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2nd-degree type I (Wenckebach)

Also called second-degree AV block type I, Mobitz I, Wenckebach.

The PR interval lengthens beat after beat until one P wave is not conducted and its QRS drops, then the cycle restarts. Longer, longer, longer, drop. Usually nodal and usually benign.

A 6-second EKG rhythm strip showing 2nd-degree type I (Wenckebach).
2nd-degree type I (Wenckebach), 6 seconds of strip. Each small box is 0.04 seconds and each large box is 0.20 seconds.

How to identify 2nd-degree type I (Wenckebach)

Rate
Ventricular slower than atrial
Rhythm
Regularly irregular, in groups
P waves
Regular, more P waves than QRS complexes
PR interval
Lengthens beat after beat until a QRS drops
QRS
Under 0.12 s
Clincher
Longer, longer, longer, drop. Usually nodal and usually benign.
Memory trick
"Longer, longer, drop, you have a Wenckebach." The PR gets longer every beat, then a QRS drops. That stretching PR is your warning.

Rhythms it gets mistaken for

Treatment by certification level

Open your level. Treat the patient, not the strip.

Paramedic

Stable

Asymptomatic: watch it, do not treat the number.

  • Monitor, IV access, transport and reassess.
  • No atropine, pacing or infusion for the asymptomatic patient.
  • If it becomes symptomatic (hypotension, altered mental status, signs of shock, ischemic chest pain, acute heart failure): atropine 1 mg every 3-5 minutes (maximum 3 mg), then transcutaneous pacing, or dopamine 5-20 mcg/kg/min or epinephrine 2-10 mcg/min.

Unstable

Symptomatic bradycardia: atropine, then pacing or an infusion.

  • Oxygenation and ventilation support, IV access, monitor.
  • Atropine 1 mg IV every 3-5 minutes, maximum 3 mg.
  • No response: transcutaneous pacing (rate 60-80, mA up until capture, then slightly above). Confirm mechanical capture with a femoral pulse. Sedation and analgesia.
  • Or a chronotropic infusion: dopamine 5-20 mcg/kg/min, or epinephrine 2-10 mcg/min, titrated to effect.
  • Look for and treat the cause: hypoxia first, then medications and high potassium.
AEMT

Stable

Pulse present and stable.

  • Oxygen only if hypoxic or short of breath, IV access, monitoring.
  • Acquire and transmit a 12-lead ECG. Interpreting it is a Paramedic skill.
  • Transport and reassess. Request a paramedic if the patient worsens.

Unstable

Symptomatic bradycardia with a pulse.

  • Support oxygenation and ventilation, establish IV access, and move the patient.
  • Expedited transport, with a paramedic intercept if that is genuinely faster than reaching the hospital.

Synchronized cardioversion and pacing are Paramedic-only, so the AEMT has no electrical option for a patient with a pulse. Never put the AED on this patient.

EMT

Stable

Pulse present: assess and support.

  • Treat the patient, not the rhythm name: airway, breathing, circulation, vital signs.
  • Oxygen if the SpO2 is under 94%, the patient is short of breath, or shows signs of shock.
  • Request ALS and transport promptly. Reassess on the way.

An AED goes only on a patient who is unresponsive, not breathing and pulseless. It cannot feel a pulse, so on a patient with a pulse it can advise a shock that would do harm.

Unstable

Pulse present but poorly perfusing: this patient needs ALS fast.

  • Treat the patient, not the rhythm name: airway, breathing, circulation, vital signs.
  • Oxygen if the SpO2 is under 94%, the patient is short of breath, or shows signs of shock.
  • Request ALS and transport promptly. Reassess on the way.

An AED goes only on a patient who is unresponsive, not breathing and pulseless. It cannot feel a pulse, so on a patient with a pulse it can advise a shock that would do harm.

Could you pick out 2nd-degree type I (Wenckebach) on a fresh strip? See how many rhythms you can identify in one minute.

Try the 60-second rhythm challenge