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Different modes of assisted ventilation in patients with acute respiratory failure

¹ Istituto di Anestesia e Rianimazione, Universita' degli Studi di Milano, Ospedale Policlinico-IRCCS, Milano, Italy, ² Dipartimento di Scienze Cliniche e Biologiche, Universita' degli Studi dell'Insubria, Varese, Italy, ³ Istituto di Anestesia e Rianimazione, Università di Milano-Bicocca, Ospedale S. Gerardo, Monza, Italy

CORRESPONDENCE: L. Gattinoni, Istituto di Anestesia e Rianimazione, Ospedale Maggiore Policlinico-IRCCS, via Francesco Sforza, 35, 20122, Milano, Italy. Fax: 39 0255033230. E-mail: gattinon@polic.cilea.it

Keywords: acute respiratory failure, assist control ventilation, assist pressure control ventilation, gas exchange, pressure support ventilation, work of breathing

Received: September 6, 2001
Accepted April 21, 2002

The aim of the present study was to verify that the patient/ventilator interaction is similar, regardless of the mode of assisted mechanical ventilation (i.e. pressure- or volume-limited) used, if tidal volume (V_T) and peak inspiratory flow (PIF) are matched. Therefore, the authors compared the effects of three different modes of assisted ventilation on the work of breathing (WOB) and gas exchange in patients with acute respiratory failure.

For Protocol 1, in seven patients, the authors compared pressure support, assist pressure control and assist control (with square and decelerating wave inspiratory flow pattern) set to deliver the same V_T and PIF. For Protocol 2, in another 10 patients, the authors compared pressure support and assist control with high (0.8 L·s^–1) and low (0.6 L·s^–1) PIFs set to deliver the same V_T.

In Protocol 1, there was no difference in WOB and gas exchange between the three modes of assisted ventilation tested. In Protocol 2, the decrease of PIFs during assist control significantly increased WOB.

In conclusion, different modes of assisted ventilation similarly reduce work of breathing and provide adequate gas exchange at fixed tidal volume and peak inspiratory flow only. During assist control, tidal volume and peak inspiratory flow (set by the physician) are the main determinants of the patient/ventilator interaction.