Pneumonia: Severity Assessment, Organisms and Antibiotic Choice

Introduction

Pneumonia is infection of the lung parenchyma, in which the alveoli fill with inflammatory exudate to produce consolidation. It is the commonest infectious cause of death in the UK and accounts for a substantial proportion of acute medical admissions.² The diagnosis requires symptoms and signs of a lower respiratory tract infection together with new shadowing on a chest radiograph - without the radiograph, the correct label is lower respiratory tract infection.

The classification that matters clinically is where the infection was acquired, because that predicts the likely organism and therefore the antibiotic:

• Community-acquired pneumonia (CAP) - acquired outside hospital, or within the first 48 hours of admission
• Hospital-acquired pneumonia (HAP) - developing 48 hours or more after admission, and not incubating at the time of admission
• Ventilator-associated pneumonia (VAP) - developing more than 48 hours after intubation
• Aspiration pneumonia - following inhalation of oropharyngeal or gastric contents
• Pneumonia in the immunocompromised - where the differential widens to include opportunistic organisms such as Pneumocystis jirovecii, fungi and mycobacteria

Organisms

Community-acquired pneumonia

Hospital-acquired and aspiration pneumonia

HAP is dominated by Gram-negative organisms - Escherichia coli, Klebsiella, Enterobacter and Pseudomonas aeruginosa - together with Staphylococcus aureus including MRSA. This is why the antibiotic choice differs completely from CAP, and why simply continuing amoxicillin in a patient who deteriorates on day 5 of an admission is a familiar error.

Aspiration pneumonia involves oral flora and anaerobes. The pattern of consolidation follows gravity and the position of the patient at the time: the right lower lobe when upright, and the posterior segment of the right upper lobe or apical segment of the right lower lobe when supine. The right side predominates because the right main bronchus is wider, shorter and more vertical.

• Risk factors for aspiration: stroke and bulbar palsy, reduced conscious level from any cause, alcohol excess, seizures, general anaesthesia, oesophageal disease including achalasia and strictures, nasogastric feeding, and poor dentition

Clinical features

Symptoms

• Cough, usually productive of purulent or rust-coloured sputum
• Fever, rigors and sweats
• Pleuritic chest pain - sharp, worse on inspiration and coughing, from inflammation of the adjacent parietal pleura
• Breathlessness
• Malaise, myalgia and anorexia
• Confusion - which may be the only presenting feature in an older patient, in whom fever and cough are frequently absent
• Haemoptysis - usually minor streaking

Examination

The signs are those of consolidation, in which the alveoli are filled with fluid and inflammatory cells but the airway remains patent, so sound is transmitted better through the lung than normal:

• Reduced chest expansion on the affected side
• Dull percussion note over the consolidated area
• Bronchial breathing - harsh and equal in inspiration and expiration, with a gap between the two
• Increased vocal resonance and tactile vocal fremitus
• Coarse crackles and a pleural rub
• Tachypnoea, tachycardia, fever and hypoxia, with hypotension and confusion in severe disease

Severity assessment

Severity scoring determines where the patient is treated and which antibiotics they receive, and should be calculated on every patient rather than estimated by impression.¹

In primary care, where urea is not available, use CRB-65: a score of 0 suggests home treatment, 1 or 2 warrants hospital assessment, and 3 or 4 requires urgent admission.

Investigations

All patients

• Chest X-ray - required for the diagnosis. Look for consolidation, air bronchograms, the distribution (lobar or multilobar), cavitation, and an associated effusion.
• Observations including respiratory rate and oxygen saturations, and a NEWS2 score
• Full blood count - neutrophilia in bacterial infection; a normal or low white cell count in severe sepsis is a poor prognostic sign
• U&Es - for the urea in CURB-65, and to detect the hyponatraemia of Legionella
• CRP - useful as a baseline and for monitoring response, and used in primary care to guide whether antibiotics are needed at all
• Liver function tests - deranged in Legionella and Mycoplasma

Moderate and high severity

• Blood cultures - two sets, before antibiotics wherever this does not delay treatment
• Sputum culture and sensitivities
• Pneumococcal and Legionella urinary antigens - both remain positive after antibiotics have been started, which makes them particularly valuable
• Arterial blood gas - if saturations are below 94% or the patient is unwell, to identify respiratory failure and acidosis
• Respiratory viral PCR - influenza and SARS-CoV-2, which also informs infection control
• Atypical serology for Mycoplasma, Chlamydophila and Coxiella where suspected
• Pleural fluid sampling - if there is an effusion of any significant size, to exclude empyema (see below)
• HIV test - in every adult admitted with pneumonia

Management

Community-acquired pneumonia

Give the first dose of antibiotic within 4 hours of diagnosis, and within 1 hour if there is sepsis, in line with the sepsis six.^1,3

Hospital-acquired pneumonia

• Non-severe, low risk of resistance: co-amoxiclav, or doxycycline or cefalexin if penicillin allergic
• Severe, or high risk of resistance: piperacillin with tazobactam, ceftazidime, or meropenem, according to local policy
• Add MRSA cover (vancomycin, teicoplanin or linezolid) if the patient is colonised or MRSA is suspected
• Duration is typically 5 days, reviewed against the clinical response

Aspiration pneumonia

Antibiotic cover must include anaerobes - typically co-amoxiclav, or a combination such as cefuroxime with metronidazole. Equally important is addressing the cause: a speech and language therapy swallow assessment, review of feeding, sitting the patient upright, and attention to oral hygiene.

Supportive care

• Oxygen to maintain saturations of 94-98%, or 88-92% in those at risk of hypercapnic respiratory failure
• Intravenous fluids for dehydration and hypotension, guided by response
• Analgesia for pleuritic pain - adequate analgesia allows the patient to take deep breaths and cough, which prevents atelectasis
• VTE prophylaxis
• Chest physiotherapy if there is sputum retention
• Nutritional support, and early mobilisation
• Consider critical care for persisting hypoxia, rising CO2, exhaustion, shock unresponsive to fluids, or a CURB-65 of 4 to 5

Review and follow-up

• Review at 48-72 hours. Fever and CRP should be falling. If not, reconsider the diagnosis - empyema, abscess, an obstructing tumour, tuberculosis, pulmonary embolism, resistant or atypical organisms, or a non-infective cause.
• Switch from intravenous to oral as soon as the patient is improving and afebrile - usually within 48 to 72 hours
• CRP measured on admission and at 48 hours predicts response better than a single value
• Repeat the chest radiograph at 6 weeks in anyone with persisting symptoms or signs, and in all those at higher risk of malignancy - particularly smokers and patients over 50. Radiological clearance lags well behind clinical recovery.
• Offer smoking cessation support, and check that pneumococcal and influenza vaccination are up to date

Complications

• Parapneumonic effusion - occurring in around 40% of hospitalised patients; simple effusions resolve with antibiotics
• Empyema - infected pleural fluid requiring chest drainage, suggested by a swinging fever and a failure to improve
• Lung abscess - a cavity with an air-fluid level, particularly after aspiration or Staphylococcus aureus and Klebsiella infection
• Sepsis and septic shock, with multi-organ failure
• Acute respiratory distress syndrome
• Respiratory failure requiring ventilatory support
• Atrial fibrillation - common in older patients, and usually resolves once the infection is treated
• Hyponatraemia - from SIADH, and characteristic of Legionella
• Acute kidney injury - from sepsis, dehydration and nephrotoxic drugs
• Extrapulmonary Mycoplasma disease - erythema multiforme, cold agglutinin haemolytic anaemia, Guillain-Barre syndrome, myocarditis and pericarditis
• Pericarditis and infective endocarditis - uncommon but recognised
• Death - overall mortality of admitted patients is around 10%, rising above 30% in those needing critical care

Red flags

Prognosis

Most patients managed in the community with low-severity pneumonia recover fully, with a mortality below 1%. Among those admitted to hospital, mortality is around 10%, and among those requiring critical care it exceeds 30%. Age, comorbidity, multilobar disease, hypoxia, hypotension, hypoalbuminaemia and a low or normal white cell count in the face of severe illness all predict a worse outcome.

Pneumonia is also a marker of frailty and of underlying disease rather than simply an isolated event. Mortality in the year following an admission for pneumonia is considerably higher than in matched controls, which makes the post-discharge period a valuable opportunity to review comorbidities, vaccination status, swallowing, nutrition and smoking.

Prevention is effective and under-used: pneumococcal vaccination for those aged 65 and over and for at-risk groups, annual influenza vaccination, COVID-19 vaccination, and above all smoking cessation, which halves the risk of invasive pneumococcal disease within 5 years of stopping.