Intraocular Pressure Changes During Prolonged Dry Static Apnea in Breath-Hold Divers
This prospective observational study of 40 breath-hold divers demonstrates that prolonged dry static apnea induces a significant, rapid-onset increase in intraocular pressure that correlates with elevated blood pressure and decreased oxygen saturation, likely driven by increased episcleral venous pressure, followed by immediate normalization upon resumption of breathing.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The human eye is a delicate, fluid-filled sphere that relies on a precise internal balance to function. Inside this sphere, a clear fluid constantly circulates, providing nourishment and maintaining the eye's shape. The pressure created by this fluid, known as intraocular pressure, is a critical factor in eye health. When this pressure rises too high or stays elevated for too long, it can damage the optic nerve, the cable that carries visual information to the brain, potentially leading to a condition called glaucoma. While doctors have long studied how breathing patterns affect this pressure, most research has focused on very short breath-holds or simple breathing exercises. A lingering question remained: what happens to the pressure inside the eye when a person holds their breath for several minutes, a feat that pushes the body to its physiological limits?
A team of researchers in the Czech Republic set out to answer this question by observing a group of experienced breath-hold divers. These individuals, known for their ability to stay underwater without air for extended periods, volunteered to perform a controlled experiment on land. The scientists asked forty divers, ranging from recreational enthusiasts to professional athletes, to lie flat on their backs and hold their breath for as long as they could. The goal was not to test their lung capacity, but to watch how their eyes and bodies reacted during the struggle to stay underwater without actually being in water. This "dry static apnea" provided a safe, controlled environment to measure changes that usually happen deep beneath the ocean surface.
As the divers held their breath, the researchers monitored their eyes, hearts, blood pressure, and oxygen levels with specialized equipment. The results were striking. Before the breath-hold began, the average pressure inside the divers' eyes was a normal 16.4 millimeters of mercury. As the minutes ticked by, this pressure climbed steadily and significantly. By the time the divers reached their limit, which averaged just under four minutes, the pressure inside their eyes had surged to an average of 29.0 millimeters of mercury. In some cases, the pressure rose even higher, reaching levels that would be considered dangerous for a person with glaucoma. This increase was not random; it grew in direct proportion to the length of time the breath was held.
While the eye pressure was rising, the divers' bodies were undergoing a dramatic transformation. Their blood pressure shot up, with the top number reaching an average of 170 millimeters of mercury, a level comparable to a severe hypertensive crisis. Their oxygen levels dropped significantly, falling from a healthy 99 percent down to an average of 86 percent. Surprisingly, their heart rates did not slow down as much as the classic "diving reflex" usually predicts, likely because the stress of the experiment and the lack of cold water immersion kept their hearts beating faster than they would in the ocean. Despite these intense internal changes, the eye pressure behaved in a remarkably consistent way: it rose as the breath was held and then plummeted back to normal levels within a minute of the divers taking their first breath again.
The researchers investigated why this pressure spike occurred, looking for a single cause among the many changes happening in the body. They considered whether the lack of oxygen or the high blood pressure was the primary driver, but the data suggested a more complex picture. The rise in eye pressure correlated with the drop in oxygen and the rise in blood pressure, but the connection was not a simple cause-and-effect relationship. Instead, the evidence points toward a mechanical blockage in the eye's drainage system. As the divers held their breath, the pressure in the veins of their neck and chest likely increased, creating a backup that made it difficult for fluid to drain out of the eye. This congestion acts like a clogged drain, causing fluid to build up and pressure to rise.
This study offers a clear view into a hidden physiological event that occurs during extreme breath-holding. It confirms that holding one's breath for several minutes causes a temporary but significant spike in eye pressure, driven by a combination of factors including increased pressure in the veins surrounding the eye. While the pressure returns to normal almost immediately after breathing resumes, the finding raises new questions for eye doctors and divers alike. For the divers, the body appears to handle these repeated surges without immediate harm, but the long-term effects of such frequent, intense pressure spikes on the optic nerve remain unknown. The research does not suggest that breath-hold diving causes blindness, but it does highlight a powerful, transient force within the body that deserves further attention.
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