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Peri-prosthetic Joint Infections (PJIs)
Traditionally, the cause of most surgical site infections (SSIs) has been either indeterminable or, as in the case of bowel contaminants, an accepted risk of surgery. It is our belief, however, that the etiology of peri-prosthetic joint infections (PJI), a specific subset of SSIs, can be determined and should not be an accepted risk of surgery.
SSI vs PJI
The term SSI commonly refers to any infection that results from surgery. Lumping the varieties of SSI’s together, however, can cause confusion. In this discussion the term “SSI” is reserved for soft-tissue infections, which are differentiated from peri-prosthetic joint infections (or deep-joint infections) that can occur after total joint replacement surgery.
The implantation of foreign material in the body fundamentally changes the pathophysiology of the infectious process. An inoculum of more than one million bacteria is required to cause an SSI, and the bacteria usually enter the wound from the adjacent skin or cut bowel.1 In sharp contrast, it has been shown that a single bacterium can cause a PJI, and it usually enters the wound as airborne contamination.2-4
SSIs manifest in one day to one month. PJIs manifest in one week to one year. Why the difference?
The answer is biofilm.5 In the presence of an implanted foreign material, the bacterium produces a coating of extracellular polymeric substances, also known as exopolysaccharide—a biofilm that effectively protects it from both antibodies and antibiotics. The bacterium can go dormant for up to a year and then, when the conditions are favorable, it multiplies into a deep infection.6-8
In contrast, bacteria cannot produce effective biofilm in soft tissue. Without a biofilm coating, bacteria in soft tissue are exposed to both antibodies and antibiotics. Since it takes more than a million bacteria to cause a soft tissue SSI, any airborne contamination in the operating room is virtually irrelevant for soft tissue SSIs. It is highly unlikely that the air could contain that level of biological contaminants.
Severity of a PJI
The soft tissue SSI is generally a minor and easily treatable complication. In contrast, the PJI is a catastrophic complication requiring explantation of the joint, IV antibiotics, prolonged hospitalization and, if the patient survives without amputation, re-implantation of the prosthetic joint at a later date. Even without amputation, these patients are severely and permanently debilitated. It has been shown that most of these patients never regain full capacity and frequently cannot accomplish the activities of daily living. 12% of patients rate their life after surviving a DJI as “worse than death.”9
Prevalence of PJIs
While PJIs typically occur only in about 1-2% of total joint replacement surgeries, the incidence can be much higher at any given hospital. Ironically, many of the hospitals reporting higher than “normal” deep joint infection rates are well known for their high quality care. This seems to indicate that reducing the PJI rate is not as simple as strictly following accepted operating room protocols—these high quality hospitals are already doing that. Logically, there must be a previously unrecognized causative factor that is an accepted part of the current protocol.
A Newly Discovered Possible Cause of PJIs
New studies suggest that the unrecognized causative factor may be forced-air patient warming devices, also known as forced-air warming or “FAW.”
Let’s start from the beginning…
Where do infectious agents come from?
The CDC has said that PJIs are primarily caused by Staphylococcus aureus, coagulase-negative staphylococci and gram-negative bacilli, predominantly skin bacteria.10 Most of the bacteria in the operating room are shed from the skin of the surgical staff.11-16 Dispersed airborne skin bacteria can also originate in the perineal region of the staff, including vaginal and rectal carriage.10,17-24
The average person sheds one billion skin cells per day and up to 10% of these have bacteria attached.25-28 The skin cells have been called “skin rafts” because they ride the air currents in the operating room, transporting the attached bacteria much like a flying carpet.13
Contaminated medical equipment can also spread infectious agents.29-34 One study showed that 58% of the FAW blowers tested were emitting large quantities of internally generated (grown) germ-sized particles from the hose end.29 In another study, 96% of the blowers were emitting up to 300 million germ-sized particles per hour.30
It has been shown in many studies that bacteria in the operating room can ride air currents.35-38 Since the airflow of the ventilation air is from ceiling to floor, it is not surprising that the shed skin particles and bacteria concentrate near the floor.36,37,39,40
How do infectious agents get from the contaminated floor to the sterile surgical wound?
The answer: forced-air patient warming. FAW produces significant amounts of waste heat. The nearly 1000 watts of waste heat from a lower body or underbody FAW blanket escapes from under the surgical drapes at their lower edge near the floor,41 warming the floor air that has the highest concentration of airborne contaminants. The heated floor air rises alongside the surgical table, directly into the downward ventilation airflow and into the sterile surgical field above the wound.42-44
“...forced-air warming established convection currents that mobilized resident air from non-sterile areas such as the floor and under the anaesthesia/surgery drape into the surgical site.” McGovern et al42
Alternatively, the waste heat from an upper body blanket has been shown to both rise along the anesthesia screen and to radiate through the drapes. If a surgical light is positioned in the typical location over the chest of the patient, the rising warm waste air crosses over the anesthesia screen into the “dead zone” under the light and then into the sterile field.45 Contaminants from the anesthesia provider as well as the patient’s head and arms have been shown to ride the convection current of waste heated air into the sterile surgical field.42,44
The radiant waste heat has been shown to increase the temperature at the surgical site by 5˚C. That heat radiating through the drapes interacts with the downward-flowing ventilation air to form vortexes above the sterile field. The rapidly spinning air can suck contaminants from the floor and deposit them at the surgical wound. One study found 2,000 times more contaminant particles in the air over the wound with Bair Hugger (FAW) than with air-free HotDog® conductive fabric patient warming. With HotDog, only 1,000 particles per cubic meter of air were present. With Bair Hugger warming, the particle count was 2,174,000 per cubic meter, an increase of 217,300%.46
With five peer-reviewed, published studies documenting this effect, it is now irrefutable that the massive amount of waste heat from FAW venting near the floor grossly disrupts the protection of the surgical ventilation system.42-46 The waste heat has been clearly shown to mobilize contaminants from the floor and head end of the patient into the sterile surgical field.
Is there proof of infection?
If the standard of proof requires a multi-center, prospective, randomized controlled trial, the answer is “no.” Given the cost involved, that study is unlikely to ever be done. The exercise of basic logic, however, reveals ample proof.
It has been shown that the concentration of airborne contaminants in the surgical field is directly proportional to the concentration of contaminants in the wound.47-57 The waste heat from FAW clearly increases the airborne contamination in the sterile surgical field.42-46,58,59 Contaminating the air in the surgical field with contaminated air from the floor inevitably leads to increased contamination of the wound.
Viewed as a logical formula, it looks like this:
Greater contamination over the wound = larger numbers of PJI’s
Forced-air warming = greater contamination over the wound
Forced-air warming = larger numbers of PJI’s
As mentioned earlier, the total joint arthroplasty patient is susceptible to even small numbers of airborne contaminants because of the implanted foreign materials. In the presence of a foreign body, a single bacterium can cause a PJI, and it is usually from airborne contamination.2-4
McGovern et al effectively tested this hypothesis by removing the FAW link in the chain. They discontinued the use of Bair Hugger FAW and reported that their PJI rate fell by 74% (3.1% ==> 0.8%) p=0.024, 1437 patients, 2.5 years.42 They switched to HotDog (air-free) conductive fabric patient warming and maintained the lower infection rate.
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