A low Pa02 may result from an incomplete diffusion equilibrium, hypoventilation, or other alterations of Va/Q relationships, including intrapulmonary, extra-pulmonary, or postpulmonary shunt. The oxygen diffusion capacity has been found to be normal in patients with cirrhosis of the liver. Our patients also presented with hyperventilation leading to respiratory alkalosis, which is consistently observed in cirrhosis. The increased ventilation and the almost normal dead space effectively ruled out hypoventilation as a source of the Va/Q inequality. buy glucophage
Using the inert gas method for determination of Va/Q relationships, Ratner and Wagner showed that the solubility of the least soluble gas (SF6) was too high to give an acute resolution in the low Va/Q end, especially in the presence of units of both low and zero Va/Q. Shunt becomes distinct only if these two regions are separated by several compartments with no ventilation or perfusion. In four out of the six patients (patients 1, 2, 4, and 6), there was a partition between shunt and underventilated units, and their shunt values thus represented their true shunt. In these four patients, the Va/Q distribution of ventilated and perfusion areas was homogeneous, and the logarithms of the standard deviations for perfusion and ventilation were close to normal values. In patient 5, some 9.5 percent of the cardiac output flowed through the four compartments of very low Va/Q, adjacent to shunt and before several zero Va/Q units. Thus, the true shunt could extend from 15.5 percent to 25 percent of the cardiac output. In patient 3, there was no zero Va/Q unit between the shunt and the low Va/Q units, so unventilated areas could not be distinguished from underventilated areas. The hypoxemia was thus attributed to a considerable true shunt, with a possible shunt effect in two cases.
In cirrhotic patients, there are numerous reports of true shunt ranging from normal to 40 percent of the cardiac output. These differences probably stem from differences in selection of patients (several patients in some studies also had pulmonary disease) and to differences in methods. In the only study using the multiple inert gas technique,26 normal values of true shunt with an increase in blood flow perfusing units of lung with low Va/Q were only observed in the patients with cutaneous spider nevi, ie> those with the most severe hepatic impairment; however, in contrast to our patients, all of the patients in the previous study26 had normal Pa02 (92.5 ±2.5 mm Hg breathing room air; n = 15).
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