Clinical Cardiovascular Research

Abstract

Background

Syncope is a transient loss of consciousness and postural tone due to cerebral hypoperfusion, characterized by a rapid onset, short duration, and spontaneous complete recovery.1-5 Syncope has a wide-range of causes from the benign vasovagal episode to the more sinister, often life-threatening cardiac syncope (Table 1).  While some patients have classic presentations for either a low-risk or high-risk etiology and can be diagnosed by history alone, many fall into the undifferentiated category. The emergency physician (EP) must rapidly differentiate between patients who can safely be discharged and those who must be admitted for further evaluation. Standardized approaches to syncope risk-stratification rely upon the combination of a thorough history, physical examination including orthostatic vitals, electrocardiogram (ECG), and laboratory studies – none of which are particularly sensitive or specific.2 3 6 Moreover, clinical decision tools have not proven better than clinical gestaly and have not been adopted broadly as standard practice.7

Table 1:

Current guidelines also recommend transthoracic echocardiography (TTE) in the evaluation of syncope only when structural heart disease is suspected.4 5 However, most history and exam findings have been shown to be poorly sensitive and specific in detecting cardiac syncope.2 3 7 And while ECGs are essential to the ED workup of syncope, they cannot definitively rule out structural heart disease.9 Point-of-care ultrasound (POCUS) improves the diagnostic accuracy and management of a variety of conditions in the acute setting.10 11 To date there is no consensus on the role of POCUS in the evaluation of syncope in the ED, even though ultrasound proficiency, including basic echocardiography, is a core component of emergency medicine training. In this article, we describe a framework to use POCUS to identify obvious structural heart disease and other life-threatening pathology when caring for an ED patient who presents with undifferentiated syncope.

THE CVA (CardioVascular Assessment) SCAN

We propose the CardioVascular Assessment (CVA) scan - a rapid, inexpensive, three-step POCUS evaluation of the heart, inferior vena cava (IVC), and the abdomen - to assist in the recognition of high-risk features in the undifferentiated syncope patient (Figure 1A and 1B). The standard views described here are routinely employed by ED providers trained in basic POCUS. The protocol for the CVA scan is as follows:

C: Cardiac Structure - evaluated on parasternal long-axis view (PLAX) of the heart

1. Pericardial effusion - Is there an effusion? If so, are there signs of tamponade?

2. LV function - Are there signs of systolic or diastolic dysfunction?

3. Chamber size - Is the right ventricle (RV), the aortic root (Ao), or the left atrium (LA) relatively enlarged?

4. Aortic and mitral valves - Is there evidence of severe stenosis with almost no leaflet movement? Or obvious disruption of leaflet coaptation?

5. Other (rare) obstructive pathology - Is there an abnormal mass or sign of hypertrophic cardiomyopathy (HCM)?

V: Vena Cava/Volume Status - assessment of the inferior vena cava (IVC) in subxiphoid region or right upper quadrant (RUQ)

1. Does the IVC vary with respiration?

A: Aorta/Abdomen

1. Abdominal aorta - evaluated on transverse view - Is there aneurysmal dilatation?

2. Intra-abdominal hemorrhage. - evaluation of the RUQ, left upper quadrant (LUQ), and pelvis - Is there free fluid (FF)?

3. Uterus (in a pregnant patient) - evaluated trans-abdominally - Is there an intrauterine pregnancy (IUP)?

Fig1A:

Fig 1B:

Cases

The examples described are of real patients seen in our department for undifferentiated syncope (or near-syncope) with equivocal history/exam and nonspecific ECG findings - the ED POCUS was critical to diagnosis of a high-risk etiology in each case. They are not case studies but rather demonstrate the potential utility and feasibility of this protocol to assist the EP in identifying obvious structural disease when the standard evaluation is inadequate.

Case 1

A male in his 30s is brought in to the ED by ambulance after bystanders witnessed him collapse at a bus stop. He had no known medical problems, does not recall the event and continues to feel lightheaded. POCUS is completed immediately upon his arrival (Figure 2A and 2B). He is found to have a circumferential pericardial effusion with RV free wall collapse during diastole. He is diagnosed with cardiac tamponade physiology and undergoes urgent pericardiocentesis.

Fig 2A:

Fig 2B:

Case 2

A 21-year-old female presents to the ED with multiple episodes of near-syncope and lightheadedness. She uses methamphetamine recreationally. POCUS (Figure 3A) shows multi-chamber enlargement of her heart, severely depressed LV ejection fraction (EF). She is diagnosed with new cardiomyopathy secondary to methamphetamine use and found to have non-sustained ventricular tachycardia during admission.

Case 3

A 68-year-old male reports worsening palpitations and near-syncope in the last week. He has never been evaluated by a doctor, and does not take any medications. On POCUS (Figure 3B), he is found to have signs of diastolic heart disease with LV hypertrophy (LVH) and LA enlargement (LAE). During his inpatient stay telemetry shows paroxysmal atrial fibrillation with rapid ventricular response.

Fig 3A:

Fig 3B:

A 63-year-old male brought in by ambulance for syncope after feeling “dizzy” all day and has another witnessed syncopal episode on arrival. His POCUS (Figure 4A) demonstrates a large RV exerting mass effect on the LV prompting the EP to obtain a short axis view (Figure 4B) which confirms RV enlargement with a “D-sign.” He is found to have a saddle pulmonary embolism on CT and is promptly started on thrombolytic therapy.

Fig 4A:

Fig 4B:

Case 5

A 64-year-old male with a history of hypertension (HTN) who has been out of his medications presents for prescription refill and reports exertional dizziness and syncope last week. On POCUS (Figure 5) he has a dilated aortic root with a flap and descending thoracic aorta (DTA) enlargement. CT confirms thoracic aortic aneurysm with type A dissection - he is emergently taken to the OR with cardiothoracic surgery.

Fig 5:

Case 6

A 61-year-old male is brought to the ED after collapsing at work without warning - he only remembers waking up on the ground. He was told he had a “benign” heart murmur since he was a child but has no other medical problems. He has a loud systolic murmur over the left sternal border. POCUS (Figure 6) shows a thick, poorly mobile aortic valve. He is diagnosed with severe aortic stenosis on formal TTE and is referred for valve replacement.

Fig 6:

Case 7

A 61-year-old male from Mexico presents after fainting while on a ladder. Immediately afterward, he briefly had numbness on his right side and “fluttering” in his chest. On POCUS (Figure 7) he has LAE and classic findings of mitral stenosis. During admission he is diagnosed with rheumatic heart disease with severe mitral stenosis, has atrial fibrillation on telemetry and his numbness is attributed to a transient ischemic attack.

Fig 7:

Case 8

A 32-year-old female presents to the ED after having palpitations for several months, increasing dizziness and a witnessed syncopal episode prior to arrival. POCUS (Figures 8A and 8B) shows severe LAE, a prolapsed posterior MV leaflet, and color Doppler shows clear mitral regurgitation (MR). She is diagnosed with mitral valve prolapse (MVP) with severe MR on formal TTE and is referred for valve replacement.

Fig 8A:

Fig 8B:

Case 9

A 37-year-old male is brought in by ambulance after a syncopal episode while walking to the bathroom. He has had “acid reflux” and fatigue recently but is otherwise healthy. POCUS (Figure 9) shows a large extra-cardiac lesion with mass-effect on the RV. During admission he is diagnosed with a mediastinal mass consistent with lymphoma.

Case 10

A 35-year-old male with no known medical problems is brought to the ED to be medically cleared for incarceration. Following a brief chase on foot, he experienced severe lightheadedness. POCUS (Figure 10) reveals a dramatically thick LV with LAE, and he is diagnosed with HCM. Of note his ECG only shows LVH with early repolarization.

Fig 9:

Case 11

A 77-year-old female with HTN is brought in by family after she reported severe dizziness and nearly fainted at the dinner. Her POC echo is unremarkable, and POCUS of her abdomen (Figure 11) shows a dilated abdominal aorta, and CT confirms an abdominal aortic aneurysm (AAA) with signs of early rupture. She is immediately taken to the OR with vascular surgery.

Fig 11:

Case 12

A 19-year-old female presents to the ED after a syncopal episode. She had a positive home pregnancy test last week, but currently has no abdominal pain or vaginal bleeding. Her bedside urine pregnancy test is positive, and POCUS (Figure 12A and 12B) shows no visible IUP and significant FF in her RUQ. She is taken to the OR by OB and diagnosed with ruptured ectopic pregnancy and 1500mL of hemoperitoneum.

Fig 12 A:

Fig 12B:

Discussion

The CVA scan, as these examples demonstrate, offers a practical and streamlined approach to evaluating a patient who presents to the ED with undifferentiated syncope (Table 2).

Part 1: The Cardiac Ultrasound

While there are multiple echocardiographic views that are used to assess the heart, the PLAX view is usually the easiest to obtain. This single view allows for a rapid, systematic approach in detecting pericardial effusion, evaluating LV function, estimating chamber size, assessing the aortic and mitral valves, and visualizing more rare obstructive pathology.

Pericardial Effusion and Cardiac Tamponade

Beck’s triad of hypotension, jugular venous distention, and muffled heart sounds is neither sensitive nor specific for tamponade, but POCUS readily demonstrates pericardial effusions and signs of tamponade.12 13

Tamponade on PLAX:

1. 1. Pericardial effusion – a hypoechoic stripe of fluid seen posteriorly between the LV free wall and pericardium and/or anteriorly above the RV
   2. Diastolic collapse of RV free wall (bowing in of the RV free wall when the MV is open)

LV Function

Systolic Dysfunction

EPs can visually estimate LV systolic function reliably using POCUS, and syncope in the setting of reduced ejection fraction (EF) is unequivocally associated with increased risk for adverse cardiac events and all-cause mortality.1-5 8 10

LV Systolic Dysfunction on PLAX:

1. 1. Globally hypokinetic LV with reduced radial movement of LV free wall and septal wall
   2. Poor thickening of the LV walls with each contraction
   3. Anterior leaflet of the mitral valve (ALMV) does not approach the septum when the MV opens during diastole

Diastolic Dysfunction

The structural changes in diastolic heart disease are associated with increased risk of atrial fibrillation and other supraventricular tachyarrhythmias even when LVEF is normal.1-5 8

LV Diastolic Dysfunction on PLAX:

1. 1. LVH (markedly thickened LV walls)
   2. LAE (larger relative to RV and Ao)

Chamber Size

On a normal PLAX view, the RV, Ao, and LA are all roughly similar in size. If any one of these structure is disproportionately larger than the others, chamber enlargement is suggested.4 5 8 10

RV Enlargement and Pulmonary Embolism (PE)

POCUS can be useful in the diagnosis of proximal PE with large clot burden by identifying signs of RV strain.14 It is important to note that many of the signs of RV dysfunction can also be seen in chronic pulmonary hypertension, another known structural cause of syncope. Ultimately, CT is necessary in definitively identifying the etiology of the RV dysfunction discovered on POCUS.14 15

PE on PLAX:

1. 1. RV enlargement relative to LA and aortic root
   2. RV strain -  severe RV enlargement causing collapse of the usually much larger LV
   3. Advanced ED sonographers: classic “D-sign” (enlarged RV flattens the inter-ventricular septum) on parasternal short axis, and the well-described McConnell’s sign on apical four-chamber view15

Aortic Root Enlargement and Thoracic Aortic Dissection

While CT angiography remains the gold standard in confirming aortic dissection (AD), if visualized on POCUS, a thoracic aortic aneurysm or dissection flap can signal this critical diagnosis and expedite definitive treatment.16-18

Thoracic AD on PLAX:

1. 1. 1. Thoracic aortic aneurysm: >4cm aortic root diameter16
      2. Visible intimal flap
      3. Advanced ED sonographers: color Doppler across the aortic valve (AV) on PLAX can detect significant aortic insufficiency and peri-aortic hematoma indicates rupturing dissection

LA Enlargement

LAE is noted in many of the case examples - as discussed, it is present in both systolic and diastolic dysfunction. When LV function is normal, the presence of LAE suggests MV pathology.4 5 8

Valvular Disease - Aortic and Mitral Valves

Though seen as a more advanced POCUS skill, multiple studies have demonstrated that EPs can qualitatively assess valvular disease with high sensitivity and specificity with basic training.19 20

Aortic Stenosis (AS)

Syncope is a cardinal symptom of AS. A patient who presents to the ED with syncope from AS has a significantly increased risk of sudden cardiac death (SCD).19 20

AS on PLAX:

1. 1. Severely thickened, poorly mobile AV leaflets with poor excursion
   2. LVH – common associated finding

Mitral Stenosis (MS)

Diastolic murmurs are notoriously difficult to recognize, and MS can easily be missed.19

MS on PLAX:

1. 1. Thickened distal portion of MV leaflets with normal proximal segments 
   2. “Hockey stick” or “elbow” appearance of the ALMV during diastole - tethered distal segment and anterior ballooning of proximal segment
   3. LAE – frequent associated finding

MVP and MR

Less common than AS or MS, MVP with MR is still an important cause of syncope and can result in deadly dysrhythmias if undiagnosed.19 21 22

MVP and MR on PLAX:

1. 1. MV leaflet prolapsing into the LA with disruption of normal coaptation
   2. LAE — frequent associated finding
   3. Advanced ED sonographers: color Doppler can show significant MR with a jet visible in the LA during systole 

Rare Causes of Obstruction: Masses and HCM

Masses

These phenomena are difficult to diagnose and aren’t common causes of syncope, but they have large implications for management and are dangerous to miss. Most cardiac and extra-cardiac masses large enough to cause obstruction may be visualized on the PLAX view unless they exclusively involve the RA or tricuspid valve. 23 24

Masses on PLAX:

1. 1. Visualization of mass – often valvular, atrial, or extra-cardiac in origin
   2. Advanced ED sonographers: additional echo views can assess for mass lesions not readily visible on PLAX

HCM

Although HCM is rare, sonographers should keep a high index of suspicion for it in young patients who present with exertional syncope, particularly if they have the classic POCUS findings noted here. 25 26

HCM on PLAX:

1. 1. LV wall thickness >15mm in at least one segment commonly with asymmetric septal hypertrophy 25 26
   2. LAE
   3. Advanced ED sonographers: presence of systolic anterior motion (SAM) of ALMV – the MV apparatus is pulled toward the LVOT during systole, worsening outflow obstruction 25 26

Part 2: The Vena Cava Assessment: Right Heart Dysfunction, Obstruction, and Volume Status

In a healthy euvolemic patient (not on positive pressure ventilation), the IVC diameter normally varies with respiration.12 27

Plethoric IVC

A plethoric IVC with no respirophasic variation suggests either RV dysfunction or obstruction. RV dysfunction can be acute or chronic, and the cardiac portion of the scan, the clinical context, and further confirmatory testing are essential in distinguishing between the two.12 27 In addition to diagnostic utility, this sonographic finding has important implications for treatment: a patient who has a dilated IVC with no respirophasic variation may be poorly tolerant of aggressive volume resuscitation.12 27

Collapsing IVC

A flat IVC with complete collapse during inspiration effectively rules out significant right heart dysfunction and outflow obstruction, especially in the setting of a normal cardiac ultrasound. This finding suggests significant intravascular volume depletion, and from a therapeutic standpoint, it indicates that the patient may benefit from volume resuscitation.27

Part 3: Abdominal POCUS: Other Life-threatening Causes of Syncope

The final component of the protocol incorporates basic abdominal POCUS views to identify other life-threatening causes of syncope that EPs must diagnose and treat promptly: abdominal aortic aneurysm, ruptured ectopic pregnancy, and intra-abdominal hemorrhage from other causes.

Abdominal Aortic Aneurysm (AAA)

There is good evidence that POCUS performed by ED providers is both sensitive and specific in assessing for AAA.28 29 Using the curvilinear transducer, the abdominal aorta is identified in the epigastrium in transverse orientation and followed down to the iliac bifurcation. A diagnosis of AAA is made if the AP diameter is >3cm.28 29

Ruptured Ectopic Pregnancy

EPs are capable of using POCUS to promptly diagnose this emergency.30 In the case of a female patient of childbearing age who presents to the ED with syncope and a positive pregnancy test, intra-abdominal FF and no visible IUP on POCUS can be considered diagnostic of a ruptured ectopic pregnancy until proven otherwise.30

Intra-abdominal Hemorrhage from Other Etiologies

Detection of FF in the abdomen in a syncope patient can suggest bleeding from other sources, and POCUS can assist in the rapid diagnosis of life-threatening intra-abdominal hemorrhage.

Table 2:

Conclusion

In the CVA scan, we propose a structured approach for the use of POCUS to augment current diagnosis and management of high-risk syncope. This tool is not suggested as a substitute for clinical judgment or a replacement for formal TTE and other more sensitive imaging modalities. It is simply meant to improve the EP’s ability to more easily recognize potentially life-threatening etiologies of undifferentiated syncope that may not be apparent on history or physical examination, and The views we suggest are designed to be simple and accessible to even novice ED sonographers. We acknowledge that limited assessment (particularly of the heart) and other factors such as inadequate images, incorrect interpretation, or equivocal findings can lead to missed diagnoses. As is the case for POCUS in other contexts, if there is any doubt or concern, a comprehensive or definitive study should be obtained, and if indicated, consultation with specialists. However, even as we further study this topic and continue to better our ultrasound capabilities in emergency medicine, we still feel that this framework provides a promising and feasible approach in the practical and thoughtful use of POCUS in the ED evaluation of syncope.

References

1. Saklani, P., Krahn, A., & Klein, G. (2013). Syncope. Circulation, 127(12), 1330-1339.
   View at Publisher | View at Google Scholar
2. Albassam, O. T., Redelmeier, R. J., Shadowitz, S., Husain, A. M., Simel, D., & Etchells, E. E. (2019). Did this patient have cardiac syncope?: the rational clinical examination systematic review. JAMA, 321(24), 2448-2457.
   View at Publisher | View at Google Scholar
3. Reed, M. J. (2019). Approach to syncope in the emergency department. Emergency Medicine Journal, 36(2), 108-116.
   View at Publisher | View at Google Scholar
4. Brignole M, Moya A, de Lange FJ, Deharo JC, Elliott PM, Fanciulli A, Fedorowski A, Furlan R, Kenny RA, Martín A, Probst V, Reed MJ, Rice CP, Sutton R, Ungar A, van Dijk JG. 2018 ESC Guidelines for the diagnosis and management of syncope. European Heart Journal. 2018 Jun 1;39(21):1883-1948.
   View at Publisher | View at Google Scholar
5. Shen, W. K., Sheldon, R. S., Benditt, D. G., Cohen, M. I., Forman, D. E., Goldberger, Z. D., ... & Yancy, C. W. (2017). 2017 ACC/AHA/HRS guideline for the evaluation and management of patients with syncope: executive summary: a report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. Journal of the American College of Cardiology, 70(5), 620-663.
   View at Publisher | View at Google Scholar
6. Chiu, D. T., Shapiro, N. I., Sun, B. C., Mottley, J. L., & Grossman, S. A. (2014). Are echocardiography, telemetry, ambulatory electrocardiography monitoring, and cardiac enzymes in emergency department patients presenting with syncope useful tests? A preliminary investigation. The Journal of emergency medicine, 47(1), 113-118.
   View at Publisher | View at Google Scholar
7. Serrano, L. A., Hess, E. P., Bellolio, M. F., Murad, M. H., Montori, V. M., Erwin, P. J., & Decker, W. W. (2010). Accuracy and quality of clinical decision rules for syncope in the emergency department: a systematic review and meta-analysis. Annals of emergency medicine, 56(4), 362-373.
   View at Publisher | View at Google Scholar
8. Sarasin, F., Junod, A. F., Carballo, D., Slama, S., Unger, P. F., & Louis-Simonet, M. (2002). Role of echocardiography in the evaluation of syncope: a prospective study. Heart, 88(4), 363-367.
   View at Publisher | View at Google Scholar
9. Stone, J. R., Lee, L., Ward, J. P., & Ward, R. P. (2018). High prevalence of clinically important echocardiographic abnormalities in patients with a normal electrocardiogram referred for transthoracic echocardiography. Journal of the American Society of Echocardiography, 31(8), 926-932.
   View at Publisher | View at Google Scholar
10. Balderston, J. R., Gertz, Z. M., Brooks, S., Joyce, J. M., & Evans, D. P. (2019). Diagnostic yield and accuracy of bedside echocardiography in the emergency department in hemodynamically stable patients. Journal of Ultrasound in Medicine, 38(11), 2845-2851.
    View at Publisher | View at Google Scholar
11. Smallwood, N., & Dachsel, M. (2018). Point-of-care ultrasound (POCUS): unnecessary gadgetry or evidence-based medicine?. Clinical Medicine, 18(3), 219-224.
    View at Publisher | View at Google Scholar
12. Alerhand, S., & Carter, J. M. (2019). What echocardiographic findings suggest a pericardial effusion is causing tamponade?. The American journal of emergency medicine, 37(2), 321-326.
    View at Publisher | View at Google Scholar
13. Nagdev, A., & Stone, M. B. (2011). Point-of-care ultrasound evaluation of pericardial effusions: does this patient have cardiac tamponade?. Resuscitation, 82(6), 671-673.
    View at Publisher | View at Google Scholar
14. Goldhaber, S. Z., & Bounameaux, H. (2012). Pulmonary embolism and deep vein thrombosis. The Lancet, 379(9828), 1835-1846.
    View at Publisher | View at Google Scholar
15. Fields, J. M., Davis, J., Girson, L., Au, A., Potts, J., Morgan, C. J., ... & Riesenberg, L. A. (2017). Transthoracic echocardiography for diagnosing pulmonary embolism: a systematic review and meta-analysis. Journal of the American Society of Echocardiography, 30(7), 714-723.
    View at Publisher | View at Google Scholar
16. Elefteriades, J. A., Mukherjee, S. K., & Mojibian, H. (2020). Discrepancies in measurement of the thoracic aorta: JACC review topic of the week. Journal of the American College of Cardiology, 76(2), 201-217.
    View at Publisher | View at Google Scholar
17. Perkins, A. M., Liteplo, A., & Noble, V. E. (2010). Ultrasound diagnosis of type A aortic dissection. The Journal of emergency medicine, 38(4), 490-493.
    View at Publisher | View at Google Scholar
18. Bernett, J., & Strony, R. (2017). Diagnosing acute aortic dissection with aneurysmal degeneration with point of care ultrasound. The American Journal of Emergency Medicine, 35(9), 1384-e3.
    View at Publisher | View at Google Scholar
19. Chen, R. S., Bivens, M. J., & Grossman, S. A. (2011). Diagnosis and management of valvular heart disease in emergency medicine. Emergency Medicine Clinics, 29(4), 801-810.
    View at Publisher | View at Google Scholar
20. Alzahrani, H., Woo, M. Y., Johnson, C., Pageau, P., Millington, S., & Thiruganasambandamoorthy, V. (2015). Can severe aortic stenosis be identified by emergency physicians when interpreting a simplified two-view echocardiogram obtained by trained echocardiographers?. Critical ultrasound journal, 7, 1-4.
    View at Publisher | View at Google Scholar
21. Hayek, E., Gring, C. N., & Griffin, B. P. (2005). Mitral valve prolapse. The Lancet, 365(9458), 507-518.
    View at Publisher | View at Google Scholar
22. Watanabe, N. (2019). Acute mitral regurgitation. Heart, 105(9), 671-677.
    View at Publisher | View at Google Scholar
23. Kocabas, U., Zekican, G., Vaiz, G., Ozbek, N., & Conkbayır, C. (2018). Syncope and right-sided heart failure due to right ventricular outflow tract obstruction by giant sinus of Valsalva aneurysm. Cardiology Journal, 25(2), 281-282.
    View at Publisher | View at Google Scholar
24. Animashaun, I. B., Akinseye, O. A., Akinseye, L. I., & Akinboboye, O. O. (2015). Right atrial myxoma and syncope. The American Journal of Case Reports, 16, 645.
    View at Publisher | View at Google Scholar
25. Mandeş, L., Roşca, M., Ciupercă, D., & Popescu, B. A. (2020). The role of echocardiography for diagnosis and prognostic stratification in hypertrophic cardiomyopathy. Journal of echocardiography, 18, 137-148.
    View at Publisher | View at Google Scholar
26. Cecchi F, Charron P, Hagege AA, Lafont A, Limongelli G, Mahrholdt H, McKenna WJ, Mogensen J, Nihoyannopoulos P, Nistri S, Pieper PG, Pieske B, Rapezzi C, Rutten FH, Tillmanns C, Watkins H. 2014 ESC Guidelines on diagnosis and management of hypertrophic cardiomyopathy: the Task Force for the Diagnosis and Management of Hypertrophic Cardiomyopathy of the European Society of Cardiology (ESC). European Heart Journal. 2014 Oct 14;35(39):2733-79.
    View at Publisher | View at Google Scholar
27. Mosier, J. M., Martin, J., Andrus, P., Clinton, M., Demla, V., Vi Am Dinh, M. D., ... & Tayal, V. (2018). Advanced hemodynamic and cardiopulmonary ultrasound for critically ill patients in the emergency department. Emerg Med, 50(1), 17-34.
    View at Publisher | View at Google Scholar
28. Rubano, E., Mehta, N., Caputo, W., Paladino, L., & Sinert, R. (2013). Systematic review: emergency department bedside ultrasonography for diagnosing suspected abdominal aortic aneurysm. Academic Emergency Medicine, 20(2), 128-138.
    View at Publisher | View at Google Scholar
29. Metcalfe, D., Sugand, K., Thrumurthy, S. G., Thompson, M. M., Holt, P. J., & Karthikesalingam, A. P. (2016). Diagnosis of ruptured abdominal aortic aneurysm: a multicentre cohort study. European Journal of Emergency Medicine, 23(5), 386-390.
    View at Publisher | View at Google Scholar
30. Stone, B. S., Muruganandan, K. M., Tonelli, M. M., Dugas, J. N., Verriet, I. E., & Pare, J. R. (2021). Impact of point-of-care ultrasound on treatment time for ectopic pregnancy. The American Journal of Emergency Medicine, 49, 226-232.
    View at Publisher | View at Google Scholar