P-wave

Atrial depolarization ➔ P-wave

  • Why do we care about the P-wave?

    • Atria pathology (enlargement)

    • Accessory pathways/abnormal focus/foci (signal origin)

    • Heart blocks & bradycardia

P-wave

  • < 3 small squares (0.12s)

  • Best seen in lead II where it is upright

  • Inverted in aVR, biphasic in V1

    • Why does the P wave look weird in these leads?

Atrial enlargement

  • Lead II and V1 are used to assess weird P-waves

  • The P-wave can show enlarged RAE & LAE

  • R atria enlarged ➔ taller P-wave (>2.5mm)

    • Pulmonary HTN, cor pulmonale

  • L atria enlarged ➔ wider P-wave (>0.12s) - Bifid (P mitrale)

    • Mitral stenosis, LVH, Hypertrophic cardiomyopathy

    • Precursor to A-fib

PR interval (0.12 - 0.20sec)

  • Time it takes from atrial depolarization ➔ AV node ➔ bundle of His

    • ↑ PR interval (> 0.20s) longer for signal to make it to ventricle

      • Blocks (discussed below)

    • ↓ PR interval (< 0.12s or 3 small boxes)

      • Sympathetic stimulation/ ↓ parasympathetic ➔ ↑ AV conduction ➔ ↓ PR interval

      • Parasympathetic stimulation (vagus nerve) ➔ ↓AV conduction ➔ ↑ PR interval

      • Preexcitation (accessory pathway sends an early signal to ventricles)

      • When the AV node takes over (AJR)

PR segment:

  • Isoelectric line between end of P wave and start of QRS

  • Pericarditis ➔ diffuse ST elevation with reciprocal PR segment depression

  • aVR ± V1: PR segment elevation with reciprocal ST depression

Heart Blocks and The PR Interval

PR Interval - Normally 3-5 small squares (0.12-0.20s)

  • > 0.20s - AV block (1 big box = 1st degree block)

  • First-degree block

    • Usually age related ↓ conduction ability

    • ASX needs no TX

  • Second-degree MOBITZ TYPE 1 (Wenckebach)

    • PR gets longer and longer and a QRS drops (usually 5:4 or 4:3)

    • ASX needs no TX but monitor for progression of block

  • Second-degree MOBITZ TYPE 2

    • PR doesn’t change

    • 2:1 every other P-wave isn’t conducted (bad)

    • 3:1 Two P-waves don’t get conducted (worse)

    • Usually from an issue below AV node ➔ wider QRS

  • Pace if sick

  • Atropine

    • OR epinephrine OR dopamine if hypotensive

  • Need a permanent pacemaker

  • Third-degree block (complete block)

    • No conduction from up top (P-waves keep trying though)

    • Junctional or ventricular rhythm (wider QRS)

    • May see cannon a-waves (pulsating JV)

  • Pace if sick

  • Atropine

    • OR epinephrine OR dopamine if hypotensive

  • Need a permanent pacemaker

Sinus arrhythmia

  • P-wave and QRS are always linked and have a normal (unchanging) PR

  • Inspiration ➔ ↑ HR

  • Expiration ➔ ↓ HR

Sick sinus syndrome

  • Patho

    • SA node dysfunction ➔ sinus brady, bradyarrhythmias, SA pauses

    • Bouts of tachycardia also present ➔ tachy-brady syndrome

  • SSX

    • Depends on how much tachy/brady occurs: Dizzy/syncope, palpitations, chest pain

  • DX

    • Holter monitor to catch the rhythms and associate them with sx

    • EKG: Sinus pause - period of no p-waves ± ventricular complex (wide QRS),

      • Tachy-brady ➔ sinus pause with funs of junctional tach

  • TX

    • Monitor asymptomatic pts

    • Tx symptomatic brady with atropine, temporary pacing

    • Definitive: Pacemaker and AICD