Am. J. Respir. Crit. Care Med., Vol 155, No. 6, 06 1997, 2096-2101.
Nasal mask pressure waveform and inspiratory muscle rest during nasal assisted ventilation
V Jounieaux, VF Parreira, P Delguste, G Aubert and DO Rodenstein
Cliniques Universitaires Saint Luc, Universite Catholique de Louvain, Brussels, Belgium.
In mechanically ventilated patients, pressure and flow tracings can be used
to assess respiratory pump muscle activity or rest. When the ventilation is
delivered through an endotracheal tube, the respiratory system can be
considered a one-compartment model, and activation of the respiratory
muscles results in deformations and variability of the pressure tracings.
This is also true when mechanical ventilation is delivered nasally. With
intermittent positive-pressure ventilation delivered through a nasal mask
(nIPPV), we have recently shown that the glottis can interfere with
ventilation even in the absence of diaphragmatic surface electromyographic
(EMG) activity. On the basis of our observations, we suggested that when
mechanical ventilation is delivered through a nasal means of access, the
respiratory system cannot be considered a one-compartment model. To confirm
this hypothesis, we submitted one healthy subject to nIPPV while his
glottis was continuously monitored through a fiberoptic bronchoscope and
his diaphragmatic activity was monitored with a bipolar esophageal
electrode. During wakefulness or sleep, we observed irregularities in the
nasal mask pressure waveform, in nasal mask peak pressure, and in actual VT
despite the absence of respiratory pump muscle activity. These
irregularities were related to significant variations in the glottic width,
rather than to the reappearance of transient phasic inspiratory muscle
activity. We conclude that during nIPPV, deformations in the mask pressure
waveform can be induced by variations in the glottic aperture without
activation of the diaphragm. Thus, when mechanical ventilation does not
bypass the glottis, the respiratory system does not behave like a
one-compartment model, and EMG remains the only reliable technique for
assessing diaphragmatic muscle activity.
