ECG Interpretation, Arrhythmias, and Cardiac Pathologies

Practice vocabulary flashcards covering ECG rate calculations, arrhythmias, conduction blocks, ischemic heart disease, myocardial infarction localization, and electrolyte disturbances.

heart rate - R-R interval

a measurement used in electrocardiography (ECG) to determine the time interval between successive R-wave peaks represents the duration of one cardiac cycle

• $\textcolor{green}{\text{heart rate (HR)}}\textcolor{green}{\bm{=\frac{60}{\text{R-R interval}}}}$ in beats per minute (bpm)

  • R–R interval = small boxes × 0.04 seconds OR large boxes × 0.20 seconds

$$\text{heart rate (HR)}\bm{=\frac{60}{\text{R-R interval}}}$$

heart rate - R-R interval

heart rate - rule of 300

a rapid method for estimating heart rate in regular rhythms by dividing 300 by the number of large boxes between consecutive QRS complexes

• ($\frac{300}{\text{Number of large boxes}}$)

rule of 300

heart rate - small box method

a calculation to assess heart rate on an ECG: $\frac{1500}{\text{Number of small boxes}}$, derived from the paper speed calculation ($25\,\text{mm/s} \times 60\,\text{s/min} = 1500$)

heart rate - 10 second rule

a heart rate calculation method ideal for irregular rhythms where one counts the total number of beats present on a 10-second ECG strip and multiplies that count by 6 to calculate beats per 60 seconds

Normal Sinus Rhythm

a regular cardiac rhythm originating from the SA node

• physiological heart rate ranging between 60 - 100 bpm

sinus tachycardia

a regular cardiac rhythm characterized by a heart rate greater than 100 bpm

sinus bradycardia

a regular cardiac rhythm characterized by a heart rate less than 60 bpm

Regular Cardiac Rhythm

A rhythm where the SA node generates impulses and conducts them through the AV node at uniform intervals, manifested on an ECG by a constant R-R interval from cycle to cycle.

Heart Block

A type of arrhythmia encompassing diseases characterized by irregularity in the initiation and conduction of cardiac electrical impulses, resulting in an irregular cardiac rhythm.

First Degree Heart Block

An atrioventricular conduction disorder defined by slowed conduction through the AV node, resulting in a prolonged PR interval greater than $0.22\,\text{s}$ ($> 0.2\,\text{s}$), where a P wave still precedes every QRS complex.

Mobitz I (Wenckebach Phenomenon)

A subtype of second degree heart block marked by progressive lengthening of the PR interval until an impulse fails to transmit, resulting in a dropped QRS complex.

Mobitz II

A subtype of second degree heart block where some impulses fail to conduct through the AV node without any preceding progressive lengthening of the PR interval.

Third Degree (Complete) Heart Block

A complete failure of conduction where no impulses transmit from the atria to the ventricles; the atria and ventricles beat independently (with the intrinsic rate of the Purkinje fibers/ventricles being slower), showing no correlation between P waves and QRS complexes and a higher P wave frequency than QRS frequency.

Atrial Fibrillation (A-fib)

The most common chronic arrhythmia caused by multiple atrial ectopic foci; defined by an irregularly irregular rhythm, absent P waves with a wavy baseline, irregular R-R intervals, an atrial rate of $350\text{--}500\,\text{bpm}$, variable ventricular rate, and zero atrial contribution to ventricular filling.

Atrial Flutter

An atrial arrhythmia wherein the atria beat rapidly but regularly, displaying a characteristic sawtooth wave appearance on the ECG.

Cardioversion Principle

The delivery of an electrical current designed to depolarize a critical mass of myocardial cells simultaneously so that upon repolarization, the SA node can recapture its role as the pacemaker of the heart.

Ventricular Fibrillation

A fatal, life-threatening arrhythmia caused by multiple ectopic ventricular foci; the ventricles fail to pump and quiver like a bag of worms, producing an erratic, chaotic ECG tracing devoid of distinct QRS complexes.

Angina Pectoris

Episodes of temporary chest pain, pressure, or discomfort caused by reduced coronary blood supply and myocardial ischemia, typically provoked by greater than $75\%$ narrowing of the coronary arteries.

ST Depression

An ECG manifestation of transient myocardial ischemia seen during episodes of angina pectoris, which typically reverts to baseline within $5\,\text{minutes}$ following nitroglycerin administration.

Myocardial Infarction (MI)

Irreversible damage and death of myocardial tissue caused by prolonged hypoxia and complete ($100\%$) occlusion of a coronary artery by atherosclerosis, thrombus, or embolus.

Left Anterior Descending Artery (LAD) Occlusion

The site of occlusion responsible for $40\text{--}50\%$ of myocardial infarctions, causing damage to the anterior and apical regions of the left ventricle and the anterior two-thirds ($\frac{2}{3}$) of the interventricular septum (IVS).

Right Coronary Artery (RCA) Occlusion

The site of occlusion responsible for $30\text{--}40\%$ of myocardial infarctions, causing necrosis to the posterior wall of the left ventricle and the posterior one-third ($\frac{1}{3}$) of the interventricular septum (IVS).

Left Circumflex Artery (LCX) Occlusion

The site of occlusion responsible for $15\text{--}20\%$ of myocardial infarctions, affecting the lateral wall of the left ventricle.

Evolutionary ECG Changes in MI

A sequential progression of ECG waveforms following myocardial infarction: acute marked ST elevation, development of pathologic Q waves with T wave inversion within hours, and the lifelong persistence of pathologic Q waves indicating an old infarct.

J Point

The distinct ECG point marking the transition and junction from the end of the QRS complex to the beginning of the ST segment.

ECG Tombstoning

An acute STEMI pattern characterized by prominent J point elevation combined with severe ST segment elevation, forming a dome-shaped waveform resembling a cemetery tombstone.

Cardiac Markers (AMI)

Circulating biomarkers measured following acute myocardial infarction: Myoglobin (early rapid elevation peaking under $1\,\text{day}$), CK-MB (peaks at approximately $1\,\text{day}$ and clears in $2\text{--}3\,\text{days}$), and cardiac Troponin I (cTnI, rises to the highest multiples of normal and remains elevated past $7\text{--}10\,\text{days}$).

Hyperkalemia

An electrolyte disorder defined by a serum potassium concentration exceeding $5\,\text{mEq/L}$; it causes defective repolarization and altered excitability, manifests with tall peaked T waves, prolonged PR interval, and widened QRS complexes, and can arrest the heart in diastole.

Tall Peaked T Wave

The classic initial ECG sign of hyperkalemia resulting from defective myocardial repolarization.