An analysis of the performance of a variable venturi-type oxygen mask

Nasal cannula and face mask gas flow rates when connecting to the Y-piece of the anesthesia circuit

STUDY OBJECTIVE

To determine the relationship between the delivered gas flows via nasal cannulas and face masks and the set gas flow and the breathing circuit pressure when connecting to the Y-adapter of the anesthesia breathing circuit and using the oxygen blender on the anesthesia machine, relevant to surgery when there is concern for causing a fire. The flow rates that are delivered at various flow rates and circuit pressures have not been previously studied.

DESIGN

Laboratory investigation.

SETTING

Academic medical center.

PATIENTS

None.

INTERVENTIONS

The gas flows from each of 3 anesthesia machines from the same manufacturer were systematically increased from 1 to 15 L/min with changes to the adjustable pressure limiting valve to maintain 0-40 cm water pressure in the breathing circuit for nasal cannula testing and at 20-30 cm water circuit pressure for face masks.

MEASUREMENTS

The delivered gas flows to the cannula were determined using a float-ball flowmeter for combinations of set gas flows and circuit pressures after connecting the cannula tubing to the Y-piece of the anesthesia circuit via a tracheal tube adapter. Decreasing the supply tubing length on the delivered flow rates was evaluated.

MAIN RESULTS

There was a highly linear relationship between the anesthesia circuit pressure and the delivered nasal cannula flow rates, with 0 flow observed when the APL valve was fully open (i.e., 0 cm water). However, even under maximum conditions (40 cm water and 15 L/min), the delivered nasal cannula flow rate was 3.5 L/min. Shortening the 6.5-ft cannula tubing increased the flow at 20 and 30 cm water by approximately 0.12 L/min/ft. The estimated FiO2 assuming a minute ventilation of 5 L/min and 30% FiO2 ranged from 21.7% to 27.0% at nasal cannula flow rates of 0.5 to 4.0 L/min. When using a face mask and the APL fully closed, delivered flow rates were 0.25 L/min less than the set flow rate between 1 and 3 L/min and equal to the set flow rate between 4 and 8 L/min.

CONCLUSIONS

When using a nasal cannula adapted to the Y-piece of the anesthesia circuit, the delivery system is linearly dependent on the pressure in the circuit and uninfluenced by the flow rate set on the anesthesia machine. However, only modest flow rates (≤ 3.5 L/min) and a limited increase in the inspired FiO2 are possible when using this delivery method. When using a face mask and the anesthesia circuit, flow rates close to the set flow rate are possible with the APL valve fully closed. Patients scheduled for sedation for head and neck procedures with increased fire risk who require more than a marginal increase in the FiO2 to maintain an acceptable pulse oximetry saturation may need general anesthesia with tracheal intubation.

The 'fixed performance' venturi: effect of downstream pressure on outflow and FIO2

Fixed performance venturi devices should provide a predetermined oxygen concentration at an outflow which exceeds an adult's peak resting inspiratory flow rate (approximately 30 l.min(-1)). Campbell's original description mentioned the sensitivity of the venturi device to downstream resistance but gave no further details. This study examined outflow and oxygen concentration from the five standard venturi devices (24-60% O(2)) when downstream pressure increased. Outflow was exquisitely sensitive to small increases in pressure. The outflow at zero downstream pressure for the 24-40% O(2) venturi devices ranged from 40 to 50 l.min(-1) but only 2-3 mmH(2)O was needed to halve this flow and increase oxygen concentration. The 60% O(2) venturi delivered a maximum of only 30 l.min(-1) at zero downstream pressure and flow was reduced further by increasing this pressure. An increase in downstream pressure of only a few mmH(2)O increased oxygen concentration and decreased outflow of all the venturi devices tested, in most to less than normal peak tidal flow in adults.

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.

Fixed performance oxygen masks: an evaluation

Fixed performance oxygen masks operate by supplying mixtures of oxygen and air at rates exceeding the inspiratory flow rate of the patient. In this study the oxygen concentration delivered by three fixed performance oxygen masks was determined non-invasively at various inspiratory flow rates. At low inspiratory flow rates all the masks studied acted as fixed performance devices. When the peak inspiratory rate increased the performance of all the masks showed some variability. The change from fixed to variable performance depended on the relation between inspiratory flow rate and the total gas flow delivered by the mask and was independent of the volume of the mask. Hence the use of low volume masks and high oxygen flow rates should produce more consistent results than high volume masks and lower flow rates.

Fixed performance oxygen masks. Hypoxic hazard of low-capacity drugs

Four recently introduced low-capacity fixed performance oxygen therapy masks have been compared with the established Ventimask design. Under conditions of varying peak inspiratory flow rate the low-capacity devices all permit a variable amount of air admixture with a consequent fall in the inspired oxygen concentration. It is concluded that low-capacity venturi masks are not true fixed performance devices under all circumstances. The Ventimasks satisfy their specifications under all test conditions. A case is made for more rigorous assessment of new so-called "fixed performance" oxygen therapy devices before marketing is permitted.

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