10.1 Altered Pharmacokinetics in Critically Ill Patients

Altered Pharmacokinetics in Critically Ill Patients

Quick Answer: Critical illness drastically changes how the body processes drugs. Key alterations include increased volume of distribution (Vd) due to fluid shifts, unpredictable drug absorption, altered protein binding from hypoalbuminemia, and fluctuating organ function affecting metabolism and excretion. Clinicians must continuously adjust dosing regimens based on a patient's dynamic clinical status.

Critically ill patients experience complex physiological changes that profoundly impact the pharmacokinetics (PK) of medications. The classic ADME model (Absorption, Distribution, Metabolism, and Excretion) is often distorted, making standard dosing regimens inadequate or unsafe. Recognizing and anticipating these PK alterations is a fundamental skill for the BCCCP pharmacist.

Absorption Alterations

Drug absorption in the ICU is notoriously unpredictable, particularly for enteral and extravascular routes.

Enteral Absorption

Several factors compromise the enteral absorption of medications:

• Delayed Gastric Emptying and Ileus: Common in trauma, sepsis, and following surgery, reducing the rate and extent of absorption.
• Vasopressor Use: High doses of vasopressors (e.g., norepinephrine, epinephrine) cause splanchnic vasoconstriction, shunting blood away from the gastrointestinal (GI) tract and diminishing drug absorption.
• Enteral Nutrition Interactions: Continuous enteral feeds can bind to certain drugs (e.g., phenytoin, fluoroquinolones, levothyroxine), preventing their absorption. Often, tube feeds must be held before and after drug administration.
• Altered Gastric pH: The use of stress ulcer prophylaxis (proton pump inhibitors, H2-receptor antagonists) increases gastric pH, which can impair the absorption of drugs requiring an acidic environment (e.g., itraconazole, ketoconazole).

Subcutaneous and Intramuscular Absorption

In shock states, peripheral hypoperfusion and edema severely reduce the absorption of subcutaneously and intramuscularly administered medications. Consequently, the intravenous (IV) route is preferred to ensure reliable drug delivery.

Volume of Distribution (Vd) Changes

The volume of distribution represents the theoretical volume into which a drug disperses. In critical illness, Vd is highly variable.

Fluid Resuscitation and Capillary Leak

Conditions like sepsis and severe burns trigger systemic inflammation, leading to endothelial dysfunction and increased capillary permeability (often termed "capillary leak" or "third spacing"). Furthermore, these patients typically receive massive fluid resuscitation.

• Hydrophilic Drugs: Drugs that are water-soluble (e.g., beta-lactams, aminoglycosides, vancomycin) distribute extensively into this expanded extracellular fluid volume. This results in a significantly increased Vd, meaning that a larger initial loading dose is required to achieve therapeutic plasma concentrations.
• Lipophilic Drugs: The Vd of lipid-soluble drugs (e.g., fluoroquinolones, macrolides, most sedatives) is less affected by fluid shifts, as they primarily distribute into adipose and other tissues rather than extracellular fluid.

Altered Protein Binding

Many drugs bind to plasma proteins, primarily albumin (for acidic drugs) and alpha-1-acid glycoprotein (AAG) (for basic drugs). Only the "free" or unbound fraction of a drug is pharmacologically active and available for clearance.

Hypoalbuminemia

Hypoalbuminemia is ubiquitous in the ICU due to decreased synthesis, increased degradation, and capillary leak.

• When albumin levels drop, the free fraction of highly protein-bound drugs (e.g., phenytoin, valproic acid, ceftriaxone) increases.
• While the total drug concentration may appear normal or low, the free (active) concentration might be therapeutic or even toxic. This necessitates monitoring free drug levels when possible or adjusting interpretations of total levels.

Elevated Alpha-1-Acid Glycoprotein (AAG)

AAG is an acute-phase reactant whose levels rise during stress, inflammation, and trauma.

• Elevated AAG can increase the binding of basic drugs (e.g., lidocaine, propranolol, fentanyl), potentially decreasing their free, active fraction and dampening their clinical effect.

Metabolism and Hepatic Clearance

Hepatic metabolism relies on intrinsic liver enzyme activity and hepatic blood flow.

• Decreased Hepatic Blood Flow: Conditions like congestive heart failure, shock, or the use of high-dose vasopressors reduce blood flow to the liver. This impairs the clearance of "high-extraction ratio" drugs (e.g., propofol, lidocaine, fentanyl), whose metabolism is flow-dependent.
• Hepatocellular Dysfunction: Acute liver failure, hypoxic hepatitis (shock liver), or severe systemic inflammation can downregulate cytochrome P450 (CYP) enzymes, reducing the clearance of "low-extraction ratio" drugs (e.g., phenytoin, warfarin), which depend on intrinsic enzyme activity.
• Therapeutic Hypothermia: Targeted temperature management (TTM) can significantly decrease CYP enzyme activity, prolonging the half-life of many medications, notably sedatives and paralytics.

Excretion and Renal Clearance

Renal function in the ICU is highly dynamic, fluctuating between hyperfiltration and failure.

Augmented Renal Clearance (ARC)

ARC is defined as a creatinine clearance (CrCl) > 130 mL/min/1.73 m². It is common in younger patients, trauma victims, burn patients, and those with early sepsis. The hyperdynamic state increases cardiac output and renal blood flow, enhancing glomerular filtration. ARC leads to the rapid elimination of renally cleared drugs (e.g., beta-lactam antibiotics, levetiracetam), often resulting in subtherapeutic concentrations. Higher doses or more frequent administration (or continuous infusions) are required.

Acute Kidney Injury (AKI)

Conversely, AKI is a frequent complication in the ICU. Decreased glomerular filtration rate (GFR) reduces the clearance of renally eliminated drugs, leading to accumulation and toxicity if doses are not reduced or intervals extended. Furthermore, renal replacement therapy (RRT), such as continuous venovenous hemofiltration (CVVH), adds another layer of complexity, requiring specific dosing strategies based on the modality and effluent rate.

Clinical Scenario

A 28-year-old male is admitted to the trauma ICU following a severe motor vehicle collision. He has received 6 liters of crystalloid resuscitation and is on a norepinephrine infusion. He develops ventilator-associated pneumonia and is prescribed intravenous cefepime (a hydrophilic, renally cleared beta-lactam).

PK Considerations:

1. Vd: Massive fluid resuscitation and trauma-induced systemic inflammation increase his extracellular fluid volume. Cefepime's Vd is significantly expanded, necessitating a higher initial loading dose to reach target concentrations.
2. Clearance: Being young and having experienced major trauma, he is at high risk for ARC. If his CrCl is > 130 mL/min, standard cefepime dosing will likely lead to subtherapeutic levels. Aggressive dosing, potentially using an extended or continuous infusion, is required to maintain drug levels above the minimum inhibitory concentration (MIC).
3. Absorption: Due to shock and vasopressor use, any enterally administered medications would have unpredictable absorption; hence, IV cefepime is the correct route.

Understanding these dynamic PK changes is crucial for optimizing drug efficacy and minimizing toxicity in the critically ill.