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Why Does the QRS Widen?
|Figure. Lead II rhythm strip obtained during cardiac arrest.|
By Ken Grauer, MD, Professor, Department of Community Health and Family Medicine, University of Florida Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book, and reports no financial relationship to this field of study.
Clinical Scenario: The lead II rhythm strip shown below was obtained from a middle-aged man in cardiac arrest. The patient had multiple co-morbidities, including end-stage renal disease (on dialysis), a history of cocaine abuse, and severe, non-ischemic cardiomyopathy. No pulse could be felt at the time this tracing was recorded. What is the rhythm? Why does the QRS complex widen (from beat Z onward)? What are the clinical implications of this rhythm in the setting of cardiac arrest?
Interpretation/Answer: The rhythm is regular at a rate of 110/minute. Looking first at the initial part of the tracing, the QRS complex appears to be of borderline duration (about half of a large box, or 0.10 second). No atrial activity is seen in this lead II monitoring lead. Although it would be helpful to see other leads (ideally a 12-lead tracing) to better ascertain QRS duration and the presence or absence of atrial activity, the lack of a clear upright P wave in lead II strongly suggests that this is not a sinus rhythm. Our best guess from this single lead tracing is that the rhythm represents a junctional (AV nodal) tachycardia at 110/minute. The QRS complex actually begins to widen (ever so slightly) with beat Y. It then dramatically widens from beat Z onward. This is not the onset of a new ventricular rhythm! We say this because there is absolutely no change in the rate over the entire tracing. Similarly, there is no introduction of atrial activity. We therefore surmise that this is a junctional tachycardia at a constant rate with development over several beats of a conduction (bundle branch block) delay that occurred during this patient's cardiac arrest. Among the reasons why this may occur in this setting is hypoxemia from lack of coronary perfusion.
The rhythm in this case therefore is PEA (pulseless electrical activity), which by definition entails the presence of an electrical rhythm in the absence of a palpable pulse. In recent years, PEA has become the most common mechanism of cardiac arrest (surpassing ventricular fibrillation). It is generally the end–result of some underlying process. Overall prognosis is unfortunately poor, especially when the underlying cause of the PEA is not reversible.
This patient expired despite resuscitative attempts.