Antimicrobial Selection by Age/Site

1. Description of the problem

Principles of antimicrobial therapy

In patients suspected of being infected, the following elements of management should always be considered but may not all necessarily apply in all cases:

• Supportive care, e.g., treat shock, hypoxia.

• Antimicrobial therapy.

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• Need for surgery – for diagnosis or therapy, e.g., drainage of abscess.

• Community – is this contagious? What interventions are required to prevent spread within the community (ward, hospital, family, broader community)?

• Prevention – e.g., immunization, antimicrobial chemoprophylaxis of contacts, care of vascular catheters.

This chapter addresses antimicrobial therapy.

The first set of decisions to be made:

• Is an infection likely?

• Where is it located? This usually provides the answer to the third and most important question:

• What are the most likely causative organisms?

• What are the likely antimicrobial susceptibilities of these organisms?

Initial management is empiric; to answer questions 3 and 4, one should use the following: (1) information from published studies of this infection; and (2) information from your specific unit.

Microbiologic information should be obtained from the patient to provide directed therapy. The most important principle in obtaining microbiological information is that specimens should be obtained from the site or presumed site of infection whenever possible. Blood cultures give information only about organisms in the blood.

The second decision to be made:

Given the answers to the first set of questions, what is the optimal agent to use?

This must take into account:

• The necessary antimicrobial spectrum of the agent(s).

• Pharmacokinetics.

• Will it reach the site of the infection?

• Adverse effects.

• Drug-drug interactions.

• Route of administration.

• Cost (use the cheapest drug, other factors being equal).

The overriding principle is to use the narrowest spectrum agent possible for the clinical circumstance.

The susceptibilities of different bacteria to different antimicrobial agents are fairly predictable. However these susceptibilities vary over time and place. Therefore, it is important to know the susceptibilities of specific organisms in your hospital and unit.

Table I indicates commonly encountered/important microbial agents in the intensive care unit, and their usual susceptibilities.

Table II lists the organisms causing different types of infections at different sites in different types of hosts and suggested empiric therapy (i.e. the initial therapy based on the clinical and epidemiological information, prior to the availability of microbiological information). Once or if a specific agent is identified, therapy should be tailored accordingly.

Table III lists the dosages, dosing intervals and routes of administration of antimicrobial agents by categories for children after the newborn period and for adults.

Table IV lists the major adverse effects of antimicrobial agents.