Ventilatory and Metabolic Response to Rebreathing ...

Ventilatory and Metabolic Response to Rebreathing the Expired Air in the Snorkel Training & Testing 162

Abstract The snorkel, which allows swimmers to keep their face down in the water while breathing, is widely used by divers, spear fishermen and monofin swimmers. A snorkel adds an additional dead space of 160 ± 170 ml and causes an increase in the concentration of CO2 in the inspired gas due to expired air trapped in the snorkel 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 (VE), respiratory rate (RR), tidal volume (TV), oxygen consumption (VÇO2) and carbon dioxide

Introduction The snorkel, which allows swimmers to keep their face down in the water while breathing, is widely used by divers, spear fishermen and monofin swimmers. A snorkel consists of a mouthpiece and a tube that has an air space usually with a volume of 160 ± 170 ml. This volume functions as an additional dead space in addition to the anatomical dead space, because the air in the snorkel is re-inspired. The concentration of CO2 in the gas in the snorkel 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 inspired air in the snorkel has no expired CO2 in it. An increase in ventilation is expected when the concentration of CO2 is higher

A. S. Toklu1 Ï lu2 A. Kayseriliog M. Ünal2 Ë. Özer3 S Ë. AktasË1 S

production (VÇCO2) were measured at rest and during light exercise both with and without the snorkel dead space. We observed a significant increase in all variables except RR, when subjects rebreathed 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 increased when CO2 concentration is high in inspired gas and rebreathing 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. Key words Snorkelling ´ work of breathing ´ dead space

in the inspired gas and this has been shown in several studies [5, 6]. In this paper we investigated ventilatory response and metabolic changes when rebreathing the gas in the snorkel.

Material and Methods Subjects Nine male and 3 female healthy young volunteers between 14 and 26 yr of age (20.9 3.4) were included in the study. The subjects were members of the monofin swimming team of Marmara University. Initial screening consisted of medical history, pulmonary function tests, physical examination and familiarization with experimental procedures.

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 HekimligÏi A.D. ´ IÇ. Ü. IÇstanbul Tõp Fakültesi ´ 34390-Þapa ´ Istanbul ´ Turkey ´ Phone: +905324125168 ´ Fax: +902125311817 ´ E-Mail: [email protected] Accepted after revision: 28 June 2002 Bibliography Int J Sports Med 2003; 24: 162±165 Georg Thieme Verlag Stuttgart ´ New York ´ ISSN 0172-4622

Experimental setup and protocol Calibration of the gas analyzer was performed before the tests by entering the dead space of the mask and LRV, barometric pressure, the room temperature and humidity. The subjects always inspired through the snorkel, to ensure that any resistance due to the snorkel was present for all phases of the experiment. When the LRV was connected to the lower end of the snorkel, the subjects expired out of the snorkel and no expired air was trapped in the snorkel. However, when the LRV was connected to the upper end of the snorkel, expired air (165 ml) remained in the snorkel and was re-inspired with the next breath (Figs. 1, 2, 3). Samples of the expired air were taken from the mask via a sample line and passed to the analyzer.

Fig. 1 The low-resistance two-way non-rebreathing valve attached to the mask. There is no additional dead space. This mechanism was used in the steps of RS[±] and ES[±].

Fig. 2 The low-resistance two-way non-rebreathing valve attached to the upper end of snorkel. There is additional dead space. This mechanism was used in the steps of RS[+] and ES[+].

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The experiment was carried out using a 4 step protocol. All subjects participated in all steps of the experiment. The steps (RS, ES, RS[+] and ES[+]) were as follows: RS[±]; subjects inspired through the snorkel but expired out through the LRV placed on the mouth piece, hence there was no additional dead space due to the snorkel, measurements done at rest, in the sitting position, ES[±]; subjects had the same snorkel set up but measurements were done during light exercise. RS[+]; subjects inspired and expired through the snorkel (dead space of the snorkel added), measurements were done at rest in sitting position, ES[+]; subjects also inspired and expired through the snorkel but measurements were done during light exercise which consisted of walking on the Quinton 65 Treadmill at a speed of 5 km/h for 15 minutes while the incline level was zero. The stages RS and ES made up the control group for comparison of results between the dead space and non dead space trials. Statistics The averages of the ventilatory values and metabolic measurements, which were recorded for 15 minutes at every stage, were used for statistical evaluation. The average values of RS and ES were compared with the average values of RS[+] and ES[+]

Training & Testing

Equipment and laboratory facilities The snorkel used in the experiment was a standard monofin swimmer snorkel, with a volume of 165 ml which was measured by filling the tube with water. The length of the ªJº shaped tube was 45 cm and the inner diameter was 2.1 cm. The subjects breathed through a ªRudolph Mask 2 way 8931º, which is a face mask either connected to a low-resistance two-way non rebreathing valve (LRV) or to the snorkel. Two adaptor pieces were used to connect the snorkel directly to the mask or LRV, thus it was possible to connect the LRV to both ends of the snorkel. Routine spirometry was performed using the 2400 Sensor Medic Spirometer. The 2900 C Sensor Medics gas analyzer was used for metabolic measurements. Metabolic measurements were performed for 15 minutes in ªbreath by breathª mode during all stages of the experiment. The parameters recorded both at rest and during light exercise for 15 minutes were; VE(BTPS) (minute ventilation at body temperature and ambient pressure, L/min), RR (respiratory rate, BPM), TV (tidal volume, L), VÇO2 (oxygen consumption, ml/min), VÇO2/kg (oxygen consumption per kilogram body weight, ml/kg/min), VÇCO2 (carbon dioxide production, ml/min). All subjects were monitored by ECG for cardiac function and heart rate during the experiment using the Quinton 5000 exercise testing system.

Fig. 3 Subject is at rest, the low-resistance two-way non-rebreathing valve attached to the upper end of snorkel. There is additional dead space. (RS[+]).

Toklu AS et al. Rebreathing While Snorkelling ¼ Int J Sports Med 2003; 24: 162 ± 165

Table 1

Changes in ventilatory values and metabolic measures when additional dead space is not added or added at rest (RS[±], RS[+]). *The ÇO2/kg and V ÇCO2 are significant (p > 0.05) ÇO2, V differences in VE(BTPS), TV, V

Variables

RS

RS [+]

Increase (%)

Asymp. Sig. (2-tailed)

VE(BTPS)

14.13 2.64

17.10 3.28

%21

0.008*

RR

18.17 2.59

18.58 3.53

%2

0.368

TV Ç O2 V

0.79 0.16

0.94 0.16

%19

0.003*

312.42 81.04

512.58 132.73

%64

0.002*

ÇO2/kg V ÇCO2 V

Training & Testing

Table 2

4.45 0.71

7.51 2.13

%69

0.002*

259.42 80.44

393.83 100.51

%52

0.004*

Changes in ventilatory values and metabolic measures when additional dead space is not added or added during light exercise (ES[±], ÇO2/kg and V ÇCO2 are significant (p > 0.05) ÇO2, V ES[+]). *The differences in VE(BTPS), TV, V

Variables

ES[±]

STEPS

ES [+]

Increase (%)

Asymp. Sig. (2-tailed)

VE(BTPS)

31.64 5.52

36.91 4.76

%17

0.002*

RR

25.83 4.82

26.67 4.87

%3

0.178

TV Ç O2 V

1.28 0.35

1.43 0.30

%12

0.009*

991.50 290.44

1344.25 225.45

%36

0.002*

13.98 2.91

19.37 2.39

%39

0.002*

805.67 241.22

1054.50 187.69

%31

0.002*

ÇO2/kg V ÇCO2 V

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STEPS

respectively using Wilcoxon Signed Ranks Test and SPSS for Windows, Release 10.0.5. The p < 0.05 was considered statistically significant.

spired air is 5 % and the average TV is 500 ml, the percentage of the CO2 in inspired air would be 1.6 % as seen below; (165X5)/100 = 8.5 and (8.25 100)/500 = 1.65

Results Table 1 and Table 2 show the differences between RS[±] and RS[+], ES[±] and ES[+] respectively. The addition of a dead space, i. e. expiring and inspiring through the snorkel (RS[+] and ES[+]), caused an increase in VE, RR, TV, VÇO2, VÇO2/kg and VÇCO2, both at rest and during light exercise. The percentages of the increase in VE, RR, TV, VÇO2, VÇO2/kg and VÇCO2 are shown in the tables. These increases were statistically significant (p < 0.05) except the increase in RR, which was very small and not statistically significant. All subjects showed increased minute ventilation when they re-inspired the expired gas trapped in the snorkel at rest or during exercise (RS[+], ES[+]). The increase in VE was achieved by increasing TV and RR remained almost unchanged.

Discussion It is clear that re-breathing the air trapped in the snorkel will cause an increase in CO2 level of inspired air. It is well known that CO2 plays an important role in the regulation of ventilation. Any cause which tends to increase the CO2 level in inspired air will result in changes in ventilation to limit the alveolar and arterial CO2 pressures. The volume of the snorkel used in this experiment was 165 ml. If we consider that the CO2 level in the in-

However, in our study the average TV was around 800 ml. In this case we expect the increase in CO2 level in the inspired air to be approximately 1 %. Several studies have been performed for detecting ventilatory and metabolic changes in conditions where subjects were exposed to higher levels of CO2 [3, 4, 5]. In these studies ventilation increased when the concentration of CO2 in the inspired gas was increased. When a gas mixture containing CO2 is inhaled, the alveolar PCO2 rises, elevating the arterial PCO2 and stimulating ventilation. In our study, VE was increased both at rest and during exercise, when the subjects expired through the snorkel. The changes in VE were obtained by increasing TV rather than increasing RR, which increased slightly. This finding is in accordance with the study of SÏmejkal et al. [8], in which they did not detect any increase in RR when their subjects breathed through a tube. The average RR in our study was slightly higher than the average of the normal population at rest. We attributed the difference in RR to the use of a face mask during the experiment. Adding extra dead space also caused some increases in VÇO2, VÇO2/kg and VÇCO2. The oxygen consumption and carbon dioxide production were increased because the work that was performed by the respiratory muscles was increased in order to increase TV. Oxygen cost of breathing can be considered as the metabolic representation of ventilatory energy expenditure [7]. It is elevated during hyperpnea [1] and inspiratory loaded breathing [2]. It is therefore likely that the feeling of performing


Acknowledgements This work was financially supported by the Turkish Underwater and Hyperbaric Medicine Society.

References 1

Aaron EA, Johnson BD, Seow CK, Dempsey JA. Oxygen cost of exercise hyperpnea: measurement. J Appl Physiol 1992; 72: 1810 ± 1817 2 Cala SJ, Wilcox P, Edyvean J, Rynn M, Engel LA. Oxygen cost of inspiratory loading: resistive vs elastic. J Appl Physiol 1991; 70: 1983 ± 1990 3 Cha EJ, Sedlock D, Yamashiro SM. Changes in lung volume and breathing pattern during exercise and CO2 inhalation in humans. J Appl Physiol 1987; 62: 1544 ± 1550 4 Ellingsen I, Liestùl K, Sydnes G, Hauge A, Nicolaysen G. Arteriel P CO2 and lung ventilation in man exposed to 1 ± 5 % CO2 in the inspired gas. Acta Physiol Scand 1987; 129: 269 ± 276 5 Ellingsen I, Sydnes G, Hauge A, Zwart JA, Liestùl K, Nicolaysen G. CO2 sensitivity in humans breathing 1 or 2 % CO2 in air. Acta Physiol Scand 1987; 129: 195 ± 201 6 Jacobi MS, Patil CP, Saunders KB. Transient, steady-state and rebreathing responses to carbon dioxide in man, at rest and during light exercise. J Physiol (Lond) 1989; 411: 85 ± 96 7 Milic-Emili J, Petit JM. Mechanical efficiency of breathing. J Appl Physiol 1960; 15: 359 ± 362 8 Ï Smejkal V, Vµvra J, Bartµcovµ L, Kryl L, PalecÂek F. The pattern of breathing and ventilatory response to breathing through a tube and to physical exercise in sport divers. Eur Appl Physiol 1989; 59: 55 ± 58

Training & Testing

better when the swimmers exhale into the water, instead of rebreathing expired air in the snorkel is not purely subjective. The use of a snorkel during swimming causes the rebreathing of some expired air. The CO2 level in the inspired air while breathing through the snorkel is high enough to make some changes in metabolic activity and pattern of breathing. Rebreathing through the snorkel causes an increase in ventilation and work of breathing. Therefore oxygen consumption and carbon dioxide production are increased during snorkelling not only as a result of the work of snorkelling itself, but also as a result of the higher concentration of CO2 in the inspired air. This study was performed in laboratory conditions. When snorkelling in water, it is more likely that immersion will also have an effect on the work of breathing, which will increase the energy expenditure. The main concern of the study was to investigate the effect of the dead space added by using a snorkel. Therefore we also performed the tests while the subjects exercised, thus increasing the energy expenditure, as might be expected in snorkelling in water. The work of breathing can be decreased by preventing the expired air from being trapped in the snorkel, thus avoiding the increase in CO2 in inspired air. A new snorkel design with a low-resistance two-way non-rebreathing valve near the mouth piece will allow swimmers to exhale into the water instead of exhaling into the snorkel, reducing the energy expenditure of the activity.

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