The role of alpha-atrial natriuretic peptide in fluid retention during mechanical ventilation with positive end-expiratory pressure.

Although controlled mechanical ventilation (CMV) with positive end-expiratory pressure (PEEP) has a central place in the treatment of acute respiratory failure (ARF), several side effects of this technique have to be faced. CMV with PEEP may induce pulmonary barotrauma, disturbance of cardiac performance, impairment of renal function and fluid retention. Atrial natriuretic peptides (ANP) are released from the atria upon stretching and play a major role in the control of sodium and fluid balance. Therefore it was logical to determine plasma levels of alpha-ANP in CMV. A study performed in 7 patients suffering from ARF suggested that alpha-ANP plasma levels were depressed during PEEP at 15 cm H2O in comparison with PEEP at 0 cm H2O (ZEEP). The decrease in plasma levels of alpha-ANP was evident in samples taken from superior vena cava, right atrium, pulmonary artery and radial artery as well. The decrease in alpha-ANP was associated with a decline in cardiac index, creatinine clearance, urinary output and urinary sodium excretion. Experiments in volume- expanded healthy volunteers also suggest that CMV with PEEP is able to depress plasma levels of alpha-ANP. The reasons behind the decline in release of alpha-ANP may be atrial compression by the distended lungs and the well-known reduction of venous return to the heart. Other possible factors promoting fluid retention during CMV with PEEP are the decrease in cardiac index and glomerular filtration rate, changes in intrarenal distribution of blood flow, and a stimulation of release of antidiuretic hormone via stretch receptors in the left atrium and baroreceptors in aorta and carotid arteries.