Third-degree block, atropine that does nothing, and a pacer that will not capture
A 74-year-old woman is brought in after a syncopal episode. She is confused, cool, and diaphoretic. Vital signs: HR 38, BP 82/50, RR 20, SpO2 94% on room air. The ECG shows complete (third-degree) AV block with a wide-complex ventricular escape rhythm at 38/min and a QRS of 160 ms; there are independent P waves at about 90/min. Two doses of atropine 1 mg IV have been given, and the rate has moved only from 38 to 41/min. Transcutaneous pacing pads have been applied anterior-laterally, the pacer is set to demand mode at 70/min and 60 mA, and the monitor shows pacing spikes at a regular 70/min with no captured QRS after them. The patient is awake and grimacing with each pacing stimulus. She has one IV and no central access.
1. Explain why atropine has failed here, and state what the guidelines say about how far to push it before moving on.
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Atropine is a vagolytic - it withdraws parasympathetic tone at the sinus node and AV node. That works when the bradycardia is vagally mediated (sinus bradycardia, some AV-nodal blocks). It does essentially nothing for an infranodal (His-Purkinje) third-degree block with a wide ventricular escape, because the escape focus is below the level of vagal influence. Her wide QRS at 38/min is the tell.
Guideline points:
- Atropine 1 mg IV every 3–5 minutes to a maximum of 3 mg is the first-line drug for symptomatic bradycardia, and it is reasonable to try it while preparing pacing.
- In high-degree (Mobitz II) or third-degree block, atropine is unlikely to help and should not delay pacing. It can occasionally worsen the block by accelerating the atrial rate without improving conduction.
- Do not give a second drug instead of electricity when pacing is available. The class I intervention for symptomatic bradycardia unresponsive to atropine is transcutaneous pacing; epinephrine or dopamine infusions are the bridging alternatives while pacing is arranged or if pacing is not tolerated.
- Infusion doses for bridging: epinephrine 2–10 mcg/min or dopamine 5–20 mcg/min, titrated to heart rate and blood pressure.
So: she has had 2 mg with essentially no response, and the correct move is to stop optimizing atropine and fix the pacer - with an epinephrine infusion running while that happens.
2. The pacer is firing at 70/min with no captured beats. Walk through the troubleshooting in the order you would do it, and state how you confirm success.
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Work from the cheapest and fastest fixes to the more invasive.
- Output too low. 60 mA is a starting point, not a therapeutic endpoint. Increase current in 10–20 mA steps until you see a paced QRS after every spike - most patients capture between 50 and 90 mA, but some need up to the device maximum (often 140–200 mA).
- Pad position and skin. Anterior-posterior placement captures more reliably than anterior-lateral in a patient with a dilated chest or COPD. Clip hair, dry the skin (she is diaphoretic), and press the pads down firmly. Do not place over a medication patch, an implanted device, or breast tissue.
- Rate vs. intrinsic rate and mode. In demand mode, if the device senses the intrinsic 38/min it should pace - but if sensing is over- or under-detecting, it can inhibit. Set the rate clearly above the intrinsic rate (60–80/min) and confirm the mode is demand (or asynchronous if you suspect oversensing).
- Technical checks: leads properly attached to the patient (not just the pacer), cable and battery seated, and the monitor showing a true paced spike rather than an artifact.
Confirming success requires two steps, not one:
- Electrical capture: a wide paced QRS following every pacing spike.
- Mechanical capture: a pulse with each paced beat - palpate the femoral artery, or better, use Doppler or the arterial line, and watch the blood pressure and mentation improve. A pacing spike with a QRS and no pulse is not a successful pace.
If transcutaneous pacing cannot achieve mechanical capture at maximum output, the next steps are an epinephrine or dopamine infusion, correction of reversible causes (hyperkalemia, ischemia, drug toxicity such as beta-blocker or calcium channel blocker overdose, hypothermia), and transvenous pacing.
3. She is in pain with each stimulus and still only 82/50. How do you manage the analgesia/sedation problem and the blood pressure without losing the pacer?
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Transcutaneous pacing at capture-level output causes real skeletal muscle contraction and pain, and untreated pain drives catecholamines, agitation, and movement that can dislodge pads and interrupt capture. But sedating a hypotensive patient can cause collapse - so treat the two problems in parallel, not in sequence.
- Optimize perfusion first or simultaneously: a 250–500 mL crystalloid bolus (small aliquots, reassess - she is 74 and may not tolerate a large load), and start norepinephrine or an epinephrine infusion to support MAP while the pacer is being optimized. A better-perfused patient tolerates sedation better and needs less of it.
- Then titrate analgesia/sedation in small, incremental doses: fentanyl 25–50 mcg boluses (hemodynamically the kindest opioid) with midazolam in 1 mg increments, or ketamine in small doses if pressure allows. Have airway equipment and suction ready, and assign a team member to watch the airway while another watches the monitor.
- Do not turn the pacer off to facilitate sedation. If sedation causes hypotension or apnea, treat that (fluids, vasopressor, bag-mask support) rather than abandoning capture.
- Definitive plan: the transcutaneous pacer is a bridge. Arrange transvenous pacing and treat the underlying cause. Once a transvenous wire is in and capturing, the painful surface pacing can stop and sedation can be rationalized.
- Reassess the escape rhythm periodically - a transient cause (ischemia, hyperkalemia, drug effect) may resolve, and the pacer may not be needed indefinitely.