Key Topics

• • Overview of the respiratory system
• • Common drugs used in the management of respiratory disorders and their pharmacology
• • Implications for dentistry

Learning Objectives

• • To describe the pharmacological management of respiratory disorders
• • To understand the main drug categories and describe their mechanism of action, indications and adverse effects
• • To understand the implications for dentistry

Introduction

This chapter considers the drugs used to treat respiratory tract diseases and the implications for the dental practitioner. First there is a brief consideration of the physiology of the respiratory tract followed by a consideration of the most prevalent lower respiratory conditions, asthma and chronic obstructive pulmonary disease (COPD). Thereafter is a consideration of other common respiratory tract disorders, rhinitis and cough (see Table 19.1 for the key sections of this chapter). There follows a description of the drugs used to manage respiratory conditions and finally the implications of prescribing these drugs for the practise of dentistry.

Respiratory physiology

The respiratory tract consists of all anatomical structures involved in respiration, from the nose to the alveoli. It is divided into 2 parts: the upper respiratory tract which includes all structures outside the thorax (nose, nasopharynx, pharynx, larynx and trachea) and the lower respiratory tract, which relates to structures within the thorax (ribs and intercostal muscles, trachea, bronchi and great vessels, pleurae and pleural cavities and the lungs).

The respiratory tree is further subdivided. The trachea divides into the right and left bronchi, then bronchioles, respiratory bronchioles, alveolar ducts and finally ends in the alveoli where gaseous exchange occurs. It is lined by respiratory epithelium with both mucus producing Goblet cells and microvilli. Smooth muscle is present at all levels.

The lungs have dual circulation. The bronchial circulation is part of the systemic circulation and supplies the lung parenchyma, airways and pleurae. The pulmonary circulation, from the right side of the heart, allows for exchange of oxygen and carbon dioxide between blood and air.

For gas exchange to occur air must flow in and out of the respiratory system. Ventilation of the lungs ensures air is delivered to the alveoli to allow gas exchange and maintenance of blood gases. The respiratory muscles as well as lung elasticity and airway resistance influence effective ventilation. Many lung diseases affect the physical properties of the lungs impacting on gas exchange. For example, obstructive disorders narrow airways resulting in air trapping, whereas restrictive disorders stiffen the lungs preventing normal expansion. Airway resistance, through increased tone of the smooth muscle lining plays an important role is asthma.

Control of ventilation is complex, involving the brainstem as well as a wide range of receptors including central and peripheral chemoreceptors, arterial receptors, pain receptors and receptors lining the lungs, airway and chest wall. All of these result in coordinated ventilation and responses to physiological conditions.

Asthma

Asthma is the commonest chronic inflammatory disease of the airways resulting in recurrent reversible obstruction to airflow and increased mucus production. It is increasing in both prevalence and severity worldwide. Children are often affected. It is characterized by intermittent episodes of wheezing, shortness of breath, chest tightness and nocturnal cough. Severe attacks can result in life-threatening hypoxaemia.

The symptoms are a result of constriction of bronchial smooth muscle, oedema of the mucosal lining of the small bronchi and plugging of the bronchial lumen by both mucus and inflammatory cells. There is hyper-reactivity of the bronchi to a wide range of stimuli including chemicals, drugs, exercise and cold air. Asthma is often divided into allergic (extrinsic), in which there is sensitization to allergens, and non-allergenic (intrinsic), however there is significant overlap. The pathogenesis of asthma is a combination of both genetic and environmental factors.

Two main phases of asthma attacks have been described: immediate phase and late phase. A number of immune cells and cytokines are involved in both the initiation and potentiation of an attack. These targets form the basis for the prevention and management of asthma.

Asthma suffers have activated T cells, T-helper 2 cells (Th2 cells), in the bronchial mucosa. These release multiple cytokines that are chemotactic to other immune cells, in particular eosinophils. Interleukin 5 (IL-5) and granulocyte-macrophage colony-stimulating factor encourage these eosinophil’s to produce cysteinyl leukotrienes. This leads to damage to the bronchial epithelium, which is responsible for its hyper-reactivity.

Acute asthma may be moderate, severe or life-threatening. Treatment of acute asthma is a medical emergency and should be treated with short-acting β₂ (see later) and systemic prednisolone. Patients with severe or life-threatening asthma should be also given high-flow oxygen and sent immediately to hospital.

In addition to the management of acute asthma exacerbations, therapeutic interventions are also required for the control of chronic asthma. The aims are to control symptoms, prevent exacerbations and maximize pulmonary function.

The British Thoracic Society in conjunction with the Scottish Intercollegiate Guideline Network (BTS and SIGN) have proposed a five-step management plan (see Figure 19.1):

• Step 1 relates to mild asthmatics with intermittent symptoms
• Step 2 for patients with either nocturnal symptoms or more then 3 exacerbations per week
• Step 3 should be initiated if patients continue to experience symptoms despite the first two steps
• Step 4 and Step 5 relate to patients with persistent symptoms