Electrical Impedance Tomography during Flow Controlled Ventilation in the Intensive Care Unit

During controlled mechanical ventilation (CMV) only the inspiration is
controlled by either a set driving pressure (Pressure Controlled Ventilation,
PCV) or tidal volume (Volume Controlled Ventilation, VCV). The expiration
depends on the passive elastic force of the respiratory system and cannot be
controlled until airway pressure is equal to the positive end-expiratory
pressure (PEEP). Consequentially airway pressure decreases exponentially during
expiration. This uncontrolled expiration results in alveolar collapse and
significant alveolar heterogeneity, especially in acute respiratory distress
syndrome (ARDS).[1] The cyclical opening and collapsing of alveoli is injurious
and contributes significantly to ventilation induced lung injury (VILI).[2] In
addition, the mechanical power (or dissipated energy) administered to the lung
during CMV is relatively high and potentially injurious.[3]
Flow controlled ventilation (FCV) is a new mechanical ventilation method that
uses a constant flow during both inspiration and expiration.[4] FCV results in
a linear increase in airway pressure during inspiration and a linear decrease
in airway pressure during expiration, as flow is controlled in both phases. In
healthy porcine lungs FCV increased the amount of normally aerated lung tissue
on CT scan, and decreased the amount of poorly aerated lung tissue as compared
to conventional CMV. Thus, FCV improved homogeneity and lung aeration in
healthy lungs.[5] In a model for ARDS FCV increased ventilation homogeneity by
reducing the amount of atelectasis and improving lung aeration.[6] Both in
healthy and ARDS lung FCV resulted in a decrease in minute volume at comparable
carbon dioxide levels.[5, 6] In addition, the calculated dissipated energy
during FCV is lower as compared to CMV.[4, 7]
FCV is based on an emergency ventilation system (Ventrain; Ventinova Medical
B.V., The Netherlands) that restores ventilation through a small bore
tracheostomy.[8, 9] The Ventrain working principle was transferred to an
automated mechanical ventilator (Evone; Ventinova Medical B.V.). Evone uses an
ejector pump and a T-piece to allow both insufflation of oxygen and
jet-assisted expiration through a 2.4mm tracheal tube. Occlusion of the ejector
pump gas outlet results in inspiration, whereas opening results in expiration
by entrainment of air through a long small bore catheter.[10, 11] Because of
the small diameter of the tube, FCV has been used during laryngeal surgery
providing the surgeon with ample space.[7, 8]
At the moment FCV is mainly used during laryngeal surgery and experience with
long term mechanical ventilation (>8 hours) in the Intensive Care Unit (ICU) is
limited. The improved ventilation homogeneity, lung aeration and reduced
dissipated energy during FCV could be especially beneficial in critically ill
patients requiring prolonged mechanical ventilation. This warrants a study with
FCV in patients requiring CMV at the ICU. We hypothesize that FCV results in
increased lung aeration as compared to PCV or VCV at comparable ventilation
settings (i.e. PEEP, peak pressure and FiO2).

Primary Objective: The primary objective of this study is to assess lung volume
measured by EIT and minute ventilation measured by the ventilator during CMV
and FCV at the ICU. This will allow for an adequate powercalculation for future
studies.

Secondary Objective(s): The secondary objectives of this study are to assess
airway pressures, lung aeration score with ultrasound, dissipated energy, and
safety of FCV as compared to conventional CMV modes.

This is a prospective intervention study comparing CMV and FCV in each patient.
ICU patients on CMV are included in this study. All patients are switched to
pressure control ventilation if not already in this mode. Lung volume and
minute volume during CMV are recorded. Subsequently, FCV is initiated with
similar mechanical ventilation settings (PEEP, peak pressure, and FiO2) used at
baseline. Lung volume and minute volume during CMV are recorded again.
Ventilation settings are adjusted based on peripheral saturation, end-tidal
carbon dioxide (etCO2), and arterial blood gases according to standard of care.
FCV is continued for 2 to 12 hours, afterwards CMV according to standard of
care is continued.

All measurements take place on the ICU at the bedside. Patients that require
CMV are included in this study following informed consent.

CMV Baseline (standard of care):
Conventional CMV is set according to standard of care and all patients are set
on PCV. Lung volume and minute volume are assessed for baseline measurements.
Airway pressures in the trachea are measured directly with a probe. This probe
is placed within 30 seconds and does not influence respiratory mechanics.

FCV (intervention):
A special connector piece is placed on the standard endotracheal tube and a
pressure measurement tube is placed inside the tube for direct tracheal
pressure measurement. The patient is then switched to FCV (Evone, Ventinova
Medical B.V.). The duration of this procedure is approximately one minute.
Initial PEEP, peak pressure, and FiO2 are set according to the last CMV
settings. Inspiratory to expiratory ratio is determined by the ventilator.
Subsequently, mechanical ventilation settings are guided by peripheral
saturation, etCO2, and arterial blood gas. During the first three hours an
arterial blood gas sample is taken every hour from the arterial line.
Afterwards, arterial blood gases are taken upon indication according to
standard of care at the ICU. FCV is continued for 2 to 12 hours. An
investigator known with the FCV mode remains available during the study period.

CMV Continued (standard of care):
Conventional CMV is set according to standard of care. Lung volume and minute
volume are measured to assess the effects of lung collapse following CMV.

The last 5 patients will not undergo the intervention (FCV) but will only be
monitored by EIT on the conventional mechanical ventilation on the ICU
(Pressure Controlled Ventilation). This to better interpret the EIT-data on the
FCV.

In all patients we strive for a peripheral saturation of 95-100%, a PaO2 <15
kPa, and a PaCO2 of 4.5-6.5 kPa all within normal values according to the
Erasmus MC mechanical ventilation and neurotrauma protocols. In addition, if
tidal volumes exceed 10mL/kg predicted body weight, driving pressure (i.e. peak
airway pressure) is reduced in order to reduce tidal volume. A tidal volume of
10mL/kg is considered to be acceptable in relatively healthy lungs at the
ICU.[16]

Lung volume is monitored by EIT. EIT is a non-invasive, radiation-free,
real-time monitoring method to estimate lung volume.[13] EIT measures
electrical currents with electrodes similar to electrocardiogram electrodes
integrated in an elastic belt on the skin surface. This belt provides us with
cross-sectional images of the thorax. An increase in lung volume can be
monitored, as an increase in air changes the electrical impedance of the
thorax. The ventilation distribution in a 5-10cm thick slice of the thorax can
be visualized.[14, 15] At baseline an EIT measurement is performed.
Subsequently, all mechanical ventilation parameters and hemodynamic parameters
are monitored and recorded continuously.

All patients are sedated and on CMV, therefore there will be no discomfort for
the patient. FCV has been successfully applied during various kinds of surgery
with general anaesthesia. For FCV a small-bore pressure measurement tube is
placed inside the standard endotracheal tube, this requires a short
disconnection compared to a standard bronchial toilet with a duration of less
than a minute. In case of desaturation or any adverse event the small-bore
catheter can be removed and standard mechanical ventilation can be applied,
restoring the original situation within one minute. An investigator known with
the FCV mode remains available during the study period. Lung volume is measured
with EIT, a non-invasive, radiation-free monitoring tool. Therefore, the risks
of this investigation are limited. Potential benefits seen in preclinical
studies are increased lung homogeneity and a decrease in dissipated energy
administered to the lung tissue.