In my experiences with dosing, antibiotics tend to offer the greatest challenge, especially in septic patients. The reason this is a challenge is because septic patients can third space resulting in extremely large volumes of distribution. As a result, when dosing these patients with antibiotics like the aminoglycosides, it is critical to track Vd throughout the course of treatment. We find that the Vd value changes over time and is an indicator oftentimes, regarding patient status.

Prior to sepsis, patients may have completely normal VD values, ie. 0.25L/kg. This may call for perfectly normal doses initially. However, recheck of a peak level may reveal dramatic increases in Vd, ie. 0.45L/kg or greater. This is usually accompanied by a change in the patient’s presentation with symptoms of sepsis occurring. As a result, it will be necessary to increase the dose to maintain adequate peaks.

However, caution is warranted where the Vd will begin to decrease over time as the patient responds to treatment. If levels are not checked every couple of days during these dramatic changes in the patient’s status, this will result in wide swings in peak levels; ie. subtherapeutic peaks when the patient is septic and toxic peaks after the patient begins to improve.

Hence, it is important to remember, that when dosing patients who are unstable, it is critical to routinely check levels to maintain therapeutic values throughout the treatment course.