From the Guidelines
Localizing shunt locations in hypoxic patients requires a systematic approach combining clinical assessment and diagnostic testing, with the most direct method being a contrast echocardiography (bubble study) with agitated saline to differentiate between intracardiac and intrapulmonary shunts. The typical locations of shunts in patients with hypoxia can be identified through various diagnostic methods, including:
- Contrast echocardiography (bubble study) with agitated saline to differentiate between intracardiac (right-to-left cardiac) and intrapulmonary shunts 1
- 100% oxygen test to indicate right-to-left shunt if PaO2 remains <500 mmHg while breathing 100% oxygen
- Pulmonary angiography to identify specific intrapulmonary shunts like arteriovenous malformations
- CT pulmonary angiography with contrast to visualize pulmonary vascular abnormalities
- Cardiac catheterization to provide definitive diagnosis of intracardiac shunts with measurement of oxygen saturation in different cardiac chambers
- Transesophageal echocardiography for superior visualization of cardiac structures compared to transthoracic approaches
The location of shunts can be categorized into:
- Intracardiac shunts, such as patent foramen ovale (PFO)
- Intrapulmonary shunts, such as arteriovenous malformations
- Systemic-to-pulmonary collaterals, which can develop in patients with single ventricle (SV) or coarctation of the aorta 1
- Veno-venous collaterals, which can be found in patients with SV
- Aortic collaterals, which can develop in patients with coarctation of the aorta
It is essential to note that shunt physiology causes hypoxemia that doesn't respond to supplemental oxygen therapy, as blood bypasses the alveolar-capillary interface where gas exchange occurs. Therefore, a stepwise approach to localizing shunt locations is crucial in managing patients with hypoxia. According to the most recent guidelines, the use of cardiac magnetic resonance (CMR) is recommended for the evaluation of shunt magnitude and direction in pediatric congenital and acquired heart disease 1.
From the Research
Typical Locations of Shunts in Patients with Hypoxia
- Intrapulmonary right-to-left shunting is a common location of shunts in patients with hypoxia due to acquired lung disease 2
- Inter-atrial shunts, such as those occurring through an atrial septal defect or a patent foramen ovale (PFO), can also cause hypoxia 3, 4, 5
- Pulmonary arteriovenous malformations (PAVMs) are another potential location of shunts that can lead to hypoxia, particularly in patients with hereditary hemorrhagic telangiectasia (HHT) 6
- Shunts can occur at the atrial level, allowing blood to flow from the right atrium to the left atrium, bypassing the lungs and leading to hypoxia 3, 4, 5
Detection of Shunts
- Contrast-enhanced echocardiography is a useful tool for detecting shunts, including intrapulmonary right-to-left shunting and inter-atrial shunts 2, 3, 5
- Agitated saline contrast echocardiography can be used to screen for PAVMs in patients with HHT 6
- Transesophageal echocardiography (TEE) can also be used to detect shunts, particularly in patients with a left ventricular assist device (LVAD) 5