Continuous Noninvasive Blood Pressure
Measurement by Pulse Transit Time
Parry Fung
1
, Guy Dumont
1
, Craig Ries
2
, Chris Mott
1
, Mark Ansermino
2
1Department of Electrical and Computer Engineering, The University of British Columbia, Canada
2Department of Anesthesia, The University of British Columbia, Canada
Abstract—Blood pressure measurement is performed ei-
ther invasively by an intra arterial catheter or noninvasively
by cuff sphygmomanometry. The invasive method is con-
tinuous and accurate but has increased risk; the cuff is safe
but less reliable and infrequent. A reliable continuous non-
invasive blood pressure measurement is highly desirable.
While the possibility of using pulse transit time to moni-
tor blood pressure has previously been investigated, most
studies were limited to calculating the correlation of the
pulse transit time and blood pressure under rather static
conditions. The relationship between the pulse transit time
and blood pressure is yet to be clearly identified. This pa-
per focuses on the modeling between the two values and
presents results on cases where dramatic variation in blood
pressure of the patient was induced by drug administration
or surgical stimulation.
Index Terms—Continuous, Noninvasive, Blood Pressure,
Pulse transit time, Pulse Oximetry
I. Introduction
A continuous measurement of blood pressure (BP) is
desirable for consistent patient care and monitoring. Al-
though BP can be measured continuously by an intra ar-
terial catheter, this costly and invasive method introduces
risk to the patient and workload for physicians. Risks of
arterial injury and skin infection does not justify its use in
many circumstances. Moreover, the rather complex setup
procedure for arterial catheter insertion may take up to
30 minutes. Expensive disposable equipment adds to the
cost of direct arterial measurement. Therefore, a nonin-
vasive method of measuring BP is desirable for patients
without significant organ dysfunction during short dura-
tion surgery, especially patients with expected fluctuation
in BP, e.g. undergoing Cesarean Section. Conventional
noninvasive BP measured by cuff sphygmomanometry up-
dates a BP reading at most every minute (usually 3 min-
utes), which is not frequent enough for certain clinical situ-
ations, e.g. hypotension treatment during a Cesarean Sec-
tion. Here, an algorithm of inferring BP by pulse transit
time (PTT) was developed for an advisory system which
aims to treat maternal hypotension induced by spinal anes-
thesia performed for Cesarean Section.
It is commonly accepted that PTT is correlated with
BP [1] and a number of commercial BP monitors use PTT
to infer BP. However, the details of the actual algorithms
have not been disclosed by the manufacturers. A human
model of BP and PTT is first presented. The model sim-
plifies the body structure and relates BP to PTT by fun-
damental physics, the conservation of energy. To reduce
inter-patient variability, the model customizes parameters
by using an easily accessible physical property of the pa-
tient, height. The practicality of the model is retained by
ignoring or approximating a few unavailable parameters
such as the elasticity of arterial wall, heart pre-ejection
period (PEP) and blood density. The paper also includes
a method for computing PTT from the ECG and plethys-
mogram (PPG). While ECG and PPG are two standard
signals in most anesthesia monitors, the PTT-BP model in
conjunction with the PTT computation algorithm provides
a low cost alternative to continuous invasive BP monitor-
ing.
II. PTT-BP Model
The model assumes laminar blood flow from the heart
chamber to the fingertip through a rigid pipe, the artery.
While it is well known that artery wall expands and con-
tracts, its small compliance of 0.0018 liter/mmHg on aver-
age [2] justifies this assumption. The model estimates the
pressure difference between the two sites, the heart and the
fingertip, by the pulse wave velocity. A pulse wave travels
from the heart to the fingertip, along the artery and its
velocity can be calculated from the distance travelled di-
vided by the PTT. The relationship between the PTT and
BP is demonstrated in the following postulate.
The work done by the pulse wave can be expressed in
terms of the kinetic energy of the wave and the gravita-
tional potential energy:
F · d = 1
2
mv2 + mgh
where F = force exerted on blood
d = distance from heart to fingertip
m = mass of blood
v = pulse wave velocity
g = 9.81m/s2
h = height difference between two sites
(1)
The force can also be written in terms of pressure differ-
ence:
F =
BP · a (2)
where a is defined as the cross section area of the artery.
Substitute equation (2) into (1) and after rearrangement:

BP =
1
2
m
a · d
v2 +
m
a · d
gh (3)
m
a·d = ρ is the density of blood and v can be expressed
as dPTT , so

BP =
1
2
ρ
d2
PTT 2
+ ρgh (4)