Back round of the Invention ll~3~ 4~
g Management of respiratory insufficiency requires knowledge and control of the inspired oxygen concentration (FiO2) delivered to the patient. In addition, control oE the oxygen concentration administered to the patient is required if one is to avoid unnecessarily high inspired oxygen concentrations, with the attendant risk of pulmonary oxygen toxicity.
When the patient is breathing through a closed system, such as through an intratracheal tube, the inspired oxygen concentration can be easily measured and controlled. When oxygen is delivered via an open face mask, for instance, a mask having large holes in the cheek portion, the true inspired oxygen concentration cannot readily be measured and may be influenced by factors other than the oxygen concentration delivered to the mask. These variables include the patient's respiratory rate and pattern of breathing, the design and fit of the mask used, the flow rate of the oxygen delivered to the mask, and air turbulence in the area surround-lS ing the patient.
U. S. Patent 3, 850, 171 to G. J. Ball et al shows a medical face mask having several apertures in the mask body. U. S. Patent 2, 843,121 to C. H. Hudson also shows a mask having a large plurality of small open-ings in the cheek portion rather than a single large opening. In fact, U. ~;. Patent 2, 843, 121 specifically states that a series of small openings instead of one large opening in each cheek portion is desirable to prevent too free a passage of the gases in or out of the mask. U. S. Patents 1, 491, 674 to Coletti and 3, 288, 1~3 to Sachs, both show face masks having cheek holes covered by extensions or enclosures. However, the masks shown therein do not ha~e means for supplying oxygen to the mask and both inhalation and exhalation is effected through these openings. ~he main purpose of the extensions or enclosure is to direct exhaled air away from the wearers face or to act to filter inspired air.
U. S. 3, 315, 672 to Cunningham et al also shows a mask with "air - 1 - r-~
reflux fairings" over cheek holes. However, these enclosures also perform a much different function than the shields of the present invention. ~his mask is designed for use by a surgeon and cannot be used for supplying oxygen to a patient. In the Cunningham et al mask, inhalation is through the intake valves in the cheek portion of the mask and exhalation is through an exhaust conduit positioned in front of the users mouth. The purpose of the air reflux fairings is to prevent any expired air from being directed onto a patient during any transitional period between inspiration and exhalation when the intake valves may not be completely closed against exhalation.
U. S. Patent 2, 416, 411 to Sharbaugh et al shows a face mask for delivery of breathable oxygen to a pilot operating at high elevations. The demand type mask includes a valve in the inlet duct, which opens only on inhalation, and valves located in the cheek portion of the mask which open on exhalation. These exhalation valves are completely enclosed by a louver which is designed to protect the valve and to retain warm ex-haled air in the vicinity of the valve to prevent the valve from freezing.
None of the masks described in these patents show or suggest the mask described and claimed herein as none have, in combination,a means for administering oxygen or an oxygen containing gas stream to the mask, cheek holes which allow dilution of the oxygen in the mask and dispersion of exhaled breath from the mask with a minimum of restric tion, and open ended shields over the cheek holes to limit and/or prevent external conditions from causing the oxygen concentration which reaches the patient to be greatly different from that predicted to be delivered to the patient.
1~?35~ 4 Summary of the Invention The standard masks available for administration of oxygen or gas mixtures containing oxygen to patients have unrestricted holes in the cheeks of the mask which act 1) to dilute the oxygen fed to the patient and
2) allow exhaled breath from the patient to be dissipated. It has been found that using such masks, the percent of oxygen fed to the patient cannot be accurately controlled, high oxygen percentages cannot be administered, and there are differences between the predicted inspired oxygen concen-trations (Fi 2~ clinically measured with those predicted sources. Using a completely enclosed mask, such as an anesthesia mask, or a mask with one way vents or multiple small cheek holes is not desirable because these structures cause undesirable restri ction of the patients breathing pattern and the flow of gas in and out of the mask.
Clinical results obtained with the standard mask show a variation in the relationship between the clinically measured and predicted inspired oxygen concentrations (FiO2). Several possibilities were considered to explain these results. For instance, the respiratory pattern in the patient might change during the experiment and tidal volume during the experiment might not be the same as the tidal volume measured with the anaesthesia face mask before and after the experiment,or the mathematical model used to predict Fi O2 might not have taken into consideration factors or variables of importance.
Accordingly, a mechanical model was constructed in order to control these variables as much as possible. However, even with the mechanical model unexpected variations in the "tracheal'' oxygen concentration occurred.
It became apparent that there was a significant, though unpredictable, mixing of incoming oxygen in the mask with ambient room air. This mixing, over and above that attributable to the difference between the inspiratory flav rate and the flow rate of oxygen delivered to the mask, is apparently caused _ 3 _ 1~3~4~L
by turbulence resulting ~rom conditions surrounding the patient such as air flow from ventilation ducts, fans or air cond~itioners, disturbances caused by persons passing by or administering to the patient, the patient's own movement, as well as other extraneous sources, or turbulence generated within the oxygen mask itself caused by the incoming stream of oxygen.
The shielded mask of the present invention eliminated all the adverse effects caused by turbulence, either interior or exterior of the mask.
An object of the invention is to provide a vented face mask which will deliver predictable oxygen concentrations.
Another object of the invention is t o provide a vented face mask for delivery of oxygen to patient where the inspired oxygen concentration is not subject to variation caused by external sources.
An additional object of the invention is to provide a vented face mask which will allow the delivery of high oxygen concentrations to a patient.
Other objects and advantages of the invention will appear from the following description of the preferred embodiment of the invention.
BrieE Description of the Drawing Figure 1 i9 a side perspective view of the invention.
Figure 2 is a front perspective view of the invention Figure 3 is a second side perspective view of the invention.
Figure 4 is a graph showing clinical re~ults of mean inspired oxygen concentrations measured on patients using a standard mask.
Figure 5 is a schematic drawing of a mechanical model used to simulate respiration for comparison of the standard mask to the mask of the invention.
Figure 6 is a graph showing a standard mask as tested under clinical conditions compared with the same mask evaluated using the mechanical model under both turbulent and turbulence free conditions.
Figure 7 is a graph showing the results of evaluation of the mask of the invention (shielded mask) under the turbulent and turbulence free conditions, using the mechanical model.
Figure 8 is a graph comparing the standard mask and the mask of the invention (shielded mask) under non-turbulent conditions using the mechanical model.
Figure 9 is a graph showing the standard mask and the shielded masks compared under similar conditions in a controlled clinical experiment under normal hospital conditions.
Description of the Preferred I3mbodiment Referring now to the drawings, the shielded mask of the present invention is shown in Figures 1 through 3. The mask 10 consists of a flexible shell shaped to fit about the nose and mouth of a patient and to be in contact with the face so as to prevent gas administered to the patient from leaking around the edges of the mask. To aid in sealing the mask to the face,a flange is provided at the outer edge of the mask. The top of the mask 14 is shaped to fit on and approximate the bridge of the nose while the lower end 16 is rounded to fit just under the patient's chin. Al~ong the side of the mask and approximately half way between the top and bottom of the mask are a pair of tabs 18. Attached to tabs 18 is an adjustable or elastic retaining strap 20. In use, the strap 20 is placed behind the patient's head thus retaining the mask on the patient and causing the flange 12 to make intimate contact with the patient's face so that the administered oxygen doe~ not leak around the edges of the mask.
Located in the upper part and in the center of the nose portion 21 of the mask is a pin 22,Mounted on this pin is a nose clip 24. The nose clip is a flexible material, preferably a thin metal which can be easily bent, and when bent readily retains its new shape. After the mask is placed on a patient, finger pressure placed on the nose clip 24 will bend it so that the clip and the portion of the mask underlying the clip is shaped to conform to the nose of the patient, thus effecting a good seal between the mask and the patient's face surrounding the nose.
Attached to the lower end of the no~e portion 21 is an inlet part 26 sized for the attachment of an aerosol connector 28, an oxygen dilution valve (not shown) or tubing connected to a regulated source of oxygen ~not shown).
Located on both sides of the nose portion 21 are a pair of cheek holes 28 which allow expired air to leave the mask and which allow diffusion of am-bient air to dilute a concentrated oxygen stream administered through the inlet port 26. The cheek holes are preferably about 1/2 inch to 1 inch in diametér but the size of the holes are not believed to be critical. In addition, rather than a single large ~o~e on e~her side of the nose portion several holes or a series of holes having a cross-sectional area approximately the ~ame as the single cheek hole 28 would serve the same purpose as each of the single cheek holes 28. Positioned over the cheek holes 28 are shields
3~ As ~hown in Figure 3 shields 30 are cup shaped flexible enclosures which cover but do not obstruct the cheek holes 28. The lower portion of the shields 32 are open to allow easy diffusion or flow of gases in and out of the mask while the upper portion of the 6hields prevent flow of air outside or past the mask from disturbing the oxygen concentration within the mask.
Using the shielded mask on patients it wa6 pos6ible to obtain a higher inspired oxygen concentration than was possible with the standard ma~k and, with the shielded ma~k, the tracheal oxygen concentration could be raised to 100% with a ~ufficiently high flow rate of oxygen delivery, also not pos~ible with the unshielded mask. In addition, the concentration of the in6pired oxygen could be more readily and accurately regulated. Greater standard deviations at delivered flow rate~ of less than 15 litres per minute were obtained u6ing the 6hielded mask but thi6 probably represent6 true breath-to-breath variahons in a situation where the delivered oxygen flow doe6 not approximate inspiratory flow requirements.
Intratracheal oxygen concentration was directly measured in patients receiving oxygen by the use of a face mask having holes in the cheek portion thereof. Each patient had had a tracheo~tomy tube removed several days earlier but was breathing 6pontaneously through the upper airway with the residual ~toma covered by an occlu6ive dressing. A catheter was passed through this 6toma into the di6tal trachea through a small plastic plug which completely occluded the rest of the stoma. The intratracheal catheter was connected to a 6mall Y connector. One limb of the Y connector was used for withdrawing sample~ of tracheal ga~ for oxygen analysis. The other limb wa~ connected to a CO2 analyzer 6et to sample at 500 c. c. per minute. The output of the CO2 analy~er wa6 continuously recorded on a strip chart recorder.
1~35~44 The total volume of the catheter Y piece and connecting tubing was 3. S c. c.
100% oxygen wa6 delivered to the patient through a nebulizer u6ing calibrated flow meter~. Wide bore tubing (3/4") connected the nebulizer to a standard aerosal oxygen mask, which wa~ substantially as shown in Figure 1 except that the cheek shields were not present. An example of `6uch a mask is 601d as Cat. No. 002610 by Inspiron Division of C. 1~. Bard, Inc.
The continuous recording of the CO2 concentration in the trachea served as a tracing of the respiratory cycie and permitted measurement of the respiratory rate and the duration of the inspiratory phase of each respira-tory cycle (the inspiratory fraction).
Samples of tracheal gas for oxygen analysis were a~pirated through the limb of the Y connector used to withdraw samples into 50 c. c. plastic syringes during several consecutive inspiratory cycle6 at a time when respiration was -15 stable and the sampled gas was immediately analyzed u6ing a paramagnetic oxygen analyzer.
In each patient the flow rate of 100% oxygen delivered to the ma6k was succes6ively raised from 5 litres per minute to 30 litres per minute in 5 litre increment~. At each of these 6iX nOw rates, tracheal oxygen concen-tration was measured during the inspiratory phase.
The patient's minute ventilation was measured with a respirometer attached to an occlusive anaesthetic face mask, both immediately before and immediately after the measurements of the tracheal oxygen concentra-tion6 delivered by the pla6tic face mask. The duplicate values of minute ventilation proved to be quite ~imilar, with the difference between the two being less than 600 c. c. in each patiént. The average of the two minute ventilation determinations wa~ uE;ed for sub6equent calculations. Tidal volume was calculated by dividing the minute ventilation by the respiratory frequency.
Figure 4 6hows results obtained in clinical studie6 using the standard face mask. The tracheal oxygen concentration increase~ with increasing flow rates o~ delivered oxygen as would be expected. However, even at high flow rates, there is marked variation in FiO2 from patient to patient (as reflected by the standard deviation). In addition. tracheal oxygen concentration never reaches lOO"~o but rather appears to plateau as the flow rate of oxygen to the mask is increasèd. In addition, it wa~ believed that the environment surrounding the patient was affecting the amount o~ oxygen actually delivered to the patient.
To evaluate the effect of the variables which might affect the inspired oxygen concentration in patients, a mechanlcal model of the ventilatorv system was con~tructed as shown in Figure 5. A sine-wave pump 34 was used to simulate respiration. A 6tandard plastic aerosal mask 36 as described above was mounted on a firm backing 38 and attached to the pump 40 with a tube the size of a normal human trachea. A catheter ~2 was used to sample the F~O2 in the mechanical "trachea". 100% o~ygen 41 was delivered ~wgh the same system of flow meters 43 and nebulizer 44 used Eor the clinical studies. The pump was set to daliver a tidal volume of 600 c. c. at a respirats>ry frequency of 15 cycles per second and an inspiratory fraction of 0. 5 of the respiratory cycle.
Oxygen flow rates to the mask were varied from 5 litres to 30 litres per minute as in the clinical studies. In addition the studies using the mechanical model were carried out in botil a very still environment and in one containing air currents generated by a small fan placed 6 feet from the face mask and the effect of turbulence caused by the fan was determined. The test was then repeated using the modified oxygen mask having shields over the side-holes as illustrated in Figure 1.
Figure 6 shows the result obtained with the mechanical model using the standard face mask in both a still and a turbulent environment. In the tur-bulent en~,ironment, (with the fan on), the Fi 2 varies considerably and unpredictably. In the still environment (fan off~, the FiO2 is higher and more stable. However, in neither case did the oxygen concentration in the mechani-cal trachea reach 100% even with oxygen delivered to the face mask at a rate _q, 1~?3'~L4~ `
o~ 30 litre~ per minute. For comparison purposes the result of the clinical trial shown in Figure 5 i3 also incorporated in Figure 6. As can be 6een, the clinical situation i~ neither a still or turbulent condition but is instead an intermediate condition.
Result~ obtained under similar circumstance~ with the shielded mask of the invention, are shown in Figure 7. These figures illustrate two important features of the modified mask. The fir~t is that room air turbulence has no significant effect on the Fl 0 2 delivered with this mask. The second iæ that the ~12 delivered with this mask i6 higher at any given flow rate of oxygen delivery than the Fi02 measured under similar circumstances with the unæhielded mask. With the shielded mask, inspired oxygen concentration in the mechanical trachea reaches 100% at an oxygen flow rate of 30 litres per minute. Even in the still environment, the FiO2 delivered with the shielded mask is higher than with the un~hielded mask under similar circumstances.
The results for the mask with shields and the standard mask evaluated under still conditions are shown in Figure 8.
Using both the ~hielded and standard maæk on each patient clinical measurements of tracheal oxygen concentration were obtained. Tke results of this study, shown in Figure 9, demonstrate that the Fi02 obtained with the æhielded mask is consiætently higher than that obtained with the standard mask under similar circumstances and values approaching 100% FiC2 could oT~ly be attained using the shielded mask. In addition, when oxygen flow to the mask is 15 litres per minute or greater, the standard deviation from patient to patient using the shielded mask, is significantly less than the standard deviation from patient to patient with the standard mask (P~. 005), indicating that the shields reduced disturbances of the oxygen feed caused by outside sources.