Three Days of Vomiting
A 68-year-old woman is admitted after three days of vomiting and poor oral intake. She takes no diuretics. Admission findings: heart rate 108 and regular, blood pressure 98/62 supine (84/56 when she sits up), respirations 24, temperature 37.0 °C, weight 70 kg (usual 72 kg). Mucous membranes are dry and skin turgor is poor. Urine output over the first 8 hours is 90 mL, specific gravity 1.030. Labs: sodium 148 mEq/L, potassium 3.1 mEq/L, chloride 112 mEq/L, BUN 42 mg/dL, creatinine 1.6 mg/dL (baseline 1.0), glucose 96 mg/dL. The provider orders 0.45% NaCl infusion and IV potassium replacement.
1. Interpret the volume and electrolyte picture. Does the BUN-to-creatinine ratio point to prerenal or intrinsic renal cause, and why? Estimate the free-water deficit.
Read the worked answer
Pattern: hypovolemia with a hypertonic, hypernatremic picture. Vomiting plus poor intake removes fluid and chloride; the sodium is 148 with concentrated urine (SG 1.030, only 90 mL in 8 hours) and orthostatic change of 14 mmHg systolic, which indicates volume depletion of at least a moderate degree.
BUN:creatinine = 42 ÷ 1.6 ≈ 26:1, above the usual ~10–20:1. This is the signature of prerenal physiology: decreased renal perfusion increases proximal urea reabsorption while creatinine is not reabsorbed, so BUN rises disproportionately. Intrinsic injury would typically blunt this ratio and produce less avid urine concentration; her urine is maximally concentrated, which supports an intact tubule responding to hypoperfusion. The history (fluid loss, no nephrotoxins, no diuretics) fits prerenal.
Free-water deficit ≈ 0.6 × 70 kg × [(148 ÷ 140) − 1] ≈ 42 × 0.057 ≈ 2.4 L, on top of the isotonic losses from vomiting. Total deficit is therefore larger than the free-water figure alone.
Interpretation note: the low potassium is also explained by vomiting plus urinary potassium loss during volume depletion, and it will fall further as the infusion drives potassium intracellularly.
2. The potassium order reads "20 mEq KCl IV now." Before starting it, what must the nurse verify, and how does the rest of this patient's picture change the priority order?
Read the worked answer
Verify first: adequate urine output and the route/rate of administration. Potassium is excreted renally; infusing potassium into a patient making 90 mL in 8 hours risks hyperkalemia. The nurse confirms a functioning line and current output, ensures the potassium is diluted and given by pump (never IV push), and confirms the ordered rate is within policy (commonly ≤10 mEq/hr peripherally). If output has not improved, the nurse raises this with the provider before infusing.
Priority order for this patient:
- Circulation/volume - the orthostatic drop and tachycardia are the immediate threat, and restoring perfusion also improves renal potassium excretion, making later potassium replacement safer.
- Potassium and cardiac monitoring - 3.1 mEq/L with a planned intracellular shift from the infusion warrants rhythm monitoring and replacement, but only on the foundation of step 1.
- Sodium correction - the ordered 0.45% NaCl is hypotonic and appropriate for the free-water deficit, but the sodium should be corrected slowly (generally no faster than ~10 mEq/L per 24 hours) to protect the brain.
The trap is treating the abnormal number with the most dramatic intervention first. In this case the low potassium is real but the low perfusion is what is producing the abnormal numbers.