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Tachycardia and bradycardia
The resting adult heart rate normally ranges from 60 to 100 beats per minute (bpm). Heart rate and rhythm abnormalities occur frequently in critically ill patients, and the incidence of sustained arrhythmias can approach 40% in some intensive care unit (ICU) settings. , Common arrhythmia risk factors in the critically ill include advanced age, sepsis, myocardial ischemia, respiratory failure, renal insufficiency, acute brain injury, polypharmacy, metabolic disturbances, cancer, trauma, and burns. , Prompt recognition and treatment are essential, as the presence of tachyarrhythmias (characterized by heart rates >100 bpm) and bradyarrhythmias (characterized by heart rates <60 bpm) is associated with both significantly increased hospital length of stay and in-hospital mortality rates in the critically ill.
Tachyarrhythmias
For classification purposes, tachyarrhythmias can be subdivided into those with a narrow QRS complex (≤120 milliseconds [ms]) and those with a wide QRS complex (>120 ms). In general, narrow QRS complex tachyarrhythmias are supraventricular tachycardias (SVTs), involving tissue from the His bundle or above. Wide complex tachyarrhythmias typically represent ventricular tachycardias (VTs) or SVTs with abnormal conduction patterns.
Sinus tachycardia
Sinus tachycardia is an atrial supraventricular tachyarrhythmia with a narrow QRS complex. On the electrocardiogram (ECG), the P wave is positive in leads I, II, and aVF and biphasic/negative in lead V1. The vast majority of sinus tachycardia is physiologic and associated with catecholaminergic triggers. Sinus tachycardia may result from pain, physical activity, fever, or hyperthyroidism. Simulants, caffeine, anticholinergics, and beta-receptor agonists can all produce transient sinus tachycardia. In critically ill patients, sinus tachycardia may signify a normal adaptive response to maintain cardiac output in the setting of decreased stroke volume, oxygen-carrying capacity, or arterial vascular tone. Although nonspecific, tachycardia can represent an early sign of impending cardiopulmonary instability, and it is an independent risk factor linked to several worse clinical outcomes in the ICU. The development of sinus tachycardia in critically ill patients is often multifactorial, and appropriate management includes identification of risk factors, discontinuation of offending agents, and treatment of underlying causes ( Table 7.1 ).
TABLE 7.1
Common Causes of Arrhythmias in the ICU
| Tachyarrhythmias | Bradyarrhythmias |
|---|---|
| Advanced age | Advanced age |
| Myocardial ischemia | Myocardial ischemia |
| Respiratory failure | Respiratory failure |
| Kidney failure | Kidney failure |
| Postcardiac surgery | Postcardiac surgery |
| Heart failure | Heart failure |
| Electrolyte abnormalities | Electrolyte abnormalities |
| Increased catecholamines: pain, exercise, anxiety | Increased vagal tone |
| Hyperthyroidism | Hypothyroidism |
| Hyperthermia | Hypothermia |
| Hypovolemia | Increased intracranial pressure |
| Anemia | Hypertension |
| Sepsis | Obstructive sleep apnea |
| Cancer | Infiltrative diseases: cardiac amyloidosis, sarcoidosis, hemochromatosis, lymphoma |
| Trauma/burns | Inflammatory diseases: Lyme disease, Chagas disease, myocarditis, endocarditis |
| Intoxication: alcohol, cocaine, stimulants | Rheumatologic diseases: rheumatoid arthritis, scleroderma, systemic lupus erythematosus |
| Medications: anticholinergics, beta-receptor agonists, chemotherapy agents, antiarrhythmics, vasopressors, inotropes | Medications: beta-receptor antagonists, calcium channel blockers, digoxin, dexmedetomidine, antiarrhythmics, opioids, tricyclic antidepressants, clonidine, lithium, phenytoin |
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