Page 301 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 29: ICU Ultrasonography 205

The inexperienced operator, who is not familiar with the basic elements surfaces across each other with respiration, and is absolute evidence that
of needle and wire insertion, may have great difficulty with understand- there is not a pneumothorax at that site of examination. Cardiophasic
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ing the ultrasound image. Experiential training on an actual patient is movement of the pleural line is termed lung pulse; this also rules out
inappropriate. The best approach is to use a task trainer for both pur- pneumothorax at the site of the examination. In the supine patient,
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poses: for the trainee to master the physical aspects of line insertion, fol- the anterior chest may be rapidly examined at multiple points, allowing
lowed then by training in how to use ultrasonography for guidance. The the intensivist to quickly and definitively rule out pneumothorax. In the
transition from the perfect anatomy of a well-designed task trainer to the unlikely event of a loculated pneumothorax, other imaging modalities
difficult patient access challenge may still be difficult, and warrants close may need to be used. Visualization of underlying consolidated lung or
supervision of the trainee. the presence of alveolar-interstitial changes that start at the pleural line
also rule out pneumothorax, even in the absence of pleural movement.
While the presence of lung sliding and/or lung pulse rules out pneu-
THORACIC ULTRASONOGRAPHY: LUNG AND PLEURA
mothorax with a high level of certainty, the opposite is not true. Absence
Respiratory failure is a common problem in the ICU. Chest radiography of these findings is suggestive of pneumothorax, but pleurodesis,
and chest CT are common imaging modalities, but each has its limita- severe underlying lung disease that reduces movement of lung (such as
tion. The supine chest radiograph may be difficult to interpret related pneumonia), or absence of lung inflation may also cause their absence.
to penetration, rotation, and tissue summation artifact, while frequent Absence of pleural movement must be interpreted within the clinical
chest CT is impractical, due to the logistical challenges of patient trans- context. For example, loss of lung sliding following central-line inser-
port. Radiation exposure is also a major consideration with chest CT. tion, when it was present beforehand, is strong evidence for a procedure-
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Thoracic ultrasonography allows the intensivist to rapidly and repeat- related pneumothorax.
edly examine the patient in order to identify typical features of lung When a pneumothorax results in partial collapse of the lung, some
disease. For important findings, it outperforms both the chest radiogra- part of the visceral pleura will still be apposed to the inside of the chest
phy and physical examination and yields results that are similar to chest wall. By moving the transducer laterally along an interspace, the exam-
CT, and it has major utility for procedural guidance. iner may be able to identify the point at which there is intermittent lung
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■ GENERAL PRINCIPLES ning plane coincident with the respiratory cycling. This finding is called
sliding, that is, where the partially collapsed lung moves into the scan-
1. Thoracic ultrasonography is performed using a cardiac transducer. the lung point, and is diagnostic of a pneumothorax. 23
The small footprint of the transducer allows easy examination Normal Aeration Pattern: A frequent cause for ICU admission is respira-
through the rib interspaces. A vascular transducer may be used for tory failure. The finding of a generalized normal aeration pattern with
better resolution of the pleural interface. lung ultrasonography or with standard chest radiography in the acutely
2. The critically ill patient is generally examined in the supine posi- dyspneic patient or the patient on ventilatory support has utility for
tion, making it difficult to examine the posterior chest; however, the the intensivist. It may suggest such diagnoses as pulmonary embolism,
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transducer may be pressed into the mattress and angled anteriorly airway disease, metabolic acidosis, or neurological dysfunction with
for partial view of this area. If indicated, the patient may be rolled augmentation of respiratory drive. Ultrasonography allows rapid iden-
to a lateral decubitus position. The transducer is held perpendicular tification of this pattern, as well as identifying the patient who presents
to the chest wall and directed through the rib interspace. This yields with lung disease with focal areas of abnormality.
a standard image with the rib shadows on either side to the image, Normally aerated lung has a distinctive pattern on ultrasonographic
the pleural line in central location, and the lung deep to the pleural examination that is characterized by presence of A lines combined with
line. By convention, the transducer indicator is oriented cephalad, lung sliding. A lines are one or more horizontal lines below the pleural
yielding a longitudinal scanning plane. The transducer is moved to line. They represent a reverberation artifact, and so are regularly spaced
adjacent interspaces in longitudinal manner such that the examiner at distance that is identical to the skin to pleural line distance. Their
lays down a scan line encompassing multiple intercostal spaces. In presence indicates normal aeration pattern at the site of the exami-
organized fashion, a series of scan lines is performed starting on nation. By moving the transducer over the chest wall, the examiner
the anterior chest wall and then proceeding to the lateral, followed determines the extent and location of the normal aeration pattern. For
by the posterolateral chest wall. In this way, the examiner performs example, a patient with a lobar pneumonia will have ultrasonographic
multiple two-dimensional tomographic sections, and so is able to abnormality over the affected lobe, but have A lines elsewhere.
develop a three-dimensional model of the thorax. A focal area of Alveolar Interstitial Abnormality: A wide variety of disease processes of
abnormality may be examined in more detail. For example, pleural interest to the intensivist result in alveolar or interstitial abnormalities
fluid is generally dependent in position in the supine patient, so identifiable with lung ultrasonography, standard chest radiography, or
the identification of a safe site for pleural device insertion requires chest CT. Lung ultrasonography is useful in identifying this pattern of
focused examination of the posterolateral chest. abnormality. Congestive heart failure, acute lung injury, ARDS, and
■ FINDINGS OF THORACIC ULTRASONOGRAPHY interstitial lung diseases all may cause alveolar or interstitial patterns,
and are important disease classes in the ICU.
Pneumothorax: Ultrasonography is useful for the detection of pneumo- Alveolar and interstitial lung diseases result in the ultrasonographic
thorax. For this application, it is superior to supine chest radiography finding of B lines, which are comet tail artifacts. These are horizontally
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and similar in performance to chest CT. An anteriorly located pneu- orientated white lines that originate at the pleural surface and end at the
mothorax may be invisible on the typical ICU chest radiograph, but is lower edge of the image. They efface A lines at any point of intersec-
readily diagnosed with ultrasonography. Ultrasonography allows the tion, and, originating at the visceral pleural line, they move with pleural
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intensivist to rapidly rule out the condition in the patient with acute movement. If the pleural line is immobile, B lines may also be immobile.
respiratory deterioration while on ventilatory support, postprocedure, A few B lines are found in normal individuals in the lower lateral thorax.
or as a routine measure during evaluation for acute dyspnea. The density of B lines is important. Two or fewer is a single ultrasound
The standard ultrasonographic view through an intercostal space scanning field is inconsequential, while three or more suggest significant
places the rib shadows on either side of the screen, with the pleural line pathology. The finding of multiple B lines is highly significant, while a
visible about 5-mm deep to the rib periosteum. Examination of the pleu- confluence of B lines leading to a white image suggests severe disease
ral line normally reveals a respirophasic movement that is called lung such as acute cardiogenic pulmonary edema. B lines may be focal in dis-
sliding. This derives from movement of the visceral and parietal pleural tribution or generalized depending on their cause. Patchy collections of

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