Situation: A 61-year-old woman with type 2 diabetes and hypertension takes losartan 100 mg and spironolactone 25 mg daily and recently began using a salt substitute. She comes to the emergency room with generalized weakness and tingling in her legs. Her serum potassium is 7.0 mEq/L (3.5–5.0 mEq/L) on a non-hemolyzed sample, serum creatinine 1.0 mg/dL (88 µmol/L), capillary blood glucose 142 mg/dL (7.9 mmol/L), and her 12-lead electrocardiogram (ECG) shows tall, peaked T waves. She is on a cardiac monitor. While the first medication is being prepared, the nurse watches the monitor. Which change would show that her hyperkalemia is progressing?

Hyperkalemia changes the ECG in a typical sequence: peaked T waves, then a prolonged PR interval with flattened or absent P waves, then a wide QRS, a sine-wave pattern, and ventricular fibrillation or asystole. Widening QRS with disappearing P waves means she is moving toward cardiac arrest.

Why this patient is at risk
This patient has three simultaneous triggers for hyperkalemia: losartan, spironolactone, and a salt substitute. Losartan blocks the renin–angiotensin–aldosterone system, which reduces renal potassium excretion. Spironolactone is a potassium-sparing diuretic that directly antagonizes aldosterone in the collecting duct. Many salt substitutes replace sodium chloride with potassium chloride, adding a large exogenous potassium load. The result is a serum potassium of 7.0 mEq/L on a non-hemolyzed sample, with classic ECG findings of tall, peaked T waves.

Hyperkalemia does not depolarize all cardiac cells uniformly; it initially speeds repolarization, producing peaked T waves, then progressively slows conduction through the atria, AV node, and ventricles. The ECG changes follow a predictable sequence that mirrors worsening membrane excitability.

ECG progression in hyperkalemia
The earliest and most recognized change is tall, peaked T waves, especially in the precordial leads ^[1]. As potassium rises further, atrial conduction slows first. The P wave becomes flattened, then disappears entirely, while the PR interval lengthens. This reflects the atria becoming electrically silenced before the ventricles.

After P wave loss, the QRS complex begins to widen. A widening QRS with flattening or absent P waves indicates that the hyperkalemia is now impairing ventricular conduction and is progressing toward a sine-wave pattern, ventricular fibrillation, or asystole. This is the point at which the patient is moving toward cardiac arrest ^[4].

ECG finding	Potassium level context	Clinical meaning
Tall, peaked T waves	Early hyperkalemia, often 5.5–6.5 mEq/L	Accelerated repolarization; earliest warning sign
P wave flattening or loss, prolonged PR	Moderate hyperkalemia, often 6.5–7.5 mEq/L	Atrial conduction suppressed; AV conduction slowing
Widening QRS complex	Severe hyperkalemia, often >7.0 mEq/L	Ventricular conduction impaired; pre-arrest rhythm
Sine-wave pattern	Critical hyperkalemia	Imminent ventricular fibrillation or asystole

Watch out! A widened QRS in hyperkalemia can be mistaken for a bundle branch block or ventricular tachycardia. The key discriminator is the accompanying loss of P waves and the clinical context of known hyperkalemia.

Key point! The progression from peaked T waves to P wave loss to QRS widening is not a laboratory diagnosis; it is a bedside monitor finding that tells the nurse the patient is deteriorating before repeat labs return.

Why the other options are incorrect
Flattened T waves with new U waves describe hypokalemia, not hyperkalemia. ST-segment depression with a shorter QT interval is not part of the hyperkalemia sequence; hyperkalemia typically shortens the QT interval early, but ST depression is not a defining feature. Progressive lengthening of the QT interval is more characteristic of hypocalcemia or certain drug toxicities, not hyperkalemia. The correct answer is the widening QRS with flattening P waves, because it represents the transition from atrial silencing to ventricular conduction failure ^[1][4].

Nursing implications at the bedside
While the first medication is being prepared, the nurse must continuously observe the monitor for QRS widening. This is the moment to notify the provider immediately and prepare for more aggressive interventions, because the ECG can reveal dangerous progression long before a repeat serum potassium is available . A patient with a potassium of 7.0 mEq/L and widening QRS complexes is in a peri-arrest state and requires rapid treatment with calcium to stabilize the myocardium, followed by potassium-shifting and potassium-removing therapies.