Use of lung mechanics to identify the optimal level of positive end-expiratory pressure in mechanically ventilated children with acute lung injury: a pilot study

Mechanical ventilation is necessitated in children with acute lung injury to
maintain gas exchange and take away increased work of breathing but is also
known to induce a pulmonary inflammation aggravating the lung injury (i.e.
ventilator-induced lung injury, VILI). One of the mechanisms of the ventilator
is to deliver a positive end-expiratory pressure (PEEP) to maintain sufficient
end-expiratory lung volume (EELV) approximating functional residual capacity
(FRC) to ensure adequate oxygenation by limiting ventilation-perfusion
mismatch. Also, the use of PEEP prevents the repetitive opening and closure of
alveoli, which is one of the main mechanisms underlying VILI. In clinical
practice, the attending physician guided by the need for supplemental oxygen
sets the *optimal* level of PEEP. Alternatively, we, and many others, use an
incremental * decremental PEEP titration to find the maximum end-expiratory
lung volume and hence set the level of PEEP. This level is identified by the
oxygenation; however, there is a poor correlation between oxygenation and the
end-expiratory lung volume. As a consequence, alternative physiological
variables such as the oxygenation index (taking the level of mechanical
ventilation into account), respiratory system compliance, lung compliance,
transpulmonary pressure or alveolar dead space fraction are proposed as
clinical surrogates for end-expiratory lung volume.

The primary objective is to study the level and time course of the
end-expiratory lung volume during the individual incremental * decremental PEEP
titration; the secondary objective is to study if there is a significant
association between the end-expiratory lung volume at each PEEP step during the
incremental * decremental PEEP titration and the level and time course of the
oxygenation index, respiratory system compliance, lung compliance,
transpulmonary pressure and alveolar dead space fraction

Prospective observational study with invasive measurements

There are a priori no specific benefits for the patients who participate in the
study because the techniques used for measuring the end-expiratory lung volume
can only be read offline. We consider the risks associated with this
non-therapeutic observational study acceptable and the burden minimal, based
upon the following arguments:
* Blood sample drawing is done via the already present indwelling arterial
line, so that no additional venous or arterial punctures are necessary. There a
no specific blood sample drawings for this study. Furthermore, the maximum
amount is 2.5 mL; for a 3 kg infant this would constitute as little as 1% of
the circulating volume.
* The insertion of an oesophageal catheter is to be considered an invasive
procedure; however, the procedure itself is comparable to inserting a
nasogastric feeding tube that is routinely done in all ventilated patients to
ensure nasogastric tube feeding and prevent gastric distension; the potential
risk includes nasal bleeding, misplacement (either to deep or not) or * very
rarely * mucosal bleeding in the oesophagus. To our best of knowledge, these
complications have so far occurred very rarely in our PICU. Misplacement into
the trachea is very unlikely because the endotracheal tube is already in place.
Nonetheless, we will record any complication that has occurred when inserting
the oesophageal catheter. Furthermore, correct position of the catheter is
confirmed if the cardiac signal is present in the pressure * time curve
displayed by the ventilator. If not, then the oesophageal catheter is removed.
If this occurs, the oesophageal catheter is re-inserted only once. The use of
additional oesophageal catheters to measure pressure in mechanically ventilated
children has been approved by local IRBs (NL26857.078.09; NL24044.029.08).
Finally, the study is conducted when there is serious acute lung injury and the
patient is usually deeply sedated so the procedure itself will pose little
burden to the patient. Similar as for any procedure clinically required, we
will measure the Comfort Score prior to insertion of the oesophageal catheter;
if necessary the dosage of sedation will be adjusted. The nurse taking care of
the patient will insert the catheter; he or she is fully capable of inserting
such a catheter. Finally, there is negligible risk of sinusitis caused by
obstruction of the ostium due to having temporarily two catheters inserted
because the occurrence of sinusitis in young mechanically ventilated children
is very uncommon and the study period is relatively short.
* All parameters collected in this study are displayed real-time on either the
ventilator or the pulmonary function monitor; only the EIT en RIP analyses are
performed off-line. For the EIT measurements 16 electrodes must be placed
circumferentially around the chest. The electrodes are fully comparable with
the electrodes routinely used for ECG monitoring; hence they pose minimal
burden. For the RIP studies two elastic bands are placed circumferentially
around the patient*s chest and abdomen. The cardiac output is measured non -
invasively
* There is no interference with clinical management of the patients for this
study.