Aspiration of dead space allows normocapnic ventilation at low tidal volumes in man

Objective: Aspiration of dead space (ASPIDS) improves carbon dioxide (CO₂) elimination by replacing dead space air rich in CO₂ with fresh gas during expiration. The hypothesis was that ASPIDS allows normocapnia to be maintained at low tidal volumes (V_T). Design: Prospective study. Setting: Adult intensive care unit in a university hospital. Patients: Seven patients ventilated for neurological reasons were studied. All patients were clinically and haemodynamically stable and monitored according to clinical needs. Interventions: ASPIDS implies that, during expiration, gas is aspirated through a catheter inserted in the tracheal tube. Simultaneously, a compensatory flow of fresh gas is injected into the inspiratory line. ASPIDS was achieved with a computer/ventilator system controlling two solenoid valves for aspiration and injection. Results: At the basal respiratory rate of 12.6 breaths min^–1, with ASPIDS V_T decreased from 602 to 456 ml, as did the airway pressures to a corresponding degree. PaCO₂ and PaO₂ remained stable. At a frequency of 20 breaths min^–1, with ASPIDS V_T was further reduced to 305 ml with preserved normocapnia. ASPIDS did not interfere with the positive end-expiratory pressure (PEEP) level. No intrinsic PEEP developed. All patients remained stable. No haemodynamic or other side effects of ASPIDS were noticed. Conclusion: The results of this study suggest that ASPIDS may be a useful and safe modality of mechanical ventilation that limits alveolar pressure and minute ventilation requirements while keeping PaCO₂ constant. Received: 21 December 1998 Final revision received: 28 April 1999 Accepted: 3 May 1999