Equine Restrictive Lung Disease - Part 1: Overview

Background

Restrictive lung diseases are those in which the lung volumes are reduced, either because of alteration in lung parenchyma or because of the diseases of pleura, chest wall, or neuromuscular apparatus. Physiologically, restrictive lung diseases are defined by reduced total lung capacity, vital capacity and resting lung volume (Functional Residual Capacity, FRC), but with preserved airflow and normal airway resistance. If due to parenchymal lung disease, restrictive disorders are accompanied by reduced gas transfer and may be marked clinically by desaturation with exercise.

Restrictive diseases can be categorized as intrinsic lung disease (diseases of the lung parenchyma) or extrinsic disorders (extra-parenchymal diseases).

Intrinsic lung diseases cause either inflammation and or scarring of the lung tissue (interstitial lung disease), or fill the air spaces with exudate and debris (pneumonitis).

Extrinsic disorders affect the chest wall, pleura and respiratory muscles which are components of the respiratory pump - their normal function needed for effective ventilation. Diseases of these structures result in lung restriction, impair ventilatory function and may lead to respiratory failure. These disorders can be further grouped as non-muscular diseases of the chest wall and the neuromuscular disorders. The most common extrinsic disorders responsible for restrictive lung disease in humans are abnormal configuration of thoracic cage (such as kyphoscoliosis or ankylosing spondylitis) and morbid obesity, both disorders not recognized in the horse. The most common extrinsic non-muscular disorders of the horse are pleuropneumonia and thoracic neoplasia. The most common extrinsic neuromuscular disorder seen in the horse is equine botulism.

The restrictive lung disorders of the horse discussed in this chapter will include pleuropneumonia and interstitial lung disease. To read about parenchymal diseases, readers are referred to other chapters in Equine Respiratory Diseases (Foal Pneumonia; Parasitic Airway Disease and Fungal Airway Diseases; Bacterial Infections Including Mycoplasmas).

Pathophysiology

Air flows to and from the alveoli as lungs inflate and deflate during respiratory cycle. Lung inflation is accomplished by contraction of respiratory muscles, diaphragm, and external intercostal muscles, whereas deflation is passive. Functional residual capacity (FRC) is the volume of air in lungs when the respiratory muscles are fully relaxed and there is no airflow. The size of FRC is determined by the balance of inward elastic recoil of the lungs and the outward elastic recoil of the chest wall. Restrictive lung diseases are characterized by reduction in FRC and other lung volumes because of pathology in lungs, pleura or the structures of the thoracic cage. The distensibility of the respiratory system is called compliance, this being the volume change produced by a change in the distending pressure. The compliance of lungs is independent from the thoracic cage which is a semi-rigid container. The compliance of the intact respiratory system is an algebraic sum of the compliances of both of these structures, and therefore will be influenced by any of the diseases of the lungs, pleura and chest wall.

Intrinsic Disorders

The physiological effects of diffuse parenchymal disorders are reduction in all lung volumes which is produced by the excessive elastic recoil of the lungs in comparison to the outward recoil forces of the chest wall. The expiratory airflows are reduced in proportion to the lung volumes. Arterial hypoxemia in these disorders is primarily caused by ventilation/perfusion mismatching with further contribution made by intrapulmonary shunt. There is impaired diffusion of oxygen, which contributes a little towards hypoxemia at rest, but is primarily the mechanism of exercise-induced desaturation. Hyperventilation at rest and exercise is caused by the reflexes arising from the lung, and the need to maintain minute ventilation by reducing tidal volume and increasing respiratory frequency.

Extrinsic Disorders

Disorders of the pleura and thoracic cage decrease total compliance of the respiratory system and hence reduce the lung volumes. As a result of atelectasis of the alveoli, gas distribution becomes non-uniform, resulting in ventilation perfusion mismatch and hypoxemia. The respiratory pump may be impaired at the level of the central nervous system, spinal cord, peripheral nerve, neuromuscular junction or respiratory muscle. The pattern of ventilatory impairment is highly dependent on the specific neuromuscular disease.