ECG on call

An ECG on call answers one question first: does this change what I do in the next ten minutes? The full reading method is on approach to ECG interpretation. This page is the short version for 3 am.

• Unstable → hypotension or shock, chest pain, acute pulmonary edema, new confusion or a drop in consciousness, with a rate above 150 or below 50. Call for help and bring the pads before you finish reading. ACLS resuscitation
• Check the trace → right patient, time printed, 25 mm/s and 10 mm/mV. Inverted P, QRS and T in lead I with an upright aVR and normal chest leads is arm-lead reversal, not a new infarct. Repeat it.
• Find the old one → most on-call ECG questions are “is this new?”. Nothing on this page replaces the comparison.

Six questions, in this order. Each links to the full method.

• Rate → above 100 or below 50? 300 divided by the large squares between two R waves. rate
• Regular? → irregularly irregular with no P waves is atrial fibrillation until proven otherwise. rhythm
• P before every QRS? → dropped beats, or P waves marching through with no relation to the QRS, is a block. rhythm
• QRS narrow or wide? → wide is 120 ms or more (three small squares). A wide regular tachycardia is VT until proven otherwise. QRS
• ST up or down? → elevation in two neighbouring leads with reciprocal depression is an infarct until proven otherwise; depression or new T inversion with symptoms is ischemia. ST / T
• QT long? → a QTc of 500 ms or more is the number that changes tonight’s drug chart. intervals

Pattern, what it means, what to do, and the sheet that holds the treatment.

• ST elevation in two contiguous leads, with chest pain → STEMI. Call cardiology and activate the cath lab now, aspirin now, do not wait for the troponin. ST depression in V1 to V3 with tall R waves: ask for posterior leads. New LBBB with ongoing chest pain: discuss with cardiology as a possible STEMI equivalent, never dismiss it. chest pain
• Peaked T waves, flat P waves, widening QRS → hyperkalemia until the result says otherwise. Calcium first, then shift the potassium, then repeat the ECG. potassium
• Wide regular tachycardia → VT until proven otherwise. Unstable: synchronized cardioversion. Stable: cardiology now; no verapamil, no diltiazem. tachycardia
• Narrow regular tachycardia near 150 → SVT or atrial flutter: vagal maneuvers, then adenosine with the rhythm strip running. Flutter at 2:1 often hides as “sinus at 150”. tachycardia
• Irregular narrow tachycardia → atrial fibrillation: rate control if stable, cardioversion if unstable, and ask why it is fast tonight (sepsis, bleeding, PE, pain). palpitations
• Irregular wide tachycardia with a delta wave → pre-excited AF. No adenosine, no verapamil, no diltiazem, no beta-blocker, no digoxin. Unstable: cardioversion. Stable: cardiology now. palpitations
• Rate below 50 with symptoms, Mobitz II, or complete heart block → atropine, pads on, pacing ready, cardiology now. Stop the rate-slowing drugs. bradycardia
• QTc 500 ms or more, or polymorphic VT → stop every QT-prolonging drug (antiemetics, antipsychotics, macrolides, fluoroquinolones), replace potassium and magnesium, magnesium IV for torsades, continuous monitoring. magnesium
• ST depression or new T inversion with symptoms → NSTE-ACS pathway: troponin now and repeated, aspirin, cardiology. troponin
• U waves, flat T waves, ST depression → low potassium, often with low magnesium. potassium
• Diffuse concave ST elevation with PR depression → pericarditis more than infarct, but with chest pain the call belongs to cardiology, not to you alone. chest pain
• Sinus tachycardia → a sign, not a diagnosis. Treat the cause: fever, pain, hypovolemia, bleeding, PE, hypoxia. tachycardia

• Old ECG → a change is what matters: new T inversion, new block, new axis. An old abnormality is reassuring; a missing old ECG is not.
• Symptoms → the same ECG is a different document with chest pain than on a routine check.
• Paced rhythm → pacing spikes make ST analysis unreliable. Go on symptoms and troponin, and ask cardiology.
• Artifact → “VT” with the patient chatting is usually tremor or a loose lead. Look at the patient, repeat the trace.
• Not sure → send a photo of this ECG and the old one to the registrar on call. A second opinion costs nothing at 3 am.

One line in the note, in the order you read it.

• Rate, rhythm, axis → “sinus 88, normal axis”.
• Intervals → PR, QRS and QTc, with the numbers.
• ST and T → where, and compared with the ECG of which date.
• Impression and action → “no acute change; troponin sent; repeat ECG if the pain returns”.

Say what you compared it with. “Unchanged from admission” is a finding. “Normal ECG” on a known LBBB is not.

Reviewed October 2026 against the 2020 AHA Guidelines for CPR and Emergency Cardiovascular Care, Adult Advanced Life Support (Panchal et al., Circulation 2020); the 2025 ACC/AHA/ACEP/NAEMSP/SCAI Guideline for the Management of Patients With Acute Coronary Syndromes (Rao et al., Circulation 2025); the 2018 ACC/AHA/HRS Guideline on Bradycardia and Cardiac Conduction Delay (Kusumoto et al., Circulation 2019); the 2015 ACC/AHA/HRS Guideline for Adult Patients With Supraventricular Tachycardia (Page et al., Circulation 2016); and the Fourth Universal Definition of Myocardial Infarction (Thygesen et al., Circulation 2018). Doses are not repeated here; they are on the linked sheets.