Key Clinical Questions
How do you triage a patient with a new tachyarrhythmia?
What are the electrocardiographic features that suggest that a wide complex tachyarrhythmia is of ventricular origin?
What are the electrocardiographic features that suggest that a wide complex tachyarrhythmia is of supraventricular origin?
What is the differential diagnosis for short RP tachycardia? What is the differential diagnosis for long RP tachycardia?
How do you acutely manage a symptomatic tachyarrhythmia?
This chapter will review the initial bedside approach to a hospitalized patient with a new, potentially life-threatening tachycardia, defined as a heart rate ≥100 beats per minute (bpm). The reader is then referred to subsequent cardiology chapters for definitive management of specific arrhythmias.
THE NORMAL CARDIAC CONDUCTION SYSTEM
The conduction pathway in the heart begins in the sinoatrial node, which spontaneously activates the right atrium, the interatrial septum, and then the left atrium. The initial portion of the P wave represents depolarization of the right atrium, and the terminal portion depolarization of the left atrium. Normally, the atrioventricular (AV) node, His bundle, and bundle branches transmit impulses in anterograde fashion from the atria to the ventricles. The QRS complex represents ventricular depolarization and the T wave represents ventricular repolarization (Figure 102-1).
Cardiac conduction system.
TACHYARRHYTHMIAS AND CONDUCTION DISTURBANCES
Tachycardias are encountered frequently in inpatient practice. Symptoms from tachyarrhythmias are variable, and some patients are asymptomatic. When present, symptoms may include palpitations, shortness of breath, chest pain, anxiety, syncope, hypotension, or may manifest by hemodynamic collapse and sudden cardiac death.
In addition to the typical AV conducting pathway, anomalous bands of tissue—accessory pathways—may be able to conduct between the atria and ventricles in a retrograde or antegrade fashion that bypasses the normal conduction system. These pathways may or may not be visible on surface electrocardiogram (ECG).
Supraventricular tachycardias (SVTs) include all tachycardias that arise at or above the His bundle. Sinus tachycardia is easily the most common SVT, but does not represent a primary pathologic arrhythmia except in rare cases. Paroxysmal SVTs usually have narrow complexes with a normal QRS duration of <100 ms; however, some may have aberrant conduction notable for a different QRS configuration from the baseline ECG. Intraventricular conduction disturbances may manifest as incomplete (100-120 ms) or complete bundle branch blocks (QRS ≥ 120 ms in duration), and may be rate-dependent. A right bundle branch block (RBBB) configuration is more common than a left bundle branch block (LBBB) aberrant pattern. The altered depolarization causes secondary repolarization ST-T abnormalities and discordance of QRS-T wave vectors. Ischemia, electrolyte disturbances, and digitalis cause primary depolarization ST-T abnormalities independent of the QRS vector. Sudden death from SVT is rare.
Ventricular tachycardia (VT) arises from the ventricles ...