ECG criteria
- T wave: tall, peaked, with a narrow base ("tented"), symmetrical — often noticeable in V2–V4, but can be diffuse
- QT interval: may be shortened, especially in the early stages, due to early T-wave termination — a non-constant and non-specific sign; when QRS is wide, QT/QTc measurement is difficult and requires cautious interpretation
- P wave: flattens, decreases in amplitude, and may subsequently become non-registrable on a surface ECG
- PQ interval: may prolong before P-wave disappearance; various degrees of AV block are possible
- QRS complex: may progressively widen; there is no specific validated width threshold for hyperkalaemia, but marked widening (typically ≥120 ms), loss of normal morphology, and dynamic deterioration on serial ECGs are particularly concerning — QRS width alone neither confirms nor rules out hyperkalaemia
- ST segment: can pseudo-elevate in V1–V2 (Brugada-like phenocopy), mimicking STEMI
- Final stage: fusion of the widened QRS and T wave into a sine wave
- Rhythm: depression of automaticity and conduction — ranging from slowing of AV conduction to atrial arrest with sinioventricular conduction, junctional or ventricular escape rhythm; in decompensation — VT, VF, asystole
What it looks like on ECG
Changes in hyperkalaemia roughly increase with potassium levels, but correlation is individual and depends on the rate of potassium rise, underlying heart disease, and other electrolytes (calcium, magnesium). There is no strict staging linked to specific concentrations — the historical reference scheme below is provided and must not be used to rule out risk.
| Potassium, mmol/L (approximate, variable) | Typical ECG changes |
|---|---|
| ~5,5–6,5 | tall, peaked, symmetrical T wave, possible QT shortening |
| ~6,5–7,5 | decreased P-wave amplitude and widening, PQ prolongation |
| ~7,5–9 | disappearance of P wave, QRS widening, possible ST pseudo-elevation in V1–V2 |
| >9 (possibly earlier) | sine wave (fusion of QRS and T), high risk of asystole and VT/VF |
All 12 leads and overall dynamics must be assessed, not just the chest leads: the severity and localization of changes are individual. The T wave in hyperkalaemia is usually narrow at the base and symmetrical — this conventionally differentiates it from the wide, domed T wave in the early stage of myocardial infarction, but the sign is not absolute. The disappearance of the P wave is attributed to atrial myocardium losing excitability before ventricular myocardium: the ECG shows depression or complete disappearance of the P wave, sometimes while sinioventricular conduction is preserved — the sinus node impulse reaches the ventricles via specialized conduction pathways, bypassing normal atrial depolarization. Such a picture may resemble atrial arrest; in severe hyperkalaemia, junctional or ventricular escape rhythms are also possible. With a further rise in potassium, the QRS can widen diffusely, without typical bundle branch block morphology, sometimes with axis deviation and a pseudo-R' in V1.
Mechanism
Potassium determines the resting membrane potential of the cardiomyocyte. In hyperkalaemia, the resting membrane potential becomes less negative (depolarized), which can accelerate the final phase of repolarization — this is associated with the tall, narrow T wave and potential QT shortening. Concurrently, due to partial inactivation of fast sodium channels, the amplitude and rate of rise of the action potential (phase 0) decrease: this can cause slowing of intra-atrial and intraventricular conduction, various degrees of AV block, and depression of automaticity. The specific picture (where exactly and how much conduction slows) depends on the level and rate of potassium increase, not just its absolute value. In extreme hyperkalaemia, ventricular myocardial conduction slows so much and becomes so homogeneous that the QRS and T merge into a sine wave — a precursor to asystole or ventricular fibrillation.
Causes
- acute and chronic kidney injury, reduced GFR
- potassium-sparing diuretics, ACE inhibitors, ARBs, mineralocorticoid receptor antagonists
- rhabdomyolysis, tumor lysis syndrome, massive hemolysis, extensive burns and trauma
- adrenal insufficiency (hypoaldosteronism)
- metabolic acidosis, especially non-organic (mineral) — may promote potassium efflux from cells; the severity of the effect depends on the type of acidosis and renal function
- excessive potassium intake (supplements, salt substitutes, transfusions)
- pseudohyperkalaemia (sample hemolysis, thrombocytosis, prolonged tourniquet use during blood draw)
Differential diagnosis
- tall T wave in hyperkalaemia vs hyperacute T waves in ischaemia — in hyperkalaemia, the T wave is often symmetrical, narrow at the base, diffuse, with possible QT shortening; in ischaemia, the T wave may be wide or narrow, changes typically have territorial dynamics, accompanied by clinical symptoms, ST changes, troponin, and serial ECG data. A normal QT does not rule out ischaemia, and T-wave morphology alone does not reliably differentiate these conditions
- ST pseudo-elevation in V1–V2 in hyperkalaemia vs true STEMI and vs Brugada pattern — suspected ST elevation requires a full ACS assessment and urgent consultation/routing; hyperkalaemia and true infarction can coexist, and potassium correction should not delay reperfusion assessment. Resolution of the pattern after potassium correction supports phenocopy but does not replace ruling out infarction
- QRS widening in hyperkalaemia vs bundle branch block — in hyperkalaemia, QRS morphology is blurred, without clear RBBB/LBBB morphology, and often changes rapidly with treatment
- absence of P wave in hyperkalaemia vs sinoatrial block/sinus node arrest — differentiated by clinical context and laboratory confirmation of potassium levels
- sine wave in severe hyperkalaemia vs ventricular tachycardia — the sine wave pattern in severe hyperkalaemia usually represents the fusion of wide QRS and T waves without a distinct isoelectric line; this is not an independent rhythm diagnosis, and VT can coexist with hyperkalaemia. Differentiation is assessed via 12-lead ECG (heart rate and rhythm regularity, presence and character of repeating QRS complexes) and clinical signs (patient condition, pulse); hyperkalaemia is confirmed by urgent potassium measurement. In an unstable patient and/or suspected hyperkalaemia, treatment is initiated immediately, without waiting for therapy response or laboratory results
Clinical significance and management
Hyperkalaemia is a potentially life-threatening condition: pronounced QRS widening, loss of P wave, and sine wave morphology are alarming signs, but the severity of risk cannot be reliably assessed by ECG alone, and a normal ECG does not rule out significant hyperkalaemia. With characteristic ECG changes or clinical instability, treatment is started immediately, concurrently with blood draws for potassium and blood gas analysis, without waiting for lab results: intravenous calcium for membrane stabilization, followed by measures to shift potassium into cells (insulin with glucose, a β₂-agonist) and remove it from the body; for refractory or severe hyperkalaemia, urgent dialysis consultation. In parallel, continuous ECG monitoring and search for the cause are necessary: renal function, medications, acidosis, other electrolytes.
Common interpretation errors
- mistake a tall T wave for a hyperacute ischaemic T wave and fail to suspect an electrolyte disturbance
- miss the absence of a P wave and interpret the rhythm as junctional or as Wenckebach rhythm/atrial fibrillation without assessing potassium
- confuse ST pseudo-elevation in V1–V2 with a true STEMI or Brugada pattern and fail to perform a full ACS assessment
- evaluate QRS widening solely as intraventricular conduction delay without considering electrolytes
- wait for laboratory confirmation of potassium before initiating treatment in the presence of clear ECG changes or patient instability
1. Which ECG wave changes most characteristically in the early stages of hyperkalaemia?
A tall, peaked, symmetrical T wave is a classic early sign of hyperkalaemia, although it is not universal.
2. What happens to the P wave in severe hyperkalemia?
Atrial myocardium loses excitability earlier than the ventricles; the P wave decreases in amplitude and may no longer be recorded.
3. What explains the appearance of a sinusoidal wave on the ECG in extreme hyperkalemia?
With marked slowing of intraventricular conduction, the QRS widens so much that it fuses with the T wave, forming a sinusoidal wave—a precursor of asystole.
Frequently asked questions
What is the first sign of hyperkalemia on the ECG?
The most classic early sign is a tall, symmetrical T wave with a narrow base, often prominent in the chest leads. It may be absent, and changes in the P or QRS do not have to appear strictly after it—the sequence is variable.
Is hyperkalemia dangerous if the ECG is nearly normal?
Yes, a normal ECG does not rule out significant hyperkalemia. The sensitivity of ECG signs is limited, so management relies on serum potassium levels and clinical presentation, not just the tracing.
How to distinguish a tall T wave in hyperkalemia from hyperacute T waves in myocardial infarction?
In hyperkalemia, the T wave is often narrow at the base, symmetrical, diffuse, with possible QT shortening. In ischemia, changes usually have regional dynamics, are accompanied by clinical symptoms, ST changes, troponin elevation, and serial ECG data; T-wave morphology alone is unreliable for differentiation, and signs may overlap.
Why does the P wave disappear in hyperkalemia?
The atrial myocardium is particularly sensitive to membrane depolarization by potassium and loses excitability earlier than the ventricles. The impulse from the sinus node may still be conducted to the ventricles via specialized pathways (sinoventricular conduction), but atrial depolarization is not recorded on the surface ECG.
What does a sinusoidal curve on the ECG mean in hyperkalemia?
This is the fusion of a markedly widened QRS complex and the T wave into a single broad wave without a clear isoelectric line—a sign of extremely severe hyperkalemia and an immediate precursor to asystole or ventricular fibrillation. It is distinguished from ventricular tachycardia by the rate and regularity of the rhythm, the presence of repeating QRS complexes, and the clinical picture, and treatment is initiated immediately without waiting for laboratory confirmation.
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Related terms
More in "Electrolytes and Metabolism"
Sources
- Goldberger's Clinical Electrocardiography, current edition
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- Orlov V.N. Guide to Electrocardiography
- Russian Ministry of Health clinical guidelines on rhythm disorders and electrolyte disturbances
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.