Effect of a patent foramen ovale in humans on thermal responses to passive cooling and heating

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.

Implications of a patent foramen ovale on environmental physiology and pathophysiology: Do we know the hole story?

The foramen ovale is an essential component of the foetal circulation contributing to oxygenation and carbon dioxide elimination that remains patent under certain circumstances, in ∼ 30% of the healthy adult population, without major negative sequelae in most. Adults with a patent foramen ovale (PFO) have a greater tendency to develop symptoms of acute mountain sickness and high-altitude pulmonary oedema upon ascent to high altitude, and PFO presence is associated with worse cardiopulmonary function in chronic mountain sickness. This increase in altitude illness prevalence may be related to dysregulated cerebral blood flow associated with altered respiratory chemoreflex sensitivity; however, the mechanisms remain to be elucidated. Interestingly, men with a PFO appear to have a shift in thermoregulatory control to higher internal temperatures, both at rest and during exercise, and they have blunted thermal tachypnea. The teleological "reason" for this thermoregulatory shift is unclear, but the shift of ∼0.5°C in core body temperature does not appear to be sufficient to have any significant negative consequences in terms of risk of heat illness. Further work in this area is needed, particularly in women, to evaluate mechanisms of heat storage and dissipation in these individuals as compared to people without a PFO. Consequences of a PFO in SCUBA divers include a greater incidence of unprovoked decompression sickness, but whether PFO is beneficial or detrimental to breath hold diving remains unexplored. Whether PFO presence will explain interindividual variability in responses to, and consequences from, other environmental stressors such as spaceflight remain entirely unknown. Abstract figure legend Associations between PFO and altitude illnesses, core body temperature and diving. This article is protected by copyright. All rights reserved.

Impaired pulmonary gas exchange efficiency, but normal pulmonary artery pressure increases, with hypoxia in men and women with a patent foramen ovale

NEW FINDINGS

What is the central question of this study? Do individuals with a patent foramen ovale (PFO⁺ ) have a larger alveolar-to-arterial difference in P O 2 ( A - a D O 2 ) than those without (PFO^- ) and/or an exaggerated increase in pulmonary artery systolic pressure (PASP) in response to hypoxia? What is the main finding and its importance? PFO⁺ had a greater A - a D O 2 while breathing air, 16% and 14% O₂ , but not 12% or 10% O₂ . PASP increased equally in hypoxia between PFO⁺ and PFO^- . These data suggest that PFO⁺ may not have an exaggerated acute increase in PASP in response to hypoxia.

ABSTRACT

Patent foramen ovale (PFO) is present in 30-40% of the population and is a potential source of right-to-left shunt. Accordingly, those with a PFO (PFO⁺ ) may have a larger alveolar-to-arterial difference in P O 2 ( A - a D O 2 ) than those without (PFO^- ) in normoxia and with mild hypoxia. Likewise, PFO is associated with high-altitude pulmonary oedema, a condition known to have an exaggerated pulmonary pressure response to hypoxia. Thus, PFO⁺ may also have exaggerated pulmonary pressure increases in response to hypoxia. Therefore, the purposes of the present study were to systematically determine whether or not: (1) the A - a D O 2 was greater in PFO⁺ than in PFO^- in normoxia and mild to severe hypoxia and (2) the increase in pulmonary artery systolic pressure (PASP) in response to hypoxia was greater in PFO⁺ than in PFO^- . We measured arterial blood gases and PASP via ultrasound in healthy PFO⁺ (n = 15) and PFO^- (n = 15) humans breathing air and 30 min after breathing four levels of hypoxia (16%, 14%, 12%, 10% O₂ , randomized and balanced order) at rest. The A - a D O 2 was significantly greater in PFO⁺ compared to PFO^- while breathing air (2.1 ± 0.7 vs. 0.4 ± 0.3 Torr), 16% O₂ (1.8 ± 1.2 vs. 0.7 ± 0.8 Torr) and 14% O₂ (2.3 ± 1.2 vs. 0.7 ± 0.6 Torr), but not 12% or 10% O₂ . We found no effect of PFO on PASP at any level of hypoxia. We conclude that PFO influences pulmonary gas exchange efficiency with mild hypoxia, but not the acute increase in PASP in response to hypoxia.

Ventilatory responses to acute hypoxia and hypercapnia in humans with a patent foramen ovale

Subjects with a patent foramen ovale (PFO) have blunted ventilatory acclimatization to high altitude compared with subjects without PFO. The blunted response observed could be because of differences in central and/or peripheral respiratory chemoreflexes. We hypothesized that compared with subjects without a PFO (PFO-), subjects with a PFO (PFO+) would have blunted ventilatory responses to acute hypoxia and hypercapnia. Sixteen PFO+ subjects (9 female) and 15 PFO- subjects (8 female) completed four 20-min trials on the same day: 1) normoxic hypercapnia (NH), 2) hyperoxic hypercapnia (HH), 3) isocapnic hypoxia (IH), and 4) poikilocapnic hypoxia (PH). Hypercapnic trials were completed before the hypoxic trials, the order of the hypercapnic (NH & HH) and hypoxic (IH & PH) trials were randomized, and trials were separated by ≥40 min. During the NH trials but not the HH trials subjects who were PFO+ had a blunted hypercapnic ventilatory response compared with subjects who were PFO- (1.41 ± 0.46 l·min^-1·mmHg^-1 vs. 1.98 ± 0.71 l·min^-1·mmHg^-1, P = 0.02). There were no differences between the PFO+ and PFO- subjects with respect to the acute hypoxic ventilatory response during IH and PH trials. Hypoxic ventilatory depression was similar between subjects who were PFO+ and PFO- during IH. These data suggest that compared with subjects who were PFO-, subjects who were PFO+ have normal ventilatory chemosensitivity to acute hypoxia but blunted ventilatory chemosensitivity to carbon dioxide, possibly because of reduced carbon dioxide sensitivity of either the central and/or the peripheral chemoreceptors. NEW & NOTEWORTHY Patent foramen ovale (PFO) is found in ~25%-40% of the population. The presence of a PFO appears to be associated with blunted ventilatory responses during acute exposure to normoxic hypercapnia. The reason for this blunted ventilatory response during acute exposure to normoxic hypercapnia is unknown but may suggest differences in either central and/or peripheral chemoreflex contribution to hypercapnia.

Inspiratory and expiratory resistance cause right-to-left bubble passage through the foramen ovale

A patent foramen ovale (PFO) is linked to increased risk of decompression illness in divers. One theory is that venous gas emboli crossing the PFO can be minimized by avoiding lifting, straining and Valsalva maneuvers. Alternatively, we hypothesized that mild increases in external inspiratory and expiratory resistance, similar to that provided by a SCUBA regulator, recruit the PFO. Nine healthy adults with a Valsalva-proven PFO completed three randomized trials (inspiratory, expiratory, and combined external loading) with six levels of increasing external resistance (2-20 cmH2 O/L/sec). An agitated saline contrast echocardiogram was performed at each level to determine foramen ovale patency. Contrary to our hypothesis, there was no relationship between the number of subjects recruiting their PFO and the level of external resistance. In fact, at least 50% of participants recruited their PFO during 14 of 18 trials and there was no difference between the combined inspiratory, expiratory, or combined external resistance trials (P > 0.05). We further examined the relationship between PFO recruitment and intrathoracic pressure, estimated from esophageal pressure. Esophageal pressure was not different between participants with and without a recruited PFO. Intrasubject variability was the most important predictor of PFO patency, suggesting that some individuals are more likely to recruit their PFO in the face of even mild external resistance. Right-to-left bubble passage through the PFO occurs in conditions that are physiologically relevant to divers. Transthoracic echocardiography with mild external breathing resistance may be a tool to identify divers that are at risk of PFO-related decompression illness.

Panting as a human heat loss thermoeffector

The human autonomic nervous system participates in the control of thermoregulatory responses that are employed to regulate core temperature following deviations of skin temperature and/or core temperature from their respective resting values. This permits a regulation of the core temperature (T_C) at 37.0 ± 1°C with superimposed circadian variations in both sexes and menstrual cycle-associated variations in premenopausal women. When rendered hyperthermic, passively by heat exposure while at rest or actively during exercise, humans engage heat loss or thermolytic responses, including eccrine sweating and cutaneous vasodilatation. A third, less studied, human thermolytic response is thermal panting, and this response is the focus of this review. Human thermal panting was first described over a century ago. It has since been shown to be a reproducible response showing some similar patterns of breathing in species that employ panting as their sole thermolytic heat loss response. The contribution of human panting as a thermolytic response, however, remains controversial. This review highlights both past and recent evidence supporting that hyperthermic humans have a panting pattern of breathing that plays an important role in human thermoregulation.