Pulmonary embolism (PE)

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How the diagnosis is reached

Two risk factors compound here rather than merely coexist: fourteen hours of immobility three days ago, and an oestrogen-containing pill that raises clotting factor levels for as long as it is taken. Sudden pleuritic pain with hypoxia and a clear chest is the useful combination, because it is the absence of findings on auscultation that makes pneumonia and pneumothorax poor fits for this degree of hypoxia. The blood gas quietly confirms the mechanism. A pH of 7.49 with a PaCO2 of 29 is a respiratory alkalosis from blowing off carbon dioxide, meaning she is hyperventilating and still only managing 91 per cent, which is dead space rather than a ventilation problem. The ECG shows the same thing from another angle: sinus tachycardia, with a prominent S wave in lead I, a Q wave in lead III and an inverted T wave there, the S1Q3T3 pattern of an acutely strained right ventricle. It is worth recognising but not worth waiting for, specific enough to be a useful hint and far too insensitive to exclude anything, and plain sinus tachycardia is the commoner finding. She is haemodynamically stable, which is the finding that determines treatment. Anticoagulation alone is therefore enough, and a direct oral anticoagulant such as apixaban or rivaroxaban, both direct factor Xa inhibitors, is preferred for most patients; low molecular weight heparin is used instead in pregnancy and active cancer. It is started on suspicion rather than on the scan when suspicion is high, since the wait is the dangerous part. Systemic thrombolysis is held back for high-risk PE with haemodynamic instability, because the bleeding risk only justifies itself against an immediate threat to life.

Key clinical clues

• Risk factors: recent long-haul flight plus oral contraceptive use
• Sudden pleuritic chest pain, tachycardia, and hypoxia
• S1Q3T3 pattern on ECG
• CT pulmonary angiography showing bilateral filling defects
• DVT identified as the embolic source on lower extremity ultrasound

The case in full

1

Presenting complaint

Patient history

A 45-year-old woman presents with sudden-onset shortness of breath and right-sided pleuritic chest pain that began two hours ago. She returned from a 14-hour international flight three days ago and takes a combined oral contraceptive pill.

2

Physical examination

Physical examination

She is anxious and tachypneic but able to speak in full sentences. Breath sounds are equal bilaterally with no focal findings on auscultation.

Measurements

Parameter	Value	Reference
Heart rate	118 bpmRef 60–100	60–100
Respiratory rate	26 /minRef 12–20	12–20
SpO₂ (room air)	91 %Ref 95–100	95–100

3

Laboratory studies

Laboratory results

D-dimer and an arterial blood gas are obtained.

Measurements

Parameter	Value	Reference
D-dimer	Markedly elevatedRef Below assay cutoff	Below assay cutoff
Arterial pH	7.49Ref 7.35–7.45	7.35–7.45
PaCO₂	29 mmHgRef 35–45	35–45

4

12-lead ECG

ECG

The ECG shows sinus tachycardia with an S1Q3T3 pattern, a prominent S wave in lead I, a Q wave in lead III, and an inverted T wave in lead III.

5

CT pulmonary angiography

Imaging

CT pulmonary angiography shows filling defects in the right and left main pulmonary arteries, consistent with bilateral emboli.

6

Lower extremity ultrasound

Imaging

Doppler ultrasound of the lower extremities reveals an occlusive thrombus in the right popliteal vein, identifying a likely embolic source.