Shunt volume analysis is a diagnostic procedure in pulmonology (the study of the lungs) that can be used to assess the extent and progression (course/progression) of a right-to-left shunt, for example, in the presence of an arteriovenous malformation (congenital malformation of the blood vessels in which the arteries are directly connected to the veins). A right-to-left shunt is defined as the crossing of blood flow from the right side of the heart to the left side, so that the necessary oxygenation of the blood (oxygenation) cannot occur. Right-to-left shunt can be divided into anatomic and physiologic shunts. While anatomical shunts are characterized by the fact that they occur after the exchange of oxygen and carbon dioxide between the blood and the air in the alveoli (pulmonary alveoli) has been completed, physiological shunts, on the other hand, exist during the perfusion of non-ventilated (non-air-filled) alveoli, which can occur, among other things, in pneumonia (lung inflammation). Several methods are used for shunt volume analysis.
Indications (areas of application)
- Arteriovenous malformations (AVM) of the lung – In the vast majority of cases, pulmonary AVM is due to the presence of hereditary hemorrhagic telangiectasia (hereditary dilatation of blood vessels), also known as Osler disease. These malformations (malformations) can occur singly or multiply (multiple) and can lead to various complications. In addition to hemorrhage, which often manifests as hemoptysis (coughing up blood), septic-embolic processes (caused by infection and vascular occlusion) such as brain abscess may occur. Typically, hypoxemia (oxygen deficiency) is caused by the appearance of the right-to-left shunt. The extent of the shunt plays an important role in the evaluation of AVM, which is why shunt volume analysis is a necessary procedure.
- Hepatopulmonary syndrome – This syndrome represents a very complex collection of various symptoms, the cause of which is portal hypertension (portal hypertension; portal hypertension). Symptoms include hypoxemia caused by the right-to-left shunt and pathological dilatation of the pulmonary vessels. Usually, worsening of hypoxemia is seen with progression of liver disease and reduction of liver function.
- Intracardiac shunts – Several can be enumerated as vitia (cardiac defects) causing right-to-left shunt. What they all have in common is the low frequency of occurrence. In addition to Ebstein’s anomaly (a very rare congenital heart malformation in which the septal and often the posterior leaflets of the tricuspid valve (between the right atrium and the right ventricle) are displaced toward the apex of the heart and the leaflets are malformed; there is usually also an open connection at the atrial level in the form of an atrial septal defect (ASD) or a persistent foramen ovale (PFO; patent foramen ovale) and Fallot’s tetralogy (congenital heart malformation that accounts for about 10% of congenital heart defects; consists of four components (hence tetralogy): A pulmonary stenosis, a ventricular septal defect, an aorta riding over the cardiac septum, and subsequent right ventricular hypertrophy), transposition (interchange) of the great arteries is included among the vities with right-to-left shunt.
- Acute Respiratory Distress Syndrome (ARDS) – this syndrome describes rapid progressive respiratory failure whose cause is not a disease process of the heart.
Contraindications
There are no contraindications when the indication is given.
Before the examination
Right-to-left shunt represents a consequence of an underlying disease. Thus, an accurate diagnosis of the underlying disease must be made before examining the shunt volume. In hepatopulmonary syndrome, portal hypertension (portal hypertension; portal vein hypertension; elevation of portal venous pressure above the normal range, 3-6 mmHg) is the underlying cause. Based on this, complications of hypertension must be excluded by endoscopy (endoscopy), sonography (ultrasound), color duplex sonography, etc.
The procedure
Shunt volume analysis is used to study the association of perfusion (blood flow) and oxygenation of blood.In a right-to-left shunt, a massive imbalance of ventilation (aeration of the lungs) and perfusion (supply of blood) can be seen, resulting in significant hypoxemia (lack of oxygen in the arterial blood). Shunt volume analysis can be quantified using this formula:
Qs/Qt = (CcO2-CaO2) / (CcO2-CvO2).
In this case, Qs/Qt represents the shunt fraction, whereas CcO2 represents the oxygen content at the end of the capillary. CaO2 is defined as arterial oxygen content and CvO2 represents arterial and mixed venous oxygen content. CcO2 is determined by capillary oxygen content. To perform the shunt volume analysis, the patient inhales 100% oxygen through a mask for a period of at least 10 or 20 minutes. During the examination, the patient wears a nose clip to ensure 100% oxygen inhalation. A common method used especially in hepatopulmonary syndrome is scintigraphic shunt detection using 99m-Tc-MAA, which can assess the accumulation of administered radioactive substances in the various organs. Furthermore, echocardiography is also used for shunt detection. The spectrum for shunt detection includes other methods as well.
After the examination
Following the examination and after other diagnostic methods have been performed to assess the underlying disease, further treatment must be determined. However, therapeutic measures are not based solely on shunt volume but on the underlying disease. In hepatopulmonary syndrome, liver transplantation (LTx) is the only method of cure.
Potential complications
Complications of shunt volume analysis depend on the procedure used for analysis.