College Answer

Usual source of infection is the insertion site, hub, infusate or haematogenous spread from distant site. Major mechanisms causing CVC infection are therefore:

(1) Contamination during insertion. Prevented by:
•   Sterile precautions during insertion (gloves,.gown, mask, appropriate cutaneous antiseptic)

(2) Contamination of insertion site after insertion:
•  Use of subclavian site for insertion (rather than jugular or femoral)
• Adequately fix catheter to prevent movement
•  Use appropriate combination of dressings and observation of site (change dressing if soiled)
•  Avoid prolonged connection of solutions prone to contamination (lipid, propofol)
•  Remove catheter as soon as need for it diminishes

(3) Subsequent contamination due to breaking of sterile connections (multi-flows, 3-way taps):
•  Limit number of lumens, decrease breaks in system, clean injection ports before accessing system (eg. Alcohol), use alternate route for blood transfusion
• Use of anti-microbial impregnated catheters

(4) Subsequent contamination from systemic infection elsewhere:
•  Aggressive treatment of other infections, remove catheters as soon as possible
• Use of anti-microbial impregnated catheters

Discussion

The second part of this question is similar to Question 8 from the first paper of 2006, which asks the candidate to "Outline  strategies  you  would  incorporate  to  prevent  central  venous  catheter  related infection.". I reproduce the answer below, for convenience. A good article from 2002 reviews the risk factors for CVC infection, which resembles the college answer.

Mechanisms of CVC infections

• Contamination during insertion
  • Unclean CVC being inserted with a substandard sterile technique
  • "Re-wiring" of an old line, rather than the insertion of a new line
• Contamination of insertion site after insertion
  • Use of contaminated site, eg. groin
  • Poor care for dressings
  • Use of solutions prone to contamination (eg. propofol, lipid or TPN)
• Contamination of lumen by breaking of sterile line connection
  • Poor line changing and port handling technique (one is supposed to use chlorhexidine-soaked gauze)
• Haematogenous contamination of intravascular portion
  • Antimicrobial-impregnanted catheters are supposed to prevent this from happening
  • One may wish to be proactive about treating bacteraemia from organisms which are known to colonise plastic.

Prevention of CVC infections

This is a point-form synopsis of CVC features and practices associated with a decreased risk of CVC infection:

• Subclavian lines.
• Minimum number of lumens.
• Use of dedicated lumens for lipid infusions.
• Immunosuppressed patients or those with burns should have antibiotic-coated lines.
• For insertion, use aseptic technique and maximal barrier precautions.
• 0.5% chlorhexidine in 70% alcohol is the preferred cleaning agent.
• Handle ends of administration sets with gauze soaked in chlorhexidine).
• Review the line daily.
• Remove the line as soon as possible.
• Change lines early - ideally, every 7 days.
• Sterile, transparent semipermeable dressings
• Change dressings regularly (every 7 days for standard dressings)