EMERGIPRESS – ECG OF THE MONTH
ECG — February 2021
Paramedics respond to a 79-year-old female with resolved lightheadedness and near-syncope. She has a history of congestive heart failure, hypertension and hypothyroidism. Her medications include lisinopril, metoprolol, digoxin and levothyroxine. She is currently asymptomatic.
Rate: ~ 48 bpm (using 10 second rule)
Rhythm: sinus rhythm, irregular, progressive lengthening of PR interval,
followed by dropped QRS complex every 4th beat
ST Segment Elevation >1mm
In 2 or more contiguous
Reciprocal Changes: N/A
This ECG demonstrates a second degree, type I heart block also known as a Mobitz Type I or Wenckebach block (named after Dr. Wenckebach who described the phenomenon in 1906). Normally, pacemaker cells within the heart generate an electrical impulse that travels through the AV node to the His-Purkinje system, onto the cardiac myocytes which contract in a coordinated fashion during systole. In a second degree, type I heart block there is a conduction delay, typically at the AV node. AV nodal cells progressively tire such that conduction through the AV node slows with each beat until the beat is dropped and the cycle resets. Second degree, type I blocks are usually benign and can be occasionally seen in healthy people while they are at rest/sleeping or conditioned athletes when cardiac chronotropy is slow. More commonly, second degree, type I heart blocks are the result of medications that slow cardiac conduction (e.g. beta blockers, calcium channel blockers, digoxin) which is the likely cause for this patient. Second degree, type I blocks can rarely be associated with an inferior myocardial infarction that affects the AV node. Most patients with second degree, type I heart blocks are asymptomatic, however some patients may experience signs of poor perfusion with the dropped beat to include lightheadedness, syncope or hypotension.
Because the patient is currently asymptomatic, she does not require immediate interventions by EMS (i.e., medications, pacing), however, she should be transported to the hospital for evaluation of her current medications and to ensure there is no evidence of a recent myocardial infarction leading to her heart block. Patients with symptomatic bradycardia that require treatment by EMS are those with signs of poor perfusion as defined in MCG 1355 – Poor Perfusion. She did experience near syncope prior to EMS arrival and will require an emergency department evaluation.
This patient has a heart block and resultant bradycardia. The correct provider impression is Cardiac Dysrhythmia – Bradycardia, and should be managed in accordance with TP-1212, Cardiac Dysrhythmia – Bradycardia. If the patient had recurrence of her poor perfusion symptoms, EMS should treat with Atropine 0.5mg (5mL) IV/IO push, repeat every 3-5min prn, maximum dose 3mg. Symptomatic patients with second degree, type I blocks typically respond to atropine. This differs from patients with 2nd degree, type II and 3rd degree heart blocks because in those conditions, the block in conduction is below the AV node and the block would not be affected by inhibiting the parasympathetic effects on the heart (as is atropine’s mechanism of action). If a patient with a bradydysrhythmia were hemodynamically unstable despite atropine, transcutaneous pacing (TCP) would be indicated for heart rate <40bpm and could be considered in this patient with a heart rate in the 40’s in consultation with Base. Always remember to consider sedation and pain management for an awake patient requiring TCP.
The patient remains asymptomatic during transport and does not require pharmacologic treatment or TCP. In the Emergency Department she is assessed and thorough evaluation does not show evidence of a myocardial infarction. It is determined that the most likely cause of her second degree, type I heart block is her medications. Given her underlying cardiac disease and her symptoms at home, she is observed overnight for her near syncope. Cardiology is consulted and her medications are adjusted. She is discharged home the next day with outpatient follow-up.
- Brown A, et al. How to Recognize and Treat Heart Block. JEMS. https://www.jems.com/patient-care/how-recognize-treat-heart-block/
- Burns E, Buttner R. AV Block: 2nd Degree, Mobitz I (Wenckebach Phenomenon). LITFL. https://litfl.com/av-block-2nd-degree-mobitz-i-wenckebach-phenomenon/
Author: Dr. Denise Whitfield, MD, MBA