[Home mechanical ventilation: indications and pathophysiological limitations]

Domiciliary assisted ventilation (DAV) may be undertaken invasively or non-invasively. Non-invasive DAV is used for patients suffering from alveolar hypoventilation due to restrictive pathology. Invasive DAV is reserved for "indications of necessity" that is when non-invasive ventilation is contraindicated due to the absence of adequate cough and for alveolar hypoventilation leading to hypercapnoea during spontaneous ventilation.

The main pathophysiological limitation to non-invasive ventilation is the interference of the glottis. In this mode the glottis imposes a variable resistance to the ventilation delivered. Its behaviour is more predictable during Volume controlled than during pressure controlled ventilation. The control parameters of a Volume controlled ventilator are very different from those used in invasive ventilation during which the respiratory system may be regarded as a single compartment (provided a cuffed tube bypasses the upper airway). In non-invasive DAV: mode VCM, tidal volume 13 mls kg(-1), rate 20 cycles min(-1), insp/exp ratio 1/1.2. In invasive DAV: mode VCM, tidal volume 8-10 mls kg(-1), rate 12 cycles min(-1), insp/exp ratio depending on the pathology 1/2.

As non-invasive DAV is essentially delivered during sleep the parameters for each patient can be optimised during polysomnography because waking, leading to a partial glottic occlusion, interferes with the ventilation delivered.

Recent understanding of the way the glottis interferes with mechanical ventilation when delivered non-invasively should lead to a revision of earlier practices based on invasive ventilation.