The primary purpose of this investigation is to determine whether water carbonation can improve orthostatic tolerance in healthy control volunteers. Orthostatic tolerance refers to the ability to maintain an adequate blood pressure when standing. In some individuals blood pressure can fall when standing, predisposing to dizzy spells or fainting episodes. Drinking water can boost blood pressure and making fainting episodes less likely. However, it is not clear whether the carbonation of the water has any further impact on the blood pressure response. This is important because it may be that carbonated water expands the stomach (gastric distension), provoking an increase in sympathetic activity. The increase in sympathetic nervous system activity boosts blood pressure. Resolving this question would have important implications for patients with syncope. This study will test whether carbonated water will have any further impact on blood pressure than the already known effect of non-carbonated water.

The primary purpose of this investigation is to determine whether water carbonation can improve orthostatic tolerance in healthy control volunteers. Orthostatic tolerance refers to the ability to maintain an adequate blood pressure when standing [1]. In some individuals blood pressure can fall when standing, predisposing to dizzy spells or fainting episodes [1]. Drinking water can boost blood pressure and making fainting episodes less likely [2-8]. However, it is not clear whether the carbonation of the water has any further impact on the blood pressure response [9,10]. This is important because it may be that carbonated water expands the stomach (gastric distension), provoking an increase in sympathetic activity. The increase in sympathetic nervous system activity boosts blood pressure. Resolving this question would have important implications for patients with syncope. This study will test whether carbonated water will have any further impact on blood pressure than the already known effect of non-carbonated water.

Volunteers (n=25) will be asked to undergo a "tilt test" to assess cardiovascular reflex control and orthostatic tolerance (measured as time to presyncope, or near fainting, in minutes). It has been previously shown that this technique to be reproducible, reliable, and to have high sensitivity and specificity for differentiating persons with differing orthostatic tolerance, or for examining the effects of interventions aimed at improving orthostatic tolerance [4,6,11-18].

Volunteers will undergo this test on three separate days. On each day participants will be asked to drink a glass of water: either a 50ml drink of room temperature water (control condition), a 500ml drink of flat (non-carbonated) room temperature water, or a 500ml drink of carbonated room temperature water. The study will be conducted in a randomised, single-blind fashion. The investigator responsible for terminating the test will be blinded as to the water condition on each test day, rendering the study single blind. It will not be possible to blind participants as to the carbonation of the water, however, participants will not be informed as to the hypothesised impact of the water conditions.

Inclusion Criteria:

• healthy, English-speaking

Exclusion Criteria:

• pregnancy or suspected pregnancy, history of cardiovascular disease, history of neurological disease, history of recurrent fainting (≥ 2 episodes of fainting with loss of consciousness in the prior 6 months)

Data will be presented in aggregate form and no identifiers will be included.

The investigator determining the end of the test (the time to presyncope) will be blinded to the condition.