Effect of nitroprusside administration and sudden withdrawal on pulmonary vascular tone and lung fluid balance

We previously reported an increase in lung lymph flow rate (QL) on cessation of nitroprusside (NP) infusion after endotoxin lung injury. Utilizing sheep with chronic lung lymph fistulas and no lung injury, we studied pulmonary transvascular fluid filtration rate and protein flux (as measured via lung lymph flow and lymph protein content, respectively) during and after NP administration. The mean aortic pressure (Ao) and pulmonary arterial pressure (PAP) were significantly reduced by NP infusion. QL rose slightly during this reduction in pressure. Sudden withdrawal of NP infusion resulted in a significant rise in QL (as seen in our previous study) and a decrease in lymph to plasma protection ratio (L/P). There were only slight changes in PAP and mean pulmonary capillary wedge pressures (WP). We suggest that withdrawal of NP increases pulmonary microvascular pressure (Pmv).