Handheld Echocardiography

Introduction

Although Moore’s law of increasing computing capabilities (including processing power and memory) has propelled the development of all imaging modalities, its most tangible manifestation has been in the field of ultrasonography. Unrestrained by the physical limitations (gantry size and power requirements) of the other modalities, echocardiography machines have evolved from large, cumbersome pieces of equipment to handheld echocardiography (HHE) devices that are the size of a mobile phone ( Fig. 46.1 ).

Three aspects of miniaturization have been particularly important for HHE. The display interface has benefited tremendously from technological evolution—cathode ray tube monitors have been replaced by lightweight, high-resolution liquid crystal display (LCD) screens. Developments in microprocessors have led to a shift in the balance between hardware and software so that it is closer to the transducer, usurping some of the functionalities previously performed by the scanner ( Fig. 46.2 ). Likewise, there has been a progressive drop in the size of digital beamforming components from 1 μm to 100 nm. Freeing the handheld device from the ECG leads through fixed time acquisitions or more complex tracking iterations using mitral annular movement of speckle tracking has led to increased portability and reduced size.

The inextricable connection between energy transmitted by a system and the information gained has led powerful, high-end systems to hold an advantage over their battery-dependent counterparts. These power-to-performance issues have benefited from greater efficiency of systems and greater integration, although the limitations imposed by their size continue to leave HHE at one end of the spectrum of ultrasound devices.

Current Handheld Devices

A modern HHE device is characteristically lightweight, portable, and can fit into a coat pocket—in contrast to previous miniaturized models. These devices provide B mode grayscale imaging and in some cases color Doppler. Unlike their fully functional mobile (but non-HHE) counterparts, which are essentially complete echocardiographic devices, most HHE devices do not provide spectral Doppler. They also have smaller screens and a display that is lower in resolution than standard echocardiographic devices ( Fig. 46.3 ). Their various properties are described in Table 46.1 . It is important to remember that comparison with standard echocardiography is only reasonable if these devices are touted as a replacement for echocardiography rather than an extension of the physical examination.

The limitations of HHE are related to the imaging modes, processing, display, and ability to do measurements. None of the devices have continuous-wave Doppler, and only one has pulsed-wave Doppler capability. Because Doppler wave-form analysis is a cornerstone in the severity assessment of valvular and diastolic heart disease, this represents an important (and potentially avoidable) limitation. The high-resolution display of high-end devices provides high-fidelity images that are difficult to reproduce on HHE devices. Finally, post-acquisition analysis and measurements are a key component of analysis. Although three devices enable measurement of distances and area, volumetric assessments are not possible. Adjustment of imaging parameters such as zoom, changing focal point, narrowing sector width to improve frame rate, changing mechanical index for contrast studies, harmonic imaging, changing dynamic range or grayscale maps, and changing frequency are all currently unavailable on such devices. Future iterations may overcome some if not all of these limitations, but currently there is a clear difference between a standard echocardiogram and HHE equipment.

The Learning Curve

The acquisition of information from the traditional physical examination is less reliable than in former times. HHE is a potential replacement for bedside diagnosis, but ultrasonography has not been as well taught and is currently restricted to certain physicians, surgeons, and sonographers. The relative cost, portability, and applicability of HHE devices make them ideal for more widespread dissemination. However, for those unfamiliar with echocardiography, there needs to be a learning process and assessment of competence. The recommended training requirements for performance and reporting of echocardiography are summarized in Table 46.2 .

Clearly Level 2– and 3–trained individuals will readily adapt to HHE; both American Society of Echocardiography (ASE) and European Association of Echocardiography (EAE) recommend that experienced cardiologists and sonographers should be able to use handheld devices. Miniaturization of technology has outpaced training and accreditation guidelines, and ultrasonography has moved from the field of radiologists and subspecialty physicians to residents and general physicians. The ASE advises additional training for Level 1–trained individuals and EAE recommends additional training for cardiologists not fully conversant with echocardiography. The American College of Emergency Physicians (ACEP) has addressed the role of focused cardiac ultrasound (FCU) in the emergency department and provided guidelines on the acquisition and interpretation of ultrasound images on a range of diagnostic possibilities. At an earlier stage in training, HHE can also be used as teaching aide to visualize anatomy and physiology at the bedside. Wittich demonstrated that 79% of medical students could produce a satisfactory parasternal long axis (PLAX) image within 3 weeks of didactic and practical sessions. It seems feasible to use HHE to acquire images and interpret basic cardiac function early during medical school.

There have been multiple previous comparisons of HHE and physical examination—a recent example is summarized in Table 46.3 . There is clear incremental value of HHE over the traditional physical examination; the area under the receiver operating characteristic (ROC) curve was 1.97 for physical examination, 2.42 for ECG and physical examination, and 6.23 for HHE-based echocardiography.

Handheld ultrasonography can be applied to any organ system, although for the purposes of this chapter it will refer solely to the examination of the cardiovascular system. FCU is a specific adaptation—involving HHE—that provides a “Focused examination of the cardiovascular system performed by a physician using an ultrasound as an adjunct to the physical examination to recognise specific ultrasonic signals that represent a narrow list of potential diagnosis in specific clinical settings.” This must be differentiated from limited transthoracic echocardiography, which refers to a reduced number of images performed on a standard and thus a high-end machine by an echocardiographer with appropriate qualifications and interpreted by a cardiologist with the necessary level of experience. The training recommendations on FCU by the ASE comprise three educational components—a didactic component (ultrasound physics, basic cardiac anatomy and views), practical training (image acquisition and correction of technique by experienced echocardiographers), and image interpretation. Recent literature has demonstrated that electronic modules were equivalent to didactic teaching, but there is no substitute for “hands-on” sonographer-based training, or the use of a training log to track the number of successful echocardiographic interrogations for specific cardiac pathologic conditions. There is considerable variation in the duration of training programs depending on the level of experience of operators, opportunity to scan, time, and resources. The length of the learning curve is variable. General practitioners with 8 hours of supervised training using HHE were able to assess left ventricle (LV) function with a sensitivity of 83% and a specificity of 78%. A regression model based on more than 230 HHE examinations performed and interpreted by 30 residents and audited against cardiologists’ measurements suggested improvements every 10 scans and that 30 scans would result in a minimal overall difference. It must be noted that with all R values of less than 0.2, the fit and predictive value of the model was limited.