Respiratory centre

The medullary respiratory centre, situated in the medulla oblongata in the brain stem, primarily controls breathing through stimulating the contraction of the diaphragm and the external intercostal muscles, which expand and contract the thoracic cavity. During inspiration the diaphragm contracts and flattens causing the thoracic cavity to lengthen, whilst the external intercostal muscles contract to expand the chest. This creates a negative intrapulmonary pressure. During expiration the diaphragm rises and the intercostal muscles relax, resulting in a positive intrapulmonary pressure. Air pressure differences allow the movement of air in and out of the lungs. All gases move from an area of greater pressure to one of lesser pressure. During inspiration, air is drawn into the lungs. It flows through the bronchi and then into the bronchioles, alveolar ducts and alveolar sacs until it reaches the alveolar capillary membrane. The lungs are able to expand and move easily because the visceral pleura that lines the lungs and the parietal pleura that lines the mediastinum and chest wall creates a potential space, known as the pleural cavity. It contains a small amount of lubricating fluid that allows easy movement between the pleura when the lungs expand.

Pulmonary stretch receptors

Pulmonary stretch receptors are located in the smooth muscle of the bronchi and bronchioles and respond to inspiration by sending impulses through the vagus nerve to the medullary respiratory centre when the lungs are filled with air, inhibiting further inflation of the lungs. This is known as the Hering–Breuer reflex and prevents over-inflation of the lungs.

Chemoreceptors

Peripheral chemoreceptors located in the aorta at the aortic arch (aortic body) and at the carotid bifurcation (carotid body) send impulses by sensory nerves to the medullary respiratory centre in response to a decrease in oxygen and pH, and an increase in carbon dioxide in the blood. The chemoreceptors in the carotid bodies are the main oxygen sensors and are stimulated to increase the breathing rate when there is a significant decrease in oxygen levels, i.e. below 90% or 8 kPa.

The normal stimulus to breathe is the rising level of carbon dioxide that easily crosses the blood–brain barrier into cerebrospinal fluid where it hydrates to form carbonic acid and then dissociates into bicarbonate and hydrogen ions. The hydrogen ions stimulate the central chemoreceptors in the medulla oblongata to send impulses to the medullary respiratory centre to increase the respiratory rate, thereby eliminating excess carbon dioxide.

Oxygen transport

During inspiration, air containing 21% oxygen is taken into the lungs and passes down the trachea and bronchi and into the alveoli, where it comes into contact with the pulmonary circulation. Once the oxygen diffuses across the alveolar capillary membrane it is carried in arterial blood to the heart and pumped around the body in two ways. Approximately 3% dissolves in the plasma, with the remaining 97% being carried in the red cells, bound to haemoglobin. Each haemoglobin molecule can reversibly bind (i.e. can give up again) four molecules of oxygen to form oxyhaemoglobin. The maximum amount of oxygen that can chemically combine with haemoglobin is called the oxygen capacity. The oxygen capacity, plus the oxygen carried as dissolved oxygen, is termed the oxygen content.

Carbon dioxide transport

Carbon dioxide, the waste product of metabolism, is transported in the blood in three forms: dissolved carbon dioxide, bicarbonate ions and carbamino compounds (carbaminohaemoglobin). The carbon dioxide diffuses from the blood capillaries across the alveolar capillary membrane to the alveoli to be exhaled. Because of the properties of haemoglobin, the unloading of oxygen and the loading of carbon dioxide are reciprocal events. When the carbon dioxide levels rise the affinity of haemoglobin for oxygen decreases.

Ventilation and perfusion

Adequate alveolar ventilation and perfusion are required for normal gas exchange. Alveolar ventilation () is the volume of gas that reaches the alveoli per minute and is approximately 4 L/min. The perfusion () is the amount of blood that flows through the pulmonary capillaries per minute and is approximately 5 L/min. The normal ratio is minus which is 0.8, although this will change in various parts of the lung due to gravitational forces. In healthy individuals these gravitational differences do not affect adequate gaseous exchange; however, in lung disease the mismatch is wider and is the most common cause of low oxygen levels in arterial blood (hypoxaemia). With effective ventilation and perfusion, oxygen is able to diffuse from the alveoli into the blood to achieve a partial pressure (PaO₂) of between 10 and 13 kPa.

Lung volume capacity and compliance

The volume of air breathed in and out and the number of breaths per minute vary from one individual to another according to age, size and activity. Normal, quiet breathing gives about 15 complete cycles per minute in the adult. Lung volume can be assessed in the following terms (Figure 3.2):

Figure 3.2 Lung volumes and capacity.

(Reproduced from Esmond 2001 with permission.)

Lung expansion and recoil

Elastic fibres in lung tissue and the surface tension of the fluid lining the alveoli give the lungs their natural recoil tendency. Ease of expansion and recoil depends on normal compliance and elasticity and on the presence of surfactant in the fluid lining the alveoli. Compliance can be reduced by the stiffening of normally soft alveolar tissue due to pulmonary oedema or to the ageing process. Conversely, compliance can be increased by extreme softening due to loss of lung tissue, as in emphysema.

Smoking cessation

Of the 13 million smokers in the UK, over 68% (General Household Survey – Office for National Statistics 2005) say they want to stop and this has often been directly related to specific reasons, for example a life event, health reasons, social pressure or financial reasons. Of this 60%, only half intend to stop in the next year, only a third of these make an attempt and only 2% succeed on their own. Stopping smoking requires motivation, effort, commitment and stamina to be successful, therefore it has to be the right time for the smoker to make an attempt. Helping people to stop smoking is a challenge faced by many nurses as it requires facilitating change and supporting patients through the process rather than actively providing care. Behaviour change required for smoking cessation is complex, with nicotine addiction as well as many other factors including social and psychological influences playing their part. Evidence-based smoking cessation guidelines have been developed to assist health care professionals to develop smoking cessation strategies that fit in with the overall tobacco control (National Institute for Health and Clinical Excellence [NICE] 2006a, 2008a, West et al 2000).

Nurses are in an ideal position to encourage and enable smokers to quit the habit through applying the five As for anybody who smokes:

This approach is supported by NICE (2002, 2008b) and it will ensure that patients are referred to smoking cessation services, often run by practice nurses or specially trained smoking cessation advisors. There are two main strategies used in smoking cessation: pharmacological interventions and lifestyle behavioural change.

Respiratory rate, rhythm and depth

Normal breathing should be regular and appear effortless with a respiratory rate in adults between 12 and 20 breaths per minute. The majority of patients with underlying respiratory disease will have an elevated respiratory rate (>20 breaths per minute) to facilitate adequate oxygenation and ventilation. Respiratory depression (<12 breaths per minute) may occur due to exhaustion and fatigued respiratory muscles caused by the work of breathing. The use of sedatives and opioid analgesics may also cause respiratory depression. Observation of the rhythm and depth of breathing should be assessed, as this will indicate the quality of each breath. Slow, shallow breathing is a sign of respiratory depression and indicates that there is an insufficient number and quality of breaths to sustain adequate ventilation. Equally, a rapid, irregular breathing pattern which does not allow expansion of the lower lobes of the lungs will also result in ventilatory failure.

Chest movement

The chest movements should be symmetrical and are best observed by standing in front of the patient rather than to one side. If one side of the chest is not expanding as well as the other this may indicate a collapsed lung due to pnuemothorax (air in the pleural space) or the presence of bronchial obstruction. The patient’s use of accessory muscles in the neck and shoulders (the sternocleidomastoid and trapezius muscles) should also be noted as this indicates that the patient is unable to use the diaphragm and external intercostal muscles sufficiently to maintain ventilation.

Oxygen levels

Observation of skin colour and pulse oximetry (oxygen saturation) will indicate whether the patient is receiving adequate oxygenation. The patient’s skin should be observed for blueness of the lips, tongue and oral mucosa, as this indicates the presence of central cyanosis caused by hypoxaemia (low oxygen levels in arterial blood). In patients with dark skin it may not be possible to see changes in lip colour and therefore the oral mucosa and tongue should always be examined. Hypoxaemia will also be detected through performing pulse oximetry as this will measure oxygen saturation levels. According to the British Thoracic Society (2008) the normal ranges are:

When examining the skin, the presence of peripheral oedema should be noted as this may indicate the presence of cor pulmonale (right ventricular hypertrophy and failure) due to vasoconstriction of the pulmonary circulation as a result of local hypoxia in the lungs. Another sign of chronic tissue hypoxia is finger clubbing, which is where the terminal phalanges of the fingers become enlarged and the base of the nail feels spongy.

Mental state

Respiratory assessment should include assessment of mental state because confusion and disorientation may be a sign of severe hypoxia resulting in a reduced supply of oxygen to the brain. Asking relatives and friends if they have noticed a change in mental state may be helpful when the nurse has not previously known the patient. The patient’s level of alertness also needs to be assessed as drowsiness may indicate hypercapnia (raised levels of carbon dioxide in arterial blood).

Causative factors

Factors that influence the patient’s respiratory health status, such as allergens, the environment and smoking, should be identified so that potential health education strategies can be implemented. As smoking is a major cause of respiratory illness a thorough smoking history should include current and past smoking habits, with the total pack years of smoking being estimated. The number of pack years can be calculated as follows:

The patient’s present and past occupation may indicate exposure to toxic substances, such as asbestos and industrial chemicals. Through identifying exposure as the causative factor of respiratory disease the patient and their family may be eligible for compensation.

Symptoms

The most common symptom associated with respiratory disease is breathlessness, although wheeze – often described by patients as chest tightness, cough and muscular chest pain – can be equally troublesome. Through assessing symptoms at regular intervals it is possible to monitor treatment effectiveness. Assessment for the presence of infection is important as it causes much respiratory disease, such as pneumonia and tuberculosis. A raised temperature and pulse rate, a raised white blood cell count, positive blood cultures and a productive cough with yellow or green sputum indicate the presence of infection.

Impact on activities of daily living

Assessing the impact of breathlessness on all the activities of daily living will allow the nurse to plan care effectively, identifying the need for referral to occupational therapy, physiotherapy, dietetics and social services as appropriate. Particular attention to mobility and nutritional status is essential, as breathlessness has been shown to cause immobility, and malnutrition due to increased energy requirements and poor appetite (Margereson & Esmond 1997). Patients with underlying respiratory disease should be observed for increasing breathlessness and oxygen desaturation (lowering of oxygen levels) on exertion, as this may indicate the need for supplementary oxygen to prevent complications of immobility. Assessment of the patient’s nutritional status should include body mass index (BMI, see Ch. 21), identification of recent significant weight loss, eating habits and changes in appetite. Patients found to have a BMI less than 20 or those with a recent weight loss greater than 10% should be referred to a dietitian.

Psychosocial factors

The chronic nature of much respiratory disease can affect the patient’s psychological state and relationships with family and friends. Through assessing the patient’s perceptions and concerns, coping strategies, such as relaxation techniques to alleviate fear and anxiety or activity pacing to allow greater mobility, can be identified to assist with adaptation to chronic respiratory illness. The introduction of nurse-led clinics, usually run by practice nurses or respiratory nurse specialists, has assisted those suffering from chronic respiratory illness to feel at ease to discuss psychosocial issues. This is usually achieved through the nurse building a therapeutic trusting relationship with the patient over a period of time so that a profile of perceptions and concerns can be developed. Consideration of current social issues and need for referral to social services or community nursing should be continually assessed so that interventions can be identified at the earliest opportunity.

Lung function tests

Lung function tests assess the functioning of the lungs and can be used to confirm a diagnosis through measuring flow rates and lung volumes. They include the following, which are collectively called spirometry:

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