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Next Lesson - Bronchiectasis and Cystic Fibrosis
Core
In this article, many types of lower respiratory tract infection will be discussed, but the main focus will be on pneumonia.
Defences of the Respiratory System
The respiratory tract has many in-built defences to infection. These include:
- Muco-ciliary clearance mechanisms – nasal hairs and ciliated columnar epithelium that line the respiratory tract.
- Expulsion mechanisms – coughing and sneezing.
- Respiratory mucosal immune system.
However, there are also many ways these defences can become compromised, including having a poor swallow (usually due to muscle weakness), abnormal ciliary function (seen in people who smoke, and patients with cystic fibrosis), airway dilation (seen in bronchiectasis), and defects in host immunity (patient with immunocompromising conditions or who are on immunosuppressants). All of these things help to weaken the defence mechanisms of the respiratory system and predispose to infection.
Bronchitis can be defined as inflammation of the medium-sized airways, which is found mainly in smokers.
Symptoms of bronchitis include a cough, fever, increased sputum production and increased shortness of breath. The chest x-ray will be normal, allowing differentiation from infection of the lung parenchyma.
Causative organisms of acute bronchitis are bacterial, and include Streptococcal pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis.
Chronic bronchitis doesn’t normally have an infective cause, but the inflammation can persist for many years causing symptoms of shortness of breath and productive cough.
Treatment for bronchitis involves bronchodilation through physiotherapy, with or without antibiotics depending on the cause.
Bronchiolitis can be defined as inflammation of the small airways, and commonly affects babies and children up to two years of age.
Symptoms are similar to those of the common cold, but can include fever, a dry and persistent cough, difficulty feeding, and wheezing. It is caused by the Respiratory Syncytial Virus, and most cases resolve without treatment.
Bronchiectasis can be defined as chronic dilation of one or more bronchi. See the article ‘Bronchiectasis and Cystic Fibrosis’ for more information on this condition.
Lung abscesses are an example of liquefactive necrosis, in which necrosis of the lung parenchyma occurs, resulting in a cavity filled with fluid and necrotic debris.
Lung abscesses can be classified as acute (<6 weeks) or chronic, (>6 weeks). They can also be classified based on their cause: primary (caused by infection, most commonly aspiration pneumonia) or secondary (caused by other conditions such as rheumatoid arthritis, or obstruction).
Symptoms may initially be similar to pneumonia but will be longer-lasting and may be associated with other symptoms such as foul-smelling breath and night sweats.
Abscess are diagnosed via imaging, most commonly a chest X-ray to rule out pneumonia followed by a CT chest, and are treated with a range of antimicrobials depending on the causative organism. If especially large, surgical drainage may be necessary.
Pneumonitis refers only to the inflammation of the lung parenchyma. The term is usually only used to describe inflammation due to non-infective causes, such as physical or chemical damage, such as occurs when noxious fumes are breathed in.
Pneumonia can be defined as inflammation of the alveoli due to infection. This acute inflammatory response causes exudation of fibrin-rich fluid into the alveolar spaces, and neutrophil and macrophage infiltration. The presence of exudate in the alveolar spaces may be localised (lobar pneumonia) or diffuse (bronchopneumonia).
Hospital and Community Acquired Pneumonias
Pneumonia is most commonly classified by the setting in which it was acquired: community acquired pneumonia or hospital acquired pneumonia.
Community acquired pneumonia is most commonly caused by Streptococcus pneumoniae or Haemophilus influenzae. Atypical causative organisms include Moraxella catarrhalis, Staphylococcus aureus and Klebsiella pneumoniae.
Hospital acquired pneumonia is defined as infection that has occurred over 48 hours after admission and is therefore not incubating at the time of admission. It is normally acquired because hospital inpatients will have other co-morbidities as the cause of their admission. Causative organisms most commonly include Staphylococcus aureus and Pseudomonas aeruginosa, but also include Enterobacteriaceae, Haemophilus influenzae, and fungal species.
Aspiration pneumonia is caused by the aspiration of contents such as food, liquids, saliva or vomit, into the respiratory tract, which collects and leads to infection. It is most commonly seen in patients with a defective swallowing mechanism, for example people with neurological conditions like Parkinson’s disease, or patients with an altered level of consciousness, which may be seen in anaesthetic induction, after excessive alcohol intake, or due to drug abuse.
Aspiration pneumonia can be caused either by aerobic bacteria such as Streptococcus pneumoniae, Staphylococcus aureus, Pseudomonas aeruginosa and Klebsiella species (usually seen in aspiration pneumonia in alcoholics), or by anaerobic bacteria such as fusobacterium.
Atypical pneumonia is pneumonia caused by atypical organisms – organisms which lack a cell wall. These include Mycoplasma pneumoniae, Chlamydia pneumonia and Legionella pneumophila (Legionnaire’s Disease).
When considering the causative organisms of pneumonia, it is also important to mention that in the immunocompromised there might be a wider range of organisms that cause the disease, such as Pneumocystis jiroveci (commonly presenting as Pneumocystis Pneumonia (PCP) in patients with AIDS) or Cytomegalovirus.
Signs and Symptoms of Pneumonia
The most common symptoms of pneumonia are shortness of breath, fever and a productive cough, usually with rapid onset of a few days
Other symptoms include malaise, rigors, pleuritic chest pain (chest pain that is worse on inspiration), nausea and vomiting.
The sputum seen in the productive cough may be yellow, rusty (presence of blood), or frankly stained with blood due to excess coughing.
On examination, the patient will be pyrexical, tachycardic, tachypnoeic, and depending on the severity, possibly cyanosed. There will be dullness to percussion of the chest due to the exudate collections, and bronchial breathing and crackles on chest auscultation.
Pneumonia can be seen on a chest radiograph, which will show shadowing in the affected area of lung. The other main investigation indicated in pneumonia is sputum culture and sensitivities, which will allow the causative organism, and therefore most appropriate antimicrobial treatment, to be identified.
Image - Chest radiograph showing shadowing in the right middle lobe, which would be inkeeping with a diagnosis of right middle lobe pneumonia
Creative commons source by Mikael Häggström, M.D. [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Patients with suspected pneumonia should also have blood tests performed:
- Full Blood Count (FBC) – white cell count will indicate response to infection.
- Urea and Electrolytes (U&Es) – urea is important for working out CURB-65 score (see later) and electrolyte values will help when assessing the stability of the patient.
- C-Reactive Protein (CRP) – useful when assessing declining levels of inflammation in recovery.
- Lactate – if there are concerns about signs of sepsis.
Consideration should also be made for arterial blood gas monitoring if the patient is very unwell and there are concerns about their oxygenation that cannot be satisfied with peripheral monitoring, or about carbon dioxide levels.
The most common assessment tool used to determine the severity of pneumonia is the CURB-65 score. This tool is used very commonly in primary and secondary care settings so is worth memorising.
The scoring system is as follows, with each positive result gaining one point:
- C – new-onset confusion
- U – Urea >7mmol/L
- R – Respiratory rate >30 breaths per minute
- B – Blood pressure <90mmHg systolic or <60mmHg diastolic
- 65 – Age ≥65
A score of 0 or 1 with signs and symptoms typical of pneumonia warrants antibiotics in the community according to clinical judgement. Any patient with a score of 2 or more should be admitted to hospital, and a patient with a score of 4 or 5 may require treatment in intensive care.
All patients with pneumonia should be managed with good fluid intake, anti-pyretics (e.g. paracetamol), analgesia and antimicrobials (refer to hospital’s antimicrobial policy when deciding on which antibiotic to use). Patients with more severe disease (for example with a CURB-65 score of 3 or more) may require inpatient care with continuous oxygen therapy and IV fluids.
Most pneumonia infections completely resolve with no long-lasting effects.
However, some result in organisation, which involves the occurrence of fibrous scarring in the lung.
Other complications of pneumonia may include a pleural effusion, empyema (a collection or pus in the pleural cavity), bronchiectasis or formation of a lung abscess.
Lower respiratory tract infections can be prevented through flu vaccines, chemoprophylaxis (commonly penicillin or erythromycin) for patients at a high risk of infections, and smoking cessation advice.
Edited by: Dr. Maddie Swannack
Reviewed by: Dr. Thomas Burnell
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