Introduction The snorkel, which allows swimmers to keep their face down in the water while breathing, is widely used by divers, spear fisher- men and monofin swimmers. A snorkel consists of a mouthpiece and a tube that has an air space usually with a volume of 160± 170ml. This volume functions as an additional dead space in ad- dition to the anatomical dead space, because the air in the snor- kelisre-inspired.TheconcentrationofCO 2 inthegasinthesnor- kel will be equal to expired air, since expired air is trapped in the snorkel tube. Some monofin swimmers reported that they felt their performance was better when they expired into the water instead of expiring into the snorkel, thus ensuring that the in- spired air in the snorkel has no expired CO 2 in it. An increase in ventilation is expected when the concentration of CO 2 is higher in the inspired gas and this has been shown in several studies [5,6].Inthispaperweinvestigatedventilatoryresponseandme- tabolic changes when rebreathing the gas in the snorkel. MaterialandMethods Subjects Nine male and 3 female healthy young volunteers between 14 and26yrofage(20.93.4)wereincludedinthestudy.Thesub- jects were members of the monofin swimming team of Marmara University. Initial screening consisted of medical history, pulmo- nary function tests, physical examination and familiarization with experimental procedures. Ventilatory and Metabolic Response to Rebreathing theExpiredAirintheSnorkel A.S.Toklu 1 A. Kayseriliog Ïlu 2 M.Ünal 2 S Ë.Özer 3 S Ë.Aktas Ë 1 Affiliation 1 The Department of Underwater and Hyperbaric Medicine, Istanbul Faculty of Medicine, Istanbul University, Istanbul, Turkey 2 The Department of Sports Medicine, Istanbul Faculty of Medicine, Istanbul University, Istanbul, Turkey 3 School of Physical Training and Sport, Marmara University, Istanbul, Turkey Correspondence A. S. Toklu, M.D. ´ Deniz ve Sualtõ Hekimlig ÏiA.D.´I Ç .Ü.I Ç stanbul Tõp Fakültesi ´ 34390-Þapa ´ Istanbul ´ Turkey ´ Phone: +905324125168 ´ Fax: +902125311817 ´ E-Mail: akint@isbank.net.tr Acceptedafterrevision: 28 June 2002 Bibliography Int J Sports Med 2003; 24: 162±165  Georg Thieme Verlag Stuttgart ´ New York ´ ISSN 0172-4622 Abstract The snorkel, which allows swimmers to keep their face down in the water while breathing, is widely used by divers, spear fisher- men and monofin swimmers. A snorkel adds an additional dead spaceof160±170mlandcausesanincreaseintheconcentration of CO 2 in the inspired gas due to expired air trapped in the snor- kel which is then re-inspired. In this study the metabolic and the ventilatory response to rebreathing the expired air in the snorkel were investigated in twelve human subjects. A 2900 C Sensor Medics gas analyzer was used in breath-by-breath mode for the measurements. Ventilation (V E ), respiratory rate (RR), tidal volume (TV), oxygen consumption (V Ç O 2 ) and carbon dioxide production (V Ç CO 2 ) were measured at rest and during light exer- cise both with and without the snorkel dead space. We observed asignificantincreaseinallvariablesexceptRR,whensubjectsre- breathed the gas in the snorkel. The increase in ventilation resulted from an increase in tidal volume rather than increasing respiratory rate. We conclude that the work of breathing is in- creased when CO 2 concentration is high in inspired gas and re- breathing while snorkelling can be prevented by a new snorkel design with a low-resistance two-way non-rebreathing valve, which will allow the expired air flow into the water. Keywords Snorkelling ´ work of breathing ´ dead space Training & Testing 162