The respiratory system and associated disorders

Respiratory disease accounts for six million bed-days a year in the UK, and is ubiquitous in both hospital and community health settings. One in five individuals in the UK live with a respiratory condition and 10 000 people receive a new respiratory diagnosis every week (British Lung Foundation, 2023), with those living in poverty at greatest risk of developing a long-term respiratory disease (Lee et al, 2019). Furthermore, admissions associated with respiratory dysfunction are rising three times faster than general admissions and exert a major pressure on the NHS during the winter (NHS England, 2022).

Respiratory diseases are also associated with mortality and, alongside cancer and cardiovascular disease, respiratory disease is one of the most common causes of death in England. According to the British Lung Foundation (2023), someone dies from respiratory disease every 5 minutes in the UK. Nurses, therefore, need to be acutely aware of the nature and aetiology of lung pathology and how to assess respiratory status and detect acute respiratory deterioration.

This article explores the fundamental respiratory anatomy and physiology and pathophysiology of the four most common respiratory disorders, namely pneumonia, asthma, chronic obstructive pulmonary disease and lung cancer. It also outlines fundamental respiratory anatomy and physiology and the essential components of respiratory assessment.

Respiratory anatomy and physiology

Bronchial tree

The lungs contain an enormous network of airways of ever-decreasing size. This network with the trachea is often referred to as the bronchial tree (Figure 1).

The largest and initial airway, the trachea, is lined with mucus-secreting pseudostratified ciliated columnar epithelium that can trap and engulf inhaled debris. Irritant receptors within the trachea can stimulate a cough that can force invading particles upwards, thus protecting the smaller airways in the bronchial tree (Moini, 2019).

The trachea divides into a series of branches (bronchi), which continue to divide into a network of bronchioles that eventually lead to lobules. Lobules are the site of gaseous exchange and contain alveolar ducts and alveolar sacs (Figure 2). The structures within the lobules are microscopic, fragile and easily damaged by infection or inhaled pollutants. The main function of the upper respiratory tract and larger airways, therefore, is to protect the delicate smaller airways (Tortora and Derrickson, 2017).

Respiration

The principal function of the respiratory system is to extract enough oxygen from the atmosphere to maintain cellular health and, simultaneously, dispose of excess carbon dioxide (Beachy, 2017). The physiological process of oxygen absorption and the removal of carbon dioxide is referred to as respiration. Respiration involves four distinct processes:

• Pulmonary ventilation (breathing)
• External respiration, also referred to as gaseous exchange
• Transport of gases, that is, the transport of oxygen and carbon dioxide between the lungs and other tissue
• Internal respiration, or the exchange of oxygen and carbon dioxide within the tissues (Wheeldon, 2020).

Pulmonary ventilation (breathing)

Breathing is divided into two distinct processes: inspiration and expiration.

During inspiration, the rib cage is pulled outwards and upwards by the external intercostal muscles, while the diaphragm contracts downwards, pulling the lungs with it. As a result, the thorax expands, facilitating the movement of air from the atmosphere into the alveoli.

Expiration is a passive process. The external intercostal muscles and the diaphragm relax, allowing the natural elastic recoil of the lung tissue to spring back into shape, forcing air back into the atmosphere (Waugh and Grant, 2018). Acute exacerbation of respiratory disease can result in people using the accessory muscles of breathing, so called because they are rarely used in health. The sternocleidomastoids, the scalenes and the pectoralis, for example, can also be used to produce a deep forceful inspiration to increase the inhalation of oxygen (Figure 3) (Wheeldon, 2021).

To achieve inspiration, respiratory muscles must overcome the elastic recoil of lung tissue and the resistance to airflow through very small airways. The energy required by respiratory muscles to overcome these hindering forces is referred to as the work of breathing. In health, lungs expand easily, a phenomenon known as lung compliance, and the amount of muscle energy used for breathing accounts for <5% of total energy expenditure. However, lung disease can affect lung compliance and airway resistance, and increase the work of breathing, which in turn leads to respiratory muscle fatigue and respiratory failure (Lumb and Thomas, 2020).

External respiration

External respiration (gaseous exchange) describes the exchange of oxygen and carbon dioxide between the alveoli and the pulmonary circulation.

Each lobule of the lung has its own arterial blood supply that originates from the pulmonary artery. The blood within comes from the systemic circulation and is therefore low in oxygen and relatively high in carbon dioxide. The amount and therefore the concentration of oxygen in the alveoli are far greater than in the passing arterial blood supply (Beachy, 2017). Because gases always move from areas of high to low concentration, oxygen diffuses out of the alveoli and into pulmonary circulation and onwards towards the left atrium. As the concentration of carbon dioxide is lower in the alveoli than in the pulmonary circulation, carbon dioxide transfers into the alveoli ready to be exhaled (Hickin et al, 2013).

Control of breathing

While humans can control the rate and depth of their breathing voluntarily (holding one's breath when swimming underwater, for example), breathing is almost exclusively a subconscious activity (Lumb and Thomas, 2020).

Breathing is controlled by the respiratory centres within the medulla oblongata and pons, which regulate the rate and depth of breathing in response to levels of carbon dioxide and oxygen. Other factors that can influence the rate and depth of breathing include pain, fear, anxiety, pyrexia and acid-base imbalance. Any change in respiratory rate, therefore, is clinically significant (Rolfe, 2019).

Respiratory failure

When respiratory disease alters lung function to the extent that the respiratory system is no longer able to meet the metabolic demands of the body, the patient is in respiratory failure (Paramothayan, 2019). The associated increase in the work of breathing that results from respiratory disease may cause fatigue, and breathing may become shallow, weak and ineffective, placing the patient at risk of reduced oxygen levels and the retention of carbon dioxide (Wheeldon, 2021).

A reduced level of oxygen in arterial blood is referred to as hypoxaemia and an elevated level of carbon dioxide in arterial blood is called hypercapnia. Patients who are hypoxaemic and hypercapnic are said to be in respiratory failure type 2 (Ward et al, 2022). Inside the erythrocyte, 20% of carbon dioxide binds to haemoglobin while the remainder combines with water to form carbonic acid (Duncan, 2017). Increases in carbonic acid reduce arterial blood pH, so respiratory failure type 2 is also referred to as respiratory acidosis. The main way to reduce carbon dioxide in the circulation is to enhance respiration through mechanical ventilation (British Thoracic Society and Intensive Care Society, 2016).

Respiratory disease

The main causes of respiratory disease are infection, smoking, pollution and airway hypersensitivity (Lumb and Thomas, 2020). Common respiratory infections include pneumonia, tuberculosis and COVID-19 (National Institute for Health and Care Excellence (NICE), 2023). Smoking is the main cause of lung cancer and chronic obstructive pulmonary disease (COPD) (NHS website, 2023a), and occupations that expose workers to hazardous substances, such as dust, harmful chemical agents or animal dander, can cause pneumoconiosis (Hubert and VanMeter, 2018). Asthma is the most common respiratory disease caused by airway hypersensitivity (Ward et al, 2022).

Non-infective respiratory disease can be either obstructive or restrictive. Diseases such as asthma and COPD, for example, lead to airflow being obstructed, whereas lung cancer and pneumoconiosis restrict airflow (Lumb and Thomas, 2020). People living with chronic respiratory disease are more likely to develop respiratory infection, which places them at greater risk of acute exacerbation of their chronic lung condition (Wheeldon, 2021).

The four main respiratory conditions associated with high mortality rates in the UK today are pneumonia, lung cancer, asthma and COPD (NHS England, 2022).

Pneumonia

Pneumonia is a lower respiratory tract infection caused by the inhalation of a pathogen, ie a bacterium, virus or fungus (NICE, 2022). Aspiration of secretions, such as vomitus, or transmission of bloodborne pathogens from an infection can also cause pneumonia (NHS website, 2023b). Pneumonia infections can be localised and present in one or more lobes (described as lobular pneumonia) while other pneumonia infections are diffuse and spread throughout the lungs.

The most common types of pneumonia are lobular pneumonia, bronchopneumonia, Legionnaires' disease, primary atypical pneumonia and pneumocystis carinii pneumonia (Wheeldon, 2022).

Lobular pneumonia is an infection that is localised to one or more lobes of the lung and is associated with a sudden and acute onset. Once the invading bacteria (normally Streptococcus pneumoniae, also known as pneumococcus) reach the lower respiratory tract beyond the trachea, they multiply in the warmth and moisture in the alveoli. Inflammation follows, causing vasodilation of capillaries and the alveoli to fill with debris, neutrophils, erythrocytes and fibrin, and a solid mass soon forms, in a process called consolidation (Hubert and VanMeter, 2018). Consolidation in the alveoli disrupts external respiration as less oxygen can diffuse from the alveoli into the pulmonary circulation (Lumb and Thomas, 2020).

Bronchopneumonia infections are characterised by widespread or diffuse infections. Rather than exhibiting acute rapid-onset symptoms, bronchopneumonias develop slowly. Infections normally start in the bronchi before spreading to the alveoli but, as with lobular pneumonia, the resultant alveolar inflammation causes a build-up of a solid mass within the alveoli walls, which reduces gaseous exchange (Hubert and VanMeter, 2018).

Legionnaires' disease is caused by a Gram-negative bacterium called Legionella pneumophila, which is found in water and usually occurs when people encounter infected sources of water and droplets within buildings. The consolidation that occurs in Legionnaires' disease can cause lung necrosis and carries a high risk of mortality (Leoffler and Hart, 2020).

Primary atypical pneumonia is caused by Chlamydia pneumoniae and Mycoplasma pneumoniae, minuscule bacteria that infect the upper respiratory tract before being aspirated. It can also result from viral infections such as influenza, when upper respiratory tract inflammation descends into the lower respiratory tract, causing diffuse inflammation of interstitial tissue rather than the alveoli themselves (Brashers, 2012).

Pneumocystis carinii pneumonia is a fungal infection that causes alveolar necrosis and diffuse interstitial tissue inflammation, which leads to alveolar consolidation (Hubert and VanMeter, 2018).

Age is a significant risk factor for pneumonia, with individuals aged under 5 years and over 65 years at greatest risk. Older people are also at risk of acute exacerbation of pneumonia and nurses must note their patient's age when determining the risk of deterioration.

People who are immunocompromised or living with cardiorespiratory disease are at significant risk of contracting pneumonia as are inpatients, particularly those at risk of aspiration, for example individuals with dysphagia, stroke or gastric reflux. Intubated people are also at greater risk. Approximately 1.5% of all inpatients in England will have hospital-acquired pneumonia at any one time (NICE, 2022).

Lung cancer

Lung cancer is the presence and growth of tumours within the lung tissue. There are two major types of bronchial carcinoma non-small cell and small cell.

Non-small cell carcinomas account for 80% of all lung cancers. Non-small cell carcinomas can be squamous cell carcinomas, adenocarcinomas or large cell carcinomas (Paramothayan, 2019). Squamous cell carcinomas tend to develop within the larger bronchi whereas other non-small cell carcinomas are found in the smaller airways, which makes them much harder to detect.

Small cell carcinomas tend to grow in the bronchial submucosa and usually manifest as a central mass. Small cell carcinomas are the most aggressive of the bronchial carcinomas and associated with a high mortality rate (Ward et al, 2022).

Substances that irritate the airway, such as asbestos or cigarette smoke, are the main cause of lung cancer. Consistent irritation by inhaled toxins damages the pseudostratified epithelium of the airways, leaving them prone to inflammation. Furthermore, the chemical properties of cigarette smoke are carcinogenic and encourage the growth of tumours. Individuals suspected of developing lung cancer often present with a cough, haemoptysis, dyspnoea, chest pain, wheeze and finger clubbing (Chapman and Robinson, 2021).

Asthma

Individuals with asthma have hypersensitive or hyper-responsive airways, which cause periodic reversible inflammation in the bronchi and bronchioles that obstructs airflow and produces a characteristic wheeze (Paramothayan, 2019).

The walls of the bronchi and bronchioles are lined with mucus-secreting glands and ciliated cells. Mast cells adjacent to the airways' blood supply, once stimulated, release cytokines (chemical messengers), which cause smooth muscle contraction, increased mucus production and greater capillary permeability. As a result, the airways soon narrow and become flooded with mucus and fluid leaking from blood vessels (Brashers and Huether, 2019) (Figure 4).

In people living with asthma, mast cells react to triggers, substances or situations that would not normally cause inflammation. Asthma is said to be either extrinsic or intrinsic, but many individuals have a combination of both. Extrinsic asthma is associated with allergy (eg to pollen, dust mites or foodstuffs) whereas intrinsic asthma is linked to infection, sudden exposure to cold, exercise, stress or cigarette smoke.

Irrespective of the causative agents, the symptoms and treatments are the same. Asthma is reversible and the main nursing aims for acute exacerbation of asthma are early treatment, and establishing severity and likelihood of deterioration (Wheeldon, 2021). Table 1 summarises the key indicators of acute severe, life-threatening, and near-fatal asthma.

Chronic obstructive pulmonary disease

COPD is an umbrella term for respiratory diseases that cause airway obstruction that is not fully reversible. Airway obstruction is progressive and characterised by abnormal inflammation; the most commonest and most likely cause of this is cigarette smoke.

The two main respiratory diseases collectively regarded as COPD are chronic bronchitis and emphysema. People living with chronic asthma are also at risk of developing COPD as their airways may become remodelled over time, resulting in irreversible airway obstruction (Abadian Sharifabad, 2023).

Emphysema

Smoking is the primary cause of emphysema (NHS website, 2023a). Cigarette smoke reduces the effectiveness of α₁-antitrypsin, a substance that protects the lung tissue from proteases, destructive enzymes released by neutrophils and macrophages during respiratory infections.

In health, α₁-antitrypsin counteracts the destructive action of proteases but, in emphysema, the less effective α₁-antitrypsin allows the destruction to continue unabated (Hickin et al, 2013). The damaged alveoli lack the elastic recoil required for exhalation, often resulting in over-inflation of the lungs and air trapping. Over-inflation increases intrathoracic pressure, which pushes the diaphragm downwards, disturbing its natural concave shape, which makes breathing difficult (Hubert and VanMeter, 2019).

Chronic bronchitis

Chronic bronchitis is characterised by increased mucus production and cilia dysfunction caused by inhaled pollutants, such as those found in cigarette smoke. The airways become irritated, resulting in chronic inflammation and a consistent cough. Cilia dysfunction also makes clearing and expectorating mucus difficult, resulting in the smaller airways becoming blocked and more susceptible to infection (MacNee and Rabinovich, 2017).

Respiratory assessment

A focused inspection of the patient is key to establishing the severity of respiratory failure. While establishing a patient's respiratory rate is essential, the nurse must also observe for signs of respiratory distress or dyspnoea, use of accessory muscles of breathing and audible unusual breathing noises, such as wheezing (Hill and Annesley, 2020).

On initial assessment, the nurse must also observe for signs of central cyanosis, a bluish or darkish hue that is visible in the lips and mouth that occurs when haemoglobin is carrying reduced amounts of oxygen, which is a cardinal sign of respiratory failure (Morgan, 2022). Cyanosis can be difficult to detect in people with darker skin tones, so nurses should also inspect the nails, lips, gums and around the eyes, where changes are more easily identifiable (Pearce, 2021). Other useful assessments include for the presence of finger clubbing, an indicator of respiratory disease, and halitosis, which could be a sign of respiratory tract infection (Wheeldon, 2022).

Ascertaining the nature of a patient's cough can enable the nurse to determine the nature of their illness. In asthma, coughs tend to be worse at night, for example. If the patient has a productive cough, the colour and consistency of their sputum could also be used determine the nature of their respiratory problem (Squires, 2022) (Table 2).

Other essential assessments include peak expiratory flow rate or ‘peak flow’ and oxygen saturation readings. The peak expiratory flow rate is the force of expiration in litres per minute and, as such, it measures the extent of airway resistance. An inability to meet a predicted value based on age, sex and height could indicate increased airway resistance as occurs during an asthma attack (Hill, 2019).

Oxygen saturation is the percentage of arterial haemoglobin carrying oxygen, which in health should be between 97% and 99% (Lister et al, 2020). Spirometry is another key respiratory assessment that enables clinicians to establish the severity of airway obstruction in obstructive disorders such as COPD. Spirometry measures of the force and volume of complete expiration after the lungs are totally filled with air. The volume of air that an individual can expel after the lungs are fully inflated is forced vital capacity (FVC). Another important measurement is the forced expiratory volume in the first second or FEV₁. Calculating an individual's FEV₁: FVC ratio can establish the severity of airway obstruction. In health, an FEV₁: FVC ratio should be ≥0.8 and anything less indicates airway obstruction (Heuer and Scanlan, 2018).

Finally, a comprehensive patient history can help establish the nature and severity of a patient's respiratory complaint. Reduced appetite and weight loss are indicative of lung cancer and tuberculosis, for example (NHS website, 2023c; 2023d). Childhood respiratory disease is often a precursor to adult lung problems and should be noted (Ward et al, 2022). Many respiratory infections are exacerbated by damp and overcrowded living conditions and many professions place individuals at risk (Rote, 2019). Smoking is the major cause of respiratory disease so taking an accurate smoking history could aid diagnosis (NHS website, 2023a; 2023b). If the patient already uses respiratory medication, it is vital that the nurse establishes their compliance because medication misuse or omissions often exacerbates respiratory problems (Wheeldon, 2022).

Case study

Sam Black (not his real name) is a 76-year-old man living with a diagnosis of COPD.

Mr Black visited his GP after a 2-week history of a productive and irritating cough. The GP assessed his lung function using spirometry and noted his FEV₁: FVC ratio was 0.45. Mr Black said he felt very breathless and fatigued and had not felt this unwell before. The GP referred him to the emergency department at the local hospital. On admission to the emergency department, the nurse recorded Mr Black's National Early Warning Score 2 (NEWS2) (Royal College of Physicians, 2017) (Table 3).

Social history

Mr Black is divorced and lives alone in a small high-rise flat. He has smoked tobacco since the age of 14 and, while he has tried to stop, he still smokes around 10 cigarettes a day. He was self-employed as a painter and decorator, but his ill-health made working difficult and he took early retirement 16 years ago. Mr Black uses the local supermarket for provisions but often relies on neighbours to shop for him. On his own admission, he drinks alcohol every day but rarely visits his local pub as the journey makes him too breathless. Instead, he prefers to drink alone at home.

Past medical history and current medication

Mr Black's history and medication is as follows:

• Chronic obstructive pulmonary disease, diagnosed 20 years ago
• Recurring chest infections, which require antibiotics
• He uses a beclometasone inhaler twice a day and a salbutamol inhaler when he feels breathless.

Further key assessments

The nurse in the emergency department notes that Mr Black is expectorating mucopurulent yellow sputum, has mild central cyanosis and that his ankles are oedematous, which the nurse considers could be significant in chronic respiratory disease. The doctor takes an arterial blood sample for analysis (Table 4) and prescribes 28% oxygen.

Conclusion

The main function of the respiratory system is to ensure cells receive a plentiful supply of oxygen while simultaneously maintaining safe levels of carbon dioxide.

However, many of the anatomical structures responsible for the exchange of oxygen and carbon dioxide are microscopic, fragile and easily damaged by infection or contamination. Airways can also become hypersensitive and prone to inflammation, impacting an individual's ability to maintain adequate respiration.

Diseases such as pneumonia, COPD, asthma and lung cancer can have a profound effect on respiratory function and their prevalence is increasing. Nurses need to be cognisant of respiratory dysfunction and how to establish the severity of an individual's respiratory disorder if they are to provide effective care and preserve patient safety.

KEY POINTS

• Many structures in the lung are microscopic, fragile and easily damaged by infection or contamination, which can interfere with their function
• Respiratory disease is associated with mortality and has a significant impact on NHS resources
• Nurses need to be aware of pulmonary pathology and be able to assess and respond to acute respiratory deterioration
• Breathing is a subconscious activity and any changes in respiratory rate, depth or rhythm are significant
• Respiratory disease makes breathing difficult and places patients at risk of fatigue. Fatigue leads to weak respiratory function and the retention of carbon dioxide

CPD reflective questions

• What are the potential causes of chronic obstructive pulmonary disease (COPD) and how does it affect breathing?
• Why does having a respiratory disease increase the risk of infection, and how might understanding the causes of respiratory disease enable you to promote your patients' health?
• What can increase the work of breathing and how does this affect wellbeing?
• For people who live with COPD, oxygen therapy should be used with caution. Why is this so and what should a nurse assess for when caring for an individual with respiratory disease who is receiving prescribed oxygen therapy?