Introduction

Restrictive lung diseases are conditions characterized by a reduction in lung volume, and may be subdivided according to the anatomic location of the pathology. Diseases of the lung parenchyma itself reduce lung volumes due to the poor compliance (‘stiffness’) of the lungs. Examples include interstitial lung diseases such as pulmonary fibrosis, connective tissue diseases affecting the lung, sarcoidosis and hypersensitivity pneumonitis. A second anatomic group involves diseases of the chest wall, where lung volumes are reduced by abnormalities of the lining of the lung (pleural thickening), the skeletal chest wall (e.g. marked kyphoscoliosis) or weakness of the muscles generating breathing activity (e.g. neuromuscular diseases).

Physiology

In patients with restrictive lung disease, lung volumes are reduced. The diagnosis requires documentation of a reduced total lung capacity (TLC), and other volumes are reduced including the vital capacity (VC), the residual volume (RV, the volume of air left after a full expiration) and the functional residual capacity (FRC, the volume left in the lung after a normal tidal expiration). FRC decreases during pregnancy (10–25% decrease by term) in patients with normal lung physiology,¹ potentially aggravating this situation. Minute ventilation increases progressively through pregnancy, mediated by increased progesterone levels.² This increased ventilation occurs via an increase in tidal volume rather than respiratory rate,³ this tidal volume increase relying on increased rib cage displacement.⁴ The fetus and utero-placental unit generate an increase in oxygen consumption, reaching about 20–33% above baseline by term^5,6 and higher during labour. The two major physiological concerns in these patients therefore are:

1. 
   

   1. Ventilation: although tidal breathing at rest in patients with restrictive lung disease may be adequately maintained, those with significantly reduced lung volumes may have difficulty increasing their minute ventilation under stress, e.g. exercise or pregnancy. The increased work of breathing generated by a non-compliant respiratory system (or reduced muscle function) may predispose to muscle fatigue and hypercapnic ventilatory failure, as the ventilatory requirement increases in pregnancy.

   
   
2. 
   

   2. Oxygenation: the interstitial process present in parenchymal lung disease impairs the diffusion of oxygen across the alveolar–capillary membrane, predisposing to hypoxaemia.⁷ Clinically, this is measured by the diffusing capacity of carbon monoxide (DLCO). While the time that red blood cells are exposed to alveolar gas during transit across the alveolar capillary is normally adequate for gas exchange, despite impaired diffusion, hypoxaemia may occur in the face of a shortened transit time, as may occur with the increased cardiac output of exercise or pregnancy (Figure 9.1). This oxygenation problem may be aggravated by destruction of alveolar capillary units and by basal microatelectasis and ventilation-diffusion mismatching, due to elevation of the diaphragm in late pregnancy. Apnoea in the pregnant patient (spontaneous or iatrogenic during endotracheal intubation) is associated with the more rapid onset of hypoxaemia due to the physiological decrease in oxygen reserve (decreased FRC) and increased oxygen consumption.⁸ This will be aggravated by the presence of interstitial lung disease.

Figure 9.1 Oxygenation of blood transiting the alveolar capillary. Solid line represents normal lung diffusion with oxygenation occurring within the first third of transit time. Dotted line represents a diffusion defect, where inadequate oxygenation may occur if the transit time is reduced

(e.g. increased cardiac output of pregnancy).

Pregnant women with restrictive lung disease are therefore at risk of hypoxic and/or hypercapnic respiratory failure. Patients with chest wall or neuromuscular disease are at risk predominantly of ventilatory failure, due to a potential inability to generate the pregnancy-induced increase in tidal and minute ventilation. Those with parenchymal lung disease are also at risk of oxygenation failure – hypoxia is more likely to occur in women with a diffuse interstitial lung disease, who may have difficulty meeting the increased oxygen demands of late pregnancy.

Chest Wall Diseases in Pregnancy

Kyphoscoliosis

This term refers to disorders of excessive spinal curvature, either in the lateral plane (scoliosis) or sagittal plane (kyphosis). Severe deformity occurs in about 1 in 10 000 people. Kyphoscoliosis may be congenital, or secondary to neuromuscular conditions, or ‘idiopathic’, probably with an inherited component. The severity of the spinal deformity is assessed by the Cobb angle, formed by the intersection of lines parallel to the top and bottom vertebrae of the curve (Figure 9.2). An angle of greater than about 100° may be associated with respiratory effects, including the development of respiratory failure.⁹ Many patients with a severe disorder may have undergone surgical treatment in the past, with vertebral fusion or rods, with variable benefit to lung functions.

Figure 9.2 Measurement of the Cobb angle in kyphoscoliosis.

The pulmonary function effects of kyphoscoliosis include a restrictive impairment with reduction in TLC and VC, but relative preservation of the RV resulting in an increased RV/TLC ratio. Rib displacement occurs, with ribs abnormally spread out on the convex side and clumped together on the concave side. Chest wall muscles are similarly overstretched or insufficiently stretched. The altered mechanical advantage of respiratory musculature exacerbates the restrictive effect and increases the work of breathing.¹⁰ The resulting rapid, shallow breathing may produce microatelectasis, affecting oxygenation.

Pregnancy is normally associated with an increased respiratory drive, manifesting in an increase in tidal volume.² A pregnant woman with severe kyphoscoliosis may have difficulty in achieving this increased minute ventilation. However, in practice kyphoscolioisis usually seems to be tolerated reasonably well through pregnancy. A report on five patients with severe kyphoscoliosis (VC ranging from 20% to 58% predicted) showed minimal deterioration of FVC through pregnancy, with some patients actually showing a small improvement.¹¹ All underwent successful pregnancies, with two delivering pre-term (35 weeks’ gestation). The possibility of the development of hypercapnic respiratory failure needs to be considered, and screening for nocturnal hypoventilation may be useful for the early identification of such patients.¹² Labour and delivery are also a significant concern, due to the increased work and oxygen consumption during this period, which may not be tolerated. Assisted delivery may be required in these patients. Furthermore, their spinal deformity may affect the ability to provide epidural or spinal anaesthesia for delivery, and they may have associated pelvic abnormalities, impeding normal delivery.^13,14 The decision for operative delivery should always be based on obstetric indications.

Infiltrative Lung Diseases in Pregnancy

Infiltrative or interstitial lung diseases (ILD) are a heterogeneous group of conditions (Table 9.1), characterized by inflammation and fibrosis of the alveolar walls, producing dyspnoea, cough, inspiratory crackles and bilateral pulmonary infiltrates on lung imaging. ILD may occur as a result of known inflammatory conditions or may be idiopathic. The aetiology, clinical course, treatment and influence of pregnancy vary between conditions. Physiologically, these conditions produce a restrictive defect with reduced lung volumes, reduced pulmonary compliance, and also reduced gas transfer and hypoxaemia, particularly during exercise. Airflow is not usually reduced by ILD, and flow rates may be increased. Exercise tolerance may be markedly reduced.

Various clinical and pathological classifications are used to categorize ILD. Connective-tissue disease-associated ILDs are most likely to affect women of child-bearing age. Other subgroups include the idiopathic interstitial pneumonias, granulomatous conditions such as sarcoid and drug-therapy-related conditions. The histopathological patterns seen include organizing pneumonia, non-specific interstitial pneumonitis and usual interstitial pneumonia.

Many of these infiltrative lung diseases occur in an older age group and are therefore relatively uncommon in pregnancy. However, some do affect women at a younger age, and maternal age is rising in most parts of the world. Data regarding the management and outcome of these conditions in pregnancy is limited to several small case series, but demonstrate that patients with less severe disease tolerate pregnancy without significant problems. Boggess et al. reported on three women with severe ILD with vital capacity ranging from 17% to 38% of predicted.¹³ Although they all required supplemental oxygen during pregnancy due to exercise-induced hypoxaemia and respiratory symptoms, all tolerated pregnancy without significant complications. Lapinsky et al. described four patients with parenchymal lung disease with vital capacities ranging from 31% to 68% predicted. Two of these patients required supplemental oxygen and all had successful pregnancy outcomes.¹¹ Some of the conditions most relevant to the pregnant patient are discussed in more detail below.

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