Use of antibiotics in surgery

Question 1

What is the prophylactic administration of antimicrobials?

Answer 1

Involves the administration of an antimicrobial agent prior to contamination of the surgical site

Question 2

What are antimicrobials?

Answer 2

General term that refers to a group of drugs that includes antibiotics, antifungals, antiprotozoals, and antivirals

Question 3

What is an antibiotic?

Answer 3

Drug that is used to treat bacterial infection

Question 4

What is the minimal inhibitory concentration (MIC)?

Answer 4

The lowest concentration that inhibits visible bacterial growth and represents the antimicrobial concentration necessary to have an inhibitory effect on bacteria in plasma or tissue

Question 5

What is the minimal bactericidal concentration?

Answer 5

The lowest concentration that kills 99.9% of the bacteria in the animal’s plasma or tissues. A drug is classified as being bactericidal if the ratio of minimal bactericidal concentration to MIC is less than 4-6, if its more than 6, it may be not possible to administer the drug at a dose that is bactericidal and nontoxic

Question 6

What are concentration dependent antimicrobials?

Answer 6

Successful therapy depends on the peak concentration above the MIC

Question 7

What are time dependant antimicrobials?

Answer 7

Time above the MIC not the peak concentration that is important for successful use. An example is beta-lactam antimicrobials

Question 8

What is the definition of contamination?

Answer 8

The introduction of microorganisms to living tissues, equipment, or materials

Question 9

What is the definition of infection?

Answer 9

The growth of microorganisms in tissue

Question 10

What is a surgical site infection and what are the three different classification of them?

Answer 10

Infection that is associated with a particular operative procedure and the facility in which it was performed.

SSIs are classified according to superficial, deep incisional or organ/space infections (centers for disease control)

Question 11

What are some host and peri-operative factors that can increase the likelyhood of infection?

Answer 11

1. Older age
2. Immunosupression
3. Intra-operative contamination
4. Excessive tissue damage

Question 12

What is the critical level of contamination?

Answer 12

10^5 bacteria per gram - but not all bacteria are equal and local wound factors do play a role

Question 13

What are some examples of good surgical technique that can reduce the likelyhood of surgical wound infection?

Answer 13

copious saline lavage, gentle tissue handling, anatomical tissue apposition, closure of dead space, proper suture selection

Question 14

What are some examples of common sources of surgical site infection?

Answer 14

• skin and hair
• bacteria colonies become re-infested 90 minutes after skin preparation
• exogenous - surgical equipment and operating room
• haematogenous routes (less common)

Question 15

What is a clean infection site and what is an example of one?

Answer 15

Non-traumatic, no inflammation encountered, no break in technique resp. GI. urine. tracts not encountered

Examples: Castration, ovariohysterectomy

Question 16

What is a clean contaminated procedure and what are some examples of some?

Answer 16

• GI, resp., urinary, bile tracts are entered or a minor break in technique
• examples include pyometra, cystotomy, pyometra etc.

Question 17

What is a contaminated procedure and when might one occur?

Answer 17

• Often involves a major break in technique (could be gross spillage from GI tract) or a traumatic wound
• examples may include enterotomy or pyometra

Question 18

What is a dirty procedure and what is an example of one?

Answer 18

• transection of “clean tissues” for the purpose of surgical access to collection of pus
• examples include leakage from perforated viscera, septic peritonitis, abscesses

Question 19

When should prophylactic surgical antibiotic use primarily be given?

Answer 19

“clean contaminated” and selected “contaminated” wounds

Question 20

Therapeutic antibiotic use is not primarily recommended for clean procedures what are some exceptions to this?

Answer 20

A procedure > 90 minutes

Procedures in which an implant was placed or when infection will be catastrophic to the outcome (eg. total hip replacement or pacemaker implantation)

Question 21

When does the period of susceptibility to microbial infection begin and when does it end?

Answer 21

Starts from the moment of the first incision and typically ends within 24 hours of the procedure

Question 22

What is the most likely pathogen to contribute to surgical site infection?

Answer 22

Staphylococci

Question 23

What are the most commonly used prophylactics used in surgery?

Answer 23

Cephalosporins (Cephazolin, Ceftazidime, Cefoxitin)

Inhibit cell wall synthesis and promote destruction of bacterial cell walls - bactericidal

Question 24

What generation of cephalosporins are commonly used in surgery?

Answer 24

Mainly only first generation is used - second and third rarely indicated and only used where absolutely necessary

Question 25

Briefly explain the timing of giving antimicrobial prophylaxis in surgery:

Answer 25

It is important to maintain the inhibitory concentrations at the incision site for the duration of the procedure:

• Give 30-60 minutes prior to surgery to achieve adequate concentration
• Re-administration depends on pharmokinetic behaviour of drug
• Current recommendations - intra-operative doses @ 1-2 x elimination 1/2 life of drug (cephazolin - typically given @ 22 mg/kg every 90-120 minutes)
• no supportive evidence for giving antimicrobials beyond the immediate post-operative (>24 hours)

Question 26

What are some additional precautions surrounding high risk procedures?

Answer 26

Prophylactic antimicrobial therapy is not warranted for most procedures where contamination is expected to be minor

Question 27

What are some examples of systemic signs that could be associated with infection?

Answer 27

Fever, renal failure, leucocytosis with left shift

Question 28

What are the steps to take in the confirming of infection?

Answer 28

1. Culture of micro-organisms and sensitivity assessment for antibiotic selection
2. Consider exit culture in high risk procedure
3. Consider best sample - tissue culture best - deep rather than superficial to avoid contaminants

Question 29

What are some considerations to factor into your response to a surgical site infection?

Answer 29

1. If the type of infection is unknown and immediate therapy is warranted - broad spectrum or combination
2. Consider toxicity and likely level of resistance
3. If not clinical improvement in 3-5 days - consider antimicrobial selection and regime