Pneumonia is an acute inflammatory condition within the parenchyma of the lung caused by infection that reaches the lower respiratory tract. In most cases, pneumonia develops as a consequence of bacterial colonization/infection of the upper respiratory tract, followed by microaspiration of infected secretions at a time of impaired host pulmonary defense mechanisms [217]. The prime host defenses against foreign particulate matter that reaches the lower respiratory tract are the cough reflex, tracheobronchial (mucociliary) clearance, and alveolar macrophage phagocytosis. Activation of the humeral (antibody) immune response provides augmentation of phagocytosis and the acute cellular response. One or more of these defense mechanisms may be impaired by a variety of factors, including underlying cardiopulmonary and neurologic disease, sedative medication, bronchial obstruction, concurrent active viral and mycoplasma bronchitis, and toxic/metabolic conditions such as alcohol excess, acidosis, and hypoxia. Individuals with an impaired immune system, such as occurs from immunosuppressive drugs, human immunodeficiency virus (HIV), chronic disease, or old age, are more susceptible to infection [4].

Determining accurate incidence rates for CAP is challenging because "pneumonia" is not a reportable disease; moreover, case definition varies across studies and national databases often link pneumonia with influenza. Epidemiology of pneumonia relies primarily on data derived from community-based cohort studies and surveillance networks. Approximately 5 to 6 million cases of pneumonia are diagnosed annually, with about 1 million occurring in older adults [36]. Approximately 4.2 million adult outpatient visits are related to CAP every year, and the mortality rate is less than 1% for adults treated on an outpatient basis [37].

The primary risk factors for CAP are age, smoking history, and chronic lung disease (e.g., chronic obstructive pulmonary disease [COPD]) and other comorbidities. Occupational dust exposure and history of childhood pneumonia have also been associated with an increased risk, as has male gender, unemployment, and single marital status [39,41]. As noted earlier, the risk for pneumonia is higher for individuals 65 years or older compared with younger adults, with the risk further increasing for those 85 years and older [39]. Alcoholism and chronic diseases, such as respiratory disease, cardiovascular disease, or kidney disease, also increase the risk for pneumonia, especially in the older population [3,42,43]. In the pediatric population, very young children are at increased risk because their immune systems have not fully developed. Conditions of frailty, dementia, alcohol use, and sedative medication all lead to diminished or ineffectual cough and the propensity for aspiration, thereby increasing the risk for pneumonia. Diseases or medications that suppress the immune system increase the risk among all ages [39,42].

The most common cause of CAP is S. pneumoniae, identified in approximately one-third of all cases and 40% to 50% of all culture-confirmed bacterial pneumonia cases that require hospitalization [9,29,30,46]. The most common causative pathogen varies in relation to the patient's age, illness severity, and clinical context (Table 3) [29,30,47].

The clinical recognition of CAP in adults is challenging because its presentation is similar to other acute respiratory illnesses such as pulmonary embolism/infarction and congestive heart failure [3,51,52]. Diagnosis relies on clinical features combined with radiographic findings; however, both the clinical presentation and chest x-ray abnormalities are variable and in part nonspecific, particularly in the elderly [3,29]. Common presenting symptoms and signs are:

Viral culture remains the criterion standard for diagnosis of viral pneumonia, but because of limitations such as the need for prompt transportation, time needed for viral detection, and the lack of sensitivity for all viruses, rapid antigen testing is often done. In adults, rapid testing has a sensitivity of 50% to 60% and a specificity of at least 90% [31]. Testing of nasal swab specimens is slightly less sensitive than testing of wash specimens, but wash specimens can be difficult to obtain in frail or cognitively impaired adults. Rapid RSV tests are usually not useful for adults, as the level of virus titers shed is low [31]. Diagnostic testing (PCR) for SARS-CoV-2 by nasopharyngeal swab should be performed on patients presenting with CAP in areas experiencing COVID-19 epidemic activity.

It is estimated that admission to an ICU is needed for 10% to 20% of patients hospitalized with CAP [76]. The IDSA/ATS guideline recommends two major and nine minor criteria to define severe pneumonia requiring ICU admission. [47]. The major criteria are septic shock requiring vasopressors or acute respiratory failure requiring intubation and mechanical ventilation. The presence of at least three of the following minor criteria suggests the need for ICU admission [47]:

Pending results of cultures and serologic testing, an initial empiric treatment regimen is selected according to patient variables and clinical setting (Table 6) [47]. Patients with mild illness and no serious coexisting disease may be managed as outpatients. The 2019 ATS/IDSA guideline recommends amoxicillin 1 g three times daily, doxycycline 100 mg twice daily, or a macrolide (e.g., azithromycin 500 mg on first day then 250 mg daily or clarithromycin 500 mg twice daily) [235]. The macrolide monotherapy recommendation is conditional based on prevalence of local pneumococcal resistance (<25%) and provided the patient has not received antimicrobials within the previous three months [47]. S. pneumoniae resistance to macrolides is four times more likely in adult patients who have received this class of drug within the previous three months, in which case a fluoroquinolone or ß-lactam plus macrolide combination should be selected. Patients with comorbidities should receive broader spectrum treatment as they are more likely to harbor resistant pathogens and to be more vulnerable to poor outcomes if the initial regimen is inadequate. For outpatient adults with comorbidities, the ATS/IDSA guideline recommends one of the following options (in no order of preference) [235]:

During the physical examination of pediatric patients, the clinician should look for signs of hypoxia and dehydration, as well as retractions, tachypnea, and use of accessory muscles of respiration [60]. The clinician should also evaluate the upper respiratory tract for evidence of rhinorrhea, otitis media, and pharyngitis [60]. Auscultation of the chest should be carried out, and the Pediatric Infectious Diseases Society (PIDS)/IDSA guideline recommends pulse oximetry for children with suspected hypoxemia [30].

One of the most common reasons for pediatric emergency room visits is fever, and fever is present in 88% to 96% of identified pneumonia cases in developed countries [70]. However, children with fever and wheezing commonly have either upper respiratory disease or reactive airway disease. As with pneumonia in adults, the accuracy of any one sign or symptom in predicting the likelihood of pneumonia is limited [61]. Nonspecific symptoms such as vomiting and abdominal discomfort are common. Careful attention should be given to the chest exam, as diminished breath sounds and fine end-inspiratory crackles are subtle, important clues to the presence of pneumonia in the pediatric patient. In one study, non-specific crackles were present in more than 90% of children with pneumococcal or mycoplasma pneumonia [70]. Infants with pneumonia commonly present with poor feeding and irritability as well as tachypnea, retractions, grunting, and hypoxemia; cough is rare [64].

For fully immunized infants and school-aged children who are hospitalized, treatment with ampicillin or penicillin G is recommended when local epidemiologic data show a low level of penicillin resistance to S. pneumoniae[30]. For children who are not fully immunized or are hospitalized in an area with a high level of penicillin-resistant S. pneumoniae, treatment with a third-generation cephalosporin (ceftriaxone or cefotaxime) should be given intravenously. If M. pneumoniae or C. pneumoniae is strongly suspected, treatment should include a macrolide (orally or intravenously) with a ß-lactam and diagnostic testing should be done as soon as possible [30]. The PIDS/IDSA guideline also recommends antimicrobial treatment for specific pathogens; however, a discussion of all possible pathogens is beyond the scope of this course.

The primary preventive strategy for pneumonia is immunization with pneumococcal and influenza vaccines, especially for adults older than 65 years of age, young children, and other individuals in high-risk groups (Table 9) [91]. Additional preventive measures include improved hand hygiene compliance and adherence to healthy lifestyle behaviors, including cigarette smoking cessation.

Parental attitudes about vaccines are an important factor in vaccination rates among children. The primary attitude is concern about the safety and efficacy of the vaccine, including fear of adverse events, the discomfort associated with vaccination, distrust of advocates of vaccination, and belief that the vaccine should not be given when a child has a minor illness [117,120,121,122]. Difficulty remembering or confusion about the vaccination schedule for children is also a major challenge [120,122]. Changes in access to health care have been noted as a factor in the low rate of influenza vaccination among teenagers [117].

In a systematic review, the American College of Physicians found several patient-related and surgery-related factors that increased the risk of postoperative pulmonary complications. The most common patient-related factors were the presence of COPD and an age older than 60 years [145]. Other significant factors were an American Society of Anesthesiologists (ASA) class of 2 (defined as a patient with mild systemic disease) or higher, functional dependence, and congestive heart failure. Cigarette use was associated with a modest increase in risk, and obesity and mild or moderate asthma were not found to increase risk [145]. Use of a PPI or histamine2 receptor antagonist is also thought to be a risk factor [45]. Surgery-related factors included prolonged duration of surgery (i.e., more than three to four hours), emergency surgery, and surgical site, with abdominal surgery, thoracic surgery, neurosurgery, head and neck surgery, vascular surgery, and aortic aneurysm repair being associated with the greatest risks [145].

The risk for VAP appears to be greatest during the first week after intubation. In one study, the risk was estimated to be 3% per day during the five-day period following intubation, decreasing to 2% per day for days 5 through 10, and to 1% per day for longer durations [147]. In a population of children who had cardiothoracic surgery, pneumonia risk correlated with mechanical ventilation for longer than three days [144]. Nearly half of all cases of VAP develop within the first four days of mechanical ventilation [148].

Viral and fungal pathogens are rare causes of HAP, VAP, and nursing home-acquired pneumonia in immunocompetent adults. Outbreaks of viral pneumonia may occur during influenza season, and influenza, parainfluenza, adenovirus, and RSV are involved in about 70% of those cases [28]. Candida spp. and Aspergillus fumigatus may cause pneumonia in patients who have had organ transplantation or who have a compromised immune system and neutropenia.

The bacterial pathogens that cause pneumonia in residents of nursing homes (and other long-term care facilities) differ according to the severity of disease. S. pneumoniae and H. influenzae are the most common causes of mild-to-moderate pneumonia in long-term care facilities [155]. In cases requiring hospitalization, C. pneumoniae, S. aureus, and influenza virus are frequently observed as well. Patients with severe illness commonly are infected with methicillin-sensitive S. aureus or MRSA, gram-negative enteric pathogens, or P. aeruginosa [23,155].

In managing a case of HAP and VAP, the clinician should review in detail the guidance provided by the ATS/IDSA, and consider consultation with appropriate subspecialty colleagues [28]. Recommendations governing selected issues of initial management emphasize the following principles [28]:

Smoking triples the risk for pulmonary complications after surgery, and smoking cessation for at least eight weeks before surgery, when possible, is recommended for current smokers [26]. The risk for complications in patients with respiratory disease or congestive heart failure can be ameliorated by optimum treatment before surgery (e.g., treatment with steroids for patients with COPD or asthma) [26].

The Institute for Healthcare Improvement (IHI) found that implementation of its ventilator bundle, a collection of five prevention strategies drawn from these guidelines, led to a 45% reduction in the incidence of VAP [174]. The bundle includes the following interventions [174]:

As with HAP, strategies to decrease or eliminate modifiable risk factors for nursing home-acquired pneumonia should be implemented. A multidisciplinary panel made three recommendations for prevention of pneumonia among nursing home residents [199]: