Pulmonary pathways and mechanisms regulating transpulmonary shunting into the general circulation: An update

Abstract

Embolic insults account for a significant number of neurologic sequelae following many routine surgical procedures. Clearly, these post-intervention embolic events are a serious public health issue as they are potentially life altering. However, the pathway these emboli utilize to bypass the pulmonary microcirculatory sieve in patients without an intracardiac shunt such as an atrial septal defect or patent foramen ovale, remains unclear. In the absence of intracardiac routes and large diameter pulmonary arteriovenous malformations, inducible large diameter intrapulmonary arteriovenous anastomoses in otherwise healthy adult humans may prove to be the best explanation. Our group and others have demonstrated that inducible large diameter intrapulmonary arteriovenous anastomoses are closed at rest but can open during hyperdynamic conditions such as exercise in more than 90% of healthy humans. Furthermore, the patency of these intrapulmonary anastomoses can be modulated through the fraction of inspired oxygen and by body positioning. Of particular clinical interest, there appears to be a strong association between arterial hypoxemia and neurologic insults, suggesting a breach in the filtering ability of the pulmonary microvasculature under these conditions. In this review, we present evidence demonstrating the existence of inducible intrapulmonary arteriovenous anastomoses in healthy humans that are modulated by exercise, oxygen tension and body positioning. Additionally, we identify several clinical conditions associated with both arterial hypoxemia and an increased risk for embolic insults. Finally, we suggest some precautionary measures that should be taken during interventions to keep intrapulmonary arteriovenous anastomoses closed in order to prevent or reduce the incidence of paradoxical embolism.

Intrapulmonary arteriovenous anastomoses in dogs with severe Angiostrongylus vasorum infection: clinical, radiographic, and echocardiographic evaluation

“…In people, IPAVAs are believed to be remnants of fetal vessels and to physiologically play a role during fetal life by bypassing blood flow away from the non-functional pulmonary capillaries [9]. After birth, the three main potential consequences are regulation (decrease) of PAP, reduction of pulmonary gas exchange efficiency, and loss of the pulmonary capillary filter.…”

“…The ability to recruit IPAVAs varies between individuals, and not all humans or dogs, healthy or diseased, are able to open IPAVAs [1,2,7,9]. These physiological anastomoses are not to be confused with macroscopic arteriovenous-fistula malformations, even though the consequences may be similar [10,11].…”

Detection of Intracardiac and Intrapulmonary Shunts at Rest and During Exercise Using Saline Contrast Echocardiography

“…The prevalence of PAVMs and grossly distended capillaries that result from rare diseases such as hepatopulmonary syndrome and hemorrhagic hereditary telangiectasia is considered to be very small, on the order of 1 in 50,000 (Khurshid & Downie, 2002;Liu et al, 2010). However, we have found that greater than 95% of healthy humans have intrapulmonary arteriovenous anastomoses that are closed at rest but open up during exercise (Stickland & Lovering, 2006;Lovering et al, 2010). Furthermore, the existence of these pathways in baboon lungs suggests that they may not be evolutionary disadvantageous since humans, gorillas and chimpanzees diverged from the old world monkeys (baboons, macaques, etc.)…”

“…Typically, 0.5-1 ml of air and 4-10 ml of saline are used to manually agitate between two syringes connected by stopcocks for a total injection volume of 10 ml (Otto, 2004;Feigenbaum, 2005;Woods et al, 2010). In a research setting, equal success has been achieved in detecting right-to-left shunts via intrapulmonary arteriovenous anastomoses using 0.5-1 ml of air and 3-5 ml of saline, for a total injection volume of 5 ml (Stickland & Lovering, 2006;Laurie et al, 2010;Lovering et al, 2010;Elliott et al, 2011a). The agitated saline mix solution should be injected as a bolus, forcibly by hand.…”

A case of intrapulmonary transmission of air while transitioning a patient from a sitting to a supine position after venous air embolism during a craniotomy

“…[8][9][10] A recent review of pulmonary pathways and mechanisms pertaining to their autoregulation highlights the interest and importance of this subject. 11 Neuroanesthesiologists are well aware of the multiple pathways for paradoxical air embolism (PAE) to occur. The following case description highlights a role for continuous transesophageal echocardiography (TEE) to detect or exclude the possibility of PAE during surgeries with a high potential for venous air emboli (VAE).…”