Uisng changes in pulmonary mechanics and gas exchange to better define the optimal pulmonary volume in patients ventilated for acute lung injury

Ventilation strategies in children and neonates vary widely between institution
and school of thought. There is increasing evidence in the literature
supporting an *open lung ventilation strategy* in both animal models and
humans. This concept is based on having a fully recruited *open* lung,
protecting the lung from further ventilator induced lung injury (VILI) caused
by repetitive opening and closing of unstable lung units during tidal
ventilation (tidal recruitment).
Open lung ventilation is achieved during conventional ventilation by optimizing
lung volume (the restitution of functional residual capacity) using positive
end expiratory pressure (PEEP), and using smaller tidal volumes (6ml/kg). This
approach minimizes tidal recruitment, and has been shown to be associated with
lower mortality and barotraumas in patients with acute respiratory distress
syndrome (ARDS). The importance of adequate PEEP in preventing VILI is widely
accepted, however this should be the lowest level of PEEP that avoids
derecruitment and at the same time does not overdistend the lung. The strategy
of open lung ventilation is equally effective in both conventional and high
frequency oscillatory ventilation (HFOV). Both laboratory experimental studies
and clinical experience using high frequency oscillatory ventilation
demonstrate that the open lung approach in HFOV is both feasible and safe -
even in premature infants.7,1,5,9
Recruitment occurs throughout the respiratory cycle, and both PEEP and PIP
(positive inspiratory pressure) contribute. There are a number of methods
available to open a derecruited lung. Recruitment manoeuvres (inspiratory
cycles with high inspiratory and end expiratory pressure) have been extensively
used, and are controversial. Their effect is unclear, and they may cause
overdistension and haemodynamic instability1. Sustained PEEP recruitment
manoeuvres have also been used without clear effect. In the presence of PEEP
sufficiently high to enable open lung ventilation, recruitment manoeuvres have
not been shown to be effective in terms of improving oxygenation.
It has been demonstrated that a recruited lung will, due to hysteresis, require
less pressure to remain open, than to initially be opened. Because of this, it
is necessary to initially use higher pressures to recruit the lung, then to
reduce the pressures to find the optimal PEEP in conventional mandatory
ventilation [CMV], to prevent end expiratory alveolar collapse. It is
increasingly accepted that only the deflation limb of the pressure volume curve
provides information about the PEEP required to maintain an *open lung*, and
that ventilation on this more compliant deflation limb enables the pressure
amplitude to be minimised as much as possible.
In the current literature there are many parameters under study as surrogate
markers to indicate that an *open lung* has been achieved. In the laboratory
subject, volume pressure curves can help to set the PEEP just above the lower
inflection point on the pressure volume curve (using static compliance);
however this is difficult to achieve and often dangerous in bedside practice.
In animal and human studies, in addition to clinical parameters (oxygen
saturation, end tidal and CO2
elimination (vCO2), blood gasses, PO2/ FiO2 ratio, systemic blood pressure and
cardiac output); markers indicating the open lung state include dynamic
compliance, elastance, CT scanning, and calculation of dead space - but many of
these tools are impractical, or difficult to interpret in clinical bedside
practice.
The mechanics of the lung are governed by the lung and chest wall elastance,
compliance, and resistance. Each of these three factors varies with lung, chest
wall, and abdominal pathology, which are often significant in patients
ventilated in the intensive care unit. Current ventilation strategies are
based on measuring only the pressures within the respiratory system, as
pressures related to chest wall compliance and resistance are difficult to
measure. As the respiratory and chest wall pathologies and pressures are
different in each patient and for each illness, it is therefore logical that
the PEEP which will be required for each patient will vary.
Studies of open lung ventilation show that the dynamic compliance reaches a
maximum just below the PEEP required for open lung ventilation, and that
elastance and dead space reaches a mimimum plateau around the PEEP required for
open lung ventilation. As these parameters affect the amount of pressure
transmitted across the lung, but not the chest wall, the transpulmonary
pressure (TPP) would be expected to measure an attenuated increase in pressure
over the pressure range where an open lung state is achieved.
The TPP, measured by the airway pressure minus the oesophageal pressure,
measures the net result of respiratory and chest wall pressures. It has been
extensively studied in respiratory physiology, but has not been studied in
optimizing an *open lung* approach to ventilation. The TPP varies with patient
effort, increases during the inspiratory and decreases during the expiratory
phases in mechanically ventilated paralysed patients. In mechanical
ventilation it measures the interaction between the positive airways pressure
from the ventilator, the restrictions imposed by inherent pulmonary resistance
and compliance, and the chest wall compliance. If an open lung approach is
used, where ventilation occurs at the optimal lung volume and compliance,
recording the TPP during and after lung recruitment may provide a correlation
between optimised *open lung* ventilation, and the attenuation of the increase
of the TPP.
The effective use of *open lung ventilation* requires measurable parameters
that are reproducible, without risk, and easy to perform and interpret at the
bedside. The transpulmonary pressure may fulfil all of these criteria, and as
such warrants further study in a clinical trial.

Describe and observe *functional* recruitment as defined by the physiological
responses (O2, dynamic compliance, CO2, haemodynamic responses), using these
parameters to find the point of the most efficient alveolar minute ventilation
at the lowest pressure cost
1. To define the transpulmonary pressure at optimal *open lung ventilation* in
patients ventilated by CMV and HFOV with Acute Lung Injury (ALI) or ARDS during
a descending PEEP trial
2. To correlate the transpulmonary pressure with other possible markers
indicating the open lung state (compliance, elastance, deadspace, blood gasses
etc)

Secondary objectives

1. To identify whether the TPP needed to optimise lung volumes (to restitute
functional residual capacity) is consistent between patients and different lung
pathologies (primary versus secondary ALI/ARDS)
2. Document that lung recruitment and an open lung state can be obtained using
stepped PEEP increments and then decrements without an inspiratory recruitment
manoeuvre in CMV

To test the hypothesis that

3. The TPP can give us a better idea of the optimal lung volume achieved (as
measured by the CO2, compliance, oxygenation etc) than simple airway pressures.

Prospective observational cohort study with a duration of 12 months.

Burden and risk for the patient are primarily given by the underlying disease
and related standard IC treatment. The studied intervention (daily
optimisation of lung volume) is already a standard procedure on our and other
IC units. For this study, the different criteria for defining the optimum lung
volume, will be measured and compared. The study will therefore not raise axtra
risks or burden for the patients.