How Ultrasound Helps with the Diagnosis of Pulmonary Embolism

 

Ultrasound Machines for Diagnosis of Pulmonary Embolism 

Technological advances in ultrasound machines have helped them make inroads into various branches of the medical field. Their potential applications have expanded from routine and simple investigations to diagnoses of major ailments and interventional procedures. Ultrasound machines can now diagnose even a fatal and underdiagnosed medical condition like pulmonary embolism.  

Here we explore the benefits that ultrasound machines offer over other technologies in diagnosing as well as treating pulmonary embolism, along with their limitations and relevant research findings. 

What is Pulmonary Embolism?  

Pulmonary embolism (PE) is a sudden blockage in one of the pulmonary arteries in the lung, usually caused by a blood clot. Since blood clots can block blood flow to the lung, PE can be fatal unless treated promptly. As such, timely diagnosis of PE is potentially lifesaving and therefore, crucial in its treatment. 

Why is Pulmonary Embolism often Undiagnosed?  

The danger of PE lies in it being frequently underestimated, underdiagnosed, and undertreated. According to Mayo Clinic, about one-third of people with undiagnosed and untreated PE do not survive. Hence, it is crucial to diagnose and treat it promptly. The difficulty, however, lies in there being nearly no single diagnostic test that is noninvasive and with sufficient sensitivity for diagnosis in all suspected cases. 

 One of the diagnostic methods is a combination of clinical probability, ventilation-perfusion lung scanning, and lower extremity sonography. However, while lung scanning, computer tomography pulmonary angiography (CTPA), and pulmonary angiography are important for diagnosis, unfortunately these tests are not widely used, making them unavailable at emergency units. This makes it difficult to diagnose PE in emergency settings, which compels the need for an alternative bedside diagnostic test that is both easy and widespread. 

Role of Transthoracic Ultrasound in Diagnosing Pulmonary Embolism 

What role can ultrasound machines play in diagnosing pulmonary embolism? Here we explore the diagnostic significance of transthoracic parenchymal sonography of the lung in patients with PE. Transthoracic Ultrasound (TUS) is advantageous in diagnosing PE in suspected cases, as it is cheap, widely available, and immediately accessible at patients’ bedside.  

The most characteristic criterion in diagnosing PE is hypoechoic, pleural-based paranchymal alteration. While most of these lesions are wedge-shaped, they may also be polygonal or rounded. Doppler color ultrasound machines may help us explore lesions for additional diagnostic information.  

Study of Transthoracic Ultrasound Diagnosis of Pulmonary Embolism 

Researchers conducted a prospective study to determine the accuracy of ultrasound diagnosis of pulmonary embolism in patients with a moderate to high clinical suspicion of PE. In total, 50 consecutive patients with a moderate to high clinical suspicion of PE were enrolled and evaluated. According to a study you can find here, researchers used multislice CTPA as the reference method in diagnosing PE.  

Apart from CTPA and TUS, the other examinations performed on all patients were serum d-dimer level, echocardiography, duplex sonography of bilateral lower extremity veins, and arterial blood gases measurements.  

Some of the pulmonary embolism ultrasound findings of the study were: 

1. TUS detected 64 lesions while CTPA detected 29 parenchymal lesions in 30 PE patients. This translates into TUS giving a mean detection of 2.14 lesions per patient, compared to CTPA’s mean of only 1 lesion per patient. Furthermore, in 6 (20%) of the PE positive cases, even when paranchymal lesion was not detected in CTPA, TUS detected sonographic parenchymal lesions of PE. Furthermore, sonography provides a better resolution in the subpleural region. However, CTPA detected PE in 3 patients with a negative TUS. These findings show that positive TUS results may prove helpful in diagnosing PE; however, a negative TUS result cannot rule out PE with certainty. This shows that transthoracic ultrasound may be used as a screening method for PE in an emergency setting.
   
2. Out of 30 patients diagnosed with PE, TUS was found to be true positive in 27, false positive in 8, true negative in 12, and false negative in three patients. Overall, TUS was found to have a sensitivity of 90%, specificity of 60%, positive predictive value of 77.1%, negative predictive value of 80%, and a diagnostic accuracy of 78% in clinically suspected patients. 
3. The study found that pathological lesions – consolidation, atelectasis, and local pleural effusion – are easily identifiable by TUS. Furthermore, changes such as intraalveolar hemorrhage, necrosis, atelectasis due to loss of surfactant, increased permeability because of mediator secretion and alveolar edema occur due to thromboembolic occlusion of pulmonary artery. These changes are found to occur mostly in the subpleural area of the lung periphery and they provide an ultrasonographic window, regardless of the presence or absence of pleural effusion. The good news is that ultrasound is able to detect early on the formation of these lesions that takes place within minutes. 

Techniques such as multislice CTPA are not feasible as these are costly, typically unavailable at every medical center, and these expose patients to potentially harmful radiation. Hence, diagnosing PE still poses significant challenges, especially in emergency settings. In addition, given the significance of a prompt diagnosis and treatment for PE, decisions about suspected PE cases need to be made in real time and that too, within a short span. 

TUS helps resolve these challenges, as it is non-invasive, widely available, cost effective, and quick. Moreover, while a negative TUS result does not rule out PE with certainty, a positive TUS with a moderate to high suspicion for PE may prove to be useful in diagnosing PE at point-of-care situations, especially in emergency departments, resulting in immediate treatment decisions. Most importantly, however, TUS with a high sensitivity and accuracy in diagnosing PE, is a reliable screening technique for diagnosing PEs with a high sensitivity but substantially low specificity.  

Triple Point-of-Care Ultrasound – A Combined Approach 

Rapid technical advances have resulted in ultrasound machine trends favoring increasingly compact sizes, lowered prices, and enhanced quality of equipment, all of which have broadened the scope of ultrasound applications. For instance, the ‘ultrasound stethoscope’ has spawned the use of point-of-care ultrasound (POCUS), that is, bedside diagnostics.  

Going a step further is triple POCUS, which includes lung, heart, and leg vein ultrasound. It is a highly promising ultrasound area, enabling bedside testing and offering real-time dynamic images. Furthermore, the triple ultrasound being a multiorgan ultrasonography, is more reliable than single organ testing with a much higher accuracy. Therefore, in cases where CTPA is contraindicated or unavailable, the triple POCUS would be a more reliable alternative to CTPA than the single-organ ultrasound.   

In a study conducted by Nazerian et. al., the diagnostic performance of triple POCUS (lung, heart, and leg vein ultrasound) was studied in 357 patients with clinical suspicion of PE with a Wells score greater than 4 or a positive D-dimer value. The study gave positive results, with sensitivity and specificity values of 90% and 86.2% respectively. 

On the whole, triple POCUS shows tremendous potential.  

How to Ultrasound for Pulmonary Embolism 

1. Bedside ultrasound diagnostics may help in narrowing differential diagnosis and lead the path to further workup. This is particularly useful in cases where patient instability may not allow the use of computed tomography imaging. Hence, ultrasound machines being noninvasive, real-time, and cheap, are excellent alternatives that enable serial bedside tests. 
2. With multi-organ ultrasound exams, one can minimize unnecessary radiologic exams. Since Computed Tomographic Pulmonary Angiography (CTPA) is often cited as being overused, it is time to employ alternative approaches like combined point-of-care ultrasound to diagnose PE.
3. It would be worthwhile to tread with caution: Point-of-care ultrasound diagnostics alone should not replace CTPA for diagnosing PE because of the high false-positive rate for ultrasound. Hence, ultrasound machines can be used as an extension of physical exams to avoid unnecessary and harmful radiologic exams. Although the studies evaluating ultrasound machines for diagnosing PE are not definitive yet, there is growing evidence supporting the use of a combination of point-of-care ultrasound and clinical assessments to rule out PE. 
4. Echocardiography (ultrasound of the heart) is useful in evaluating suspected high-risk PE cases, especially when the patient’s condition is so critical that only bedside diagnostics are allowed. It’s recommended to use B-mode, M-mode, and Doppler ultrasound when at risk for pulmonary embolism. Therefore, if CTPA were not available or practical, an echocardiography can help detect abnormalities of the right side of the heart. The Mindray M6, a color Doppler portable ultrasound machine comes with a micro convex transducer and is apt for cardiac imaging. 
5. Lung ultrasound is a good alternative when CTPA is contraindicated. The advantages of lung ultrasound over other imaging tests are that it poses no biological risks to the patient and has nearly no contraindications, making it safe to be used in patients with renal insufficiency or in pregnancy. Furthermore, it can also be extremely useful in hemodynamically unstable patients, as it can be used by the bedside.

USAT – Emerging Therapy for Pulmonary Embolism 

Finally, applications of ultrasound machines are not limited to merely detection of PE anymore but also extend to its therapy. The Ultrasound-assisted Catheter-directed Thrombolysis (USAT) is one such emerging therapy for patients with submassive pulmonary embolism, as per the research conducted at the University of North Carolina School of Medicine in Chapel Hill2. 

In a prospective study of 61 patients with submassive PE, one group received therapeutic anticoagulation while the other received 8 mg to 24 mg of tissue plasminogen activator infused via USAT catheter. However, for selection for USAT, undergoing treatment for cancer was “the only significant clinical predictor.”

Three patients who underwent anticoagulation died within six months while two experienced bleeding. However, the second group that received USAT showed substantial improvements in RV dilation and RV dysfunction, on echocardiography. Hence, the USAT can be a safe alternative to anticoagulation for patients with PE. However, further studies on why patients with cancer are less likely to receive USAT, are needed. 

How National Ultrasound Can Help 

The fight against Pulmonary Embolism centers around the ability to diagnose it quickly and ultrasound devices can prove to be useful by facilitating this in an easy, prompt, and cost-efficient manner. Moreover, ultrasound machines do not cause radiation, making them a safe screening method for diagnosis of PE. 

Our wide range of cutting-edge, affordable ultrasound machines from different manufacturers can help fulfill the demands of your practice accurately and efficiently. Contact a National Ultrasound professional today for a customized quote on any of our fixed or portable Doppler ultrasound machines.