# Clinical Cases: Pharmacokinetics

## Case 1: Digoxin Toxicity in Renal Impairment

### Clinical Scenario
A 78-year-old female with a history of atrial fibrillation and heart failure with reduced ejection fraction presents to the emergency department with nausea, vomiting, and visual disturbances described as "yellow halos around lights."

### Patient Demographics
- **Age:** 78 years
- **Sex:** Female
- **Weight:** 55 kg

### Chief Complaint
Nausea, vomiting, confusion, and visual changes for 2 days

### History of Present Illness
The patient has been on digoxin 0.25 mg daily for rate control of atrial fibrillation for the past 5 years. She was recently started on lisinopril 10 mg for hypertension management. She reports decreased appetite, fatigue, and seeing "yellow-green halos" around lights. Her daughter notes increased confusion over the past 48 hours.

### Physical Examination
- **Vital Signs:** BP 100/65 mmHg, HR 48 bpm (irregular), RR 16, T 36.8C, SpO2 97% on RA
- **General:** Elderly female appearing fatigued and confused
- **Cardiovascular:** Irregular rhythm, bradycardic, no murmurs
- **Neurological:** Oriented to person only, sluggish responses
- **HEENT:** Pupils reactive, patient reports blurred vision

### Workup and Results
| Test | Result | Reference Range |
|------|--------|-----------------|
| Digoxin level | 3.8 ng/mL | 0.8-2.0 ng/mL |
| Potassium | 5.8 mEq/L | 3.5-5.0 mEq/L |
| Creatinine | 2.4 mg/dL | 0.6-1.2 mg/dL |
| BUN | 48 mg/dL | 7-20 mg/dL |
| Magnesium | 1.6 mEq/L | 1.5-2.5 mEq/L |

**ECG Findings:** Atrial fibrillation with slow ventricular response (48 bpm), bidirectional ventricular ectopy, ST segment "scooping" consistent with digoxin effect

### Diagnosis
**Digoxin toxicity** secondary to decreased renal clearance (acute kidney injury) and drug interaction with ACE inhibitor

### Pharmacokinetic Principles Illustrated
1. **Renal elimination:** Digoxin is primarily eliminated by the kidneys (60-80%). Decreased GFR leads to drug accumulation and toxicity.
2. **Drug interactions:** ACE inhibitors can reduce renal function and increase digoxin levels.
3. **Narrow therapeutic index:** Digoxin has a therapeutic range of 0.8-2.0 ng/mL with toxicity occurring at levels only slightly above this range.
4. **Half-life prolongation:** In renal impairment, digoxin half-life increases from 36-48 hours to 4-6 days.

### Treatment
1. Hold digoxin immediately
2. Administer Digoxin Immune Fab (Digibind) for life-threatening arrhythmias
3. Correct hyperkalemia with calcium gluconate, insulin/glucose, and sodium bicarbonate
4. IV fluids for volume resuscitation
5. Cardiac monitoring in ICU
6. Temporary pacing if symptomatic bradycardia persists

### Clinical Image

![Digoxin Effect ECG](case_01_image.jpg)

*ECG showing characteristic digoxin effect with "scooped" ST segments. In toxicity, various arrhythmias may be seen including bidirectional ventricular tachycardia.*

**Image Source:** Wikimedia Commons
**License:** Public Domain
**URL:** https://commons.wikimedia.org/wiki/File:ECG_digoxin_effect.png

---

## Case 2: Phenytoin Toxicity Due to Enzyme Saturation

### Clinical Scenario
A 45-year-old male with epilepsy presents with ataxia, nystagmus, and slurred speech after his neurologist increased his phenytoin dose.

### Patient Demographics
- **Age:** 45 years
- **Sex:** Male
- **Weight:** 70 kg

### Chief Complaint
Unsteady gait, double vision, and difficulty speaking for 3 days

### History of Present Illness
The patient has had well-controlled epilepsy on phenytoin 300 mg daily for 10 years. Due to breakthrough seizure activity, his dose was increased to 400 mg daily one week ago. Since then, he has developed progressive unsteadiness, difficulty speaking, and feeling "drunk without drinking."

### Physical Examination
- **Vital Signs:** BP 125/80 mmHg, HR 78 bpm, RR 14, T 37.0C
- **General:** Alert but appears intoxicated
- **Neurological:**
  - Horizontal nystagmus on lateral gaze bilaterally
  - Dysarthria (slurred speech)
  - Ataxic gait (unable to tandem walk)
  - Finger-to-nose dysmetria
  - Deep tendon reflexes normal

### Workup and Results
| Test | Result | Reference Range |
|------|--------|-----------------|
| Total phenytoin level | 35 mcg/mL | 10-20 mcg/mL |
| Free phenytoin level | 4.2 mcg/mL | 1-2 mcg/mL |
| Albumin | 3.8 g/dL | 3.5-5.0 g/dL |
| ALT | 45 U/L | 7-56 U/L |
| AST | 38 U/L | 10-40 U/L |

### Diagnosis
**Phenytoin toxicity** due to saturation kinetics (zero-order elimination)

### Pharmacokinetic Principles Illustrated
1. **Non-linear (saturation) kinetics:** Phenytoin follows Michaelis-Menten kinetics. At therapeutic doses, the hepatic enzymes become saturated, converting first-order to zero-order elimination.
2. **Disproportionate concentration increases:** Small dose increases (33% in this case) can lead to disproportionately large increases in plasma concentration (75% increase).
3. **Narrow therapeutic index:** Therapeutic range 10-20 mcg/mL; toxicity occurs at levels only slightly above.
4. **Protein binding considerations:** Phenytoin is highly protein-bound (90%). Free drug levels are more clinically relevant.

### Treatment
1. Hold phenytoin until levels decrease to therapeutic range
2. Supportive care for ataxia (fall precautions)
3. Seizure precautions
4. No antidote needed; allow drug clearance
5. Resume phenytoin at lower dose (350 mg) once level <20 mcg/mL
6. Consider free phenytoin level monitoring

### Clinical Image

![Phenytoin Structure](case_02_image.jpg)

*Chemical structure of phenytoin (5,5-diphenylhydantoin), an anticonvulsant with saturable hepatic metabolism leading to non-linear pharmacokinetics.*

**Image Source:** Wikimedia Commons
**License:** Public Domain
**URL:** https://commons.wikimedia.org/wiki/File:Phenytoin_structure.svg

---

## Case 3: Gentamicin Therapeutic Drug Monitoring

### Clinical Scenario
A 62-year-old male with infective endocarditis is being treated with gentamicin and requires dose adjustment based on therapeutic drug monitoring.

### Patient Demographics
- **Age:** 62 years
- **Sex:** Male
- **Weight:** 80 kg
- **Height:** 175 cm

### Chief Complaint
Follow-up for gentamicin level monitoring during treatment for endocarditis

### History of Present Illness
The patient was admitted 5 days ago with Enterococcus faecalis endocarditis and started on ampicillin plus gentamicin for synergy. He has mild chronic kidney disease at baseline. Current regimen: Gentamicin 120 mg IV every 8 hours (started 3 days ago).

### Physical Examination
- **Vital Signs:** BP 130/75 mmHg, HR 82 bpm, RR 14, T 37.2C
- **General:** Improving, less fatigued
- **Cardiovascular:** Regular rhythm, grade II/VI systolic murmur
- **Neurological:** No vestibular symptoms, hearing intact

### Workup and Results
| Test | Result | Reference Range |
|------|--------|-----------------|
| Gentamicin peak (30 min post-dose) | 8.5 mcg/mL | 3-5 mcg/mL (synergy dosing) |
| Gentamicin trough (pre-dose) | 2.1 mcg/mL | <1 mcg/mL |
| Creatinine | 1.6 mg/dL | 0.6-1.2 mg/dL |
| BUN | 28 mg/dL | 7-20 mg/dL |
| Audiometry | Normal | - |

### Diagnosis
**Supratherapeutic gentamicin levels** with elevated trough indicating accumulation and nephrotoxicity risk

### Pharmacokinetic Principles Illustrated
1. **Therapeutic drug monitoring:** Aminoglycosides require monitoring due to narrow therapeutic index and concentration-dependent toxicity.
2. **Peak levels:** Correlate with bactericidal efficacy (concentration-dependent killing).
3. **Trough levels:** Correlate with nephrotoxicity and ototoxicity risk; should be <1 mcg/mL for synergy dosing.
4. **Renal elimination:** Gentamicin is eliminated almost entirely by glomerular filtration; dose adjustment required in renal impairment.
5. **Steady-state:** Reached after 4-5 half-lives; levels drawn after reaching steady state.

### Treatment
1. Extend dosing interval to every 12 hours
2. Reduce dose to 100 mg IV every 12 hours
3. Repeat levels after 3 doses at new regimen
4. Monitor serum creatinine daily
5. Obtain weekly audiometry during prolonged therapy
6. Target trough <1 mcg/mL for synergy dosing

### Key Learning Points
- Aminoglycoside dosing requires careful PK/PD optimization
- Extended-interval (once-daily) dosing reduces nephrotoxicity while maintaining efficacy for standard indications
- Synergy dosing uses lower doses with different target levels
- Trough levels are better predictors of toxicity than peak levels