CONTENTS
key features of hypokalemia
- 🔑 Prolonged Q-TU interval (often the most obvious feature).
- QT may be normal in aVL and/or aVR, which supports the presence of a U-wave. (Berberian 2021)
- 🔑 Two main morphologic variations may be seen (without correlation to K level):
- [1] Prominent U-waves & T-wave flattening ➡️ bifid, complex TU-waves.
- [2] Diffuse downsloping/scooped STD may produce a down-up morphology (often greatest in the left precordial leads).
- 🔑 Prominent, peaked P-waves in inferior leads.
arrhythmias related to hypokalemia
- Hypokalemia increases automaticity & reduces AV conduction, similar to digoxin. This may promote a range of arrhythmias, including:
- [1] Heart blocks:
- 1st degree AV block.
- Mobitz I.
- AV dissociation.
- [2] Torsade de pointes.
- [3] Increased automaticity:
- PACs.
- PVCs.
- [4] Sustained tachyarrhythmias:
- Atrial fibrillation or atrial flutter.
- Paroxysmal atrial tachycardia (especially with AV block).
- Ventricular tachycardia.
- Ventricular fibrillation.
intervals
- Increased QT/QU prolongation (a marker for Torsades de Pointes).
- Other intervals can also increase:
- PR prolongation.
- QRS widening (seldom >200 ms; tends to preserve morphology).
prominent P-waves
- It may mimic RAA.
down-up morphology
- (i) ST depression
- Downsloping “scooped” ST depression can occur.
- It may cause a pattern of diffuse STD with STE in aVR.
- (ii) T-wave flattening & sometimes inversion
- (iii) Prominent U-wave
- It may be larger than the T-wave (creating a bifid appearance).
- Subtle U-waves can be detected by causing distortions in the “T-wave” (which actually represents a T-U fusion wave). When in doubt, scour all the leads for this.
related differential algorithms
- ECG findings:
- Diagnostic algorithms:
- Posterior MI 📖
key features of digoxin
- 🔑 Scooped ST depression (which may cause a down-up T-wave configuration).
- 🔑 Short QT interval. 📖
- 🔑 Unusual rhythms 2/2 increased automaticity & AV blockade:
- Accelerated junctional rhythm (+/- AF).
- Ectopic atrial tachycardia with block.
- AF with slow ventricular rate.
- AF with 3rd-degree block.
arrhythmias
- Bradyarrhythmias and blocks:
- Sinus bradycardia, sinus pauses, or sinus arrest.
- 2nd or 3rd-degree AV block.
- Tachyarrhythmias:
- PACs, MAT, AF.
- PVCs or ventricular bigeminy are often the first signs of digoxin toxicity. (de Luna 2022)
- Accelerated junctional rhythm or junctional tachycardia highly suggest digoxin (usually 70-130 b/m).
- Accelerated idioventricular rhythm (AIVR).
- Monomorphic VT.
- Bidirectional VT. 📖
- Tachyarrhythmia plus block:
- Focal atrial tachycardia with a block (atrial rate usually 150-200 b/m, variable AV block with Mobitz I is most common).
- AFib with slow ventricular rate.
- 3rd-degree heart block plus accelerated junctional or ventricular rate.
- Regularized AF:
- AF + 3rd degree block + accelerated junctional rhythm.
- This creates a regular rhythm in a patient with chronic AF.
- Almost exclusively seen with digoxin.
intervals
- PR prolonged (vagotonic effect).
- QRS: Rarely causes bundle branch block.
- QTc is shortened 📖 (although it may appear prolonged due to a prominent U wave).
morphology
- [1] Sagging STD with upward concavity and depressed J point (in leads with tall R wave).
- This will occur diffusely and may mimic diffuse STD with STE in aVR pattern. 📖
- [2] Flattened or inverted T-wave:
- The first part of the T-wave is “dragged down” by the depressed ST segment. Initially, this may produce a biphasic T-wave (initially negative and later on positive). As the STD becomes more prominent, the entire T-wave may be inverted.
- TWI may be sharp, resembling MI or pericarditis
- [3] Prominent or inverted U-wave
- Digitalis effect reveals the presence of digoxin, but doesn't correlate with clinical digoxin toxicity. So this may be helpful to identify patients with exposure to cardiac glycosides – but it is otherwise nonspecific (especially in a patient who is known to be on digoxin).
related differential algorithms
- ECG findings:
- Diagnostic algorithms:
- Posterior MI 📖
- 🔑 Primary finding = short ST segment:
- The T-wave may appear to take off directly from the QRS complex.
- This can create the appearance of domed STE, mimicking a myocardial infarction (but a short QT interval may help differentiate this from MI).
- 🔑 Short QT interval:
- QT may be low (<360 ms).
- However, severe hyperkalemia can prolong the T-wave, with a neutral effect on the QT interval.
- Other features that may occur:
- Prominent U-waves.
- Osborne waves.
differential diagnosis of hypercalcemia ECG
- [1] Hypercalcemia can mimic ischemic STE.
- Hypercalcemia may be favored by a symmetric, domed appearance without any ST segment.
- [2] Digoxin can also shorten QT interval and appear grossly similar.
- [3] Other causes of QT shortening (including catecholamine use or acidosis; see the next section).
- ⚠️QT shortening alone doesn't establish the diagnosis of hypercalcemia. This can be a nonspecific feature seen in extremely critically ill patients.
causes of QT<360 ms (9 boxes)
[#1/4] hypercalcemia
- Shortened ST segment.
- (Discussed in the section directly above ☝)
[#2/4] digoxin ⚡️
- 🔑 Scooped ST depression (which may cause a down-up T-wave configuration).
- 🔑 Short QT interval. 📖
- 🔑 Unusual rhythms 2/2 increased automaticity & AV blockade:
- Accelerated junctional rhythm (+/- AF).
- Ectopic atrial tach with block.
- AF with slow ventricular rate.
- AF with 3rd-degree block.
[#3/4] other acquired causes
- Hyperthermia.
- Hyperthyroidism.
- Hyperkalemia.
- Catecholamine effect.
- Androgen use.
- Acidosis. (O'Keefe 2021, Sadhu 2023)
[#4/4] short QT syndrome
ECG findings:
- Short QT (<360 mS).
- T-waves are usually peaked and tall, especially in V2-V3. (de Luna 2022)
- VT/VF may occur.
rough clinical criteria
- (1) QTc <330 ms (M) or <340 ms (F), regardless of symptoms.
- (2) QTc <360 ms (M) or <370 ms (F) plus a history of arrest, syncope, or atrial fibrillation at an early age.
clinical implications
- Extremely rare.
- May atrial fibrillation and/or ventricular tachyarrhythmias (including sudden death).
- ICD is the treatment of choice.
U-waves
- U-waves are usually most prominent in V2-V3 & perhaps lead II (but can occur in any lead).
- U-waves are generally concordant with the T-wave and <25% of the T-wave height.
- U-waves must be distinct from the preceding T-wave, with a brief isoelectric baseline after the preceding T-wave (which may be best seen in Leads II and V5). If there is never an isoelectric separation from the T-wave, then it's not clearly a U-wave (it may be best to refer to this as a complex T-wave). (O'Keefe 2021)
prominent U-wave
definition of prominent U-waves
- Most common definition: >1.5 mm. (36907158; O'Keefe 2021)
- >25% of the height of the adjacent T-wave is also probably abnormal.
more common causes of prominent U-waves
- Electrolyte abnormalities (can cause U-wave alternans):
- Hypokalemia (unique in that the U-wave may be larger than the T-wave).
- Hypomagnesemia.
- Hypercalcemia.
- Medications:
- Digoxin.
- Catecholamines.
- Class IA or class III antiarrhythmics and related medications with sodium-channel blocker properties (e.g., tricyclic antidepressants; phenothiazines).
- Methadone. (18956527)
- Coronary artery disease: Posterior ischemia (mirror image of an inverted U-wave).
- LVH.
- Early repolarization.
- Bradyarrhythmias (U-wave size is generally inversely related to the heart rate). (O'Keefe 2021)
- Rare causes:
- Thyrotoxicosis.
- CNS events (prominent U and T waves, with a normal ratio).
- Hypothermia.
- Post-exercise.
- Congenital long-QT syndrome.
- Forced inspiration. (36907158)
inverted U-wave
definition of pathological U-wave inversion
- U-waves may normally be inverted in aVR, III, or aVF. They should be upright in other leads (and concordant with the T-wave).
causes of pathological U-wave inversion
- Myocardial infarction:
- It is best seen in V4-V6, where it is an insensitive but relatively specific marker of LAD disease.
- Negative U-waves in the right precordial leads in the presence of chest pain are highly suggestive of LAD occlusion. (de Luna 2022)
- It may be biphasic (positive –> negative).
- LVH.
- Severe RVH may cause U-wave inversion in the right precordial leads.
- Pressure overload: Uncontrolled hypertension:
- U-waves are usually biphasic (negative –> positive).
- Volume overload (including due to valvular heart disease).
- Hyperthyroidism.
findings
- Increased QTc due to a prolonged ST segment.
- The only other things that extend the ST segment are hypothermia or long QT syndrome type 3.
- Pure hypocalcemia usually doesn't affect the T-wave.
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References
- 18956527 Athanasos P, Farquharson AL, Compton P, Psaltis P, Hay J. Electrocardiogram characteristics of methadone and buprenorphine maintained subjects. J Addict Dis. 2008;27(3):31-5. doi: 10.1080/10550880802122596 [PubMed]
- Gaggin, H. K., & Januzzi, J. L., Jr. (2021). MGH Cardiology Board Review. Springer Science & Business Media.
- 36907158 Kihlgren M, Almqvist C, Amankhani F, Jonasson L, Norman C, Perez M, Ebrahimi A, Gottfridsson C. The U-wave: A remaining enigma of the electrocardiogram. J Electrocardiol. 2023 Jul-Aug;79:13-20. doi: 10.1016/j.jelectrocard.2023.03.001 [PubMed]
