**2. Epidemiology**

Acute pulmonary embolism (PE) is an acute critical clinical condition characterized by the propagation of blood clots from peripheral veins or systemic circulation to the lung vasculature affecting the alveolar gas exchange. Acute PE can be

symptomatic or silent. The thrombus responsible for PE often originates from leg veins, especially the deep calf veins, followed by proximal dispersion to popliteal and femoral veins [1]. Thrombus at popliteal vein and proximal to it are at a high risk of embolic phenomenon resulting in acute PE. A non-propagating deep calf vein thrombus increases recurrence rate and the likelihood of postphlebitic complication [2]. A thrombus from the upper extremity is often due to intravascular venous catheters, cardiac devices, effort thrombosis, or thoracic outlet obstruction [3]. Pelvic veins represent another source of emboli in patients with recent pelvic surgery, pregnancy, infection, or prostate disease. Rarely pulmonary vascular occlusion occurs due to nonthrombus etiology such as parasites (schistosomiasis), sickled erythrocytes (sickle cell disease), talc (illicit drugs), air (central lines), or tissue (amniotic fluid or fat embolism).

Earlier clinical literature suggested PE as an underdiagnosed condition; however, recent studies indicate it to be an excessively diagnosed condition due to the introduction of modern imaging techniques in detecting PE [4, 5]. Newer studies indicate an increased incidence at >113 cases per 100,000 population [5]. Another reason is defensive medicine, as the inability to identify a clinically symptomatic patient could turn out to be a malpractice issue as only 8% die with appropriate therapy, and the figure is 30% with no therapy [6–8]. Even with an increased incidence, the overall mortality rate has remained the same, declining case fatality rates [5]. The DVT/PE incidence rate in the United States of America (USA) yearly is 600,000 patients per year [9]. Approximately 30% of these patients die within the next 3 months (180,000 per year) [4]. In medical or surgical intensive care units (MICU/SICU), deep vein thrombosis (DVT) occurs in 30% of patients [10, 11]. In an extensive registry of diagnosed DVT patients, PE was seen in 29% in the lower extremity(LE) and 9% in the arms [12]. PE occurrence was similar in these groups on observing them over the next 90 days. PE is a frequent preventable mortality source in hospitalized patients [13]. Despite anticoagulant therapy in critically ill, acute PE is linked with considerable morbidity and mortality due to a limited cardiopulmonary reserve [14]. After the acute critical episode, patients who make it out are at higher risk of type four pulmonary hypertension and postthrombotic syndrome. A recent study confirms that after 6 months of a PE episode, dissolution of the entire clot was observed in 50% of patients, and the remaining still had lingering occlusion [15, 16].

### **3. Clinical features**

Symptomatic patients with acute respiratory failure should increase diagnostic possibility if they have risk elements. These risk factors have been mentioned in **Table 1** [1, 13, 14].

Clinical features depend on the patient's physiologic response to the venous thrombus, especially cardiopulmonary reserve, and vary from asymptomatic to hemodynamic instability and death. An excellent clinical history can reveal risks, including hormone replacement therapy, bed rest, air or road travel, oral contraceptive use, and other comorbid conditions. Clinical symptoms include acute respiratory distress (most common), chest discomfort, dry cough, fever, leg swelling with or without pain, bloody expectoration, and rarely syncopal episode. The physical examination can reveal tachycardia, tachypnea, hypotension, phlebitis, rales, a loud P2, and an S4. It may also reveal other signs indicative of risk factors. Of the above clinical features, only three can distinguish between positive and negative PE based on angiogram, including rales, a loud P2, and S4 [17]. Clinical presentation to a hospital is seen via five different syndromes, which include 1) Pleuritic chest discomfort or bloody

