Physiology of breathlessness associated with pleural effusions

Abstract

Purpose of review: Pleural effusions have a major impact on the cardiorespiratory system. This article reviews the pathophysiological effects of pleural effusions and pleural drainage, their relationship with breathlessness, and highlights key knowledge gaps.

Recent findings: The basis for breathlessness in pleural effusions and relief following thoracentesis is not well understood. Many existing studies on the pathophysiology of breathlessness in pleural effusions are limited by small sample sizes, heterogeneous design and a lack of direct measurements of respiratory muscle function. Gas exchange worsens with pleural effusions and improves after thoracentesis. Improvements in ventilatory capacity and lung volumes following pleural drainage are small, and correlate poorly with the volume of fluid drained and the severity of breathlessness. Rather than lung compression, expansion of the chest wall, including displacement of the diaphragm, appears to be the principle mechanism by which the effusion is accommodated. Deflation of the thoracic cage and restoration of diaphragmatic function after thoracentesis may improve diaphragm effectiveness and efficiency, and this may be an important mechanism by which breathlessness improves. Effusions do not usually lead to major hemodynamic changes, but large effusions may cause cardiac tamponade and ventricular diastolic collapse. Patients with effusions can have impaired exercise capacity and poor sleep quality and efficiency.

Summary: Pleural effusions are associated with abnormalities in gas exchange, respiratory mechanics, respiratory muscle function and hemodynamics, but the association between these abnormalities and breathlessness remains unclear. Prospective studies should aim to identify the key mechanisms of effusion-related breathlessness and predictors of improvement following pleural drainage.

Box 1

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FIGURE 1

Expiratory flow-volume loops before (red) and 4 h after (blue) thoracocentesis of 1.3 l in a 62-year-old man with large malignant right pleural effusion. Thoracocentesis resulted in increases in vital capacity, total lung capacity and peak expiratory flow, but they remain severely reduced due to underlying pulmonary and pleural disease. FVC, forced vital capacity.

FIGURE 2

Computed tomography scan (axial view) shows expansion of the right thoracic cage (with increased anteroposterior diameter and widened intercostal space) to accommodate a large pleural effusion.

FIGURE 3

Computed tomography scan (coronal view) shows the right diaphragm that is pushed down by a moderate pleural effusion to a level lower than the left diaphragm. It shows the effect of the weight of fluid on the diaphragm even when the patient is in the supine position. This is likely to be worse when the patient is in an upright position.

FIGURE 4

(a, b) Computed tomography scan (sagittal view) and ultrasound showing flattened right diaphragm caused by a moderate pleural effusion.

FIGURE 5

(a, b) The normal shape of the right diaphragm is restored following therapeutic drainage of 1 l of pleural fluid.

FIGURE 6

(a, b) CT scan (coronal and sagittal views) shows a large pleural effusion pushing down and inverting the left diaphragm. (c, d) The left diaphragm reverts to its normal shape and position following complete evacuation of the effusion.