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Question and answer links
- W1: After a laparoscopic hysterectomy, a 50-kg woman with pain and nausea will need IV fluid for a day. Which order carries · Answer
- W2: Two women each have a serum sodium of 121 mEq/L. In one it fell over 36 hours after surgery; in the other it drifted dow · Answer
- Q3: Post-operative day 2: sodium 121, serum osmolality 250, urine osmolality 540, urine sodium 70, BUN 8, uric acid 2.6; no · Answer
- Q4: It is 21:45, the urine results are in, and she is symptomatic with a sodium of 121 and a hemolyzed potassium of 6.4 with · Answer
- Q5: If you were thinking SIADH , and you then learn that her sodium fell from 121 to 119 overnight after a liter of 0.9% sal · Answer
- Q6: Ms. Novak (60 kg before surgery), sodium 119; a generalized seizure just stopped with lorazepam, and she remains obtunde · Answer
- Q7: At 14:00 her sodium is 126, up 7 from 119 at 07:05. Urine output is 260 mL/h with a urine osmolality of 110. What now? · Answer
Session S1 · 2026-10-22 · supplementary · 20 min Anchor: Fluids, electrolytes and acid-base — hospital-acquired hyponatremia and the hypertonic saline bolus · Integrates: IV fluid prescribing (hypotonic maintenance fluids), post-operative ADH triggers and the SSRI, pseudohyperkalemia, the scheduled NSAID order One-line hook: A maintenance-fluid order written on the day of surgery, two days of nausea, and a night-time page about a critical potassium: a new PA has to decide which number is the emergency.
Fictional patient. Every name, date and number in this case is invented for teaching. Self-study: about 20 minutes. Commit to an answer before you open each folded callout.
Learning Objectives
Answers for this section: W1 · W2 · Q3 · Q4 · Q5 · Q6 · Q7
By the end of this case the student can:
- Identify hypotonic maintenance fluid plus post-operative ADH (pain, nausea, opioids, an SSRI) as the cause of hospital-acquired hyponatremia, and choose a safer fluid plan.
- Classify hyponatremia from serum osmolality, urine osmolality, urine sodium and a bedside volume exam, and predict what a liter of isotonic saline does to the sodium on each branch.
- Recognize pseudohyperkalemia from the draw and a normal ECG, and repeat the sample correctly.
- Treat a hyponatremic seizure with the right hypertonic saline regimen — dose, timing, sodium checks and when to stop.
- Apply the 24-hour correction ceiling, anticipate the water diuresis when the ADH stimuli fade, and decide who acts before the ceiling is crossed.
Warm-Up (individual commit before the case)
Answers for this section: W1 · W2
Two readiness items.
W1The maintenance order
After a laparoscopic hysterectomy, a 50-kg woman with pain and nausea will need IV fluid for a day. Which order carries the highest risk of lowering her sodium?
0.9% saline at 100 mL/h, stopped once she is drinking
A saline lock, with oral fluids offered to her thirst
Lactated Ringer's at 100 mL/h, stopped once she drinks
D5 ½NS at 100 mL/h, continued until she is eating
Show answer and explanation · Link to this question
Correct answer: D
Learning point: Dextrose is metabolized within minutes, so what stays in the body is the bag's salt and water. D5 ½NS carries 77 mEq/L of sodium — half of 0.9% saline — so about half of every liter is free water. After surgery, pain, nausea, opioids and stress switch ADH on, the kidney keeps that water, and the sodium falls: hospital-acquired hyponatremia. Why not the others: A — isotonic, so it brings no free water of its own; B — drinking to thirst keeps intake matched to need; C — a balanced crystalloid, close to isotonic. Course reading: Evidence for this case
W2Fast or slow
Two women each have a serum sodium of 121 mEq/L. In one it fell over 36 hours after surgery; in the other it drifted down over three months on a new medicine. Which statement is correct?
The slow one is likelier to seize, because her sodium has been low for much longer
The fast one is likelier to seize, because her brain cells have not shed solute
Both carry the same seizure risk, because symptoms track the number, not the speed
Neither needs a correction limit, because both are above the 120 danger line
Show answer and explanation · Link to this question
Correct answer: B
Learning point: Brain cells adapt to a low sodium over about 48 hours by shedding solutes so that they do not swell. A sodium that falls faster than that swells the brain, so acute hyponatremia seizes at numbers a slowly adapted brain tolerates. The flip side: the adapted brain is the one injured by fast correction (osmotic demyelination), which is why the correction ceiling applies once hyponatremia has lasted more than 48 hours, or no one knows how long. Why not the others: A — duration protects against symptoms; C — the speed of the fall matters as much as the number; D — there is no safe "120 line"; the limit follows chronicity. Course reading: Evidence for this case
Part 1 — Presentation
Answers for this section: Q3 · Q4
Setting: Surgical floor, Wednesday 20:45 — post-operative day 2. You are the PA student on the gynecology team and will inherit her in the morning; start where the night intern started.
Patient: Ms. Teresa Novak, 44, an office manager, 60 kg, two days after an uncomplicated laparoscopic hysterectomy for fibroids with heavy bleeding. She has taken sertraline for generalized anxiety disorder for two years. Pre-admission testing six days before surgery: sodium 137, potassium 4.2, creatinine 0.7 mg/dL.
Course so far: Since surgery Monday she has been nauseated and has eaten almost nothing; the maintenance fluid kept running "because she isn't eating," and the nurses encouraged sips of water and ice chips to settle her stomach — about 2 L by mouth over two days. On Tuesday at 06:00 her sodium was 131; no one acted on it. Since this afternoon she has had a headache, worsening nausea (no vomiting) and "feels foggy."
Post-operative orders:
| Drug | Dose | Route | Frequency | Indication |
|---|---|---|---|---|
| D5 ½NS (dextrose 5% in 0.45% saline) | 125 mL/h | IV | continuous since Monday 11:00 | Maintenance fluid |
| Ketorolac | 30 mg | IV | every 6 h, scheduled | Post-operative pain |
| Hydromorphone | 0.4 mg | IV | every 3 h as needed (6 doses Tuesday, 4 today) | Breakthrough pain |
| Ondansetron | 4 mg | IV | every 8 h as needed | Nausea |
| Sertraline | 100 mg | PO | daily | Generalized anxiety disorder |
Vitals: T 37.2 °C · heart rate 84 lying, 88 standing · BP 128/76 lying, 126/78 standing · RR 16 · SpO₂ 98% on room air · weight 61.4 kg (60.0 before surgery).
Exam: Alert, oriented, slow to answer. Lips dry (she breathes through her mouth); mucous membranes moist. Neck veins normal at 30°. Lungs clear. No edema. Abdomen soft, port sites clean. No focal neurologic signs.
The 20:00 blood draw was hard: the nurse used a hand vein, with the tourniquet on for about three minutes while Ms. Novak pumped her fist. The lab comment reads: "Hemolysis index moderate — potassium may be falsely elevated." ECG at 20:50: sinus rhythm 84/min, normal T waves, QRS 86 ms.
| Test | Result | Reference range |
|---|---|---|
| Sodium | 121 mEq/L | 135–145 |
| Potassium (hemolyzed) | 6.4 mEq/L | 3.5–5.0 |
| Chloride | 89 mEq/L | 98–107 |
| Bicarbonate | 23 mEq/L | 22–28 |
| Anion gap, Na − (Cl + HCO₃) | 9 mEq/L | 8–12 |
| BUN | 8 mg/dL | 7–20 |
| Creatinine | 0.6 mg/dL | 0.6–1.1 |
| Glucose | 128 mg/dL (glucose-corrected Na = 121 + 1.6 × 0.28 ≈ 121.4) | 70–99 |
| Serum osmolality, measured | 250 mOsm/kg | 275–295 |
| Uric acid | 2.6 mg/dL | 2.5–6.0 |
| TSH | 1.9 mIU/L | 0.4–4.5 |
| Cortisol (20:00, post-operative) | 24 µg/dL | 6–23 (08:00) |
| Urine osmolality (resulted 21:40) | 540 mOsm/kg | 50–1,200 |
| Urine sodium (resulted 21:40) | 70 mEq/L | — |
Q3Which branch is she on?
Post-operative day 2: sodium 121, serum osmolality 250, urine osmolality 540, urine sodium 70, BUN 8, uric acid 2.6; no orthostatic change, moist mucosa, normal neck veins, weight up 1.4 kg; TSH and cortisol normal. Which diagnosis fits?
Hypovolemic hyponatremia — two days of nausea and poor eating have left her dry
Hypervolemic hyponatremia — fluid overload from the IV with early heart failure
SIADH — ADH from pain, nausea, opioids and sertraline, with free water to keep
Pseudohyponatremia — a lab artifact from the hemolyzed, fist-pumped blood sample
Show answer and explanation · Link to this question
Correct answer: C
Learning point: Measured serum osmolality confirms hypotonic hyponatremia. Concentrated urine despite hypotonic plasma indicates antidiuretic activity; the examination, urine sodium and low BUN/urate support a postoperative SIADH pattern after assessing thyroid, adrenal and renal function. Hypotonic intake plus pain, nausea and medicines provide plausible contributors. Sodium depends on both water and exchangeable sodium/potassium balance; the available intake and weight data cannot establish an exact retained free-water volume. Why not the others: A — a dry patient saves sodium (urine sodium under 20), runs a high BUN and is orthostatic; B — no edema, normal neck veins, clear lungs; D — the measured osmolality is low, so the hyponatremia is real, and hemolysis distorts potassium, not sodium. Course reading: Evidence for this case
Q4The orders at 21:45
It is 21:45, the urine results are in, and she is symptomatic with a sodium of 121 and a hemolyzed potassium of 6.4 with a normal ECG. Which set of orders is right?
Stop hypotonic fluid and ketorolac; urgent symptom review for 3% saline, close sodium checks, and repeat potassium now
Give 1 L of 0.9% saline over 4 h for poor intake; restart the D5 ½NS after; repeat potassium in the morning
Switch the fluid to D5W at 125 mL/h as she is not eating; calcium gluconate, insulin and dextrose for the 6.4
Keep every order unchanged; recheck sodium and potassium at 06:00, since she is awake and still talking
Show answer and explanation · Link to this question
Correct answer: A
Learning point: Stop the hypotonic fluid and ketorolac, restrict further free water and obtain urgent senior review of the neurologic symptoms. A single 150-mL 3% bolus is guideline-supported for moderately severe symptoms attributable to hyponatremia; severe deterioration requires emergency bolus treatment. Recheck sodium promptly and use the correction limits in Q7. Repeat the hemolyzed potassium immediately: the normal ECG does not exclude hyperkalemia, but this discordant specimen needs rapid confirmation while the patient is monitored. Why not the others: B — saline is for a dry patient, and Q3 showed she is not; restarting the D5 ½NS restarts the cause, and a morning potassium is 8 hours late; C — D5W is pure free water, and treating a presumed artifact without rapid confirmation can add risk; the patient remains monitored during repeat testing; D — eight hours of waiting on a falling, symptomatic sodium is how post-operative seizures happen. Course reading: Evidence for this case
Part 2 — Data
Answers for this section: Q5 · Q6
What happened that night. At 21:15 — before the urine results posted — the night intern, paged about a "critical potassium," wrote: "K 6.4 on a hemolyzed sample — repeat. Na 121 with poor PO intake, nausea and dry lips — likely dehydration." The orders: stop the D5 ½NS, give 1 L of 0.9% saline over 4 hours, and repeat the potassium.
At 22:05 phlebotomy redrew the potassium from the antecubital vein — tourniquet released within a minute, no fist pumping: 4.1 mEq/L, not hemolyzed. The saline finished at 01:15 and the IV was capped; no other IV fluid ran overnight. She sipped about 200 mL of water, roughly what she lost through skin and breath overnight. Ondansetron at 22:00 and 04:00; hydromorphone at 23:30.
05:00 BMP: sodium 119, potassium 4.0, chloride 87, bicarbonate 24 (anion gap 8), BUN 7, creatinine 0.6, glucose 102.
Q5Evidence update
Concordance-style practice: a judgment about how a finding should move your thinking, not a graded test. There is no expert panel behind the key, the teaching explanation. "Unchanged" means the finding does not move the hypothesis — not "I'm not sure."
If you were thinking SIADH, and you then learn that her sodium fell from 121 to 119 overnight after a liter of 0.9% saline, this hypothesis becomes:
Much less likely
Less likely
Unchanged
More likely
Much more likely
Show answer and explanation · Link to this question
Correct answer: D
Formative evidence update: The letter is a teaching anchor. Accept a justified adjacent response; this is not an expert-panel-validated score. Learning point: The fall after saline weakens volume depletion as the main explanation and supports persistent antidiuresis in this context. Isotonic saline can worsen SIADH when sodium and potassium are excreted in urine sufficiently concentrated relative to plasma, leaving retained electrolyte-free water. Total urine osmolality alone cannot quantify that effect because it includes urea; urine sodium plus potassium and measured balances are needed. The observed response is evidence, not a universal diagnostic test. What else could it mean, and what would move you: the fall counts because nothing hypotonic was running — the D5 ½NS was stopped when the saline went up; with the drip still running, a fall would be expected in either diagnosis. "More likely," not "much more likely," because the 21:40 urine studies had already made the case. Course reading: Evidence for this case
07:05. While the day team is rounding, Ms. Novak has a generalized tonic-clonic seizure. The nurse gives IV lorazepam per the seizure protocol, and the seizure stops at 07:08. She is unresponsive to voice and withdraws to pain, breathing on her own; SpO₂ 95% on 2 L. Fingerstick glucose 104. Point-of-care sodium 119.
Q6After the seizure
Ms. Novak (60 kg before surgery), sodium 119; a generalized seizure just stopped with lorazepam, and she remains obtunded; glucose normal. What is the sodium treatment now?
3% saline as a continuous infusion at 1 mL/kg/h until the sodium reaches 130, checked every 6 hours
3% saline 150 mL over 20 minutes; reassess symptoms and sodium after each, up to three boluses total
1 L of 0.9% saline over 30 minutes, because isotonic fluid is safer than hypertonic in a seizure
Fluid restriction to 800 mL a day and a loading dose of an antiseizure drug to prevent recurrence
Show answer and explanation · Link to this question
Correct answer: B
Learning point: A seizure requires emergency hypertonic saline with close monitoring. This case uses the European protocol: 150 mL of 3% saline over 20 minutes, reassess symptoms and sodium, and repeat as needed up to three boluses total to obtain about a 5 mEq/L early rise. Stop rescue boluses when the immediate target is achieved and follow correction limits. A closed-system estimate predicts about 1.9 mEq/L per bolus here, but ongoing urine losses and other fluids can make the actual change very different; measure rather than dose to the formula. Why not the others: A — an infusion run "to 130" is the overcorrection route; C — isotonic saline lowered her sodium overnight; D — restriction works over days, and an antiseizure drug does not shrink a swollen brain. Course reading: Evidence for this case
Part 3 — Course and Decisions
Answers for this section: Q7
07:12 first bolus. 07:30 sodium 121; still drowsy, opening her eyes to voice. 07:35 second bolus. 08:00 sodium 123: awake, oriented to person and place, following commands. No further boluses; she moves to step-down with a sodium every 2 hours. The D5 ½NS stays off, ketorolac and hydromorphone are stopped, scheduled acetaminophen covers the pain, ondansetron covers the nausea, and free water is limited to about 1 L a day. Sertraline is continued: she has taken it for two years with a normal sodium.
10:00 sodium 124. 12:00 sodium 125; the headache is gone, no nausea since 09:00, no opioid needed. 13:00–14:00: urine output 260 mL/h (about 50 mL/h overnight), urine osmolality 110 mOsm/kg. 14:00 sodium 126.
Q7The second danger
At 14:00 her sodium is 126, up 7 from 119 at 07:05. Urine output is 260 mL/h with a urine osmolality of 110. What now?
Keep restricting water to 1 L/day — it is still SIADH — and recheck the sodium at 06:00 tomorrow
Give furosemide 20 mg IV to clear the extra water and bring her sodium above 130 by this evening
Call nephrology now; sodium every 2 h, hourly urine output, lift the water limit, add no sodium
Let the diuresis run — post-operative hyponatremia is acute, so a 12-point rise by morning is safe
Show answer and explanation · Link to this question
Correct answer: C
Learning point: Her hyponatremia has lasted more than 48 hours (131 on Tuesday at 06:00, 49 hours before the seizure), so her brain has begun to adapt and the ceiling applies: no more than 8 mEq/L in 24 hours, never more than 10. She is at +7 with 17 hours to run. With the ADH stimuli gone, dilute urine at 260 mL/h is a water diuresis that can add 10 or more by morning; slowing it (D5W, sometimes desmopressin) is nephrology's call, made before the ceiling is crossed. Why not the others: A — restriction now removes the water that would slow the rise; B — a loop diuretic adds free-water loss to the diuresis; D — the clock, not the operation, decides acute versus chronic, and hers is past 48 hours. Course reading: Evidence for this case
Resolution and Teaching Points
Answers for this section: W1 · W2 · Q3 · Q4 · Q5 · Q6 · Q7
Nephrology replaced her urine output with D5W and gave one dose of desmopressin; the sodium held at 127 at 07:05 on Friday — exactly 8 in 24 hours — then 131 on Saturday and 134 on Sunday, when she went home. She had no neurologic deficit at discharge or at her two-week visit, when her sodium was 138. Sertraline was held during the acute episode. Its restart or replacement was reviewed with the prescriber, considering psychiatric benefit, recurrent hyponatremia risk and early sodium checks; years of prior use do not exclude a contribution during acute illness.
Her chart now carries: postoperative hyponatremia with seizure — reassess fluid and drug orders promptly; monitor electrolytes while IV fluids are needed. Prescribe adult maintenance fluids from weight, oral intake, losses, electrolytes and comorbidity, and stop IV fluids when oral intake suffices. Avoid unchecked hypotonic intake during antidiuresis, but do not teach that all adult maintenance must be isotonic: isotonic saline can also worsen SIADH. The sodium of 131 on Tuesday required reassessment.
Clinical Pearl — The dextrose disappears; the salt decides D5 ½NS is half free water once the sugar is burned. After surgery, pain, nausea, opioids and stress hold ADH on, so the water stays and the sodium falls.
Clinical Pearl — The saline paradox Isotonic saline can lower sodium during persistent antidiuresis. Total urine osmolality alone cannot predict how much; urine sodium plus potassium, plasma sodium and the measured fluid balance matter. A fall after saline supports reconsidering the presumed cause.
Clinical Pearl — Seizure plus hyponatremia means a bolus, then stop Use one local emergency protocol consistently. Here: 150 mL of 3% saline over 20 minutes, reassess after each, up to three boluses total, targeting about a 5 mEq/L early rise before stopping rescue boluses. Measure sodium closely; do not run a drip to an arbitrary normal value.
Clinical Pearl — The second danger arrives when the cause goes away Once the ADH stimuli fade, a water diuresis can push the sodium past the ceiling (8 in 24 hours, never 10) within hours. Dilute urine pouring out is the alarm; nephrology decides on D5W and desmopressin.
Pitfall — "Na 121, dry lips: give a liter of saline" Dry lips in a mouth-breather are not volume depletion. Her steady standing vitals, moist mucosa and BUN of 8 already argued against it, the urine studies 25 minutes later said SIADH, and in SIADH the liter lowered her sodium.
Pitfall — Chasing the potassium The page said "critical potassium." A hemolyzed, fist-pumped 6.4 with normal kidneys and a normal ECG is repeated cleanly, not treated — while the sodium, the real emergency, keeps falling.
Concept Links
Answers for this section: W1 · W2 · Q3 · Q4 · Q5 · Q6 · Q7
Assigned reading: the explanations in this case and the question-linked course evidence guide. The objective map distinguishes tested decisions from concepts taught in explanations.
Concepts: lytes-01, lytes-02, lytes-04, lytes-05, lytes-08, lytes-09, lytes-12, structural-20.