INTRAPLEURAL PRESSURE

• The resting position of the lungs and chest wall occurs at FRC.
• If isolated, the lungs, being elastic, would collapse to a volume <FRC.
• The isolated thoracic cage would normally have a volume >FRC.
• Since the chest wall is coupled to the lung surface by the thin layer of intrapleural fluid between parietal and visceral pleura, opposing lung and chest wall recoil forces are in equilibrium at FRC.
• This produces a pressure of about −0.3 kPa [−2 mmHg ] in the pleural space.
• Normal inspiration reduces intrapleural pressure further to −1.0 kPa [−6 mmHg] but with forced inspiration it can reach negative pressures of −4.0 kPa or more.
• Intrapleural pressure may be measured by an intrapleural catheter or from a balloon catheter placed in the mid-oesophagus [Oesophageal pressures
• tend to reflect intrapleural pressures]
• In the upright adult the intrapleural pressure at the base of the lung is approximately 0.7 kPa greater than the pressure at the apex
• It will increase during coughing
• -1.0 kPa intrapleural pressure is equivalent to a distending transpulmonary pressure of +1.0 kPa