There are currently in use many medical procedures for the extracorporeal treatment of blood. Some of these procedures involve removal of blood from a patient, circulation of the removed blood through an extracorporeal blood circuit including a device for treating the blood, and return of the circulated, treated blood to the patient. Typical procedures of this type are blood dialysis, blood oxygenation, and hemoperfusion. Other treatment procedures involve the separation of the removed blood into one or more of its component parts, treatment of said one or more components in an extracorporeal environment, and return of the separated, treated component(s) to the patient. Illustrative of this type of procedure is one in which blood is separated into a plasma fraction and a cellular or "formed element" fraction, the plasma fraction is chilled and filtered, the filtered plasma fraction is warmed and recombined with the cellular fraction, and the recombined fractions are returned to the patient. This type of procedure is disclosed in U.S. Pat. No. 4,350,156.
The extracorporeal circuits used to carry out the aforementioned treatment procedures may require the use of a fluid reservoir as well as the specific device needed to effect the desired treatment. For example, in the case of open heart surgery, where either a bubble oxygenator or a membrane oxygenator is employed to take over the gas exchange function of the patient's lungs, a device known as a cardiotomy reservoir is generally included in the extracorporeal circuit to receive and hold banked blood and/or blood collected by the application of suction from the opened chest cavity of the patient ("cardiotomy blood"). Cardiotomy reservoirs usually include a defoamer for defoaming cardiotomy blood and banked blood and an internal chamber in which the defoamed blood may be temporarily stored to await further use at various stages of the surgical procedure. A cardiotomy reservoir has at least one and, usually, several inlet ports communicating with its interior. Where there are two or more inlets, one may be used for the addition of banked blood and another may be used for admission of cardiotomy blood. The cardiotomy reservoir further includes an outlet port through which blood is withdrawn to be conducted downstream thereof, usually to the blood inlet of the oxygenator.
In addition to a blood defoamer element, cardiotomy reservoirs frequently include a blood filtration media for filtering the blood after it has been defoamed, such devices being referred to as "cardiotomy reservoirs with filters." The purpose of filtering the blood after it has been defoamed is to remove undesirable particulate matter such as blood clots, aggregated blood cells, and debris from the site of surgery. As a practical matter, a blood filtration media should not be designed to remove undesirable particulate matter of less than about 20 microns in size. This is because the size of white blood cells typically is as high as about 18 microns and a filtration media capable of filtering particles of less than about 20 microns will not only remove the aforementioned undesirable particulate matter but may very likely also remove white blood cells or other desirable components of the blood. It is desirable therefore to provide a blood filtration media which is capable of efficiently removing particulate matter whose size is equal to or greater than 20 microns.