Question: What electrolyte abnormality is associated with these prominent U waves?

1. Hypernatremia
2. Hyperkalemia
3. Hyponatremia
4. Hypokalemia ✅

Answer: Moderate to severe hypokalemia is classically associated with prominent U waves and a prolonged QU interval — the interval from the start of the QRS interval to the end of the U wave — that is longer than ½ the distance between adjacent QRS complexes.

This patient is suffering from critical hypokalemia secondary to heat exhaustion. He is likely hypovolemic due to volume loss from excessive sweating. Along with water loss, heavy sweating also results in salt loss and can lead to low potassium levels in the blood. Incomplete electrolyte repletion throughout the week has allowed potassium levels to drop progressively. If the patient has been drinking water only and not consuming adequate amounts of salt-containing foods, the body will be unable to replace the lost potassium. The prominent U waves and prolonged QU interval are ECG signs of hypokalemia.

Treatment per protocol 1222 should involve cooling the patient and, in this case, volume expansion with normal saline to help replete the lost water and salt. Although potassium repletion will be needed, LA County EMS crews do not carry potassium-containing fluids; this will be initiated upon arrival at the emergency department.

While you are treating the patient, he complains of dizziness and says he thinks he is going to pass out again. You capture an ECG and obtain the following result:

Question: What rhythm seen in this ECG may be precipitated by severe hypokalemia?

1. Atrial fibrillation with RVR
2. Sinus tachycardia
3. Torsades de Pointes ✅
4. Ventricular fibrillation

Answer: Torsades de Pointes (TdP) is a form of polymorphic ventricular tachycardia caused by a prolonged QT interval. In this case, a premature atrial contraction likely led to R-on-T phenomenon (where a premature beat occurs while the heart is depolarizing from the last heartbeat, landing on the T wave of the preceding beat), triggering TdP. The characteristic morphology of the QRS complex twisting around the isoelectric line is dramatically seen in the following example:

You recognize this as TdP and immediately perform synchronized cardioversion at 120 J due to the patient being hypotensive and lacking IV access. The patient converts back to sinus rhythm and is transported to the hospital in stable condition.

After base contact is made, the patient is transported to the MAR where he is diagnosed with heat exhaustion and severe hypokalemia. His serum potassium level is 1.9, well below the normal range of 3.5-5.

Torsades de Pointes is treated in the hospital with magnesium infusion and defibrillation if the patient is pulseless, unstable, or the rhythm does not resolve. Although defibrillating a patient with a pulse may seem unusual, the polymorphic nature of the VT makes true synchronization impossible, so defibrillation is the appropriate electrical therapy.

Magnesium is believed to stabilize the heart’s electrical activity, though the exact mechanism remains unclear. [1]

• Excessive exertion and sweating without appropriate repletion may lead to dangerous electrolyte abnormalities due to salt wasting.
• Prominent U waves and prolonged QU interval are classic indicators of hypokalemia.
• Torsades de Pointes (TdP) is a potentially deadly rhythm that can result from critical hypokalemia. It is treated via protocol 1213.