Article, Emergency Medicine

Measuring the physiological impact of extreme heat on lifeguards during cardiopulmonary resuscitation. Randomized simulation study

Journal logoUnlabelled imagePhysiological impact of ex”>American Journal of Emergency Medicine 38 (2020) 2019-2027

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American Journal of Emergency Medicine

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Measuring the physiological impact of extreme heat on lifeguards during cardiopulmonary resuscitation. Randomized simulation study

Roberto Barcala-Furelos a,b,c, Maria Fernandez-Mendez b,c,d,?, Francisco Cano-Noguera e, Martin Otero-Agra a,

Ricardo Moran-Navarro e, Santiago Martinez-Isasi b,c

a Faculty of Education and Sport Sciences, REMOSS Research Group, Universidade de Vigo, Pontevedra, Spain

b CLINURSID Research Group, Nursing Department, Universidade de Santiago de Compostela, Santiago de Compostela, Spain

c Santiago de Compostela’s Health Research Institute (IDIS), Santiago de Compostela, Spain

d School of Nursing, REMOSS Research Group, Universidade de Vigo, Pontevedra, Spain

e Faculty of Sport, Universidad de Murcia, Murcia, Spain

a r t i c l e i n f o

Article history:

Received 19 April 2020

Received in revised form 13 June 2020 Accepted 13 June 2020

Keywords:

Cardiopulmonary resuscitation Hyperthermic environment Oxygen uptake

Loss body fluid Exertion Temperature

a b s t r a c t

Objective: Lifeguard teams carry out their work in extremely hot conditions in many parts of the world. The aim of this study was to analyze the impact of high temperatures on Physiological parameters during cardiopulmonary resuscitation (CPR).

Method: A randomized quasi-experimental cross-over design was used to test physiological lifesaving demands (50 min acclimatization +10 min CPR) in two different thermal environments: Thermo-neutral environment (25 ?C) vs Hyperthermic environment (37 ?C).

Results: The data obtained from 21 lifeguards were included, this covers a total of 420 min of resuscitation. The CPR performance was constantly maintained during the 10 min. The Oxygen uptake (VO 2) ranged from 17 to 18 ml/min/kg for Chest compressions and between 13 and 14 ml/min/kg for ventilations (V) at both 25 ?C and 37 ?C, with no significant difference between environments (p > 0.05). The percentage of maximum heart rate (%HR max) increased between 7% and 8% at 37 ?C (p < 0.001), ranging between 75% and 82% of HR max. The loss of body fluids (LBF) was higher in the hyperthermic environment; LBF: (37 ?C: 400 +- 187 g vs 25 ?C: 148 +- 81 g, p < 0.001). Body temperature was 1 ?C higher at the end of the test (p < 0.001). The perceived fatigue (RPE) increased by 37? an average of 2 points on a scale of 10 (p = 0.001).

Conclusions: Extreme heat is not a limiting factor in CPR performance with two lifeguards. Metabolic consump- tion is sustained, with an increase in CC, so V can serve as active rest. Nevertheless, resuscitation at 37 ?C results in a higher HR, is more exhausting and causes significant loss of fluids due to sweating.

(C) 2020

Introduction

Exposure to high temperatures is more lethal than other weather- related phenomena [1]. Heat-related injuries are multifactorial and range from worsening of chronically ill patients [2] to increased drown- ing rates from beach and pool use, thus producing a direct relationship between drowning and temperature [3].

In aquatic environments, lifeguards are the professionals in charge of Emergency First Response [4]. Most lifeguards work during the summer period when temperatures are maximum. One of the main conse- quences of climate change is the increase in temperature as well as

* Corresponding author at: School of Nursing, University of Vigo, Campus de Pontevedra, C/Joaquin Costa 41, CP: 36004 Pontevedra, Spain.

E-mail addresses: [email protected] (R. Barcala-Furelos), [email protected] (M. Fernandez-Mendez), [email protected] (M. Otero-Agra), [email protected] (S. Martinez-Isasi).

the extension of these periods [5]. In this situation, lifeguards are exposed to a far greater workload, both for longer periods and at signif- icantly higher temperatures. Living in areas with temperatures above 35 ?C or working in environments above 32 ?C are zones considered to have a high ambient temperature [6] and this is common for many lifeguards around the world.

Cardiopulmonary Resuscitation (CPR) is one of the competencies of lifeguards [7,8]. On many occasions, this CPR can be prolonged due to the response time of the Emergency Medical Services (EMS) in difficult-to-access environments (such as long, remote beaches) [9] or delays due to the strong demand for EMS during extreme heat [1]. In ad- dition to maintaining resuscitation as long as necessary until the arrival of EMS, another objective of rescuers is to provide quality maneuvers [10]. CPR generates fatigue which may increase, either because of the duration or the previous conditions [11-14]. Exhaustion during CPR may be accelerated by thermo-environmental variations that cause cer- tain body changes; for instance increased heart rate, body temperature,

https://doi.org/10.1016/j.ajem.2020.06.042

0735-6757/(C) 2020

R. Barcala-Furelos, M. Fernandez-Mendez, F. Cano-Noguera et al. American Journal of Emergency Medicine 38 (2020) 20192027

blood pressure and dehydration [15,16]. There is a lack of knowledge about how this thermal phenomenon can physiologically affect rescuers during resuscitation. The purpose of this study is to understand the in- fluence of thermo-variability on metabolic consumption, perceived fa- tigue and dehydration in prolonged resuscitation by rescuers.

Method

Study design

A quasi-experimental randomized crossover study design was used to test the difference in metabolic intake, rate of perceived exertion and body fluid lost during a 10 min CPR simulation in a thermo-neutral envi- ronment (25 ?C) versus (vs) a hyperthermicenvironment (37 ?C)(Fig. 1).

Sample

A convenience sample from the Murcia region of Spain participated in this study. The inclusion criteria were that they should be profes- sional lifeguards, updated according to the recommendations of the

European Council on Cardiopulmonary Resuscitation (ERCGR2015) [10], should not present any physical or psychological contraindication to carrying out the study and should authorize their participation in written consent. The final sample was 21 rescuers (14 men, 7 women). The general characteristics were: weight 76 +- 18 kg, height 172 +- 9 cm. The average age was 27 +- 6 years.

This study respected the ethical principles of the Helsinki Conven- tion. Each participant authorized in writing the transfer of his or her data which was needed for this study and it was subsequently approved by the ethics committee of the Faculty of Education and Sport Sciences, (University of Vigo, Spain), with code 11-2802-18.

Roller refresher

Before the intervention, in order to standardize skills and become fa- miliar with the manikin and bag-valve mask (BVM), a 15-minute CPR roller refresher was performed [17,18] according to ERCGR2015 recom- mendations [10]. This training was conducted by an instructor accredited by the Spanish Society of Intensive and Critical Medicine and Coronary Units.

Image of Fig. 1

Fig. 1. Study lifeguards’ CONSORT flow diagram.

2020

R. Barcala-Furelos, M. Fernandez-Mendez, F. Cano-Noguera et al.

American Journal of Emergency Medicine 38 (2020) 20192027

2021

Fig. 2. Research protocol and phases before, during and after CPR test. CPR: cardiopulmonary resuscitation; CC: chest compressions; V: ventilations; RPE: Rating of perceiving exertion.

Controlled CPR test

Two CPR tests in the sequence 30 chest compressions (CC) and 2 ventilations (V), with duration of 10 min were compared:

  • A 25 ?C Test (T25?C): Consisting of a Thermo-Neutral environment with a uniform temperature of 25 ?C [19].
  • A 37 ?C Test (T37?C): Consisting of a Hyperthermic environment at a temperature of 37 ?C based on the lower range of annual extreme temperatures recorded in the Murcia region of Spain from 1975 to 2018 [20]. This area of the Mediterranean covers 274 km of coastline and is called the “Costa Calida (Warm Coast)”, boasting over 3000 h of sunshine per year, plus an average of 107 days with extreme tem- peratures >30 ?C [20].

The thermo-environmental simulation was carried out in the Physi- ology Laboratory (Faculty of Sport Sciences in Murcia, Spain). Test T25?C was performed in a thermo-neutral room and T37?C was performed in a 12 m2 space conditioned for thermal variability. The temperature was monitored using a thermometer and a TFA Dostmann 45.2028hygrom- eter (Wertheim, Germany).

Experimental procedure and materials

Physiological adaptation

The lifeguards accessed the laboratory, where a team of two nurses took anthropometric measurements (height, weight and body temper- ature). After the measurements, the rescuers waited 50 min for the CPR test (T25?C or T37?C) to begin. This time was used for physiological ad- aptation to the environmental conditions. During this period, they were instructed to behave in a similar way to watching from a high chair in a lifeguard tower. They were allowed to drink, although no lifeguard dur- ing this period chose to rehydrate or have a refreshment, neither in T25?C nor in T37?C.

2.5.3. CPR test (Supplementary video online)

After the acclimatization period of 50 min, a 10-min CPR test was performed on a Laerdal ResusciAnne(R) manikin (Stavanger, Norway) programmed according to ERCGR2015(10). Resuscitation variables were recorded with Laerdal Medical’s QCPR SkillReporter software (Sta- vanger, Norway). Ambu(R) Mark IV adult BVM (Ballerup, Denmark) was used to administer V. For both tests, the following sequence was established: 2 min of CC [carried out 3 times: Round 1, Round 3 and Round 5] – 2 min of V with BVM [carried out 2 times: Round 2 and Round 4] (Fig. 2). The resuscitation team was completed by an expert instructor. This instructor offered no feedback or additional instructions and his only function was to support the lifeguard being investigated in the CPR tandem. Only the single lifeguard outcomes were analyzed in the study. Analyzing only one of the providers was based on the meth- odology used in a previous resuscitation study carried out in special cir- cumstances [21]. In order to avoid fatigue bias, the second test was distanced by an average of 24 h.

Variables

Cardiopulmonary resuscitation

The following formula was used to evaluate the quality (Q) of the CC: Q-CC (%) = [CC with the correct depth (between 5 and 6 cm) as a percentage + CC with the correct rhythm (between 100 and 120 com- pressions per minute) as a percentage + CC with the complete re- expansion of the chest as a percentage]/3. These parameters are based on the ERCGR2015 [10] recommendations. Furthermore, the mean rate (MR) of CC and the total number of CC (TCC) at the end of each test were also recorded.

For the airway management analysis, V was taken as a reference for the insufflations with effective air intake. Effective ventilation (EV) is

defined as that which achieves air intake with visible chest elevation and electronic recording of the measurement software. The formula used calculated the percentage based on the number of effective venti- lation (NEV) with the number of total ventilation attempts (NTV). The numerical expression is EV (%) = (NEV x 100) / NTV.

Both the formula for analysis of Q-CC (%) and EV (%) are based on previous studies [13,22].

  • Physiological variables

    Percentage of maximum heart rate during CPR (%HRmax). The heart rate was measured with a Polar HR Bluetooth H7 sensor (Kempele, Finland), monitored in real-time. To calculate the maximum HR we used the formula of Karvonen, Kentala, and Mustala: HRmax = 220 – age for men and HRmax = 226 – age for women [23].

  • Oxygen uptake CPR (ml/min/kg) (VO2). This variable was recorded using breath-by-breath indirect calorimetry (Cortex Metalyzer 3B, Leip- zig, Germany), which was calibrated before each test. In the analysis of the data, 5 Rounds were stratified, into which each test was divided: Round 1-CC, Round 2-V, Round 3-CC, Round 4-V, Round 5-CC.
  • Loss of body fluid (LBF). For the calculation of the LBF, the body weight was recorded before and after the test. The Tanita MC780MA high-precision multi-frequency segmental scale (Tanita Corporation, Tokyo) [24] was used.
  • Body temperature. It was estimated in degrees Celsius (?C) at the tympanic level and was based on the average of three continuous mea- surements, before and after the CPR. In T37?C a basal measurement was added (in a thermo-neutral environment) before the acclimatization period to the extreme temperature, so as to know the variability during the 50 min. of physiological adaptation. All temperature measurements were made with the Type FT-65 Beurer GmbH (Ulm, Germany).
  • Rating of perceived exertion (RPE). At the perceptual level, the Modified scale of perceiving effort (RPE) [25] was recorded (measure- ment of the range 0/10 – rest/maximal). Previously, the lifeguards were trained in the understanding and use of this scale.

    Statistical analysis

    All statistical analyses were performed with SPSS for Windows, ver- sion 20 (SPSS Inc., IBM, USA). The Shapiro-Wilk test was used to evalu- ate the normality of the data. The variables of age, weight, and height were described with measures of central tendency (mean) and disper- sion (standard deviation and 95% confidence interval). A Student t- test for related samples was performed to compare the parametric var- iables and the Wilcoxon range-sum test for related samples for non- parametric variables. The repeat measures test ANOVA with Bonferroni correction was used for the moment comparisons (CC and V). In all tests, a significance value of 0.05 was established. Cohen’s test for com- parisons in parametric tests and Rosenthal’s test in non-parametric tests were used to measure the effect size. The classification for the effect size was as follows: Trivial (<0.2); Small (0.2-0.5); Moderate (0.5-0.8); Large (0.8-1.3); Very large (>1.3).

    Results

    Ambient conditions description

    The atmospheric pressure during the laboratory tests was: T25?C: 1022 mmHg vs T37?C: 1021 mmHg; p = 0.005, ES = 0.43 and humid- ity; T25?C: 48% vs T37?C: 46%; p = 0.02, ES = 0.93. The hyperthermic environment was significantly warmer than the thermo-neutral envi- ronment (p < 0.001, ES = 10.93).

    Table 1

    CPR data.

    T25?C

    T37?C

    p value

    Mean +- SD

    95%[CI]

    Mean +- SD

    95%[CI]

    Q-CC (%)

    67 +- 15

    [60-73]

    62 +- 20

    [53-72] p = 0.41a

    MR

    115 +- 6

    [112-117]

    114 +- 8

    [111-118] p = 0,96b

    TCC

    568 +- 43

    [549-588]

    566 +- 48

    [544-588] p = 0.84a

    EV (%)

    96 +- 14

    [89-102]

    91 +- 17

    [83-98] p = 0.16b

    Q-CC%: Quality of Chest Compression in percentage. MR: Mean Rate. TCC: Total Chest Compression. EV (%) Effective Ventilations in percentage.

    Significance level = 0.05.

    a Student t-test for related samples.

    b Wilcoxon test for related samples.

    CPR

    The results of CPR in both environments are shown in Table 1. No significant differences (p > 0.05) were observed in the comparison of the two thermal environments in any of the variables. The Q-CC exceeded 60% during the 10 min (T25?C: 67% and T37?C: 62%; p = 0.41). In the round to round analysis, no significant differences were found, although Fig. 3 shows a slightly lower trend (5%) in the Q-CC in the T37?C environment and time (Round 1 vs Round 5). In the EV anal- ysis (%), it also shows a lower trend of 5% for the T37?C environment in the two ventilated rounds (Round 2 and Round 4).

    VO2 (ml/min/kg) during CPR

    Table 2 shows the results of the Oxygen uptake in every environ- ment and round. The VO2 showed no significant difference (p > 0.05) in the comparison between T25?C vs T37?C, neither in the CC rounds nor the V rounds. During the CC rounds, the VO2 ranged from 17 to 18 ml/min/kg, and from 13 to 14 ml/min/kg for the V rounds (Fig. 3). The VO2 during CC was significantly higher than the V (p < 0.01, ES >= 0.89) at both T25?C and T37?C.

    %HRmax

    In the analysis of the %HRmax variable, the lifeguards at T25?C were between 70% (Round 2) and 75% (Round 5) and for T37?C, they were be- tween 77% (Round 2) and 82% (Round 5). In both cases, the lowest value coincides with the first round of V and the highest one with the last round of CC. Performing CPR at 37 ?C caused a significant increase of % RH 7-8% (Fig. 3) in all rounds (1 to 5) compared to T25?C; p <= 0.001, ES >= 0.83 (Table 2). The CPR implied a higher %HR demand especially in the comparison of the first V cycle with the last CPR cycle at T25?C (Round 2: 70 +- 10 vs Round 5: 75 +- 9; p = 0.02, ES = 0. 53) and be- tween the first cycle of V and the second and third cycle of CC at T37?C (Round 2: 77 +- 8 vs Round 3: 81 +- 8; p < 0.001, ES = 0.44 &

    Round 2: 77 +- 8 vs Round 5:82 +- 8; p < 0.001, ES = 0.63).

    Thermo-regulation

    The LBF variable was obtained by weighing the lifeguards before and after the test. In T37?C a significant decrease was recorded compared to T25?C (T37?C: 400 +- 187 vs T25?C: 148 +- 81 g; p < 0.001, ES = 0.59).

    After 50 min. of acclimatization, body temperature was taken just be- fore starting the 10 min. of CPR. In the warm environment, the rescuers started the test with a higher temperature (T25?C: 36.4 +- 0.4 ?C vs T37?C: 37.2 +- 0.6 ?C; p < 0.001, ES = 1.57) and after the test, the in- crease reached 1 ?C (T25?C: 36.5 +- 0.5 ?C vs T37?C: 37.5 +- 0.7 ?C;

    p < 0.001, ES = 1.71).

    RPE

    The perception of fatigue was higher at T37?C (CPR hard) than at T25?C (CPR somewhat hard) with a large effect size (T37?C: 6 +- 2 vs T25?C: 4 +- 2; p = 0.001, ES = 0.81) (Fig. 4).

    Image of Fig. 3

    Fig. 3. Integrated view of physiological variables and CPR. CPR: cardiopulmonary resuscitation; EV: effective ventilation; %HR: percentage of heart rate; Q-CC: quality of chest compression; VO2: Oxygen uptake.

    Table 2

    Physiological variables in T25?C and T37?C in 5 rounds.

    Round 1 Round 2 Round 3 Round 4 Round 5 Repeated measures ANOVA

    Mean

    +- SD

    95% [CI]

    Mean

    +- SD

    95% [CI]

    Mean

    +- SD

    95% [CI]

    Mean

    +- SD

    95% [CI]

    Mean

    +- SD

    95% [CI]

    Rounds

    (1 vs 2 vs 3 vs 4 vs 5)

    VO2

    L/min/kg

    T25?C 17 +- 4 [16-19] 13 +- 2 [12-14] 17 +- 4 [15-19] 13 +- 3 [12-15] 17 +- 3 [15-18] 1 vs 2 < 0.001

    (1.25) Large

    1 vs 3 = 1.00 (-)

    1 vs 4 < 0.001

    (1.19) Large

    1 vs 5 = 1.00 (-)

    2 vs 3 < 0.001

    (1.11) Large

    T37?C 17 +- 4 [15-19] 14 +- 2 [13-15] 17 +- 3 [16-19] 14 +- 3 [13-15] 17 +- 3 [15-18] 1 vs 2 = 0.003

    (0.92) Large

    1 vs 3 = 1.00 (-)

    1 vs 4 = 0.010

    (0.89) Large

    1 vs 5 = 1.00 (-)

    2 vs 3 < 0.001

    (1.23) Large

    2 vs 4 = 1.00 (-)

    2 vs 5 < 0.001 (1.14)

    Large

    3 vs 4 = 0.001 (1.05)

    Large

    3 vs 5 = 1.00 (-)

    4 vs 5 < 0.001 (1.06)

    Large

    2 vs 4 = 1.00 (-)

    2 vs 5 = 0.002 (1.01)

    Large

    3 vs 4 < 0.001 (1.19)

    Large

    3 vs 5 = 1.00 (-)

    4 vs 5 = 0.007 (0.98)

    Large

    T25?C vs T37?C

    p = 0.50 p = 0.25 p = 0.77 p = 0.32 p = 0.76

    % HR max. T25?C 71 +- 10 [66-76] 70 +- 10 [65-75] 73 +- 11 [68-79] 71 +- 9 [66-76] 75 +- 9 [70-79] 1 vs 2 = 0.30 (-)

    1 vs 3 = 0.94 (-)

    1 vs 4 = 1.00 (-)

    1 vs 5 = 0.13 (-)

    2 vs 3 = 0.13 (-)

    T37?C 78 +- 8 [74-82] 77 +- 8 [73-81] 81 +- 8 [77-85] 78 +- 8 [74-83] 82 +- 8 [76-86] 1 vs 2 = 0.32 (-)

    1 vs 3 = 0.001

    (0.36) Small

    1 vs 4 = 1.00 (-)

    1 vs 5 = 0.002

    2 vs 3 < 0.001

    (0.44) Small

    2 vs 4 = 1.00 (-)

    2 vs 5 = 0.020 (0.53)

    Moderate

    3 vs 4 = 0.79 (-)

    3 vs 5 = 0.89 (-)

    4 vs 5 = 0.048 (0.41)

    Small

    2 vs 4 = 0.50 (-)

    2 vs 5 < 0.001 (0.63)

    Moderate

    3 vs 4 = 0.15 (-)

    3 vs 5 = 1.00 (-)

    4 vs 5 = 0.09 (-)

    T25?C vs T37?C

    p = 0.001

    (0.86) Large

    p = 0.001

    (0.83) Large

    p = 0.001

    (0.88) Large

    p < 0.001

    (0.88) Large

    p < 0.001

    (0.88) Large

    VO2: Oxygen uptake consumption in L/min/kg. % HR: Heart rate in percentage.

    ANOVA: significance level p = 0.05. Effect Size: Cohen test: Trivial (<0.2); Small (0.2-0.5); Moderate (0.5-0.8); Large (0.8-1.3); Very large (>1.3).

    Discussion

    The purpose of this study was to analyze the physiological demands and perceived fatigue in CPR in a high-temperature situation, such as those that may be faced by rescuers in many parts of the world during the summer.

    The main findings of this study were; a) CPR performance was not affected by extreme heat, b) Physiological demands for oxygen con- sumption and heart rate were higher in CC than in V, and had a higher tendency in the hyperthermic environment, c) Perceived effort during CPR at high temperatures was higher (hard vs somewhat hard) and,

    d) Body fluid loss without rehydration was significant for only 1 h at 37 ?C.

    Resuscitation is a vital technique in the event of cardiac arrest, and for this reason, ERCGR2015 to maintain a high standard of quality pro- poses relays every 2 min [10], which is why in our study it was per- formed with two rescuers, exchanging the role (V and CC) every 2 min. As a general rule, resuscitation maneuvers generate fatigue, which will decrease the quality levels in the case of being prolonged

    [12] and will require high physiological demands [11]. However, neither the duration of resuscitation nor the thermovariability affected the CPR performance in this research. An explanation for this phenomenon could be related to the individual VO2max of each rescuer. Lifeguards are physically trained professionals [26] and their VO2max may reach 53/55 ml/min/kg [26-28], which implies that metabolic demand during CPR may be relatively low [29]. The average VO2 consumption during the test was 15 ml/min/kg (17 for CC, 13 for V) with little variation in each cycle. These data are in accordance with the study by Sousa et al.

    [30] in which rescuers in a basal physiological state reached values of around 16 ml/min/kg without great variations for 4 min. Using these values as a reference, theoretically, rescuers can resuscitate at less than 30% of their VO2max which is not the case with other rescuer pro- files which, with less physically demanding protocols, require a higher percentage of VO2max to perform the same activity. Pierce et al. [29] found in a group of sedentary individuals that the oxygen uptake was 26 ml/min/kg (54% of their VO2max) for 10 min. and in the study by Elvira et al. [11] Healthcare staff the VO2 during a 16 min CPR test was 14 ml/min/kg (45% of their VO2max).

    The analysis of VO2 has been carried out in several studies on CPR [11,29-32], however, to our knowledge, this is the first study with life- guards resuscitating in teams, which disaggregates the metabolic con- sumption of the CC and V phases. This is relevant because it has been suggested that the CC are the part with the greatest physiological de- mand during CPR [14,32,33], although the evidence related to this idea is limited [34].

    The VO2 was significantly higher during CC and also in HR (%), with a higher trend in T37?C. Two ideas converge here: 1) Compressions are the part of CPR that produce the most muscle mobilization [35] and therefore the most associated muscle fatigue [29]. The metabolic cost of CC has metabolic equivalents with activities of moderate physical in- tensity, such as recreational cycling or swimming [33]. 2) The decrease in VO2 and HR (%) during ventilation may be because this phase in- volves a period of active rest [34], although Hong et al. [34] found a con- tinuous increase and higher HR values during the ventilation phase for 10 min., possibly because their study was performed with a single res- cue, while ours was performed in a team of two.

    Fig. 4. Thermo-regulation and effort charts. LBF: Loss of body fluid; RPE: Rating of perceiving effort.

    Image of Fig. 4

    Concerning the thermovariability, higher values of HR (%) were re- corded at T37?C. The increase in HR should be analyzed along with the RPE and the decrease in weight due to the loss of body fluid.

    There is an association between temperature and increased HR [36],

    and in turn between increased HR and higher RPE [37] and finally be- tween dehydration and early heat fatigue [36]. This physiological re- sponse is consistent with the highest RPE reported by rescue workers at T37?C. Following the values of Foster et al. [25], in the hyperthermic environment, they perceived CPR as “hard”, while in the thermoneutral environment it was “somewhat hard”. RPE using perceived stress scales is a widely used tool with high validity to quantify exercise intensity

    [37] and is commonly used in resuscitation studies with rescuers [13,30,38,39].

    During exercise, numerous physiological mechanisms of heat loss are activated to prevent excessive core temperature increases, but in hot and humid environments this can be a challenge in body thermo- regulation [40]. When the air temperature exceeds 36 ?C, the human body gains heat through radiation and convection, therefore the ther- moregulatory mechanism will be evaporation [41]. In adverse environ- ments above 35 ?C, physical activity can cause losses of up to 1.5 l per hour [36,42]. Our results show a loss of 400 g in 1 h, after a period of ac- climatization of 50 min and 10 min of exercise, and although CPR could be a low-impact activity for trained rescuers, prolonged low-intensity exercise causes an increase in body temperature and HR [36]. VO2 and

    %HR suggest that CPR is an aerobic activity, and the literature shows that performance deficit in aerobic activities occurs with dehydration of 3% of total body water or a loss of 2% of body mass index (BMI) [43]. A longer resuscitation time, higher temperature or a previous state of dehydration may be a limiting factor in maintaining CPR perfor- mance. During heat exercise, sweat output usually exceeds water in- take, hence the body water deficit [42].

    Although they were able to hydrate themselves, none of the life-

    guards in this study needed to drink. This is not uncommon, as the sen- sation of thirst does not occur until 2% of BMI is lost [42], therefore, the intake of fluid on demand during exercise at high temperatures usually generates some dehydration [41]. Fluid replacement may attenuate the increase in heart rate and body temperature [36].

    Practical implications

    Lifeguards are exposed to interventions in hostile environments or extreme environmental conditions. This study can be used to under- stand the physiological effects related to resuscitation at high tempera- tures and to establish active recovery measures and specific training to achieve better adaptation to the environment. It is necessary to consider adaptive training programs for lifeguards who are exposed to extreme temperatures and rehydration measures so as to maintain water bal- ance. This measure should include fluid intake that coincides with losses [16], and based on this study, it could be set at around 400/500 ml per hour when conditions exceed 35 ?C. A period of adaptation to high tem- peratures should also be taken into account before the start of the beach monitoring season. This adaptation begins within a few days and is completed within one or two weeks [41]. Adaptation to extremely hot environments results in a reduction in core temperature and a decrease in heart rate during physical activity [44] which can lead to greater safety for the lifeguard and improved physical performance. Knowing the metabolic expenditure of each rescue or resuscitation task can pro- vide valuable information for the design of training programs and can manage the role of each rescuer according to their physical capacity.

    Limitations

    This research has been carried out in a laboratory and in a simulated situation with a manikin, hence it does not necessarily represent the characteristics of a real victim nor does it evaluate the psychological im- plications of the rescuer. The VO2max of the participants was not

    calculated, so we cannot reliably know their condition at the time of the research, although they were all frequently trained lifeguards. The participants wore a mask during the CPR test, which could detract from their comfort in applying resuscitation skills. The duration of the tests was 10 min, so longer resuscitations may have different results.

    Conclusions

    Extreme heat is not a limiting factor for CPR performance by life- guards, and metabolic consumption may remain stable during prolonged resuscitation. However, maintaining quality standards means higher cardiac output, more perceived fatigue and significant fluid loss through sweating. This study should serve to promote adap- tive training for rescue teams operating in extreme heat environments and physiological recovery measures after hot workdays.

    Supplementary data to this article can be found online at https://doi. org/10.1016/j.ajem.2020.06.042.

    Funding sources

    This research did not receive any specific grant from funding agen- cies in the public, commercial, or not-for-profit sectors.

    CRediT authorship contribution statement

    Roberto Barcala-Furelos: Conceptualization, Methodology, Investi- gation, Writing – review & editing, Visualization. Maria Fernandez- Mendez: Methodology, Investigation, Writing – original draft, Supervi- sion, Project administration. Francisco Cano-Noguera: Investigation, Resources. Martin Otero-Agra: Formal analysis, Investigation, Data curation. Ricardo Moran-Navarro: Software, Investigation, Resources, Data curation. Santiago Martinez-Isasi: Validation, Investigation, Supervision.

    Acknowledgements

    We would like to thank all the lifeguards, sport science professionals and participants involved in the study.

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