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 were studied under three conditions:

4 h of spontaneous breathing at 4 cmH2O continuous positive airway pressure (CPAP); CMV: CPAP for the 1st and 4th h and CMV with PEEP for the 2nd and 3rd h, resulting in a mean airway pressure of 20 cmH2O; and Increased IVCP: 4 h of CPAP, with IVCP increased during the 2nd and 3rd h by inflation of a chronically implanted cuff. Urine volume, sodium excretion, and fractional sodium excretion decreased during the 2nd and 3rd h of CMV and during 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 CMV or Increased IVCP. Plasma renin activity decreased during CONTROL and Increased IVCP conditions but remained elevated during the 2nd and 3rd h of CMV. We conclude that, in conscious extracellular volume-expanded dogs, IVCP elevation contributes considerably to the water- and sodium-retaining effect of short-term CMV with PEEP.