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.

Measurement of FiO2: It has been established that FiO2 achieved with nasal cannula does not
differ significantly whether the subject breathes through their mouth or nose. This property
has not been established for masks. With a closed mask, FiO2 achieved would also be the same
regardless of mouth or nose-breathing. With an open mask, the assumption must be made that
oxygen concentration is equal at the nose and the mouth in order to ensure equivalence of
mouth and nose-breathing. Also, previous studies on oxygen delivery systems have used a
variety of methods for measuring FiO2. From a review of the literature of the oxygen delivery
devices discussed above, only the nasal cannula has been studied by measurement of oxygen at
the level of the pharynx, whereas the other masks have been studied by measurement of oxygen
at the lips.

Study Rationale: The studies on OxyMaskTM determined FiO2 based on gas sampled at the lips.
However, due to the possibility of gas mixing and/or nasal breathing with the open mask
design, it is unclear if the gas sampled at the lips is the same gas that arrives at the
trachea, which is the most clinically relevant location. The gas inspired from the trachea is
what is actually seen by the alveoli and participates in oxygenation of the blood. Gas mixing
would seem more likely with an open mask design such as the OxyMask. A study comparing
pharyngeal FiO2 between masks would more accurately demonstrate the effective FiO2 than the
currently available data. This in turn, will better inform perioperative clinicians of the
best approach to maximize oxygen delivery to our critically ill post-operative patients who
require supplemental oxygen to treat or prevent systemic hypoxia.

Study Design: The investigators will measure steady state FiO2 in the pharynx and at the lips
simultaneously on each subject as they undergo each of the following oxygen delivery
conditions sequentially: 1) room air; 2) high flow oxygen and simple mask; 3) high flow
oxygen and non-rebreather mask; 4) high flow oxygen and OxyMask; and 5) high flow oxygen and
anesthesia mask with head-strap and Jackson Rees circuit.

Study Population: The study will include up to 20 subjects. A mean difference of 10%
pharyngeal FiO2 between any of the masks will be considered clinically important. The
expected standard deviation of the within-subject FiO2 is 3.5%. With a significance criterion
of 0.05, 10 subjects would provide more than 90% power to detect a mean difference of 10%.

Setting: The study will take place in a fully equipped operating room with an anesthesia
machine that has been approved for patient use by Infection Control and Biomedical
Engineering. It is located in the South OR area of Kohler Pavilion at OHSU and is equipped
with the Datex Ohmeda anesthesia machine and Poet Gas Analyzer that will be used for the
study. The study will be conducted after hours to avoid any conflict with surgical patient
care. The oxygen sensors will be calibrated before each subject is tested.

Recruitment: Subjects will be recruited from OHSU's perioperative service. Advertisement
flyers will be distributed in each operating room location.

Consent: The consent process will be undertaken by individuals with appropriate human
subjects protection and HIPPA education, and performed in person on the OHSU campus or on the
phone. Subjects may withdraw from the study at any time.

Procedures: The subjects will each participate on one study day and will require one hour per
subject. Subjects will remain NPO for at least 6 hours prior to their study period. Subject
characteristics will be recorded. The anesthesia machine will be turned on and the oxygen
sensor on the gas analyzer will be calibrated. Baseline vital signs (HR, BP, SpO2) will be
measured. FEV1 and FVC will be measured with the anesthesia machine and a standard circuit,
using the Y-piece as the mouthpiece. The following protocol for gas sampling will be used:
One gas sampling line will be attached at one end to the gas analyzer and at the other end to
an 8 French suction catheter, which will be lubricated with 2% lidocaine jelly. This nasal
catheter will be inserted through a nare, with the tip position immediately behind the uvula.
Placement will be confirmed by visualizing the tip of the catheter using a penlight and
having the subject open the mouth widely and say, "Ah." If required, a tongue depressor will
be used. If the catheter cannot be visualized, then it will be inserted to a standard depth
of 9-10cm. The nasal catheter will be taped to the subject's face. A second gas sampling line
will be taped to the subject's face and positioned so that sampling occurs at the patient's
lips. After placement of the two sampling catheters is confirmed, the subject will breathe
room air normally for 90 seconds and FiO2 will be measured over a period of 30 seconds. The
subject will not be instructed whether to use their nose or mouth to breathe, since patients
in the hospital typically not instructed which way to breathe. Afterward, testing of the
various oxygen masks will be performed with oxygen set at high flow (15L/min) through simple
mask, non-rebreather mask, OxyMask, and anesthesia mask with headstrap and Jackson Rees
circuit. Since the auxiliary oxygen dial on the anesthesia machine only goes up to 10L/min, a
15L/min adjustable oxygen valve will be attached directly to the main oxygen supply line in
the operating room. After placement of each mask and starting oxygen high flow, the subject
will breathe normally for 90 seconds and then FiO2 will be measured over the next 30 seconds,
similarly to the room air control. At the end of each trial period, each subject will be
asked to take a single vital capacity breath (starting with maximum exhalation and followed
by maximum inhalation). Between testing of each mask, there will be a five minute period of
breathing room air as a washout period and to confirm stability of hemodynamic status
(measurement of BP and HR). Demographic data to be obtained includes height (cm), weight
(kg), age, gender, and self-declared ethnicity. Blood pressure, heart rate, and oxygen
saturation will be recorded at baseline and prior to and following each mask-trial. Baseline
FEV1/FVC measurement will be conducted on all patients prior to initiating the study and
recorded. FiO2 data will be downloaded digitally from the gas analyzer to an excel
spreadsheet. If this is not possible, then oxygen percentage will be manually recorded during
each breath to determine the range of oxygen percentages (minimal and maximal) during both
phases of ventilation, during the 30 second measurement period. Respiratory rate during
measurement time will also be recorded. At the end of the 30 second sampling period, a final
measurement will be taken during a forced vital capacity breath.

Data Analysis: Phase of ventilation will be defined using the capnography tracing, which is
displayed concurrently with measurement of inspired oxygen. Maximal and minimal oxygen
concentration will be averaged for inspiration, during the 30 second mask breathing trial,
for each subject. These values will be averaged across the group for each intervention to
define statistics for the entire group. Values recorded during normal tidal volume
ventilation will be analyzed separately from values recorded during each forced vital
capacity ventilation. The Analysis of Variance test and appropriate post-hoc analysis will be
used to determine the difference between FiO2 administered by each mask tested.

Inclusion Criteria:

- 18 to 70 years of age

- Male and female volunteers

- ASA physical status I, II and III

- Capable and willing to provide written informed consent in English

Exclusion Criteria:

- Acute cardiopulmonary disease, as defined by blood pressure greater than 150/90, HR
greater than 120 and room air oxygen saturation less than 92.

- Allergy to lidocaine or adhesive tape

- History or physical exam finding of nasal polyps

- Currently taking oral or parenteral anticoagulant medications (other than aspirin)

- History of frequent nose bleeds

- Current symptoms of nasal congestion

- Physical examination findings of rales or wheezing

- Facial hair that prevents forming a seal with an anesthesia mask