Respiratory mechanical unloading and proportional assist ventilation in infants

Conventional patient-triggered ventilation attempts to synchronize the upstroke in ventilator pressure with the onset of spontaneous inspiration. Other parameters of the mechanical cycle such as the peak inspiratory pressure are preset by the clinician. They will be imposed on the infant regardless of the actual spontaneous respiratory drive. Proportional assist ventilation (PAV) and respiratory mechanical unloading of spontaneous breathing (RMU, resistive and elastic unloading) are based on fundamentally different concepts. In contrast to the conventional perception of the ventilator being a pump, RMU/PAV servo-controls the applied ventilator pressure continuously throughout each inspiration. These modalities proportionally enhance the effect on ventilation of each respiratory effort. They rely on rather than interfere with the subject's respiratory control system. The patient controls all variables of the respiratory pattern while the ventilator works fully enslaved as a proportional amplifier. Back-up conventional mechanical ventilation is initiated during episodes of hypoventilation and apnoea. The clinician sets the degree of the assist during RMU/PAV in terms of "gains". Selecting specific gains for the elastic and resistive unloading components allows the ventilator pressure waveform to be tailored to the individual degree of restrictive and obstructive pulmonary disease. This results in a reduction in the transpulmonary pressure cost of ventilation compared with conventional modes.

Further studies on RMU/PAV are required to evaluate clinically important long-term outcome variables in infants and to determine whether the benefits outweigh potential drawbacks and the complexity involved in these new modes of mechanical ventilation.