Fluctuating hypokalemia with late-onset hypomagnesemia and evolving acid–base disturbance: a case report

The Body's Tug-of-War: A Story of Lost Salts and Shifting Gears

Imagine your body is a bustling city where electricity keeps the lights on and the traffic moving. In this city, tiny charged particles called electrolytes are the workers keeping everything running. Two of the most important workers are Potassium and Magnesium. Potassium is like the main power grid; without enough of it, the heart's rhythm can stumble, and muscles can go limp. Magnesium is the foreman that helps the power grid stay stable; if the foreman goes on strike, the power grid becomes difficult to fix, even with extra electricity.

Sometimes, the body loses these workers through the gut (like when you're vomiting) or the kidneys. When the levels drop too low, it's a medical emergency. But here's the tricky part: the body is a complex machine, not a simple bucket. Sometimes, fixing one problem reveals another, or the levels bounce around like a yo-yo before they finally settle. Doctors have to be detectives, watching how the body's chemistry changes hour by hour to figure out where the workers are hiding and why they won't stay put. This story is about a young woman who taught us that sometimes, you can't just pour in the missing pieces; you have to fix the foreman first.

---

The Case of the Bouncing Potassium

This paper tells the story of a 20-year-old woman who arrived at a hospital on March 21, 2026, feeling terrible. She had been vomiting for days, was extremely weak, and nearly passed out. When she walked in, her blood pressure was dangerously low at 80/40 mmHg, and her heart was racing—a sign of hypovolemic shock, which basically means her body had lost so much fluid that it was running on empty.

The first lab test revealed a scary problem: her potassium level was only 1.7 mmol/L. For context, a normal level is usually much higher. This is a critical situation because such low potassium can stop the heart. The doctors also saw that her blood was acidic (a metabolic acidosis), with a bicarbonate level of 10.1 mmol/L. At first, the doctors thought her kidneys might be the problem, specifically a condition called distal renal tubular acidosis, where the kidneys leak too much acid and salt.

But the story didn't go in a straight line. As the medical team treated her, her potassium levels started to bounce.

• On the morning of March 22, it was still 1.7 mmol/L.
• By the evening, it climbed to 2.1 mmol/L.
• The next morning, it jumped to 2.7 mmol/L, and by evening, it looked even better at 3.3 mmol/L.

It seemed like they were winning the battle. But then, on March 24, the potassium dropped again to 2.9 mmol/L, and then rose to 3.4 mmol/L. It was fluctuating, refusing to stay stable.

The Hidden Foreman: Magnesium

The turning point came on the evening of March 24. The doctors finally checked her magnesium levels and found they were low at 1.3 mg/dL. This was the "late-onset" discovery. The paper suggests that this low magnesium was the reason her potassium kept bouncing. Think of magnesium as the glue that holds potassium in the cells. Without enough magnesium, the body leaks potassium no matter how much you try to give it back, and it can impair the ability to stabilize potassium levels.

Once they realized this, they corrected the magnesium. By March 25, her magnesium was up to 2.3 mg/dL, and for the first time, her potassium stopped its wild dance and finally stabilized.

The Acid-Base Rollercoaster

While the salts were bouncing, her blood chemistry was also shifting gears.

• Start: On March 21, her blood was acidic (pH 7.405, but with a very low bicarbonate of 10.1 mmol/L and low CO2 of 16.1 mmHg). This was a classic sign of metabolic acidosis, where the body was compensating by breathing fast to blow off acid.
• End: By March 26, the picture had changed. Her pH had flipped to 7.524 (alkalemia), her bicarbonate was 14.4 mmol/L, and her CO2 had dropped even further to 17.9 mmHg.

The authors explain that this wasn't just one problem fixing itself; it was a mixed acid-base disorder. She still had some metabolic acidosis (low bicarbonate), but she had also developed respiratory alkalosis (breathing too fast, blowing off too much CO2). This suggests her body was adapting and over-correcting as she recovered.

What the Paper Rules Out and Confirms

The authors used a clever trick to figure out the root cause. They looked at her urine. If her kidneys were the problem (leaking magnesium), her urine would be full of magnesium. Instead, her urine magnesium was very low at 1.2 mg/dL. This confirmed that her kidneys were actually doing their job and trying to save magnesium. The problem wasn't the kidneys; it was gastrointestinal losses (her vomiting and constipation) that were washing the salts away.

The paper also notes that her hemoglobin (the protein that carries oxygen in blood) dropped from 12.5 g/dL on admission to 8.6 g/dL by March 26. The authors suggest this wasn't because she was bleeding, but because her blood was getting diluted as she received fluids and her body was fighting a systemic illness.

The Takeaway

This case report highlights a few key lessons for anyone dealing with complex electrolyte issues:

1. Potassium doesn't always fix itself linearly. It can fluctuate wildly before it settles.
2. Magnesium is the gatekeeper. You may struggle to fix low potassium if you don't address the low magnesium first. The paper suggests that late-onset hypomagnesemia can impair potassium stabilization and coincided with the difficulty in maintaining stable levels.
3. Watch the whole picture. The acid-base balance (pH, CO2, bicarbonate) can change from a simple acidosis to a complex mixed disorder as the patient recovers.

The authors conclude that while severe hypokalemia is dangerous, it often requires a detective's eye to spot the hidden magnesium deficiency and the shifting acid-base patterns. They didn't just find a number; they found the pattern of recovery, highlighting the importance of serial monitoring—checking the numbers again and again—when the body is in a tug-of-war.