ARTICLE 1

BACKFLOIA IN LOVV-VOLUME SUCTION LINES:THE IMPACT OF PRESSURE CHANGESGINGER L. BYRD MANN, R.D.H., B.S., M.S.; TAMARA L. CAMPBELL,D.M.D.; JAMES J. CRAWFORD, M.A., PH.D.

A previous study indicated that

fluid can flow backward in low-

volume suction lines when pa-

tients close their lips around the

saliva ejector tip. The authors in-

vestigated this phenomenon fur-

ther. They documented physical

and mechanical parameters of

saliva ejector backflow in low-

volume suction lines and con-

cluded that backflow is possible

but can be avoided with simple

precautions.

Oackflow from saliva ejector tubing into dental patients' mouthsmay serve as a source of cross-contamination. It could expose theoral mucosa or non-intact tissues of a patient to previously suc-tioned fluids such as saliva or blood components from another per-son.Watson and Whitehouse reported that backflow in low-volume

suction lines can occur when patients close their lips around a sali-va ejector tip to form a seal.' Findings in their study also confirmedthe presence of oral bacteria in suction lines. To date, however,there have been no published reports of any adverse health effectsassociated with backflow from the saliva ejector.

Possible cross-contamination by means of saliva ejector tips withlow-volume suction is a matter of concern in the dental practice, par-ticularly since increased numbers ofimmunocompromised patientsmay be receiving dental care.2 Because of the continuing concernabout infection control in dentistry, this potential cross-contaminationsource must be investigated and prevention strategies suggested.We undertook a study to further investigate and document the

physical and mechanical parameters of saliva ejector backflow inlow-volume suction lines.

MATERIALS AND METHODS

We used computer technology to document the occurrences of pres-sure changes in the suction lines of saliva ejectors and to relatethem to visible movements of air and fluid pockets trapped in low-volume suction lines.We tested 20 dental units for changes in pressure during use of

the low-volume evacuator (saliva ejector) with intermittent activa-tion of the high-volume evacuator. These tests were performed ondental units in a university dental clinic and a hospital dentalclinic. We tested several types of dental equipment. Although mostof the dental units tested were made by one company (A-dec Inc.),the backflow problem under investigation appeared to be indepen-dent of the brand of the units tested.

The dental units we tested were operated with more than oneunit attached to central vacuum pump systems set at 15 pounds per

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square inch of negative pres-sure. We performed the tests inlarger clinics served by onelarge evacuation system, as wellas in smaller clinics served bysmaller evacuation systems to

represent the private practicesetting.

Pressure recording equip-ment documented changes,which we compared with vis-ual observations. The pressure

Figure. Line-pressure readings during the two phases of the experi-ment. Experiment A (top): saliva ejector in use with high-volume evac-uator activated. Experiment B (bottom): Unit 1, high-volume evacuatoractivated; Unit 2, saliva ejector In use.

sensor consisted of a pressuretransducer fitted with a needleinserted into transparent suc-tion tubing. The transducer wasattached by a cable to the trans-ducer interfacing unit and con-nected to a portable computer.The transducer interface gath-ered the analog input signalfrom the pressure transducerand scaled and buffered the sig-nal, converted the analog signalto a digital signal, and sent thedigital signal to the computer ina serial RS-232 format. Thecomputer then converted thedigital signal into usable graph-ics; this allowed the pressurechanges to be documented andcompared with visual observa-tions. Clear polyethylene evacu-ator tubing made the fluidmovements easily visible.

In initial experiments, we at-tached a transparent suctionline to a suction tip that was in-serted through a stopper fittedinto a 25-milliliter bottle to sim-ulate a patient's closed mouth.Then we evacuated 25 mL ofwater from the bottle by admit-ting a small amount of air, andwe again sealed the bottlearound the tip.We evaluated the backflow

into the bottle by injecting 1 mLof a dye solution into the tubing3 centimeters beyond the tipwith a 27-gauge needle. Dyebackflow was evaluated 30times for each unit with a 2-footlength of tubing nearest the tipelevated to 3 cm, then to 50 cmabove the simulated mouth(bottle) and finally draped di-rectly down from the bottle. Weobserved that the dye flowedback into the bottle under onlytwo conditions: when suctiontubing was elevated to 3 cm or50 cm above the bottle, andwhen we observed backward os-cillations (22 to 60 millimeters)

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of water in the line as a resultof the operation of other high-velocity evacuation hoses in anadjacent operatory.As a result of this observa-

tion, when we began our experi-ments with human subjects,we asked volunteer subjects toevaluate the correspondence ofoscillations with use of evacua-tion equipment in adjacent op-eratories. Each subject filled hisor her mouth with plain tapwater for aspiration by the sa-liva ejector. The researcherplaced the saliva ejector in thesubject's mouth and asked thesubject to close his or her lipsaround the ejector tip to form aseal. To avoid any risks of cross-contamination, with each trialwe used clean, new suction tub-ing that was fitted with newejector tips. All subjects weretested with the evacuationtubing positioned downwardtoward the floor, below theirmouths. The subjects' headswere positioned above the levelof attachment of the tubing tothe control unit to help us de-termine if that would protectthem from backflow.We set up this scenario to de-

pict the environment in a pa-tient's mouth during a clinicalprocedure. It also mimicked thesituation in which, we believed,contaminated backflow exitedthe low-volume evacuator linesand entered a patient's mouth.We conducted subsequent ex-

periments using a single dentalunit. Subjects were instructedto use the low-volume evacuatoras described previously. After abaseline pressure reading wasrecorded, the high-volume evacu-ator was activated to induce atransient change in evacuatorpressure. If the pressure read-ing deviated from baseline, wedeactivated the high-volume

evacuator. The subjects were in-structed to continue holding thesuction tip in their mouths untilthe vacuum was restored to thebaseline pressure reading.

For the final experiments, wesimultaneously connected twodental units to the transducerinterface. These units were sideby side or across from eachother and shared a commonevacuation system. Cannulaeand pressure transducers wereplaced in the low-volume evacu-ator lines of both dental units tomeasure the pressure changes(Figure). Subjects used the low-volume evacuator as in previousexperiments. We recorded base-line pressure readings in bothdental units, then activated thehigh-volume evacuator in theoperatory not being used by thesubject. Low-volume evacuatorpressure was continually re-corded in both dental units dur-ing this experiment to depictchanges in pressure when evac-uation equipment is used simul-taneously in different operato-ries in a typical dental office.

One researcher operated thecomputer while the other ob-served the flow of water in theclear low-volume evacuatorlines. At random intervals, theresearcher marked the clearplastic tubing and measuredfluid to record movement.

RESULTS

In three repetitions of 30 testseach, using a sealed bottle tosimulate the patient's mouth,we observed a backflow of col-ored fluid 83 of 90 times when alength of tubing near the suc-tion tip was positioned at thesame level or above the simu-lated mouth, and when oscilla-tions were produced by operat-ing other suction equipment.Only after clinic hours, when all

operatories in the building werevacated, did tests of individualunits detect surges of fluidsback 20 cm through descendingsuction hose, or the full lengthof the ejector tip into the sealedbottle. Back surge coincidedwith opening the valve of an ad-jacent high-volume suctionhose. We attributed the exces-sive backflow to excessive nega-tive pressure at the test unitwhen 160 other units in thebuilding served by the samelarge central vacuum pump sys-tem were not in use. Such backsurges did not occur when testswere performed during theclinics' normal operating hours.We further evaluated the ex-

tent of oscillations with subjectsin a clinical setting with thesaliva ejector tubing extendingin a downward position wellbelow the subjects' heads. Whensubjects closed their lips aroundthe low-volume evacuator tip,movement of fluid down the linestopped as soon as the mouthwas empty. Then, whenever anadjacent high-volume evacuatorwas activated, a substantialprecipitous negative change ofpressure-a vacuum-occurredin the low-volume evacuatorlines.

Experiment A in the figuredepicts line-pressure changesresulting from the activation ofthe high-volume evacuatorwhile the saliva ejector wasbeing used. The pressure de-creased continually, indicatingvacuum formation, when thehigh-volume evacuator was ac-tivated. Deactivation of thehigh-volume evacuator resultedin a slow but steady return tothe base line pressure. Experi-ment A illustrates the scenarioof a restorative procedure beingperformed using both the low-volume evacuator and high-vol-

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ume evacuator simultaneously.Experiment B in the figure

represents line-pressurechanges encountered when twounits, attached to the same cen-

tral evacuation system, are

used simultaneously for restor-ative and, for instance, dentalhygiene procedures. As can beseen in Experiment B, a largerdecrease in the low-volumeevacuator's line pressure was

evident in Unit 1 when thehigh-volume evacuator was ac-

tivated. Comparatively, Unit 2exhibited a smaller decrease inline pressure when only thelow-volume evacuator was usedand Unit l's high-volume evacu-

ator was activated.The data collected represent

demonstrable pressure changesrelated to oscillatory backflowof fluid in the suction tubingwhile the patient's mouth was

closed. Pressure changes ob-served ranged from 1.8 to 6.5psi. The computer model andgauges used were capable of in-dicating only changes greaterthan 0.3 psi due to backgroundvariation. At intervals, oscilla-tory movement of air and fluidtrapped in tubing were mea-

sured. These varied from ap-proximately 15 to 22 mm in the0.4-mm-diameter tubing, whichrelated to approximately one-

third to one-half the length ofthe 0.3-mm-diameter suctiontip. It also was evident thatfluid in the hose oscillated in con-

cert with changes in pressure.These observations matched

conditions that caused backflowof colored solution into the 25-mL bottle when tubing was ele-vated for the in vitro model. Thesuction tubing extended in a

downward position from the tiptoward the floor, well below thelevel of subjects' mouths. Fluidsoscillated in the tubing near the

floor, also well below the sub-jects' mouths. No fluid backflowinto the mouth of any of thesubjects was evidenced by fluidmovement or detected by thesubjects.

DISCUSSION

This study confirmed the poten-tial for backflow of fluids in theejector suction tubing whensubjects closed their mouths on

the saliva ejector tip. We foundthat backflow was related tohow the tubing was positionedrelative to the patient's head, as

well as to the simultaneous use

We found that back-flow was related to

how the tubing was

positioned relative to

the patients head, as

well as to the simul-taneous use of otherevacuation equip-

ment.

of other evacuation equipment.Watson and Whitehouse' con-

ducted two random telephonesurveys of 117 dental offices inEdmonton, Alberta, Canada, todetermine the use of low-vol-ume evacuators. Their surveydocumented that practitionersin 91 percent of the officesasked their patients to closetheir lips around the low-vol-ume evacuator tip. This indi-cates that at the time of thesurvey, conditions necessary forfluids to flow back from low-vol-ume evacuator lines occurredcommonly. By this point, morethan three years later, most ofthe dental community appearsto know that patients shouldnot close their lips around the

saliva ejector tip.Watson and Whitehousel

speculated that backflow occurs

when there is more negativepressure in the mouth than inthe evacuator tubing, and thatit relates to the way patientsclose their lips on the tip. Ourstudy focused on the potentialfor fluctuating tube pressure tofacilitate fluid backflow. Exper-iments A and B (Figure) indi-cated marked pressure changesin the low-volume evacuatorlines when the high-volumeevacuator was activated. Thistransient vacuum led to oscilla-tory saliva flow that could facili-tate backflow into the mouth.Possibilities for such abrupttransient pressure changesexist when the low-volumeevacuator is used at one unitwhile the high-volume evacua-

tor is used at another unit-forexample, when an operativeprocedure and a dental hygieneprocedure are carried out simul-taneously in two separate oper-

atories. This occurs in systemswith several dental units thatoperate with a central evacua-

tion system, because the vacu-

um capacity must be distribut-ed among the units in use.

Gravity appeared to be an es-

sential contributing factor tosaliva backflow. Gravity pulledfluid back toward the patient'smouth whenever a length of thesuction tubing holding the tipwas positioned just above the pa-tient's mouth or when an excess

of fluid collected in the tubingthat was attached to the unitwell above the patient's head.

Another critical precipitatingfactor was the pressure changesin the system, which caused os-

cillation in the segments of airand water trapped in the suc-

tion tube. Under the unusualcircumstance of being used in a

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vacant clinic served by a largevacuum system, an ejector unitmay be subject to much largerpressure surges when vacuumequipment in an adjacent oper-atory is used. Under normaltreatment conditions, althoughpressure changes varied, theywere not sufficient of them-selves to expel fluid back fromthe suction tubing through thefull length of the suction tipinto the mouth. Instead, thesudden movements of segmentsof fluid trapped in the lines pre-

cipitated and assisted gravity-driven backflow of fluid in thelines under normal clinic condi-tions. Thus, as long as pro-

nounced fluid oscillation oc-

curred, backflow was poten-tiated and the actual degree ofpressure change involved wasnot a factor. The phenomenon isanalogous to filling a syringe,when the needle is pointed up-ward and the syringe is tappedto dislodge an air bubble so thefluid will flow down and the aircan be expelled.

Data obtained by testing sub-jects agreed with in vitro find-ings that the tubing must bekept below the patient's head toavoid backflow. Data from thesubject observers agree withthe in vitro evidence that back-flow did not enter the subjects'mouths when suction tubingwas positioned below the mouth.

CONCLUSIONS

Retraction of oral fluids fromcontaminated lines into pa-

tients' mouths can be a poten-

tial source of cross-contamina-tion. This study demonstratedthat fluid aspirated into low-volume saliva evacuation tub-ing can move backward or oscil-late when patients close theirmouths over saliva ejector tips.We found that oscillations offluid in the suction line were re-

lated to frequent pressure dropsin the system that coincidedwith simultaneous use of otherevacuation equipment attachedto the same system. Only whensuction tubing was elevated atleast slightly above a simulatedmouth did rapid oscillations offluid in the tubing activate airbubbles, which caused them torise and thereby allowed fluidsnear the mouth to flow backinto a simulated closed mouth.This indicated a need for carefulpositioning of low-volume suc-

tion tubing well below the pa-

tient's mouth. Creating addi-tional negative pressure in theclosed mouth by oral move-ments that could draw fluidback into the mouth (such as

sucking motions) should beavoided.Many dentists who work

with a dental assistant haveabandoned the use of salivaejectors in favor of the high-vol-ume suctioning device to re-

move water and saliva from thepatient's mouth during treat-ment. Use of the saliva ejectoris still common among dentalhygienists, because most prac-tice without an assistant. Peri-odic use of the high-volumeevacuator instead of the saliva

ejector is one way to avoid theproblem presented in this arti-cle. Another possible solutionthat merits investigation isusing a single-use disposable or

sterilizable suction line insertedinto the existing suction sheathto hold the suction tip.

Dental unit manufacturersare taking a close look at pos-sible solutions to this dilemma.Because more extensive re-

search on ejector units andbackflow is lacking, the preven-tion of this phenomenon is anarea of concern that should bestudied further. The possible so-

lutions we have suggested pro-vide an opportunity for furtherresearch. .

Ms. Mann is clinical assistant professor,The University of North Carolina at ChapelHill, School of Dentistry, Department ofDental Ecology-Dental Hygiene, CB #7450,Brauer Hall, Chapel Hill, N.C. 27599-7450.Address reprint requests to Ms. Mann.

Dr. Campbell is a clinical assistant profes-sor and the director, General Practice Resi-dency Program, The University of NorthCarolina at Chapel Hill, School of Dentistry,Department of Dental Ecology.

Dr. Crawford is coordinator for infectioncontrol and director, Clinical Oral Micro-biology Laboratory, The University ofNorthCarolina at Chapel Hill, School of Dentistry,Department of Endodontics.

This research was supported by the Uni-versity of North Carolina Dental SterilizationMonitoring Service, Chapel Hill, N.C.

The authors thank Oliver Monbureau forhis computer expertise and A-dec Inc. for itscontribution of supplies.

1. Watson CM, Whitehouse RLS. Possibilityof cross-contamination between dental pa-tients by means of the saliva ejector. JADA1993;124(4):77-80.

2. Rubin RH, Young LS, eds. Clinical ap-

proach to infection in the compromised host.2nd ed. New York: Plenum Publishing; 1988.

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