A 48-year-old man presents to the emergency department with generalized weakness and muscle cramps. He reports a recent history of diarrhea for three days. Laboratory investigations reveal a serum potassium level of 2.8 mmol/L. An ECG is performed.
Which of the following ECG findings is most indicative of hypokalemia?
A. Prolonged QT interval
B. Narrow QT interval
C. Absence of P wave
D. Elevation of ST segment
E. All of the above
Correct Answer: A. Prolonged QT interval
Explanation:
Hypokalemia affects cardiac conduction by altering the resting membrane potential and repolarization phase of myocardial cells. This leads to characteristic ECG changes that can help confirm the diagnosis.
- Prolonged QT Interval:
- Hypokalemia causes delayed ventricular repolarization, which manifests as a prolonged QT interval on the ECG. This is due to the lengthening of the T wave’s duration.
- The prolonged QT interval in hypokalemia is primarily due to flattening or inversion of the T wave and the appearance of prominent U waves, which may be mistaken as a continuation of the T wave.
- Prolonged QT intervals increase the risk of ventricular arrhythmias, such as Torsades de Pointes, making early recognition and correction of hypokalemia critical.
Other Common ECG Findings in Hypokalemia:
- Flattened or Inverted T Waves:
- The earliest ECG sign of hypokalemia is flattening or inversion of the T wave due to delayed repolarization.
- This finding is more prominent in leads V2–V4.
- Prominent U Waves:
- U waves are positive deflections following the T wave, most apparent in precordial leads.
- Their presence is highly specific for hypokalemia.
- ST Depression:
- Hypokalemia can cause mild ST-segment depression, further contributing to repolarization abnormalities.
- Arrhythmias:
- Severe hypokalemia can lead to atrial or ventricular arrhythmias, including atrial fibrillation, premature ventricular contractions, and ventricular tachycardia.
Analysis of Incorrect Options:
B. Narrow QT Interval:
- Hypokalemia prolongs the QT interval rather than narrowing it, making this option incorrect.
C. Absence of P Wave:
- Hypokalemia does not typically cause the absence of P waves. This finding may be associated with conditions like atrial fibrillation or hyperkalemia.
D. Elevation of ST Segment:
- ST-segment elevation is not a feature of hypokalemia. It is more commonly seen in conditions like acute myocardial infarction or pericarditis.
E. All of the above:
- Not all the listed findings are associated with hypokalemia, so this option is incorrect.
Clinical Context and Management:
Hypokalemia is defined as a serum potassium level below 3.5 mmol/L. It can result from:
- Gastrointestinal Losses: Diarrhea or vomiting.
- Renal Losses: Diuretic use or hyperaldosteronism.
- Shift into Cells: Alkalosis or insulin administration.
Symptoms:
- Muscle cramps, weakness, and, in severe cases, paralysis.
- Cardiac manifestations include arrhythmias and conduction abnormalities.
Management:
- Potassium replacement is the mainstay of treatment, either orally or intravenously, depending on severity.
- Continuous ECG monitoring is essential during IV potassium administration to prevent hyperkalemia.
Summary:
ECG findings in hypokalemia are critical for diagnosis and management. Key changes include a prolonged QT interval, flattened or inverted T waves, and prominent U waves, which reflect delayed repolarization. Early recognition is vital to prevent life-threatening arrhythmias. Hypokalemia is often caused by gastrointestinal losses, renal disorders, or intracellular shifts. Management includes potassium supplementation and addressing the underlying cause. Understanding these ECG features aids in timely diagnosis and effective treatment of this electrolyte imbalance.