Pharmacokinetics and pharmacodynamics in the critically ill patient

Drug Dosing in Critically Ill Patients with Renal Failure: A Pharmacokinetic Approach

Accurate pharmacotherapy management in the intensive care unit (ICU) patient is crucial to minimize adverse drug events. Pharmacokinetic principles including absorption, distribution, metabolism, and excretion (ADME) all play an important role in determining the fate of medications used in the critical care setting. Renal failure in this setting further alters pharmacokinetic parameters, resulting in drug dosing changes. This article highlights and applies principles of drug dosing in normal patients and in the pharmacokinetically challenging environment of critically ill patients with renal failure. Specific drug dosing tables serve as a guide for the clinician to renally adjust medication doses in the critically ill patient with renal failure.

Therapeutic drug monitoring of phenytoin in critically ill patients

Therapeutic drug monitoring of phenytoin is necessary to ensure therapeutic and nontoxic levels. Hypoalbuminemia, renal failure, and interactions with other highly protein-bound drugs (e.g., valproic acid) alter protein binding of phenytoin. When these conditions are present, free serum concentrations, which represent the pharmacologically active entity, cannot be predicted from total serum concentrations. Besides general alterations in drug distribution and elimination, protein binding is often altered in critically ill patients. Case reports describe phenytoin toxicity secondary to inappropriate dosage adjustments based solely on total serum concentrations in patients with hypoalbuminemia. Free drug measurements and theoretical equations to facilitate the interpretation of total phenytoin serum levels have been introduced. However, they are not widely implemented in clinical practice because evidence of improvements in patient outcomes is limited. Knowledge of the pharmacokinetic properties of phenytoin is indispensable for correct interpretation of total serum concentrations when protein binding is altered. Free serum concentrations should be measured, or theoretically calculated if measurements are unavailable, to avoid misinterpretation of total serum levels and consequent inappropriate adjustments in the dosage of phenytoin in critically ill patients.

Antifactor Xa activity in critically ill patients receiving antithrombotic prophylaxis with standard dosages of certoparin: a prospective, clinical study

Introduction

Deep venous thrombosis with subsequent pulmonary embolism or post-thrombotic syndrome is a feared complication in the intensive care unit. Therefore, routine prophylactic anticoagulation is widely recommended. Aside from unfractionated heparin, low molecular weight heparins, such as certoparin, have become increasingly used for prophylactic anticoagulation in critically ill patients. In this prospective study, we evaluated the potency of 3,000 IU certoparin administered once daily to reach antithrombotic antifactor Xa (aFXa) levels of 0.1 to 0.3 IU/ml in 62 critically ill patients.

Methods

AFXa levels were determined 4, 12 and 24 h after injection of certoparin. Prothrombin time, activated partial thromboplastin time, antithrombin, fibrinogen, hemoglobin, platelet count, serum urea and creatinine concentrations were documented before and 12 and 24 h after injection of certoparin.

Results

Four hours after certoparin injection (n = 32), 28% of patients were within the antithrombotic aFXa range. After 12 and 24 h, 6% achieved antithrombotic aFXa levels. Because of a severe pulmonary embolism in one study patient, an interim analysis was performed, and the dosage of certoparin was increased to 3,000 IU twice daily. This regime attained recommended antithrombotic aFXa levels in 47%, 27%, 40% and 30% of patients at 4, 12, 16 and 24 h, respectively, after twice daily certoparin injection (n = 30). Antithrombin and fibrinogen concentrations slightly increased during the observation period. Low antithrombin concentrations before certoparin were independently correlated with underdosing of certoparin. Patients with aFXa levels <0.1 IU/ml 4 h after certoparin injection required vasopressors more often and had lower serum concentrations of creatinine and urea than patients with antithrombotic aFXa levels.

Conclusion

Standard dosages of certoparin of 3,000 IU given once or twice daily are ineffective for attaining the recommended aFXa levels of 0.1 to 0.3 IU/ml in critically ill patients. Low antithrombin levels before certoparin administration were independently associated with low aFXa levels. Renal function and vasopressor therapy may further influence the effectiveness of certoparin in ensuring adequate antithrombotic prophylaxis.

Pharmacokinetic and pharmacodynamic considerations when treating patients with sepsis and septic shock

Sepsis and septic shock are accompanied by profound changes in the organism that may alter both the pharmacokinetics and the pharmacodynamics of drugs. This review elaborates on the mechanisms by which sepsis-induced pathophysiological changes may influence pharmacological processes. Drug absorption following intramuscular, subcutaneous, transdermal and oral administration may be reduced due to a decreased perfusion of muscles, skin and splanchnic organs. Compromised tissue perfusion may also affect drug distribution, resulting in a decrease of distribution volume. On the other hand, the increase in capillary permeability and interstitial oedema during sepsis and septic shock may enhance drug distribution. Changes in plasma protein binding, body water, tissue mass and pH may also affect drug distribution. For basic drugs that are bound to the acute phase reactant alpha(1)-acid glycoprotein, the increase in plasma concentration of this protein will result in a decreased distribution volume. The opposite may be observed for drugs that are extensively bound to albumin, as the latter protein decreases during septic conditions. For many drugs, the liver is the main organ for metabolism. The determinants of hepatic clearance of drugs are liver blood flow, drug binding in plasma and the activity of the metabolic enzymes; each of these may be influenced by sepsis and septic shock. For high extraction drugs, clearance is mainly flow-dependent, and sepsis-induced liver hypoperfusion may result in a decreased clearance. For low extraction drugs, clearance is determined by the degree of plasma binding and the activity of the metabolic enzymes. Oxidative metabolism via the cytochrome P450 enzyme system is an important clearance mechanism for many drugs, and has been shown to be markedly affected in septic conditions, resulting in decreased drug clearance. The kidneys are an important excretion pathway for many drugs. Renal failure, which often accompanies sepsis and septic shock, will result in accumulation of both parent drug and its metabolites. Changes in drug effect during septic conditions may theoretically result from changes in pharmacodynamics due to changes in the affinity of the receptor for the drug or alterations in the intrinsic activity at the receptor. The lack of valid pharmacological studies in patients with sepsis and septic shock makes drug administration in these patients a difficult challenge. The patient's underlying pathophysiological condition may guide individual dosage selection, which may be guided by measuring plasma concentration or drug effect.

Trends in veterinary clinical and fundamental pharmacology: past and future in The Netherlands

Veterinary pharmacology has undergone a gradual development in the Netherlands during this century. Starting from a historical perspective the paper aims to provide an overview of future trends and important issues in the area of veterinary pharmacology and toxicology. It is pointed out that this discipline comprises several subdisciplines as the comparative aspect of both, pharmacology and toxicology, is inherent to veterinary medicine which has to address a broad variety of animal species. Thus, the comparison of drug effects, side effects, and drug disposition as well as the comparison of the species-specific susceptibility to xenobiotics are obvious challenges in this discipline. Several areas in clinical pharmacology are highlighted to indicate future research needs. Finally, the principles of Good Veterinary Practice are presented as the 'golden standard' in veterinary clinical pharmacology.

The effect of oxygen on propofol-induced inhibition of microsomal cytochrome P450 3A4

We have shown previously that both hypoxia and propofol may inhibit the metabolism of midazolam. We now wished to see whether there was any additive or synergistic effect when they occurred together. Microsomes were incubated with 20 microns midazolam for 60 min, and propofol 0, 50, 100 or 1000 microM was added. Incubates were further subdivided so that the environment contained 0, 10, 21 or 70% oxygen. The results confirmed our earlier study showing that propofol only had a significant inhibitory effect at a concentration greater than that seen clinically (1000 microM). Anoxia was the only environment in which significant depression of the metabolism of midazolam occurred at all concentrations of propofol. This reduced it to almost zero. Post hoc analysis of the data showed that, with the greatest concentration of propofol (1000 microM), there was increasing inhibition of metabolism of midazolam with increases of oxygen from 10 to 70%.

Serum protein binding of propofol in critically ill patients

BACKGROUND

Disease-induced modifications in the level of serum proteins may change the degree of binding of drugs highly bound to serum proteins.

METHODS

The serum protein binding of propofol, an intravenous anaesthetic agent was studied in vitro in samples of serum from healthy volunteers (n=28) and from critically ill patients (n=17). The free fraction was obtained by the ultrafiltration technique and was measured by high-performance liquid chromatography. Concentrations of serum albumin, alpha1-acid-glycoprotein and free fatty acids were also measured in all samples.

RESULTS

The percentage of free propofol was significantly increased (P<0.001) in critically ill patients (1.31 (1.06-2.25)%) vs control subjects (1.07 (0.49-1.47)%). Albumin levels were significantly decreased (P<0.001) in patients (16.3 (8.8-24.6) g x l(-1) vs 45.8 (31.4-55.5) g x l[-1]), while levels of alpha1-acid-glycoprotein were increased (P<0.001) (1.9 (0.9-2.8) g x l(-1) vs 0.9 (0.5-1.4) g x l[-1]), as were levels of free fatty acids (0.68 (0.50-1.14) mmol x l(-1) vs 0.37 (0.11-1.05) mmol x l(-1); P<0.05 ). No correlation was found between levels of alpha1-acid-glycoprotein or free fatty acids and the bound/free ratio of propofol. However, a linear relationship was found between levels of albumin and the bound/free ratio (r2=0.25; P<0.001).

CONCLUSION

In conclusion, in these critically ill patients, an increase in the percentage of free propofol occurs. The significance of this observation remains uncertain, but may be validated in future studies. However, the observation supports the common idea that potent drugs should be given with great care in critically ill patients.

What changes drug metabolism in critically ill patients--III? Effect of pre-existing disease on the metabolism of midazolam

Liver samples were obtained at hepatectomy from patients with end stage alcoholic liver disease (n = 5), primary biliary cirrhosis (n = 5) and chronic rejection needing retransplantation (n = 5). Normal liver material was also obtained from five organ donors. From these samples microsomes were made containing cytochrome P450 3A. The amount of this enzyme was measured by Western immunoblotting and its function assessed by measuring the rate of production of two metabolites of midazolam, 1-hydroxy midazolam and 4-hydroxy midazolam. There was a wide range in all groups for both the expression and function of this enzyme. Liver tissue affected by cirrhotic disease showed greater preservation of enzyme function than that affected by hepatocellular disease. There was a good correlation between the expression of the enzyme and production of the 1-hydroxy metabolite, but a poor correlation between production of the 4-hydroxy metabolite and expression. This poor correlation may reflect the failure to measure the specific enzyme responsible for producing 4-hydroxy midazolam.