Heart Blocks

The PR interval tells you whether electrical signals are arriving late, getting randomly dropped, or completely blocked — and only one of those three requires an emergency pacemaker.

Heart blocks are AV conduction delays classified by degree. First-degree: every P wave conducts, but PR interval is prolonged >0.20 seconds. It's benign and rarely treated. Second-degree Type I (Wenckebach): PR progressively lengthens until one QRS is dropped, then the cycle resets. Often transient (post-inferior MI, medication effect) and usually stable. Second-degree Type II: PR interval is constant, but QRS complexes are dropped unpredictably — this is dangerous because it can deteriorate to third-degree block without notice. The block is below the AV node (infranodal), so the QRS is often wide. Third-degree (complete): no atrial impulses reach the ventricles. P waves and QRS complexes march independently (AV dissociation). The ventricular escape rate is typically 20–40 bpm, producing symptomatic bradycardia, syncope, and hemodynamic instability. Second-degree Type II and third-degree blocks are the two that commonly require pacemaker intervention. Atropine may temporarily increase rate in symptomatic bradycardia at the AV nodal level but is unreliable for infranodal blocks. Nursing priorities: continuous telemetry monitoring, assessing for decreased cardiac output (hypotension, altered LOC, dizziness), and having transcutaneous pacing equipment immediately available for Type II and third-degree blocks.

Key Distinctions

Don't confuse Type I (progressive PR lengthening, predictable dropped beat, usually benign) with Type II (fixed PR, unpredictable dropped beats, pacemaker territory). Students mix up third-degree block with atrial fibrillation — both look irregular, but third-degree shows regular P waves AND regular QRS complexes that are completely dissociated from each other. Atropine works at the AV node level; it won't rescue an infranodal (Type II or complete) block reliably.

Clinical Pearl

"Longer, longer, longer, DROP — that's a Wenckebach." If the PR never changes but beats still drop, that's Type II — and that patient needs a pacer.