Pneumonia - Community-Acquired Disease Background

Classification

Respiratory symptoms of pneumonia include an acute cough, which can either be nonproductive or productive of purulent or rust-colored sputum, and at least one abnormal chest finding (eg diminished breath sounds, rhonchi, crackles, or wheeze). Systemic symptoms may also occur which include pleuritic chest pain, chills or rigors, and confusion. Abnormal vital signs that prompt suspicion include a respiratory rate of >20 breaths/minute, heart rate of >100 beats/minute, and fever >37.8°C. Chest X-ray findings may show lobar consolidation, bilateral infiltrates, or cavitation. It is potentially life-threatening, especially in older adults and those with comorbid disease.

Pneumonia - Community-Acquired_Initial Assessment 1

Pneumonia - Community-Acquired_Initial Assessment 2

• Respiratory rate of <30 breaths/minute
• Heart rate of <125 beats/minute
• Temperature of >36°C or <40°C
• Systolic blood pressure of >90 mmHg
• Diastolic blood pressure >60 mmHg 

Moderate-risk Community-acquired Pneumonia (CAP)

Patients classified under moderate-risk community-acquired pneumonia may have an acute onset of altered mental state, suspected aspiration, extrapulmonary evidence of sepsis (eg endocarditis, arthritis, encephalitis, otitis media), or an unstable comorbid condition (eg uncontrolled diabetes mellitus, active malignancies, congestive heart failure class II-IV, chronic obstructive pulmonary disease in acute exacerbation, decompensated liver disease). Chest X-ray findings may show bilateral or multilobar involvement, progression of the lesion to 50% of initial finding within 24 hours, pleural effusion, or abscess.  

They also typically present with the following vital signs:

• Respiratory rate of ≥30 breaths/minute
• Heart rate of ≥125 beats/minute
• Temperature of ≤36°C or ≥40°C
• Systolic blood pressure of <90 mmHg
• Diastolic blood pressure of ≤60 mmHg 

High-risk Community-acquired Pneumonia (CAP)

Clinical Features  

Patients classified under high-risk community-acquired pneumonia exhibit either one major criterion or ≥3 minor criteria.  

The minor criteria are as follows: 

• Respiratory rate of ≥30 breaths/minute
• Arterial oxygen partial pressure/Fractional inspired oxygen (PaO₂/FiO₂) ratio of ≤250 mmHg
• Multilobar infiltrates
• Confusion or disorientation
• Blood urea nitrogen (BUN) of ≥20 mg/dL
• White blood cell count (WBC) <4000 cells/μL
• Platelet count <100,000/μL
• Core temperature <36°C
• Hypotension needing aggressive fluid resuscitation 

The major criteria are as follows:

• Septic shock needing vasopressors
• Respiratory failure needing mechanical ventilation 

Introduction

Pneumonia is an acute infection of the pulmonary parenchyma accompanied by symptoms of acute illness and abnormal chest findings. It commonly presents with at least one abnormal chest finding of diminished breath sounds, rhonchi, crackles, or wheezing, and X-ray may show lobar consolidation, bilateral infiltrates, or cavitation. It occurs in the very young and the very old. It is a potentially life-threatening disease, especially in older adults and those with comorbid illnesses.

Etiology

The development of community-acquired pneumonia may be due to microaspiration, presence of a defect in the host defenses, possible exposure to a virulent microorganism, or due to presence of an overwhelming inoculum. Microaspiration is a mechanism by which the constituents of both the microbiota and pathogens reach the lungs. Other mechanisms by which a pathogen gains access to the lungs are via hematogenous spread, contiguous spread, and macroaspiration.  

The following are the virulence factors of some causative agents of community-acquired pneumonia: 

• Chlamydia pneumoniae possesses ciliostatic factor
• Mycoplasma pneumoniae shears off the cilia
• Influenza virus causes a marked reduction in the tracheal mucus velocity for up to 12 weeks post-infection
• Streptococcus pneumoniae and Neisseria meningitides produce proteases and split secretory IgA; other virulence factors include inhibition of phagocytosis, pneumolysin, and thiol-activated cytolysin
• Mycobacterium sp, Nocardia sp, and Legionella sp are all resistant to microbicidal activity (phagocytes)

Pathophysiology

• Administration of immunosuppressive agents (eg recipients of solid organ or stem cell transplants or those receiving chemotherapy, and long-term steroids)
• Comorbid conditions:
  • Chronic respiratory disease (eg bronchial asthma, chronic bronchitis, cystic fibrosis, bronchiectasis, chronic obstructive pulmonary disease, pulmonary edema)
  • Genetic disorder (eg Kartagener's syndrome)
  • Influenza
  • Chronic renal disorders
  • Hepatic conditions
  • Diabetes mellitus
  • Malignancy (eg myeloma, lung cancer)
  • Immunocompromised states such as human immunodeficiency virus (HIV) infection, hypogammaglobulinemia (IgG2 immunodeficiency), hyperimmunoglobulin E (Job) syndrome, surgical asplenia, or sickle cell disease 
• Continual contact with children (eg young children attending childcare, preschool teachers)
• Cigarette smoking and alcoholism
• Elderly (>65 years old)
• Immunosuppression and malnutrition
• Medications (eg inhaled corticosteroids, proton pump inhibitors and H₂ blockers, antipsychotic drugs, and sedatives) 
• Oxygen and inhalation therapy particularly containing steroids or using plastic spacers
• Other risk factors for young adults include training in the military and presence of low cholesterol or albumin levels
• People who are homeless and overcrowding inside jails and human shelters 

Risk Factors

Community-acquired pneumonia may be classified as low-, moderate-, or high-risk depending on the patient’s overall clinical status, vital signs, presence of co-morbidities, and chest X-ray findings.

Low-risk Community-acquired Pneumonia (CAP)  

Low-risk community-acquired pneumonia is associated with low morbidity and mortality rate of <5% and is therefore suitable for outpatient care. Patients considered at low-risk community-acquired pneumonia are those with stable vital signs and stable comorbid conditions (eg controlled diabetes mellitus [DM], coronary artery disease [CAD], renal insufficiency, COPD, chronic liver disease, chronic alcohol abuse, and asplenia).  

They may be treated as outpatients with a reasonable assurance of follow-up and sufficient social support. Hospitalization of patients with low-risk community-acquired pneumonia may be required if there are complications of pneumonia itself, exacerbation of underlying diseases, advanced age (≥65 years of age), inability to reliably take oral medications or receive outpatient care, inability to maintain oral intake, uncontrollable vomiting, cognitive dysfunction, and social problems (eg unavailability of a caregiver, homelessness).  

The possible pathogens include Streptococcus pneumoniae, Haemophilus influenzae, Chlamydophila pneumoniae, Mycoplasma pneumoniae, Moraxella catarrhalis, and enteric Gram-negative bacilli (among those with comorbid illnesses).

Moderate-risk Community-acquired Pneumonia (CAP)  

Moderate-risk community-acquired pneumonia is associated with a complicated outcome and higher mortality rate, thus in-hospital parenteral therapy is recommended. In patients with prior respiratory tract colonization, culture, swab, or polymerase chain reaction (PCR) tests must be taken prior to starting empiric therapy.  

The possible pathogens include Streptococcus pneumoniae, Haemophilus influenzae, Chlamydophila pneumoniae, Mycoplasma pneumoniae, Moraxella catarrhalis, enteric Gram-negative bacilli, Legionella pneumophila, and anaerobes (in patients with risk of aspiration).

High-risk Community-acquired Pneumonia (CAP)  

High-risk community-acquired pneumonia is associated with a mortality rate of 36%, thus it is managed in a hospital setting and those with hemodynamic instability or respiratory failure should be admitted to the intensive care unit (ICU).  

The possible pathogens include Streptococcus pneumoniae, Haemophilus influenzae, Chlamydophila pneumoniae, Mycoplasma pneumoniae, Moraxella catarrhalis, enteric Gram-negative bacilli, Legionella pneumophila, Staphylococcus aureus, Pseudomonas aeruginosa, Pneumocystis jirovecii, Klebsiella pneumoniae, Acinetobacter sp, and anaerobes (in patients with risk of aspiration).  

Patients who have a history of chronic (>7 days within the past month) use of broad-spectrum antibiotic therapy, bronchiectasis, malnutrition, and on steroid therapy are at risk of developing Pseudomonas aeruginosa infection.