Continuous Measurement of Atrial Volume with an Impedance Catheter During Positive Pressure Ventilation and Volume Expansion

Right atrial volume (RAV) and other hemodynamic parameters were measured in ten anesthetized swine with an impedance catheter in the right atrium during ventilation and increasing levels of positive end-expiratory pressure (PEEP) to determine the effects of PEEP on right atrial dynamics. RAV and cardiac output (CO) decreased progressively during ventilation with 5, 10 and 15 centimeters of water PEEP, whereas central venous pressure and pulmonary capillary wedge pressure increased. Despite this decrease in RAV, atrial ejection fraction increased significantly and active atrial output (AAO equals stroke volume times heart rate) was maintained. Thus, the decrease in CO resulted from a decrease in the passive component of ventricular filling. Compensatory increases in AAO helped to minimize the fall in CO in some swine (r = 0.73, p less than 0.01). Peripheral adenocorticotropin (ACTH) increased from 14.0 +/- 4.5 picograms per milliliter at zero centimeters of water PEEP to 46.8 +/- 18.3 picograms per milliliter at 10 centimeters of water PEEP (p less than 0.05). Volume expansion that restored CO to control levels despite PEEP led to a significant increase in RAV and in passive ventricular filling, as well as a decrease in ACTH to 11.2 +/- 1.5 picograms per milliliter (p less than 0.05). Changes in the ACTH levels correlated with changes in the RAV (r = 0.58, p less than 0.05), but did not correlate with changes in CO, arterial pressure or central venous pressure. The data herein suggest that the decrease in CO with PEEP results from decreased venous return, RAV and passive ventricular filling, whereas a compensatory increase in right atrial contractility helps to maintain CO. Ventilation with PEEP stimulates hormonal secretion that can be terminated by adequate volume expansion.