Measuring work-of-breathing in mechanically ventilated children

The most common approach to weaning infants and children is gradual reduction
of ventilatory support. Weaning with intermittent mandatory ventilation (IMV)
or synchronized IMV (SIMV) occurs by reducing the ventilatory rate. With
pressure support (PS) ventilation, the inspiratory pressure is initially set to
provide the required support and then reduced gradually. PS is often combined
with IMV/SIMV during weaning (SIMV-PS). Alternatively, another approach to
weaning is attempted with alternating periods of complete ventilatory support
and graded spontaneous breathing with assistance. This *sprinting* is performed
on the theory that the respiratory muscles can be slowly trained to sustain
complete spontaneous breathing. Also, theoretically this *sprinting* allows a
better distribution of the tidal volume in the lung. Interestingly, both
approaches are used simultaneously: the decision to use either one or both
approaches is dependent upon the preferences of the attending physician as
described in many observational single center studies. Children ventilated in
the PICU of the Beatrix Children*s Hospital are weaned from the mechanical
ventilator using both the gradual weaning and the sprinting
approach.Importantly, there is no data comparing the *sprinting* approach with
more traditional approaches of weaning in children with respect to patient
work-of-breathing. Work-of-breathing is defined by the physiologic work a
patient has to deliver to expand the lungs and the chest wall. It can be
assessed by various means:
•Bedside: tachypnoea and the presence of nasal flaring and intercostal and/or
interjugular retractions indicate increased work of breathing
•Clinical surrogate parameters: the ratio of the inspiratory time to total
breathing cycle time, the oesophageal pressure - rate product (PRP),
oesophageal pressure - time product (PTP) and expiratory airway resistance
(i.e. the difference in transpulmonary pressure and compliance, divided by
flow). These clinical surrogate parameters require the presence of an
oeosphageal catheter to measure the pressure. These catheters are routinely
present in ventilated patients as they are used for nasogastric tube feeding;
modern catheters can also measure the oesophageal pressure (i.e. double
function).
•Mathematically: the area under the oesophageal pressure - volume curve. This
is the classic and traditional approach to measure work of breathing. Its
normal value is within the range of 0.5 - 1.0 J/L. The variable is measured by
a commercially available ventilator (AVEA, CareFusion, Yorba Linda, CA, USA).
To measure this parameter, an oesophageal catheter is necessary.

The primary objective for this study is to compare the level and time course of
the patient work-of-breathing during PS ventilation with the work-of-breathing
during SIMV-PS ventilation with a lower rate of breaths per minute delivered by
the ventilator. The secondary objectives for this study are to compare the
level and time course of the PRP, PTP, imposed work of breathing and the
PaO2/FiO2 ratio as well as the distribution of tidal volume in the lung during
PS ventilation with the work-of-breathing during SIMV-PS ventilation with a
lower rate of breaths per minute delivered by the ventilator.

This is a prospective observational 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 December 1,
2011 and is completed by November 30, 2012.

There are no specific benefits for the patients who participate in the study.
However, the risks associated with this study are 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 (routine
clinical care)
•All patients already have a nasogastric tube inserted that is capable of
measuring the oesophageal pressure (routine clinical care)
•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
•There is no interference with clinical management of the patients for this
study.