Wide Complex Tachycardias - DRAFT
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Introduction
main article: cardiac arrest
Tachycardia, or a heart rate above 100 in adults, is a physiologic response to increased metabolic demand or reduced oxygen delivery. Sinus tachycardia can occur normally due to changes in the body outside the heart, during exercise or stress, after administration of various drugs, or following a host of pathological processes.
Tachycardia may also result directly from abnormal heart function, mediated by aberrant electrical activity in the atria, ventricles, or the electrical conducting system.
Sinus tachycardia normally resolves following removal of the precipitating cause. Arrhythmias may also spontaneously resolve, though they may also lead to increasingly chaotic rhythm of the heart and result in pulseless ventricular tachycardia and/or ventricular fibrillation. These signify cardiac arrest, and death will rapidly follow if defibrillation and other treatments are not provided.
The Case of James R.
Ventricular Tachycardia
Ventricular tachycardia (VT) is a run of three or more PVCs. Sustained VT lasts linger than 30 seconds, can induce syncope, or requires termination by drugs or cardioversion.
Ventricular tachycardia is commonly seen in patients with structural heart disease, including myocardial infarction, heart failure, hypertrophy, electrical diseases, valvular heart diseases, and congenital heart diseases. Reentry circuits are most commonly old scars.
ECG findings
QRS complexes are wide, and occur at rates of 100-200, or sometimes faster. These wide QRS complexes distinguish ventricular tachycardia from supraventricular causes.
Idiopathic VT exists and is rarely life-threatening. However, new-onset, VT is an emergency, as it can quickly deteriorate into ventricular fibrillation. Cardioversion is normally done, or drugs such as amiodarone, procainamide, or lidocaine can be used to medically convert the rhythm. Underlying risk factors should be identified and addressed, and pacemakers are very useful for future episodes.
Torsades de Pointes
Torsades de pointes is a form of ventricular tachycardia with varying amplitudes of QRS waves. It is commonly caused by drugs, electrolyte imbalances (hypokalemia or hypomagnesemia), on top of prolonged QT intervals. It is usually symptomatic but frequently self-limiting.
Premature Ventricular Contractions
Premature ventricular contractions (PVCs) arise when an ectopic ventricular focus spontaneously fires an action potential. This appears as a widened QRS complex, as the impulse travels in an altered path. If every alternating beat is a PVC, it is called bigeminy. Consecutive PVCs are called couplets, and three are triplets.
PVCs are common in healthy people, particularly adolescents. Benign PVCs are single, uniform, disappear with exercise, and have no structural abnormalities present.
If underlying cardiac or metabolic conditions are present, PVCs can represent dangerous precursours to severe dysrhythmias or sudden death.
History and Physical Exam
A tachycardia may represent an unstable, life-threatening condition. Ensure the patient's ABC's (airway, breathing, and circulation) are being managed, as described under 'basic life support'.
- history
- physical exam
History
When interviewing a patient with undiagnosed tachycardia, inquire into the following:
- when did it begin?
- has this ever happened before? (get details)
- is there chest pain or shortness of breath?
- have they passed out?
- is there fever, chills, or other evidence of infection?
A review of systems is also warranted.
Past medical history should include:
- heart disease
- thyroid disease
Social history should include:
- sleep patterns
- alcohol use
- drug use
- cigarette smoking
- caffeine intake
- stress levels
Physical Exam
Examine the patient's overall state, including:
- level of consciousness
- vital signs
- colour
Specific attention should be paid to:
- cardiac exam
- respiratory exam
Investigations
- lab investigations
- diagnostic imaging
Lab Investigations
If investigations are warranted, bloodwork may include:
- CBC
- electrolytes
- creatinine
- troponin
- TSH
- calcium, magnesium
Diagnostic Imaging
ECG is the mainstay of imaging. Other tests to investgate a tachycardia can include:
- chest X-ray (investigate for heart failure)
- echocardiogram
Management
Treatment of tachyarrhythmias is meant to stabilize the patient, protect against lethal consequences, and mitigate the underlying cause(s) of the arrhythmia.
As described, it is paramount to assess and address the ABC's when managing a patient with tachycardia. This can include:
- Opening the airway
- Providing supplemental oxygen, postive pressure ventilation, and/or intubation
- IV access x2 and fluid administration
- Chest compressions
- Defibrillation
Antiarrhythmics must be used with caution due to the high risk of further arrhythmic complications and death.
Electrical cardioversion and defibrillation can be used to depolarize the bulk of myocardial tissue, interrupting rentry and allow the sinus node to regain pacemaker control. Implantable cardioverters can be used to automatically cardiovert or defibrillate a heart.
Pathophysiology - TO REVIEW
main article: electrical control of the heart
There are many underlying processes that can lead to tachycardias:
Ectopic Rhythms
If an area of tissue develops an intrinsic rate of firing faster than that of the SA node, ectopic (premature) beats can occur. They can occur due to high catecholamine concentrations, hypoxemia, ischemia, electrolyte disturbances, and drugs such as digitalis.
Abnormal Automaticity
Injured cardiomyocytes can acquire automaticity and spontaneously depolarize, though means not fully understood, but likely involving a slow calcium current.
Triggered Activity
Under certain conditions, action potentials can trigger abnormal depolarizations that result in extra heart beats or rapid arrhythmias. Afterdepolarizations appear as oscillations and can be early, during repolarization, or delayed. Early afterdepoloarizations are most common during conditions that prolong APs, such as long GT syndrome.
Reentry
Reentry occurs when impulses circuluate around a unidirectional conduction block, recurrently depolarizing a region of cardiac tissue. Reentry around distinct anatomic pathways usually appears as monomorphic tachycardia on an ECG, while fibrillation is likely caused by multiple circulation reentry wave fronts.
Resources and References
Topic Development
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