Pneumonia

Pneumonia is an acute inflammation of the lung parenchyme, with involvement of the bronchioli and alveoli, most often caused by microbes. It can have significant mortality. It’s a very common condition. In young people, it usually have a mild natural course, but for elderly, it has a high mortality.

Community-acquired pneumonia affects 0,1 – 1% of the adult population each year. It’s more common in young children and older adults, especially those with comorbidities. It’s more common in the winter months. The mortality is higher in the elderly than children.

Hospital-acquired pneumonia can affect anyone who is admitted to the hospital. It’s a severe condition which causes significant mortality.

Classification

According to the place where the patient acquired the infection:

• Community-acquired pneumonia (CAP)
  • Absence of prior hospital admission in the last 7 days
• Nosocomial pneumonia
  • Hospital-acquired pneumonia (HAP)
    • Acquired > 48 hours after hospital admission
  • Ventilator-associated pneumonia (VAP)
    • Acquired > 48 hours after endotracheal intubation
  • (Healthcare-associated pneumonia (HCAP))
    • Pneumonia in health care facilities other than hospital
    • Nowadays considered a type of CAP

This classification is important because the causative agents and therefore the management are different for each type.

According to the pathological distribution:

• Bronchopneumonia
  • Exudate in a patchy distribution
• Lobar pneumonia
  • Exudate involving an entire lobe
• Interstitial pneumonia (pneumonitis)
  • Inflammation localized to alveolar septae

Special types:

• Atypical pneumonia
  • Caused by atypical bacteria
• Aspiration pneumonia
  • Caused by aspiration of gastric fluids
• Chemical pneumonitis

Etiology

Microbial agents

• Community-acquired pneumonia
  • Streptococcus pneumoniae (most cases)
  • H. influenzae
  • Staphylococcus (if after influenza)
  • Gram negatives (if old patient or chronic disease)
• Nosocomial pneumonia
  • Gram negative enterobacteria (E. coli, Klebsiella)
  • MSSA and MRSA
  • Pseudomonas aeruginosa
  • ESBL-producing gram negatives
• Atypical pneumonia
  • Chlamydia pneumoniae
  • Mycoplasma pneumoniae
  • Legionella pneumophila
  • Pneumocystis jirovecii (in AIDS patients) (= pneumocystis pneumonia, PCP)
• Aspiration pneumonia
  • Anaerob bacteria

Risk factors

• Children < 5 years
• Adults > 40 years
• Comorbidities (COPD, bronchiectasis, heart disease, renal disease)
• Swallowing impairment (for aspiration pneumonia)
• Decreased mucociliary clearance (occurs in smokers and in immobilisation)
• Immunosuppression
• Endotracheal intubation (risk is proportional to duration of intubation)

Clinical features

The clinical features are caused by microbial inflammation, the host inflammatory response which impairs gas exchange, as well as the systemic effects of infection. Patients usually present soon after onset of symptoms.

• Fever
• Shivers
• Pleuritic chest pain
• Cough
• Sputum (purulent or bloody)
• Dyspnoea

In elderly, non-specific symptoms like confusion or delirium can occur.

In atypical pneumonia, the symptoms are usually non-specific like nausea, muscle aches, etc., rather than pulmonary. Atypical pneumonia can cause less severe pulmonary symptoms, as well as extrapulmonary symptoms like diarrhoea, hepatosplenomegaly, etc.

Pathophysiology

Pneumonia causes hypoxaemia (type I respiratory failure) due to V/Q mismatching. When part of the lung becomes consolidated, that part of the lung receives no ventilation, but it still receives perfusion, causing a ventilation/perfusion mismatch. This effectively forms a shunt, where part of the blood is not oxygenated in the lung.

Diagnosis

In CAP, imaging and microbiology are not necessary, and the condition is rather diagnosed based on clinical features. In nosocomial pneumonia, imaging and microbiology is important judge the severity and to target the treatment.

Physical exam

• Crackles and crepitation
• Dullness on percussion
• Bronchial breathing

Laboratory examination

In nosocomial pneumonia, sputum culture, blood culture, and ABG are important to guide treatment. CRP and leukocytes are usually measured to monitor severity. Procalcitonin can be used to differentiate bacterial from non-bacterial causes.

There are rapid antigen tests of urine for detection of S. pneumoniae and legionella, which can be used for more complicated cases. This is usually not necessary in a typical case of CAP. Atypical bacteria can’t be cultured but the specific pathogen can be determined by serology or PCR.

Imaging

The gold standard for diagnosis is the presence of a new opacity on chest x-ray in the setting of typical clinical features. The opacity is caused by the consolidation of the tissue, caused by presence of exudate in the alveoli. Chest x-ray is negative in the first 24 – 48 hours. The sensitivity of chest x-ray for pneumonia is low, and so a negative x-ray does not rule it out, especially in the early phases.The chest x-ray findings usually persist for weeks even after symptoms have cleared, so they’re usually not used for follow-up for pneumonia. CT may also visualise pneumonia but is unnecessary for uncomplicated cases.

The consolidation usually has indistinct margins, except if they are next to the pleura of interlobar fissures, where the margins will be sharp. It appears "fluffy". If the pneumonia is central, one can see hypodense bronchi in the consolidation, which are called air bronchograms.

Lobar pneumonia is the most common type, in which an entire lobe or almost an entire lobe is consolidated on imaging. Bronchopneumonia also exists, where there are multiple diffuse patchy opacities in one or both lungs.

In atypical pneumonia, chest x-ray may show patchy reticular opacities in the perihilar lung. CT can show also show the pathology, but CT is rarely necessary to make the diagnosis.

In elderly typical lung symptoms are not necessary for diagnosis as they may have atypical symptoms.

If a consolidation has been found on chest radiograph and a diagnosis of pneumonia has been made, a repeat radiograph should be made after resolution of the pneumonia to rule out lung cancer.

Differential diagnosis

• Acute bronchitis
• Exacerbation of COPD
• Pulmonary embolism

Severity assessment

Most CAP are managed outpatient, but many cases require inpatient treatment. To know who can be managed in an outpatient setting, who can be managed in the normal ward, and who must be admitted to ICU, severity assessment is important. Multiple scoring systems exist, like CURB-65, PSI, SMART-COP, CPIS.

For CURB65, a point is awarded for each of these factors:

• Confusion
• Elevated urea (> 7 mM)
• Respiratory rate > 30/min
• Hypotension (SBP < 90 mmHg)
• Old age (> 65 years)

0 or 1 points can likely be treated outpatient, 2 likely requires in-ward treatment, while 3 – 5 points likely requires ICU.

Treatment

Treatment is initially empirical and is based on guidelines as well as local microbial patterns and resistance rates. As such, the choice of antibiotic varies wildly from country to country. For example, in Norway, there is much less antibiotic resistance among commonly implicated bacteria, and so more narrow spectrum antibiotics are used than in other countries.

The most important components are correcting gas exchange and fluid balance abnormalities, as well as antimicrobial treatment.

For correcting gas exchange, interventions like oxygen supplementation or non-invasive or invasive mechanical ventilation may be used.

In a typical low-risk (young, no comorbidities) outpatient case of pneumonia, pneumococcus or haemophilus is presumed as the cause, so amoxicillin alone is used.

In older people or people with comorbidities, the risk for gram negative pathogens is higher, so amoxicillin + clavulanic acid is used for the increased gram negative coverage.

Atypical pneumonia is treated with doxycycline or macrolides. If we’re not certain whether the pneumonia is typical or atypical, amoxicillin ± clavulanic acid can be combined with either doxycycline or macrolide.

In case of penicillin allergy, a 3rd generation cephalosporin is used. In case of cephalosporin allergy, a respiratory fluoroquinolone is used.

Nosocomial pneumonia empiric treatment depends on the risk for pseudomonas. If there is low risk for pseudomonas, the most likely pathogens are mostly the same as for CAP, so the treatment is mostly the same as for CAP. If there is high risk for pseudomonas, the treatment is different. There are many options:

• Low risk for pseudomonas
  • 3rd generation cephalosporin
  • Respiratory fluoroquinolone (moxi, gemi, levofloxacin)
  • Ertapenem
• High risk for pseudomonas (any anti-pseudomonal antibiotic is a good option)
  • Imipenem or meropenem
  • Cefepime
  • Piperacillin + tazobactam
  • Levofloxacin
  • Ceftazidime
  • Add vancomycin or linezolide (if high local prevalence for MRSA)

TMP/SMX for PCP. Amoxicillin + clavulanic acid + clindamycin for aspiration pneumonia.

In uncomplicated cases, 5 days of antimicrobial treatment is enough. For more severe cases or atypical pneumonia, 7 – 10 days is required. Inpatient, the length of the treatment is based on the clinical features. When they are improving satisfactorily, treatment can be stopped. Also, normalized procalcitonin level can also be used as an indicator for stopping treatment.

Complications

• Pleural effusion
• Abscess
• Respiratory failure (especially in staph. aureus)
• Sepsis