25.3 General Pharmacologic Principles: Pharmacokinetics, Pharmacodynamics, Drug Categories & the Rights of Drug Safety
Key Takeaways
- Pharmacokinetics is what the body does to the drug — absorption, distribution, metabolism and excretion — while pharmacodynamics is what the drug does to the body through receptor and enzyme interactions.
- The rights of drug safety are right patient, right drug, right dose, right route, right time and right documentation, with right reason, right response and right to refuse added in expanded lists.
- The intravenous route gives 100 percent bioavailability and the most rapid onset, which is why every intravenous contrast injection demands verification before rather than after administration.
- Beam attenuation by a contrast agent depends on its iodine concentration in milligrams per millilitre and on the proximity of the beam energy to the iodine K-edge at 33.2 keV, not on the volume injected alone.
- A radiologic technologist administers medication only within the scope authorised by Republic Act No. 7431, under an appropriate order and institutional protocol, and never on personal initiative.
25.3 General Pharmacologic Principles: Pharmacokinetics, Pharmacodynamics, Drug Categories & the Rights of Drug Safety
General Pharmacologic Principles carries 6 items in the Radiological Sciences subject, and its four competencies are explicit: explain pharmacokinetic and pharmacodynamic principles of drugs; explain the uses and impact of drug categories and contrast agents' categories on the patient; illustrate the beam attenuation and pharmacologic profile of contrast agents; and implement the six rights of drug safety. Sections 25.1 and 25.2 covered contrast classification and adverse reactions. This section supplies the pharmacological framework beneath them.
1. Pharmacokinetics — What the Body Does to the Drug
The four processes, remembered as ADME.
| Process | Definition | Radiology application |
|---|---|---|
| Absorption | Movement of drug from the site of administration into the bloodstream | Intravenous administration bypasses absorption entirely, giving 100% bioavailability and the fastest onset. Oral barium is not absorbed at all, which is what makes it safe as a luminal agent |
| Distribution | Movement from blood into tissues and compartments | Iodinated contrast distributes into the extracellular fluid and does not enter normal cells or cross an intact blood-brain barrier — which is precisely why a lesion that breaks that barrier enhances |
| Metabolism (biotransformation) | Chemical alteration, mainly hepatic | Iodinated contrast is not significantly metabolised; it is excreted unchanged |
| Excretion | Elimination from the body | Iodinated contrast is excreted by glomerular filtration through the kidneys, which is why renal function determines both clearance and risk |
Terms to be able to define:
- Bioavailability — the fraction of an administered dose that reaches the systemic circulation unchanged; 100% for the intravenous route by definition.
- Half-life — the time for the plasma concentration to fall by half. Iodinated contrast has a plasma half-life of roughly 1-2 hours in normal renal function, prolonged markedly in renal impairment.
- Onset, peak and duration — the practical timing parameters. For intravenous contrast, onset is seconds, which is why a reaction can begin before the injection finishes.
- Therapeutic index — the ratio between the toxic dose and the effective dose. A narrow index means small errors matter greatly.
- First-pass effect — hepatic metabolism of an orally administered drug before it reaches the systemic circulation; a reason why oral doses exceed intravenous doses of the same agent.
Route comparison
| Route | Onset | Bioavailability | Comment |
|---|---|---|---|
| Intravenous (IV) | Seconds | 100% | Fastest, most controllable, least reversible once given |
| Intramuscular (IM) | Minutes | High | Adrenaline for anaphylaxis is given intramuscularly into the anterolateral thigh |
| Subcutaneous (SC) | Slower | High | Small volumes |
| Oral (PO) | 30-90 minutes | Variable; subject to first-pass metabolism | Barium, oral contrast, premedication |
| Rectal (PR) | Variable | Partial first-pass bypass | Barium enema, some antiemetics |
| Inhalational | Seconds to minutes | High | Bronchodilators for contrast-induced bronchospasm |
| Sublingual | 1-3 minutes | Bypasses first pass | Glyceryl trinitrate |
| Topical / transdermal | Slow | Variable | Local anaesthetic cream before cannulation in children |
| Intrathecal | Immediate local | — | Myelography only, and only with an agent labelled for intrathecal use; injecting a non-intrathecal agent causes fatal neurotoxicity |
The intrathecal rule is examined and must be absolute: verify the label reads "for intrathecal use" before any myelographic injection.
2. Pharmacodynamics — What the Drug Does to the Body
Drugs act by binding to receptors, by inhibiting or activating enzymes, by altering ion channels, or by direct physicochemical effect.
| Term | Meaning | Radiology example |
|---|---|---|
| Agonist | Binds a receptor and produces the effect | Adrenaline on alpha and beta receptors in anaphylaxis |
| Antagonist | Binds a receptor and blocks the effect | Naloxone reversing opioid respiratory depression; flumazenil reversing benzodiazepine sedation |
| Partial agonist | Produces a submaximal effect | — |
| Potency | Amount of drug needed for a given effect | — |
| Efficacy | Maximum achievable effect | — |
| Side effect | Predictable, dose-related, non-therapeutic effect | Warmth and metallic taste with contrast injection |
| Adverse drug reaction | Harmful, unintended response | Contrast-induced urticaria |
| Idiosyncratic reaction | Unpredictable, not dose-related | Rare anaphylactoid response |
| Synergism / potentiation | Combined effect greater than additive | Opioid plus benzodiazepine — the classic sedation hazard |
| Antagonism | One drug reduces another's effect | Beta blockers blunt the response to adrenaline, which is why an anaphylactic patient on a beta blocker may need glucagon |
| Tolerance / dependence | Reduced effect with repeat exposure / physiological need | Relevant in sedation practice |
3. Drug Categories Encountered in Imaging
| Category | Examples | Imaging relevance |
|---|---|---|
| Contrast agents | Iodinated (ionic and non-ionic), barium sulphate, gadolinium chelates, microbubble ultrasound agents, carbon dioxide | The core of the department's formulary |
| Emergency drugs | Adrenaline (epinephrine), atropine, salbutamol, hydrocortisone, diphenhydramine, oxygen, intravenous fluids | Must be immediately available wherever contrast is injected |
| Analgesics | Paracetamol, NSAIDs, opioids such as fentanyl and morphine | Interventional and painful procedures |
| Sedatives and anxiolytics | Midazolam, diazepam | Conscious sedation; requires monitoring and a reversal agent on hand |
| Local anaesthetics | Lidocaine | Access-site infiltration; watch total dose and inadvertent intravascular injection |
| Antiemetics | Metoclopramide, ondansetron | Contrast-related nausea |
| Antispasmodics | Hyoscine butylbromide, glucagon | Reduce bowel peristalsis in barium and cross-sectional bowel studies |
| Anticoagulants and antiplatelets | Heparin, warfarin, direct oral anticoagulants, aspirin, clopidogrel | Determine bleeding risk in interventional work |
| Diuretics | Furosemide | Diuretic renography |
| Radiopharmaceuticals | Technetium-99m labelled agents, fluorine-18 FDG, iodine-131 | Nuclear medicine |
| Others with imaging consequences | Metformin (management around iodinated contrast in renal impairment), beta blockers (blunt the response to adrenaline and are used for heart-rate control in cardiac CT) | — |
4. Beam Attenuation and the Pharmacologic Profile of Contrast
The TOS asks specifically for this link.
Attenuation depends on:
- Iodine concentration, expressed in milligrams of iodine per millilitre (typically about 300-370 mgI/mL for intravascular agents). Higher concentration means greater attenuation per unit volume.
- Proximity of the beam's photon energy to the iodine K-edge at 33.2 keV. Attenuation is maximal just above the K-edge and falls away at higher energies — which is why excessively high kVp wastes contrast, and why dual-energy and spectral CT can separate iodine from calcium.
- Total iodine delivered — concentration times volume times injection rate — which drives the peak arterial enhancement in a CT angiogram.
- The patient's circulation — cardiac output, body weight and vascular access calibre all shift the timing of peak enhancement, which is why bolus tracking exists.
The pharmacologic profile that must be balanced against attenuation:
| Property | Consequence |
|---|---|
| Osmolality | High osmolality draws water into the vascular space, causing pain, heat, vasodilation, endothelial injury, and haemodynamic shift. Non-ionic low-osmolar and iso-osmolar agents were developed to reduce this |
| Viscosity | Determines injectability through fine catheters; warming reduces viscosity |
| Ionicity | Ionic agents dissociate into ions, doubling the osmotic particles per molecule; non-ionic agents do not dissociate |
| Chemotoxicity | Direct molecular toxicity, independent of osmolality |
| Protein binding and hydrophilicity | Greater hydrophilicity and lower protein binding reduce biological interaction and toxicity |
Barium sulphate is pharmacologically different: it is an inert, insoluble suspension that is not absorbed, has no systemic pharmacology, and is contraindicated wherever perforation or leakage into the peritoneum is suspected, or where aspiration is likely — situations in which water-soluble iodinated contrast is substituted.
5. The Rights of Drug Safety
The classical six rights, which the TOS names explicitly:
| Right | What you verify |
|---|---|
| 1. Right patient | Two identifiers obtained from the patient, checked against the request and the prescription |
| 2. Right drug | The label read three times — when taking the container, when drawing up, and when discarding. Check for look-alike and sound-alike names |
| 3. Right dose | Calculated and, for high-risk drugs, independently double-checked; weight-based where applicable |
| 4. Right route | Intravenous, intramuscular, oral, rectal, intrathecal only with an intrathecal-labelled agent |
| 5. Right time | Correct timing relative to the scan phase, the fast, or the previous dose |
| 6. Right documentation | Drug, dose, route, time, site, batch or lot number, expiry, and the administering person recorded immediately after administration, never before |
Expanded lists add: right reason (a valid indication), right response (assess and document the effect), right to refuse (the patient may decline), and right education (the patient understands what is being given and what to report).
Before every injection also check: expiry date, container integrity, clarity and absence of particulate matter, that the correct concentration was selected, and that patency of the cannula has been confirmed with a saline flush. After every injection: flush, observe the patient for the critical first 15-20 minutes, and document.
6. Scope of Practice — The Boundary That Matters
Under RA 7431, medication administration by a radiologic technologist is permitted only:
- Within the scope authorised by the credential held, and consistent with the Board's rules;
- On the order of a physician or under an approved institutional protocol;
- After documented training and competency assessment in cannulation, contrast administration and reaction management;
- With a physician immediately available for contrast and interventional work;
- Never on the technologist's own initiative, and never a drug the technologist is not authorised and trained to give.
The Enhanced TOS reinforces this in the very next sub-topic, which asks the examinee to recognize the legal, ethical, and professional liability issues of the Radiologic Technologist's role in contrast media administration. Administering outside scope is simultaneously a negligence exposure and a violation of RA 7431.
The technologist is also the last check. Even where another professional prescribes and another draws up, the person pushing the syringe verifies the six rights. "I was told to give it" is not a defence.
Which statement correctly distinguishes pharmacokinetics from pharmacodynamics?
A radiologic technologist prepares to inject iodinated contrast for a CT study. Which sequence correctly reflects the six rights of drug safety?
Why does excessively high kilovoltage reduce the effectiveness of an iodinated contrast agent?
A ward nurse hands a radiologic technologist a syringe of contrast for a myelogram and states that it was drawn from the department's standard stock. What is the correct action?