Continuous non-invasive monitoring of blood pressure in the operating room: A cuffless optical technology at the fingertip

Abstract

Routine monitoring of blood pressure during general anaesthesia relies on intermittent measurements with a non-invasive brachial cuff every five minutes. This manuscript provides first experimental evidence that a physiology-based pulse wave analysis algorithm applied to optical data (as provided by a standard fingertip pulse oximeter) is capable of accurately estimating blood pressure changes in-between cuff readings. Combined with the routine use of oscillometric cuffs, the presented novel approach is a candidate technology to increase patient safety by providing beat-to-beat hemodynamic measurements without the need of invasive monitoring procedures.

Full text

Current Directions in Biomedical Engineering 2016; 2(1): 267–271
Open Access
Josep Solà*, Martin Proença, Fabian Braun, Nicolas Pierrel, Yan Degiorgis, Christophe Verjus,
Mathieu Lemay, Mattia Bertschi and Patrick Schoettker
Continuous non-invasive monitoring of blood
pressure in the operating room: a culess optical
technology at the ngertip
DOI 10.1515/cdbme-2016-0060
Abstract: Routine monitoring of blood pressure during
general anaesthesia relies on intermittent measurements
with a non-invasive brachial cu every ve minutes. This
manuscript provides rst experimental evidence that a
physiology-based pulse wave analysis algorithm applied
to optical data (as provided by a standard ngertip pulse
oximeter) is capable of accurately estimating blood pres-
sure changes in-between cu readings. Combined with
the routine use of oscillometric cus, the presented novel
approach is a candidate technology to increase patient
safety by providing beat-to-beat hemodynamic measure-
ments without the need of invasive monitoring proce-
dures.
Keywords: beat-to-beat; blood pressure; cuess; general
anesthesia; operating room; pulse wave analysis.
1Introduction
Blood pressure and heart rate monitoring belong to the
minimal standards for basic anaesthetic monitoring [1].
Still today, the non-invasive measurement of blood pres-
sure is performed with brachial ination cus placed
around the arm. However, these devices only provide
intermittent measurements of blood pressure during the
interventions, i.e. every 2–5 min. For patient safety rea-
sons, beat-to-beat blood pressure readings are manda-
tory in critical patients during long and complex inter-
ventions. Unfortunately, the routine procedure to obtain
*Corresponding author: Josep Solà, CSEM – Centre Suisse
d’Electronique et Microtechnique, Jacquet-Droz 1, CH-2002
Neuchâtel, Switzerland, E-mail: Josep.Sola@csem.ch
Martin Proença, Fabian Braun, Christophe Verjus, Mathieu Lemay
and Mattia Bertschi: CSEM – Centre Suisse d’Electronique et
Microtechnique, Jacquet-Droz 1, CH-2002 Neuchâtel, Switzerland
Nicolas Pierrel, Yan Degiorgis and Patrick Schoettker: CHUV –
Centre Hospitalier Universitaire Vaudois, Rue du Bugnon 46,
CH-1011 Lausanne, Switzerland
reliable continuous blood pressure readings relies on
arterial invasive catheters, with their non-negligible asso-
ciated morbidity issues [2]. Non-invasive and continuous
blood pressure measurement devices have been tested
in the past, but their accuracy and reliability have been
repeatedly questioned [3], in particular when aiming at
replacing existing procedures. Even more, high costs and
cumbersomeness of existing non-invasive beat-by-beat
devices impede their introduction into daily anaesthesia
practices.
This manuscript presents rst experimental evidence
on the feasibility of tracking beat-by-beat changes of blood
pressure in humans during induction of general anaes-
thesia by simple optical means. The presented technol-
ogy relies on the physiological analysis of transmission
photoplethysmographic signals measured at the ngertip.
Such signals are recorded by commercial pulse oxime-
ters, which are routinely applied to almost every patient.
While the presented approach does not aim at replacing
oscillometric gold-standard monitoring, it introduces an
appealing approach to continuously estimate blood pres-
sure measurements in-between two cu inations, thus
allowing the detection of sudden blood pressure changes
and thereby increasing the patient’s safety.
Because no additional monitoring equipment is
required in the operating room, this optical technology
appears as an appealing candidate to improve non-
invasive monitoring capabilities during general anaesthe-
sia. Figure 1 illustrates the status of the novel technology
within the constellation of existing monitoring means in
the operating room.
2Materials
The clinical study (ClinicalTrials.gov NCT02651558)
described in this section is compliant with all relevant
Swiss ethics, regulations and institutional policies and
in accordance with the tenets of the Helsinki Declaration.
©2016 Josep Solà et al., licensee De Gruyter.
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 License.

268 |J. Solà et al.: Continuous non-invasive monitoring of blood pressure in the operating room
Figure 1: Summary of available blood pressure (BP) monitoring
technologies in the operating room.
Informed consent was obtained from all individuals before
enrolment at CHUV – University Hospital of Lausanne.
Patients were placed on the operating table and mon-
itoring was carried out via the Philips monitor IntelliVue
MX800. Monitoring consisted of a 3-lead electrocardio-
gram, non-invasive blood pressure cu measurements
at the left arm and oxygen ngertip saturation readings
at the right index. Under local anaesthesia, a dedicated
catheter (BD Arterial Cannula 20G/1.1 mm ×45 mm, Bec-
ton Dickinson Infusion Therapy Syst. Inc., UT, USA) was
inserted into the right radial artery, allowing beat-to-beat
continuous blood pressure monitoring. General anaesthe-
sia was then provided by the anaesthetist in charge of
the patient, in accordance with the standard practice of
the department: propofol (2 mg/kg) for induction, fen-
tanyl (3 µg/kg) for analgesia, and rocuronium (0.6 mg/kg)
for neuromuscular blockade; endotracheal intubation
followed.
Recording of continuous invasive blood pressure
signals, non-invasive ngertip photoplethysmographic
signals (i.e. Pleth Wave), and electrocardiogram was per-
formed at 500 Hz by ixTrend®– software for Philips In-
telliVue monitors. As the clinical study is ongoing, only
the rst three patients showing good raw signal quality
(by an unsupervised quality index) were used to show rst
experimental evidence.
3Methods
3.1 Overall methodology
The novel optical technology relies on the extraction of
physiology-based pulse wave analysis (PWA) features from
raw transmission photoplethysmographic signals. These
physiological features are later projected into blood pres-
sure values by means of intermittent oscillometric mea-
surements provided by a brachial cu (see Figure 2).
3.2 Extraction of PWA features
Raw photo-plethysmographic signals are acquired by a
ngertip pulse oximetry probe. While for the current study
a commercial transmission probe is used, any other means
to acquire a surrogate measure for peripheral arterial pul-
satility is adapted for this methodology.
After a pre-processing and denoising procedure, a
PWA technique is applied to each arterial pulse [4]. The
implemented PWA algorithm extracts features from each
pulse based on the wave reection theory of the arterial
system [5]. The algorithm assumes that arterial pulses
propagate along the arterial tree being reected at each ar-
terial branching. The resulting arterial pulses are thus the
superposition of forward and backward waves interfering
at various locations of the arterial tree.
Given the arterial topology of a particular patient,
changes in blood pressure modify the velocity at which
arterial pulses propagate, reshaping the individual mix-
ture of incident and reected waves. These morphological
changes are quantied via the implemented PWA algo-
rithm as modications of the patients’ arterial pulsatility
patterns. Pulsatility pattern changes are indirect indica-
tors of the underlying changes in propagation charac-
teristics, and thus, of the underlying changes of central
blood pressure. Therefore the outcome of the implemented
PWA algorithm is a vector of physiologically meaningful
Figure 2: Overall methodology: blood pressure estimates
interpolate patient’s hemodynamic changes in-between two
oscillometric measurement.

J. Solà et al.: Continuous non-invasive monitoring of blood pressure in the operating room |269
features containing information on the patient’s central
blood pressure status.
3.3 Calibration to brachial cu values
While containing meaningful physiological information,
the feature vector extracted from the PWA algorithm does
not yet relate to blood pressure values in mm Hg.
In the context of a methodology to extract beat-
to-beat blood pressure estimates in-between two cu
measurements a dedicated (re-)calibration algorithm is
thus needed. The aim of this algorithm is to map
subject-dependent changes of the PWA feature vector
xiinto actual changes of blood pressure. During each
Figure 3: Performance of the culess blood pressure estimation
technique on a particular study patient during induction of
anesthesia. Each data point corresponds a patient’s heart beat.
Temporal evolution of invasive systolic blood pressure is displayed
as ±8 mm Hg range (shaded in grey).
recalibration at instant i, the availability of intermittent
cu-based measurements yiis used as a reference to
re-estimate the parameters 
piof a mapping function fas it
is depicted by equation 1. A forgetting factor λcan be used
to smooth pressure estimates in time.

pi=(1−λ)
pi−1+λargmin pi‖yi−f(pi,xi)‖2(1)
4Results and discussion
Based on the previously described methodology, rst ex-
perimental evidence on the performance of the novel cuf-
ess technique is provided here. Figures 3–5 illustrate
Figure 4: Performance of the culess blood pressure estimation
technique on a particular study patient during induction of
anesthesia. Each data point corresponds a patient’s heart beat.
Temporal evolution of invasive systolic blood pressure is displayed
as ±8 mm Hg range (shaded in grey).

270 |J. Solà et al.: Continuous non-invasive monitoring of blood pressure in the operating room
Figure 5: Performance of the culess blood pressure estimation
technique on a particular study patient during induction of
anesthesia. Each data point corresponds a patient’s heart beat.
Temporal evolution of invasive systolic blood pressure is displayed
as ±8 mm Hg range (shaded in grey).
the results of cuess systolic blood pressure estimations
compared to invasive reference measurements. Each gure
corresponds to data recorded during induction of anaes-
thesia on one patient. Each data point in these gures cor-
responds to a single heartbeat. Calibration with reference
measurement was performed every 5 min.
The rst row in each gure depicts correlation plots
between invasive systolic blood pressure values and heart
rate estimates (left plot) or cuess blood pressure esti-
mates (right plot), respectively. Note that in current clinical
practice, when only intermittent blood pressure measure-
ments are available, anaesthetists fully rely on heart rate
measurements to infer blood pressure changes.
The second row in each gure depicts Bland-Altman
plots when comparing invasive systolic blood pressure
values against cuess blood pressure estimates. Due to
the re-calibration procedures, overall bias are within a
reasonable range (|µ|<4 mm Hg). Standard errors of the
estimates σare all smaller than 8 mm Hg, and thus comply
with the specications of the AAMI standard [6].
The third row in each gure depicts the tempo-
ral evolution of invasive systolic blood pressure during
anesthesia induction (±8 mm Hg range shaded in grey),
and simultaneous cuess estimates. Note that calibration
periods are illustrated by black boxes marked as “CAL”.
It is important to note that the novel cuess method
was able to detect blood pressure changes in between
calibration periods for all the three patients analyzed.
5Conclusion
This manuscript presented rst experimental evidence for
the feasibility of estimating blood pressure changes in the
operating room by means of a simple optical probe at the
ngertip, which is routinely applied to every patient.
During all anaesthetic procedures and surgeries, the
patient’s oxygenation, ventilation, circulation and tem-
perature must be continuously monitored. To cope with
these requirements, and in order to increase patients’
safety, international standards for safe practice of anaes-
thesia have been implemented and consist of cumbersome
equipment.
While comprehensive statistical analysis of the perfor-
mances of the novel approach will be presented at nalisa-
tion of the ongoing clinical study, these preliminary results
support the hypothesis that physiology-based PWA algo-
rithms applied to standard photo-plethysmographic sig-
nals might contain sucient information to deduce blood
pressure changes during general anaesthesia, without the
need of additional monitoring equipment.
Technological advances such as the one herein pre-
sented appear as an interesting alternative, aiming at sim-
plifying patient access, and potentially decreasing mor-
bidity associated to invasive monitoring in the operating
room.
Authors’ Statement
Research funding: The author state no funding involved.
Conict of interest: Authors state no conict of interest.
Material and Methods: Informed consent: Informed con-
sent is not applicable. Ethical approval: The conducted
research is not related to either human or animal use.

J. Solà et al.: Continuous non-invasive monitoring of blood pressure in the operating room |271
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