Improving AKI Care: Will Handheld POCUS Devices Have a Role?

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The prevalence of fluid overload among hospitalized patients with acute kidney injury (AKI) and its negative impact on prognoses has been increasingly recognized (1). Additionally, the potential contribution of fluid overload to kidney dysfunction (congestive nephropathy) highlights the need for reliable bedside methods for objective and reliable assessment of volume status (2). Point-of-care ultrasonography (POCUS) is a clinician-performed imaging procedure using ultrasound that addresses focused clinical questions at the bedside. Although appropriately trained nephrologists can perform scans ranging from simple kidney and bladder ultrasonography to comprehensive hemodynamic assessments, the resurgence of interest in POCUS is sparked by studies exploring the role of lung ultrasonography in patients receiving hemodialysis (3–5).

Affordable handheld ultrasound devices (HUDs) with improved portability have helped with the widespread use of POCUS. Most clinical ultrasonography systems, whether conventional or HUDs, use piezoelectric crystals within the transducers. These crystals vibrate, generating ultrasound waves when a potential difference is applied across the electrodes. Upon receiving an echo, they produce an electric signal that is displayed as an image. Transducers vary in their internal crystal makeup and arrangement, influencing how they display images and the frequencies at which they operate. Conversely, certain HUDs use a more recently developed microchip technology that generates ultrasound waves through a change in capacitance. These units, constructed on silicon using micromachining techniques, retain conventional ultrasound wave properties and image characteristics but enable the amalgamation of various transducer properties into a single unit, facilitating miniaturization of the equipment.

A recent study by Soares et al. (6) investigated the efficacy of a HUD using microchip technology for lung and inferior vena cava (IVC) ultrasonography in patients with AKI who were undergoing renal replacement therapy. In this observational study that included 50 patients who were critically ill, the investigators performed lung and IVC ultrasonography at the beginning of dialysis and 60 minutes into the session using the Butterfly IQ^TM microchip HUD as well as conventional piezoelectric crystal-based machines (Philips InnoSight or GE HealthCare Logiq^TM P6). Dialysis prescription was not altered based on the ultrasonography findings. A strong correlation was found between the microchip and traditional piezoelectric-based ultrasound modalities in documenting the improvement in lung B-lines and IVC dynamics at two time points during hemodialysis.

B-lines on the lung ultrasonography indicate extravascular lung water, usually secondary to elevated left heart filling pressures. At the same time, IVC ultrasonography is a standard echocardiographic parameter for estimating right atrial pressure in patients with spontaneously breathing. The study's commendable use of an 8-zone lung ultrasonography method, as opposed to the more cumbersome 28-zone approach, enhances its practicality. Although acknowledging that the the study's microchip HUD is not the sole HUD available on the market, and most HUDs use piezoelectric technology, the study's findings underscore the potential role of ultraportable ultrasonography in nephrology practice. This aligns with the dynamic workflow of nephrologists who often traverse a variety of care settings.

It is, however, crucial to avoid overly optimistic conclusions extrapolating these findings to all POCUS applications. Lung ultrasonography relies on artifact interpretation, independent of high-resolution imaging capabilities; advanced settings suppressing artifacts are often disabled on larger machines to create a dedicated lung preset. Likewise, capturing images of the IVC does not usually warrant high-resolution devices, particularly considering that the average weight of patients in this study was 70 kg. Nonetheless, a comprehensive bedside hemodynamic evaluation requires more than lung and IVC ultrasonography, specifically a focused cardiac ultrasonography and selected Doppler applications (7, 8). Furthermore, IVC POCUS is not reliable in estimating right atrial pressure in patients who are mechanically ventilated. Therefore, the image quality of the device is a crucial factor for nephrologists seeking to enhance their proficiency in advanced POCUS applications. In this context, it is notable that the Butterfly IQ^TM device ranked lowest among four commonly used HUDs in the United States regarding image quality (9). Although the study contributes to the evolving landscape of POCUS applications in nephrology, a nuanced approach is essential, given the diversity of available devices and the ever-advancing nature of this technology. Table 1 presents a summary of the strengths and drawbacks of various methods for evaluating volume status in individuals with kidney diseases.

Table 1

Commonly used methods for evaluation of volume status

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