To date, the only evidence-based recommendation for POC-US is for lung ultrasonography (LUS).7 A panel of experts voted for the level of evidence in each condition where lung ultrasonography is used. The level of evidence was classified as following: Level A: high-quality evidence and further research is very unlikely to change the estimated effect or accuracy of evidence. Level B: Moderate quality and further research is likely to have an important impact on estimated effect or accuracy and may change level of evidence. Level C: Low quality of evidence and further research is likely to change the estimated effect or accuracy of evidence (Table 12–3).
Table 12–3Evidence bases recommendations for lung ultrasonography. ||Download (.pdf) Table 12–3 Evidence bases recommendations for lung ultrasonography.
|Level of Evidence ||Clinical Syndromes and Recommendations |
|A (High quality) Further research is very unlikely to change estimates || |
Signs suggesting PTX: presence of lung point, B-lines, lung pulse, or absence of lung sliding.
Ultrasound rules out PTX more accurately than supine anterior chest radiography (CXR).
Lung ultrasound for detection of lung consolidation can differentiate consolidation due to pulmonary embolism, pneumonia, or atelectasis.
Lung ultrasound is an alternative diagnostic tool to computerized tomography (CT) in diagnosis of pulmonary embolism when it is contraindicated or unavailable.
In mechanically ventilated patients, lung ultrasonography is more accurate than CXR in detecting and distinguishing various types of consolidations.
A hypoechoic space between the parietal and visceral pleura with respiratory movement of the lung within the effusion is very specific for such condition.
Internal echoes in the effusion suggest that fluid is an exudate or hemorrhagic.
Lung ultrasound is more accurate that supine radiography and nearly as accurate as CT.
Lung ultrasound is more accurate in distinguishing effusion from consolidation than CXR.
Monitoring lung disease
In cardiogenic pulmonary edema, severity of congestion is proportional to number of B-lines.
B-lines can be used to monitor response to therapy in patients with cardiogenic pulmonary edema.
In patients with increased extravascular lung water, assessment of lung reaeration can be assessed by a decrease in the number of B-lines.
In acute lung injury or acute respiratory distress syndrome (ARDS), tracking changes in sonographic findings may quantitatively assess lung reaeration.
|B (Moderate quality) Further research is likely to have an important impact in changing || |
Diffuse bilateral B-lines indicate interstitial syndrome of various causes, ie, pulmonary edema/interstitial pneumonia/diffuse parenchymal lung disease.
Localized B-lines are seen in pneumonia/pneumonitis/atelectasis/pulmonary contusions/pulmonary infarctions/pleural disease/neoplasia.
Pulmonary fibrosis can be evaluated with ultrasound, diffuse multiple B-lines with pleural abnormalities are seen often.
Sonographic findings of ARDS include anterior subpleural consolidations/absence or reduction of lung sliding/pleural line abnormalities/nonhomogeneous B-lines.
Monitoring lung disease
Serial evaluation of B-lines in hemodialyzed patients with pulmonary congestion may be of clinical utility.
In cardiogenic pulmonary edema, semi-quantitative B-line assessment is a prognostic indicator of adverse outcomes and mortality.
|C (Low Quality) Further research is very likely to have an important impact as the evidence is currently very uncertain || |
Ultrasound is a better initial study compared to chest radiography and may lead to better patient outcomes.
Ultrasound compares well to computerized tomography in assessment for PTX.
Lung point ultrasound is a useful tool to differentiate large and small PTX.
Low-frequency ultrasound scanning may allow for better evaluation of the extent of a consolidation.
Lung ultrasound used as an initial diagnostic strategy in consolidations improves outcomes compared to CXR.
Monitoring lung disease
Ultrasound Probe Selection and Orientation
The preferred probe to evaluate the thorax is a 3.5 to 5 MHz with a small footprint and the cursor pointing cephalad. If more superficial structures of the thorax are being investigated, a 7.5- to 10-MHz linear probe can be used.
The classical views of the chest are in the midclavicular line, midaxillary line, and posterior axillary or mid scapular line. However, scanning other parts of the chest may be indicated as long as good images are obtained for interpretation.
Clinical Implications and Description of Syndromes
In the hands of an experienced operator, LUS in the critically ill is superior to chest roentgenogram (CXR) and comparable to computerized tomography of the chest (CT-Chest) in several conditions.8,9 It is important to understand the air-fluid ratio in the lung to understand and interpret LUS. The air-fluid ratio in pneumothorax (PTX) is pure air, no fluid; normal lung has air and very little fluid, the interstitial syndrome has air with more fluid; alveolar consolidation has little air but a lot of fluid and pleural effusion consists of pure fluid, no air. (Figure 12–1).
Understanding air-fluid ratio in lung ultrasonography. (Reproduced with permission from Lichtenstein DA: Lung Ultrasound in the Critically Ill: The BLUE Protocol. Switzerland: Springer; 2016).
The only way to image the lung is in between the ribs (Figure 12–2). The lung has to be centered in the screen and the two rib shadows are seen on the side; this is referred to as the bat sign. The pleural line can be identified as a bright structure. Standard ultrasound probes cannot separate the visceral from the parietal pleura so only one line is seen. In a healthy lung, the pleural surfaces move against each other during the respiratory cycle causing a shimmering line, this is called lung sliding. Lung pulse is referred to the pleural line moving synchronously with each cardiac cycle. A normal aerated lung will show reverberations artifact from ultrasound reflection between the pleural line and the skin surface, these are called A-lines. In the presence of lung sliding, A-lines indicate normally aerated lung. In the absence of lung sliding, A-lines do not necessarily indicate normal lung and pneumothorax could be present.
This is the bat sign representing normal lung. Arrowheads are pointing the A-lines.
The Interstitial Syndrome—B-lines represents thickened subpleural interlobular septa and/or ground-glass opacity,10 either of cardiogenic or noncardiogenic etiology. They are described as comet-tail artifact and must fulfill all of these requirements: (1) Starts from the pleural surface, (2) moves with lung sliding, (3) erases A-lines (4) reaches the bottom of the screen. If any one of the requirements is not met, it is not a B-line.
B-line artifact can be seen in interstitial syndromes (Table 12–4). A few B-lines in the bases of the lung are considered normal but more than three B-lines in a single field should be considered abnormal (Figure 12–3).
B-lines are comet-tail artifacts that are seen in interstitial lung syndrome.
Table 12–4Causes of interstitial syndrome. ||Download (.pdf) Table 12–4 Causes of interstitial syndrome.
|Pathophysiology ||Etiologies |
|Cardiogenic ||Acute hemodynamic pulmonary edema |
|Noncardiogenic || |
Adult respiratory distress syndrome
Interstitial lung disease
Diffuse bilateral B-line pattern with smooth pleura is consistent with cardiogenic interstitial edema.11 Diffuse bilateral B-line pattern with an irregular pleura is usually seen in adult respiratory distress syndrome. Focal irregular pleura with B-lines can be interpreted as early or atypical pneumonia in an appropriate clinical setting.
The Alveolar Consolidation Syndrome—The alveolar consolidation syndrome refers to any condition that can fill the alveoli with fluid (Table 12–5). About 98.5% of alveolar consolidations reach the pleura, enabling ultrasound examination. In a supine patient, 90% of consolidations can be found in the most dependent area of the thorax.8 The tissue-like sign is an echoic pattern, with regular trabeculation reminiscent of an ill-defined liver (Figure 12–4). Ultrasound has a 90% and 98% sensitivity and specificity, respectively.8 In alveolar consolidation syndromes 2 types of air bronchograms can be seen, static and dynamic. Dynamic air bronchograms indicate the presence of air moving within the bronchus with each breathing cycle, ruling out absorptive atelectasis. A static air bronchogram indicates obstruction of the bronchus causing absorptive atelectasis.
Alveolar consolidation syndrome seen in pneumonia. (Image courtesy of Sahar Ahmad, MD).
Table 12–5Causes of consolidation. ||Download (.pdf) Table 12–5 Causes of consolidation.
PTX occurs in 6% of the ICU patients12 and up to 30% of those can be misdiagnosed on conventional CXR owing to the supine position of the critically ill patient.13 The gold standard to diagnose PTX is CT-Chest; this involves transferring the patient with all the risks associated with it. Luckily ultrasound can help us identify PTX by scanning the anterior chest. Once the bat sign is identified, the absence of lung sliding plus a diffuse A-line pattern should prompt us to consider the diagnosis of PTX (Figure 12–5). Lung point indicates the point where the lung is touching the chest wall and moves with each respiratory cycle, this is 100% specific for PTX. The sensitivity of lung point for PTX detection is not 100% since this is usually absent in large PTX. It is important to mention that the presence of lung pulse or B-lines rule out PTX in 100% of the cases.
Another phenomenon described in left side PTX is the flickering image in the parasternal long axis view of the heart. The heart is seen in mid-diastole and disappears in midsystole. This is explained by the transient interposition of air between the chest wall and the heart, making it hard to visualize when it is empty and not in contact with the chest wall. This is called heart point (Figure 12–6).14
This image shows pneumothorax, note the absence of lung sliding that is represented as stratosphere sign (arrows) and sliding lung represented by seashore sign (in between arrows) in M-mode. Intermittent seashore and stratosphere sign is seen in the lung point.
Heart point. In a parasternal long axis view of the heart Image A represents the heart in diastole, when it is fully seen, Image B represents the heart in systole, contact of the heart with the chest wall has disappeared as well as the image. (Reprinted with permission of the American Thoracic Society. Copyright © 2016 American Thoracic Society. Khan R, Rahmanian M, Kaufman M, et al: The Heart Point Sign: An Ultrasonographic Confirmation Of Pneumothorax, 2013.)
Pleural effusion is a very common finding in ICU patients.15 It collects in dependent areas unless loculated. By using the signs described below, ultrasound can achieve a sensitivity and specificity of 93%.9
A big pleural effusion is easily identified and must be distinguished from ascites. Using simple static and dynamic signs, we can tell them apart. The lung can be identified as a consolidated mass usually with air bronchograms, floating in fluid. This is called the jellyfish sign. The diaphragm should be identified above the liver and the spleen, if there is no jellyfish sign in a big effusion; the structure is likely to be the liver (or spleen), not the lung (Figure 12–7).
Image A represents a pleural effusion; look at the similarities with ascites in Image B. It is important to determine static and dynamic signs to differentiate between pleural effusions from ascites.
Ultrasound can also help identify the nature of the pleural effusion. A complex anechoic or hypoechoic space with septations most likely represents an exudative effusion (Figure 12–8).16 Sometimes this effusion can be seen as a “thick effusion” meaning a tissue-like echogenicity but with particles swirling around. This is called the plankton sign, which is seen in empyema and hemothorax (Figure 12–9).
Septated pleural effusion usually seen in an exudative effusion. These strands resembling spider webs are floating in the pleural effusion.
Plankton sign is usually seen when debris exist within the pleural effusion; it can be pus or blood (see arrow).
This structure usually is easily visualized above the liver or spleen. There have been studies examining diaphragm thickness and strength. It is well known that thinning of the diaphragm starts within 18 hours of muscle inactivity.17 This was demonstrated by ultrasonography in 7 patients measuring the diaphragm using the 7.5- to 10-MHz linear probe placed in the midaxillary line to measure the apposition area (Figure 12–10).18 The utility of this should be further studied. Another promising use for ultrasonography of the diaphragm includes evaluation of its excursion during weaning from mechanical ventilation. Kim et al found that in patients with diaphragm dysfunction, defined when diaphragm excursion was less than 10 mm, weaning failure rate was higher when compared to patients with diaphragm excursion of more than 10 mm.19
Diaphragm thickness (in between arrows). Image acquired in the apposition area.
Critical Care Echocardiography
Echocardiography, either transthoracic echocardiography or transesophageal is a very helpful tool in the critically ill patient. This will be discussed in detail in Chapter 94 of this book.
Ultrasound Probe Selection and Orientation
The preferred ultrasound probe to evaluate the abdominal area will be a 3.5 to 5 MHz with a small footprint with the cursor pointing cephalad.
Clinical Implications of Abdominal Ultrasonography and Recognition of Important Structures
The intensivist should be capable of performing and interpreting simple abdominal ultrasonography. Detailed ultrasound examination of the abdominal organs is out of the scope of this book. It is our practice to perform abdominal POC-US paying attention to liver, spleen, kidney, bladder, gallbladder, aorta, and intestines.
Recognition of the Peritoneal Space
The parietal and visceral layers are in touch with each other and slide with each respiratory cycle; this is called gut sliding.
It is imperative to look for free fluid in an unstable patient. We have found FAST very useful in different situations outside the classic trauma population.
The peritoneal fluid can range from anechoic to hyperechoic, depending on the etiology. Transudates will have an anechoic nature, an hemoperitoneum can be visualized depending on the amount and time elapsed since bleeding started; it could be mildly hyperechoic with plankton sign or more hyperechoic if the blood had already organized. An exudative effusion causing infectious peritonitis could have septations. Ultrasound is more sensitive for detecting septations than CT scan.
Pneumoperitoneum (Figure 12–11) can be detected with ultrasound using the same concepts we have learned in pneumothorax. There are a couple of signs that will help us identify this condition: (a) Gut sliding: as described above, can rule out pneumoperitoneum. (b) Splanchnogram: it is when an abdominal organ can be seen with ultrasound, thus, ruling out pneumoperitoneum between the probe and the organ.
The presence of these 2 signs can rule out pneumoperitoneum accurately.
Pneumoperitoneum detected through ultrasonography. Please note the reverberation artifact resembling the lung A-line. There is no splagnogram. Used with permission from Sahar Ahmad, MD.
The probe should be oriented with the pointer cephalad and placed in the posterior axillary line under the costal border; once the liver or the spleen is identified, the kidney is easier to find.
Acute renal failure (ARF) is frequently encountered in the critically ill patient. The etiologies are classified into prerenal, renal, and postrenal. Ultrasound is useful as an initial screening tool.
The evaluation of the renal parenchyma by ultrasound is complex but we can briefly mention some concepts. ARF can be differentiated from chronic renal failure (CRF) because in the latter small kidneys with thin parenchyma and irregular borders are seen. Corticomedullary dedifferentiation can be seen in multiple conditions. In acute medical renal disease the renal parenchyma becomes more echogenic. The renal pelvises are well visualized under ultrasound. Chronic dilatation causes rounded renal calyces and the acute dilatation causes flatter renal calyces (Figure 12–12).
Acute hydronephrosis of the right kidney.
The bladder is very well visualized in the suprapubic area with the probe held transversely. A full bladder can be identified as an anechoic structure, if the patient has an urinary catheter, this can be easily visualized (Figure 12–13). Furthermore, ultrasound of the bladder can be used to check for urinary retention without using a catheter.
Urinary catheter balloon inside the bladder.
Ultrasound of the Abdominal Aorta
This is a very important structure to evaluate in an unstable patient and in someone who has been complaining of abdominal or flank pain. Abdominal aortic aneurysm (AAA) rupture has a high mortality rate (around 50%) and ultrasound has been shown to be a reliable method of evaluating AAA.
The thoracic aorta is better evaluated with a transesophageal echocardiogram or contrast computerized tomography. The abdominal aorta is easily visualized by transabdominal ultrasound.
The aorta is evaluated with the ultrasound probe placed in the midline of the abdomen and the cursor pointing toward the patient's right when a transverse view is desired and pointing toward the head when a longitudinal view is desired. The aorta should be studied from the epigastrium until the bifurcation to the iliac arteries (Figure 12–14). A diameter greater than 3 cm is considered aneurysmal. Ultrasound is not very sensitive to evaluate for aortic dissections but if a false lumen is seen, it is highly specific for such disease.
Image A shows a normal abdominal aorta with the inferior vena cava located on the left. Image B shows a dissection flap (arrow) inside the abdominal aorta.
The esophagus is usually very difficult to visualize under ultrasound. It becomes visible and feasible for ultrasound assessment once it enters the abdomen to join the stomach. Ultrasound evaluation of the abdominal part of the gastrointestinal (GI) tract is very useful in the critically ill patient.
One observational study showed the utility of identifying gastric contents with ultrasound and performing suctioning prior to emergent endotracheal intubation.20 It is thought that this can potentially decrease the chances of aspiration during intubation. The use of ultrasound had also been described for confirmation of nasogastric tube and Sengstaken-Blakemore tube positioning.21,22,23
When evaluating the GI tract, it is important to observe the wall thickness that usually ranges around 2 to 4 mm. It is also very important to observe for gut sliding and peristalsis; the latter are permanent dynamic crawling contractions seen from the antrum of the stomach until the terminal ileus. The presence of peristalsis is rare in a true surgical abdomen.
Liver, Gallbladder, and Spleen
The liver is rarely a target in a critically ill patient despite its size. It acts as an acoustic window to access the heart, Morrison pouch and inferior vena cava analysis. However in some occasions, gross abnormalities can arise such as the following: (a) Liver abscess: yields a round heterogenous hypoechoic image within the regular hepatic echostructure easily identifiable. A highly hyperechoic image can represent microbial gas. (b) Portal gas: as a result of mesenteric infarction. Hyperechoic images disseminated within the liver parenchyma mainly in the periphery and resembling alveolar consolidation with air bronchograms. (c) Liver cirrhosis: yields a coarse and nodular pattern with atrophy (Figure 12–15). (d) Hepatic tumors: Metastatic tumors are usually multiple iso- or hypoechoic masses. However an echoic heterogeneous mass within a cirrhotic parenchyma suggests hepatoma. A simple biliary cyst is usually described as a simple anechoic structure with thin walls (Figure 12–16).
A small liver compatible with liver cirrhosis, the similarity with a consolidated lung can confuse the nonexperienced operator between ascites and pleural effusion.
Liver tumor with metastasis (arrows).
The gallbladder can be viewed during ultrasound of the right upper quadrant. The following findings are suggestive of acute cholecystitis: (1) Enlarged gallbladder over 90 mm in long axis and over 50 mm in short axis. (2) Thickening of the wall greater than 3 mm. (3) Sludge or stones seen within gallbladder. (4) Perivesicular fluid collection. (5) Murphy sign. The sensitivity drops if all the criteria are used but specificity increases.
Cardiogenic or noncardiogenic gallbladder wall edema can confound the diagnosis of acute cholecystitis.
Spleen analysis in the critically ill is seldom revealing. It acts as an acoustic window for different organs and also to localize the diaphragm while planning for thoracentesis. In a trauma patient, splenic laceration can be detected and in rare instances a splenic abscess is seen.
Deep Venous Thrombosis Assessment
Ultrasound Probe Selection and Orientation
The high-frequency 7.5 to 10 MHz linear probe is preferred in our institution with the cursor pointing toward the right of the patient.
Introduction of Venous Ultrasonography of the Lower Extremities
Deep vein thrombosis (DVT) and pulmonary embolism (PE) are a spectrum of the same disease. In a rapidly decompensating patient, either in the hospital ward or in the ICU, DVT and PE should always be in the differential diagnosis. Blood test such as d-dimer is nonspecific in the hospitalized patient. Ultrasound of the lower extremities can successfully diagnose DVT. When done by a trained intensivist, the sensitivity and specificity are very similar to examinations done by the radiology technician and interpreted by a radiologist.24 A DVT can be found in the upper and lower extremities. We will focus on lower extremity anatomy here.
The patient should be positioned in the decubitus position with the leg slightly flexed and externally rotated (Figure 12–17). The leg should be examined by performing gentle and firm compression, enough to collapse (Figure 12–18) the vein but not the artery. A vein with DVT will not collapse. (Figures 12–19 and 12–20).
Position of deep venous thrombosis (DVT) study for femoral and popliteal vein.
Femoral vein in a patient with suspected deep venous thrombosis. Image A without compression and Image B showing vein completely collapsible.
Femoral vein in a patient with suspected deep venous thrombosis. Image A without compression and Image B showing the vein is not collapsible.
The operator should be familiar with the lower extremity venous anatomy (see Figure 12–20). The ultrasound probe should be placed at the level of the inguinal ligament to localize the femoral vein, which is medial to the femoral artery. Once this is identified, a compression movement is performed to check for collapsibility, a collapsible vein rules out DVT. A noncollapsible vein, rules in DVT. Color Doppler can be used but will increase study time without adding much information. Unfortunately, the external iliac veins are not visualized fully but can be partially visualized if the probe is tilted and pointing the ultrasound beam “under” the inguinal ligament. Once the common femoral vein (CFV) is studied, the drainage of the greater saphenous vein can be seen medially. A couple of centimeters below, the CFV bifurcates in deep femoral vein and superficial femoral vein (SFV). It is important to recall that the SFV is still part of the deep venous system. Compression should be done every 2 to 3 cm while identifying the perforating branches until the vein enters the adductor canal if visibility allows. The popliteal vein should also be studied with the leg bent and the sole of the foot on the bed (see Figure 12–17). The popliteal vein should be located superficially to the artery. Again, firm pressure is applied to evaluate compressibility.
Important veins of the lower extremity, it is important to be familiar with the anatomy in order to interpret where the DVT is located. Dark blue: Deep vein. Light Blue: superficial vein.
How can we integrate these findings in our clinical assessment? A hypoxic patient with positive DVT and bilateral A-line pattern in the lung, PE should be suspected. Furthermore, a quick echocardiogram should be done and if there are signs of right ventricle overload, submassive or massive PE is highly suspected.
Optic Nerve Ultrasonography
Probe Selection and Orientation
It is usually performed with the high-frequency 7.5 to 10 MHz linear probe and it is placed over the eyelid. The optic nerve sheath (ONS) will be identified and then measured (Figure 12–21).
Optic nerve sheet measurement. In this particular case, it was not enlarged.
It is estimated that 10 studies could be sufficient in experienced US operators and 25 studies for inexperienced US operators to achieve competency.25
The gold standard for detection of an elevated intracranial pressure (ICP) is invasive measurement. These methods can lead to complications such as infection or bleeding. Other methods are described but we found US measurement of the optic nerve sheath diameter (ONSD) an excellent screening test for elevated ICP.
When ONSD is compared to the gold standard, a pooled analysis using cutoff value range from 5.2 to 5.9 mm showed a sensitivity and specificity of 74% to 95% and 74% to 100%, respectively.26 In another meta-analysis27 the pooled sensitivity and specificity was 90% and 85%, respectively. One study using 5 mm as a cutoff28 yielded 100% and 95% sensitivity and specificity, respectively.