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Pulmonary embolus is another clinical condition that can present to EMS clinicians with respiratory distress. Classic risk factors for venous thromboembolism (VTE) include the Virchow triad of venous stasis, trauma, and hypercoagulability. There are many risk factors for VTE, but the ones that have been clinically validated by Wells criteria and the PERC rule for risk stratification include recent surgery or immobilization of an extremity, malignancy, exogenous estrogen use, and prior DVT [64]. Other notable risk factors include genetic deficiency of anticlotting factors, pregnancy, obesity, and extended travel.

Pulmonary embolus is a challenging clinical diagnosis because the manifestations can be subtle. The most common symptom is dyspnea, and the most common clinical signs are tachycardia and tachypnea. The pulmonary exam is usually unremarkable, although examination of the extremities, particularly the legs, may reveal swelling, erythema, and possibly pain in a limb with a DVT. With the increased use of peripherally inserted central venous catheters, pulmonary emboli are also reported more frequently because of upper extremity DVTs [65]. Small emboli often present with respiratory distress. Larger emboli that cause lung infarction can present with findings such as pleuritic chest pain and hemoptysis, and those with massive saddle embolism cause findings suggestive of obstructive shock (Box 5.2). The latter can be detected by findings such as right axis deviation, right ventricle strain, and right bundle branch block on a 12‐lead ECG. Additional useful ECG features for pulmonary embolus include the presence of T‐wave inversions in both V₁ and lead III, an S wave in lead I, and a Q wave and inverted T wave in lead III (S1Q3T3) [66]. Acute right ventricular dysfunction can also be visualized using portable ultrasonography.

EMS treatment priorities include high‐flow oxygen, vascular access, and cardiac monitoring. A fluid bolus is reasonable in the patient who presents with a suspected massive pulmonary embolus and perfusion failure. In some patients, the presentation can take the form of a witnessed nontraumatic cardiac arrest with narrow complex pulseless electrical activity as the initial rhythm. The presence of respiratory distress, altered mental status, and a shock index (heart rate/systolic blood pressure) >0.8 have also been shown to be predictive of cardiac arrest in suspected pulmonary embolus patients [67]. A rapidly declining EtCO₂ can also be a harbinger of impending arrest. Although the use of prehospital thrombolysis in these instances has been reported to be effective in selected cases, a randomized controlled clinical trial failed to show improved outcomes during cardiac arrest when t‐PA was administered compared to placebo for patients with refractory pulseless electrical activity [68, 69].