GI Drug Absorption in Critical Illness
OVERVIEW
- multi-factorial determinants influence the bioavailability of drugs
- governed by factors altering transport of substances across cell membranes (diffusion, active transport, ultrafiltration)
- Fick’s Law of diffusion plays important role (net diffusion = change in pressure x area x solubility / distance x (square root of molecular weight))
(1) BLOOD FLOW
- in critically ill patient blood is often shunted away from the GI tract (increased sympathetic tone) -> decreased absorption
(2) SURFACE AREA
- may be decreased from resection post surgery
(3) PRESENCE OF BINDING OR CHELATING AGENTS
- in the presence of overdose binding agents (activated charcoal) or chelating agents (desferrioxamine in iron overdose) may be used therapeutically to abate toxicity.
(4) pH
- may become altered in critical illness -> increased secretion of gastric acid -> decreased absorption of drugs that become more ionized with a lower pH
(5) ALTERED GI MOTILITY
- ileus, diarrhoea, nausea and vomiting -> decrease bioavailability of drugs
(6) PERMEABILITY
- sepsis and SIRS can result in increased gastrointestinal permeability -> increased interstitial deposition -> ileus
(7) CONCENTRATION GRADIENT
(8) SOLUBILITY OF DRUG
- may be influenced by pH
(9) DISTANCE TO DIFFUSE
- may be increased with increased GI oedema
References and Links
Introduction to ICU Series
Introduction to ICU Series Landing Page
DAY TO DAY ICU: FASTHUG, ICU Ward Round, Clinical Examination, Communication in a Crisis, Documenting the ward round in ICU, Human Factors
AIRWAY: Bag Valve Mask Ventilation, Oropharyngeal Airway, Nasopharyngeal Airway, Endotracheal Tube (ETT), Tracheostomy Tubes
BREATHING: Positive End Expiratory Pressure (PEEP), High Flow Nasal Prongs (HFNP), Intubation and Mechanical Ventilation, Mechanical Ventilation Overview, Non-invasive Ventilation (NIV)
CIRCULATION: Arrhythmias, Atrial Fibrillation, ICU after Cardiac Surgery, Pacing Modes, ECMO, Shock
CNS: Brain Death, Delirium in the ICU, Examination of the Unconscious Patient, External-ventricular Drain (EVD), Sedation in the ICU
GASTROINTESTINAL: Enteral Nutrition vs Parenteral Nutrition, Intolerance to EN, Prokinetics, Stress Ulcer Prophylaxis (SUP), Ileus
GENITOURINARY: Acute Kidney Injury (AKI), CRRT Indications
HAEMATOLOGICAL: Anaemia, Blood Products, Massive Transfusion Protocol (MTP)
INFECTIOUS DISEASE: Antimicrobial Stewardship, Antimicrobial Quick Reference, Central Line Associated Bacterial Infection (CLABSI), Handwashing in ICU, Neutropenic Sepsis, Nosocomial Infections, Sepsis Overview
SPECIAL GROUPS IN ICU: Early Management of the Critically Ill Child, Paediatric Formulas, Paediatric Vital Signs, Pregnancy and ICU, Obesity, Elderly
FLUIDS AND ELECTROLYTES: Albumin vs 0.9% Saline, Assessing Fluid Status, Electrolyte Abnormalities, Hypertonic Saline
PHARMACOLOGY: Drug Infusion Doses, Summary of Vasopressors, Prokinetics, Steroid Conversion, GI Drug Absorption in Critical Illness
PROCEDURES: Arterial line, CVC, Intercostal Catheter (ICC), Intraosseous Needle, Underwater seal drain, Naso- and Orogastric Tubes (NGT/OGT), Rapid Infusion Catheter (RIC)
INVESTIGATIONS: ABG Interpretation, Echo in ICU, CXR in ICU, Routine daily CXR, FBC, TEG/ROTEM, US in Critical Care
ICU MONITORING: NIBP vs Arterial line, Arterial Line Pressure Transduction, Cardiac Output, Central Venous Pressure (CVP), CO2 / Capnography, Pulmonary Artery Catheter (PAC / Swan-Ganz), Pulse Oximeter
CCC Pharmacology Series
Respiratory: Bosentan, Delivery of B2 Agonists in Intubated Patients, Nitric Oxide, Oxygen, Prostacyclin, Sildenafil
Cardiovascular: Adenosine, Adrenaline (Epinephrine), Amiodarone, Classification of Vasoactive drugs, Clevidipine, Digoxin, Dobutamine, Dopamine, Levosimendan, Levosimendan vs Dobutamine, Milrinone, Noradrenaline, Phenylephrine, Sodium Nitroprusside (SNiP), Sotalol, Vasopressin
Neurological: Dexmedetomidine, Ketamine, Levetiracetam, Lignocaine, Lithium, Midazolam, Physostigmine, Propofol, Sodium Valproate, Sugammadex, Thiopentone
Endocrine: Desmopressin, Glucagon Therapy, Medications and Thyroid Function
Gastrointestinal: Octreotide, Omeprazole, Ranitidine, Sucralfate, Terlipressin
Genitourinary: Furosemide, Mannitol, Spironolactone
Haematological: Activated Protein C, Alteplase, Aprotinin, Aspirin, Clopidogrel, Dipyridamole, DOACs, Factor VIIa, Heparin, LMW Heparin, Protamine, Prothrombinex, Tenecteplase, Tirofiban, Tranexamic Acid (TXA), Warfarin
Antimicrobial: Antimicrobial Dosing and Kill Characteristics, Benzylpenicillin, Ceftriaxone, Ciprofloxacin, Co-trimoxazole / Bactrim, Fluconazole, Gentamicin, Imipenem, Linezolid, Meropenem, Piperacillin-Tazobactam, Rifampicin, Vancomycin
Analgesic: Alfentanil, Celecoxib, COX II Inhibitors, Ketamine, Lignocaine, Morphine, NSAIDs, Opioids, Paracetamol (Acetaminophen), Paracetamol in Critical Illness, Tramadol
Miscellaneous: Activated Charcoal, Adverse Drug Reactions, Alkali Therapies, Drug Absorption in Critical Illness, Drug Infusion Doses, Epidural Complications, Epidural vs Opioids in Rib Fractures, Magnesium, Methylene Blue, Pharmacology and Critical Illness, PK and Obesity, PK and ECMO, Sodium Bicarbonate Use, Statins in Critical Illness, Therapeutic Drug Monitoring, Weights in Pharmacology
Toxicology: Digibind, Flumazenil, Glucagon Therapy, Intralipid, N-Acetylcysteine, Naloxone, Propofol Infusion Syndrome
Critical Care
Compendium
Chris is an Intensivist and ECMO specialist at The Alfred ICU, where he is Deputy Director (Education). He is a Clinical Adjunct Associate Professor at Monash University, the Lead for the Clinician Educator Incubator programme, and a CICM First Part Examiner.
He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. He was one of the founders of the FOAM movement (Free Open-Access Medical education) has been recognised for his contributions to education with awards from ANZICS, ANZAHPE, and ACEM.
His one great achievement is being the father of three amazing children.
On Bluesky, he is @precordialthump.bsky.social and on the site that Elon has screwed up, he is @precordialthump.
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