Apneas (breath-holds) are increasingly being tested in human subjects to understand how the human body operates. Apneas decrease heart rate and increase blood pressure. These findings are driving current research into the effects of oxygen concentrations on the heart rate and blood pressure responses to apneas and the effect of breath-hold training on these responses. The interest in apnea research is three-fold:

1. Apneas are a nervous system stressor that can help researchers better understand the fundamental operation of the human body;
2. Elite divers can use findings from research to better their training and performance; and
3. The scientific understanding of apneas may translate to a better understanding of sleep apnea.

Despite this interest, little is known about the repeatability (the consistency within a single day) and reproducibility (the consistency between days) in the heart rate and blood pressure responses to apneas. This uncertainty limits the scientific interpretations from previous results. This study aims to determine the repeatability and reproducibility of heart rate and blood pressure responses to apneas. The goals of the study are:

1. To provide greater certainty to previous results; and
2. Inform best practices for future studies.

The study requires 20 healthy volunteers (10 females) and will measure heart rate, blood pressure, breathing parameters (expired gas concentrations, breathing volume and rate), and oxygen saturation. During the protocol, participants will complete two maximal voluntary apneas and five test apneas. The test apneas will all be the same length based on the longer of the two maximal voluntary apneas. Before each apnea, participants will also breathe low oxygen concentrations (hypoxia). Hypoxia provides a bigger decrease in heart rate during apneas than room air which makes it easier to see changes in heart rate responses between apneas (i.e., bigger signal-to-noise ratio). Participants will complete two identical test sessions on back-to-back days. The differences in heart rate and blood pressure responses to the five apneas within each session will determine repeatability and the differences between sessions will determine reproducibility. The investigators hypothesize that repeatability will be good and that repeatability within a session will be better than reproducibility between sessions.

Inclusion Criteria:

• Males and females between the ages of 18-70

Exclusion Criteria:

• Having known cardiovascular or nervous system disease
• Low or high blood pressure (< 90/60 or >139/85, respectively)
• Taking any prescribed medications (other than oral contraceptives) that may affect cardiovascular system function
• Females who may be pregnant (self-reported)
• Involved in other studies