Population Pharmacokinetics of Piperacillin in the Early Phase of Septic Shock - Does Standard Dosing Result in Therapeutic Plasma Concentrations?

Early appropriate antimicrobial therapy is of utmost importance for reducing mortality in critically ill patients with sepsis and septic shock. Patophysiological changes associated with the septic process, such as changes in volume of distribution (Vd), drug clearance (CL), decrease in plasma-protein concentration and organ dysfunction, lead to pharmacokinetic (PK) changes that may alter the efficacy of the antimicrobial given. As a consequence, antibiotic plasma concentrations are variable and hard to predict in these patients, which makes optimal antibiotic exposure a challenge, especially in the early phase of treatment. In sepctic shock patients, appropriate dosing is even more important, as effective antimicrobial therapy within the first hour of documented hypotension is associated with increased survival to hospital discharge.

Piperacillin/tazobactam is a β-lactam - β-lactamase inhibitor combination frequently used for empirical treatment in the critically ill. It is a time-dependent antibiotic where antibacterial activity is related to the time for which the free, unbound concentation of the drug is maintained above the minimal inhibitory concentration (f T>MIC). Maximizing f T>MIC both increases the therapeutic impact and reduces the risk of drug resistance development. Because of the PK changes seen in the critically ill, standard dosing of antimicrobials may result in subtherapeutic plasma-concentrations (17) and it has been suggested that current empiric dosing recommendations for ICU patients are inadequate and needs to be reconsidered (18). Patients with septic shock are especially vulnerable (7) and optimal dosing in these patients is crucial for reducing mortality.

Piperacillin/tazobactam 4g/0.5g every 8 hour (h) is the empiric standard dosing for sepsis and septic shock. The aim of this study was to determine if this dosing results in therapeutic plasma concentrations in septic shock patients, within the initial 24 hours of therapy. A PK population model was established with the dual purpose to assess current standard treatment and to simulate alternative dosing regimens and modes of administration.

Critically ill patients with known or suspected septic shock who required noradrenaline infusion and who were prescribed piperaillin/tazobactam 4g/0.5g (Tazocin®) by the treating physician were eligible for the study. Patients on renal replacement therapy and patients under the age of 18 were not included.

Piperacillin/tazobactam 4g/0.5g was administered intravenously (i.v.) over 3 minutes every 8 h. Blood samples (4 mL) were collected by trained staff from an arterial catheter around the time of administration of the third consecutive infusion. Each patient had a total of eight blood samples drawn; before administration of the drug (time 0), at 10, 20, 30 minutes and 1, 2, 4 and 8 h after administration of the drug.