13.1 Pharmacokinetics, Pharmacodynamics & High-Alert Medications

Pharmacokinetics, Pharmacodynamics & High-Alert Medications

Core Clinical Mandate: In Irish acute healthcare, safe medication administration is both a fundamental professional accountability under the Nursing and Midwifery Board of Ireland (NMBI) Scope of Nursing and Midwifery Practice Framework and a critical competency assessed across the RCSI Overseas Aptitude Test. Registered General Nurses (RGNs) must not only execute technical administration routines, but also understand the biochemical and physiological journeys of drugs through the human body. Recognizing pharmacokinetic shifts, narrow therapeutic windows, toxic accumulation, and high-alert drug protocols prevents catastrophic adverse drug events.

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Principles of Pharmacokinetics: The ADME Framework

Pharmacokinetics describes what the human body does to a drug, encompassing four dynamic physiological phases: Absorption, Distribution, Metabolism, and Excretion (ADME).

+-----------------------------------------------------------------------------+
|                        THE DYNAMIC ADME FRAMEWORK                           |
+-----------------------------------------------------------------------------+
|                                                                             |
|  1. ABSORPTION: Drug enters systemic circulation from site of administration|
|     [Oral / Enteral -> Portal Circulation -> Systemic Circulation]          |
|                                    |                                        |
|                                    v                                        |
|  2. DISTRIBUTION: Reversible transfer between blood plasma and tissues      |
|     [Plasma Protein Binding (Albumin) <-> Free Active Unbound Drug]         |
|                                    |                                        |
|                                    v                                        |
|  3. METABOLISM: Enzymatic biotransformation (primarily hepatic CYP450)      |
|     [Phase I: Functionalisation | Phase II: Conjugation -> Polar Metabolites]|
|                                    |                                        |
|                                    v                                        |
|  4. EXCRETION: Irreversible elimination of parent drug and metabolites       |
|     [Renal Glomerular Filtration & Secretion | Biliary / Faecal Route]      |
|                                                                             |
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1. Absorption & Bioavailability

• Absorption is the process by which an administered pharmacological agent transfers from its site of administration into the circulating vascular compartment.
• Bioavailability (F): The fraction of an administered dose of unchanged drug that reaches systemic venous circulation. Intravenous (IV) administration has an absolute bioavailability of 100% (F = 1.0). Oral administration typically has lower bioavailability (F < 1.0) due to incomplete gastrointestinal absorption, gut lumen breakdown, or extensive hepatic extraction.
• First-Pass Hepatic Metabolism: Following oral absorption across the intestinal mucosal epithelium, drugs travel via the hepatic portal venous system directly to the liver before reaching systemic circulation. Hepatic enzymes may metabolise a substantial percentage of the drug prior to systemic delivery. For example, oral propranolol, morphine, and glyceryl trinitrate (GTN) undergo extensive first-pass hepatic extraction, explaining why oral morphine doses (e.g., 30 mg) are approximately three times higher than equivalent parenteral IV/SC doses (e.g., 10 mg), and why GTN is formulated as a sublingual tablet or spray to bypass portal circulation directly into the superior vena cava.

2. Distribution & Plasma Protein Binding

• Volume of Distribution (Vd): The theoretical fluid volume that would be necessary to contain the total amount of an administered drug at the same concentration observed in blood plasma:

                    Total Amount of Drug in Body (mg)
Volume of Dist. =  ------------------------------------
                    Plasma Drug Concentration (mg/L)

• Low Vd (< 0.2 L/kg): Drug remains predominantly confined to the intravascular plasma compartment (e.g., large polar molecules or highly protein-bound drugs such as heparin, warfarin, and gentamicin).
• High Vd (> 1.0 L/kg): Drug distributes extensively into peripheral tissues, skeletal muscle, and adipose reservoirs (e.g., lipophilic molecules such as digoxin, amiodarone, diazepam, and fentanyl).

• Plasma Protein Binding: In the circulation, drugs exist in dynamic equilibrium between protein-bound fractions (primarily bound to albumin for acidic drugs, and alpha-1-acid glycoprotein for basic drugs) and free (unbound) active drug:
  • Only free drug can cross capillary endothelial membranes, interact with pharmacological target receptors, exert biological effects, and undergo hepatic or renal clearance.
  • When multiple highly protein-bound medications (e.g., warfarin, phenytoin, furosemide) are co-administered, they compete for limited albumin binding sites. Displacement increases the free active fraction, risking acute toxicity despite normal total plasma drug assays.

3. Metabolism & The Hepatic Cytochrome P450 System

Metabolism biotransforms lipophilic compounds into polar, water-soluble metabolites that can be eliminated by the kidneys.

• Phase I Reactions (Functionalisation): Oxidation, reduction, or hydrolysis, predominantly catalyzed by the Cytochrome P450 (CYP450) superfamily of hepatic microsomal hemoproteins:
  • CYP3A4: Metabolises over 50% of clinically prescribed medications (statins, calcium channel blockers, macrolide antibiotics, fentanyl).
  • CYP2D6: Converts codeine and tramadol to active morphine and O-desmethyltramadol; metabolises beta-blockers and SSRIs.
  • CYP2C9: Metabolises warfarin, phenytoin, and NSAIDs.
• Enzyme Induction vs. Inhibition:
  • CYP450 Inducers (e.g., rifampicin, carbamazepine, phenytoin, phenobarbital, St John's Wort): Accelerate hepatic enzyme synthesis over days to weeks, increasing drug clearance, reducing plasma levels, and causing therapeutic failure (e.g., graft rejection in organ transplant patients on tacrolimus, or unintended pregnancy on oral contraceptives).
  • CYP450 Inhibitors (e.g., clarithromycin, erythromycin, fluconazole, ciprofloxacin, amiodarone, diltiazem, grapefruit juice): Instantly block enzyme catalytic activity, slowing substrate clearance, causing drug accumulation, and triggering life-threatening toxicity (e.g., clarithromycin co-administered with simvastatin causes acute rhabdomyolysis; fluconazole co-administered with warfarin causes major haemorrhage).
• Phase II Reactions (Conjugation): Attachment of an endogenous water-soluble molecule (glucuronic acid, sulphate, glutathione) to produce inactive, easily excretable metabolites.

4. Excretion, Half-Life & Steady State

• Renal Clearance: The primary route of excretion for most polar drugs and water-soluble metabolites via glomerular filtration, active proximal tubular secretion, and passive distal tubular reabsorption.
• Elimination Half-Life (t1/2): The time required for the plasma drug concentration to decrease by 50% during elimination. In first-order kinetics, half-life remains constant regardless of the administered dose.
• Steady State (Css): The physiological equilibrium where the rate of drug administration exactly matches the rate of drug elimination. Reaching steady state requires 4 to 5 elimination half-lives:

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|                   ACCUMULATION TO STEADY STATE (4-5 HALF-LIVES)             |
+-----------------------------------------------------------------------------+
|  Dosing Interval: Every half-life (t1/2)                                    |
|                                                                             |
|  After 1 half-life:   50.0% of steady-state concentration reached           |
|  After 2 half-lives:  75.0% of steady-state concentration reached           |
|  After 3 half-lives:  87.5% of steady-state concentration reached           |
|  After 4 half-lives:  93.75% of steady-state concentration reached          |
|  After 5 half-lives:  96.875% (~97%) -> CLINICAL STEADY STATE ACHIEVED      |
|                                                                             |
|  *CLINICAL RULE: Similarly, when a drug is discontinued, 97% of the agent   |
|   is cleared from the body after 5 elimination half-lives (wash-out period).|
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|         PHARMACOKINETIC CONCENTRATION PROFILE & THERAPEUTIC WINDOW          |
+-----------------------------------------------------------------------------+
| Plasma Conc.                                                                |
|     ^                                                                       |
|     |            Toxic Concentration Threshold                              |
|  ---+----------------------------------------------------  TOXIC RANGE      |
|     |                 /\        /\        /\                                |
|     |      PEAK ->   /  \      /  \      /  \                               |
|     |               /    \    /    \    /    \    <-- STEADY STATE           |
|     |              /      \  /      \  /      \                              |
|     |             /        \/        \/        \/   <- TROUGH LEVEL         |
|  ---+------------/---------------------------------------  THERAPEUTIC RANGE|
|     |           /   Minimum Effective Concentration (MEC)                   |
|     |          /                                           SUB-THERAPEUTIC  |
|   0 +---------+---------+---------+---------+---------+--> Time             |
|             Dose 1    Dose 2    Dose 3    Dose 4    Dose 5                  |
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Age-Related Pharmacokinetic Alterations in Geriatric Patients

Older adults (aged ≥ 65 years) represent the highest consumers of prescription medications in Irish hospitals. Physiological changes associated with normal senescence and frailty alter every stage of pharmacokinetics, substantially increasing vulnerability to adverse drug events.

Geriatric Pharmacokinetic Shifts & Clinical Consequences

[!WARNING] The Geriatric Serum Creatinine Trap: Serum creatinine alone is a dangerously misleading indicator of renal function in frail, sarcopenic older adults! Creatinine is a breakdown product of skeletal muscle. In an elderly patient with severe muscle wasting, a 'normal' serum creatinine of 75 µmol/L may mask a severe occult renal impairment with an actual creatinine clearance of < 30 mL/min. Always evaluate estimated GFR (eGFR) or calculate creatinine clearance using the Cockcroft-Gault formula prior to administering renally cleared high-alert drugs.

Pharmacodynamics & Narrow Therapeutic Index (NTI) Medications

Pharmacodynamics describes what a drug does to the body—the biochemical and physiological mechanisms of action, receptor interactions, and dose-response relationships.

Agonists, Antagonists & Partial Agonists

• Full Agonist: A ligand that binds to a specific cellular receptor and elicits maximum biological response (high affinity and high intrinsic efficacy, e.g., Morphine at µ-opioid receptors; Salbutamol at beta-2 adrenoceptors).
• Antagonist: A molecule that binds to a receptor with high affinity but zero intrinsic efficacy, blocking endogenous ligands or agonists from activating the receptor:
  • Competitive Antagonist: Competes reversibly with the agonist for the same binding site. Can be overcome by increasing agonist concentration (e.g., Naloxone reversing morphine; Flumazenil reversing midazolam).
  • Non-Competitive Antagonist: Binds irreversibly or at an allosteric regulatory site, reducing maximum possible receptor response regardless of agonist concentration (e.g., Aspirin irreversibly acetylating platelet COX-1).
• Partial Agonist: Binds to receptors but produces a sub-maximal biological response even at 100% receptor occupancy (e.g., Buprenorphine at µ-opioid receptors). If co-administered with a full agonist like morphine, a partial agonist displaces the full agonist and acts functionally as an antagonist, precipitating acute opioid withdrawal.

Therapeutic Index & Narrow Therapeutic Index (NTI) Drugs

The Therapeutic Index (TI) is the ratio between the toxic dose and the minimum effective therapeutic dose (TI = TD50 / ED50). Medications with a Narrow Therapeutic Index (NTI) possess a narrow margin of safety between therapeutic clinical benefit and life-threatening toxicity. Minor variations in dose, hydration status, renal clearance, or drug interactions can tip the patient into severe toxicity.

Irish Acute Hospital NTI Reference Matrix

Therapeutic Drug Monitoring (TDM) Protocols in Ireland

Therapeutic Drug Monitoring (TDM) is the clinical practice of measuring specific drug concentrations at designated intervals to optimize dosage regimens and prevent toxicity.

Trough vs. Peak Levels

• Trough Concentration (Cmin): The lowest drug concentration in blood plasma during a dosing cycle. It reflects drug clearance and accumulation. Standard Protocol: Collect immediately before the next scheduled dose (strictly within 30 minutes prior to administration).
• Peak Concentration (Cmax): The highest drug concentration achieved in blood plasma following distribution. Collected after the drug distribution phase has finished (e.g., 30 to 60 minutes after the end of an intravenous infusion). In modern extended-interval once-daily aminoglycoside therapy, monitoring peak levels has largely been superseded by pre-dose trough monitoring or nomogram-based 6–14 hour post-dose sampling.

Gentamicin Once-Daily Extended-Interval Protocol (Hartford / HSE Nomogram)

Gentamicin is an aminoglycoside antibiotic exhibiting concentration-dependent bacterial killing (higher peak concentrations produce greater bactericidal effect) and a pronounced post-antibiotic effect (PAE) (bacterial suppression persists even after serum drug levels fall below the minimum inhibitory concentration).

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|             HSE ONCE-DAILY GENTAMICIN CLINICAL DECISION PATHWAY             |
+-----------------------------------------------------------------------------+
| 1. BASELINE ASSESSMENT:                                                     |
|    - Measure serum creatinine and compute baseline creatinine clearance/eGFR|
|    - Calculate dose using IDEAL BODY WEIGHT (IBW) (or Adjusted Body Weight  |
|      if actual weight > 120% of IBW to prevent severe overdosing)           |
|    - Standard Initial Regimen: 5 to 7 mg/kg IV infused over 30 to 60 minutes|
|                                                                             |
| 2. THERAPEUTIC DRUG MONITORING (TDM):                                       |
|    - Trough Level Timing: Draw serum sample IMMEDIATELY PRIOR (within 30    |
|      minutes) to the next scheduled dose (typically at hour 18 to 24).      |
|    - TARGET PRE-DOSE TROUGH: < 1.0 mg/L (strictly < 0.5 mg/L in frail/renal)|
|                                                                             |
| 3. CLINICAL ACTION BASED ON TROUGH LEVEL:                                   |
|    * If Trough < 1.0 mg/L: Clearance adequate -> Administer scheduled dose. |
|    * If Trough >= 1.0 mg/L: Impaired clearance / drug accumulation ->       |
|      WITHHOLD DOSE, notify prescriber/clinical pharmacist, extend dosing    |
|      interval (e.g. from 24 to 36 or 48 hours), and repeat trough assay.    |
|                                                                             |
| 4. TOXICITY SURVEILLANCE:                                                   |
|    - Nephrotoxicity: Monitor daily serum creatinine and hourly urine output.|
|    - Ototoxicity: Ask patient daily about hearing changes, tinnitus, or     |
|      vertigo/dizziness. Ototoxicity involves permanent hair cell death!    |
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Vancomycin Intermittent Infusion Protocol

Vancomycin is a glycopeptide antibiotic exhibiting time-dependent killing with an optimal pharmacokinetic-pharmacodynamic target of Area Under the Curve to Minimum Inhibitory Concentration ratio (AUC/MIC ≥ 400).

• Target Trough Ranges:
  • Standard / Mild-to-Moderate Infections (e.g., skin/soft tissue, uncomplicated UTI): 10 to 15 mg/L.
  • Severe / Deep-Seated MRSA Infections (e.g., bacteremia, infective endocarditis, osteomyelitis, hospital-acquired pneumonia, meningitis, severe sepsis): 15 to 20 mg/L.
  • Troughs consistently > 20 mg/L dramatically increase the risk of irreversible acute tubular necrosis (nephrotoxicity).
• Timing: Trough sample collected within 30 minutes prior to the 4th dose (at steady state).
• Infusion Rate Safety & Red Man Syndrome:
  • Vancomycin must be diluted appropriately (typically 500 mg in 100 mL, or 1,000 mg in 250 mL 0.9% NaCl or 5% Dextrose).
  • Maximum infusion rate: Must NOT exceed 10 mg/minute (e.g., a 1,000 mg dose requires a minimum infusion duration of at least 60 to 100 minutes; 1,500 mg requires at least 90–120 minutes).
  • Red Man Syndrome (Vancomycin Flushing Reaction):
    • Pathophysiology: An adverse non-immune drug reaction caused by rapid infusion, triggering direct, non-IgE degranulation of mast cells and basophils, releasing massive amounts of histamine.
    • Clinical Presentation: Intense erythematous macular flushing, itching (pruritus), and burning rash over the face, neck, and upper torso ('cape' distribution), accompanied by tachycardia, dizziness, and hypotension. (Does NOT involve stridor, wheeze, or facial angioedema typical of true IgE-mediated anaphylaxis).
    • Immediate Nursing Management: Immediately stop the infusion. Check vital signs (INEWS). Administer prescribed IV antihistamines (e.g., chlorphenamine). Once flushing and hypotension resolve, restart the infusion at half the previous rate over a prolonged period (e.g., over 2 to 3 hours). Document the reaction as an infusion rate-related adverse reaction, NOT an absolute penicillin/vancomycin allergy.

High-Alert Medications: The Irish HSE 'A-PINCH' Framework

High-alert medications are drugs that bear a heightened risk of causing significant patient harm or fatal outcomes when used in error. While errors may not necessarily occur more frequently with these drugs, the consequences of an error are devastating.

The Irish Health Service Executive (HSE) and the Irish Medication Safety Network (IMSN) champion the 'A-PINCH' clinical safety framework to standardize safeguards across acute hospitals:

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|                 THE IRISH HSE 'A-PINCH' MEDICATION FRAMEWORK                |
+---+-------------------------------------------------------------------------+
| A | ANTI-INFECTIVES (Aminoglycosides, Vancomycin, Amphotericin B)           |
+---+-------------------------------------------------------------------------+
| P | POTASSIUM & CONCENTRATED ELECTROLYTES (IV KCl, Hypertonic Saline)       |
+---+-------------------------------------------------------------------------+
| I | INSULIN (All formulations, IV infusions, subcutaneous regimens)        |
+---+-------------------------------------------------------------------------+
| N | NARCOTICS / OPIOIDS (Morphine, Oxycodone, Fentanyl, Hydromorphone)      |
+---+-------------------------------------------------------------------------+
| C | CHEMOTHERAPY / CYTOTOXIC AGENTS (Oral, IV, Intrathecal Antineoplastics) |
+---+-------------------------------------------------------------------------+
| H | HEPARIN & ANTICOAGULANTS (UFH, LMWH, Warfarin, DOACs)                   |
+---+-------------------------------------------------------------------------+

Clinical Breakdown of A-PINCH Drug Classes & Mandatory Safeguards

1. A – Anti-infectives (Aminoglycosides, Vancomycin, Amphotericin B)

• Risks: Acute kidney injury (nephrotoxicity), permanent ototoxicity, Red Man syndrome, infusion reactions, and emergence of resistant pathogens.
• Mandatory Safeguards: Pre-dose TDM blood levels audited before every dose; daily creatinine monitoring; strict compliance with minimum infusion durations via electronic infusion pumps; patient weight accurately measured (not estimated).

2. P – Potassium & Concentrated Electrolytes (IV Potassium Chloride, 3% NaCl, Concentrated Phosphate)

• Risks: Rapid administration of concentrated potassium instantly arrests the heart in diastole, causing instantaneous fatal cardiac arrest. Historically one of the deadliest medication errors in hospital care.
• Mandatory Safeguards:
  • Complete Restriction of Ampoules: Concentrated Potassium Chloride (KCl) ampoules (e.g., 20 mmol in 10 mL) are strictly banned and eliminated from general ward stock cupboards across Irish hospitals. They are restricted solely to intensive care, pharmacy, and cardiac arrest resuscitation kits.
  • Commercially Pre-Mixed Bags Only: Wards may only stock and infuse manufacturer pre-mixed infusion bags (e.g., 1,000 mL 0.9% NaCl with 20 mmol or 40 mmol KCl) with bold neon warning borders.
  • Electronic Infusion Control: Must NEVER be infused by gravity drip. Must run through an electronic volumetric pump with anti-free-flow protection.
  • Peripheral Rate Ceiling: Peripheral IV potassium infusion must never exceed 10 to 20 mmol/hour.
  • NEVER administer Potassium Chloride as an IV push or bolus.

3. I – Insulin (All Formulations & Concentrations)

• Risks: Severe iatrogenic hypoglycaemia, neuroglycopenia, irreversible encephalopathy, seizures, and death from overdose.
• Mandatory Safeguards:
  • Dedicated U-100 Syringes: Subcutaneous insulin must be measured and drawn up using dedicated, calibrated U-100 insulin syringes with attached needles. Never use standard 1 mL, 2 mL, or Luer-lock syringes, which leads to massive 10-fold overdoses.
  • Prohibition of 'U' Abbreviation: The abbreviation 'U' or 'IU' is strictly banned in Irish healthcare (prescriptions must read 'units' written in full; writing '10 U' looks like '100' on handwritten kardexes).
  • Independent Double Check: Dose, brand, formulation (rapid, short, intermediate, long-acting, or pre-mixed), and blood glucose reading must be independently checked by two registered nurses at the bedside.

4. N – Narcotics / Opioids (Morphine, Oxycodone, Fentanyl, Alfentanil)

• Risks: Fatal respiratory depression, severe sedation, bradycardia, hypotension, and bowel obstruction/ileus.
• Mandatory Safeguards:
  • Two-Nurse Verification: Mandatory independent double-checking of stock, prescription, dose calculations, and patient identity under the Misuse of Drugs Regulations.
  • Sedation & Respiratory Monitoring: Routine tracking of respiratory rate and POSS (Pasero Opioid-Induced Sedation Scale) score (1 = Awake/alert; 2 = Slightly drowsy, easily aroused; 3 = Frequently drowsy, drifts off during conversation [unacceptable, withhold opioid]; 4 = Somnolent, minimal response to physical stimulation [emergency]).
  • If respiratory rate drops < 10 breaths/min or sedation score is 3 or 4, withhold further doses, stimulate patient, apply oxygen, and prepare Naloxone (400 micrograms IV titrated in 100 microgram increments).

5. C – Chemotherapy / Cytotoxic Agents (IV, Oral, Subcutaneous)

• Risks: Bone marrow suppression, severe teratogenicity, secondary malignancies, severe extravasation necrosis, and occupational exposure to healthcare workers.
• Mandatory Safeguards:
  • Specialist Certification: Cytotoxic drugs may only be prepared by clinical oncology pharmacists and administered by nurses who have completed certified cytotoxic administration training.
  • Cytotoxic Waste Segregation: Discard all administration lines, vials, gloves, and pads into designated purple cytotoxic hazardous waste containers and purple sharps bins.
  • Extravasation Preparedness: Immediate access to institutional cytotoxic extravasation kits and antidotes (e.g., dexrazoxane for anthracyclines; topical DMSO; warm or cold packs per drug protocol).

6. H – Heparin & Other Anticoagulants (UFH, LMWH, Warfarin, DOACs)

• Risks: Major catastrophic haemorrhage (intracranial, retroperitoneal, gastrointestinal), fatal overdoses from concentration confusion, and Heparin-Induced Thrombocytopenia (HIT).
• Mandatory Safeguards:
  • Unfractionated Heparin (UFH): High risk of dosing confusion (1,000 units/mL vs 25,000 units/mL vials). IV continuous infusions must be titrated against regular activated Partial Thromboplastin Time (aPTT) or anti-Xa ratios.
  • Low-Molecular-Weight Heparin (LMWH, e.g., Enoxaparin, Tinzaparin): Weight-adjusted dosing; requires dose reduction in renal impairment (eGFR < 30 mL/min). Baseline and serial platelet count monitoring to detect Heparin-Induced Thrombocytopenia (HIT; a > 50% drop in platelets between days 4 and 14 of therapy).
  • Reversal Agent: Protamine Sulfate must be immediately accessible for urgent neutralization of unfractionated heparin.