Cardiac Assessment and ECG Interpretation - NREMT: Paramedic Level

QRS ≥ $0.12$ s with broad R in I/V6 and deep S in V1. Prolonged QRS with monophasic R in lateral leads and QS or deep S in right precordial leads reflects left bundle conduction delay.

QRS ≥ $0.12$ s with rsR′ in V1 and wide S in I/V6. These features indicate delayed right bundle conduction, producing secondary R wave in right precordial leads and slurred S in left leads.

AV dissociation with independent atrial and ventricular rates. Complete dissociation results from total AV conduction failure, causing independent atrial and ventricular rhythms, often necessitating pacing.

Constant PR with intermittent nonconducted P waves. Stable PR interval with sudden conduction failure suggests infranodal block, often requiring intervention due to risk of progression to complete block.

PR interval > $0.20$ s with $1:1$ conduction. Prolonged PR interval indicates delayed AV nodal conduction, but consistent P-QRS relationship maintains 1:1 atrioventricular synchrony.

QTc ≤ $0.46$ s. Females have a slightly longer normal QTc due to physiological differences, with this limit aiding in identifying risks for arrhythmias like torsades de pointes.

QTc ≤ $0.44$ s. The upper limit for corrected QT interval in males accounts for heart rate normalization, preventing misdiagnosis of prolonged QT syndromes.

< $0.12$ s (typically $0.06$–$0.10$ s). Normal QRS duration reflects efficient intraventricular conduction, with values under $0.12$ s indicating no bundle branch block or other delays.

$0.12$ to $0.20$ s (120–200 ms). This range represents normal atrioventricular conduction time, allowing atrial depolarization to precede ventricular activation without delay.

Left midaxillary line, level with V4. V6 in the midaxillary line at V4 level evaluates the low lateral left ventricular wall, completing the horizontal plane of precordial leads.

Left anterior axillary line, level with V4. V5 at the anterior axillary line, horizontally aligned with V4, assesses the lateral wall of the left ventricle for comprehensive ECG coverage.

Midway between V2 and V4. V3 is located equidistant between V2 and V4 to bridge septal and anterior views, providing transitional precordial lead information.

5th intercostal space, left midclavicular line. V4 placement at the fifth intercostal space in the midclavicular line targets the anterior left ventricle for detecting ischemia or infarction in that region.

4th intercostal space, left sternal border. V2 is positioned at the left sternal border in the fourth intercostal space to monitor the septum and left ventricle, ensuring proper 12-lead ECG interpretation.

4th intercostal space, right sternal border. Standard placement for V1 views the right ventricle and septum from the right sternal border at the fourth intercostal space to capture accurate anterior cardiac electrical activity.

I, aVL, V5, and V6. Lateral leads monitor the left ventricular lateral wall, supplied by left circumflex artery, aiding in lateral infarction identification.

V1 and V2. Septal leads V1-V2 assess the interventricular septum, often involved in anteroseptal infarcts from left anterior descending artery occlusion.

V3 and V4. Anterior leads V3-V4 evaluate the anterior left ventricular wall, critical for diagnosing anterior myocardial infarction.

ST elevation ≥ $1$ mm (≥ $0.1$ mV) in ≥ $2$ contiguous leads. This criterion indicates acute myocardial injury in limb leads, prompting urgent reperfusion therapy for suspected STEMI.

ST elevation ≥ $2$ mm (≥ $0.2$ mV) in ≥ $2$ contiguous leads. Higher threshold in precordial leads accounts for normal variant elevations, ensuring specificity for diagnosing STEMI in chest leads.

Inferior STEMI. Elevation in inferior leads suggests occlusion of the right coronary artery, affecting the inferior left ventricular wall.

Anterior (anteroseptal) STEMI. ST elevation across these leads indicates left anterior descending artery involvement, compromising anteroseptal myocardium.

II, III, and aVF. These leads view the inferior heart surface, corresponding to right coronary artery territory for detecting inferior wall abnormalities.

Heart rate $= rac{60}{R-R(s)}$. Dividing 60 by the R-R interval in seconds provides an accurate ventricular rate estimation for irregular rhythms on ECG.