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The use of ultrasound for the diagnosis of cardiogenic pulmonary edema in dogs and cats | VETgirl Veterinary Continuing Education Podcasts

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In this VETgirl online veterinary continuing education podcast, we review the use of ultrasound for the diagnosis of cardiogenic pulmonary edema in dogs and cats. Nowadays, we’re doing more and more FAST (Focused Assessment of Sonography in Trauma) ultrasounds in the ER setting, and it’s great (and easy) for diagnosing cavital effusion (e.g., hemoabdomen, pleural effusion, pericardial effusion, etc.). But can it be easily used for lung ultrasound (LUS) to detect pathology (or water) in the lung? Does it work to diagnose cardiogenic pulmonary edema?

As many of us have experienced, the diagnosis of congestive heart failure (CHF) in a dog or cat presenting for acute dyspnea can prove diagnostically challenging. This is true for multiple reasons – one is the inherent difficulty of performing diagnostics on unstable or dyspenic patients, and another is that thoracic radiography and other diagnostics can often yield equivocal results. Lung ultrasound (LUS) is an exciting new diagnostic imaging option for the dyspneic patient. LUS is a cage-side thoracic ultrasound that allows practitioners to assess for pulmonary edema by looking for artifacts that the pulmonary fluid creates in ultrasound images. These sonographic changes are called “B lines.” Other terms that you may have heard include “ring-down artifacts,” “comet tails,” and “lung rockets.” The B lines appear as “narrow-based vertical hyperechoic artifacts that extend from the pleural-pulmonary interface to the far aspect of the ultrasound screen without fading.” They are also identifiable because they move in synchrony with respiration. In people, LUS can be helpful in differentiating cardiogenic from non-cardiogenic causes of dyspnea, and the number of B lines correlates with severity of pulmonary edema.

So, Ward et al wanted to evaluate the usefulness of LUS in a study entitled “Accuracy of point-of-care lung ultrasonography for the diagnosis of cardiogenic pulmonary edema in dogs and cats with acute dyspnea.” To these authors’ knowledge, there were no veterinary studies that had prospectively studied the clinical and diagnostic accuracy of using thoracic ultrasound (specifically LUS) in a patient population of dyspneic dogs and cats. So, they conducted a prospective study from North Carolina State University over a 16-month time period. The goals of this study? The authors wanted to evaluate the accuracy of a protocol for point-of-care LUS for the diagnosis of cardiogenic pulmonary edema (CPE) in a group of dyspneic dogs and cats. They also wanted to evaluate the diagnostic accuracy of LUS in dogs versus cats, to characterize the types of non-cardiac disease that were correctly or incorrectly identified, and to determine interobserver agreement between LUS results.

Dogs and cats that were dyspneic that presented to the Emergency/Critical Care or Cardiology Departments were enrolled. Inclusion criteria also included having thoracic radiographs performed within 6 hours of LUS, and having a trained examiner in LUS available. Patients were excluded if they had a respiratory disturbance thought to be secondary to pain, a recent history of trauma, or moderate-to-severe pleural effusion. Medical records and imaging studies were reviewed by the investigators to confirm the final diagnosis. Patients were stabilized, and age, sex, breed, weight, TPR, BCS, and time of LUS examination were evaluated. Participating examiners were cardiology and ER clinicians who completed a 2-hour training session and completed proficiency exams, and then post-hoc analysis of the images were performed independently by blinded examiners (1 novice and 1 experienced). The presence and number of B lines were documented at 4 sites on each hemithorax, with > 3 B lines/site being recorded as positive. If > 2 positives sites were noted on each hemithorax, it was defined as cardiogenic pulmonary edema; otherwise, the underlying cause was deemed non-cardiac as the cause of dyspnea.

Overall, 76 dogs and 24 cats were enrolled in the study. Results showed that all animals had images obtained at all sites, indicating that the test was “technically feasible.” 61% of patients had a final diagnosis of cardiogenic pulmonary edema with no difference in proportion between cats and dogs. This leaves 39% of the patients that had dyspnea without cardiogenic pulmonary edema. In this non-cardiac group, the most common causes of non-respiratory diseases were pulmonary hypertension or PTE in dogs, and asthma or diffuse pulmonary disease of unknown etiology in cats.

90% of the patients with cardiogenic pulmonary edema had at least 1 positive site on LUS, with a mean of 5.3 positive sites on LUS. In the non-cardiac group, 67% patients had a positive site, with a mean of 2.7 positive sites on ultrasound. In other words, patients that had cardiogenic pulmonary edema DID have a significantly higher number of positive sites than those without CPE. However, it was not only the number of positive sites, but also the distribution of positive sites that were found to be significantly different between groups. In the group with cardiogenic pulmonary edema, the right and left middle sites were the most common sites where pulmonary edema was detected. In the non-cardiac group, the right middle site was the most likely to be positive for B lines. In both groups, the right and left caudal sites were the least likely to have a positive score.

Using a cutoff of 2/4 sites, the sensitivity of LUS was 84% and the specificity was 74% as compared to thoracic radiographs, which had a 85% sensitivity and 87% specificity. VETgirl’s take – this isn’t much more beneficial over chest radiographs! There were also 10 cases of false positives (where there was an incorrect diagnosis of cardiogenic pulmonary edema). In patients that had a false positive, there were other causes of diffuse interstitial or alveolar disease. What does this mean? This means that LUS can’t differentiate well between CPE and diffuse interstitial or alveolar disease… which then warrants getting some chest radiographs!

However in dogs, if patients with a presenting heart rate < 150 bpm were categorized as “non-cardiac disease”, the specificity of LUS improved to 87%, but the sensitivity decreased to 52%. This impact of heart rate on specificity was not observed in cats. In other words, incorporation of heart rate was found to increase the specificity of LUS in dogs. The authors also found excellent interobserver agreement between the experienced and inexperienced examiners. There were only 6 cases out of 100 in which the final diagnosis between observers was different.

So, what do we take away from this VETgirl podcast?

This study found that lung ultrasonography is a promising and feasible bed-side diagnostic tool. LUS for the diagnosis of cardiogenic pulmonary edema has a good overall sensitivity that is comparable to thoracic radiography. It also was good at ruling out cardiogenic pulmonary edema in patients with noncardiac disease (ie, it had good overall specificity). Types of noncardiac diseases that were more likely to cause false-positive results on lung ultrasound included those that cause a diffuse interstitial or alveolar disease such as ARDS, pulmonary neoplasia, pneumonitis, pulmonary hypertension (suspected pulmonary fibrosis), and PTE. The specificity was lower in dogs than in cats and humans, which is thought to be the result of the more common types of non-cardiac diseases seen in dogs versus other species. In this study, the sensitivity was slightly lower than the values found in human studies, which may be secondary to differences in the distribution of pulmonary edema or other physical differences between species. When looking at a variety of physical examination parameters, they found adding heart rate to the analysis did slightly improve the specificity of the LUS findings in dogs, but not to a degree that was significant.

Some pros and cons of this study? This study used the Volpicelli criterion to determine a positive result, which requires bilateral distribution of positive sites. However, given variations in distribution of cardiogenic pulmonary edema, there was concern this method could lead to false negative results. However, this study looked at alternative scoring methods but still found that the Volpicelli method was the best predictor of cardiogenic pulmonary edema. Additional studies could be performed to look at how other point-of-care tests (like cardiac biomarkers and LA-Ao ratios) help when used in conjunction with LUS.

In conclusion, this study showed that LUS was feasible in all cats and dogs with minimal stress to the dyspneic patient. It provides a useful diagnostic tool for differentiating CPE from non-cardiac dyspnea. Given the excellent interobserver agreement, it suggests that even with minimal training and experience, reliable results are able to be obtained. One possible limitation for practitioners is the availability of in-house ultrasonography. However, VETgirl’s take? You’re going to need chest radiographs to confirm location and etiology anyway, and lung ultrasound isn’t much “better” for sensitivity and specificity than chest radiographs. Also, VETgirl’s tip? In dogs, when you have a HR < 150 bpm, it’s less likely to be CHF anyway (e.g., my cut off for CHF in small dogs with CVHD is that they are typically tachycardiac on presentation with a HR > 150-160 bpm). That said, LUS is a practical bed-side diagnostic tool to help differentiate CPE from other non-cardiac causes of dypsnea in dogs and cats.

References:
1. Ward J, Lisciandro G, Keene B, Tou S, DeFrancesco T. Accuracy of point-of-care lung ultrasonography for the diagnosis of cardiogenic pulmonary edema in dogs and cats with acute dyspnea. J Am Vet Med Assoc 2017;250(6):666-675.

2. Al Deeb M, Barbie S, Featherstone R, et al. Point-of-care ultrasonography for the diagnosis of acute cardiogenic pulmonary edema in patients presenting with acute dyspnea: a systematic review and meta-analysis. Acad Emerg Med 2014;21:843-852.

3. Lisciandro GR, Fosgate GT, Fulton RM. Frequency and number of ultrasound lung rockets (B-lines) using a regionally based lung ultrasound examination named Vet BLUE (veterinary bedside lung ultrasound exam) in dogs with radiographically normal lung findings. Vet Radiol Ultrasound 2014;55:315-322.

Abbreviations:
ARDS – Acute Respiratory Distress Syndrome
CPE – Cardiogenic Pulmonary Edema
LUS – Lung Ultrasonography
NT-proBNP – N-terminal pro-B-type natriuretic peptide
Vet BLUE – Veterinary bedside lung ultrasound exam

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