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Proportional Assist Ventilation - in vitro assessment of the effect of unloading on ventilator performance

Presented at the Neonatal Society 2002 Summer Meeting (programme).

Leipälä JA, Iwasaki S, Milner A, Greenough A

Children Nationwide Neonatal Intensive Care Centre, King's College Hospital, London, UK

Background: During proportional assist ventilation (PAV) the applied pressure is servo controlled throughout each spontaneous breath. The applied pressure increases in proportion to the flow and tidal volume generated by the patient, which enhances the effect of the patient's respiratory efforts on ventilation. The clinician can vary the degree of enhancement or "unloading". Resistive unloading relieves the resistive work of breathing and elastic unloading the elastic work of breathing (1). Excessive unloading, however, could result in "runaway" ventilator pressures or oscillations (2).

Aim: To evaluate varying levels of unloading during PAV on ventilator performance, using a lung model simulating RDS.

Methods: A Stephanie ventilator was connected via a pneumotachograph and size three endotracheal tube to a one litre bottle (compliance 0.79 ml/cmH2O and resistance 87 cmH2O/l/sec). The flow signal from the pneumotachograph and attached differential pressure transducer was integrated to give volume. Airway pressure was measured from a side-port on the pneumotachograph. Flow, volume and pressure were simultaneously recorded on a Gould polygraph. A 20 ml syringe was attached to a separate inlet into the bottle and was used to simulate spontaneous breaths. Volumes of 5, 10, 15 and 20 ml were withdrawn from the lung model simulating inspiration. At each setting the results of five breaths were meaned. The ventilator was set to CPAP and PAV mode and varying levels of elastic (0.25-4.0 cmH2O/ml) and resistive (25-200 cmH2O/l/sec) unloading were studied. The experiments were then repeated with pressure limits of 20, 30, 40 and 50 cmH2O.

Results: At a resistive unloading of 50 cmH2O/l/sec or greater oscillations appeared, indicating resistive overcompensation. Pressures of greater than 40 cmH2O were observed when inspiratory volumes of 20 ml were used with an elastic unloading of 0.75 cmH2O/ml or greater and with volumes of 10 ml or more when the elastic unloading was 1.0cmH2O/ml or greater. Limiting the peak pressure to 20 cmH2O resulted in short inflation times (Ti <0.2 sec) when an elastic unloading of 0.75 cmH2O/ml or greater was employed.

Conclusion: The degree of unloading is crucial to appropriate delivery of PAV, this will vary according to the infant's underlying lung disease.

1. Schulze A Schaller P 1997 Assisted mechanical ventilation using resistive and elastic unloading. Seminars in Neonatology 2:105-114
2. Schulze A, Rich W, Schellenberg L et al 1998 Effects of different gain settings during assisted mechanical ventilation using respiratory unloading in rabbits. Pediatric Research 44:132-138

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