Metabolic Mayhem

ECG Exigency 012

Two quick ECG cases for you!

Case 1

A 30-year old female with a history of Conn’s syndrome secondary to bilateral adrenal adenomas presents with generalised weakness and muscle pains after a change in her medications.

Q1. Describe the ECG.

The ECG shows:

  • Sinus rhythm at around 70 bpm
  • Normal axis
  • PR interval 200ms (upper limit of normal)
  • Long QT (640ms, QTc 700ms)
  • Widespread downsloping ST depression / T wave inversion
  • Prominent U waves, especially in the precordial leads

NB. Note that the long QT in this case is an ‘apparent’ long QT due to fusion of the T wave and U wave (see comments below).

Q2. What is the diagnosis?

  • The combination of a markedly prolonged QT interval, widespread ST depression and T wave inversion with prominent U waves is pathognomonic of severe hypokalaemia.
  • This patient’s potassium was 1.7 mmol/L (she had recently stopped taking her spironalactone).
  • The muscle pain was rhabdomyolysis secondary to hypokalaemia.
  • She was treated in the high-dependency area for 3 days on a continuous potassium drip and made a good recovery.

Full review of the ECG features of hypokalaemia

T wave inversion and prominent U waves in severe hypokalaemia

T wave inversion and prominent U waves in severe hypokalaemia

Case 2

A 24-year old male with type 1 diabetes presents to ED with 2 days of nausea and vomiting.

Q3. Describe the ECG.

The ECG shows:

  • Sinus tachycardia at 120 bpm
  • Inferior axis (+90 degrees)
  • Normal intervals
  • Markedly peaked T waves, particularly in V3-4

Q4. What is the diagnosis?

  • Hyperkalaemia secondary to diabetic ketoacidosis (this patient’s potassium was 8.5 mmol/L)
  • This is his ECG after treatment, by which time the potassium was 5.5 mmol/L:

Full review of the ECG features of hyperkalaemia

Potassium now 5.5, T wave changes resolving

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Comments

  1. says

    Nice cases Ed,
    In case 1 -- is it a long QT or a long QU that you’ve measured?
    My understanding is that when there are TU waves the onset of the U wave should be used as the approximate end of the QT interval. Generally in hypokalemia the actual QT-interval is normal; but the QT-interval appears prolonged because of the presence of the T-U fusion complexes.

    C

  2. says

    Hey Chris,

    You’re absolutely right.

    I was measuring the ‘apparent QT interval’ or QU interval.

    In hypokalaemia, technically the actual QT interval (measured from the Q to the beginning of the U wave) may be normal, while the QU interval (from the Q to the end of the U) is prolonged.

    I usually prefer to look at the QU interval, for two main reasons:

    -- It is often impossible to accurately determine where the T ends and the U begins.
    -- The QU interval may be more relevant as a marker for increased risk of Torsades de Pointes.

    I have added a new ECG to the hypokalaemia page to illustrate my second point.

    (http://lifeinthefastlane.com/ecg-library/basics/hypokalaemia)

    Take a look at the second ECG under the heading: “More Examples of Hypokalaemia”.

    In this patient with a long QU interval and prominent U waves, a PAC kicks off a run of Torsades de Pointes.

    The things I found interesting about this ECG are:

    -- The ‘actual’ QT is not significantly prolonged (QT = 380ms, QTc = 460ms)
    -- However, the QU interval is huge (QU = 540ms, QUc = 654ms)
    -- The PAC seems to fall on the U wave rather than the T wave (i.e. it is ‘R on U’ rather than ‘R on T’ that initiates the run of Torsades).

    My opinion would be that while you can measure both the ‘actual’ QT and the ‘apparent’ QT (=QU), the ‘apparent’ QT may be more clinically useful.

    Ed

  3. Dr Tom says

    Ed and LITFL chums, this site rocks -- It’s time us Brit ED Registrars trainees get some accelerated leaning going on so I’m recommending your site to all my colleagues!

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