Page 1
HIGH FREQUENCY OSCILLATORY VENTILATION:
Clinical Management Strategies for Adult Patients By: Stephen Derdak, D.O Pulmonary/Critical Care Medicine With increased understanding of the physiology of lung recruitment and the adverse consequences of ventilator induced lung injury (VILI) it has been hoped that improved ventilator strategies will reduce the morbidity and mortality of this common cause of respiratory failure (1. The NIH ARDS network clinical trial recently reported that a conventional ventilator strategy employing a low tidal volume “lung protective” approach [tidal volumes (TV) < 6 ml /kg ideal body weight, inspiratory plateau pressures (Pplat) < 30 cm H2O] reduced absolute mortality 9% compared with a larger tidal volume (12 ml/kg) strategy (2). Unfortunately, in practice, many critically ill patients with ARDS are unable to achieve oxygenation goals using conventional protective lung approaches (arbitrarily defined as FiO2 ≤ 60% with Pplat ≤ 30 cmH2O) and the mortality from ARDS remains unacceptably high. In view of the successful use of high frequency oscillatory ventilation (HFOV) in neonatal and pediatric acute respiratory failure syndromes in the 1980’s and early 1990’s we became interested in the potential application of this ventilatory mode for adults with severe ARDS. The optimal specific techniques of using HFOV in large patients are continuing to evolve and as with all interventions in critical care, frequent reassessment of the patient and modification of therapeutic strategies as the patient’s condition changes is essential.
Clinical Trials of HFOV in Adults In the early 1990’s, our group at Wilford Hall Medical Center conducted a non-randomized pilot trial using the SensorMedics 3100B ventilator in adult patients with severe ARDS who were failing conventional ventilation (3). In this trial, 17 adult patients with severe ARDS (mean PaO2/FiO2 67 mmHg on 18 cmH2O PEEP) were treated with HFOV using an “open lung” strategy. Despite the severity of ARDS, 30-day survival was 47%. Despite the higher mPaw applied, there was no significant compromise in cardiac output or oxygen delivery. In view of the encouraging results seen with the adult pilot study, a multicenter randomized, controlled trial of oscillatory ventilation in ARDS (MOAT) was initiated in 1997 and completed in December 2000 (4). In this trial, adult patients with ARDS (PaO2/FiO2 < 200 mmHg on PEEP ≥ 10 cm H2O) were randomized to HFOV (N= 75) or a conventional pressure control strategy (N= 73). Although this study was not specifically powered to evaluate mortality differences, we observed a nonsignificant trend towards improved overall mortality at 30-days in the HFOV group compared with the CV group (37% versus 52% 30-day mortality, respectively, p=0.098). This trend toward improved mortality with HFOV was still evident at 6 months (6 month overall mortality rate 47% in the HFOV group and 59% in the CV group, p=0.143). There was no significant difference between groups in new or worsening barotrauma, endotracheal tube obstruction, or adverse hemodynamic effects. In a retrospective analysis of pretreatment indicators of survival, only peak inspiratory pressure (< 38 cmH2O), but not pre-study days on CV, P/F, or OI showed a significant difference in mortality between HFOV and CV (30-day mortality 26% in the HFOV group versus 52% mortality in the CV group, p=0.018). Although this was a post-hoc finding, it suggests that earlier application of HFOV (when the lung is more compliant) may improve the outcome. Most recently, an additional nonrandomized study of HFOV in 24 adults with ARDS has been published by the Toronto group (6). Patients entered into this study had severe ARDS (mean PaO2/FiO2 98 mmHg, OI 32) and required ≥ 60% FiO2, or Pplat ≥ 35 cmH2O on CV before starting HFOV. Again noted was the finding that survivors were on CV for significantly fewer days prior to starting HFOV (survivors 1.6 days versus 7.8 days in nonsurvivors). Overall mortality at 30-days was 67%. 2. Technique Of HFOV In Large Patients When to initiate HFOV Patients with severe ARDS who are requiring an FiO2 ≥ 60% with a mPaw > 24 may be considered for a trial of HFOV – especially if a “lung protective” target Pplat < 30 - 35 cmH2O cannot be maintained. In practice, patients considered for HFOV have generally already been tried on “high” PEEP (e.g. 20 – 25 cmH2O) and/or pressure control ventilation with extended inspiratory times to raise mean airway pressure. At this point, patients are usually already deeply sedated and on neuromuscular blockade to facilitate oxygenation.