Laboratory Evaluation of the Vortran Automatic Resuscitator Model RTM

BACKGROUND: One device that has been proposed to address the need for emergency ventilation is the Vortran Automatic Resuscitator.

OBJECTIVE: To test the hypothesis that increasing load (ie, increasing resistance or decreasing compliance) significantly affects minute alveolar ventilation.

METHODS: A Vortran Automatic Resuscitator was connected to a passive lung model and we measured load with 8 combinations of 4 compliances (14, 28, 46, and 63 mL/cm H₂O) and 2 resistances (20 and 42 cm H₂O/L/s). Source gas flow was either 20 or 40 L/min. We measured tidal volume (V_T), frequency, inspiratory time, expiratory time, peak inspiratory pressure, and intrinsic positive end-expiratory pressure. We calculated the ratio of inspiratory time to total cycle time (T_I/T_tot), minute ventilation, minute alveolar ventilation, and estimated P_aCO2. Raw data were summarized with descriptive statistics. A subset of the experimental data (outcome measures for conditions with high and low values for resistance, compliance, and source gas flow) was analyzed with a 2-level factorial design, with standard “design of experiments” procedure, including analysis of variance. Differences associated with p values ≤ 0.05 were considered significant.

RESULTS: Assuming the model lung represented a 68-kg adult, the measured V_T ranged from a low of 1.7 mL/kg to a high of 16.7 mL/kg. T_I/T_tot was greatly affected by the input flow. At 40 L/min the average T_I/T_tot was 30%, and at 20 L/min T_I/T_tot was 52%. As the load increased, V_T decreased and frequency increased. However, neither the minute ventilation nor the minute alveolar ventilation stayed constant. Minute ventilation ranged from 5.2 L/min to 11.3 L/min at 40 L/min source flow. More importantly, minute alveolar ventilation ranged from zero to 9.8 L/min, resulting in a calculated P_aCO2 range of over 100 mm Hg to 16 mm Hg, respectively. Indeed, calculated P_aCO2 was never in the normal range (35–45 mm Hg). “Design of experiments” analysis showed that V_T was affected by compliance and resistance (p < 0.001 and p < 0.05, respectively). Frequency was affected only by compliance (p < 0.001). Minute alveolar ventilation was affected by compliance and resistance (p < 0.001 and p < 0.01, respectively). Minute alveolar ventilation increases as compliance increases and/or resistance decreases, but these variables were essentially independent.

CONCLUSIONS: The Vortran Automatic Resuscitator showed an automatic increase in frequency and decrease in V_T that resulted in inappropriate levels of minute alveolar ventilation over a range of compliance and resistance values expected in paralyzed patients ventilated for respiratory failure. The variable performance under changing load, along with the lack of alarms, should prompt caution in using the Vortran Automatic Resuscitator for emergency ventilatory support in situations where the patient cannot be constantly monitored by trained and experienced operators.