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Hepatopulmonary Syndrome

Definition

  • Liver disease --> excessive pulmonary microvascular dilation +/- AVMs --> altered gas exchange --> hypoxemia
    • No relationship b/w severity of HPS and severity of liver disease
  • Diagnostic criteria:
    • PaO2 <80 mmHg or Aa gradient >15 mmHg on room air
    • Positive contrast echocardiography or abnoral uptake in the brain with radioactive lung-perfusion scanning
    • Liver disease



Pathophysiology

Increased pulmonary vasodilatation caused by:
  • Increased nitric oxide production in the lungs (no clear explanation)
  • Increased lung angiogenesis factors (no clear explanation)
Mechanism of Hypoxemia
  • Diffusion-perfusion limitation
    • Capillaries dilated too much --> O2 can't traverse to bind to RBC in the core of the vessel
    • If pt has elevated cardiac output --> decreased transit time in capillary bed --> even less O2 binding
    • Can be corrected w/ supplemental O2
  • Ventilation-perfusion mismatch
    • Vascular dilation --> increased perfusion relative to preserved ventilation
    • If pt has atelectasis/edema/PNA --> localized hypoxic alveoli --> impaired hypoxia induced vasoconstriction due to HPS --> even more V/Q mismatch
    • Can be corrected w/ supplemental O2
  • Shunts
    • HPS associated w/ pulmonary and extrathoracic AVMs --> blood flow that bypasses pulmonary capillary beds and never gets oxygenated
    • CanNOT be corrected w/ supplemental O2
Symptoms of HPS
  • Dyspnea, cyanosis, clubbing
  • Platypnea
    • Dyspnea in upright position that is alleviated when supine (likely due to orthodeoxia)
  • Orthodeoxia
    • Hypoxemia in upright position that is alleviated when supine
    • Vascular tone remains fixed in all parts of the lung in HPS --> when upright, you decrease the surface area of gravity dependent alveolar units with better ventilation capabilities --> worse V/Q matching
Diagnosis of HPS
  • Best test = transthoracic echo w/ agitated saline injected in peripheral vein
    • Bubbles are suppose to get trapped in pulmonary capillary bed because they are too big to pass through
    • In HPS, capillaries dilate --> bubbles can path through
    • If bubbles seen in left atrium 3 or more cardiac cycles after bubbles seen in right atrium --> intrapulmonary vascular dilation
  • Pts w/ liver disease may have other lung disease that impairs gas exchange, somake sure to evaluate for:
    • Hepatic encephalopathy --> aspiration PNA
    • Emphysema from a1-AT deficiency
    • Hepatic hydrothorax (pleural effusion)
    • Atelectasis from massive ascites
    • Muscle wasting --> restrictive lung disease
    • Pulmonary HTN (entity called portopulmonary HTN)
    • Hepatic synthetic dysfunction --> PE
    • Chronic medical illness unrelated to liver disease (RAD, asthma, COPD, ILD, sarcoid, cancer, etc)
Prognosis of HPS
  • Higher mortality compared to liver pts w/o HPS
    • Severe hypoxemic resp. failure is usually NOT the cause of death
    • Death from complication of liver disease (GIB, coagulopathy, sepsis, heptagonal syndrome, etc)
  • Worse functional class and quality of life compared to liver pts w/o HPS

Treatment

  • Supplemental O2
    • No data to confirm clinical benefit (but makes sense...)
  • Pentoxifylline
    • Reduces nitric oxide synthesis
    • Clinical trials have resulted in conflicting conclusions of its benefit in HPS
  • Embolization if AVM exists
  • Vasodilator therapy (iNO, sildenafil) usually not used
  • Liver transplant
    • Only proven successful treatment
    • Improves 5 year survival rate compared to no transplant
    • Hypoxia may persist or even worsen after transplant
    • Normalization of gas exchange may take 6-12 months
      • More severe pre-op hypoxia = longer time to resolution of hypoxia

References

1) Rodríguez-Roisin R, Krowka MJ. Hepatopulmonary syndrome--a liver-induced lung vascular disorder. N Engl J Med. 2008 May 29;358(22):2378-87.