INFERIOR VENA-CAVAL PRESSURE INCREASE CONTRIBUTES TO SODIUM AND WATER-RETENTION DURING PEEP IN AWAKE DOGS

This study compared the hemodynamic, renal, and hormonal effects of an experimentally induced increase in inferior vena caval pressure (IVCP ) [to the same extent as during controlled mechanical ventilation (CMV ) with positive end-expiratory pressure (PEEP)] with those of CMV with PEEP. Six volume-expanded conscious chronically tracheotomized dogs w ere studied under three conditions: Control: 4 h of spontaneous breath ing at 4 cmH(2)O continuous positive airway pressure (CPAP); CMV: CPAP for the Ist and 4th h and CMV with PEEP for the 2nd and 3rd h, result ing in a mean airway pressure of 20 cmH(2)O; and Increased IVCP: 4 h o f CPAP, with IVCP increased during the 2nd and 3rd h by inflation of a chronically implanted cuff. Urine volume, sodium excretion, and fract ional sodium excretion decreased during the 2nd and 3rd h of CMV and d uring Increased IVCP compared with Control. Glomerular filtration rate , mean arterial pressure, and antidiuretic hormone, atrial natriuretic peptide, and aldosterone plasma concentrations were not affected by C MV or Increased IVCP. Plasma renin activity decreased during Control a nd Increased IVCP conditions but remained elevated during the 2nd and 3rd h of CMV. We conclude that, in conscious extracellular volume-expa nded dogs, IVCP elevation contributes considerably to the water- and s odium-retaining effect of short-term CMV with PEEP.