Back flow in low-volume suction lines may lead to potential cross-contamination

Chris Miller, PHD

Studies confirm that there appears to be a chance for cross-contamination from saliva ejector (low-volume evacuation) systems. A common practice is to have patients close their lips around the low-volume saliva ejector tip and "spit" to help evacuate the mouth. Closing the lips around the ejector tip can apparently cause a decrease in the vacuum line pressure. This may allow previously evacuated fluid in the line to flow backwards, possibly reaching the patient`s mouth.

The initial study of this "back flow" was reported in 1993. The low-volume evacuation systems at dental units were disinfected, and a red solution was aspirated into the suction line and a clear plastic tip was placed on it. Each test subject closed their lips around the tip. The ejector tip was observed for the presence of the red dye coming back up from the vacuum line. The test subject then expectorated onto white tissue so that any red solution that entered the mouth might be observed. The test was performed 97 times with 15 dental units at nine locations.

The results indicated that back-flow into a patient`s mouth may occur about 21 percent of the time when patients close their lips around the saliva ejector tip to evacuate their mouth.

Th same study confirmed that microbes can be recovered from the fluids present in the low-volume suction lines. A portion of suction lines were flushed, and the water was microbiologically analyzed for the concentration of bacteria present. All of the lines were contaminated with numbers ranging from 46 to 200,000 colony-forming-units of bacteria per portion.

The investigators also determined that 91 percent of 117 dental offices asked patients to close their lips around the saliva ejector tip. In addition, the investigators noted that only 23 percent of the offices rinsed or disinfected their suction lines in between patients.

Preliminary results from a second study were reported in 1995. In this study, a high-volume evacuator was used simultaneously with the low-volume ejector. Test subjects closed their lips around saliva ejector tips. These investigators actually measured the suction line pressure and used clear saliva ejector lines so that movement of the fluid in the lines could be easily observed.

Closing the lips around the saliva ejector tips with simultaneous use of the HVE caused a decrease in pressure in the saliva ejector lines, resulting in a back and forth movement of the fluid in those lines.

The investigators concluded, "Back flow is a direct result of the decrease in pressure found during simultaneous use of the saliva ejector and the activation of the high-volume evacuator." They also reported that "pressure equalization devices may be needed to decrease the risk of cross-contamination through this back flow phenomenon."

Based upon these studies and personal communication with dental manufacturers (who indicate that indeed this type of back flow can occur), it appears that a "newly recognized" mode of potential cross-contamination has been discovered.

There certainly hasn`t been any documented case of disease spread or oral infections caused by this mode. However, microbes present in the low-volume suction line could be mixed with fluid.

And that line occasionally may contact a patient`s mouth if decreased suction-line pressure occurs.

Avoid contaminating suction lines

1) Don`t use low-volume evacuation but rely on the HVE, for which no reports of back flow have appeared.

2) Don`t have patients close their lips around the saliva ejector tip.

3) Don`t use the saliva ejector simultaneously with the HVE. This may be difficult to control since it is not always known when the HVE is being used at other operatories in the office.

4) Disinfect and thoroughly rinse the evacuation line between patients.

Chris Miller is director of Infection Control Research and Services and professor of oral biology at Indiana University.