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:
- Filtration - nasal hairs filter the air we breathe in.
- Muco-ciliary clearance mechanisms - mucus traps inhaled particles and microbes. Cilia (tiny hair-like structures) on the ciliated columnar epithelium (the tall, column-shaped cells that line the airways) beat to move the mucus and trapped material along.
- Expulsion mechanisms - coughing and sneezing.
- Respiratory mucosal immune system (immune cells and antibodies in the mucus that lines the airways).
However, these defences can become compromised in several ways. A poor swallow, for example after a stroke or in other conditions that affect the brain or nerves, can let food, drink or saliva enter the airways. Ciliary function (how well the cilia beat) can be abnormal in people who smoke and in patients with cystic fibrosis. Airway dilation (widening of the airways) is seen in bronchiectasis. Defects in host immunity (the body’s own immune defences) are seen in patients with immunocompromising conditions (conditions that weaken the immune system) or who take immunosuppressants (medicines that reduce the activity of the immune system). All of these weaken the defence mechanisms of the respiratory system and make infection more likely.
Bronchitis can be defined as inflammation of the medium-sized airways. Acute bronchitis is usually caused by a virus. Chronic bronchitis is long-lasting inflammation of these airways, and is found mainly in smokers.
Symptoms of bronchitis include a cough, fever, increased sputum (phlegm) production and increased shortness of breath. A chest X-ray is not usually needed in general practice. When one is done, it is usually normal, which helps to tell bronchitis from infection of the lung parenchyma (the working tissue of the lung), as in pneumonia.
Causative organisms of acute bronchitis are most commonly viral. Bacterial infection is less common, but when present may include Streptococcus 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 acute bronchitis is symptom relief, such as fluids and pain relief. NICE advises against routinely offering antibiotics unless the person is very unwell or at higher risk of complications. NICE also advises against bronchodilators (medicines that widen the airways) for acute bronchitis, unless the person has an underlying airways disease such as asthma. Chronic bronchitis can lead to airflow limitation (chronic obstructive pulmonary disease, COPD), and short-acting bronchodilators are the first treatment used in COPD to relieve breathlessness.
Bronchiolitis can be defined as inflammation of the small airways, and commonly affects babies and children up to two years of age.
Early symptoms are similar to those of the common cold, with a slightly high temperature. A persistent cough, fast breathing, difficulty feeding and wheezing (noisy breathing) can follow. Bronchiolitis is most commonly caused by Respiratory Syncytial Virus (RSV). There is no specific treatment, and most cases get better on their own. Some babies become seriously unwell. Babies with pauses in breathing (apnoea), severe difficulty breathing or poor feeding need urgent assessment and may need hospital care.
Bronchiectasis can be defined as chronic dilation (widening) of one or more bronchi (the large airways of the lungs). See the article ‘Bronchiectasis and Cystic Fibrosis’ for more information on this condition.
Lung abscesses are an example of liquefactive necrosis (tissue death in which the dead tissue turns to liquid), 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 (in people with no underlying medical condition, most often after aspiration, which is breathing in food, saliva or vomit) or secondary (in people with an underlying condition, such as airway obstruction, a weakened immune system or rheumatoid arthritis, a long-term inflammatory joint disease).
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.
Abscesses are diagnosed with imaging. A chest X-ray usually shows a cavity (a hollow space) with an air-fluid level (a flat line where the fluid settles under the air). A CT scan of the chest then gives more detail and helps to look for other causes of a cavity, such as a tumour. Treatment is usually a prolonged course of antimicrobials (antibiotics), chosen according to the causative organism. If this fails, the abscess may need to be drained through the skin or with a scope. Surgery is needed in only a minority of patients.
Pneumonitis refers to 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, for example when noxious (harmful) fumes are breathed in.
Pneumonia can be defined as inflammation of the alveoli due to infection. This acute inflammatory response causes exudation (leakage) of fibrin-rich fluid into the alveolar spaces (the air sacs), and infiltration (entry) of neutrophils and macrophages. The presence of exudate (this fluid and its cells) in the alveolar spaces may be localised to one lobe of the lung (lobar pneumonia) or patchy (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. Other typical bacterial causes include Moraxella catarrhalis, Staphylococcus aureus and Klebsiella pneumoniae. Atypical bacterial causes include Mycoplasma pneumoniae, Chlamydia pneumoniae and Legionella pneumophila. Viruses, such as influenza viruses and respiratory syncytial virus (RSV), can also cause community acquired pneumonia.
Hospital acquired pneumonia is defined as infection that has occurred 48 hours or more after admission and was therefore not incubating at the time of admission. Hospital inpatients are more at risk because they often already have other illnesses (co-morbidities). Causative organisms most commonly include Staphylococcus aureus and Pseudomonas aeruginosa, but also include Enterobacteriaceae (a family of gut bacteria that includes Escherichia coli), Haemophilus influenzae, and fungal species.
Aspiration pneumonia is caused by the aspiration (breathing in) of contents such as food, liquids, saliva or vomit into the respiratory tract. The material collects in the lung 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. It is also seen in patients with an altered level of consciousness (reduced alertness). This may happen during anaesthetic induction (when a general anaesthetic is started), after excessive alcohol intake, or due to drug misuse.
Aspiration pneumonia can be caused by aerobic bacteria such as Streptococcus pneumoniae, Staphylococcus aureus, Pseudomonas aeruginosa and Klebsiella species. It can also be caused by anaerobic bacteria such as Fusobacterium. Klebsiella pneumoniae is found in the throat more often in people who drink more alcohol, and it has been linked to pneumonia in people with alcohol use disorder.
Atypical pneumonia is pneumonia caused by atypical organisms. These are typically not easily identified on Gram stain (a test that colours bacteria so that they can be seen under a microscope) and often have intracellular life cycles (they live and multiply inside human cells). Their cell structures mean that antibiotics other than beta-lactams (the penicillin family, which includes amoxicillin) are often used. These organisms include Mycoplasma pneumoniae, Chlamydia pneumoniae and Legionella pneumophila (Legionnaires’ disease).
People who are immunocompromised (their immune system is weakened) can be infected by a wider range of organisms. These include Pneumocystis jirovecii, a fungus that causes Pneumocystis Pneumonia (PCP). PCP is commonly seen in patients with AIDS, and is increasingly seen in other immunocompromised patients, for example after a transplant. Cytomegalovirus, a virus, is another cause in this group.
Signs and Symptoms of Pneumonia
Common symptoms of pneumonia are a cough (which may be productive, meaning it brings up sputum), shortness of breath and fever. They can start suddenly or gradually over a few days. Older people may not have these typical symptoms and may become confused instead.
Other symptoms include malaise (feeling generally unwell), rigors (shaking chills), 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 visibly stained with blood, which can be 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 (X-ray), which will show shadowing in the affected area of lung. Microbiological sampling is selective. NICE recommends sending a sample for testing in hospital-acquired pneumonia, but not routinely testing adults with low-severity community-acquired pneumonia. In adults with moderate- or high-severity community-acquired pneumonia, consider sputum culture and susceptibility testing (growing bacteria from the sputum, then testing which antibiotics kill them). Take account of prior antibiotics, clinical progress, comorbidities (other long-term illnesses), recent hospitalisation and whether a good-quality sample can be obtained. NICE also suggests considering blood cultures if there are other signs such as suspected sepsis, a pneumococcal urinary antigen test (a urine test for Streptococcus pneumoniae, which can support a switch to a narrower-spectrum antibiotic, one that targets fewer types of bacteria), and a Legionella urinary antigen test (a urine test for Legionella) if the person has risk factors for Legionella infection. A useful result may identify a pathogen and guide the choice of antibiotic. However, samples can be contaminated and cultures often fail to grow the cause, so a cause is not always found.
Image - Chest radiograph showing shadowing in the right middle lobe, which would be in keeping 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) - a marker of inflammation. NICE suggests considering a baseline CRP on admission, and a repeat after 3 or 4 days if there is concern that treatment is failing.
- Lactate - if there are concerns about signs of sepsis. A raised level suggests the tissues are not getting enough oxygen.
An arterial blood gas (a blood sample taken from an artery to measure oxygen, carbon dioxide and acidity) should also be considered if the patient is very unwell. It is useful if oxygen levels cannot be judged properly from a finger probe (peripheral monitoring, such as pulse oximetry), or if there are concerns about carbon dioxide levels.
The most common assessment tool used to determine the severity of pneumonia is the CURB-65 score. NICE advises using CURB-65 in hospital. In primary care it advises CRB-65, which is the same score without the urea result. Both are worth memorising.
The scoring system is as follows, with each positive result gaining one point:
- C - new confusion (an Abbreviated Mental Test score of 8 or less, or new disorientation in person, place or time)
- U - Urea >7mmol/L
- R - Respiratory rate 30 or more breaths per minute
- B - Blood pressure <90mmHg systolic or 60mmHg or less diastolic
- 65 - Age ≥65
The score helps to decide where a person is treated. It is used together with clinical judgement, and other factors such as other illnesses or pregnancy also matter. For a score of 0 or 1 (low risk), NICE suggests discharge home, with referral to primary care-led services and safety netting advice (advice on when to seek help again). For a score of 2 (intermediate risk), NICE suggests considering a virtual ward (monitoring at home), a same-day emergency care unit, a hospital at home service or inpatient care. For a score of 3 or more (high risk), NICE suggests considering inpatient care, with referral to critical care if appropriate. A patient with a score of 4 or 5 may require treatment in intensive care. In primary care, NICE suggests considering referral to hospital for a CRB-65 score of 2 or more.
Management includes good fluid intake, antipyretics (medicines that reduce fever, for example paracetamol) and analgesia (pain relief) as needed, plus antimicrobials (medicines that kill microbes, mainly antibiotics). NICE advises offering an antibiotic to adults with community-acquired pneumonia, starting as soon as possible after diagnosis and within 4 hours of presentation to hospital. For low-severity disease the first-line choice is oral amoxicillin (a penicillin-type antibiotic), usually for 5 days. Local antimicrobial policy and resistance data also guide which antibiotic is chosen. Patients with more severe disease (for example with a CURB-65 score of 3 or more) may require inpatient care with oxygen therapy and IV fluids. The target oxygen saturation is 94 to 98% for most acutely ill patients, or 88 to 92% for those at risk of hypercapnic respiratory failure (a build-up of carbon dioxide in the blood), such as people with COPD.
Most pneumonia infections completely resolve with no long-lasting effects.
However, in some people the inflamed lung tissue does not return to normal. Instead it undergoes organisation, in which fibrous (tough, scar-like) tissue forms and leaves scarring in the lung.
Other complications of pneumonia may include a pleural effusion (a collection of fluid in the pleural cavity, the space around the lung), empyema (a collection of pus in the pleural cavity), bronchiectasis or formation of a lung abscess.
The risk of lower respiratory tract infections can be reduced through vaccination and smoking cessation advice. Examples are the influenza (flu) vaccine and the pneumococcal vaccine, which protects against pneumococcal infections such as pneumonia. The pneumococcal vaccine is recommended for babies, adults aged 65 and over, and people at higher risk of getting seriously ill from pneumococcal infections. The RSV vaccine is offered to pregnant women and to older adults (aged 75 or over, aged 65 to 74 with certain health conditions, or living in a care home). Antibiotic prophylaxis (antibiotics given to prevent infection) is reserved for specific clinical indications rather than routine prevention.
Edited by: Dr. Maddie Swannack
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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