1) This study used electrical impedance tomography (EIT) to measure regional lung ventilation distribution in pigs with both healthy and injured lungs as they were rotated between positions and exposed to different levels of positive end-expiratory pressure (PEEP). 2) The results showed that regional ventilation distribution was highly dependent on gravity, especially in injured lungs. Increasing PEEP levels and more lateral rotation positions led to greater heterogeneity and overdistension of ventilation in healthy lungs. 3) In injured lungs, lateral rotation provided significant recruitment of collapsed lung tissue and improved oxygenation at all PEEP levels tested. Higher PEEP was needed to maintain this recruitment effect.

1. Position-dependent distribution of lung ventilation
– A feasability study
Waldmann A, Ortolá C, Martinez M, Vidal A, Santos A, Manuel
Márquez M, Róka P, Böhm S, Sipmann F, Sensors Applications
Symposium, Zadar, Croatia, 2015

2. Position-dependent distribution of lung ventilation
– A feasability study
Andreas D. Waldmann1
, Carlos Ferrando Ortolá2
, Manuel Muñoz Martinez3
, Anxela Vidal4
, Arnoldo Santos
4
, Manuel
Perez Márquez4
, Péter L. Róka1
, Stephan H. Bohm1
, Fernando Suarez-Sipmann5
1
Swisstom AG, Landquart, Switzerland,
2
Department of Anesthesia and Critical Care. Hospital Clínico Universitario, Valencia, Spain.
3
Departement of Anesthesiology, Hospital Universitario de la Princesa, Madrid, Spain.
4
Department of Critical Care. Instituto de Investigación Sanitaria Fundación Jiménez Díaz, Madrid, Spain,
5
Department of Surgical Sciences, Hedenstierna Laboratory, Uppsala University, Uppsala, Sweden
Abstract—The aim of this feasibility study was to determine
whether the measurement setup and study protocol were able to
show the effect that lung disease, body position and different
levels of positive end expiratory pressure (PEEP) have on lung
function. By means of a motorized rotation table and gravity
sensors six pigs were rotated in steps of 30° from left to right
lateral position. Regional ventilation distributions, measured by
electrical impedance tomography (EIT), oxygenation and
compliance measurements were performed at each position.
Both, experimental and measurement setup as well as the
parameters chosen to characterize lung function appear suitable
for analyzing the effects of PEEP and rotation in healthy and
injured lungs. The initial results show that the distribution of
regional ventilation was highly gravity-dependent especially in
sick lungs. Furthermore lateral rotation showed significant
recruitment effects on previously collapsed lung tissue as
witnessed by the increases in oxygenation at all PEEPs.
Keywords—EIT, electrical impedance tomography, lung
function mechanical ventilation, oxygenation, PEEP, positive end
expiratory pressure, rotation, recruitment, sensors.
I. INTRODUCTION
The therapeutic role of lateral rotation and prone
positioning during mechanical ventilator support of patients
with severe lung injury has been a matter of much debate [1].
Although most clinical trials have failed to show a clear
beneficial effect of prone positioning on outcomes [2]–[4], a
recent study in patients with severe lung injury showed that an
early application of prolonged prone-positioning sessions
significantly decreased 28-day and 90-day mortality [5].
However, the pathophysiologic and therapeutic mechanisms
behind the effectiveness of position therapy remain unclear.
Most parameters such as lung mechanics and blood gases used
in clinical practice describe lung function only globally, but do
not allow insights at a regional level.
Electrical impedance tomography (EIT) is a novel method
to investigate and monitor regional lung function and heart
activity. For this purpose, 32 sensors are placed around the
thorax. Weak alternating currents are applied via two of these
sensors and the resulting potentials are measured at the
remaining sensors. From the measured voltages, sequences of
real-time images are calculated which show the distribution of
electrical impedance within the body [6] representing organ
function rather than structure. It has been shown that intra-
thoracic impedance changes with ventilation [7]–[10] and the
cardiac cycle [11]. The signals caused by the breathing lungs
are about 10 times stronger than that caused by the beating
heart. Therefore, EIT has been proposed for measuring regional
ventilation rather than cardiac function within the human
thorax. Unlike traditional medical imaging methods such as
computer tomography, EIT imaging is non-invasive and can
thus be employed continuously right at the bedside.
The distribution of ventilation depends on the lung´s
condition, body position [12], [13] and on the positive end
expiratory pressure (PEEP) [14]–[17] applied. Turning from
the supine to the right lateral and the left lateral position,
Riedel et al. [13] showed in 10 healthy subjects that the
regional distribution of ventilation changes significantly with
body position. They also showed that the gravity-dependent
lower part of the lung is ventilated more than the non-
dependent upper lung. Blankman et al. [17] showed in patients
after cardiac surgery that ventilation moves towards the non-
dependent lung when decreasing PEEP from 15 cmH2O to
0 cmH2O.
The aim of this feasibility study performed in six pigs, three
with healthy lungs and three with acute lung injury, was to
determine whether the measurement setup and study protocol
were able to show the effect that lung disease, body position
and different levels of PEEP have on global lung function and
particularly on the fraction of ventilation delivered to either
lung.
II. METHODS
A. Study design
The study protocol was approved by the Ethic Committee of
Fundación Jiménez Díaz research institute. Acute lung
damage was induced by repeated saline lung lavage for
surfactant depletion and subsequent 2 hours of injurious
mechanical ventilation to establish a two-hit ventilator induced
lung injury model. All pigs were placed in the supine position
at 0°, and were then rotated in steps of 30° in a clockwise
direction until +90° and back to 0°, the same procedure was
978-1-4799-6117-7/15/$31.00 ©2015 IEEE
This full text paper was peer-reviewed at the direction of IEEE Instrumentation and Measurement Society prior to the acceptance and publication.
429

3. Fig. 1. Protocol: pigs were rotated in 30°-steps from the supine to the right
and then to the left lateral position and back to 0°. The numbers from 1 to 13
indicate the measurements taken in sequence.
then repeated in a counter-clockwise direction until -90° and
back to 0° (for details see Fig. 1). The exact rotation of the
subject was measured with an accelerometer positioned on the
subject´s sternum. To ensure stable recording conditions no
data were obtained during the first 2 of 3 minutes at a given
angle. Baseline ventilation was performed with a tidal volume
of 6 ml/kg in a volume controlled mode, at a respiratory rate
of 30 breaths per min, an inspiratory to expiratory ratio of 1:2
and a PEEP of 5 cmH2O. The fraction of inspiratory oxygen
(FIO2) was set to 0.4 for the healthy model and 1 for the lung
injury model. An entire measurement sequence consisting of a
total of 13 EIT recordings and arterial and mixed venous
blood gas analyses was performed at each angle studied and at
each of the following PEEP levels: 5, 10 and 15 cmH2O. The
order in which PEEP levels were applied was randomized. To
avoid carry over effects baseline ventilation conditions were
applied for 5 minutes before changing to the next level of
PEEP.
B. Motorized rotation table
We used a costum-built motorized table (Guido Kübler
GmbH, Bobingen, Germany) to rotate the animal to the
predefined angles (Fig. 2c). The table has two linear actuators
(MAX30-A200415C510F, SKF Group, Gothenburg, Sweden)
and one DC motor (403.957, Valco, Germany). In contrast to
tables with only one motor, our table rotates the animal
concentrically around its longitudinal axis without changing
the z-direction of the table or its lateral extension. Rotation of
the table was controlled by a laptop via a serial
communication protocol. Before starting the experiment the
table was calibrated by the electronic inclinometer Incli Tronic
plus (BMI, Hersbruck, Germany) with an accuracy of ± 0.1° at
0° and 90° and of ± 0.3° between 1° and 89°. A vacuum
mattress and two belts were used to tightly fix the animals
during the rotation, see Fig. 2a.
C. EIT device
EIT measurements were performed with the Pioneer-Set
(Swisstom AG, Landquart, Switzerland). This device consists
of a custom-built pig interface, the SensorBelt (SB) and a
SensorBeltConnector (SBC) to drive the SB, see Fig. 2b. The
special pig interface was made of an elastic tube (Silcolatex
7x10, Tefelex Medical, Germany) carrying 32 contact pads
made of stainless steel tubing. A standard electrically non-
conductive ultrasonic gel was used to reduce the impedance
between these pads and the skin.
(a) Pig placement (b) SensorBelt
(c) Motorized table
Fig. 2. (a) Measurement set-up, consisting of a vacuum matress, flow and
pressure sensors and the SensorBelt, (b) SensorBelt, consicting of gravity
sensor, pig interface and plug for SensorBeltConnector. (c) Motorized table,
consisting of a wooden table top, two linear motors and one DC motor. The
table can be moved vertically from 84 cm to 90 cm and rotated concentrically
from -90 to +90° at a maximum speed of 1° per second using serial
commands.
Each of these pads was connected via a cable to the SB.
The SB contained 32 active sensors, which were switched
between a current injecting and a voltage measurement
function. In order to reduce the complexity of the belt bus
within the SB the 32 sensors are arranged in daisy chain
architecture. Digital commands, sent by the SBC, are then used
to sequentially read out all sensors. The measured analog
voltages are converted into digital signals using a 14 bit analog
to digital converter. The SBC is plugged into the SB equipped
with an integrated 3-axis accelerometer (ADXL343, Analog
Devices, Boston, MA; USA) to measure the subject´s exact
body position. Gravity and the respective other accelerations
were measured at 30 Hz and converted into a longitudinal and
a transversal angle, to describe the rotation angle of the subject.
An electrical drive current with peak-to-peak amplitude of
3 mA and a frequency of 144 kHz was used in all
measurements. 30 tomographic differential images were
recorded per second. EIT-based moving images of regional
ventilation were generated from the collected potential
differences and the known excitation currents using a
derivative of the publically available GREIT image
reconstruction algorithm [18]. More information about the EIT
device can be found in [19], [20].
D. EIT data analysis
During each one of the predefined study angles EIT movies
were created from 20 consecutive breaths. During post hoc
analysis for each pixel the impedance difference between
inspiration and the preceding end-expiration was calculated
which delivered the so-called tidal EIT images. From these 20
tidal images the fraction of ventilation delivered to the right
and left lung was determined simply splitting the image into a
right and left hemithorax.
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4. E. Dynamic complicance
Dynamic compliance was calculated on a breath-by-breath
basis using the internal flow and pressure sensors of the Open
Lung Tool (Servo-i Maquet Critical Care, Solna, Sweden).
III. RESULTS
During the experiments neither technical nor medical
difficulties were encountered. All devices and sensors
delivered the expected data. Fig. 3 and 4 depict the percentage
of ventilation delivered to the right lung, dynamic compliance
and oxygenation index for the healthy and for the lung injury
model. Right at the start and during the study the lungs of the
supposedly “healthy” pigs presented with a wide range of
mechanical and gas exchange properties ranging from fully
healthy to rather sick.
A. EIT data / Ventilation distribution
At PEEP 5 cmH2O the healthy right lung received 55% of
the tidal volume and the left lung received 45%, see Fig.3. A
slight asymmetry remained at all angles returning to almost
50% after a complete rotation sequence. At PEEP 10 cmH2O
and 15 cmH2O ventilation to the right lung increased to 70% in
the right lateral positions, whereas rotation to the left side
decreased right lung ventilation to 60 and 50%, respectively.
In contrast, the injured lung showed a higher rotation-
dependency, see Fig.4. Whereas in the supine position
ventilation was rather equally distributed between the right
(52%) and the left lung (48%), in the left lateral positions 78%
of the ventilation shifted to the non-dependent upper right lung
reaching as much as 73% when back in the supine position.
Even the slightest rotation by 30° to the right side inverted this
distribution abruptly with the right lower lung now receiving
less than 50%. Increasing and decreasing the rotation angle did
not change this ventilation pattern. The distribution of
ventilation between the lungs at PEEP 10 and 15 cmH2O
resembled that of the healthy lungs at PEEP 5 and 10 cmH2O,
but showing a slightly larger angle-dependent hysteresis in the
sick lung at PEEP 10 cmH2O.
B. Dynamic compliance
The dynamic compliance in the healthy pig ventilated at PEEP
5 cmH2O remained unchanged during the entire rotation
sequence but its overall level increased with PEEP 10 cmH2O
from around 15 to 20 ml/cmH2O without showing a major
hysteresis, see Fig. 3. During the initial leftward rotation at
PEEP 15 cmH2O a marked gain in compliance was noticed
which stabilized between 20 and 25 ml/cmH2O for the
remainder of the study. The sick lungs showed a similar
behavior however at compliance values approximately 5
ml/cmH2O lower than their healthy counterparts see Fig. 4.
C. Blood gas PaO2/FiO2 ratio
At PEEP 5 cmH2O the oxygenation index PaO2/FIO2
remained between 250 and 350 mmHg in the healthy lung,
reached values around 250 mmHg at PEEP 10 cmH2O and did
not change with the rotation angle. At PEEP 15 cmH2O
PaO2/FIO2 remain between 300 and 350 mmHg and did not
change with the rotation angle.
The PaO2/FIO2 of about 120 mmHg at 0° corresponded to a
very sick lung. At PEEP 5 cmH2O, oxygenation improved with
lateral rotations to both sides reaching maximum values
between 250 and 300 mmHg. Rotation at PEEP 10 cmH2O
opened collapsed lung units and stabilized PaO2/FIO2 value
above 350 mmHg showing the same behavior also at PEEP 15
cmH2O, see Fig. 4.
IV. DISCUSSION
These initial results show that the distribution of regional
ventilation was highly gravity-dependent especially in sick
lungs.
The function of the healthy lung at PEEP 5 cmH2O was
normal and appeared rotation-independent. With higher PEEPs
a clear dependency on the rotation angle was revealed which
reached its maximal expression at ± 60° and ± 90° and together
with the upward-convex shape of the compliance curve can be
interpreted as the beginning (10 cmH2O) of overt (15 cmH2O)
overdistension of the respective non-dependent healthy upper
lung. Thus, high PEEP in conjunction with a high rotation
angle increased the heterogeneity of ventilation in the healthy
lungs.
Starting from low oxygenation levels lateral rotation
showed significant recruitment effects on previously collapsed
lung tissue as witnessed by the increases in oxygenation at all
PEEPs. PEEP 5 cmH2O, however, was not able to maintain
this recruitment. Once PEEP reached 10 cmH2O it was high
enough to keep the newly recruited lung units open shifting
oxygenation and compliance into the normal range. Increasing
PEEP further to 15 cmH2O created a lung situation similar to
the one seen in the healthy lung at 10 cmH2O reflecting a
stable but slightly overdistended upper lung.
V. CONCLUSION
The distribution of regional ventilation was gravity-
dependent and the combined effects of PEEP and rotation
angle were different in health and disease. Both, experimental
and measurement setup as well as the parameters chosen to
characterize lung function appear suitable for analyzing the
effects of PEEP and rotation in healthy and injured lungs.
VI. OUTLOOK
In this feasibility study in six subjects we calculated the
distribution of ventilation between the right and left lung, only.
In the future we should also analyze the distribution of
ventilation between the gravity dependent and the non-
dependent lung by dividing the lung at each angle by a
horizontal line perpendicular to the gravity vector. This way,
the true gravity- dependency of lung function and a correlation
with parameters of global lung function should become more
obvious. Furthermore it would be interesting to study the local
matching of ventilation and perfusion, as suggested by [21],
[22], under different rotation and PEEP conditions. Therefore,
regional perfusion measurements should be included in the
next study protocol. Once data analysis and study protocol
have been refined in the above way they should be applied in
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5. more pigs to reveal the underlying mechanisms of gravity- and
PEEP-dependency of lung function.
ACKNOWLEDGMENT
This project was funded by the “Fondo de Investigaciones
Sanitarias FIS, PI10/01885.
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6. 0° 30° 60° 90°-30°-60°-90°
PEEP 5
PEEP 10
0° 30° 60° 90°-30°-60°-90°
R
R
R
R
R
R
R
R
R
R
R
R
R
R
PEEP 15
30° 60° 90°-30°-60°-90° 0°
R
R
R
R
R
R
R
Fig. 3. Healthy lung: Percentage of ventilation delivered to the right lung, dynamic compliance and oxygenation index PaO2/FIO2 are plotted for different body
positions and PEEP level. Small arrows indicate the measurement sequence. Representative sequence of tidal EIT images during roation from the left (+90°) to the
right (-90°) lateral position show regional ventilation where bright pixels depict a large and dark ones a low ventilation amplitude. Data are plotted for each
condition for three pigs. The thin lines show the data of the individual pigs, the thick violet line represents the mean value.
433

7. PEEP 5
PEEP 10
PEEP 15
0° 30° 60° 90°-30°-60°-90°
0° 30° 60° 90°-30°-60°-90°
0° 30° 60° 90°-30°-60°-90°
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
Fig. 4. Sick lung: Percentage of ventilation delivered to the right lung, dynamic compliance and oxygenation index PaO2/FIO2 are plotted for different body
positions and PEEP level. Small arrows indicate the measurement sequence. Representative sequence of tidal EIT images during roation from the left (+90°) to the
right (-90°) lateral position show regional ventilation where bright pixels depict a large and dark ones a low ventilation amplitude. Data are plotted for each
condition for three pigs. The thin lines show the data of the individual pigs, the thick violet line represents the mean value.
434

8. SWI SSTO M SCIE NT IFIC LIBRARY 2 015
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