Case #005

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.