ECG examples

Tachycardias can be classified by wide vs narrow, irregular vs regular, and then presence vs absence of atrial activity. Sinus rhythm has P waves that are upright in lead II and biphasic in V1; atrial flutter has flutter waves (at a rate of around 300) that are typically inverted in lead II and fully upright in V1, but atypical flutter may have different morphologies

Tachycardias may be primary electrical problems (eg SVT) or have secondary causes (eg sepsis driving AF with rapid ventricular response)

Bradycardias can be classified by wide vs narrow, irregular vs regular, and then presence vs absence of atrial activity (with or without AV block). They may be primary bradyarrhythmias (eg 3rd degree AV block) or have secondary causes (eg bradycardial secondary to inferior STEMI or hyperkalemia)

AV blocks include nodal (first degree with long PR, second degree type 1 with PR prolongation before drop), or infranodal (second degree type 2 with fixed PR before drop, third degree with AV dissociation). They may be primary electrical problems requiring pacemaker (eg third degree AV block), secondary to a reversible cause, or physiologic (eg first degre AV block may be normal, or secondary to inferior MI or hyperkalemia)

Right bundle branch block (RBBB) causes slow conduction towards the right (wide second R wave in V1) and away from the left (wide S wave in I/V6). The anterior leads with rsR' have secondary, discordant and proportional ST/T, ie ST depression and R wave inversion proportional to the QRS

Acute coronary occlusion has traditionally been defined by presence or absence of ST elevation. But there can be ST elevation without occlusion (eg LVH, early repolarization, pericarditis, LBBB), or occlusion without ST elevation. Systematic interpretation can identify false positive STEMI or subtle occlusion MI: 

• Heart rate/rhythm: 

  • tachycardia rare unless cardiogenic shock 

  • bradycardia common with inferior MI (SA node perfused by RCA or circumflex) 

  • ischemic rhythms include polymorphic VT or AIVR

• Electrical conduction: 

  • PR: RCA occlusion can produce AV blocks that are usually transient

  • LBBB is not a STEMI equivalent unless Modified Sgarbossa Criteria (concordant ST elevation, concordant ST depression V1-3, or excessive discordant ST elevation > 25% of the S wave)

  • RBBB can be acutely caused by proximal LAD occlusion, often with left anterior hemiblock

• Axis:

• R-wave: 

  • anterior MI can lead to loss of R waves or Q waves; 

  • posterior MI can lead to early R waves

• Tall/small voltages: 

  • LVH or early repolarization can have tall voltages with proportional ST segments

• ST/T: 

  • occlusion MI can have regional ST elevation and hyperacute T waves (wide, inflated, large relative to QRS), and reciprocal ST depression and T wave inversion

  • non-occlusive causes of ST/T changes include pericarditis (diffuse concave ST elevation without any reciprocal change except aVR); Brugada (coved or saddle back ST elevation V1-2); hyperkalemia (diffuse narow and peaked T waves)

Acute pulmonary embolism may not cause any ECG changes. But the larger the PE the more likely it will produce acute RV strain, which can cause a variety of ECG changes:

• H: sinus tach or new atrial fibrillation

• E: new RBBB

• A: S1 or right axis

• R: delay R wave

• T: no changes to voltages

• S: primary antero-inferior T wave inversion.

Depending on its severity, rheumatic heart disease can have multiple ECG changes: aortic regurgitation can result in left ventricular hypertrophy, mitral stenosis or regurgitation can result in left atrial enlargement with atrial fibrillation/flutter, and pulmonary hypertension resulting in right ventricular hypertrophy

Left ventricular hypertrophy (LVH) produces tall voltages towards the left, with discordant and proportional ST/T change: I/aVL/V5-6 have tall R waves with ST depression/T wave inversion, and the mirror image is V1-3 have deep S waves with ST elevation and tall T waves

Hyperkalemia can cause multiple changes, each of which is not sensitive or specific. But in a patient with high pre-test likelihood of hyperkalemia (eg chronic kidney disease, with dehydration and weakness), multiple ECG signs of hyperkalemia can guide empiric calcium:

• Heart rate/rhyhm: bradycardia, junctional bradycardia, or wide complex rhythm

• Electrical: wide PR, wide QRS with atypical BBB

• Axis deviation: right or left

• R-wave abnormalities

• Tall/small: voltages unaffected

• ST/T: diffuse peaked T waves (narrow base, sharp peak, look pinched); may cause ST elevation or Brugada pattern