Light-headedness on standing for two days, with one unwitnessed collapse

29-year-old woman. Light-headedness on standing for two days, with one unwitnessed collapse

Forestry worker, no regular medication, no cardiac history. Removed a tick from her thigh six weeks ago and had an expanding red rash around the bite that faded without treatment. Two weeks of intermittent joint aches since.

Stage 1 of 2

She walked into the emergency department after collapsing in the yard at home. She was unconscious for a few seconds and came round without confusion. Her pulse feels slow and it drops a beat every few cycles. Chest is clear, there is no murmur, and she has no chest pain or breathlessness.

Tracing 1 of 2 for the quiz “Light-headedness on standing for two days, with one unwitnessed collapse”. — 12-lead recorded at triage, 25 mm/s, 10 mm/mV Open this tracing in the studio

What does this tracing show?

ASecond-degree AV block, Mobitz I Correct answer Your answer
BSinus rhythm with non-conducted atrial ectopics Your answer
CSecond-degree AV block, Mobitz II Your answer
DComplete heart block with a junctional escape Your answer

The tracing shows grouped beating with progressive prolongation of the PR interval before a dropped QRS, which is the Wenckebach pattern of Mobitz I second-degree AV block. The P waves are regular and sinus in morphology, excluding non-conducted atrial ectopics which would appear early and differ in shape. In Mobitz II the PR interval is constant before the dropped beat. Complete heart block would show AV dissociation with P waves and QRS complexes marching independently.

Which finding on this tracing weighs most heavily towards the block sitting in the AV node rather than below it?

AThe ventricular rate is around 60/min Your answer
BThe P waves are upright in II, III and aVF Your answer
CThe atrial rate stays regular across the strip Your answer
DThe QRS measures around 90 ms in every lead Correct answer Your answer

A narrow QRS of about 90 ms indicates that the bundle branches and fascicles are intact, which strongly favours block within the AV node rather than in the infranodal conduction system. Infranodal block typically produces a broad QRS because it implies disease in the His bundle or distal branches. The ventricular rate, upright P waves in the inferior leads and regular atrial rate do not reliably localise the level of block.

Stage 2 of 2

Borrelia serology is sent and she is put on the monitor. Forty minutes later the nurse calls you to the bedside because the patient has gone pale and grey. This is the strip that was printing at the time. Her blood pressure is 88/54 while it runs, and she is awake and talking.

Tracing 2 of 2 for the quiz “Light-headedness on standing for two days, with one unwitnessed collapse”. — Lead II monitor strip recorded during the episode Open this tracing in the studio

What does this recording commit you to now?

AAdmit to a monitored bed and start intravenous ceftriaxone Correct answer Your answer
BBook a permanent dual-chamber pacemaker before discharge Your answer
CStart intravenous methylprednisolone and repeat the ECG tomorrow Your answer
DDischarge on oral doxycycline with a Holter in two weeks Your answer

The strip shows progression of the conduction disturbance with higher-degree block emerging, and the patient is now haemodynamically compromised. In a young patient with a tick bite and erythema migrans six weeks ago, this is Lyme carditis until proved otherwise. Intravenous ceftriaxone on a monitored bed is the standard of care for Lyme disease with cardiac involvement. A permanent pacemaker is premature because the block in Lyme carditis almost always resolves with antibiotics. Oral doxycycline is insufficient for cardiac Lyme disease, and discharge is unsafe with a progressing block.

Diagnosis

Lyme carditis causing second-degree AV block

Lyme carditis is a treatable cause of AV block that should be considered in any young patient with conduction disease and a plausible exposure history. The hallmark is a fluctuating block that can progress rapidly from first degree through second degree to complete heart block over hours, as this case illustrates.

Intravenous ceftriaxone is the standard treatment for Lyme carditis with higher-degree block, and the conduction disturbance typically resolves within days to weeks. A permanent pacemaker is almost never needed. The key is to recognise the pattern early, because the history of erythema migrans may not be volunteered unless specifically asked.

References

Stanek G, Wormser GP, Gray J, Strle F. Lyme borreliosis. Lancet. 2012;379(9814):461-473.
Wormser GP, Dattwyler RJ, Shapiro ED, et al. The clinical assessment, treatment, and prevention of Lyme disease, human granulocytic anaplasmosis, and babesiosis: clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis. 2006;43(9):1089-1134.

Light-headedness on standing for two days, with one unwitnessed collapse

The case

On arrival

Diagnosis

References

Discussion