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Reading chest radiographs in the critically ill (Part I): Normal chest radiographic appearance, instrumentation and complications from instrumentation

The standard PA chest radiograph is rarely taken in the ICU patient and is replaced by an anteroposterior (AP) radiograph. These radiographs are ideally obtained in the AP projection with a patient–to–x-ray plate distance of 72 inches with the patient in the upright position at maximum inspiration; but more often, a distance of 40 inches is used in the supine or sitting position due to the impaired mobility of ICU patients [Figure 1]. A radiograph obtained in this way magnifi es the mediastinum and heart due to gravitational and geometrical effects. Moreover, supine position alters the physiology of the pulmonary vasculature, diverting the blood to lung apices — an appearance that is regarded as normal on AP supine radiograph but considered abnormal on a PA radiograph. Supine radiographs have further limitations, including problems of differentiating pleural effusions from air space shadowing and detecting a pneumothorax. Exposing a radiograph in full inspiration in an ICU patient poses further challenges as these patients are often uncooperative or are suffering from postoperative pain [Figure 2]. A more-thanperfect inspiratory effort creates artifacts, making the diagnosis of basilar atelectasis and pulmonary edema diffi cult, besides causing changes in the I chest radiographs of the critically ill patients who are in intensive care units (ICUs) poses a challenge not only for the intensive care physicians but also for the radiologist. These challenges arise because of several factors:[1] ICU patients are prone to several cardiopulmonary disorders which when superimposed on the underlying pathology that prompted admission create a complex radiological appearance, which may be diffi cult to interpret on the basis of imaging fi ndings alone.[2] The standard posteroanterior (PA) radiograph is replaced by the suboptimal anteroposterior (AP) radiograph in the ICU patient.[3] Instrumentation, mechanical ventilation, cardiac and other vital sign monitoring and feeding tubes, etc., detract from other fi ndings on the ICU chest radiograph.[4] Radiologists/ Intensive care physicians are under pressure for rapid interpretation of chest x-rays when treating critically ill patients, often with inadequate clinical information, partly due to the fact that things can change rapidly in the critically ill.[5] Radiological interpretation is hampered by the bewildering array of line placements in the ICU patient, where incorrect placement is not uncommon, which may not be obvious to the observer without clinical input.[6] Air space shadowing in the ICU patient may have identical appearances in a variety of cardiopulmonary pathologies. Although the imaging modality of choice in the ICU patient remains that of chest radiography, computed tomography is often performed as computed tomographic pulmonary angiography (CTPA) with suspected pulmonary embolism. Ultrasound is used to confi rm pleural and pericardial effusions and when pleural intervention is planned.


The Normal Chest X-ray
The standard PA chest radiograph is rarely taken in the ICU patient and is replaced by an anteroposterior (AP) radiograph. These radiographs are ideally obtained in the AP projection with a patient-to-x-ray plate distance of 72 inches with the patient in the upright position at maximum inspiration; but more often, a distance of 40 inches is used in the supine or sitting position due to the impaired mobility of ICU patients [ Figure 1]. A radiograph obtained in this way magnifi es the mediastinum and heart due to gravitational and geometrical effects. Moreover, supine position alters the physiology of the pulmonary vasculature, diverting the blood to lung apices -an appearance that is regarded as normal on AP supine radiograph but considered abnormal on a PA radiograph. Supine radiographs have further limitations, including problems of differentiating pleural effusions from air space shadowing and detecting a pneumothorax. Exposing a radiograph in full inspiration in an ICU patient poses further challenges as these patients are often uncooperative or are suffering from postoperative pain [ Figure 2]. A more-thanperfect inspiratory effort creates artifacts, making the diagnosis of basilar atelectasis and pulmonary edema diffi cult, besides causing changes in the I nterpreting chest radiographs of the critically ill patients who are in intensive care units (ICUs) poses a challenge not only for the intensive care physicians but also for the radiologist. These challenges arise because of several factors: [1] ICU patients are prone to several cardiopulmonary disorders which when superimposed on the underlying pathology that prompted admission create a complex radiological appearance, which may be diffi cult to interpret on the basis of imaging fi ndings alone. [2] The standard posteroanterior (PA) radiograph is replaced by the suboptimal anteroposterior (AP) radiograph in the ICU patient. [3] Instrumentation, mechanical ventilation, cardiac and other vital sign monitoring and feeding tubes, etc., detract from other fi ndings on the ICU chest radiograph. [4] Radiologists/ Intensive care physicians are under pressure for rapid interpretation of chest x-rays when treating critically ill patients, often with inadequate clinical information, partly due to the fact that things can change rapidly in the critically ill. [5] Radiological interpretation is hampered by the bewildering array of line placements in the ICU patient, where incorrect placement is not uncommon, which may not be obvious to the observer without clinical input. [6] Air space shadowing in the ICU patient may have identical appearances in a variety of cardiopulmonary pathologies. Although the imaging modality of choice in the ICU patient remains that of chest radiography, computed tomography is often performed as computed tomographic pulmonary angiography (CTPA) with suspected pulmonary embolism. Ultrasound is used to confi rm pleural and pericardial effusions and when pleural intervention is planned.
The aim of this paper is [1] to discuss the radiographic findings of cardiopulmonary disorders common in the ICU patient and suggest guidelines for interpretation based not only on the chest radiograph but also on the pathophysiology and clinical grounds. [2] apparent size of the heart and mediastinum. [1][2][3][4] The sensitivity and specifi city of the ICU chest radiograph are low, but its common use stems from studies that have shown that as much as 65% of ICU chest radiographs reveal a signifi cant pathology that results in a change in patient management. Current recommendations from the American College of Radiology suggest that daily chest radiographs be obtained on patients with acute cardiopulmonary problems and those receiving mechanical ventilation. The college further recommends that only an initial chest radiograph is needed for the placement or change of indwelling devices. [5] In patients that have undergone thoracotomy for whatever reason, the initial postoperative radiographs after return from operating theatre will show the lines and tubes placed perioperatively, such as an endotracheal tube, thoracostomy tubes, mediastinal drains and central venous catheters. These devices need to be identifi ed and their positions checked for incorrect placement. Immediately following and a few days after CABG, lower lobe atelectasis is common, usually more pronounced on the left, generally resolving within a few days without complications. There is slight widening of the mediastinum, but significant increase in diameter might indicate complications such as a mediastinal bleeding. A small left pleural effusion is invariable following CABG, but a larger pleural effusion or a subsequent increase in size may need intervention so as not to cause respiratory compromise [ Figure 3]. Therefore, comparison with previous radiographs is essential to assess a change in size of a pleural effusion.

Identifying Lines and Tubes and Other Devices
Tubes, lines and drainage catheters play a vital role in monitoring and treating critically ill patients. Accurate placement of these devices and monitoring malfunction are crucial. The initial portable chest radiograph plays an essential role in recognizing correct placement and complications. All placed devices should be identifi ed on the preliminary radiograph as a priority in these patients before looking for cardiopulmonary disorders [ Figures 4 and 5].  : An AP chest radiograph on a 55-year-old female immediately following CABG, shows hazy opacifi cation at the lung base and blunting of the left costophrenic angle due to a small pleural effusion, which is a common feature following thoracotomy (black arrow)    Ideally placed between proximal venous valves of the subclavian or jugular veins and the right atrium. Jugular venous placement has lower complications.

Swan ganz catheter
The tip is wedged into the distal pulmonary artery. The balloon is defl ated once the pressure is taken, and the tip is pulled back to the main pulmonary artery. The tip of the catheter located within the mediastinal shadow indicates correct placement.

The thoracostomy tube
The last side-hole in a thoracostomy tube can be identifi ed by an interruption in the radiopaque line. This interruption in the radiopaque line should lie within the thoracic cavity, if not and or with evidence of subcutaneous air, a misplaced tube is suspected. Incorrectly placed tubes for empyemas may delay drainage and result in loculation of the purulent fl uid. Thoracostomy tubes placed within pleural fi ssures often cease to drain when the lung surfaces become apposed.

Cardiac pacemakers
The tip of the cardiac pacemaker should be at the apex of the heart, and there should be no sharp angulations along the length of the pacemaker wires. The lateral radiograph should show the tip imbedded within the cardiac trabeculae.
For correct placement to have occurred, the tip should appear 3 to 4 mm beneath the epicardial fat pad.
A tip that appears to be placed beyond the epicardial fat stripe may have perforated the myocardium. Cardiac pacers placed within the coronary sinus appear to be directed posteriorly on the lateral chest radiographs. respiratory failure. These tubes are cuffed and placed in the trachea, either via the oropharynx or introduced surgically through a tracheostomy. If it is anticipated that the patient needs intubation for a period longer than a week or has upper airway obstruction, a tracheostomy is usually preferred. A chest radiograph is essential to correctly locate the tip of endotracheal tubes. Misplaced ET may cause serious compromise of respiratory function and has been reported in 10% of the patients. Thus, daily chest radiographs are recommended in these patients as these devices may migrate.  This child has had a midline sternotomy (hollow arrowhead); a mediastinal drainage catheter is seen in place (white hollow arrow); there is a pericardial pacing wire (black arrow) and a nasogastric tube -all in a satisfactory position of the midline and the diagonal line is the most likely position of the carina. This is a cumbersome method for the busy ICU physician/radiologist and is seldom used.
ETs are misplaced in approximately 10% of the patients. A comparatively common misplacement is when the tube enters the right main stem bronchus, due to its more vertical orientation. This position impairs the left lung ventilation, leading to collapse of the left lung; similarly, if the endotracheal tube enters the bronchus intermedius, the right upper lobe may collapse. ET that has been placed at a higher level may slip into the pharynx or the esophagus, causing gastric air distension with the potential danger of refl ux of gastric contents and aspiration. More serious complications of ETs include tracheal stenosis, tracheal rupture, cord paralysis, cervical and mediastinal emphysema, hematoma and abscess formation.
When upper airway injury is suspected, a lateral radiograph may be useful. The soft tissue space between the trachea and cervical spine is increased in diameter to over the width of one vertebral body in the presence of a hematoma or infection.
Severe injury such as tracheal rupture should be suspected in patients with pneumothorax, pneumomediastinum, subcutaneous emphysema in the neck or precipitous respiratory failure following intubation [ Figure 8]. Most tracheal ruptures are placed posteriorly. [6][7][8][9] The thoracostomy tube Thoracostomy tubes are often placed into the pleural space to treat a pneumothorax or drain pleural fl uid [ Figure 9]. Chest radiographs are often obtained following placement of thoracostomy tubes to identify their position. It is important to recognize that on a supine AP radiograph, air accumulates anteriorly and fl uid gravitates posteriorly. This fact is taken into account when placing thoracostomy tubes relevant to the pathology in hand. Determining whether a tube is anterior or posterior is often diffi cult with a single AP chest radiograph. Thoracostomy tubes placed within pleural fi ssures often cease to drain when the lung surfaces become apposed. Correct placement of thoracostomy tube fenestrations within the thoracic cavity is important to proper functioning of these tubes. The last side-hole in a thoracostomy tube can be identifi ed by an interruption in the radiopaque line. This interruption in the radiopaque line should lie within the thoracic cavity, if not and or with evidence of subcutaneous air, a misplaced tube should be suspected. Incorrectly placed tubes for empyemas may delay drainage and result in loculation of the purulent fl uid. [10] The feeding tube Nasogastric tubes (NGs) are used to feed patients or for gastric aspiration in appropriate clinical circumstances. Radiographs are seldom required for accurate placement of NG tubes and are not used except when the patient is unconscious and there is risk of placement of the tube into the bronchial tree. There are other exceptions where a chest radiograph is useful and may prevent serious consequences, which include instances where small-bore feeding tubes are inserted and in statuspost esophagectomy [ Figures 10-12]. The lower tip of the NG is generally placed in the upper small bowel, which may be confi rmed with an abdominal radiograph. [11][12][13][14] Monitoring  Figures 13-17]. Other complications of CVP line placement are a pneumothorax; and intimal injury to veins, causing perforation or thrombosis. These complications can be avoided by using ultrasound guidance for CVP catheter placement, and jugular vein placement is preferred because of lower complication rates. [15][16][17] Monitoring pulmonary capillary wedge pressure Pulmonary capillary wedge pressure monitors are introduced    via the venous system to help accurate assessment of the patient's volume status and can help differentiate between cardiac and noncardiac pulmonary edema. Swan-Ganz catheters are generally used as pulmonary capillary wedge pressure monitors. These catheters are introduced percutaneously via the right heart and into the pulmonary artery. This allows pulmonary wedge pressure to be calculated by infl ating a balloon located at the tip of the catheter. The tip is advanced into a distal pulmonary artery and wedged there. The balloon is defl ated once the pressure is taken, and the tip is pulled back to the main pulmonary artery. The tip of the catheter located within the mediastinal shadow indicates correct placement. The catheter tip should ideally be placed proximal to an interlobar pulmonary artery [ Figures 18-20]. Malpositioning of Swan-Ganz catheters may occur in a quarter of the patients, resulting in false pulmonary capillary wedge pressure readings, risk for pulmonary infarction, pulmonary artery perforation, cardiac arrhythmias and endocarditis. [18][19][20][21][22][23] The intra-aortic counterpulsation balloon pump The intra-aortic counterpulsation balloon pump (IACB) is used to decrease afterload and increase cardiac perfusion in patients with cardiogenic shock. The device is synchronized with either the aortic pressures or the patient's EKG, to infl ate during diastole and defl ate during systole [ Figure 21]. The device is introduced via the right femoral artery to be positioned above the celiac axis in the region of the aortic isthmus or left main bronchus. During systole, the lucent air-fi lled balloon appears as fusiform. [24][25][26] Cardiac pacing devices ICU patients with cardiac arrhythmias or a heart block may require temporary cardiac pacemakers. The pacing wires of these devices are introduced via the cephalic or subclavian vein into the apex of the right ventricle. AP and lateral chest radiographs are usually required to evaluate accurate pacemaker placement. The tip of the cardiac pacemaker should be at the apex of the heart, and there should be no sharp angulations along the length of the pacemaker wires. The lateral radiograph should show the tip imbedded within the cardiac trabeculae. For correct placement to have occurred, the tip should appear 3 to 4 mm beneath the epicardial fat pad [ . A tip that appears to be placed beyond the epicardial fat stripe may have perforated the myocardium.  Cardiac pacers placed within the coronary sinus appear to be directed posteriorly on the lateral chest radiographs. [27][28][29][30] Pneumothorax and other intrathoracic air collections Extra-alveolar thoracic air collections in the ICU patients are not uncommon -often the result of intubation, other thoracic interventional procedures and barotrauma from positive endexpiratory pressure ventilation. This extra-alveolar air can collect as pulmonary interstitial emphysema, pneumothorax, pneumomediastinum, pneumopericardium or subcutaneous air.
Subcutaneous emphysema is not uncommon and often follows percutaneous intrathoracic placement of drains and other devices. Air dissects through the fascial planes, which usually has no clinical consequences [ Figure 25]. Subcutaneous emphysema may be associated with a pneumomediastinum; however, in the presence of isolated cervical subcutaneous emphysema, the patient needs to be examined for upper airway injury, particularly relevant following a diffi cult intubation or the placement of a new nasogastric tube. The chest x-ray fi ndings of subcutaneous emphysema may be striking; and the air dissecting the fascial planes, particularly between muscle bundles in the pectoralis region, is well seen obscuring underlying lung parenchymal changes. In the presence of subcutaneous emphysema, the detection of a pneumothorax also can become diffi cult.

Pneumothorax
A pneumothorax represents accumulation of air in the pleural space and it may occur spontaneously, or secondary to trauma, it may or may not be associated with lung parenchymal disease. Air rises to the nondependent position, and the radiographic appearance depends upon how the radiograph has been exposed. In the erect patient, air rises to apicolateral surface of the lung and appears as a thin, white pleural line with no lung markings beyond [ ; however, the presence of lung markings beyond the pleural line does not exclude a pneumothorax. The diagnosis of a pneumothorax may be particularly diffi cult in the presence of parenchymal disease, which may alter the compliance or affect the compliance of the lung, making collapse more diffi cult. A skin fold may mimic a pneumothorax. A skin fold line when followed continues outside of the chest [ Figure 29].
In an ICU patient, diagnosis of pneumothorax is often made on a supine radiograph. In a supine patient air rises anteromedially. An apical air collection in a supine patient is a sign of a large pneumothorax. Air can sometimes be trapped in a subpulmonic location between the lung and the diaphragm. Anterolateral extension of air into the costophrenic sulcus may increase the radiolucency at the costophrenic sulcus. This is called the deep sulcus sign. Other features of a subpulmonic pneumothorax include visualization of the superior surface of the diaphragm and the superior part of the inferior vena cava.
A tension pneumothorax is accumulation of air within the pleural space due to free ingress of air with limited egress of air. The intrapleural pressure exceeds atmospheric pressure in the lung during expiration (ball-valve mechanism). The diagnosis of tension pneumothorax is a clinical one based on respiratory and cardiac compromise. Diagnosis of a tension pneumothorax in a critically ill patient can be extremely challenging, partly due to the fact that lung pathology such as ARDS may reduce lung compliance, preventing total lung collapse as occurs in a tension pneumothorax. Similarly, a mediastinal shift, a hallmark of tension pneumothorax, may not occur with the use of PEEP. Signs of a tension pneumothorax include depression of a hemidiaphragm, a shift of the heart border, the superior vena cava and the inferior vena cava. [31][32] Pneumomediastinum A pneumomediastinum represents air in the mediastinum and may be related to pulmonary interstitial air dissecting centripetally in the intubated patient [ Figures 30 and 31].
Mediastinal air may also represent a leak from a major airway injury or air dissecting through fascial planes from the retroperitoneum. Unlike a pneumopericardium, air from a pneumomediastinum often dissects up into the neck. Moreover, a pneumopericardium can extend inferior to the heart. A pneumomediastinum usually remains asymptomatic. However, occasionally a retrosternal crunch may be heard on auscultation. Radiographic features of a pneumomediastinum include air around the great vessels, the medial border of the superior vena cava, and the azygos vein seen as surrounding lucencies. Air may also be seen outlining the aortic knuckle, descending aorta and the pulmonary arteries. A posteromedial pneumomediastinum is usually the result of esophageal rupture, where air dissects into the paraspinal costophrenic angle and beneath the parietal pleura of the left diaphragm. The result is a V-shaped lucency called the V-sign of Naclerio. [33] Pneumopericardium A pneumopericardium refers to an accumulation of gas/air between the myocardium and pericardium [ Figures 32-34]. Pneumopericardium can be an occasional complication of pneumothorax but is more often found in the postoperative cardiac patient. The chest x-ray fi ndings are those of a lucent line around the heart, extending up to the main pulmonary arteries. Air may accumulate inferior to the cardiac shadow, which crosses the midline above the diaphragm, which is said to be diagnostic for pneumopericardium, the so-called continuous diaphragm sign. [34] Pleural effusions Pleural effusions are accumulations of fl uid within the pleural space [ Figures 35-41]. Pleural effusions occur frequently in the ICU patients, which may be secondary to heart failure, fl uid overload, hypoproteinemia, infection, pulmonary embolism, thoracic and upper abdominal surgery, neoplastic disease, subphrenic infl ammatory processes, trauma and ascites. The fl uid could be blood, chyme, pus, transudates or exudates. The radiographic appearance of a pleural effusion is dependent on the position of the patient. Pleural fl uid accumulates in the dependent areas of the chest. A pleural effusion is easier to identify in the erect patient as fl uid collects at the base of the lung, causing costophrenic angle blunting and decreased visibility of the lower lobe vessels. In the supine position, identifi cation of a pleural effusion is more challenging. In the supine position, pleural fl uid accumulates in the posterior basilar space, which appears as homogenous density that increases in intensity towards the lung base. The normal bronchovascular markings are retained in this veil-like density. With increasing amount of : Chest x-ray fi ndings of a pneumopericardium shown as a lucent line around the heart extending up to the main pulmonary arteries (solid white arrows). Air may accumulate inferior to the cardiac shadow, which crosses the midline above the diaphragm, which is said to be diagnostic for pneumopericardium, the so-called continuous diaphragm sign (hollow arrow)  The chest radiograph and CT scan were taken on the same day, 7 hours apart. There is nothing suspicious on the chest radiograph to suggest a pericardial effusion. There is blunting of the left costophrenic angle, suggestive of a small pleural effusion. However, the axial CT scan shows a small pericardial effusion and moderate bilateral pleural effusions Figure 39: Encysted pleural effusion seen en face as an oval opacity; its margin is partially well defi ned and partially ill defi ned (AP radiograph). On the lateral radiograph, the effusion appears as a homogenous density with biconvex edges  pleural fl uid, the diaphragm loses its contour and costophrenic angle may be obliterated. However, it should be remembered that the pleural space may accommodate up to a liter of fl uid above the diaphragm without blunting of the costophrenic angle. With larger pleural effusions, the fl uid may appear as pleural cap at the lung apex, making it easier to identify on a supine radiograph. The fl uid may sometimes accumulate on the medial side of the lung, appearing as a widened mediastinum. Often, smaller pleural effusions are missed on supine chest radiographs despite meticulous technique. When effusions are not readily apparent on a supine chest radiograph but clinically suspected, a lateral decubitus fi lm is indicated. The fi lm should be taken with the side of the patient suspected to have an effusion in the dependent position. The lateral decubitus fi lm would not only confi rm smaller pleural effusions but can also differentiate between loculated and free effusions. The latter information is important when pleural drainage is planned, as loculated effusions may need more than one drain. A pleural effusion at the lung base is termed a subpulmonic effusion and is a common occurrence in the ICU patient. On the chest radiograph, a subpulmonic pleural effusion appears as a raised hemidiaphragm with fl attening and lateral displacement of the dome. A lateral decubitus fi lm can usually resolve this.
Loculated pleural effusions may pose a diagnostic challenge, especially when fl uid is retained within the fi ssures; and in particular, when the fissures are incomplete. A loculated effusion in the minor fi ssure and right middle lobe atelectasis may be diffi cult to differentiate on a supine chest radiograph. Interlobar effusion appears as a homogenous density with biconvex edges and preservation of the minor fi ssure, while atelectasis appears as an inhomogeneous density with concave margins and obliteration of both the right heart border and minor fi ssure. CT or an erect lateral radiograph, if possible, may resolve the issue. [35][36][37][38][39] Pericardial effusions Pericardial effusions are accumulations of fluid between the visceral and parietal pericardium [ . They usually cannot be seen on plain chest radiographs, and smaller effusions are diffi cult to differentiate from cardiomegaly.
A variety of pathologies may cause pericardial effusions, including lymphatic or venous obstruction by tumors, changes in osmotic pressures, or infl ammation of the pericardium causing increased permeability. Generally, pericardial effusions only become symptomatic when the intrapericardial pressure rises by 3 or 4 mm Hg. A hemopericardium may follow cardiac surgery or trauma. The rapidity at which the pericardial effusion accumulates dictates hemodynamic consequences. Radiographically, a pericardial effusion appears as cardiomegaly with a change in cardiac silhouette, resulting in a featureless, globular or "water bottle" shape. The best and quickest way to determine the presence of a pericardial effusion is by echocardiography. [40][41][42][43]