Is Blood Flow Through IPAVA and PFO Related to Breath-hold and SCUBA Diving-induced Pulmonary Hypertension?

Pulmonary arterial hypertension (increased lung blood pressure) is a multifactorial disease without a cure. Investigating reversible forms of pulmonary hypertension induced under extreme conditions such as a prolonged breath-hold and/or SCUBA diving may help to better understand why some individuals develop this devastating disease, and others do not.

Pulmonary arterial pressure is typically very low in healthy humans. Low oxygen levels (hypoxia) cause a constriction of the lung blood vessels resulting in smaller diameters, but lung blood flow stays constant or increases. Having constant flow with smaller vessel diameters causes pulmonary arterial pressure to increase but it will return to normal once normal oxygen levels are restored. Moreover, there is an association with patent foramen ovale (PFO, small hole between the atria in the heart) and excessive pulmonary arterial pressures in low oxygen conditions. However, the reasons for the exacerbated increase in pulmonary arterial pressure in these subjects with a PFO (PFO+) is unknown, but may be due to an exaggerated constriction response to low oxygen.

During a breath-hold, the oxygen in the lung decreases and in elite breath-hold divers, it decreases to very low levels. Compared to subjects without a PFO (PFO-), the oxygen may drop even lower in those PFO+ subjects because deoxygenated blood travels through the small hole to mix with oxygenated blood, exacerbating the level to which blood oxygen decreases. Thus, using a breath-hold model of lung hypoxia is one approach to examining a hypoxia-induced increase in pulmonary arterial pressure - a method critically dependent upon the elite breath-hold diver's ability to hold their breath for significant durations.

Intrapulmonary arteriovenous anastomoses (IPAVA) are vessels within the lung that bypass capillaries. The investigator's group has investigated the possible roles these unique vessels may have in physiological and pathophysiological conditions. The investigators have found that IPAVA blood flow occurs when healthy subjects breathe low oxygen gas. The investigators have also found that IPAVA blood flow is inversely related to pulmonary arterial pressure. Specifically, individuals with high pulmonary arterial pressures have low IPAVA blood flow and vice versa.

The right ventricle of the heart pumps blood through the pulmonary artery to the lungs. Under resting conditions the right heart performs a minimal amount of work because the pressure in the lung blood vessels is low. When pulmonary arterial pressure increases, the work of the right side of the heart has to increase substantially to keep blood pumping through the lung. Thus, high pulmonary arterial pressures will increase the work of the right heart and may lead to right heart dysfunction thereby limiting the amount of blood the heart can pump. If the pressure is high enough to limit the amount of blood flowing through the lung then this can be detected by a reduction in pulmonary blood flow and/or changes in the function of the heart during contraction (systole) and relaxation (diastole). Accordingly, an intervention that reduces pulmonary arterial pressures during a breath-hold may have a beneficial effect on right heart function.