Introductory Chapter: Pneumonia

1. Introduction

Pneumonia is an umbrella term regarding a variety of syndromes with different etiologies and refers to infection of the lung parenchyma. Pneumonia can be caused by a variety of microorganisms including bacterial, viral, and fungal pathogens [1]. Bacterial pneumonia can be divided into typical and atypical infections. Typical pneumonia is caused by microorganisms with the possibility of culturing on standard media or observation using gram staining techniques, such as Streptococcus pneumonia, Staphylococcus aureus, Haemophylus influenza, Moraxella catarrhalis, Group A streptococci, and gram-negative bacteria (both anaerobic and aerobic species). Atypical pneumonia is caused by pathogens that do not fit the aforementioned criteria; such as Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella [2]. A variety of viruses, of both RNA and DNA virus families, can lead to pneumonia characteristics in patients. Some examples of these viruses include respiratory syncytial virus (RSV), rhinovirus, influenza viruses, parainfluenza viruses, adenovirus, varicella-zoster virus (VZV), cytomegalovirus (CMV), especially in HIV-infected patients, measles, and coronavirus family [3]. Fungal infections, though mostly overlooked, are important sources of pneumonia in immunocompromised patients. Some of the important organisms in this group include Histoplasma, Blastomyces, and Coccidioides [4].

2. Types of pneumonia

Pneumonia is classified into three groups based on etiology, disease characteristics, and clinical setting of the pathogen transmission. These subtypes include community-acquired pneumonia (CAP), hospital-acquired pneumonia (HAP), and ventilator-associated pneumonia (VAP) [1].

CAP is a type of pneumonia that is acquired in a community setting, caused by both atypical and typical bacterial organisms, viruses, and fungi [1, 5]. HAP is considered a type of pneumonia acquired 48 h after hospital admission, with no incubation at time of admission [6]. On the other hand, VAP is a pneumonia acquired in patients under endotracheal incubation 48 h after the procedure [7]. The underlying pathogens responsible for HAP and VAP include gram-negative bacilli (Escherichia coli, Pseudomonas Aeruginosa, Acinetobacter, Enterobacter, Klebsiella, etc.) and gram-positive cocci such as S. aureus [8]. Also, aspiration of both small (micro-aspiration) and large (macro-aspiration) amounts of oropharyngeal and upper gastrointestinal secretions is responsible for aspiration pneumonia, which accounts for approximately 5–15% of CAP cases [9].

Pneumonia’s pattern of pulmonary involvement in these infectious diseases varies widely and can be categorized into lobar pneumonia, lobular pneumonia, and focal/diffused interstitial pneumonia [10].

Risk factors predisposing patients to each of these pneumonia subtypes are summarized in Table 1.

Pneumonia subtype	CAP	HAP	Aspiration pneumonia
Risk factors	Age < 5 Age > 65 + comorbidities Male gender Immunocompromised Life style (smoking, etc.) Prematurity^p Household air pollution^p Ambient particulate matter^p Suboptimal breast feeding^p Pulmonary disease^A DM^A CVD^A Chronic liver disease^A	Male gender Burns, trauma, surgery History of antibiotic therapy Malnutrition Disease severity Virulent pathogens in oropharynx Pulmonary aspiration (and predisposing conditions to pulmonary aspiration) ARDS	Impaired swallowing Decreased consciousness Impaired cough reflex
References	[11]	[11]	[11]

Table 1.

Rrisk factors of pneumonia. (DM = diabetes mellitus, CVD = cardiovascular disease, ARDS = acute respiratory distress syndrome, p superscript: pediatric cases, A superscript: adult cases).

3. Disease burden

The results of the Etiology of Pneumonia in the Community (EPIC) study in the united states indicated the annual incidence of CAP to be 2.4 cases per 1000 adults, mostly in age groups of >65 years old [12]. Similar studies in Europe estimated the annual incidence to be 1.07–1.2 cases per 1000 people [13]. The annual incidence of HAP is about 5–10 patients per 1000 hospital admissions worldwide and VAP cases include 10–25% of patients under ventilation [14].

Mortality of CAP in outpatient care, hospital wards, and ICU is <1%, 4–18%, and up to 50% respectively [15, 16, 17]. HAP and VAP are the most common causes of death in hospital-acquired infections with global mortality rates of 20–10% and 20–50% respectively [18, 19, 20, 21, 22].

4. Pathophysiology

Inability of the immune system in clearance of pathogens from the lower respiratory system is the basis for the incidence of pneumonia [23]. In addition to the pathogens, both local and systemic immune responses lead to parenchymal damage, constitutional symptoms, fluid congestion, pus formation in lungs, and reduction in alveolar compliance [24].

Pathological findings throughout this process consist of four stages including congestion, due to intra-alveolar edema, red hepatization, gray hepatization, both with characteristics of increased firmness of the parenchyma, and resolution [25].

5. Diagnosis

Most important symptoms stated by patients include fever, chills, diaphoresis, fatigue, myalgia, malaise, productive or non-productive coughs, dyspnea, and pleuritic chest pain [23]. Clinical data used in the definition of pneumonia include symptoms of acute lower respiratory tract disease for periods of less than 7 days, minimum of one systemic symptom, and newly appeared signs in chest examination unexplained by other conditions. Along with clinical criteria, laboratory tests, such as complete blood count (CBC), serum electrolytes, procalcitonin levels, renal and liver function tests, and radiologic findings, such as consolidations, abnormal silhouettes, opacities, and infiltrates, are also of importance in defining pneumonia [26].

6. Management

Management of CAP is carried out after a risk stratification process mostly using CURB-65 criteria. This scale consists of five criteria; Confusion, Uremia (BUN>20 mg/dl), respiratory rate > 30/minute, blood pressure < 90/60 mmHg, and ages>65 years. Treatments of HAP and VAP take more time and are more complicated. First-line treatment of pneumonia includes empirical antibiotic therapy using broad-spectrum antibiotics [1].

Figure 1 summarizes the approaches in the management of pneumonia.

Figure 1.

Approach to pneumonia [1].