Mechanism of decrease in cardiac output caused by opening the chest

The cardiac output in ten dogs fell an average of 18.8% (±1.1 sem) when the chest was opened. Opening the chest, which increased the extracardiac pressure, shifted the cardiopulmonary-function curve (relating right atrial pressure to cardiac output) toward higher atrial pressure levels. This caused the cardiopulmonary curve to equate with the animal's systemic-function curve at a lower cardiac output; the greater the extracardiac pressure the lower the output. Thus, changes in extracardiac pressure can alter cardiac output even though the pumping ability of the heart is not altered.