Comparison of Pharyngeal Oxygen Delivery by Different Oxygen Masks

Oxygen therapy refers to the administration of oxygen at higher concentrations than room air. According to the most recent American Association for Respiratory Care Clinical Practice Guidelines, indications for supplemental oxygen include hypoxemia, severe trauma, acute myocardial infarction, short term therapy, and postoperative recovery. Though there are few contraindications to oxygen therapy, potential complications exist, including ventilatory depression in hypercarbic patients, absorption atelectasis, and oxygen toxicity (particularly in some patients receiving specific chemotherapeutic agents). In the spontaneously breathing hypoxemic patient without an endotracheal tube, it is important to be able to deliver high concentrations of oxygen in order to either avoid intubation or bridge to intubation. A number of oxygen delivery devices are available that have different properties that make them useful for different situations.

Variable performance (low-flow) systems: Low-flow devices are variable performance because the delivered flow rates are less than the patient's peak inspiratory flow rate (PIFR) and the oxygen is diluted by room air entrained from around the devices. Higher PIFR (e.g. respiratory distress) results in a decrease in the FiO2 delivered through a variable performance device. Rebreathing of carbon dioxide is also possible with low-flow masks at lower oxygen flow rates. Variable performance systems include the nasal cannula, simple mask, partial rebreathing mask, and non-rebreathing mask.

Nasal cannulas are commonly used for stable patients because they are more comfortable, less irritating to skin, and less anxiety-inducing than masks; they also allow the patient to eat, talk, and use incentive spirometry. Nasal cannula can provide 24-40% FiO2 with flow-rates up to 6 L/min. The simple mask is commonly used in the immediate postoperative setting for supplemental oxygen. Several small holes on either side of the mask as well as the imperfect seal allow room air to be entrained and exhaled air to escape from the mask. Simple masks can provide 35-50% FiO2 at flow-rates 5-10 L/min.1 Long-term use of masks can lead to skin irritation and pressure sores.The partial rebreather consists of a simple mask with a reservoir bag, and provides 40-70% FiO2 at flow-rates 6-10 L/min.1 The partial rebreather is no longer used at many hospitals because the range of oxygen delivery can be encompassed by the simple mask and the non-rebreather. The non-rebreathing mask is similar to the partial rebreather but includes one-way valves that prevent exhaled air from returning to the reservoir bag. The non-rebreather can deliver 60-80% FiO2 at 10-15 L/min.1

Fixed performance (high-flow) systems: By delivering oxygen at a flow rate greater than PIFR, fixed performance systems make it possible to provide a specified FiO2 throughout the respiratory cycle. The two commonly used devices are the Venturi mask and OxyMaskTM. Venturi masks use the Bernoulli principle to deliver oxygen at high flows through a Venturi valve that has a narrow constriction followed by a wider area with vents that entrain room air. Because of the relative ease of use and consistent oxygen delivery, they are useful for patients with COPD or others with chronic hypoxemia. The Venturi mask is able to deliver an FiO2 of 24-50%.1 Venturi masks will not be tested in this study.

The OxyMaskTM is a relatively new device that uses the Venturi diffuser with a five-pronged open mask design, which allows the patient to talk and may cause less claustrophobia than the more closed masks. OxyMaskTM is able to deliver 25-80% FiO2 at flow rates of 1.5-15 L/min. A closed mask with a perfect seal could theoretically deliver 100% FiO2. For the purposes of this study, a Jackson Rees circuit will be used to deliver oxygen to an anesthesia mask. The Jackson Rees circuit (also called the Mapleson F circuit) is routinely used during transport to deliver oxygen from the oxygen tank to intubated patients. This circuit consists of tubing connected to a manual ventilation bag that can be squeezed to deliver pressurized gas flow to the patient. The bag is equipped with an adjustable pressure limiting valve, which allows the provider to control the pressure delivered by squeezing the bag.