Normal intrapleural pressure is

The normal external pressure is equal to the normal intrapleural pressure (the pressure in the chest  cavity), which is −4 mm Hg. Note in the figure that a rise in intrapleural pressure, to −2 mm Hg, shifts the entire cardiac output curve to the right by the same amount.  This  shift  occurs  because  to  fill  the  cardiac chambers  with blood requires an extra 2 mm Hg right atrial pressure to overcome the increased pressure on the outside of the heart. Likewise, an increase in intrapleural pressure to +2 mm Hg requires a 6 mm Hg increase in right atrial pressure from the normal −4 mm Hg, which shifts the entire cardiac output curve 6 mm Hg to the right.