What Changes To The Ventilator Setting Would You Expect In The Presence Of Ards And Why?

Review Article Review

Imaging of Acute Respiratory Distress Syndrome

Respiratory Care April 2012, 57 (four) 607-612; DOI: https://doi.org/x.4187/respcare.01731

Abstruse

Chest radiography and computed tomography (CT) have a crucial role to play in the diagnosis and management of acute respiratory distress syndrome (ARDS). The identification of pulmonary opacification is a requirement for the definition of ARDS on the chest radiograph, while CT has a role to play, not only in the diagnosis of ARDS, only likewise in the identification of complications. This paper reviews the radiological appearances of ARDS that have been documented for some time, and besides more recent inquiry that has identified a role for CT in directing ventilation and in prognostication.

acute respiratory distress syndrome
ARDS
computed tomography
CT
imaging
intensive care
prognosis
etiology

Introduction

Acute respiratory distress syndrome (ARDS) is a astringent course of acute lung injury (ALI), which has up to a 50% risk of mortality.¹ Pathologically, ARDS is characterized past diffuse alveolar damage, which is created by abnormal alveolar-capillary permeability, resulting in an influx of protein and fluid into the alveoli and interstitial spaces. This process is caused by alveolar injury secondary to ane (or more) of many precipitating insults, including sepsis and toxic inhalational injuries. Ensuing fibrin deposition and collagen formation can lead to pulmonary fibrosis.

ARDS and ALI exist on the same spectrum of clinical features, pathophysiology, and radiographic appearances. The term ARDS is reserved for the nearly severe finish of this spectrum, when the P_{aO_two}/F_IO₂ is < 200 mm Hg, every bit opposed to < 300 mm Hg in ALI. In this sense, all ARDS patients take ALI, but all ALI patients do non have ARDS.² The definition of both ARDS and ALI, notwithstanding, requires the presence of bilateral infiltrates on the breast radiograph.²

Imaging thus plays an important office in the diagnosis of ARDS and may also provide clues as to the etiology and chronicity of the condition. At the outset of this paper, it should be stressed that radiological features by themselves are nonspecific, and correlation with clinical findings is mandatory. For example, the etiology of ARDS may be classified as directly (pulmonary ARDS) or indirect (nonpulmonary ARDS), and these categories in plow may have distinct radiological appearances. All the same, in clinical practise complex patients with ARDS frequently have a combination of pulmonary and nonpulmonary pathologies, and the exact cause tin exist difficult to pinpoint by radiological means alone.³ Also as establishing the diagnosis, imaging has a part to play in the monitoring of ARDS and in identifying clinical complications. It is also increasingly being shown to provide prognostic information. This paper volition review the role of imaging (principally computed tomography [CT]) in diagnosing and managing patients with ARDS.

Plainly Chest Radiography

The plain breast radiograph is a valuable exam that tin can support the diagnosis, confirm the position of tubes and lines, monitor progression of pulmonary disease, and detect complications in ARDS. The plain radiographic appearances in ARDS vary depending on the stage of the disease, but these features are usually stereotypical irrespective of the cause. Although in that location is considerable overlap, the radiographic stages of the disease broadly correlate with the histopathological stages outlined below.

Exudative or Astute Phase (1–7 Days)

There is ordinarily a radiographic latent period in the first 24 hours following the initial insult that causes ARDS, and the breast radiograph is oftentimes normal.^iv The exception is if ARDS is triggered by a direct lung injury (for example pneumonia), in which case consolidation may be observed. There is rapid deterioration in the subsequent 24–72 hours. Injury to the endothelium results in increased capillary permeability and the influx of protein-rich fluid into the alveolar infinite and interstitium, which promotes pulmonary edema. Air infinite and interstitial opacities on the chest radiograph in ARDS are predominantly bilateral and symmetrical.⁵ Cardiogenic pulmonary edema is the main radiographic differential diagnosis, but the lack of temporal change; absenteeism of cardiomegaly, septal lines, or pleural fluid; and the presence of peripheral alveolar opacities all favor ARDS. In practise, ARDS and cardiogenic pulmonary edema tin can coexist, and attempting to discriminate between the 2 is oft difficult using chest radiography solitary.

Proliferative or Intermediate Phase (viii–14 Days)

After the rapid development of radiographic changes in the acute stage of ARDS, the appearances usually stabilize and remain static for a variable length of fourth dimension (Fig. 1). During the intermediate phase, diffuse coarse reticular opacities may develop on the chest radiograph, although this does non imply irreversible fibrosis, every bit the opacities may resolve. It is important to be aware of this stable period in the radiographic appearances of the disease, equally new air space opacities are likely to represent superadded infection or other complications exterior the natural history of ARDS.

Fig. 1.

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Fig. 1.

A: Chest radiograph of patient with ARDS shows bilateral infiltrates. There is bilateral consolidation and a right pleural effusion. B: Chest radiograph of the aforementioned patient shows persistent bilateral infiltrates after 7 days.

Fibrotic or Late Phase (> 15 Days)

If the patient survives the acute phase of ARDS, virtually radiographic abnormalities begin to resolve in the belatedly phase. The speed and elapsing of recovery are variable and depend on various factors, including comorbidities. Indeed, the breast radiograph in the last phase may range from completely normal to widespread coarse reticular opacities.^four

Computed Tomography in ARDS

Since the CT features of ARDS were start described,⁶ the use of CT in ARDS has go widespread both for clinical and research purposes. CT has been shown to be helpful, not only as a confirmatory and problem-solving tool, only emerging studies take shown the potential for classifying and prognosticating ARDS. Of class, the benefits of CT imaging in the clinical setting have to be weighed against the practicalities and risks associated with transporting a patient from the intensive care unit to the radiology department.

The Early Phase

The classical CT appearance of astute stage ARDS is that of opacification that demonstrates an anterio-posterior density gradient inside the lung, with dumbo consolidation in the most dependent regions, merging into a background of widespread ground-glass attenuation and and then normal or hyperexpanded lung in the non-dependent regions (Fig. 2). Ground-glass opacification on CT is a non-specific sign that reflects an overall reduction in the air content of the affected lung. In the instance of acute ARDS, this is likely to stand for edema and protein within the interstitium and alveoli. Another important observed characteristic in acute ARDS is bronchial dilatation within areas of ground-drinking glass opacification (Fig. 3). While this may be an indicator of the evolution of early fibrosis (and so-called traction bronchiectasis), the sign cannot exist taken equally conclusive proof of fibrosis considering the reversal of bronchial dilatation in the afterward stages of ARDS is a well recognized phenomenon.^seven

Fig. 3.

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Fig. iii.

Computed tomogram in ARDS shows bilateral reticulation and ground-drinking glass opacification, containing areas of bronchial dilatation in the upper lobes. In the acute phase of ARDS, bronchial dilatation may indicate fibrosis or may be reversible. Note besides the pneumomediastinum identified by computed tomogram.

The theory behind the inhomogeneity of appearances in ARDS is that the increased weight of overlying lung causes compressive atelectasis posteriorly, which produces dense opacification. The theory is supported by the fact that when the position of a patient with ARDS is changed from supine to prone, the density gradient chop-chop redistributes accordingly.⁸

In the nondependent portions, lung may be of normal attenuation, or information technology may be even lower if the patient is being mechanically ventilated. The identification of dense consolidation in nondependent areas of the lung should always alarm the clinician to the possibility of infective consolidation, either from a pre-existing pneumonia (which may have been the precipitating condition in ARDS) or new ventilation-associated pneumonia (Fig. 4).

Fig. 4.

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Fig. iv.

Computed tomogram of the mid zones of a patient with ARDS shows bilateral ground-glass opacification. Note the presence of non-dependent consolidation in the right lower lobe, which raises the possibility of superadded infection. The esophageal stent is incidental.

The Late Phase and Appearances in Long-Term Survivors of ARDS

After the acute stage, CT appearances are variable. Although consummate resolution of abnormalities may occur, the more typical later on phase CT appearances are that of a coarse reticular design and ground-glass opacification in the inductive (nondependent) part of the lungs.^9,x Cartoon conclusions about the meaning of appearances in ARDS survivors is always limited by the lack of accompanying histopathology in nigh patients. However, in this setting, information technology is probable that the basis-glass opacities correspond areas of fine fibrosis, which are too small to be resolved on CT. Information technology can too be speculated that fibrosis develops in these patients equally a sequelae of barotrauma secondary to mechanical ventilation in the nondependent lung, with consolidation and atelectasis conferring relative protection on the more than dependent regions.

A more recent written report has also reported a less classical distribution of disease in ARDS survivors. Masclans et al¹¹ found that 76% of patients had abnormalities on high-resolution CT at half-dozen months follow-up, which were typically areas of reticular and ground-drinking glass opacification. However, the majority of patients had a diffuse distribution of affliction, with only approximately a tertiary having the more classical exclusively anterior changes. A smaller number (18%) were also reported to take exclusively posterior abnormalities. The study as well reiterated the finding that airways disease (presumed to mean traction bronchiectasis) was common, again implying underlying lung fibrosis in these patients.

Pulmonary cysts of varying sizes and bullae are also features of the after stages of ARDS and probably develop as a event of prolonged ventilation¹² (Fig. 5). However, pulmonary cysts are also know to arise secondary to pneumonia and do not necessarily take to be associated with an episode of prolonged ventilation.¹⁰

Fig. 5.

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Fig. 5.

A: Computed tomogram shows bilateral dependent consolidation in a patient with ARDS, also as ground-glass opacities in the not-dependent lung. B: Follow-up computed tomogram after ane year shows resolution of the consolidation and ground-glass opacification with cyst formation in the anterior left lung.

Small but pregnant differences have been described at follow-up betwixt unlike groupings of ARDS patients. Higher levels of lung fibrosis are nowadays on CT in patients who spent the greatest length of time on mechanical ventilation, and who had the highest PEEPs.⁹ Furthermore, past dividing patients strictly into groups of pulmonary and nonpulmonary ARDS it has been shown that pulmonary ARDS survivors are more likely to accept pulmonary fibrosis on CT.^thirteen Information technology has been speculated that this is considering patients with pulmonary ARDS have less lung susceptible to alveolar recruitment and thus are more than prone to ventilator-induced lung injury.

CT in ARDS: Tin It Identify Etiology?

The etiology of ARDS can take implications for clinical management because the mechanical ventilation of pulmonary and extra-pulmonary ARDS patients may differ. Lung compliance is lower in patients with pulmonary ARDS, and increasing PEEP tends to reduce lung compliance farther still. Conversely, in actress-pulmonary ARDS, increasing the PEEP tin can accept a desirable effect, as it tends to increase lung compliance.¹⁴

With this in mind, it could be of clinical use if CT could determine whether the cause of ARDS is intra- or actress-pulmonary. Studies have shown that a typical CT design (symmetrical basis-drinking glass opacification, with an antero-posterior slope from dense consolidation to normally aerated lung) is more unremarkably associated with extra-pulmonary ARDS.^15,16 In pulmonary ARDS the distribution of lung changes are more likely to exist asymmetrical, with a mixed motion picture of ground-glass opacification, not-dependent dense consolidation, and parenchymal cysts.^15,sixteen Other features, such equally pleural effusions and air bronchograms, are said to be equally mutual to both types of ARDS.^xvi

However, whether these associations are strong enough to reliably use CT to determine the cause of ARDS is debatable. In clinical practise it is often difficult to attribute a single cause to ARDS in the beginning place. In summary, CT cannot be routinely recommended as a method of identifying the cause of ARDS, merely rather equally a tool to confirm the diagnosis and its complications.

CT in ARDS: A Tool to Identify Complications

CT imaging is not required to fulfill the diagnostic criteria for ARDS, but given the diffuse opacification that is feature of the condition, the chest radiograph can be limited in terms of identifying complications, and this is where cross-sectional imaging with CT can be of assist.

In an ARDS patient who is deteriorating, CT can detect various ventilation-associated complications and foci of infection that may not be apparent on the antero-posterior, supine, intensive care unit breast radiograph. Sequelae of mechanical ventilation that may exist more easily detected on CT include pneumonia, abscesses, pneumothorax, pneumomediastinum, and pulmonary interstitial emphysema. The position of the endotracheal tube tin also be established, and lung collapse due to malpositioning tin exist identified.

CT in ARDS: Tin It Be Used to Direct Ventilation?

The mainstay of management in ARDS is mechanical ventilation. This life-saving therapy is as well an invasive one, with potential to crusade barotrauma and long-term damage to the lungs. Recruitment is a term that refers to the number of previously collapsed acini that can exist inflated for a given pressure. Increasing PEEP recruits more lung at the higher force per unit area end of the spectrum. However, increased ventilation pressure can crusade barotrauma to the lung that would have successfully been ventilated at lower pressures. This means that in that location is a trade-off betwixt recruiting more than acini for gas exchange and causing ventilator-related lung injury. On CT, collapsed lung can be readily identified, and in the case of ARDS this is likely to be dependent lung, collapsed nether the increased weight of the overlying lung. Gattinoni et al¹⁷ take suggested performing limited CT of the lungs, with just two or iii slices each on inspiration, expiration, and with different PEEPs, equally a means of guiding ventilator settings. By acquiring CT images at different ventilator settings, the assessment of the potential for recruitment can be made by identifying the extent of atelectatic lung and its response to ventilation. Additionally, even with these limited sections, CT may provide the ability to distinguish atelectatic lung from areas of consolidation that would not exist amenable to recruitment.^xviii

CT as a Means of Prognosticating in ARDS

Limited studies have examined whether CT findings in early on ARDS tin predict long-term outcome in individuals. Ichikado et al measured the extent of illness on CT studies performed inside ane week of the onset of ARDS, and constitute that the presence of fibrosis (indicated by basis-glass opacification, traction bronchiectasis, and honeycombing) was a strong independent predictor of mortality.^xix The presence of traction bronchiectasis was also found to be a predictor of poor outcome in another study, along with disease affecting over fourscore% of the lungs, and signs of elevated right eye force per unit area (pulmonary artery dilatation and an enlarged right ventricle).²⁰ This study also suggested that patients who have a pure consolidative pattern of affliction in the early stages have a ameliorate chance of survival. The implication of this observation is that, because consolidation is non a marker of fibrotic modify, in that location is a more favorable outcome for this grouping of patients, thus indirectly supporting the finding that early fibrosis in ARDS is associated with a higher risk of expiry.

Summary

The role of the chest radiograph and CT in the diagnosis and direction of ARDS is an important one. Information from the plain radiograph non only forms ane of the diagnostic criteria, but it can hands provide ongoing information about the clinical state of the ARDS patient. The office of CT has been vital to the increasing understanding of the illness procedure in ARDS, and information technology continues to be an enlightening research tool. In the clinical setting, CT is a recognized and effective means of confirming the diagnosis of ARDS, although it is perhaps about readily used for identifying complications. Although less ordinarily used for other purposes, information technology also has the potential to determine ventilation strategies and prognosis.

Footnotes

Correspondence: Anand Devaraj Doc MRCP FRCR, Department of Radiology, St George's Infirmary, Blackshaw Road, London SW17 0QT, Britain.
The authors have disclosed no conflicts of interest.

References

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Source: https://rc.rcjournal.com/content/57/4/607

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