Expiratory muscle activity and function in critically ill ventilated patients

Cooperation between inspiratory and expiratory muscles exists to maintain
adequate alveolar ventilation and such to satisfy the demands of breathing. The
diaphragm is the most important respiratory muscle and its capacity is often
impaired by ICU-acquired weaknesses. This is associated with adverse outcome,
including prolonged duration of mechanical ventilation and higher mortality. In
patients with high load of breathing or low inspiratory muscle capacity,
expiratory muscles are often recruited. However, the coordination between the
inspiratory and expiratory muscles is poorly understood and highly complex,
particularly during the expiratory phase of breathing and when the load imposed
on the respiratory muscles increases. In addition, the functional effects of
expiratory muscle recruitment are poorly understood.

The primary objective is to investigate the coordination (functional and
timing) between the different respiratory muscles in ICU patients under
mechanical ventilation during assisted breathing and during a spontaneous
breathing trial. To this end, we 1) quantitate expiratory muscle effort using
the expiratory pressure-time-product derived from Pga recordings (PTPex), 2) we
define the relationship between PTPex and the total pressure-time-product of
the respiratory muscles (PTPtot = PTPex + PTPin) and 3) we define the
activation pattern of the respiratory muscles using sEMG.

Secondary objectives are:
- To evaluate the effects of an SBT on expiratory diaphragm activity
- To evaluate the effects of an SBT on thoracoabdominal synchrony as derived
from respiratory inductance plethysmography (RIP)
- To evaluate the correlation between PTPex and other parameters such as
surface electromyography (sEMG) of the abdominal wall muscles and their derived
parameters, RIP-derived endpoints, and clinical respiratory parameters
- To study whether the activation of the abdominal wall muscles and changes in
coordination have effects on weaning outcome

Observational physiological study

Placement of balloon catheters is a standard operational procedure in selected
ICU patients for the measurements respiratory muscle effort (i.e. severe acute
respiratory distress syndrome, difficult weaning, high risk for
ventilator-asynchronies). We will only enroll patients that already have such
catheter in situ. As such, there are no additional risks related to the
recordings obtained from this catheter.
All study procedures are non-invasive, or study parameters will be obtained
from routine care measurements / chart review.
In this study respiratory muscle monitoring procedures will be performed in a
structured manner during a period of 1 hour. Respiratory muscle weakness is
common in mechanically ventilated patients and is associated with prolonged
mechanical ventilation. Studying the effects of critical illness and mechanical
ventilation on the expiratory muscles and the coordination between the
inspiratory and expiratory muscles is therefore highly needed. Patients have no
direct benefit in participating in this study. However, the measurements of
esophageal and gastric pressure are available for the clinicians at the bedside
and study observations may be used to further optimize ventilator settings.