Three months of the heartbeat stumbling, with an occasional sensation that it stops

62-year-old woman. Three months of the heartbeat stumbling, with an occasional sensation that it stops

No cardiac history. Drinks four coffees a day. No syncope, no chest pain, no breathlessness.

Stage 1 of 2

The pulse is irregular at rest. Examination is otherwise normal. Potassium, magnesium and thyroid function are normal, and an echocardiogram shows a structurally normal heart.

Tracing 1 of 2 for the quiz “Three months of the heartbeat stumbling, with an occasional sensation that it stops”. — Twelve-lead in clinic, 25 mm/s, 10 mm/mV. Open this tracing in the studio

Bloods

TSH (m IU/L): 1.9
Magnesium (mmol/L): 0.88
Potassium (mmol/L): 4.3

What does this tracing show?

AFrequent atrial ectopics, with occurence of blocked ectopics Correct answer Your answer
BMobitz II second-degree atrioventricular block Your answer
CIntermittent sinus arrest Your answer
DFrequent ventricular ectopics Your answer

The rhythm strip shows a sinus baseline interrupted by premature P waves, some followed by a narrow QRS and others blocked, producing pauses. The narrow QRS of the conducted ectopics excludes a ventricular origin. Mobitz II would show P waves at the sinus rate with intermittently dropped beats rather than early P waves deforming the preceding T wave. Sinus arrest would produce pauses without any preceding premature atrial activity.

Which feature separates the pauses here from second-degree atrioventricular block?

AEach pause measures two sinus cycles Your answer
BProgressive PR lengthening into each pause Your answer
CA premature P wave distorting the T wave before each pause Correct answer Your answer
DThe pauses fall in a fixed ratio to the conducted beats Your answer

In second-degree AV block, P waves march at the sinus rate and the dropped beat falls where the next sinus P wave would be expected. Here, each pause is preceded by a premature P wave that distorts the T wave of the prior beat, identifying the mechanism as a non-conducted premature atrial complex rather than a conduction failure. The pause length is not a simple multiple of the sinus cycle, which also argues against block.

Stage 2 of 2

She is started on bisoprolol 2.5 mg daily and reviewed six weeks later. She reports the palpitations are much less frequent. Her pulse is regular at 78. This tracing is recorded at that visit.

Tracing 2 of 2 for the quiz “Three months of the heartbeat stumbling, with an occasional sensation that it stops”. — Twelve-lead at review, 25 mm/s, 10 mm/mV. Open this tracing in the studio

Observations

HR: 78
SpO₂ (%): 98
Respiratory rate: 14
Temperature (°C): 36.6
Systolic BP (mmHg): 124
Diastolic BP (mmHg): 74

What has changed between the two tracings?

AThe same number of atrial ectopics, now all conducted Your answer
BFewer atrial ectopics, and those that remain are conducted Correct answer Your answer
CThe second-degree atrioventricular block has resolved Your answer
DThe ectopic focus has moved below the atrioventricular node Your answer

The second tracing shows a more regular rhythm with fewer premature atrial complexes, and those that remain are all followed by a QRS complex. The non-conducted ectopics that produced the pauses on the first tracing have gone. The QRS morphology of the remaining ectopics is still narrow and unchanged, so the focus has not migrated below the AV node, and there was no second-degree block to resolve.

Her pulse is faster at review than before the beta blocker was started. Why?

AThe dose is too low to have taken effect Your answer
BThe sinus node is discharging faster than before Your answer
CThe beta blocker has speeded conduction through the atrioventricular node Your answer
DLess automaticity leads to less PACs Correct answer Your answer

On the first tracing, frequent non-conducted premature atrial complexes produced pauses that lowered the palpable pulse despite a sinus rate of about 78. The beta blocker has suppressed the ectopic focus, so fewer PACs fire and none are blocked, and the pulse now reflects the underlying sinus rate. The sinus node itself has not accelerated, and AV conduction has not changed.

Diagnosis

Frequent atrial ectopy with non-conducted atrial ectopic beats, suppressed by beta blockade

Premature atrial and ventricular complexes frequently lead to beats that aren't palpable or heard on auscultation. This effect can be even more pronounced when PACs are non-conducted.

Here, beta blockers can paradoxically lead to higher pulse due to suppressed automaticity and regularisation of the rhythm.

References

Conen D, Adam AG, Roche F, et al. Premature atrial contractions in the general population: frequency and risk factors. Circulation. 2012;126(19):2302-2308.
Binici Z, Intzilakis T, Nielsen OW, et al. Excessive supraventricular ectopic activity and increased risk of atrial fibrillation and stroke. Circulation. 2010;121(17):1904-1911.

Three months of the heartbeat stumbling, with an occasional sensation that it stops

The case

On arrival

Bloods

Observations

Diagnosis

References

Discussion