An Unusual Cause of Refractory Hypoxemia in Cirrhosis

Discussion

Portal hypertension is a progressive inevitable consequence of cirrhosis with formation of many collaterals.² Portopulmonary collaterals, also called portopulmonary venous anastomoses, connect with the pulmonary circulation. We highlighted the clinical presentation of an unusual anatomic shunt that permits venous admixture with oxygenated blood, causing hypoxemia that is refractory to the administration of supplemental oxygen, and recurrent hepatic encephalopathy.

In portal hypertension, collaterals can be either intrahepatic or extrahepatic. Dynamic contrast computed tomography is regarded as the best tool for evaluation of the status of portosystemic collaterals in patients with portal hypertension.^3,4 Portopulmonary venous anastomosis has been recognized as one of the cephalad hepatofugal collaterals in patients with portal hypertension.⁵ Portopulmonary venous anastomosis was first reported by Schoenmackers and Vieten in 1953.⁶ Incidences have been reported to be 1–30% in portal hypertension.^5,7,8 In previous reports, portopulmonary venous anastomosis was detected on postmortem angiography and percutaneous transhepatic portography. However, computed tomography imaging of direct portopulmonary shunts has not been described in the literature.

Saline contrast echocardiography is used widely in screening intracardiac and intrapulmonary shunts. However, portopulmonary venous anastomosis is typically detected during transhepatic portography rather than saline contrast echocardiography because it provides enough contrast echoes to be picked up even at a location distant from the injection site.⁵ This explains the negative contrast echocardiography study in our case.

Extrapulmonary accumulation of Technetium-99m macroaggregated albumin (MAA) is also used to detect right-to-left shunts. The appearance of the radiotracer in the systemic circulation to document visualization of brain, kidneys, and spleen after intravenous administration of MAA indicates right-to-left intrapulmonary or intracardiac shunts because macroaggregated albumin particles (20–60 μm) supposedly bypass the pulmonary bed (< 15 μm) in these cases, instead of becoming trapped in a normal scenario without such shunts.⁹ However, the MAA scan was also expected to be negative in our case, as there was no intrapulmonary shunting, and macroaggregated albumin particles would still be trapped in the normal pulmonary bed.

When performing contrast echocardiography by the portal approach, 3 main routes might produce contrast echoes in the left heart: (1) portopulmonary venous anastomosis, (2) intracardiac right-to-left shunt, and (3) intrapulmonary right-to-left shunt (hepatopulmonary syndrome). The latter 2 produce contrast echoes first in the right cardiac chambers via the portacaval route and subsequently in the left cardiac chambers. Intracardiac shunt requires one cardiac beat or less, whereas intrapulmonary shunt requires a time interval of 4–6 cardiac beats.^10,11 Portopulmonary venous anastomoses show characteristic contrast echo patterns: an earlier and stronger appearance of echoes in the left cardiac chambers than in the right. These results allow real-time differential diagnosis of portopulmonary venous anastomosis from other causes of intrapulmonary shunt.⁵

As the portal vein carries deoxygenated blood, portopulmonary venous anastomosis causes venous admixture, resulting in a significant reduction in arterial oxygen saturation, especially when the portal venous blood has very low oxygen saturation and/or the shunt is large enough to carry a significant amount of cardiac output.^8,12 The hemodynamics in such cases involves a functional right-to-left shunt. Embolic and infective agents that migrate to the draining vein from the portal circulation have the potential to directly reach the left atrium through the [redundant] portopulmonary venous anastomosis, and systemic embolization can occur. Patients with portosystemic shunts may in fact present with neurological abnormalities and high ammonia levels even in the absence of significant liver damage.¹³ The passage of neurotoxic substances coming from the bowel into the systemic circulation can be involved in the pathogenesis of recurrent hepatic encephalopathy, as in our case.¹⁴

We reported the first case of direct spontaneous portopulmonary venous anastomosis connecting the portal vein to the pulmonary veins and subsequently draining into the left atrium. This case highlights the importance of considering direct portopulmonary venous anastomosis in the differential diagnosis of oxygen-refractory hypoxemia and recurrent hepatic encephalopathy in patients with cirrhosis and the use of contrast-enhanced computed tomography in the ultimate diagnosis.