Diagnostic ultrasound was developed in the mid-20th century and 70 years later, a portable ultrasound can be found throughout most ICUs (1). Over the past 20 years, critical care ultrasound used at the bedside has evolved to be a standard in ICUs and emergency departments (2). With widespread uptake across various disciplines, some have criticized critical care ultrasound for its lack of standardization and unclear diagnostic and therapeutic implications. Despite these criticisms, it is hard to deny that ultrasound is an essential tool to the intensivist providing the capability to quickly confirm a diagnosis and perform a timely intervention.

As with all diagnostic studies, however, we should determine the efficacy of critical care ultrasound across three domains (3):

1) technical and diagnostic efficacy: how accurate are the images, and what is the ability to diagnose from the images, 2) diagnostic thinking and therapeutic efficacy: how do the results affect thinking and management, and 3) patient outcome and societal efficacy: how does the study change outcomes, and what is the cost to society.

If we look to the past for guidance, a clear correlation can be found between critical care ultrasound and the use of the pulmonary artery catheter (PAC), as these questions prompted utilization and then evaluation of PACs in the ICU. At one time, PACs were widely used in the ICU to provide objective measurement of hemodynamic parameters and guide therapy, but without formal assessment of clinical effectiveness. This prompted several clinical trials that showed no improvement in outcomes in critically ill patients (4,5). One of the clinical trials that evaluated the PAC was known by the acronym PAC-Man. In this issue of Critical Care Medicine by Heldeweg et al (6), the UltraMan study is published, which is observational evaluation of how critical care ultrasound changes diagnosis and management. There are similarities in the stories of PACs and critical care ultrasound but also important differences. The question moving forward after the observational study UltraMan is whether critical care ultrasound requires a prospective randomized clinical trial like PAC-Man?

In this international, observational study, Heldeweg et al (6) have adeptlyadded to the evidence for critical care ultrasound by evaluating the effect of lung, pleural, diaphragm, and inferior vena cava examinations, also known as thoracic ultrasound (TUS), on changes in management and therapeutic decision-making in the ICU. Patients 18 years and older admitted across four academic and nonacademic ICUs with a specific indication for TUS were included. Prior to TUS, clinicians completed a case report form, which included a pre-TUS diagnosis and management plan. Ultrasound findings such as, but not limited to, pleural effusion, hypovolemia, pneumothorax, or cardiogenic pulmonary edema were documented. The clinician then decided if the findings provided clinical contribution and, if so, whether the contribution was supporting or altering the initial clinical impression and management plan. Changes in management were broad but could include things such as an alteration in fluid balance goals, positive end-expiratory pressure adjustment or drainage of pleural fluid.

Over the course of 2 years, a total of 725 examinations were completed across medical and surgical admissions. Most patients were mechanically ventilated (67%) with a mean Pao2/Fio2 of 199.5 mm Hg (± 107.8 mm Hg) and Sequential Organ Failure Assessment score of 8.7, representing significant clinical severity. Of all examinations, a “change in clinical impression” occurred in 47.6% (95% CI, 43.9–51.3%) of studies and a “confirmation of diagnosis” in 48.6% (95% CI, 48.6–52.3%). Changes in management occurred in 39% of examinations with most changes impacting net fluid balance.

Patients who had no changes to fluid management were labeled the unguided fluid balance group and had a cumulative fluid balance +186 mL at 8 hours. This contrasts with those that TUS changed management by targeting a positive (+907 mL) or negative (–411 mL) fluid balance. The differences were statistically significant suggesting that TUS influenced early therapeutic changes that differed from the initial clinical impression.

It is important to interpret these findings in the context of critically ill patients with comorbidities complicating diagnostics. This is demonstrated by 72% of TUS being used for diagnostic purposes with 48% of examinations changing initial clinical impression. This suggests clinicians turn to ultrasound for additional data points to guide management decisions in the most complex scenarios. This is a familiar argument for PACs in determining hemodynamics in undifferentiated shock. A main difference, however, is TUS is highly sensitive and specific for the most common diagnoses altering management in UltraMan: atelectasis, pleural effusion, and pulmonary edema; all findings clinicians are confident in determining (7). If we take it a step further, clinicians can then leverage TUS to differentiate loculated versus simple fluid and guide definitive management without additional imaging studies. Last, procedural planning can occur at the time of diagnosis while lowering the risk of procedural complications (8).

Although the signal for benefit seems clear, a critic of ultrasound will argue a substantial heterogeneity exists between novice and expert sonographer. Although true in theory, one should consider the broad utility of TUS across varying diagnostic and procedural techniques. For instance, lung ultrasound can be taught in a brief didactic session and shows high interobserver reliability after five solo examinations when compared with expert sonographers (9). On the other end of this spectrum is transthoracic echocardiography (TTE), which requires significant training to image and interpret accurately (10). Even within the spectrum of TTE exists basic skills such as identification of pericardial fluid to more advanced skills like variation between mitral and tricuspid inflow velocities evaluating for tamponade physiology (11). Clinicians have acknowledged this spectrum by demanding more ultrasound training and didactics, which have been shown to increase proficiency and accuracy in diagnosis (12). As the clinician’s experience and abilities improve, ultrasound should be seen as any other diagnostic tool. One should determine a pretest probability, level of confidence in the finding and ultimately, the significance for the patient. This pertains to all aspects of diagnostics, whether it be assessment of jugular venous distention, lung crackles, elevated brain natriuretic peptide, or interstitial edema on chest radiography. The perspicacious clinician should temper their confidence in ultrasound findings, just as any other finding, based on their experience and direct their therapeutic decisions off numerous data points. The way TUS affects clinical decision-making, however, has been an area of uncertainty, and UltraMan takes us one step closer to answering this question.

Admittedly, however, what ultimately led to the fall of the PAC was randomized controlled trials showing no benefit to patient outcomes at a substantial increase in cost. As the clinical indications for ultrasound vary significantly compared with PACs, we are unlikely to see such a definitive answer when it comes to ultrasound. If we can acknowledge our lack of understanding regarding patient outcomes and societal efficacy of critical care ultrasound, it is easier to see this as an extension of the physical examination that adds to the clinicians diagnostic ability. Perhaps the most prominent finding of UltraMan is that TUS provides clinicians with data that confirms their initial impression or is convincing enough to change management in the most diagnostically complex patients. This may obviate further diagnostic workup and lead to faster interventions. A thought-provoking finding as we consider what metrics should be used to determine the efficacy of ultrasound in further research endeavors.

What is the next step for evaluating the efficacy of critical care ultrasound? UltraMan shows that TUS is commonly used for fluid management decisions. A clinical trial that includes ultrasound in a therapeutic algorithm is a logical next step. In comparison to evaluation of the PAC, this would be similar to the Fluids and Catheters Treatment Trial of the Acute Respiratory Distress Syndrome (ARDS) Network that compared conservative to liberal fluid strategies and central venous catheters to PACs (5) in ARDS patients. A proof of concept can be seen in a recent pilot study comparing critical care ultrasound goal-directed therapy versus early goal-directed therapy in septic shock that demonstrated earlier lactate clearance and less overall fluid balance at 12 and 24 hours using ultrasound (13).

Over the past 2 decades, we have moved from PAC-Man to UltraMan, and the medical community has learned a great deal along the way. As critical care ultrasound has grown dramatically and become a tool for the intensivist, there has been a demand for ultrasound didactics, training, and certification to ensure proficiency in image acquisition and interpretation. Heldeweg et al (6) have taken the first step in furthering our knowledge of the clinical impacts of critical care ultrasound on diagnostic thinking and therapeutic efficacy. As bedside providers, it is prudent to continue to advance our skills and understanding of ultrasound while at the same time taking a step back to acknowledge where we stand regarding its limitations. If the medical community continues to approach it thoughtfully while advancing our knowledge of ultrasound’s application in the ICU, we can avoid making the same mistake twice and provide highly effective, timely care to the sickest patients.

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