The Role of Lung Ultrasound on the Daily Assessment of the Critically Ill Patient

Fig. 8.1

Left: Multiple detector computed tomography after intravenous contrast arterial revealed bilateral consolidations with air bronchogram (arrows), associated with pleural effusions. Right: Lung ultrasound longitudinal scan at the lower lateral regions. The main ultrasound features included bilateral consolidations with air bronchogram (arrows) and pleural effusions (From Georgopoulos et al. [4])

As a result, LU is likely to have a significant impact on clinical decision-making and therapeutic management of these patients [3, 5]. LU may also be used to assess and monitor lung aeration, which is of particular importance in patients with acute respiratory distress syndrome. This application may guide the titration of positive end-expiratory airway pressure (PEEP) and may serve as a safeguard against excessive fluid loading in critically ill patients [6]. Finally, it has been shown that ultrasound may be used to measure the thickening fraction of the diaphragm during tidal breathing, which is useful as a non-invasive estimation of the work of breathing in critically ill patients [7 ].

Despite the proven diagnostic ability of LU and its influence on decision-making and therapeutic management, there are significant barriers to the widespread use of this pragmatic, non-invasive bedside tool. The fact that the interpretation of LU findings is heavily dependent on operator experience represents one important limitation. In addition, LU may not identify with accuracy, deep pulmonary lesions.

The aim of this chapter is to introduce the ultrasonography imaging of the lungs and pleura, and the main LU findings associated with basic respiratory disorders in critically ill patients (Table 8.1).

Table 8.1

The use of lung ultrasound in various lung and pleural pathologic conditions

Lung parenchyma abnormalities

1. Consolidation

(a) Atelectasis

(b) Pneumonia

2. Interstitial syndrome

(a) Congestive heart failure

(b) Acute respiratory distress syndrome

(d) Pneumonia

(e) Interstitial lung diseases

(f) Evaluation of lung congestion

(g) PEEP titration and lung recruitment in ARDS patients

3. Lung overdistention

4. Pulmonary embolism

Pleural diseases

1. Pneumothorax

2. Pleural effusion

3. Evaluation of diaphragm contraction – paralysis

4. Diaphragm ultrasound as a predictor of successful weaning

8.2 Equipment

Lung ultrasonography can be performed using any commercially available 2D scanner. Today, portable machines are lightweight, relatively inexpensive and can easily be used at the bedside. High-frequency transducers provide excellent resolution, but do not visualise deep structures (poor penetration). Both the microconvex 3–8 MHz probe and the high-frequency linear probe (8–12.5 MHz) are suitable. The use of the microconvex transducer facilitates semi-posterior analyses with minimal patient mobilisation. The probe depth should range between 60 and 140 mm, and, in an effort to reduce the natural artefacts, tissue harmonics are preferable. Colour Doppler and power Doppler can be helpful for the detection of blood flow signals within consolidated areas [3, 8].

8.3 Ultrasound Waves and Lung Interaction

It is well known that there is poor interaction between the air-filled lungs and the ultrasound beam [9]. Ultrasound, in general, is reflected at tissues, and the amount of reflected ultrasound is associated with the relative change in acoustic impedance [10]. In the case of the normal lung, the ultrasound beam meets the aerated lung (low impedance 0.004 Rayl, and no acoustic mismatch). On the other hand, in the presence of extravascular lung water, the ultrasound beam is reflected at the interlobular septa, thickened by oedema (in this case, high impedance and high acoustic mismatch). When the lung is associated with complete loss of aeration, LU displays a tissue-like pattern similar to the liver (high impedance 1.65 Rayl, high-speed sound velocity).

8.4 Examination Protocols

There are, in essence, two examination LU protocols. In the first protocol, the lungs are divided into 12 regions [2]. The anterior surface of each lung is defined by clavicle, parasternal, anterior axillary line, and the diaphragm is divided into two areas, upper and lower. The lateral surface is defined by the anterior and posterior axillary lines and divided into an upper and lower area. Finally, the posterior lung surface is defined by the posterior axillary and the paravertebral lines and divided into an upper and lower area. The lung apex is scanned from the supraclavicular space [3]. In the second protocol, which is simpler, the operator examines the anterior and lateral areas of each hemithorax from the second to the fourth or fifth intercostal spaces, and from parasternal to the axillary line [11].

8.5 Lung Ultrasound Imaging

8.5.1 The Normal Lung Pattern

The probe is placed vertically over the intercostal space. The resultant image depicts the superior and inferior ribs, their acoustic shade, and the pleural line, 0.5 cm from an imaginary line connecting the ribs [2, 12]. The pleural line corresponds to the visceral pleura and represents the lung surface. Lines parallel to the pleural line are referred to as A-lines. These represent reverberation artefacts with constant location. Apart of these static signs, the normal lung generates a dynamic sign known as ‘lung sliding’. The sliding movement of the visceral pleura towards the parietal pleura during the respiratory cycle characterises it. In time-motion mode, the normal lung pattern is illustrated by the ‘seashore sign’ (Fig. 8.2) [12]. The latter is characterised by the chest wall layers over the pleural line and a granular pattern below it. In many cases, pleura act as a mirror producing the mirror effect [13].

Fig. 8.2

The normal lung pattern. Pleural line is shown by black arrows. At the right of the screen appears the ‘seashore sign’. It is characterised by the chest wall layers over the pleural line and a granular pattern below it. Parallel lines to the pleural line (white arrows) are reverberation artefacts known as A-lines

8.5.2 Pathological Conditions: Lung Parenchyma

8.5.2.1 Atelectasis/Consolidation

Atelectasis/consolidation is associated by the complete loss of the lung aeration. LU displays a tissue-like structure pattern, similar to the liver [14, 15]. It is associated with (1) abolition of the lung sliding and dynamic diaphragmatic movement and (2) the presence of static air bronchogram within the atelectasis/consolidation. In critically ill patients, this pathology is usually also associated with pleural effusion. In this case, particularly in the dependent lung regions, the compressed lung floats within the effusion, a LU finding which is very common in critically ill mechanically ventilated patients (Fig. 8.3) [5]. The static air bronchogram is caused by entrapped air inside a lung area that is no longer aerated, thus creating hyperechoic punctiform images (artefacts) [16].

Fig. 8.3

Atelectatic lower lobe floating into the pleural fluid (black anechoic area)

8.5.2.2 Interstitial Syndrome

Interstitial syndrome is characterised by the presence of multiple B-lines. B-lines are well-defined hyperechoic comet-tail artefacts, arising from the pleural line and extending into the far field [17]. They move according to the lung-sliding movement, erasing the A-lines. B-lines may arise from thickened pleura due to the accumulation of fluid (oedema) or in interstitial lung diseases, from fibrosis-thickened subpleural septa. The distance between B-lines may help to differentiate between these two mechanisms; the presence of B-lines, 7 ± 1 mm apart (B7-lines), is consistent with the thickening of the interlobular septa, whereas B-lines 3 ± 1 mm apart (B3-lines) indicate oedema and correspond to ground-glass pattern in CT scan. The former pattern is resistant to diuretic therapy, while the latter may respond to therapy towards the cause of pulmonary oedema (i.e. diuretics, dialysis, PEEP), even within minutes or hours [10, 18, 19]. White lung is defined as completely white echographic lung fields, with coalescent B-lines and no horizontal reverberation (Fig. 8.4). A recent study examined the ability of the bedside LU to quantify the PEEP-induced lung recruitment. This study clearly shows that using LU for PEEP titration in ARDS patients is accurate enough and has the advantage of being non-invasive and easily performed at the bedside [6].

Fig. 8.4

White lung in a patient with severe ARDS. Notice the white echographic lung fields and no horizontal reverberation (no A-lines)

8.5.2.3 Pneumonia

Echographic lung imaging from standard windows allows the evaluation of pneumonia, since most pneumonias in critically ill reach the pleura [15, 20, 21]. The LU signs that support the diagnosis of pneumonia are (1) bilateral or local B-lines pattern, (2) the presence of anterior lung consolidation with irregular boundaries, (3) the existence of vascular flow within the infected area, (4) the presence of pleural effusion and (5) the dynamic air bronchogram [16]. Dynamic air bronchogram is illustrated by linear or punctiform hyperechoic artefacts within a consolidation with dynamic movement according to the respiratory cycle, representing the air moving into the bronchial tree (Fig. 8.5). LU may track the response to therapy in critically ill patients with pneumonia. Bouhemad et al. have shown that lung re-aeration can be accurately estimated with bedside LU in patients with ventilator-associated pneumonia treated by antibiotics [22].