Spontaneous breathing and work-of-breathing in mechanically ventilated children

The need for mechanical ventilation (MV) for acute or impending respiratory
failure originating from (primary) lung failure is one the most common
indications for children to be admitted to a paediatric intensive care unit
(PICU). Recent data from the United States and Europe demonstrate that up to
64% of children admitted to the PICU are mechanically ventilated for at least
24 hrs. Despite worldwide daily use of MV in children, numerous issues remain
unsolved and much of the current clinical practice is based upon anecdotal
experience in combination with data originating from studies in critically ill
adults. However, the respiratory system is physiologically different between
small babies, children and adults implying that all data obtained from adults
cannot be easily extrapolated to children.

Children are mainly ventilated in a synchronized intermittent mandatory
ventilation mode (SIMV), usually in a pressure controlled (PC) fashion. This
means that the inspiratory pressures that are going to be applied by the
ventilator are set by the attending physician. The delivered tidal volume (Vt)
depends upon the compliance and resistance of the respiratory system of the
patient. The rate at which the ventilator delivers the pre-set inspiratory
pressures is defined by the patient*s age and disease condition. . In addition
to these machine breaths, it is possible for the patient to maintain
spontaneous breathing. These breaths are supported by a pre-defined level of
pressure support (PS). However, it is current clinical practice to fully
ventilate the child during the early phase of the disease course; only a few
days prior to discontinuation of ventilation the child is allowed to breathe
spontaneously.

Studies performed in critically ill adults have shown that spontaneous
breathing during mechanical ventilation significantly improves the distribution
of the tidal volume and pulmonary gas exchange. In a supine patient the thorax
is divided into a non-dependent lung zone (the upper half of the thorax) and a
dependent lung zone (the lower half of the thorax). During mechanical
ventilation the tidal volume is directed towards the non-dependent lung zones.
This has two major consequences: the alveoli in the non-dependent lung zones
overstretch and the alveoli in the dependent lung zones collapse. This causes
significant injury to alveoli. In contrast, when adults breathe spontaneously
during mechanical ventilation the tidal volume is directed towards the
dependent lung zones because the posterior muscular sections of the diaphragm
move more than the anterior tendon plate. Consequently, in patients in the
supine position the dependent lung zones tend to be better ventilated during
spontaneous breathing protecting against alveolar collapse and improving
oxygenation by decreasing ventilation/perfusion mismatch.

Thus, there are strong arguments to facilitate spontaneous breathing as much as
possible during mechanical ventilation. This strongly suggests that a physician
should set the number of machine breaths as low as possible allowing the
patient to breathe spontaneously, and to apply a sufficient level of pressure
support. However, the effects of spontaneous breathing during mechanical
ventilation observed in adults have to date not been explored in mechanically
ventilated children. Hence, we propose that there are two main issues that need
to be addressed. First, is spontaneous breathing during mechanical ventilation
associated with beneficial effects similar to adults (i.e. better distribution
of tidal volume and improved gas exchange)? Second, how much pressure support
must be delivered by the ventilator to minimize the patient*s work of
breathing?

This prospective intervention study without invasive measurements is designed
to test the hypothesis that a) spontaneous breathing during mechanical
ventilation is associated with improved aeration of the lung dependent zones
and improved gas exchange, and b) the work of breathing during spontaneous
breathing is influenced by the level of pressure support.

Primary objective:
* To study differences in global lung aeration before and during spontaneous
breathing during mechanical ventilation

Secondary objective:
* To study the work of breathing (WOB) under various pressure support (PS)
levels in spontaneously breathing mechanically ventilated children

This is a prospective intervention study without invasive measurements in a 20
bed tertiary paediatric intensive care facility at the Beatrix Children*s
Hospital/University Medical Centre Groningen. The study will start September 1,
2011 and is completed by March 31, 2011.

Not applicable

There are a priori no specific benefits for the patients who participate in the
study as we do not know if the beneficial effects of spontaneous breathing
during mechanical ventilation observed in adults also occur in mechanically
ventilated children. Yet, if they do (and we expect so) then the enrolled
patients will experience similar beneficial effects.
The risks associated with this study are to be considered 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
*All patients already have a nasogastric tube inserted that is capable of
measuring the oesophageal pressure
*All parameters collected in this study are real-time displayed on either the
ventilator or the pulmonary function monitor; only the EIT analyses are
performed off-line. For the EIT measurements 16 electrodes must be placed
circumferentially around the chest. However, these electrodes are fully
comparable with the electrodes routinely used for ECG monitoring; hence they
pose no additional burden
*There are no invasive measurements for this study
Nevertheless, the decrease in pressure support may induce some degree of
dyspnoea as characterized by tachypnoea, nasal flaring and/or intercostal
retractions. We have therefore defined a stopping rule.