Pneumonia – A Complete Concise Review For Medical Students

Pneumonia is the most important of the pulmonary infections. It can cause a lot of morbidity and mortality if not treated.

What is Pneumonia?

Pneumonia is defined as an infection of the pulmonary parenchyma. This includes the alveoli, bronchioles and interstitial tissue.

Pathophysiology of Pneumonia

The pathophysiology of pneumonia is initiated with the entry of micro-organisms into the lower respiratory tract and alveoli. The alveolar macrophages engulf and kill these bacteria in the alveoli. Pneumonia occurs when the alveolar macrophages cannot kill the micro-organisms.

In this case, the host inflammatory response mediated by several cytokines causes alveolar infiltration by neutrophils and increased alveolar-capillary permeability. The latter leads to the exudation of fluid. This results in the clinical syndrome of pneumonia.

Initially, the exudation has red cells and neutrophils, adding a pinkish colour to the yellow-coloured purulent sputum. Later on, there is mainly a purulent exudate made of neutrophils. Viral pneumonia leads to colourless mucoid sputum unless there is a secondary bacterial infection.

Classification of Pneumonia

Pneumonia is classified based on where the infection was believed to have been acquired.

• Community-acquired pneumonia
• Healthcare-associated pneumonia – hospital-acquired and ventilator-associated pneumonia
• Special categories are pneumonia in immunocompromised patients and aspiration pneumonia.

This classification is useful because the causative organisms vary according to the environment from which the organism may have caused the infection. Pneumonia can also be classified based on the pathological and radiological pattern into lobar and bronchopneumonia

Community-acquired pneumonia

Common etiologies for community-acquired pneumonia are bacterial, viral and some other organisms.

The Presence of some risk factors might be helpful to narrow down the aetiology.

• Alcoholism – Streptococcus pneumoniae, oral anaerobes, Klebsiella
• COPD – Haemophilus, Pseudomonas
• Structural lung disease – Pseudomonas, Staphylococcus aureus
• Dementia, stroke, altered consciousness (which have a high risk of aspiration) – oral anaerobes, gram-negative enteric bacteria.

Clinical features of Pneumonia

The symptoms often begin with features of an upper respiratory infection which soon proceeds to more severe coughing, more purulent symptoms, and pleuritic pain. With the onset of pneumonia, the patient is usually quite ill and has a high fever. Some would develop the clinical features without a preceding upper respiratory infection.

• Fever
• Systemic features
• Cough, tachypnoea, shortness of breath
• Pleuritic chest pain
• Evidence of consolidation may be present in patients with lobar pneumonia (diminished breath sounds with superadded crepitations, dull percussion and bronchial breathing).
  • Sometimes a pleural effusion may be associated and clinical features of an effusion may be present.
• Sepsis and septic shock are complications of pneumonia. Patients who come late to the hospital and those with abnormal immune functions may present with these complications.
• Some patients may have features typical of the causative organism.
  • For example, mycoplasma infection may cause erythema multiforme and autoimmune hemolytic anaemia.
  • Legionella infection may be associated with multisystem involvement and symptomatic hyponatremia.
  • The other feature in these ‘atypical’ organisms causing pneumonia are radiological alterations which are out of proportion to clinical manifestations.

Investigations to confirm the diagnosis of Pneumonia

• Inflammatory markers – FBC/CRP: These indicate the presence of an infection.
• Chest X-Ray PA is the key to confirming the diagnosis
• The radiological manifestations may differ based on the infecting agent.
• The most common abnormalities are evidence of consolidation and effusion.
• Pneumatocoeles, Pneumothorax are seen in Staph aureus and Klebsiella infections.
• Diffuse pulmonary infiltrates are seen in Legionella and Mycoplasma infections

Microbiological confirmation

• Sputum for gram stain and culture
• Pleural fluid aspiration and culture – in cases where there is a pleural effusion
• Blood culture – very low diagnostic yield
• Rarely done tests:
  • Antigen testing in urine for legionella (high sensitivity and specificity, but rarely done)
  • Viral antigen testing (but rarely done)

Treatment for Pneumonia

General Management

• No restrictions on diet.
• Rest appropriate to the level of illness.
• Hydration is important for the severely ill.

Symptomatic treatment:

Paracetamol for fever. Avoid cough suppressants (which are common with most cough syrups). If a syrup is requested a mucolytic such as bromhexine can be prescribed. Oxygen is useful in a breathless patient.

Specific treatment

Once diagnosed, the first step in treatment in a Sri Lankan setting is to decide on the likely organism. This will help us to start on an appropriate antibiotic early. Next, we need to decide on the severity of pneumonia and select the antibiotic-appropriate antibiotic risk stratification of the patient based on clinical and biochemical parameters.

A score commonly used overseas is the CURB-65. It is used as a guideline for admission overseas. This is rarely useful because the blood urea result would take at least 5 to 6 hours. A more practical approach is to consider as severe any patient having any of the other criteria!

Other features indicating severe CAP include associated comorbidities, metabolic acidosis, multilobar pneumonia, hyperglycaemia, hyponatremia and hypoxia. Patients who have CAP of low severity may be managed as outpatients whereas others require inpatient therapy.

Monitoring

This is extremely important in management and includes

• symptom severity
• temperature
• vital parameters
• respiratory rate, and
• physical signs.
• Check on fluid balance and nutrition.

Empirical antibiotic therapy

Once the severity of pneumonia is decided it is extremely important to commence empirical antibiotic therapy. The antibiotics to use should be decided based on local antibiotic guidelines. In Sri Lanka, a macrolide such as clarithromycin (500 mg b.d.) or azithromycin (500 mg daily) may be used as therapy in patients with mild pneumonia.

Such patients can be managed as an out-patient.In patients with severe community-acquired pneumonia who are hospitalized a combination of a beta-lactam antibiotic (oral co-amoxiclav, or IV ceftriaxone) and a macrolide is recommended (e.g. oral clarithromycin).

A respiratory fluoroquinolone such as levofloxacin can be used as monotherapy, but it is best to consult your senior colleagues before prescribing it. In rare cases where Pseudomonas is suspected (patients with COPD, bronchiectasis, and in those with greenish yellow sputum) an antipseudomonal beta-lactam (piperacillin, tazobactam, meropenem, imipenem, ceftazidime) should be combined with a fluoroquinolone (levofloxacin).

If Staph aureus is suspected IV vancomycin should be added to the empirical regimen. This requires the decision of a consultant because the drug is potentially nephrotoxic and very expensive.If microbiological investigations yield an organism the antibiotics should be changed based on the susceptibility pattern

Duration of therapy

Most patients with CAP respond within 72 hours of initiating therapy. Parenteral antibiotics can be switched to oral therapy after this duration if the patient is afebrile and clinically improving.

The total duration of treatment should be at least 10 days because in Sri Lanka we do not have adequate data on causative organisms, their antibiotic sensitivity and therapeutic response can be difficult to monitor.

A short duration of antibiotics runs the risk of relapse or complications such as effusion or bronchiectasis.

Failure to improve

Consider an alternate diagnosis (e.g. TB), or complications (e.g. empyema). Also, check whether the antibiotic is appropriate, and think of superadded nosocomial infection.

Another important cause to consider in patients with poorly resolving CAP is underlying structural lung disease (e.g. bronchiectasis) or bronchial carcinoma.

Complications

• Metastatic infection
• Lung abscess
• Empyema