Antihistamines are commonly used and currently levocetirizine is most frequently prescribed in the Netherlands. They are commonly used by divers, to solve ear, nose and throat problems, especially to open tubal passage. However, the effects of these drugs on cognitive performance have not been investigated during diving.The objective of this study is to investigate the effects of levocetirizine, hydroxyzine and placebo on cognitive and psychomotor functioning during controlled simulated diving in a hyperbaric chamber in professional navy divers at 10 mt (2 bar) and 30 mt (4 bar).It is hypothesized that hydroxyzine will produce significant impairment, and that the magnitude of impairment is related to hyperbaric pressure.

This double-blind, randomized, placebo-controlled cross-over study will comprise of 1 training day and 3 test days. Twenty-four healthy male divers from the Royal Netherlands Navy will participate in this study.The objective of this study is to investigate the single dose effects of levocetirizine (5 mg), hydroxyzine (50 mg) and placebo.

Visit 1 is a training day, and Visit 2-4 are test days. At the test day, subjects receive one of the three medications. One hour after administration of the drugs, subjects will start with the dive in the recompression chamber. After one hour, medication has reached their peak plasma concentration, and effects can be expected. There will be a wash-out period of at least 7 days between the test days.

Subjects will perform one simulated dive at each test day. They first descend in 2 min to a pressure of 4 bar (30 mt), stay for 20 min, the dive continues when they ascend to a pressure of 2 bar (10 mt) with another stay for 20 min. At both depths the 20 min periods are the measuring period to perform the test. At the end they will go to the surface according the decompression profile and stop for 3 min at 9 mt, 8 min at 6 mt and 23 min at 3 mt. These decompression stops are regular for the profile and necessary to prevent decompression sickness.Before and after diving, and at 2 and 4 bar, tests measuring attention, memory, psychomotor, concentration and comprehension ability of the participants are conducted.

Inclusion Criteria:

• male healthy volunteer
• He is aged between 18 and 55 years old
• BMI between 18 and 30
• Written informed consent
• Normal static binocular acuity, corrected or uncorrected
• Normal hearing
• Possession of a valid divers certificate and medical fit for diving
• Be considered as reliable and mentally capable of adhering to the protocol.

Exclusion Criteria:

• Female
• Current drug use
• Use of psychoactive medication, including antihistamines
• Prior enrolment in the same study
• Physical or mental illness
• Excessive alcohol use (>21 alcoholic drinks per week)
• Intake of caffeine-containing beverages over 5 glasses per day
• Smoker