7 Tachycardia and Bradycardia
Cardiac arrhythmias, a common problem encountered in the intensive care unit (ICU), increase the length of stay and represent a major source of morbidity.1 Clinical issues such as electrolyte derangements (particularly those related to potassium and magnesium ion concentrations), acidemia, hypoxia, cardiac ischemia or structural defects, and catecholamine excess (exogenous or endogenous) can play important roles in the cause of arrhythmias. Treatment of these arrhythmias depends most importantly on the cardiac physiology of the patient but also on the ventricular response rate and duration of the arrhythmia.
The two major categories of cardiac arrhythmias are defined by heart rate: bradycardia (heart rate <60 beats per minute [bpm]) and tachycardia (heart rate >100 bpm). Asymptomatic bradycardia does not carry a poor prognosis, and in general no therapy is indicated.2 Bradycardia with or without hypotension should prompt a consideration of metabolic disturbances, hypoxemia, drug effects, and myocardial ischemia. Other causes of bradycardia are shown in Table 7-1.
TABLE 7-1 Common Causes of Bradycardia
The recommended initial therapy for bradycardia that is leading to inadequate cardiac output and organ perfusion is 1 mg atropine intravenously (IV). The underlying cause for bradycardia should be investigated; if it is of abrupt onset, hypoxemia or acidosis can be quickly excluded by obtaining an arterial blood gas measurement. If the patient is unresponsive, endotracheal intubation and mechanical ventilation are indicated and should be instituted promptly. If the patient is already intubated, disconnect the ventilator and manually ventilate the patient (using an Ambu bag) to ensure adequate ventilation and oxygenation. Mucous plugging of the endotracheal tube or airways should be excluded in an acutely hypoxemic patient. Once these conditions are excluded, evaluate the electrocardiogram (ECG) for evidence of second- or third-degree heart block or ischemic changes. Aminophylline (100 mg IV) has been reported to correct ischemic heart block.3 Insertion of a temporary transvenous pacemaker may be indicated in the setting of ischemic heart block, because further deterioration can occur unpredictably.
Sinus tachycardia is probably the most common dysrhythmia encountered in the ICU and often occurs as a response to a sympathetic stimulus (e.g., hypoxia, vasopressors, inotropes, pain, dehydration, or hyperthyroidism). The first step is to review the patient’s medication list, including infusions, to exclude an iatrogenic etiology for the tachycardia. Treatment focuses on identifying and trying to correct the underlying cause. In trauma and postsurgical patients, tachycardia can be a sign of bleeding and hypovolemia. It is usually reasonable to administer an intravascular volume challenge (e.g., 500 mL of colloid solution in adults) and check the hemoglobin concentration. Sinus tachycardia and hypertension can be manifestations of opioid withdrawal, failure of a ventilator weaning trial, or inadequate sedation. Most patients at high risk for coronary disease warrant prophylactic treatment with a β-adrenergic blocker to prevent myocardial ischemia secondary to a high “rate-pressure product” and high myocardial oxygen demand.4,5 In particular, perioperative patients with significant cardiac risk should have titrated therapy with a β-adrenergic blocker to maintain the heart rate at less than 80 bpm unless significant contraindications exist.6
Sustained regular tachycardia (heart rate >160 bpm) associated with a narrow QRS complex on the ECG often has a reentrant mechanism as the etiology. Reentrant narrow complex tachycardia is more prevalent in females and usually is not associated with structural heart disease. The key treatment is to block AV conduction.1 These dysrhythmias can often be converted with carotid sinus massage. Adenosine can be administered (6 mg IV, followed by 12 mg IV if no response to the lower dose) if sequential carotid sinus massage fails to abort the dysrhythmia or is contraindicated. Patients presenting with reentrant supraventricular tachycardia in the ICU often have a past history of this dysrhythmia. β-Adrenergic blockers or calcium channel blockers are reasonable choices for both acute conversion and maintenance therapy. Specific β-adrenergic blockers include metoprolol (5 mg IV every 5 minutes until therapeutic effect is achieved) or esmolol (loading dose of 500 µg/kg over 1 minute, then 50 µg/kg/min infusion). Esmolol can be rebolused (500 µg/kg and the drip titrated to a maximum of 400 µg/kg/min). For diltiazem, use 5- or 10-mg boluses, using higher doses only after it is determined that administration of the agent does not lead to arterial hypotension.
