Roger Johnson
Air-oxygen blenders, also known as oxygen proportioners, are common devices used in patient airway applications. A typical blender has one or two low-flow output ports and one high-flow output port. Understanding how these ports operate, and sharing that knowledge with operators, can save facilities a substantial amount of unnecessary costs.
Blender Basics
Blenders have input ports for two separate gases, which are usually oxygen and medical air. These inputs can either be delivered separately through the blender or as a blended air-oxygen concentration of 21% to 100% by adjusting the front control knob. It is important to remember that blenders adjust gas concentrations, and that it is their auxiliary components that control the output flow.
Internal blender parts need a certain amount of flow in order for the output concentration to be accurate. In order to accommodate any low-flow conditions, one of the low-flow output ports is connected with an output bleed port, located on the bottom of the blender. If two low-flow ports are present on the blender, only one of these will be connected with the bleed.
Whenever a device is connected to the bleed-activated low-flow port, the bleed flow is active even if the external device is idle. Operators can simply put their finger over the bleed output filter and feel the gas flow to determine whether the bleed is active or idle. Among the many models of blenders available from several manufacturers, bleed rates can vary between 3 and 9 liters per minute. Coincidentally, the bleed gas concentration is also regulated by the blender’s oxygen setting. For purposes of this article, I will assume a bleed rate of 5 liters per minute.
The most common problem I have observed is when a flow meter is connected to a nonbleed output port and the operator is delivering a flow of less than 3 liters per minute. In this situation, it is difficult to predict what oxygen output concentration will result, especially if a blended output is selected. Factors such as input gas pressures, the actual requested flow, and the last concentration setting all play a role in the requested output.
In my experience, I have seen operators double-check gas delivery concentrations with a separate handheld oxygen analyzer, increasing patient safety. However, when I explained to the operator how bleed operation worked and how it affected low-flow applications, the operator changed his procedures. In most cases, he shifted the flow meters from one output port to another, and a simple in-service on these specifics completed the clinical issue.
Hidden Costs
A critical point to be aware of, however, is that blenders not in clinical use still have an associated operational financial cost if the bleed is active, because the constant flow of either oxygen or medical air has a calculable price. Although the cost of medical air is difficult to quantify, since pumps are generating this product, oxygen is purchased directly and some cost assumptions can be approximated.
To get a sense of the costs involved, let’s first calculate the total flow in liters for 1 year when a flow meter is set to 1 liter per minute. If we multiply 1 liter times 60 minutes, then multiply the result by 24 hours, and that result by 365 days, we come up with 525,600 liters of gas used in 1 year. Although rates change over time, and pricing is certainly different based on one’s location and usage, I know that at one time this total flow over a year was equal to approximately $60 when 100% oxygen was used.
With this in mind, imagine the following scenario: A 40-bed NICU has a blender dedicated to each room. At any given time, only half of the blenders are being used, even though the room may be occupied. All of these blenders are plugged into their gas outlets, with flow meters attached to one or both low-flow ports (the bleed is now active), and the concentration dial on the front of the blender is set to 100%. The flow meters are all turned off, and the blender appears to be completely idle. Indeed, the bleed is all but silent. Nonetheless, oxygen is being consumed.
In this case, the oxygen consumption calculation would be as follows:
20 (blenders) x 5 (liters/minute bleed rate) x $60 = $6,000/year.
If during the span of this year the blenders’ high-pressure input oxygen and air hoses had been disconnected from the wall, or if the flow meters had been disconnected from the blender, this cost would have been avoided entirely. As you can imagine, additional blenders or increased bleed rates would increase this cost substantially!
Our goal in clinical engineering should go beyond ensuring the functionality and safety of medical equipment. Sometimes we have an educational or training role to fulfill, as is the case with these blenders, which were operating as intended but not completely understood. When clinical engineering, hospital staff, and the manufacturer are working together for the betterment of the patient, our combined goal of a better patient experience, cost-effective management of resources, and enhanced outcomes can be realized.
Roger Johnson is an anesthesia specialist working for Providence Health & Services in Anchorage, Alaska.
I agree with what Roger has said about saving money, except that to be safe the blender must have the gas supply hoses disconnected from the supply when not in use, not just the flow meter removed from the aux outlets. This is to prevent possible gas bleed over between the air and oxygen systems through the unused blender. This is for the safety of infants that are set to 0.21 FiO2, and can not tolerate accidental bleed-over of oxygen into the medical air system through the unused blender. The potential bleed-over is eliminated when the blender is disconnected or there is flow coming from the blender, such as the bleed flow from the aux outlet being activated by a flow meter connection.