Antiseizure Drug Pharmacology in the ICU

ABPN Neurocritical Care Study Guide Exam

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1 Chapter 1: Neurologic Examination & Coma Assessment in the ICU

Current Examination, Blueprint & Preparation Neurologic Examination in Critically Ill Patients Coma Evaluation & Consciousness Alterations Delirium & Toxic-Metabolic Encephalopathy in the ICU

2 Chapter 2: Cerebral Physiology & Intracranial Dynamics

Cerebral Blood Flow, Metabolism & Autoregulation Intracranial Pressure & Compliance Cerebral Edema & Herniation Syndromes

3 Chapter 3: Intracranial Hypertension & Cerebral Perfusion Management

Tiered ICP Management Protocols Hyperosmolar Therapy & Fluid Management Advanced & Refractory ICP Interventions

4 Chapter 4: Airway, Mechanical Ventilation & Pulmonary ICU Management

Emergency Airway Management & Neuro-Intubation Mechanical Ventilation Strategies in Acute Brain Injury Tracheostomy & Ventilator Weaning in Neurocritical Care Pulmonary Complications in the Neuro-ICU Obstructive Lung Disease, Pulmonary Hypertension & Bronchoscopy

5 Chapter 5: Hemodynamics, Shock & Cardiovascular Management

Hemodynamic Monitoring & Blood Pressure Targets Vasoactive Agents: Inotropes, Vasopressors & Antihypertensives Cardiac Complications of Brain Injury

6 Chapter 6: Neuropharmacology, Analgesia & Sedation

ICU Sedation & Analgesia Neuromuscular Blockade & Monitoring Propofol Infusion Syndrome & Drug Toxicities Antiseizure Drug Pharmacology in the ICU

7 Bleeding, Thrombosis & Hematologic Emergencies

Warfarin & Heparin Reversal Direct Oral Anticoagulant (DOAC) Reversal Antiplatelet Management & Platelet Dysfunction Venous Thromboembolism Prophylaxis & Management Transfusion, Coagulopathy, TMA & Cellular-Therapy Neurotoxicity

8 Chapter 8: Systemic ICU Care, Fluids, Electrolytes & Endocrine

Hyponatremia & Osmoregulation: SIADH vs Cerebral Salt Wasting Diabetes Insipidus, Hypernatremia & Endocrine Crises Critical Care Prophylaxis, Nutrition & Infection Prevention

9 Systemic Trauma & Environmental Injury

Polytrauma, Chest Injury & Abdominal Hemorrhage Burns, Electrical Injury & Hanging

10 Cardiac Emergencies & Mechanical Support

IABP, Ventricular Devices & ECMO Acute Coronary, Valvular, Rhythm & Aortic Emergencies

11 Kidney Injury, Acid–Base & Renal Replacement

Acute Kidney Injury & Renal Drug Clearance Acid–Base Disorders, Electrolytes & Renal Replacement

12 Systemic Infection & Endocrine Emergencies

Sepsis, Endocarditis & Antimicrobial Principles Thyroid, Adrenal & Dysglycemic Emergencies

13 Gastrointestinal & Hepatic Emergencies

Acute Abdomen, GI Bleeding & Abdominal Compartment Syndrome Acute Liver Failure, Cirrhosis & Hepatic Drug Effects

14 Chapter 9: Neuroimaging in the Neuro-ICU

Non-Contrast CT & CT Angiography CT Perfusion & Advanced Perfusion Imaging Magnetic Resonance Imaging in Critical Illness

15 Chapter 10: Electrophysiology & Neuromonitoring

Continuous EEG & ACNS Standardized Critical Care Terminology Invasive Neuromonitoring: ICP & External Ventricular Drains Multimodal Monitoring: PbtO2, Microdialysis & Transcranial Doppler

16 Angiography, Electrodiagnosis & CSF Analysis

Conventional Angiography & Cranial–Spinal Shunts EMG, Nerve Conduction & Acute Muscle Disease CSF Analysis: Infection, Inflammation & Hemorrhage

17 Chapter 11: Bedside Procedures, Ultrasound & Death by Neurologic Criteria

Critical Care Procedural Skills Critical Care Ultrasound & Echocardiography Determination of Death by Neurologic Criteria

18 Chapter 12: Acute Ischemic Stroke & Reperfusion Therapies

Acute Ischemic Stroke Management & Thrombolysis Endovascular Thrombectomy & Large Vessel Occlusion Malignant Hemispheric & Cerebellar Infarction

19 Chapter 13: Spontaneous Intracerebral Hemorrhage & Intraventricular Hemorrhage

Acute Intraparenchymal Hemorrhage Management Intraventricular Hemorrhage & Hydrocephalus Surgical Evacuation & Minimally Invasive Surgery in ICH

20 Chapter 14: Aneurysmal Subarachnoid Hemorrhage & Delayed Cerebral Ischemia

Aneurysmal Subarachnoid Hemorrhage: Acute Resuscitation & Securing Delayed Cerebral Ischemia & Cerebral Vasospasm Non-Aneurysmal Subarachnoid Hemorrhage & Vascular Malformations

21 Venous, Spinal & Traumatic Vascular Disease

Cerebral Venous Thrombosis & Spinal Cord Infarction Traumatic Dissection, Carotid–Cavernous Fistula & Penetrating Injury

22 Chapter 15: Traumatic Brain & Spinal Cord Injury

Severe Traumatic Brain Injury & Contusions Acute Traumatic Spinal Cord Injury & Neurogenic Shock Spine Trauma, Fractures & Cord Syndromes

23 Chapter 16: Status Epilepticus, Seizures & Neuromuscular Emergencies

Status Epilepticus: Convulsive & Non-Convulsive Refractory & Super-Refractory Status Epilepticus Acute Neuromuscular Respiratory Failure

24 Chapter 17: Post-Cardiac Arrest Care, CNS Infections & Encephalitis

Targeted Temperature Management & Post-Cardiac Arrest Care Post-Cardiac Arrest Neuroprognostication Acute CNS Infections, Encephalitis & Inflammatory Emergencies

25 Neuroinflammation & Neurooncology

Fulminant Demyelination & Autoimmune Encephalitis Brain, Spinal & Leptomeningeal Cancer Emergencies

26 Pregnancy, Transplantation & Pediatric Care

Pregnancy, Postpartum & Post-Transplant Neurologic Care Pediatric Neurocritical Care

27 Chapter 18: Neurosurgical & Perioperative Emergencies

Postoperative Neurosurgical Complications Device Complications: EVDs, Shunts & ICP Monitors Cerebral Hyperperfusion Syndrome & Post-Interventional Emergencies

28 Chapter 19: Ethics, Palliative Care & Organ Donation in the Neuro-ICU

Communication, Surrogate Decision-Making & Conflict Resolution Neuroprognostication Ethics, Withdrawal of Life Support & Palliative Care Organ Donation & Management of the Deceased Donor

Antiseizure Drug Pharmacology in the ICU

Key Takeaways Explain This Topic AI

Loading and maintenance doses serve different purposes; adjust subsequent dosing to organ function, dialysis, interactions, and response.

Measured free phenytoin is preferred when hypoalbuminemia or uremia makes total concentrations unreliable.

Valproate can cause hyperammonemic encephalopathy despite normal transaminases and apparently therapeutic concentrations.

Carbapenems can markedly lower valproate exposure; coordinate an alternative antimicrobial or antiseizure plan.

Lacosamide can prolong PR and worsen conduction disease; brivaracetam metabolism includes non-CYP hydrolysis and CYP2C19 hydroxylation.

Last updated: October 2026

Antiseizure Drug Pharmacology in the ICU

Acute seizures and status epilepticus in the neuro-ICU demand aggressive, protocolized intravenous pharmacotherapy. Seizures dramatically escalate cerebral metabolic rate of oxygen ( C M R O 2 CMRO_2 CMR O 2 ​ ), exhaust cellular glucose and adenosine triphosphate ( A T P ATP A TP ), provoke massive lactic acidosis, drive acute intracranial pressure ( I C P ICP I CP ) surges, and induce excitotoxic neuronal necrosis through sustained glutamate release and intracellular calcium flooding. Selecting the appropriate antiseizure medication ( A S M ASM A SM ) requires thorough knowledge of loading paradigms, protein-binding nuances, metabolic interactions, and unique organ toxicities.

• First-Line Parenteral Antiseizure Medications ICU Antiseizure Drug Classification & Mechanisms
• ├── Synaptic Vesicle Glycoprotein 2A (SV2A) Ligands
• │ ├── Levetiracetam: 60 mg/kg load, renally cleared, no CYP interactions, neuropsychiatric side effects

│ └── Brivaracetam: Higher SV2A affinity, faster brain entry, non-CYP hydrolysis and CYP2C19-mediated hydroxylation, lower irritability

• ├── Voltage-Gated Sodium Channel Modulators
• │ ├── Fosphenytoin: Fast inactivation blocker, 20 mg PE/kg load, non-linear kinetics, Sheiner-Tozer correction

│ └── Lacosamide: Slow inactivation enhancer, 200-400 mg load, minimal interactions, PR prolongation / AV block

• └── Multimodal Neurotransmitter Modulator
• └── Sodium Valproate: GABA enhancer, T-type Ca2+ / Na+ blocker, hyperammonemia (VHE), carbapenem interaction
• Levetiracetam

Mechanism: Binds stereoselectively to synaptic vesicle protein 2A ( S V 2 A SV2A S V 2 A ) , an integral transmembrane vesicle glycoprotein. By modulating SV2A, it inhibits calcium-dependent exocytosis of excitatory neurotransmitters (primarily glutamate) without directly interfering with basal synaptic transmission.

ICU Dosing & Kinetics:

Loading Dose: 60 mg/kg 60\text{ mg/kg} 60 mg/kg IV (maximum 4500 mg 4500\text{ mg} 4500 mg ) infused over 10 to 15 minutes. In status epilepticus, the established randomized ESETT trial confirmed the safety and efficacy of high-dose loading ( 60 mg/kg 60\text{ mg/kg} 60 mg/kg ).

Maintenance: 1000 to 1500 mg 1000\text{ to }1500\text{ mg} 1000 to 1500 mg IV every 12 hours (adjusted for renal clearance).

Elimination: 66 % 66\% 66% excreted unchanged in urine via glomerular filtration; 24 % 24\% 24% undergoes non-hepatic enzymatic hydrolysis of the acetamide group into an inactive carboxylic acid metabolite ( u c b L057 ucb\text{ L057} u c b L057 ). It has low plasma protein binding (less than 10%) and exhibits no hepatic CYP450 interactions.

Renal Replacement Therapy: Highly cleared by hemodialysis ( 50 % 50\% 50% removed per 4-hour session); a supplemental dose is selected for the dialysis regimen; the adult label recommends 250–500 mg after conventional hemodialysis.

Adverse Effects: Somnolence, dizziness, and prominent neuropsychiatric toxicity ("levetiracetam rage": agitation, irritability, emotional lability, depression, and acute psychosis). In neuro-ICU patients exhibiting unprovoked agitation or delirium, levetiracetam toxicity should always be considered.

Fosphenytoin

Mechanism: Fosphenytoin is a water-soluble disodium phosphate ester prodrug of phenytoin. Following intravenous administration, blood and vascular endothelial alkaline phosphatases rapidly cleave the phosphate ester group ( t 1 / 2 conversion ≈ 8 to 15 minutes t_{1/2\text{ conversion}} \approx 8\text{ to }15\text{ minutes} t 1/2 conversion ​ ≈ 8 to 15 minutes ), liberating active phenytoin. Phenytoin selectively binds to and stabilizes the inactive state of voltage-gated sodium channels , prolonging their inactivation and preventing repetitive, high-frequency neuronal firing.

ICU Dosing:

Dosed exclusively in Phenytoin Sodium Equivalents ( P E PE PE ) to avoid dosing conversion errors ( 1.5 mg 1.5\text{ mg} 1.5 mg fosphenytoin = 1.0 mg PE = 1.0 mg = 1.0\text{ mg PE} = 1.0\text{ mg} = 1.0 mg PE = 1.0 mg phenytoin sodium).

Loading Dose: 20 mg PE/kg 20\text{ mg PE/kg} 20 mg PE/kg IV.

Infusion Rate: Up to 150 mg PE/min 150\text{ mg PE/min} 150 mg PE/min (three times faster than parenteral phenytoin's maximum rate of 50 mg/min 50\text{ mg/min} 50 mg/min ).

Pharmacokinetics & Therapeutic Drug Monitoring:

Phenytoin exhibits non-linear, capacity-limited Michaelis-Menten elimination kinetics mediated by hepatic CYP2C9 and CYP2C19. Once hepatic enzymes become saturated within therapeutic concentrations, small dose increases cause disproportionate spikes in serum concentrations.

Target Levels: Total phenytoin 10 to 20 μ g/mL 10\text{ to }20\text{ }\mu\text{g/mL} 10 to 20 μ g/mL ; Free (unbound) phenytoin 1.0 to 2.0 μ g/mL 1.0\text{ to }2.0\text{ }\mu\text{g/mL} 1.0 to 2.0 μ g/mL .

Protein Binding & The Sheiner-Tozer Equation: Phenytoin is 90 % 90\% 90% bound to serum albumin. In critical illness, hypoalbuminemia and uremic toxin displacement increase the unbound (biologically active) free fraction. When albumin is depressed, the measured total phenytoin level appears falsely subtherapeutic while the active free level is therapeutic or toxic. The Sheiner-Tozer equation calculates adjusted total concentration: Adjusted Total Phenytoin = Measured Total Phenytoin 0.2 × Albumin (g/dL) + 0.1 \text{Adjusted Total Phenytoin} = \frac{\text{Measured Total Phenytoin}}{0.2 \times \text{Albumin (g/dL)} + 0.1} Adjusted Total Phenytoin = 0.2 × Albumin (g/dL) + 0.1 Measured Total Phenytoin ​

*(In renal failure with creatinine clearance < 20 mL/min < 20\text{ mL/min} < 20 mL/min , the denominator constant shifts: 0.1 × Albumin + 0.1 0.1 \times \text{Albumin} + 0.1 0.1 × Albumin + 0.1 ). Direct measurement of free phenytoin is the diagnostic gold standard.

Adverse Effects: Cardiac arrhythmias, PR and QT prolongation, and hypotension driven by rapid sodium channel blockade. Fosphenytoin has an alkaline p H pH p H ( 8.6 – 9.0 8.6\text{--}9.0 8.6 – 9.0 ), lower than parenteral phenytoin and lacks propylene glycol diluent, reducing local tissue toxicity, although purple glove syndrome and serious local injury remain possible caused by conventional parenteral phenytoin ( p H 12 pH\ 12 p H 12 ).

Sodium Valproate

Mechanism: Multi-mechanistic action: increases brain gamma-aminobutyric acid ( G A B A GABA G A B A ) levels by inhibiting GABA transaminase ( G A B A - T GABA\text{-}T G A B A - T ) and succinic semialdehyde dehydrogenase ( S S A D H SSADH SS A DH ), blocks voltage-gated sodium channels, and attenuates low-threshold T-type calcium currents in thalamic neurons.

ICU Dosing: Loading dose of 40 mg/kg 40\text{ mg/kg} 40 mg/kg IV (maximum 3000 mg 3000\text{ mg} 3000 mg ) infused at 3 to 6 mg/kg/min 3\text{ to }6\text{ mg/kg/min} 3 to 6 mg/kg/min ; maintenance 15 to 30 mg/kg/day 15\text{ to }30\text{ mg/kg/day} 15 to 30 mg/kg/day divided every 8 hours. Target serum trough: 50 to 100 μ g/mL 50\text{ to }100\text{ }\mu\text{g/mL} 50 to 100 μ g/mL (up to 120 μ g/mL 120\text{ }\mu\text{g/mL} 120 μ g/mL in refractory status).

Complications & Black-Box Warnings:

Valproate-Induced Hyperammonemic Encephalopathy (VHE): Valproic acid inhibits carbamoyl phosphate synthetase I ( C P S - I CPS\text{-}I CPS - I ) in the urea cycle and depletes mitochondrial carnitine. Patients develop acute lethargy, cognitive slowing, asterixis, and paradoxical seizure worsening. VHE occurs in the presence of completely normal transaminases (AST/ALT) and therapeutic valproate levels! Treatment: immediate cessation of valproate and administration of intravenous L-carnitine (specialist-directed regimens commonly use 100 mg/kg up to 6 g initially, then 50 mg/kg up to 3 g every eight hours; adjust to toxicity guidance).

Hematologic & Visceral Toxicity: Dose-dependent thrombocytopenia (especially at troughs > 100 μ g/mL > 100\text{ }\mu\text{g/mL} > 100 μ g/mL ), acute necrotizing pancreatitis, and idiosyncratic fulminant hepatic failure.

Warning

Carbapenems can markedly and rapidly lower valproate concentrations, sometimes with loss of seizure control. Avoid the combination where feasible and coordinate an alternative antimicrobial or antiseizure plan. Simply escalating valproate often does not overcome the interaction. The effect is variable and involves impaired recycling of valproate glucuronide; it is not inhibition of a supposed glucuronosyltransferase reactivation pathway.

Lacosamide

Mechanism: Selectively enhances slow inactivation of voltage-gated sodium channels, in contrast to phenytoin, carbamazepine, and lamotrigine which primarily modulate fast inactivation. By promoting slow inactivation, lacosamide selectively dampens chronically depolarized, hyper-excitable epileptic neurons without disrupting physiologic basal neurotransmission.

ICU Dosing & Profile: Loading dose 200 to 400 mg 200\text{ to }400\text{ mg} 200 to 400 mg IV infused over 15 minutes; maintenance 100 to 200 mg 100\text{ to }200\text{ mg} 100 to 200 mg IV every 12 hours. Negligible protein binding ( < 15 % < 15\% < 15% ), low CYP interaction profile, and excreted 95 % 95\% 95% by kidneys ( 40 % 40\% 40% unchanged).

Adverse Effects: Dose-dependent PR interval prolongation, second- or third-degree AV block , and syncope. Baseline 12-lead ECG and telemetry monitoring are mandatory, especially in patients with pre-existing conduction disease or concurrent nodal-blocking medications.

Clinical profile: IV brivaracetam is used in selected acute seizure settings, with status use and loading regimens guided by specialist protocols. Maintenance and hepatic adjustments follow the product information. Some observational switching studies suggest improved behavioral tolerability compared with levetiracetam, but there is no universally validated 1:10 equivalent conversion or guarantee that psychiatric adverse effects will disappear.

Antiseizure Drug Primary Mechanism Recommended ICU Loading Dose Elimination & Clearance High-Yield ICU Pitfall / Toxicity

Levetiracetam SV2A vesicle binding 60 mg/kg 60\text{ mg/kg} 60 mg/kg IV (max 4500 mg 4500\text{ mg} 4500 mg ) Renal excretion ( 66 % 66\% 66% unchanged); cleared by HD Neuropsychiatric rage, agitation, depression

Fosphenytoin Sodium channel fast inactivation 20 mg PE/kg 20\text{ mg PE/kg} 20 mg PE/kg IV (max 150 mg PE/min 150\text{ mg PE/min} 150 mg PE/min ) Hepatic (CYP2C9/2C19); non-linear Michaelis-Menten Arrhythmias, hypotension; prefer measured free phenytoin when binding is altered

Sodium Valproate GABA ↑ \uparrow ↑ , N a + Na^+ N a + , T-type C a 2 + Ca^{2+} C a 2 + block 40 mg/kg 40\text{ mg/kg} 40 mg/kg IV (max 3000 mg 3000\text{ mg} 3000 mg ) Hepatic glucuronidation & mitochondrial β \beta β -oxidation Hyperammonemia with normal LFTs (treat with L-carnitine); carbapenem washout

Lacosamide Sodium channel slow inactivation 200 – 400 mg 200\text{--}400\text{ mg} 200 – 400 mg IV over 15 min 15\text{ min} 15 min Renal excretion ( 95 % 95\% 95% , 40 % 40\% 40% unchanged) PR prolongation, heart block; requires baseline ECG

Brivaracetam High-affinity SV2A binding 100 – 200 mg 100\text{--}200\text{ mg} 100 – 200 mg IV Hepatic hydrolysis with a separate CYP2C19-mediated hydroxylation pathway Behavioral adverse effects remain possible; some switch studies suggest improved tolerability