Exercise-induced intrapulmonary arteriovenous shunt in healthy women

Quantification of shunt fraction using contrast ultrasound and indicator dilution in an in vitro model

Transthoracic saline contrast echocardiography is commonly used to assess intrathoracic shunt flow in vivo. Though the technique has many advantages (safe, simple, repeatable), the measurement technique lacks specificity, and the contrast agent has limited stability. This study sought to determine if the indicator dilution modeling technique could be applied to ultrasound contrast data to quantify shunt fraction and to determine if buoyant force has a significant effect on microbubble pathway determination at a "vascular" bifurcation. A model of the pulmonary circuit was perfused with blood at three distinct flow rates (low, medium and high) over shunt fractions ranging from ∼2-10 %. The buoyancy effect on contrast was quantified using a simplified in vitro model of a vascular bifurcation that had an upper and lower outflow tract where saline contrast formed from carbon monoxide (CO) gas passed through the bifurcation, was collected and quantified. The indicator dilution model was found to have a mean bias of - 3.2 % for the low flow stage, - 2.6 % for the medium flow stage and - 1.4 % for the high flow stage compared to volumetric measurements, suggesting agreement increases with increasing flow rate. Investigations of the buoyant effects revealed that at lower flow rates, contrast bubbles that encounter a bifurcation will favor the upper outflow tract over the lower. However, this effect is reduced by increasing the flow rate two-fold. These data identify that application of indicator dilution theory to contrast ultrasound data and the pathway ultrasound contrast travels in a network of tubules is flow dependent.

Influence of blood Po2 on the stability of agitated saline contrast

The utility of transthoracic saline contrast echocardiography (TTSCE) to assess blood flow through intrapulmonary arteriovenous anastomoses (Q̇_IPAVA) in humans is limited due to the potential destabilizing effects of the gas concentration gradients established in varied blood-gas environments. This study assessed the specific effect of a hyperoxic and mixed venous blood-gas environment on the stability of saline contrast. We hypothesized that the rate of contrast mass lost in hyperoxic blood would be similar to mixed venous due to the establishment of equal and opposing gas gradients (O₂, N₂, CO₂) created when the partial pressure of dissolved gases is manipulated. Using an in vitro model of the pulmonary circulation perfused with defibrinated sheep blood and a membrane oxygenator to control blood gases, we assessed the percent contrast conserved (an index of contrast stability) between inflow and outflow sites at multiple flow rates (1.8, 2.8, 4.3, and 6.8 L/min) in a hyperoxic (Po₂: 646 ± 16 mmHg; Pco₂: 0 ± 0 mmHg) and a mixed venous blood gas condition (Po₂: 35 ± 3 mmHg; Pco₂: 40 ± 0 mmHg). We found significant contrast decay with time in both conditions, with slightly higher contrast conservation in the hyperoxia trials (64 ± 32%) versus the mixed venous trials (55 ± 21%). These findings suggest that contrast stability is not likely a factor affecting the interpretation of TTSCE performed in healthy humans breathing hyperoxia and lends support to the existence of a local O₂-dependent mechanism contributing to the regulation of Q̇_IPAVA.NEW & NOTEWORTHY Hyperoxic blood has a small stabilizing effect on agitated saline contrast compared with mixed venous blood, lending support to studies that show the reversal of exercise-induced blood flow through intrapulmonary arteriovenous anastomoses (Q̇_IPAVA) with hyperoxia. These data support the possible presence of a local O₂-dependent regulatory mechanism within the pulmonary vasculature that may play a role in Q̇_IPAVA regulation.

Perfusion of Intrapulmonary Arteriovenous Anastomoses Is Not Related to VO2max in Hypoxia and Is Unchanged by Oral Sildenafil

Background: Perfusion of intrapulmonary arteriovenous anastomoses (IPAVA) is increased during exercise and in hypoxia and is associated with variations in oxygen saturation (S_PO₂), resulting in blood bypassing the pulmonary microcirculation. Sildenafil is a pulmonary vasodilator that improves S_PO₂ and endurance performance in hypoxia. The purpose of this study was to determine if 50 mg sildenafil would reduce IPAVA perfusion (Q_IPAVA) and if the decrement in maximal exercise capacity (VO_2max) in hypoxia is related to Q_IPAVA. We hypothesized that during progressive levels of hypoxia at rest (F_IO₂ = 0.21, 0.14, 0.12), sildenafil would increase S_PO₂ and reduce bubble score (estimate of Q_IPAVA) compared to placebo, and that the decrement in VO_2max in hypoxia would be positively correlated with bubble score at rest in hypoxia. Materials and Methods: Fourteen endurance-trained men performed a graded maximal exercise test at sea level and at a simulated altitude of 3000 m, followed by two experimental visits where, after randomly ingesting sildenafil or placebo, they underwent agitated saline contrast echocardiography during progressive levels of hypoxia at rest. Results: All participants experienced a decrement in power output in hypoxia that ranged from 9% to 19% lower than sea level values. Compared to normoxia, bubble score increased significantly in hypoxia (p < 0.001) with no effect of sildenafil (p = 0.580). There was a negative correlation between S_PO₂ and bubble score (p < 0.001). The decrement in peak power output at VO_2max in hypoxia was unrelated to IPAVA perfusion in resting hypoxia (p = 0.32). Several participants demonstrated Q_IPAVA greater than zero in room air, indicating that arterial hypoxemia may not be the sole mechanism for Q_IPAVA. Conclusion: These results indicate that the VO_2max decrement caused by hypoxia is not related to Q_IPAVA and that sildenafil does not improve VO_2max in hypoxia through modulation of Q_IPAVA.

AltitudeOmics: effect of reduced barometric pressure on detection of intrapulmonary shunt, pulmonary gas exchange efficiency, and total pulmonary resistance

Blood flow through intrapulmonary arteriovenous anastomoses (QIPAVA) occurs in healthy humans at rest and during exercise when breathing hypoxic gas mixtures at sea level and may be a source of right-to-left shunt. However, at high altitudes, QIPAVA is reduced compared with sea level, as detected using transthoracic saline contrast echocardiography (TTSCE). It remains unknown whether the reduction in QIPAVA (i.e., lower bubble scores) at high altitude is due to a reduction in bubble stability resulting from the lower barometric pressure (PB) or represents an actual reduction in QIPAVA. To this end, QIPAVA, pulmonary artery systolic pressure (PASP), cardiac output (QT), and the alveolar-to-arterial oxygen difference (AaDO2) were assessed at rest and during exercise (70-190 W) in the field (5,260 m) and in the laboratory (1,668 m) during four conditions: normobaric normoxia (NN; [Formula: see text] = 121 mmHg, PB = 625 mmHg; n = 8), normobaric hypoxia (NH; [Formula: see text] = 76 mmHg, PB = 625 mmHg; n = 7), hypobaric normoxia (HN; [Formula: see text] = 121 mmHg, PB = 410 mmHg; n = 8), and hypobaric hypoxia (HH; [Formula: see text] = 75 mmHg, PB = 410 mmHg; n = 7). We hypothesized QIPAVA would be reduced during exercise in isooxic hypobaria compared with normobaria and that the AaDO2 would be reduced in isooxic hypobaria compared with normobaria. Bubble scores were greater in normobaric conditions, but the AaDO2 was similar in both isooxic hypobaria and normobaria. Total pulmonary resistance (PASP/QT) was elevated in HN and HH. Using mathematical modeling, we found no effect of hypobaria on bubble dissolution time within the pulmonary transit times under consideration (<5 s). Consequently, our data suggest an effect of hypobaria alone on pulmonary blood flow. NEW & NOTEWORTHY Blood flow through intrapulmonary arteriovenous anastomoses, detected by transthoracic saline contrast echocardiography, was reduced during exercise in acute hypobaria compared with normobaria, independent of oxygen tension, whereas pulmonary gas exchange efficiency was unaffected. Modeling the effect(s) of reduced air density on contrast bubble lifetime did not result in a significantly reduced contrast stability. Interestingly, total pulmonary resistance was increased by hypobaria, independent of oxygen tension, suggesting that pulmonary blood flow may be changed by hypobaria.

Reduced blood flow through intrapulmonary arteriovenous anastomoses during exercise in lowlanders acclimatizing to high altitude

NEW FINDINGS

What is the central question of this study? The aim was to determine, using the technique of agitated saline contrast echocardiography, whether exercise after 4-7 days at 5050 m would affect blood flow through intrapulmonary arteriovenous anastomoses (Q̇IPAVA) compared with exercise at sea level. What is the main finding and its importance? Despite a significant increase in both cardiac output and pulmonary pressure during exercise at high altitude, there is very little Q̇IPAVA at rest or during exercise after 4-7 days of acclimatization. Mathematical modelling suggests that bubble instability at high altitude is an unlikely explanation for the reduced Q̇IPAVA. Blood flow through intrapulmonary arteriovenous anastomoses (Q̇IPAVA) is elevated during exercise at sea level (SL) and at rest in acute normobaric hypoxia. After high altitude (HA) acclimatization, resting Q̇IPAVA is similar to that at SL, but it is unknown whether this is true during exercise at HA. We reasoned that exercise at HA (5050 m) would exacerbate Q̇IPAVA as a result of heightened pulmonary arterial pressure. Using a supine cycle ergometer, seven healthy adults free from intracardiac shunts underwent an incremental exercise test at SL [25, 50 and 75% of SL peak oxygen consumption (V̇O2 peak )] and at HA (25 and 50% of SL V̇O2 peak ). Echocardiography was used to determine cardiac output (Q̇) and pulmonary artery systolic pressure (PASP), and agitated saline contrast was used to determine Q̇IPAVA (bubble score; 0-5). The principal findings were as follows: (i) Q̇ was similar at SL rest (3.9 ± 0.47 l min^-1 ) compared with HA rest (4.5 ± 0.49 l min^-1 ; P = 0.382), but increased from rest during both SL and HA exercise (P < 0.001); (ii) PASP increased from SL rest (19.2 ± 0.7 mmHg) to HA rest (33.7 ± 2.8 mmHg; P = 0.001) and, compared with SL, PASP was further elevated during HA exercise (P = 0.003); (iii) Q̇IPAVA was increased from SL rest (0) to HA rest (median = 1; P = 0.04) and increased from resting values during SL exercise (P < 0.05), but was unchanged during HA exercise (P = 0.91), despite significant increases in Q̇ and PASP. Theoretical modelling of microbubble dissolution suggests that the lack of Q̇IPAVA in response to exercise at HA is unlikely to be caused by saline contrast instability.

A methodological approach for quantifying and characterizing the stability of agitated saline contrast: implications for quantifying intrapulmonary shunt

Agitated saline contrast echocardiography is often used to determine blood flow through intrapulmonary arteriovenous anastomoses (Q̇IPAVA). We applied indicator dilution theory to time-acoustic intensity curves obtained from a bolus injection of hand-agitated saline contrast to acquire a quantitative index of contrast mass. Using this methodology and an in vitro model of the pulmonary circulation, the purpose of this study was to determine the effect of transit time and gas composition [air vs. sulphur hexafluoride (SF6)] on contrast conservation between two detection sites separated by a convoluted network of vessels. We hypothesized that the contrast lost between the detection sites would increase with transit times and be reduced by using contrast bubbles composed of SF6 Changing the flow and/or reducing the volume of the circulatory network manipulated transit time. Contrast conservation was measured as the ratio of outflow and inflow contrast masses. For air, 53.2 ± 3.4% (SE) of contrast was conserved at a transit time of 9.25 ± 0.02 s but dropped to 16.0 ± 1.0% at a transit time of 10.17 ± 0.06 s. Compared with air, SF6 contrast conservation was significantly greater (P < 0.05) with 114.3 ± 2.9% and 73.7 ± 3.3% of contrast conserved at a transit time of 10.39 ± 0.02 s and 13.46 ± 0.04 s, respectively. In summary, time-acoustic intensity curves can quantify agitated saline contrast, but loss of contrast due to bubble dissolution makes measuring Q̇IPAVA across varying transit time difficult. Agitated saline composed of SF6 is stabilized and may be a suitable alternative for Q̇IPAVA measurement.

Exercise before and after SCUBA diving and the role of cellular microparticles in decompression stress

Risk in SCUBA diving is often associated with the presence of gas bubbles in the venous circulation formed during decompression. Although it has been demonstrated time-after-time that, while venous gas emboli (VGE) often accompany decompression sickness (DCS), they are also frequently observed in high quantities in asymptomatic divers following even mild recreational dive profiles. Despite this VGE are commonly utilized as a quantifiable marker of the potential for an individual to develop DCS. Certain interventions such as exercise, antioxidant supplements, vibration, and hydration appear to impact VGE production and the decompression process. However promising these procedures may seem, the data are not yet conclusive enough to warrant changes in decompression procedure, possibly suggesting a component of individual response. We hypothesize that the impact of exercise varies widely in individuals and once tested, recommendations can be made that will reduce individual decompression stress and possibly the incidence of DCS. The understanding of physiological adaptations to diving stress can be applied in different diseases that include endothelial dysfunction and microparticle (MP) production. Exercise before diving is viewed by some as a protective form of preconditioning because some studies have shown that it reduces VGE quantity. We propose that MP production and clearance might be a part of this mechanism. Exercise after diving appears to impact the risk of adverse events as well. Research suggests that the arterialization of VGE presents a greater risk for DCS than when emboli are eliminated by the pulmonary circuit before they have a chance to crossover. Laboratory studies have demonstrated that exercise increases the incidence of crossover likely through extra-cardiac mechanisms such as intrapulmonary arterial-venous anastomoses (IPAVAs). This effect of exercise has been repeated in the field with divers demonstrating a direct relationship between exercise and increased incidence of arterialization.

Intrapulmonary shunt and SCUBA diving: another risk factor?

Laboratory and field investigations have demonstrated that intrapulmonary arteriovenous anastomoses (IPAVA) may provide an additional means for venous gas emboli (VGE) to cross over to the arterial circulation due to their larger diameter compared to pulmonary microcirculation. Once thought to be the primary cause of decompression sickness (DCS), it has been demonstrated that, even in large quantities, their presence does not always result in injury. Normally, VGE are trapped in the site of gas exchange in the lungs and eliminated via diffusion. When VGE crossover takes place in arterial circulation, they have the potential to cause more harm as they are redistributed to the brain, spinal column, and other sensitive tissues. The patent foramen ovale (PFO) was once thought to be the only risk factor for an increase in arterialization; however, IPAVAs represent another pathway for this crossover to occur. The opening of IPAVAs is associated with exercise and hypoxic gas mixtures, both of which divers may encounter. The goal of this review is to describe how IPAVAs may impact diving physiology, specifically during decompression, and what this means for the individual diver as well as the future of commercial and recreational diving. Future research must continue on the relationship between IPAVAs and the environmental and physiological circumstances that lead to their opening and closing, as well as how they may contribute to diving injuries such as DCS.

Clinical consideration for techniques to detect and quantify blood flow through intrapulmonary arteriovenous anastomoses: lessons from physiological studies

Intrapulmonary arteriovenous anastomoses (IPAVA) are large diameter (>50 μm) vascular conduits, present in >95% of healthy humans. Because IPAVA are large diameter pathways that allow blood flow to bypass the pulmonary capillary network, blood flow through IPAVA (QIPAVA) can permit the transpulmonary passage of particles larger than pulmonary capillaries. IPAVA have been known to exist for over 50 years, but their physiological and clinical significance are still being established; although, currently suggested roles for QIPAVA include allowing emboli to reach the systemic circulation and providing a source of shunt. Studying QIPAVA is an important area of research and as the suggested roles become better established, detecting and quantifying QIPAVA may become significantly more important in the clinic. Several techniques that can be used to quantify and/or detect QIPAVA in animals, ex vivo human/animal lungs, and intact healthy humans; microspheres, radiolabeled macroaggregated albumin particles, and saline contrast echocardiography, are reviewed with limitations and advantages to each. The current body of literature using these techniques to study QIPAVA in animals, ex vivo lungs, and healthy humans has established conditions when QIPAVA is present, such as during exercise or with arterial hypoxemia and conditions when QIPAVA is absent, such as at rest or during exercise breathing 100% O2 . Many of these physiological studies have direct application to patient populations and we discuss each of these findings in the context of their potential to influence the clinical utility, and interpretation, of the results from these techniques highlighted in this review.

Intrapulmonary arteriovenous anastomoses in humans--response to exercise and the environment

Intrapulmonary arteriovenous anastomoses (IPAVA) have been known to exist in human lungs for over 60 years. The majority of the work in this area has largely focused on characterizing the conditions in which IPAVA blood flow (Q̇IPAVA ) is either increased, e.g. during exercise, acute normobaric hypoxia, and the intravenous infusion of catecholamines, or absent/decreased, e.g. at rest and in all conditions with alveolar hyperoxia (FIO2 = 1.0). Additionally, Q̇IPAVA is present in utero and shortly after birth, but is reduced in older (>50 years) adults during exercise and with alveolar hypoxia, suggesting potential developmental origins and an effect of age. The physiological and pathophysiological roles of Q̇IPAVA are only beginning to be understood and therefore these data remain controversial. Although evidence is accumulating in support of important roles in both health and disease, including associations with pulmonary arterial pressure, and adverse neurological sequelae, there is much work that remains to be done to fully understand the physiological and pathophysiological roles of IPAVA. The development of novel approaches to studying these pathways that can overcome the limitations of the currently employed techniques will greatly help to better quantify Q̇IPAVA and identify the consequences of Q̇IPAVA on physiological and pathophysiological processes. Nevertheless, based on currently published data, our proposed working model is that Q̇IPAVA occurs due to passive recruitment under conditions of exercise and supine body posture, but can be further modified by active redistribution of pulmonary blood flow under hypoxic and hyperoxic conditions.

Hypoxia, not pulmonary vascular pressure, induces blood flow through intrapulmonary arteriovenous anastomoses

KEY POINTS

Blood flow through intrapulmonary arteriovenous anastomoses (IPAVA) is increased by acute hypoxia during rest by unknown mechanisms. Oral administration of acetazolamide blunts the pulmonary vascular pressure response to acute hypoxia, thus permitting the observation of IPAVA blood flow with minimal pulmonary pressure change. Hypoxic pulmonary vasoconstriction was attenuated in humans following acetazolamide administration and partially restored with bicarbonate infusion, indicating that the effects of acetazolamide on hypoxic pulmonary vasoconstriction may involve an interaction between arterial pH and PCO2. We observed that IPAVA blood flow during hypoxia was similar before and after acetazolamide administration, even after acid-base status correction, indicating that pulmonary pressure, pH and PCO2 are unlikely regulators of IPAVA blood flow.

ABSTRACT

Blood flow through intrapulmonary arteriovenous anastomoses (IPAVA) is increased with exposure to acute hypoxia and has been associated with pulmonary artery systolic pressure (PASP). We aimed to determine the direct relationship between blood flow through IPAVA and PASP in 10 participants with no detectable intracardiac shunt by comparing: (1) isocapnic hypoxia (control); (2) isocapnic hypoxia with oral administration of acetazolamide (AZ; 250 mg, three times a day for 48 h) to prevent increases in PASP; and (3) isocapnic hypoxia with AZ and 8.4% NaHCO3 infusion (AZ + HCO3 (-) ) to control for AZ-induced acidosis. Isocapnic hypoxia (20 min) was maintained by end-tidal forcing, blood flow through IPAVA was determined by agitated saline contrast echocardiography and PASP was estimated by Doppler ultrasound. Arterial blood samples were collected at rest before each isocapnic-hypoxia condition to determine pH, [HCO3(-)] and Pa,CO2. AZ decreased pH (-0.08 ± 0.01), [HCO3(-)] (-7.1 ± 0.7 mmol l(-1)) and Pa,CO2 (-4.5 ± 1.4 mmHg; P < 0.01), while intravenous NaHCO3 restored arterial blood gas parameters to control levels. Although PASP increased from baseline in all three hypoxic conditions (P < 0.05), a main effect of condition expressed an 11 ± 2% reduction in PASP from control (P < 0.001) following AZ administration while intravenous NaHCO3 partially restored the PASP response to isocapnic hypoxia. Blood flow through IPAVA increased during exposure to isocapnic hypoxia (P < 0.01) and was unrelated to PASP, cardiac output and pulmonary vascular resistance for all conditions. In conclusion, isocapnic hypoxia induces blood flow through IPAVA independent of changes in PASP and the influence of AZ on the PASP response to isocapnic hypoxia is dependent upon the H(+) concentration or Pa,CO2.

Increased cardiac output, not pulmonary artery systolic pressure, increases intrapulmonary shunt in healthy humans breathing room air and 40% O2

Blood flow through intrapulmonary arteriovenous anastomoses (IPAVAs) has been demonstrated to increase in healthy humans during a variety of conditions; however, whether or not this blood flow represents a source of venous admixture (Q̇ VA /Q̇T) that impairs pulmonary gas exchange efficiency (i.e. increases the alveolar-to-arterial PO2 difference (A-aDO2)) remains controversial and unknown. We hypothesized that blood flow through IPAVAs does provide a source of Q̇ VA /Q̇T. To test this, blood flow through IPAVAs was increased in healthy humans at rest breathing room air and 40% O2: (1) during intravenous adrenaline (epinephrine) infusion at 320 ng kg(-1) min(-1) (320 ADR), and (2) with vagal blockade (2 mg atropine), before and during intravenous adrenaline infusion at 80 ng kg(-1) min(-1) (ATR + 80 ADR). When breathing room air the A-aDO2 increased by 6 ± 2 mmHg during 320 ADR and by 5 ± 2 mmHg during ATR + 80 ADR, and the change in calculated Q̇ VA /Q̇T was +2% in both conditions. When breathing 40% O2, which minimizes contributions from diffusion limitation and alveolar ventilation-to-perfusion inequality, the A-aDO2 increased by 12 ± 7 mmHg during 320 ADR, and by 9 ± 6 mmHg during ATR + 80 ADR, and the change in calculated Q̇ VA /Q̇T was +2% in both conditions. During 320 ADR cardiac output (Q̇T) and pulmonary artery systolic pressure (PASP) were significantly increased; however, during ATR + 80 ADR only Q̇T was significantly increased, yet blood flow through IPAVAs as detected with saline contrast echocardiography was not different between conditions. Accordingly, we suggest that blood flow through IPAVAs provides a source of intrapulmonary shunt, and is mediated primarily by increases in Q̇T rather than PASP.

Resting pulmonary haemodynamics and shunting: a comparison of sea-level inhabitants to high altitude Sherpas

The incidence of blood flow through intracardiac shunt and intrapulmonary arteriovenous anastomoses (IPAVA) may differ between Sherpas permanently residing at high altitude (HA) and sea-level (SL) inhabitants as a result of evolutionary pressure to improve gas exchange and/or resting pulmonary haemodynamics. To test this hypothesis we compared sea-level inhabitants at SL (SL-SL; n = 17), during acute isocapnic hypoxia (SL-HX; n = 7) and following 3 weeks at 5050 m (SL-HA; n = 8 non-PFO subjects) to Sherpas at 5050 m (n = 14). SpO2, heart rate, pulmonary artery systolic pressure (PASP) and cardiac index (Qi) were measured during 5 min of room air breathing at SL and HA, during 20 min of isocapnic hypoxia (SL-HX; PETO2 = 47 mmHg) and during 5 min of hyperoxia (FIO2 = 1.0; Sherpas only). Intracardiac shunt and IPAVA blood flow was evaluated by agitated saline contrast echocardiography. Although PASP was similar between groups at HA (Sherpas: 30.0 ± 6.0 mmHg; SL-HA: 32.7 ± 4.2 mmHg; P = 0.27), it was greater than SL-SL (19.4 ± 2.1 mmHg; P < 0.001). The proportion of subjects with intracardiac shunt was similar between groups (SL-SL: 41%; Sherpas: 50%). In the remaining subjects, IPAVA blood flow was found in 100% of subjects during acute isocapnic hypoxia at SL, but in only 4 of 7 Sherpas and 1 of 8 SL-HA subjects at rest. In conclusion, differences in resting pulmonary vascular regulation, intracardiac shunt and IPAVA blood flow do not appear to account for any adaptation to HA in Sherpas. Despite elevated pulmonary pressures and profound hypoxaemia, IPAVA blood flow in all subjects at HA was lower than expected compared to acute normobaric hypoxia.

Exercise after SCUBA diving increases the incidence of arterial gas embolism

Arterialization of gas bubbles after decompression from scuba diving has traditionally been associated with pulmonary barotraumas or cardiac defects, such as the patent foramen ovale. Recent studies have demonstrated the right-to-left passage of bubbles through intrapulmonary arterial-venous anastamoses (IPAVA) that allow blood to bypass the pulmonary microcirculation. These passages open up during exercise, and the aim of this study is to see if exercise in a postdiving period increases the incidence of arterialization. After completing a dive to 18 m for 47 min, patent foramen ovale-negative subjects were monitored via transthoracic echocardiography, within 10 min after surfacing, for bubble score at rest. Subjects then completed an incremental cycle ergometry test to exhaustion under continuous transthoracic echocardiography observation. Exercise was suspended if arterialization was observed and resumed when the arterialization cleared. If arterialization was observed a second time, exercise was terminated, and oxygen was administered. Out of 23 subjects, 3 arterialized at rest, 12 arterialized with exercise, and 8 did not arterialize at all even during maximal exercise. The time for arterialization to clear with oxygen was significantly shorter than without. Exercise after diving increased the incidence of arterialization from 13% at rest to 52%. This study shows that individuals are capable of arterializing through IPAVA, and that the intensity at which these open varies by individual. Basic activities associated with SCUBA diving, such as surface swimming or walking with heavy equipment, may be enough to allow the passage of venous gas emboli through IPAVA.

Prevalence of left heart contrast in healthy, young, asymptomatic humans at rest breathing room air

Our purpose was to report the prevalence of healthy, young, asymptomatic humans who demonstrate left heart contrast at rest, breathing room air. We evaluated 176 subjects (18-41 years old) using transthoracic saline contrast echocardiography. Left heart contrast appearing ≤3 cardiac cycles, consistent with a patent foramen ovale (PFO), was detected in 67 (38%) subjects. Left heart contrast appearing >3 cardiac cycles, consistent with the transpulmonary passage of contrast, was detected in 49 (28%) subjects. Of these 49 subjects, 31 were re-evaluated after breathing 100% O2 for 10-15min and 6 (19%) continued to demonstrate the transpulmonary passage of contrast. Additionally, 18 of these 49 subjects were re-evaluated in the upright position and 1 (5%) continued to demonstrate the transpulmonary passage of contrast. These data suggest that ~30% of healthy, young, asymptomatic subjects demonstrate the transpulmonary passage of contrast at rest which is reduced by breathing 100% O2 and assuming an upright body position.