Squiggler

Breathlessness during a viral chest infection

The case

55-year-old woman. Breathlessness during a viral chest infection

Dilated cardiomyopathy with a documented left bundle branch block. No chest pain and no syncope.

On arrival

HR
84
GCS
15
SpO₂ (%)
98
Respiratory rate
16
Temperature (°C)
36.7
Systolic BP (mmHg)
126
Diastolic BP (mmHg)
74

Stage 1 of 1

She is mildly breathless but haemodynamically stable.

The tracing for the quiz “Breathlessness during a viral chest infection”.
Twelve-lead recorded on arrival, 25 mm/s, 10 mm/mV. Open this tracing in the studio

Observations

HR
84
SpO₂ (%)
98
Respiratory rate
16
Temperature (°C)
36.7
Systolic BP (mmHg)
126
Diastolic BP (mmHg)
74
What conduction abnormality is present?

The QRS duration is about 143 ms, well above the 120 ms threshold for complete bundle branch block. The tracing shows broad, notched R waves in the lateral leads and deep S waves in V1 to V3, which is the classic left bundle branch block pattern. Right bundle branch block would produce an RSR' in V1 with wide terminal S waves laterally. Pre-excitation would shorten the PR interval and produce a delta wave at the onset of the QRS, neither of which is seen here.

How should the ST-T changes be interpreted?

In left bundle branch block the ST segment and T wave are expected to point opposite to the dominant QRS deflection. On this tracing, leads with a tall R wave show ST depression and T-wave inversion, while leads with a deep S wave show ST elevation. This appropriate discordance is a secondary repolarisation change driven by the abnormal depolarisation sequence, not by ischaemia or pericarditis. Hyperkalaemia would produce peaked T waves and QRS widening without this lead-by-lead mirror pattern.

Which additional ECG feature would be most concerning for acute coronary occlusion?

The Sgarbossa criteria identify acute infarction in the presence of LBBB. The most specific finding is concordant ST elevation of at least 1 mm in a lead where the QRS is predominantly positive, because this change runs against the expected discordant direction and cannot be explained by the conduction abnormality alone. Discordant T-wave inversion, deep S waves in V1 to V3 and broad notched lateral R waves are all expected features of LBBB itself and carry no independent diagnostic weight for ischaemia.

Diagnosis

Chronic complete left bundle branch block with secondary repolarisation changes

Left bundle branch block alters depolarisation so profoundly that the ST segments and T waves are expected to point opposite to the main QRS deflection. This appropriate discordance is not ischaemia and does not, by itself, warrant activation of a catheterisation laboratory.

The feature that should raise the alarm is concordant ST elevation: ST shift in the same direction as the QRS, which the conduction abnormality cannot explain. The modified Sgarbossa criteria quantify this and remain the most validated tool for diagnosing acute coronary occlusion in the presence of LBBB. In a patient with known LBBB and no new concordant changes, the tracing is reassuring.

References

  1. Sgarbossa EB, Pinski SL, Barbagelata A, et al. Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle-branch block. N Engl J Med. 1996;334(8):481-487.
  2. Smith SW, Dodd KW, Henry TD, Dvorak DM, Pearce LA. Diagnosis of ST-elevation myocardial infarction in the presence of left bundle branch block with the ST-elevation to S-wave ratio in a modified Sgarbossa rule. Ann Emerg Med. 2012;60(6):766-776.
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Discussion

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