Clinical Radiology (1988) 39, 245-246

Ultrasound Scanning of Post-operative Wounds - the Risks of

Cross-infection

P. SPENCER and R. C. SPENCER

Department of Radiology, Royal Hallamshire Hospital, Sheffield SlO 2JF

Ultrasound scanning of surgical wounds is an established procedure for the detection of abscesses. The possible risks of cross-infection resulting from this technique were examined by testing the sterility of the ultrasound probes, the coupling gel and the stand-off medium Kitecko (3 M). The coupling gel was also assessed for any bactericidal properties. Sixty-six per cent of swabs taken from machines in constant use and 33% of the total number of swabs taken were contaminated with skin flora including Staphyloccocus aureus. Sterility was achieved using a 70% alcohol wipe. The coupling gel was inherently sterile but had no bactericidal action. The solid stand-off medium Kitecko grew Enterobacteriaceae and Pseudomonas species. The implications of these find- ings in relation to scanning post-operative wounds are discussed.

Ultrasound scanning of post-operative wounds is a sen- sitive and accurate technique to detect and locate abscesses in the abdominal wall (Yeh and Rabinowitz, 1982). This method can be more reliable than clinical examination and will differentiate between abscesses and cellulitis (Wolson, 1977). When scanning over a post-operative wound the ultrasound probe should ideally be sterile to prevent infecting a clean wound. Conversely, scanning an infected wound should not put the next patient at risk as a result of probe contamina- tion. The coupling gel and solid stand-off medium Kitecko (3 M) offer other potential sources of infection during any scanning procedure. Thus our investigation was addressed to answer the following questions: (a) Are the ultrasound probes sterile? Is there a risk of cross- infection? (b) If not, how quickly and simply can they be made sterile? (c) Is the coupling gel sterile? (d) Will the coupling gel support bacterial growth once contaminated? (e) Is the solid gel stand-off medium, Kitecko, sterile?

PATIENTS AND METHOD

All the ultrasound probes in use in Sheffield were investigated. Initially , swabs were taken at random between patients during the routine scanning sessions. This involved soaking a dry swab in sterile saline, wiping the swab across the face of the ultrasound probe and immediately placing it in charcoal transport medium. All swabs were sent for routine bacteriological examina- tion and organisms were identified according to stan- dard laboratory procedure. Swabs were taken from each probe on three separate occasions.

Correspondence: Dr P. A. S. Spencer, Senior Registrar, Depart- ment of Radiology, Royal Hallamshire Hospital, Glossop Road, Sheffield S10 2JF.

A second series of swabs were then taken at random during routine ultrasound scanning sessions, but on this occasion each probe was first wiped with a paper towel followed by a swab pre-saturated with 70% isopropyl alcohol BP and 0.5% chlorhexidine diacetate BPC (Sterets, Schering). The probe was allowed to dry and a swab taken as before.

The coupling gel was examined by squeezing a small amount from each hand bottle into a sterile container, on three separate occasions, and the fluid cultured. Samples of 10 ml of gel were taken from the large container and inoculated with clinical strains of Staphylococcus aureus, Group A haemolytic streptococ- cus, Escherichia coli and Pseudomonas aeruginosa at a concentration of about 105 colony-forming units/ml. Each day a sample of the gel was examined for viability of the organisms.

The stand-off medium Kitecko was, on three occa- sions, removed from its container and swabbed for bac- teriological investigations as described above. After this, each time the Kitecko was used it was washed under the tap and stored in an air-tight container in a solution of chlorhexidine at a dilution of 1 in 5000. This solution was changed every 14 days. After several months of using this method of storage, the Kitecko was removed from its container and swabbed on three separate occa- sions, each time towards the end of a fortnight's use of chlorhexidine solution.

RESULTS

Of the 33 swabs taken from the probes before the alochol wipe, 11 (33%) were contaminated with skin flora including Staphylococcus aureus. Five of these swabs gave a confluent growth of bacteria and from six swabs the growth was scanty. A breakdown of the figures (Table 1) shows that of the probes in constant use, 10 out of 15 (66%) swabs grew bacteria. Of those probes used intermittently only one of 18 swabs was contaminated. All swabs taken from the probes after the alcohol wipe were sterile.

All samples of coupling gel from the small bottles were sterile.

Once inoculated with bacteria the coupling gel sus- tained the viability of all test organisms for 72 h (Table 2).

From all three of the swabs taken from the Kitecko stored in tap water there were heavy growths of Pseudo- monas species, Enterobacteriaceae and aerobic spore bearers. The swabs taken from the Kitecko once it was kept in 0.02% chlorhexidine solution were sterile.

DISCUSSION

The species of bacteria isolated from the ultrasound probes were those found on normal skin, and were not

246 CLINICAL RADIOLOGY

Table 1 - Swabs taken and the culture result from the ultrasound probes

Probe Random swabs After alcohol wipe Frequency

No. taken Total positive No. taken Total positive of use

Machine 1 3.5 MHz sector 3.5 MHz linear 5.0 MHz sector

Machine 2 3.5 MHz sector 3.5 MHz 'port ' 3.5 MHz linear

Machine 3 3.5 MHz sector

Machine 4 3.5 MHz sector 5.0 MHz sector 7.0 MHz sector

Total

3 2 3 0 Constant 3 3 3 0 Constant 3 0 3 0 Little

3 2 3 0 Constant 3 1 3 0 Constant 3 2 3 0 Constant

6 1 3 0 Intermittent

3 0 3 0 Intermittent 3 0 3 0 Intermittent 3 0 3 0 Little

33 11 30 0

Table 2 - Daily concentration of organisms (organisms/ml) in the ultrasound coupling gel, following inoculation with a concentration of l0 s organisms/ml

Organism Day 1 Day 2 Day 3 Day 4 Day 5 (inoculation)

Escherichia coli 105 105 103 102 0 Staphylococcus aureus 105 105 103 102 0 Group A streptococcus l0 s 105 104 103 0 Pseudomonas aeruginosa 105 l0 s l0 s 0 0

considered to be a serious infective risk in the majority of patients. This is not true when a post-operative wound is scanned. The heavy infestation of bacteria, including Staphylococcus aureus, on half of the contami- nated probes may cause a significant wound infection in an otherwise fit patient. It has been shown that healing of skin defects is delayed in mice whose wounds were inoculated with bacteria (Segree et al., 1970) and that infection reduces the bursting strength of wounds (Bucknall, 1980). Wounds infected with Staphylococcus aureus have delayed and impaired healing; the decreased tensile strength persists long after gross evi- dence of infection has disappeared (Smith and Enquist, 1967). Indeed of all the factors affecting wound healing, infection is the most significant (Bucknall, 1983).

With the smaller number of organisms simple con- tamination may occur, the invading organisms being disposed of by local humeral and cellular defences. In the presence of necrotic tissue, however, in patients with chronic disease or patients who are immuno-suppressed either by disease or drugs, contamination can become infection (Bucknall and Ellis, 1984). Scanning over a wound must therefore be performed with a sterile probe. Wipes with 70% alcohol have been used suc- cessfully to sterilise intra-arterial pressure transducer heads (Talbot and Skros, 1985) and this present study shows that the 70% alcohol wipe following a dry wipe sterilises the ultrasound probes. This procedure is quick, inexpensive and convenient, and we recommend that this should be done before any attempt is made to scan a wound.

Although the coupling gel is inherently sterile, it would sustain the viability of any bacteria transmitted on to the ultrasound probe whilst scanning over a wound that is already infected with pathogenic bacteri~i,

increasing any chance of cross-infection. It is, therefore, most important that the probe is wiped clean after scanning.

The stand-off medium Kitecko should be an ideal aid to scanning wounds, as an abscess will be near the skin surface and probably painful to pressure.. Washing and storing the gel in water precludes its use in this situation. Since we have kept the Kitecko in the dilute chlorhex- idine there has been no deterioration of the product and as sterility has been achieved we suggest that keeping the gel in such a solution routinely is a simple and practical way of removing the infective risk from patients.

Acknowledgements. We wish to thank Mrs B. W. Skevington and Mrs Y. Steel for their help in preparing the manuscript.

REFERENCES

Bucknall, TE (1980). The effect of local infection upon wound healing - an experimental study. British Journal of Surgery, 67, 851-855.

Bucknall, TE (1983). Factors influencing wound complication. Annals of the Royal College of Surgeons, 65, 71-77.

Bucknall, TE & Ellis, H (1984). Wound Healing for Surgeons. pp. 45-54, Bailliere Tindall, London.

Segree, WA, James, O, Morris, DE & Haase, DA (1970). Bac- teriological studies of infected wounds. West Indian Medical Jour- nal, 19, 65-70.

Smith, M & Enquist, IF (1967). A quantitative study of impaired healing resulting from infection. Surgery, Gynaecology and Obstetrics, 1125, 965-973.

Talbot, GH & Skros, M (1985). 70% Alcohol disinfection of trans- ducer heads: experimental trials. Infection Control, 6, 237-239.

Wolson, AH (1977). Ultrasound diagnosis of pelvic and wound abscess after appendectomy. Surgery, Gynaecology and Obstetrics, 144, 376-380.

Yeh, HC & Rabinowitz, JG (1982). Ultrasonography and computed tomography of inflammatory abdominal wall lesions. Radiology, 144, 859--863.