To assess the effects of continuous positive (CPAP) or negative airway pressure (CNAP) breathing (+/- 10#x2013;12 cmH2O, duration 25 min) on blood content in the body's capacitance vasculature, regional distribution of labeled red blood cells was evaluated in seven spontaneously breathing supine volunteers. Counts were acquired by whole body scans and detectors overlying the liver, intestine, left ventricle, and lower arm, and arterial pressure, heart rate, calf blood flow and vascular resistance, hematocrit, vasopressin, and atrial natriuretic peptide plasma concentrations were also obtained. With CPAP, thoracic, cardiac, and left ventricular counts diminished significantly by 7#x2013;10%, were accompanied by significant increases in counts over both the gut and liver, and remained decreased during CPAP but reversed to baseline with zero airway pressure. Calf blood flow and vascular resistance significantly decreased and increased, respectively, whereas limb counts, arterial pressure, heart rate, and hormone concentrations remained unchanged. With CNAP, in contrast, regional counts and other variables did not change. Thus, moderate levels of CPAP deplete the intrathoracic vascular bed and heart, shifting blood toward the gut and liver but not toward the limbs. No short-term compensation increasing cardiac filling during CPAP was seen. In contrast, CNAP did not alter intrathoracic or organ blood content and, therefore, does not simply mirror the effects evoked by CPAP.
