush test. In this test, a square pulse in pressure
waveform is generated by opening and closing the flush valve.
Subsequently, the flow through the catheter-transducer system
is acutely increased and generates an acute rise in pressure
within the system in the form of a square wave. A system with
appropriate dynamic response components will return to the
baseline waveform within one to two oscillations.
Figure 4 shows the relationship between natural frequency
and damping coefficient for a system requiring normal
dynamic response. It describes a range of natural frequency
and damping coefficient combinations that are required for an
optimally damped system. An optimally damped system has a
high natural frequency to allow for the largest possible range
in damping coefficients. Commonly, for systems not requiring
a fast response, natural frequencies greater than 30Hz are
accurate, meaning that they yield an arterial pressure that is
within 1mmHg of the true pressure. [1] Further, the absolute
minimum natural frequency that can lead to an adequate
pressure waveform is 7Hz. A damping coefficient of 0.5 is an
adequate level of damping for the widest range of natural
frequencies. [1]
Figure 4 shows the relationship between natural frequency
and damping coefficient for systems requiring a fast or high
dynamic response. Error of 2 percent in arterial pressure
waveform is considered to be small and as such, acceptable,
whereas error greater than 10 percent is excessively large and
is unacceptable. [7] Note that a 2 percent error translates to an
error in arterial pressure of 3-4.6mmHg, which is greater than
the error permitted for lower dynamic response systems
(1mmHg). This is the case since fluid-filled cathetertransducer systems that require a fast response are affected
more heavily by compliance, fluid inertia, and fluid resistance
than solid probe systems. Consequently, the aim of recently
developed solid probes is to combat error in arterial pressure
measurement for high dynamic response systems. Figure 5
shows that an absolute minimum natural frequency that can
lead to an accurate pressure waveform is 4Hz. In general, for
systems requiring a fast response, natural frequencies greater
than 60Hz are accurate. A damping coefficient of 0.5, as in
Gardner’s study, is also an adequate level of damping for the
widest range of natural frequencies. [7]
In general, for both systems, as the natural frequency
decreases, the requirement for damping coefficient is less than
1. This indicates that the second-order dynamic system is a
damped one.
0
III. ACHIEVING AN ACCURATE PRESSURE WAVEFORM
0
5
10
15
20
25
30
35
40
45
Natural Frequency (Hz)
Figure 4: Degree of Pressure Waveform Damping as a Function of
Natural Frequency and Damping Coefficient for Patients with a Normal
Heart Rate [1]
Figure 5: Percentage Error in Pressure Waveform as a Function of
Natural Frequency and Damping Coefficient for Patients with a High
Heart Rate [7]
IV. DISCUSSION - QUALITY ASSURANCE TO ACHIEVE
ACCURATE DYNAMIC RESPONSE COMPONENTS
The catheter-transducer system obtained from the
manufacturer has a set natural frequency value. Yet, the
compliance, fluid inertia, and fluid resistance factors that
affect natural frequency and damping coefficient vary with
4
system setup and use. This variation needs to be controlled by
the user to assure high quality arterial pressure waveform
measurement.
The following procedural steps should
therefore be followed with every patient being measured: (1)
priming and monitoring of system components, (2) zeroing and
(3) leveling or referencing the transducer, and (4) dynamic
response testing. [5],[6],[8]
1) Priming and Monitoring System Components
The presence of air bubbles in the system as well as any
blood clotting at the tip of the catheter both contribute to
overdamping of the pressure waveform. An overdamped
waveform leads to an underestimation of systolic blood
pressure and an overestimation of diastolic blood pressure.
Air, an incompressible fluid, acts as a shock absorber that
dampens the waveform, subsequently producing erroneous
pressure values. It affects the resistance of the fluid by altering
its viscosity. Air bubble entrapment is a high-risk source of
error as (i) even air bubbles measuring 1 mm in diameter can
cause substantial waveform distortion and (ii) they can occur
at many locations throughout the system.[6] Consequently, airfree priming of the entire system is one of the most important
steps in avoiding technical error. It includes the following
checks: tightening of all connections, closing stopcocks to air,
avoiding multiple stopcocks and injection ports, and
periodically flicking and flushing all tubing and stopcocks to
eliminate tiny air bubbles escaping the flushing solution. In
particular, special monitoring of tubing, stopcocks,
interconnectors, and the transducer needs to be conducted as
these locations are prone to air entrapment. Also, the use of
tubing that is as short as practically possible is advised, as this
decreases the number of possible locations of air bubble
occurrence.
If a patient is having their arterial pressure monitored for an
extended period of time, the likelihood that blood clotting at
the tip of the catheter will occur is increased.[6] As such, the
catheter should be visually inspected and flushed regularly.
The physical properties of the tubing also affect the
transmission of the pressure waveform. If the tubing is overly
compliant, the pressure waveform will be overdamped. If the
diameter of the tubing is too large (greater than 1x10 -3),
optimal damping cannot be achieved. [8] Thus, as a general
rule, stiffer, shorter, and narrower tubing produces improved
accuracy of the pressure waveform.
2) Leveling
For each arterial pressure monitoring session, the equipment
must be setup such that an accurate reference point from which
all subsequent measurements is established. This reference
point refers to the position of the air-fluid interface of the
catheter-transducer system (the stopcock attached to the top of
the transducer) that is to read atmospheric pressure. For
arterial pressure monitoring, this height corresponds to the
patient’s right atrium of the heart.
Clinical personnel
approximate it externally by an anatomical landmark called the
phlebostatic axis. In the event that the transducer is placed
below the phlebostatic axis, the resulting arterial pressure
waveform will be inaccurately high. Conversely, the arterial
pressure waveform is erroneously low if the transducer is
higher than the phlebostatic axis. [5]
3) Zeroing
Zeroing ensures that the catheter-transducer system assigns
the air-fluid interface as the reference point measuring
atmospheric pressure. This is done by opening a stopcock to
atmospheric pressure, and, as with most instrumentation,
selecting ‘zero’ from the console. Zeroing eliminates zerodrift, which is the potential, usually minimal, transducer offset
or distortion occurring over time. A pressure monitoring
system should be zeroed regularly. [5]
4) Dynamic Response Testing
To determine if dynamic response components have been
altered throughout the course of system setup and use, a square
wave test like that used by Gardner and Van Genderingen
should be performed. If dynamic response components are
inadequate, that is if after perturbation the waveform does not
return to its original waveform within one or two oscillations,
the above procedural steps should be readdressed. [6]
In general, in order to achieve accurate arterial pressure
measurements, it is essential that the catheter-transducer
system have a high natural frequency, be suitably damped,
properly zeroed, appropriately referenced, and regularly tested
for dynamic response.
V. CONCLUSIONS
Direct pressure monitoring is essential for determining
immediate changes in a patient’s blood pressure values. The
waveform offers valuable diagnostic, treatment, and surgical
monitoring information. Yet, if not accurate, erroneous data in
the arterial pressure waveform can potentially lead to
detrimental treatment. For systems required to respond to
normal heart rates, the minimum natural frequency ensuring
accuracy is 7Hz. For high dynamic response systems, this
frequency is 4Hz. In both systems, the damping coefficient that
permits the widest range of natural frequencies is 0.5. Clinical
personnel need to be aware of these dynamic response
components leading to optimal damping and consequently
accurate waveforms. Their involvement in system setup
(priming, leveling, zeroing) as well as regular dynamic
response testing is critical to quality assurance of the measured
arterial pressure waveform. These findings are a significant
benchmark in physiological measurement and pave the way for
direct blood pressure measurements of the ventricles and atria,
as well as pressure measurement of other fluids such as the
craniocerebral fluid.
5
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[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
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