1. Introduction
Cardiovascular ultrasound is an imaging investigation with unique characteristics: It is safe, low cost, widely available, repeatable, and accurate. In fact, it is the most used imaging technique in clinical practice. In cardiology, the use of various echocardiographic modalities allows a comprehensive study of cardiac structure, function, and hemodynamics. Mono-dimensional (M-mode), two-dimensional (2D), and three-dimensional (3D) echo define cardiac chamber dimensions and volumes, systolic function, and valvular morphology: Color and Doppler (pulsed and continuous) techniques allow accurate investigation of cardiac hemodynamics and diastolic function; tissue Doppler imaging (TDI) and two-dimensional strain (2DS) are able to detect clinical and subclinical systolic and diastolic dysfunction [
1]. All derived echocardiographic information has a well-documented diagnostic and prognostic utility [
2].
For many decades, echocardiography equipment was immobile, ultrasound scans were performed in specific echo-laboratories, and cardiologists were the only competent operators. With technological development, echocardiography equipment has become movable, portable, and miniaturized, and ultrasound use has become more extensive, at the bedside, in wider clinical scenarios, in critically ill patients, and in emergencies/urgencies, and its use has expanded to physicians with different backgrounds [
3].
Nowadays, various types of equipment are available, with different sizes, different echo modalities, and different diagnostic capabilities. This review focuses on the use of handheld ultrasound devices (HUDs), and describes differences with other equipment, their limitations, and the numerous advantages that come from their use.
HUDs can be performed easily, rapidly, and allow basic information to be obtained for diagnosis and clinical management of various diseases; an HUD can be used by physicians from different disciplines, in many situations and clinical scenarios, especially in emergencies/urgencies and its use has transformed almost all aspects of our daily practice.
2. Differentiation of the Various Echocardiographic Equipment: From Standard, Comprehensive Transthoracic Echocardiography to Focused Cardiac Ultrasound
Standard echocardiography performed in echo-labs by cardiologists or sonographers/cardiologists provides information about cardiac size, structure, function, and hemodynamics. All these data are obtained using stationary systems that are equipped with various modality system and transducers: 2D, M-mode, Doppler (pulsed and continuous), color, and TDI, transesophageal approach (TEE), and, in many cases with advanced modalities, such as 2DS and three-dimensional echo (3D). During the exam, all modalities can be utilized (if necessary, also the advanced modalities), all standardized echo sections are performed, simultaneously with ECG-guided comprehensive measurements of cardiac structure, heart function, and hemodynamics. Thus, the final report is complete and accurate [
1,
2,
4].
Portable machines are smaller, and allow a basic comprehensive exam of 2D, M-mode, pulse, and continuous Doppler and color. Generally, they do not have advanced modalities, the quality of images is good, and the exam is clinically complete. The final report describes all basic cardiac morphological and functional information.
Handheld ultrasound devices are the smallest machines, very simple to use, with a limited number of basic controls for adjusting the depth and gain, to freeze and store images (in JPEG) and little loops (in MPEG-4), and the available measurements are few and limited to a simple distance and area assessment. The devices only have the 2D modality with grey-scale images and color-Doppler, simultaneous ECG, M-mode, TDI, and advanced technologies are lacking. Images have lower spatial (640 × 480 pixels) and temporal resolution than other equipment. However, 2D and color Doppler are in real-time; the field-of-view (2D and color flow), maximum depth (25 cm), automatic frame rate (28 frames per second), and transducers (phased array—1.7 to 3.8 MHz) are similar; images have good technical quality; and a correct final diagnosis can be made in most cases (
Figure 1) [
5,
6,
7,
8]. The exam is performed with a limited number of echo sections; often the evaluation is qualitative, with a bimodal (yes/no) or semi quantitative (normal/reduced) response. It represents an extension of the physical examination, is focused to recognize specific signs that lead to an answer to a clinical diagnostic suspicion in a specific clinical setting, and, because it is focused on getting a few findings, it is also named FoCUS [
9,
10] (
Table 1).
3. Focused-Echo in Various Clinical Scenarios: When to Use It
FoCUS/HUDs can be used in many situations and different scenarios, with various acquisition protocols, in stable and unstable patients. It can be used in in- and out-patients and by operators with different specialties. Few measurements are possible, such as left and right atrium and ventricle dimensions, wall thickness, ascending aorta, and inferior vena cava (IVC), but HUDs can qualitatively provide useful information about atrial dilatation, left and right ventricle global systolic function, ventricular dilatation or hypertrophy, significant valvular stenosis and regurgitation, pericardial effusion, and tamponade [
8,
9,
10]. In the literature, many studies have demonstrated that HUDs provide a more accurate diagnosis than physical examination for the majority of common cardiovascular diseases and FoCUS results correlate well with standard echocardiography [
8,
10,
11,
12,
13,
14,
15,
16,
17].
In
stable patients, FoCUS is appropriate for the screening of structural heart disease. It allows early diagnosis, defines prognostic stratification, and directs to the appropriate therapy. The exam can rapidly define/exclude qualitative ejection fraction, wall motion abnormalities, ventricular systolic dysfunction, left ventricular hypertrophy, cardiomyopathies with hypertrophic pattern, dilated cardiomyopathies, pulmonary congestion, significant valvulopathies, ascending aorta and aortic root dilatation, pericardial and pleural effusion, and IVC size and respiratory collapsibility [
3,
7,
8,
11,
12,
14,
16] (
Table 2).
Below are some examples of the effective use of HUD:
In patients with clinical suspicion of
congestive heart failure, FoCUS helps to define atrial dilatation, wall motion abnormalities and thicknesses, cardiac dimension and function, significant mitral regurgitation, lung congestion (B-lines at lung ultrasound, also called “comets”), IVC collapse, and it can differentiate between systolic or diastolic heart failure (
Figure 2) [
18,
19,
20].
In
atrial fibrillation, FoCUS defines left atrial dimensions (dilatation) and left ventricular function, which are strong predictors of sinus rhythm restoration and lead to proper treatment [
21,
22].
In patients with
valvular stenosis, FoCUS can qualitatively show valvular morphology, such as thickened and calcified leaflets with reduced mobility and turbulent transvalvular flow by color Doppler. All these indirect signs lead to the suspicion of significant valvulopathy. Quantitative assessment is not possible, nor are pulmonary pressures, but in patients with signs/symptoms of heart failure, atrial fibrillation, or syncope, these data define the underlying pathophysiology [
9,
10,
23].
In valvular regurgitation, color-HUDs can differentiate significant mild/trivial valvulopathy.
In patients with
pericarditis, FoCUS can be used for basic follow-up of pericardial effusion (
Figure 3) [
9,
20].
This approach is a screening tool that provides all information for diagnosis and immediate start for proper therapy, but it cannot be considered as a definitive investigation and standard echocardiography must follow later for a comprehensive heart disease evaluation [
3,
4,
24].
In
unstable patients, HUDs provide crucial information in critically ill patients or impending critical situations, is an essential technology for improving early diagnosis, help to rule-in/rule-out different pathological conditions, evaluates the pathophysiology of clinical status, and allows proper clinical management [
15,
25]. When an HUD is used at the point of care, the acronym FoCUS is changed to POCUS (point of care ultrasound).
For 20 years, many POCUS protocols have been proposed in the literature to standardize the procedure. They preferentially evaluate cardiac, pulmonary, and abdomen diseases, or are elective for searching for trauma lesions. Actually, because there are many clinical scenarios, data acquisition depends on the specific imaging target rather than specific protocol.
In
ill patients, POCUS is extremely useful in terms of a differential diagnosis of clinical conditions that manifest with the same symptoms (
Table 3).
In
acute respiratory failure, lung ultrasound can differentiate pneumothorax from pleural effusion or cardiogenic and non-cardiogenic pulmonary edema [
26].
In
shock patients, POCUS is useful for diagnosis, management, and monitoring of treatment efficacy and clinical progression. Chamber size, left ventricular systolic function, IVC dimensions and collapsibility, pericardial effusion, and pulmonary congestion (lung B lines—comets) in lung ultrasounds identifies cardiogenic shock [
27].
In
cardiac tamponade, POCUS is useful to identify pericardial effusion size and distribution and directs to the best approach for pericardiocentesis [
28].
In the
emergency department, POCUS helps to differentiate the nature of chest pain. In addition to history, clinical examination, electrocardiogram, and biomarkers, echocardiography can visualize wall-motion abnormalities and left ventricular function, dilated right ventricle with free wall hypokinesia, ascending aorta dimension and morphology, significant aortic regurgitation, and pericardial effusion. In this way, it is a first step to a differential diagnosis among acute coronary syndromes, acute aortic syndromes, pulmonary embolism, congestive heart failure, and pericarditis [
29].
In patients with
cardiac arrest due to complex arrhythmias, HUDs can exclude/diagnose all cardiac arrhythmogenic diseases, such as hypertrophic, dilated, or arrhythmogenic right ventricular cardiomyopathies [
30].
In
cardiopulmonary resuscitation and during BLS (basic life support) and ALS (advanced life support) protocols, POCUS can help to diagnose potentially treatable causes of cardiac arrest, such as cardiac tamponade, massive pulmonary embolism, severe ventricular dysfunction, and hypovolemia [
31,
32].
