The role of diaphragm muscle EMG monitoring on respiratory physiology in mechanically ventilated patients with ARDS: a pilot study

Mechanical ventilation is a life saving intervention in patients with acute
respiratory failure. The goal of mechanical ventilation is to increase arterial
oxygenation and reduce work of breathing. Modern ICU ventilators allow the
physicaibn to choose between several ventilatory modes. Roughly, modes are
either controlled or supported. It is assumed that in controlled mechanical
ventilation the ventilator performs all the work of breathing, allowing rest of
the respiratory muscles. A disadvantage of controlled mechanical ventilation is
the development respiratory muscle atrophy due to disuse. In addition,
controlled mechanical ventilation may be associated with patient agitation due
to asynchrony in respiratory centre and ventilator output ("fighting the
ventilator"). In supported ventilation (i.e. pressure support ventilation) the
patient activates the ventilator by creating a negative inspiratory pressure.
With activation of the ventilator a preset inspiratory pressure is delivered
that unloads the inspiratory muscles. A limitation of this mode is the delay
between patient inspiratory effort and initiation of the mechanical supported
breath. In addition, several studies have shown wasted respiratory efforts
during pressure support ventilation. In wasted efforts, the patient aims to
trigger the ventilator, but is unsuccessful, due to muscle weakness or the
development of intrinsic PEEP. Wasted efforts increase work of breathing and
are associated with patient discomfort (*fighting the ventilator*). Finally,
with pressure support ventilation, the support is the same with every breath.
This could be considered unphysiological because during normal breathing the
tidal volume varies considerably.
NAVA ventilation is a relatively new supported mode of ventilation that was
designed to better follow patients physiology of breathing. First, trigger
delay is significantly shorter during NAVA compared to pressure support
ventilation (Spahija, Crit Care Med, 2010). Second, NAVA has been shown to
reduce the number of wasted efforts (Spahija, Crit Care Med, 2010). In
addition, this study showed that patients ready to be weaned from mechanical
ventilation, had significantly lower work of breathing during NAVA ventilation
compared to pressure support ventilation. The effects of different ventilatory
modes on work of breathing, oxygenation and patient synchrony has not been
studied during the acute phase of ARDS.

The objective of this pilot study is to demonstrate that NAVA ventilation is
superior compared to pressure support and pressure control ventilation in work
of breathing (expressed as pressure-time product) patient ventilator synchrony
and other physiological variables such as arterial oxygenation.

Prospective comparative cross-over study.

Two interventions will be performed:

1. Placement of modified nasogastric tube
2. Blood withdrawel from indwelling arterial catheter (total 12 ml in 90
minutes).

All ICU patients have a nasogastric tube for feeding and assessment of gastric
retentions. However, to perform physiological measurements a modified tube
needs to be inserted. The placement of this modified tube does not impose
additional hazards (compared to routine nasogastric tubes). After completion of
the study, this dedicated nasogastric tube does notneed to be replaced, but can
be used for feeding and assessment of gastric retentions according to clinical
protocols.

The risks of nasogastric tube placement are minimal. Nevertheless, patients
with elevated risk for complication of nasogastric tube placemnt are excluded
from this study (see exclusion criteria).

To the best of our knowledge no solid studies have been published concerning
the complication rate of nasogatric tube placement in ICU patients. From our
clinical experience we consider the risks of nasogastric tube placement
minimal, especially when *high risk patients* are excluded (upper airway /
esophageal pathology, bleeding disorders, hepatic failure). Nasal bleeding is
the most frequent complication, though incidence is still low.
As mentioned, nasogastric tube placement is routine care in ICU patients. The
dedicated nasogastric tube will be used for clinical purposes after completion
of the study.

Blood withdrawel will be performed via the indwelling arterial catheter
(routinely available in all ventilated ICU patients). In total, maximal 12 ml
of blood will be withdrawn in a time frame of 90 minutes. No complications are
to expected from this intervention.