Behind the Mask

Patients with acute respiratory distress could benefit from a new mask that provides an Fio2 of more than 80% at an oxygen-flow rate of 8 L/min.

The treatment objective for severe hypoxemia resulting from acute respiratory failure is optimization of alveolar (and, thereby, arterial) Po2. Conditions causing acute respiratory distress are often associated with hyperventilation.1 The patient’s high minute ventilation and associated high inspiratory flows severely limit the Fio2 that most masks can deliver.2-5 When sufficiently high Pao2 cannot be provided by mask use alone, the therapeutic option defaults to endotracheal intubation, which is associated with considerable discomfort, morbidity, and cost.

d06a.gif (9090 bytes)
Figure 1. Inspired fraction of oxygen for
breathing at rest and hyperventilation while
wearing each type of mask. Bars represent
standard deviation.
* signifies significant difference from the new mask (P<.05).
NRM: nonrebreathing mask; PRM: partial-rebreathing mask;
simple: mask with side holes and no oxygen reservoir.

There have recently been several new indications for the inhalation of high Fio2. Greif et al6 reported that postoperative patients treated with high concentrations of oxygen had half the episodes of nausea and vomiting of those treated with 30% oxygen. A similar report7 on the benefits of more than 80% oxygen has shown reduced nausea in patients more than 60 years old who were transported for minor trauma; maternity transport cases may also benefit (Y. Yucel, MD, unpublished data 2001). A randomized controlled trial8 sponsored by the US National Institutes of Health showed that patients undergoing bowel resection who were treated with more than 80% oxygen in the perioperative period had 50% fewer postoperative infections, although both treatment groups received the recommended preoperative dose of antibiotics.

Effective delivery of high concentrations of oxygen depends on the mask’s ability to match the oxygen flow to the patient’s minute ventilation and peak inspiratory flow without diluting that oxygen with room air. Peak flows in breathless patients can reach several hundred liters per minute.1 One common approach is to try to match the peak inspiratory flows with oxygen flow to the mask; however, most oxygen flowmeters are calibrated to deliver only 15 L/min. At the flush setting, they still limit flow to approximately 25 L/min, far less than peak flow requirements. Delivery of higher flows into the mask requires a tandem setup of multiple flowmeters, increasing the complexity and cost of the system.

Another approach is a mask with an oxygen reservoir on the inspiratory side, with or without a one-way valve between the reservoir and the mask. In theory, the reservoir fills with oxygen during exhalation, and that oxygen is then available to meet peak inspiratory flow demands during inspiration. In practice, there is an obligatory entrainment of room air throughout inspiration, limiting the Fio2. The volume of entrained air depends on the relative resistance to flow in the side ports of the mask and the oxygen inlet. Differences in performance between the nonrebreathing and partial-rebreathing masks may be small. On one hand, the valve at the oxygen inlet of the nonrebreathing mask prevents expired gas from entering the reservoir; on the other, it increases resistance to flow from the bag to the mask and results in entrainment of more air (and a further decrease in Fio2), assuming both mask types fit the face equally well, as air entrained around the mask will also dilute inspired oxygen.

A new mask was recently developed to provide a compact, effective means of delivering Fio2 levels of more than 80% with oxygen flows no greater than minute ventilation. Its performance was compared with that of standard configurations of a nonrebreathing mask, a partial-rebreathing mask, and a simple oxygen mask.

The new mask is composed of inspiratory and expiratory limbs, each containing a one-way valve of very low resistance, and a sequential dilution conduit (leading from the atmosphere to the inspiratory limb) with a one-way valve that has a slightly positive cracking pressure. There is a gas reservoir on the inspiratory limb. During expiration, the gas reservoir fills with oxygen. During inspiration, gas from the oxygen source and the reservoir are drawn in preferentially. If the oxygen flow is equal to or greater than the minute ventilation, no atmospheric air is entrained and the patient gets pure oxygen. If the minute ventilation exceeds the oxygen flow, the reservoir collapses, the sequential dilution valve opens, and the remainder of the inspired gas is drawn from the atmosphere. Because gas is inhaled sequentially—first oxygen, then room air—the alveoli receive oxygen, while room air inspired at the end of inspiration is delivered to the anatomical dead space. Theoretically, due to the sequential delivery of oxygen and air, the minimum oxygen flow needed to provide an Fio2 of 100% is equal to the alveolar (not minute) ventilation. The alveolar ventilation is only about two thirds of the minute ventilation at rest.

The major determinants of Fio2 are minute ventilation and circuit design, with a minor effect from the patient’s breathing pattern, especially as it relates to peak inspiratory flow. To minimize the effects of intersubject variability in pulmonary function and breathing pattern, mechanical models have previously been used to study the efficacy of oxygen masks.9,10 While these models replicate respiratory parameters exactly, they introduce difficulties in calculating the Fio2. Under most conditions, the Fio2 varies throughout inspiration. Net Fio2 is a flow-averaged and time-averaged value determined using complex calculations based on two synchronized signals, one from a flow-measurement device and the other from an oxygen analyzer with a rapid response. A better measure of true Fio2 is calculated from the expired oxygen concentration using the alveolar gas equation; however, this requires a system that consumes oxygen. For this reason, the measurement can only be made in vivo.11 To facilitate determination of Fio2, the masks were compared using three trained male subjects.

The subject was seated in a chair and each mask was tested in an unblinded fashion. Each mask was placed comfortably on the face, with the metal strip molded to the bridge of the nose and the elastic headbands adjusted to provide the best possible seal, as assessed by occluding the inspiratory ports and having the subject make inspiratory efforts. The mask’s oxygen flow was provided by a calibrated flowmeter set at 8 L/min. Gas was sampled at 200 mL/min from a catheter placed 1 to 2 cm inside the nostril and was analyzed for carbon dioxide and oxygen content. For each mask, the subject was instructed to breathe in a relaxed manner (resting ventilation) and data were recorded for 1 minute after a stable breathing pattern had been established (with stability defined as less than 2 mm Hg of change in end-tidal partial pressure of carbon dioxide [Petco2] over a period of 3 minutes). Subsequently, the subject increased his ventilation by any combination of tidal volume and frequency to decrease Petco2 by 5 mm Hg (as prompted by visual feedback from the capnograph). After a steady state had been obtained, data were recorded for 1 minute. The analog signals for Pco2 and Po2 were digitized and recorded using an analog-to-digital converter. The peak-detection function was used to identify Petco2 and end-tidal fraction of oxygen (Feto2) for each breath. These values were exported to a commercial spreadsheet program. Net Fio2 was calculated from the Feto2 by solving the alveolar gas equation for Fio212: Fio2=(Feto2xRQ+Faco2)/(RQ+ Faco2x(1– RQ)), where RQ is the respiratory quotient and is assumed to be 0.8 and Faco2=Petco2/barometric pressure.

Results were compared using appropriate t-tests with corrections for multiple comparisons; P<.05 was considered significant. All results are reported as mean±SD.

The new mask provided an Fio2 of 97%±1%, 42% more than the next-best oxygen mask tested. When subjects increased ventilation, Petco2 values fell from 42.6±0.5 mm Hg to 37.4±0.2 mm Hg. At this higher level of ventilation, Fio2 was 90%±5% with the new mask, which was not significantly different from the resting Fio2, but was 34% more than the next-best oxygen mask tested (see Figure, page 24).

The new mask is configured as a compact manifold and mask with a profile similar to that of nonrebreathing or partial-rebreathing masks. All of the masks tested, except for the new mask, provide lower Fio2s because of obligatory entrainment of room air through their side holes throughout inspiration. These side ports allow expiration, but are also necessary to provide negative-pressure relief when minute ventilation exceeds oxygen flow. In contrast, the new mask is completely sealed and the relief ports for inspiration and expiration are located on the other side of one-way valves. This prevents dilution of the inspired oxygen by air as long as the oxygen flow is equal to or greater than the minute ventilation. If it is less than minute ventilation, sequential gas delivery means that the oxygen flow can, theoretically, be about 30% less than the minute ventilation at rest without compromising the effective (alveolar) Fio2. This is also likely to be why the Fio2 is higher with the new mask than with the nonrebreathing and partial-rebreathing masks at the higher ventilation level of testing. It may also ensure a high Fio2 during rapid shallow breathing (as seen in patients with rib fractures or such restrictive lesions as pulmonary edema or fibrosing alveolitis).

The new mask was the only mask to provide an Fio2 of more than 80% at an oxygen-flow rate of 8 L/min. This high Fio2 may provide clinically significant benefits, or be useful when conservation of oxygen is needed in field situations.

Ron Somogyi, BSc, and David Preiss, MScEng, are graduate students; Alex Vesely, MSc, Eitan Prisman, BSc, Janet Tesler, MSc, and George Volgyesi, PEng, are research associates; and Joseph A. Fisher, MD, is associate professor; Department of Anesthesiology, University Health Network, University of Toronto. Hiroshi Sasano, MD, PhD, is a visiting professor at University Health Network from the Department of Anesthesiology and Resuscitology, Nagoya City University Medical School, Japan. Steve Iscoe, PhD, is associate professor, Department of Physiology, Queens University, Kingston, Ontario.

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