Diagnostic Imaging in Sepsis of Pulmonary Origin
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Lung infections (community- or hospital-acquired) represent the most common cause for sepsis. Diagnostic imaging plays a crucial role in the initial evaluation of patients with criteria for sepsis and suspected pulmonary infection. In patients with clinical signs and symptoms of respiratory infection, the chest X-ray allows confirming the diagnosis of pneumonia. On the other hand, the chest X-ray can be useful to assess response to treatment (in some patients), define a pattern suggesting specific germs (particularly TB), identify complications (empyema and ARDS), and propose a differential diagnosis.
KeywordsPulmonary infection Pneumonia ARDS Pulmonary sepsis Lung abscess Pleural infection
Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Organ dysfunction is a clinical concept and is based on an increase of two or more points in the SOFA score .
Septic shock should be defined as a subset of sepsis in which particularly profound circulatory, cellular, and metabolic abnormalities are associated with a greater risk of mortality than with sepsis alone .
Lung infections (community- or hospital-acquired) represent the most common cause for sepsis, and in a variable percentage of patients, they are associated to acute lung injury (ALI) and acute respiratory distress syndrome (ARDS).
Diagnostic imaging plays a crucial role in the initial evaluation of patients with criteria for sepsis and suspected pulmonary infection.
The diagnosis of pneumonia is based on the presence of clinical manifestations of an infection (fever, chills, leukocytosis), signs, or symptoms located in the respiratory system (cough, increased sputum production, shortness of breath, chest pain, or abnormal lung exam) and the presence of new or changing opacities in the chest X-rays . In young patients without cardiopulmonary disease, the diagnosis of pneumonia is relatively simple with the features mentioned above. However, in elderly patients or in those with underlying diseases (congestive heart failure, COPD, neoplasm, pulmonary fibrosis), the clinical picture may be variable, and pneumonia diagnosis could be complex.
The definition of severe pneumonia can be subjective and imprecise and is considered in the practice due to respiratory and/or circulatory failure requiring admission into the ICU .
It is important to consider specific groups of patients with sepsis associated to pulmonary infection including immunocompromised (HIV-infected and non-HIV-infected), patients with pneumonia associated to health care, and patients with pneumonia associated to mechanical ventilation. In these contexts, the etiology of the infections varies, and radiographic manifestations can have characteristic features.
In patients with clinical signs and symptoms of respiratory infection, the chest X-ray allows confirming the diagnosis of pneumonia. On the other hand, the chest X-ray can be useful to assess response to treatment (in some patients), define a pattern suggesting specific germs (particularly TB), identify complications (empyema and ARDS) and propose a differential diagnosis.
Concepts relating to the usefulness of HRCT in patients with pneumonia have evolved. Traditionally, HRCT is used in patients with torpid clinical evolution, comorbidities, or suspected complications such as empyema, ARDS, and lung abscess. Recent studies suggest that HRCT can play a major role in patients with suspected pneumonia. Nie et al. in their work with 178 patients with community-acquired pneumonia (CAP) concluded that the effect of the treatment based on HRCT findings is not inferior to the effect of the treatment guided by microbiological characterization . A series by Karhu et al. (65 patients with severe community-acquired pneumonia) discovered new HRCT findings compared to those seen on the X-rays in 60% of the patients, which generated an additional treatment conduct in 75% of the cases . The retrospective nature of the trials and the scarce number of patients, currently, do not allow recommending substantial changes on the use of HRCT in patients with pneumonia. However, there is no doubt that new multidetector helical tomography techniques, with drastic reduction of ionizing radiation for the patient, can offer safe and relevant information in patients with pneumonia. Therefore, further studies are required to redefine the usefulness of HRCT for the diagnosis and follow-up of patients with suspected pneumonia.
Focal or lobar pneumonia . Resulting from a rapid production of fluid edema with scarce cellular reaction (Fig. 5.1).
Bronchopneumonia pattern . Related to inflammation located in the airways and patchy alteration of adjacent lung parenchyma. With disease progression, parenchymal alterations may coalesce and form a lobar pattern (Fig. 5.2).
It is important to bear in mind particular alterations or patterns in imaging studies that allow suggesting specific germs on certain clinical contexts.
Halo sign. Related to the presence of a ground-glass halo around a nodule, mass, or consolidation. In the context of a neutropenic immunocompromised patient, it can be associated to invasive mycosis (aspergillosis, mucormycosis).
The identification of microorganisms in patients with CAP varies according to the series. In a recent US series (patients with CAP who required hospitalization), a pathogen was identified in 38% of the cases, with a higher frequency of viral infection (23%) over bacterial infection (11%) .
Viruses represent the most frequent cause of respiratory infections in children and adults. In immunocompetent patients, most respiratory viral infections are self-limited. However, in a group of immunocompetent patients and in immunocompromised patients, respiratory viral infections present with severe tracheobronchitis, bronchiolitis, or pneumonia. The direct cytopathic effect of the virus causing damage to the respiratory epithelium can be associated to necrotizing bronchitis/bronchiolitis, diffuse alveolar damage, and/or alveolar hemorrhage. The manifestations of viral pneumonia on imaging studies depend on the degree of airway and/or lung parenchymal alteration and on the association of permeability pulmonary edema or alveolar hemorrhage. X-ray findings include normal X-ray, bronchial wall thickening, ill-defined nodules, and “patches” of peribronchovascular consolidation. HRCT may show ground glass, consolidation, thickening of peribronchovascular interstitium, thickening of interlobular septum, nodules tree-in-bud pattern, and areas of focal air entrapment with mosaic attenuation pattern .
Respiratory infections due to adenovirus can present with severe pneumonia in immunocompetent patients. Sixty percent of the patients with adenovirus pneumonia require mechanical ventilation. Radiographic manifestations are similar to those of bacterial pneumonia with focal consolidation that progresses to bilateral consolidation, associated to ground-glass opacities and pleural effusion .
In immunocompromised patients, cases of pneumonia due to herpes simplex virus (HSV), varicella-zoster virus (VZV), and cytomegalovirus (CMV) have been described, as well as respiratory syncytial virus (RSV) or adenovirus (ADV).
In a series by Mayer et al. including 51 immunocompromised patients with clinical pneumonia and positive syncytial respiratory virus (RSV) tests, the most frequent findings in the early stages of the disease were nodules and tree-in-bud pattern. With disease progression, ground glass (64%), consolidation (56%), and small nodules (55%) were described as relevant findings. The association of sinusitis, risk factors, and the radiographic manifestations described may suggest the diagnosis RSV pneumonia .
It is the most frequent cause of bacterial CAP in most of the series. The characteristic radiographic pattern is of focal or lobar compromise, with consolidation predominantly affecting the inferior lobes. The clinical course is variable, with adequate response to antibiotics (in most patients) or with complications that include ARDS and death.
M. pneumoniae is an important cause of pneumonia in adolescents and young adults. M. pneumoniae causes an interstitial inflammatory response by mononuclear cells appearing on the X-rays as perihilar and basal reticular opacities. The germ is an extracellular pathogen, whose survival depends on the adherence to the bronchial epithelium. For this reason, the bronchial wall thickening and the presence of centrilobular nodules are frequent findings in patients with M. pneumoniae pneumonia. Other HRCT findings include ground-glass opacities and consolidation . The study by Miyashita et al. comparing rapid diagnostic tests for M. pneumoniae concluded that the ImmunoCard Mycoplasma kit was not useful to diagnose this entity and that in patients with three or four criteria of the Japanese respiratory society score, the findings on HRCT may suggest the diagnosis of M. pneumoniae .
M. tuberculosis infection is an important cause of morbimortality associated to respiratory disease. It generally has a subacute or chronic course, but tuberculosis may manifest with acute respiratory clinical features, and differentiating it from respiratory infections of another etiology (bacterial) may prove difficult in the clinical practice.
The frequency of respiratory failure in patients with active TB ranges between 1.5 and 5%, with variable presentations (miliary TB-disseminated TB-ARDS). In a series by Mahmoud (350 patients with CAP admitted to the ICU), 3.1% of the patients developed ARDS associated to TB. In more than half of the patients, the suspicion of TB was based on radiographic manifestations .
The work by Yeh et al. to predict active pulmonary TB by means of the development of an HRCT score showed good performance with sensitivity (100%), specificity (96.9%), positive predictive value (76.5%), and negative predictive value (100%) .
Invasive Aspergillus infection is an important cause of pulmonary sepsis in immunocompromised patients. It has been traditionally described in neutropenic patients. However, invasive aspergillosis has also been found in non-neutropenic patients (with COPD, in solid organ transplant recipients and in ICU patients). Radiographic manifestations of invasive aspergillosis are variable. In the angioinvasive form, radiographic manifestations include large nodules (with halo sign), consolidation, and cavitation. In aspergillosis with airway invasion (usually associated to parenchymal disease), the findings described include tree-in-bud, centrilobular nodules and consolidation [(17, 18).].
The most frequent complications associated to pneumonia, and that may be evaluated with imaging studies, correspond to secondary pleural infection, lung parenchymal necrosis (abscess), and ARDS.
The microbiology of pleural disease is different to that described in patients with CAP. In a multicenter study of intrapleural sepsis, the most common germs associated to pleural infection were Streptococcus intermedius (24%), Streptococcus pneumoniae (21%), and other Streptococci (7%), followed by anaerobic bacteria (20%) and staphylococci
- 21.Kuhahda I, Zaragoulidis Tsirgogianni K, et al. Lung abscess-etiology, diagnostic and treatment options. Ann Transl Med. 2015;3(13):183.Google Scholar