Patients with respiratory failure are usually mechanically ventilated, mostly with fraction of inspired oxygen (FiO2) > 0.21. Minimizing FiO2 is increasingly an accepted standard. In underserved nations and disasters, salvageable patients requiring mechanical ventilation may outstrip oxygen supplies.

The hypothesis of the present study was that mechanical ventilation with FiO2 = 0.21 is feasible. This assumption was tested in an Acute Respiratory Distress Syndrome (ARDS) model in pigs.

Seventeen pigs were anesthetized, intubated, and mechanically ventilated with FiO2 = 0.4 and Positive End Expiratory Pressure (PEEP) of 5cmH2O. Acute Respiratory Distress Syndrome was induced by intravenous (IV) oleic acid (OA) infusion, and FiO2 was reduced to 0.21 after 45 minutes of stable moderate ARDS. If peripheral capillary oxygen saturation (SpO2) decreased below 80%, PEEP was increased gradually until maximum 20cmH2O, then inspiratory time elevated from one second to 1.4 seconds.

Animals developed moderate ARDS (mean partial pressure of oxygen [PaO2]/FiO2 = 162.8, peak and mean inspiratory pressures doubled, and lung compliance decreased). The SpO2 decreased to <80% rapidly after FiO2 was decreased to 0.21. In 14/17 animals, increasing PEEP sufficed to maintain SpO2 > 80%. Only in 3/17 animals, elevation of FiO2 to 0.25 after PEEP reached 20cmH2O was needed to maintain SpO2 > 80%. Animals remained hemodynamically stable until euthanasia one hour later.

In a pig model of moderate ARDS, mechanical ventilation with room air was feasible in 14/17 animals by elevating PEEP. These results in animal model support the potential feasibility of lowering FiO2 to 0.21 in some ARDS patients. The present study was conceived to address the ethical and practical paradigm of mechanical ventilation in disasters and underserved areas, which assumes that oxygen is mandatory in respiratory failure and is therefore a rate-limiting factor in care capacity allocation. Further studies are needed before paradigm changes are considered.

Use of an oxygen-powered demand-valve to ventilate through an endotracheal tube is considered inappropriate due to concern regarding excessive airway pressure.

It was hypothesized that ventilation through an endotracheal tube using a bag-valve (BV) device and the recently modified demand-valve (DV) would produce similar tidal volumes (Vt), minute ventilation (MV), and peak airway pressures (PAP).

This is a prospective, randomized vitro experimental model. Subjects were blinded to volume and pressure gauges. Thirty-nine EMTs (mean age 27 years with mean experience five years) volunteered to ventilate a mechanical test lung through an endotracheal tube for 10 minutes. Each subject was randomized to BV or DV and to either normal (0.1 L/cm H2O) or poor (0.04 L/cm H2O) lung compliance. This DV delivers set flow of 40 L/min at maximum 50±5 cm H2O. Subjects were instructed to use their “usual” technique for an average size adult in respiratory arrest with normal heart rate and blood pressure. The Vt and PAP were recorded for each breat; the MV and maximum PAP (PAP-max) for each minute was noted. Data were analyzed using repeated measures ANOVA and Tuke multiple comparisons with alpha set at 0.05.

Overall average tidal volumes and minute ventilations were acceptable with both ventilalory devices at both normal and poor compliance for the first, fifth, and 10th minute of continuous ventilation. Average airway pressures and peak airway pressures during the first, fifth, and 10th minute of ventilation all were significantly higher with those of the bag-valve than with the use of the demandvalve at both normal and poor compliance.

In this model, ventilation with bag-valve and demand-valve both provided more than adequate Vt and MV; values wer similar except for higher Vt with BV at normal compliance. However, DV yielded significantly lower PAP and PAPmax at both poor and normal compliance. These findings need corrobration in an in vivo model, but suggest that with proper training, demand-valve ventilation through an endotracheal tube may be preferable.