# Clinical Cases: Acid-Base Physiology

## Case 1: Diabetic Ketoacidosis

### Clinical Image
![Davenport acid-base diagram](case_01_image.jpg)
*Source: [Wikimedia Commons - Davenport Diagram](https://commons.wikimedia.org/wiki/File:Davenport_diagram.svg) - Public Domain*

### Case Presentation
A 22-year-old woman with type 1 diabetes presents to the emergency department with nausea, vomiting, abdominal pain, and confusion for 1 day. She ran out of insulin 3 days ago. On examination, she is lethargic but arousable, with dry mucous membranes, poor skin turgor, and fruity breath odor. Vital signs show heart rate 118 bpm, blood pressure 95/60 mmHg, respiratory rate 32/min with deep labored breaths (Kussmaul respiration), and temperature 37.2C.

Laboratory results show serum glucose 520 mg/dL, serum sodium 128 mEq/L, potassium 5.8 mEq/L, chloride 92 mEq/L, bicarbonate 8 mEq/L, BUN 32 mg/dL, and creatinine 1.8 mg/dL. Arterial blood gas shows pH 7.12, PaCO2 22 mmHg, PaO2 98 mmHg.

Systematic ABG interpretation:
1. **pH**: 7.12 indicates severe acidemia
2. **Primary disorder**: HCO3 of 8 is markedly low, indicating metabolic acidosis as the primary disorder
3. **Anion gap**: Na - (Cl + HCO3) = 128 - (92 + 8) = 28 mEq/L (elevated, normal 8-12)
4. **Expected compensation**: Winter's formula: expected PaCO2 = (1.5 x 8) + 8 = 20 mmHg, range 18-22. Measured PaCO2 is 22 mmHg, indicating appropriate respiratory compensation
5. **Delta-delta ratio**: (28-12)/(24-8) = 16/16 = 1.0, indicating pure anion gap metabolic acidosis

The elevated anion gap is due to accumulation of beta-hydroxybutyrate and acetoacetate (ketoacids). Kussmaul respiration represents maximal respiratory compensation - deep, rapid breathing to eliminate CO2 and partially correct acidemia. Despite PaCO2 of 22 mmHg (dramatic hyperventilation), pH remains severely low because the metabolic acidosis is so profound.

Treatment includes IV insulin, aggressive fluid resuscitation with normal saline, and potassium replacement (serum K will fall precipitously with insulin therapy). Serial monitoring of pH, anion gap, and electrolytes guides therapy. The patient's pH normalizes over 12 hours with resolution of ketoacidosis.

### Key Learning Points
- DKA causes anion gap metabolic acidosis from ketoacid accumulation
- Kussmaul respiration is the respiratory compensation (deep, rapid breathing)
- Winter's formula calculates expected PaCO2 for metabolic acidosis compensation
- Delta-delta ratio near 1 indicates pure anion gap acidosis; less than 1 suggests concurrent non-gap acidosis
- Serum potassium may be elevated despite total body depletion due to acidosis-induced cellular shifts

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## Case 2: COPD Exacerbation with Acute-on-Chronic Respiratory Acidosis

### Case Presentation
A 70-year-old man with severe COPD (FEV1 28% predicted) on home oxygen presents with worsening dyspnea and increased sputum production for 3 days. On examination, he is in respiratory distress with pursed-lip breathing and use of accessory muscles. Vital signs show heart rate 105 bpm, blood pressure 148/88 mmHg, respiratory rate 28/min, and oxygen saturation 82% on his usual 2 L/min nasal cannula.

Arterial blood gas on 2 L/min nasal cannula shows pH 7.28, PaCO2 72 mmHg, PaO2 52 mmHg, HCO3 33 mEq/L. Previous baseline ABG from 6 months ago showed pH 7.38, PaCO2 58 mmHg, HCO3 34 mEq/L on 2 L/min O2.

Interpretation of this ABG requires understanding his chronic baseline. At baseline, he has chronic respiratory acidosis (PaCO2 58) with full metabolic compensation (HCO3 34, pH near normal). The expected compensation for chronic respiratory acidosis is 3.5 mEq/L rise in HCO3 for each 10 mmHg rise in PaCO2 above 40. His baseline: (58-40)/10 x 3.5 = 6.3 mEq/L increase, predicting HCO3 of 24 + 6 = 30 mEq/L. His actual HCO3 of 34 is appropriate.

Now with acute exacerbation: PaCO2 has risen acutely from 58 to 72 mmHg. For this acute rise of 14 mmHg, only chemical buffering can occur (not enough time for renal compensation), predicting HCO3 rise of about 1 mEq/L per 10 mmHg, or approximately 1.4 mEq/L. Since his chronic compensation was already at 34, we'd expect HCO3 around 35. Actual HCO3 is 33, confirming acute-on-chronic respiratory acidosis without additional metabolic component. The pH of 7.28 reflects the acute, uncompensated portion of the CO2 rise.

The patient is treated with BiPAP (non-invasive ventilation), bronchodilators, systemic corticosteroids, and antibiotics for presumed infectious exacerbation. Oxygen is titrated carefully to maintain SpO2 88-92% to avoid suppressing hypoxic respiratory drive. Within 48 hours, his PaCO2 returns toward baseline and pH normalizes.

### Key Learning Points
- Chronic respiratory acidosis shows full metabolic compensation (3.5 mEq/L HCO3 per 10 mmHg PCO2)
- Acute respiratory acidosis shows only chemical buffering (1 mEq/L HCO3 per 10 mmHg PCO2)
- Acute-on-chronic respiratory acidosis shows acidemia despite elevated bicarbonate
- Comparison to baseline ABG is essential for accurate interpretation
- Supplemental oxygen must be titrated carefully in chronic CO2 retainers

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## Case 3: Salicylate Toxicity with Mixed Disorder

### Case Presentation
A 19-year-old woman is brought to the emergency department after being found confused at home with multiple empty aspirin bottles. Her mother estimates she may have ingested 100-150 aspirin tablets (325 mg each) approximately 4 hours ago. On examination, she is agitated, diaphoretic, and hyperventilating. Vital signs show heart rate 125 bpm, blood pressure 100/65 mmHg, respiratory rate 36/min, temperature 38.5C, and oxygen saturation 99% on room air. Tinnitus is present.

Laboratory results show serum sodium 138 mEq/L, potassium 3.2 mEq/L, chloride 98 mEq/L, bicarbonate 12 mEq/L, glucose 85 mg/dL, BUN 22 mg/dL, and creatinine 1.1 mg/dL. Serum salicylate level is 85 mg/dL (therapeutic <20 mg/dL, toxic >40 mg/dL). Arterial blood gas shows pH 7.42, PaCO2 20 mmHg, PaO2 105 mmHg.

Analysis reveals a complex mixed acid-base disorder characteristic of salicylate toxicity:
1. **pH**: 7.42 - near normal, but this does not exclude a disorder
2. **Primary disorders identified**: Low PaCO2 (20) indicates respiratory alkalosis; low HCO3 (12) indicates metabolic acidosis
3. **Anion gap**: 138 - (98 + 12) = 28 mEq/L (elevated)

This is a mixed anion gap metabolic acidosis AND primary respiratory alkalosis. Salicylate toxicity classically produces this dual disorder through two mechanisms: (1) direct stimulation of the medullary respiratory center causes primary hyperventilation (respiratory alkalosis), and (2) salicylate uncouples oxidative phosphorylation, causing lactic acid production, while salicylic acid itself is an organic acid (metabolic acidosis).

The near-normal pH is deceptive - it results from the opposing effects of respiratory alkalosis and metabolic acidosis, not from absence of disease. Neither disorder is compensating for the other; both are primary processes.

Treatment includes aggressive IV fluid resuscitation, IV sodium bicarbonate to alkalinize blood and urine (enhancing salicylate excretion), correction of hypokalemia, and emergent hemodialysis given severely elevated salicylate level with altered mental status. The patient undergoes hemodialysis with subsequent improvement in salicylate level and mental status.

### Key Learning Points
- Salicylate toxicity causes mixed respiratory alkalosis AND anion gap metabolic acidosis
- Near-normal pH can mask severe underlying dual disorders
- Early: respiratory alkalosis predominates; late: metabolic acidosis worsens
- Treatment includes IV bicarbonate (alkalinizes urine for excretion) and hemodialysis for severe toxicity
- The two processes are both primary, not compensation for each other