Augmented renal clearance

Adding to the complexity of caring for critically ill patients is the fact that many of them have a creatinine clearance that exceeds 130 mL/min/1.73 m². This phenomenon, termed augmented renal clearance (ARC), has only recently been widely recognized and its pathogenesis remains incompletely understood. However, ARC has been shown to result in increased dose requirements for drugs that are primarily eliminated by renal excretion, including many antimicrobial agents and enoxaparin. Recognition of ARC is hampered by the fact that the standard creatinine-based equations used to estimate renal function are not accurate in this clinical setting and the diagnosis is best established using both serum and urine creatinine measurements to calculate clearance. So a high index of clinical suspicion and awareness is usually required before this step is taken to confirm the diagnosis of ARC.

Optimization and Evaluation of Piperacillin-Tobramycin Combination Dosage Regimens against Pseudomonas aeruginosa for Patients with Altered Pharmacokinetics via the Hollow-Fiber Infection Model and Mechanism-Based Modeling

Augmented renal clearance (ARC) in critically ill patients can result in suboptimal drug exposures and treatment failure. Combination dosage regimens accounting for ARC have never been optimized and evaluated against Pseudomonas aeruginosa by use of the hollow-fiber infection model (HFIM). Using a P. aeruginosa isolate from a critically ill patient and static-concentration time-kill experiments (SCTKs), we studied clinically relevant piperacillin and tobramycin concentrations, alone and in combinations, against two inocula (10^5.8 and 10^7.6 CFU/ml) over 72 h. We subsequently evaluated the effects of optimized piperacillin (4 g every 4 h [q4h], given as 0.5-h infusions) plus tobramycin (5 mg/kg of body weight q24h, 7 mg/kg q24h, or 10 mg/kg q48h, given as 0.5-h infusions) regimens on killing and regrowth in the HFIM, simulating a creatinine clearance of 250 ml/min. Mechanism-based modeling was performed in S-ADAPT. In SCTKs, piperacillin plus tobramycin (except combinations with 8 mg/liter tobramycin and against the low inoculum) achieved synergistic killing (≥2 log₁₀ versus the most active monotherapy at 48 h and 72 h) and prevented regrowth. Piperacillin monotherapy (4 g q4h) in the HFIM provided 2.4-log₁₀ initial killing followed by regrowth at 24 h and resistance emergence. Tobramycin monotherapies displayed rapid initial killing (≥5 log₁₀ at 13 h) followed by extensive regrowth. As predicted by mechanism-based modeling, the piperacillin plus tobramycin dosage regimens were synergistic and provided ≥5-log₁₀ killing with resistance suppression over 8 days in the HFIM. Optimized piperacillin-tobramycin regimens provided significant bacterial killing and suppressed resistance emergence. These regimens appear to be highly promising for effective and early treatment, even in the near-worst-case scenario of ARC.

Vitamin C Pharmacokinetics in Critically Ill Patients: A Randomized Trial of Four IV Regimens

BACKGROUND

Early high-dose IV vitamin C is being investigated as adjuvant therapy in patients who are critically ill, but the optimal dose and infusion method are unclear. The primary aim of this study was to describe the dose-plasma concentration relationship and safety of four different dosing regimens.

METHODS

This was a four-group randomized pharmacokinetic trial. Patients who were critically ill with multiple organ dysfunction were randomized to receive 2 or 10 g/d vitamin C as a twice daily bolus infusion or continuous infusion for 48 h. End points were plasma vitamin C concentrations during 96 h, 12-h urine excretion of vitamin C, and oxalate excretion and base excess. A population pharmacokinetic model was developed using NONMEM.

RESULTS

Twenty patients were included. A two-compartment pharmacokinetic model with creatinine clearance and weight as independent covariates described all four regimens best. With 2 g/d bolus, plasma vitamin C concentrations at 1 h were 29 to 50 mg/L and trough concentrations were 5.6 to 16 mg/L. With 2 g/d continuous, steady-state concentrations were 7 to 37 mg/L at 48 h. With 10 g/d bolus, 1-h concentrations were 186 to 244 mg/L and trough concentrations were 14 to 55 mg/L. With 10 g/d continuous, steady-state concentrations were 40 to 295 mg/L at 48 h. Oxalate excretion and base excess were increased in the 10 g/d dose. Forty-eight hours after discontinuation, plasma concentrations declined to hypovitaminosis levels in 15% of patients.

CONCLUSIONS

The 2 g/d dose was associated with normal plasma concentrations, and the 10 g/d dose was associated with supranormal plasma concentrations, increased oxalate excretion, and metabolic alkalosis. Sustained therapy is needed to prevent hypovitaminosis.

TRIAL REGISTRY

ClinicalTrials.gov; No.: NCT02455180; URL: www.clinicaltrials.gov.

Determining the mechanisms underlying augmented renal drug clearance in the critically ill: use of exogenous marker compounds

Introduction

The aim of this study was to explore changes in glomerular filtration (GFR) and renal tubular function in critically ill patients at risk of augmented renal clearance (ARC), using exogenous marker compounds.

Methods

This prospective, observational pharmacokinetic (PK) study was performed in a university-affiliated, tertiary-level, adult intensive care unit (ICU). Patients aged less than or equal to 60 years, manifesting a systemic inflammatory response, with an expected ICU length of stay more than 24 hours, no evidence of acute renal impairment (plasma creatinine concentration <120 μmol/L) and no history of chronic kidney disease or renal replacement therapy were eligible for inclusion. The following study markers were administered concurrently: sinistrin 2,500 mg (Inutest; Laevosan, Linz, Austria), p-aminohippuric acid (PAH) 440 mg (4% p-aminohippuric acid sodium salt; CFM Oskar Tropitzsch, Marktredwitz, Germany), rac-pindolol 5 or 15 mg (Barbloc; Alphapharm, Millers Point, NSW, Australia) and fluconazole 100 mg (Diflucan; Pfizer Australia Pty Ltd, West Ryde, NSW, Australia). Plasma concentrations were then measured at 5, 10, 15, 30, 60 and 120 minutes and 4, 6, 12 and 24 hours post-administration. Non-compartmental PK analysis was used to quantify GFR, tubular secretion and tubular reabsorption.

Results

Twenty patients were included in the study. Marker administration was well tolerated, with no adverse events reported. Sinistrin clearance as a marker of GFR was significantly elevated (mean, 180 (95% confidence interval (CI), 141 to 219) ml/min) and correlated well with creatinine clearance (r =0.70, P <0.01). Net tubular secretion of PAH, a marker of tubular anion secretion, was also elevated (mean, 428 (95% CI, 306 to 550) ml/min), as was net tubular reabsorption of fluconazole (mean, 135 (95% CI, 100 to 169) ml/min). Net tubular secretion of (S)- and (R)-pinodolol, a marker of tubular cation secretion, was impaired.

Conclusions

In critically ill patients at risk of ARC, significant alterations in glomerular filtration, renal tubular secretion and tubular reabsorption are apparent. This has implications for accurate dosing of renally eliminated drugs.

Augmented renal clearance in the ICU: results of a multicenter observational study of renal function in critically ill patients with normal plasma creatinine concentrations*

OBJECTIVE

To describe the prevalence and natural history of augmented renal clearance in a cohort of recently admitted critically ill patients with normal plasma creatinine concentrations.

DESIGN

Multicenter, prospective, observational study.

SETTING

Four, tertiary-level, university-affiliated, ICUs in Australia, Singapore, Hong Kong, and Portugal.

PATIENTS

Study participants had to have an expected ICU length of stay more than 24 hours, no evidence of absolute renal impairment (admission plasma creatinine < 120 µmol/L), and no history of prior renal replacement therapy or chronic kidney disease. Convenience sampling was used at each participating site.

INTERVENTIONS

Eight-hour urinary creatinine clearances were collected daily, as the primary method of measuring renal function. Augmented renal clearance was defined by a creatinine clearance more than or equal to 130 mL/min/1.73 m. Additional demographic, physiological, therapeutic, and outcome data were recorded prospectively.

MEASUREMENTS AND MAIN RESULTS

Nine hundred thirty-two patients were admitted to the participating ICUs over the study period, and 281 of which were recruited into the study, contributing 1,660 individual creatinine clearance measures. The mean age (95% CI) was 54.4 years (52.5-56.4 yr), Acute Physiology and Chronic Health Evaluation II score was 16 (15.2-16.7), and ICU mortality was 8.5%. Overall, 65.1% manifested augmented renal clearance on at least one occasion during the first seven study days; the majority (74%) of whom did so on more than or equal to 50% of their creatinine clearance measures. Using a mixed-effects model, the presence of augmented renal clearance on study day 1 strongly predicted (p = 0.019) sustained elevation of creatinine clearance in these patients over the first week in ICU.

CONCLUSIONS

Augmented renal clearance appears to be a common finding in this patient group, with sustained elevation of creatinine clearance throughout the first week in ICU. Future studies should focus on the implications for accurate dosing of renally eliminated pharmaceuticals in patients with augmented renal clearance, in addition to the potential impact on individual clinical outcomes.

A comparison of CKD-EPI estimated glomerular filtration rate and measured creatinine clearance in recently admitted critically ill patients with normal plasma creatinine concentrations

Background

The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) estimated glomerular filtration rate (eGFR) has been widely integrated into clinical practice. Although useful in screening for CKD, its’ application in critically ill patients with normal plasma creatinine concentrations remains uncertain. The aim of this study was to assess the performance of CKD-EPI eGFR in comparison to creatinine clearance (CL_CR) in this setting.

Methods

This prospective observational study was performed in a tertiary level, university affiliated intensive care unit (ICU). Study participants had to have an expected ICU length of stay > 24 hours, a plasma creatinine concentration < 121 μmol/L, and no history of prior renal replacement therapy or CKD. CKD-EPI eGFR was compared against 8-hour measured urinary CL_CR. Data capture occurred within 48 hours of admission.

Results

One hundred and ten patients (n = 110) were enrolled in the study. 63.6% were male, the mean age was 50.9 (16.9) years, 57.3% received invasive mechanical ventilation, and 30% required vasopressor support. The mean CL_CR was 125 (45.1) ml/min/1.73 m², compared to a CKD-EPI eGFR of 101 (23.7) ml/min/1.73 m² (P < 0.001). Moderate correlation was evident (r = 0.72), although there was significant bias and imprecision (24.4 +/− 32.5 ml/min/1.73 m²). In those patients with a CKD-EPI eGFR between 60–119 ml/min/1.73 m² (n = 77), 41.6% displayed augmented renal clearance (CL_CR ≥ 130 ml/min/1.73 m²), while 7.8% had a CL_CR < 60 ml/min/1.73 m².

Conclusions

These data suggest CKD-EPI eGFR and measured CL_CR produce significantly disparate results when estimating renal function in this population. Clinicians should consider carefully which value they employ in clinical practice, particularly drug dose modification.

Augmented renal clearance in septic and traumatized patients with normal plasma creatinine concentrations: identifying at-risk patients

Introduction

Improved methods to optimize drug dosing in the critically ill are urgently needed. Traditional prescribing culture involves recognition of factors that mandate dose reduction (such as renal impairment), although optimizing drug exposure, through more frequent or augmented dosing, represents an evolving strategy. Elevated creatinine clearance (CL_CR) has been associated with sub-therapeutic antibacterial concentrations in the critically ill, a concept termed augmented renal clearance (ARC). We aimed to determine the prevalence of ARC in a cohort of septic and traumatized critically ill patients, while also examining demographic, physiological and illness severity characteristics that may help identify this phenomenon.

Methods

This prospective observational study was performed in a 30-bed tertiary level, university affiliated, adult intensive care unit. Consecutive traumatized and septic critically ill patients, receiving antibacterial therapy, with a plasma creatinine concentration ≤110 μmol/L, were eligible for enrolment. Pulse contour analysis (Vigileo / Flo Trac^® system, Edwards Lifesciences, Irvine, CA, USA), was used to provide continuous cardiac index (CI) assessment over a single six-hour dosing interval. Urinary CL_CR measures were obtained concurrently.

Results

Seventy-one patients contributed data (sepsis n = 43, multi-trauma n = 28). Overall, 57.7% of the cohort manifested ARC, although there was a greater prevalence in trauma (85.7% versus 39.5%, P <0.001). In all patients, a weak correlation was noted between CI and CL_CR (r = 0.346, P = 0.003). This was mostly driven by septic patients (r = 0.508, P = 0.001), as no correlation (r = -0.012, P = 0.951) was identified in trauma. Those manifesting ARC were younger (P <0.001), male (P = 0.012), with lower acute physiology and chronic health evaluation (APACHE) II (P= 0.008) and modified sequential organ failure assessment (SOFA) scores (P = 0.013), and higher cardiac indices (P = 0.013). In multivariate analysis, age ≤50 years, trauma, and a modified SOFA score ≤4, were identified as significant risk factors. These had greater utility in predicting ARC, compared with CI assessment alone.

Conclusions

Diagnosis, illness severity and age, are likely to significantly influence renal drug elimination in the critically ill, and must be regularly considered in future study design and daily prescribing practice.

See related commentary by De Waele and Carlier, http://ccforum.com/content/17/2/130

Antibacterial therapeutic drug monitoring in cerebrospinal fluid: difficulty in achieving adequate drug concentrations

This report illustrates the difficulty in managing CNS infection in neurosurgical patients, the altered drug pharmacokinetics associated with critical illness, and the role that therapeutic drug monitoring (TDM) of CSF can play in assisting clinical decision making. The authors present a case of external ventricular drain-related ventriculitis in a critically ill patient who initially presented with a subarachnoid hemorrhage. They discuss the physiological changes found in such patients, in particular augmented renal clearance (demonstrated in this patient by a measured creatinine clearance of 375 ml/min/1.73 m(2)), noting the effect this had on drug pharmacokinetics and leading to dosing requirements 2-3 times those recommended in standard regimens. The authors consider the bacterial "kill" characteristics of 2 different antibacterial agents (meropenem and vancomycin) and describe the unique approach of using plasma and CSF TDM to achieve optimal drug exposure at the site of infection while limiting toxic side effects. The authors demonstrate that simply using plasma TDM as a surrogate marker for drug concentration in the CNS may lead to underdosing, exemplified in this patient by CSF vancomycin concentrations as little as 13% of that in plasma. Finally, by measuring CSF and plasma ratios, the authors illustrate the disparity in pharmacokinetic properties between drugs, reminding the clinician of the importance of CNS penetration when selecting antibacterial agents in such cases. This work raises an important hypothesis in the accurate prescription of antibacterial agents in neurosurgical critical care, namely underdosing in the context of augmented elimination and impaired target site penetration. However, prior to any recommendations regarding empirical dose modification, more data are clearly needed, particularly with respect to the safety and efficacy of such an approach. In this respect, the authors would advocate further research using TDM in the management of CNS infection in this setting, in addition to work defining plasma and CSF concentrations associated with antibacterial efficacy and toxicity.

Do we understand the impact of altered physiology, consequent interventions and resultant clinical scenarios in the intensive care unit? The antibiotic story

Early, appropriate antibacterial therapy is a key factor in effectively managing septic critically ill patients1. The prescriber must not only employ an agent of appropriate spectrum, but also in an adequate dose to achieve bacterial eradication at the site of infection. However, the relationship between drug administration and therapeutic success is complex in the critically ill, such that a patient's physiology heavily influences the way drugs distribute into tissue and are eliminated. This represents a significant challenge to the emergency or intensive care physician, and in this manner, personalising therapy, through a greater understanding of how a drug will behave in an individual patient, is likely to lead to improved outcomes.

Augmented creatinine clearance in traumatic brain injury

BACKGROUND

Hypertonic saline and/or norepinephrine infusion are routinely used to achieve a desired cerebral perfusion pressure (CPP) in the management of traumatic brain injury (TBI). We hypothesized that creatinine clearances (CrCls) would be significantly augmented in this setting.

METHODS

This was an observational cohort study in TBI patients older than 16 years with normal serum creatinine concentrations, requiring maintenance of CPP. Eight-hour urinary CrCl collections were performed while on and off active management. Demographic data, use of vasoactive medications, fluid balance, feeding regimen, and hemodynamic variables were recorded throughout the study period. Augmented CrCl was defined as >150 mL/min/1.73 m(2) in women and >160 mL/min/1.73 m(2) in men.

RESULTS

Twenty patients were enrolled, and augmented clearances were demonstrated in 17 (85%). The mean maximum CrCl was 179 mL/min/1.73 m(2) while receiving CPP therapy (95% confidence interval [CI], 159-198), returning to a mean of 111 mL/min/1.73 m(2) (95% CI, 91-131; P < 0.001) when measured after discharge from the intensive care unit. The mean CrCl in the intensive care unit while not receiving CPP therapy was 150 mL/min/1.73 m(2) (95% CI, 134-167; P = 0.03). The mean time to reach peak CrCl while receiving active treatment was 4.7 days (95% CI, 3.0-6.4). In a multivariate analysis, norepinephrine use, saline loading, mean arterial blood pressure, and central venous pressure were associated with augmented CrCl on the day of measurement.

CONCLUSIONS

Augmented CrCls are common in TBI patients receiving active management of CPP and persist even after discontinuation of such therapy. Further work is needed to clarify the impact of such clearances on renally excreted drugs in this setting.