Normal Sinus Rhythm
Normal sinus rhythm (NSR) is the baseline reference rhythm originating from the SA node. Characteristics include a regular rhythm with a rate of 60 to 100 beats per minute, upright P waves preceding each QRS complex, a consistent PR interval of 0.12 to 0.20 seconds, and narrow QRS complexes of less than 0.12 seconds. Each P wave is followed by a QRS, and each QRS is preceded by a P wave. NSR indicates normal electrical conduction through the heart and requires no intervention.
Sinus Bradycardia
Sinus bradycardia originates from the SA node but at a rate below 60 beats per minute. The rhythm is regular with normal P waves, consistent PR intervals, and narrow QRS complexes. It may be normal in athletes, during sleep, or in patients taking beta blockers or calcium channel blockers. Treatment is indicated only if the patient is symptomatic, showing signs such as hypotension, altered mental status, chest pain, or signs of poor perfusion. Symptomatic bradycardia is treated with atropine 0.5 mg IV every 3 to 5 minutes (maximum 3 mg), transcutaneous pacing, or dopamine/epinephrine infusion if atropine is ineffective.
Sinus Tachycardia
Sinus tachycardia originates from the SA node at a rate of 100 to 150 beats per minute. The rhythm is regular with normal P waves and narrow QRS complexes. It is typically a compensatory response to an underlying condition such as pain, fever, anxiety, hypovolemia, hypoxia, or medication effects. Treatment is directed at the underlying cause rather than the rhythm itself. Identifying and correcting the root cause (such as fluid replacement for dehydration or oxygen for hypoxia) will resolve the tachycardia. Do not use medications to slow sinus tachycardia.
Atrial Fibrillation
Atrial fibrillation (A-fib) is characterized by chaotic, disorganized atrial electrical activity. The atria quiver rather than contracting effectively. Key features include an irregularly irregular rhythm, absence of distinct P waves (replaced by fibrillatory waves), and a variable ventricular rate that may be rapid (over 100, called rapid ventricular response or RVR), controlled (60 to 100), or slow (under 60). QRS complexes are typically narrow unless there is a bundle branch block. A-fib increases the risk of stroke due to blood pooling and clot formation in the atria. In the prehospital setting, manage symptoms and transport. Unstable patients with rapid A-fib may require synchronized cardioversion.
Ventricular Tachycardia (V-Tach)
Ventricular tachycardia is a wide-complex tachycardia (QRS greater than 0.12 seconds) with a rate of 100 to 250 beats per minute. It originates from an irritable focus in the ventricles. V-tach can be monomorphic (uniform QRS appearance) or polymorphic (varying QRS morphology, including torsades de pointes). The patient may be stable with a pulse or unstable. Pulseless V-tach is treated the same as ventricular fibrillation with defibrillation and CPR. Stable V-tach with a pulse may be treated with amiodarone 150 mg IV over 10 minutes. Unstable V-tach with a pulse requires immediate synchronized cardioversion starting at 100 joules.
Ventricular Fibrillation (V-Fib)
Ventricular fibrillation is a chaotic, disorganized electrical activity in the ventricles with no effective cardiac output. The ECG shows an irregular, wavy baseline with no identifiable P waves, QRS complexes, or T waves. The amplitude may be coarse (tall, more likely to respond to defibrillation) or fine (low amplitude, sometimes confused with asystole). V-fib is a shockable rhythm. Treatment includes immediate high-quality CPR, defibrillation at maximum joules as soon as possible, epinephrine 1 mg IV/IO every 3 to 5 minutes, and amiodarone 300 mg IV/IO for the first dose, then 150 mg for the second dose. Early defibrillation is the most important intervention for survival.
Asystole
Asystole is the absence of all electrical activity in the heart, appearing as a flat line on the monitor. Before confirming asystole, check that leads are properly connected, gain is turned up, and verify in at least two leads (to rule out fine V-fib perpendicular to a single lead). Asystole is a non-shockable rhythm. Treatment includes high-quality CPR, epinephrine 1 mg IV/IO every 3 to 5 minutes, identification and treatment of reversible causes (the Hs and Ts), and consideration of termination of resuscitation efforts per protocol. Defibrillation is not indicated for asystole.
Pulseless Electrical Activity (PEA)
PEA is defined as any organized electrical rhythm on the monitor without a palpable pulse. The monitor may show what looks like sinus rhythm, bradycardia, or other organized patterns, but the heart is not generating effective mechanical contractions. PEA is a non-shockable rhythm. Treatment includes high-quality CPR, epinephrine 1 mg IV/IO every 3 to 5 minutes, and aggressive identification and treatment of reversible causes. The reversible causes are remembered as the Hs and Ts: Hypovolemia, Hypoxia, Hydrogen ion (acidosis), Hypo/Hyperkalemia, Hypothermia, Tension pneumothorax, Tamponade (cardiac), Toxins, and Thrombosis (pulmonary or coronary). Identifying and correcting the underlying cause is the key to successful resuscitation in PEA.