Performance of the large-reservoir oxygen mask (Ventimask)

The performance of large-reservoir Venturi masks ('Ventimask') giving nominal inspired oxygen (O2) concentrations ranging from 24% to 60% was assessed in a face model, in six normal subjects, and in ten patients with severe chronic airflow obstruction at the O2 flow rates recommended. Instantaneous oxygen concentrations were measured with a mass spectrometer ('Centronics MGA200') and volume-weighted to give mean inspired concentrations (FiO2). In human studies volume-weighting was achieved by simultaneous measurement of tidal volume from chest and abdominal motion by the use of a respiratory inductance plethysmograph ('Respitrace'). Reinspiration of dead space from the mask was assessed by measuring FiCO2, which varied from 0.2 to 0.4% (normals) and was 0.6 +/- 0.4% in patients. In normal subjects and patients breathing at rest the FiO2% in 24, 28, 35, and 40% ventimasks was within 1.9% absolute of nominal (range-1.2 to +1.9%) but the 60% mask read low (50.3%). Various factors can make FiO2 less than nominal but the human and model studies indicated that reinspiration of dead space from the mask was more important than hyperventilation. In tachypnoeic patients (frequency greater than 30/min), the O2 flow should be increased to 50% above recommended.