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Proportional pressure support in acute lung injury: an observational study

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Proportional Pressure Support (PPS-) is a partial ventilatory support that overcomes patient respiratory system elastance (Ers) and resistance (Rrs) by applying pressure `proportional' to volume (volume assist, VA) and flow (flow assist, FA) respectively. The aim of this study was to evaluate its clinical use at the bedside in critically ill intubated patients affected by acute lung injury (ALI).


Thirty previously healthy patients affected by post-traumatic (emopneumothorax and lung contusion, N=21), infective (pneumonia, N=8) and inflammatory (pancreatitis, N=1) ALI (American-European Consensus Conference on ARDS) were studied. After several days (range 2-7, mean 4) of assist/conrolled (A/C) ventilation (BIPAP-), patients were switched to PPS once they were stable. Ers and Rrs were determined during a short period of volume control ventilation (square flow wave, tidal volume 10 ml/kg) using the inspiratory hold technique. On the ventilator (EVITA 4, Draeger) FA and VA were set to 80% value of patient Ers and Rrs respectively. The Automatic Tube Compensation (ATC-) was also used at 100%compensation. O2 inspiratory fraction (FiO2) and positiveend-respiratory pressure (PEEP) were the same as in A/Cmode. Progressive reduction of FA, VA, PEEP and FiO2was used as the weaning technique, during the healing phase of the disease. The following parameters were ascertained daily: arterial blood gas analysis, respiratory rate (RR), tidal volume (Vt), minute ventilation (Ve), patients' comfort, number of respiratory distress that needed changes in ventilatory mode and failure to wean. The FA and VA values at which the patients were able to sustain spontaneous breathing and extubation were registered.


PaO2/FiO2 ratio remained unchanged or slightly better compared to A/C mode, and progressively returned to normal during recovery. PaCO2 ranged between normal values and pH ranged between 7.45 and 7.40 for each patient during the study period. The respiratory pattern changed greatly in terms of RR and Vt from one patient to another and in the same patient during the days, while Ve remained more stable. All patients, except two, were always comfortable and they were successfully weaned and extubated at mean values of FA=7 ± 2 cmH2O/l/sec and VA=8 ± 3 cmH2O/l. The two patients that experienced respiratory distress were affected by pneumonia and needed a change of ventilatory mode.


PPS- with ATC- may be used to assist the ventilatory needs of patients with post-traumatic, infective or inflammatory ALI. The clinician must not be frightened by an unusual respiratory pattern in the presence of a comfortable patient, because with PPS- patients are able to breathe as they like to maintain a normal pH, most likely because the neuroventilatory coupling is improved by this mode, as described by Younes [1]. Extensive studies are needed to evaluate the role of PPS- with ATC- among the ventilatory modes now available to support the patient with ALI.


  1. Younes M: Proportional Assist Ventilation. In Principles and Practice of Mechanical Ventilation. Tobin MJ: Mc Graw-Hill, Inc.; 1994, 349.

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Capra, S., Manera, S. Proportional pressure support in acute lung injury: an observational study. Crit Care 4 (Suppl 1), P111 (2000).

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