Article, Emergency Medicine

Effects of compression-to-ventilation ratio on compression force and rescuer fatigue during cardiopulmonary resuscitation

Original Contribution

Effects of compression-to-ventilation ratio on Compression force and rescuer fatigue during cardiopulmonary resuscitation?

Chih-Hsien Chi MDa, Jui-Yi Tsou MSb,c, Fong-Chin Su PhDb,?

aDepartment of Emergency Medicine, National Cheng Kung University, Tainan 70403, Taiwan bInstitute of Biomedical Engineering, National Cheng Kung University, Tainan 70101, Taiwan cDepartment of Physical therapy, Fooyin University, Kaohsiung 83102, Taiwan

Received 1 April 2009; revised 25 June 2009; accepted 26 June 2009

Abstract

Introduction: Although increasing consecutive compressions during cardiopulmonary resuscitation (CPR) is beneficial to patients, it possibly affects the workload and, ultimately, the Quality of CPR. This study examines the effects of compression-to-ventilation ratio on external chest compression performance of rescuers.

Methods: Subjects were 17 health care providers. Each participant performed CPR with 3 compression- to-ventilation ratios: 15:2, 30:2, and 50:5. The duration of CPR was 5 minutes in each group, with a rest period of 50 minutes in between. The manikin was equipped with a 6-axis force load cell to measure the force applied. An 8-camera digital motion analysis system was used to collect the 3-dimensional trajectory information. Data were compared using the Crossover design. Ratings of Perceived exertion and body area discomfort were measured.

Results: The mean compression forces (in Newtons) delivered at 1 minute 20 seconds to 1 minute 40 seconds and at 4 minutes 20 seconds to 4 minutes 40 seconds were 494.65 +- 53.58 and 478.64 +- 50.29, respectively (P = .047), for compression-to-ventilation ratios of 15:2; 473.57 +- 49.69 and 435.59 +- 56.79, respectively (P b .001), for ratios of 30:2; and 468.44 +- 38.05 and 442.18 +- 43.40, respectively (P = .012), for ratio of 50:5. Diminished compression force in the ratio 50:5 was observed at 1 minute 20 seconds, and in the 30:2 ratio, it was observed at 4 minutes 20 seconds. The mean joint angles in each group did not differ significantly between 1 minute 20 seconds and 4 minutes 20 seconds. The Ratings of Perceived Exertion Scale was 3.38 +- 1.64 in 15:2, 4.06 +- 1.43 in 30:2, and 4.35 +- 1.54 in 50:5 (P = .045). Waist discomfort was noted in 50:5 after 4 minutes 20 seconds of external chest compression.

Conclusions: Rescuer fatigue must be considered when raising the consecutive compression during CPR. Switching the compressor every 2 minutes should be followed where possible.

(C) 2010

? The authors would like to thank the National Science Council of Taiwan for financially supporting this research under contract no. NSC93- 2320-B-006-066 and NSC94-2320-B-006-041.

* Corresponding author. Tel.: +886 6 2760665; fax: +886 6 2343270.

E-mail address: [email protected] (F.-C. Su).

Introduction

Quality of cardiopulmonary resuscitation (CPR) is a very important factor in increasing survival rates from cardiac arrest [1]. However, the cardiac output during manual

0735-6757/$ – see front matter (C) 2010 doi:10.1016/j.ajem.2009.06.022

external chest compression (ECC) is only 27% to 32% of the normal cardiac output under ideal circumstances [2,3]. Hence, the efficiency of chest compression has become a significant issue in the practice of ECC.

The practice of CPR has changed in the last decade. A major change is in the ratio of compression to ventilation (C/ V). The 2005 guidelines recommend a C/V ratio for adult CPR of 30:2 rather than the previously recommended 15:2 [4,5]. This change was based on various human and animal studies, which indicated that pauses in chest compression for ventilation impair the effects of ECC, subsequently reducing the coronary perfusion, neurologic function, and prognosis of resuscitation [6-9].

An investigation involving hemodynamic and Gas exchange during CPR has suggested that a 50:5 ratio provides better oxygenation (oxygen transport) after 3 to 4 minutes of CPR [10]. Mathematical analysis has indicated that oxygen delivery and oxygen transport improve gradually as the C/V ratio is raised [6,8,9]. One animal study has found that 50:2 ratio provides a better chest compression effect and favorable carotid artery pressure [6]. These studies together imply that the recommended ratio of C/V may change if more evidence can be obtained to indicate that the effectiveness of CPR can be improved. The recommended ratio for adult CPR has recently changed from 15:2 to 30:2 [4]. Another C/V ratio (50:5 or 50:2 or continuous ECC) may be preferred if supported by scientific evidence.

The degree of fatigue and difficulties in performing CPR by rescuers is an important factor when addressing the resuscitation effects of various C/V ratios. Previously obtained data indicate that CPR requires strenuous effort and that the quality of chest compression can decline soon after ECC is started [11-14]. Riera et al [15] found that health professionals can comfortably apply uninterrupted ECC for 2 minutes. Rescuer fatigue occurs after 3 minutes of continuous ECC [16]. Although a 30:2 ratio delivers better chest compression than a 15:2 ratio, it is more exhausting [17].

Although increasing consecutive compressions during cardiopulmonary resuscitation is beneficial to patients, it possibly affects the workload and, ultimately, the quality of CPR. The study examines the effects of C/V ratio on ECC performance of rescuers.

Materials and methods

Study design

This study examines the mean compression force delivered at various C/V ratios. The perceived exertion and fatigue caused by different C/V ratios are analyzed and compared. The mean measured angles of joint movement during ECC are also computed. This experiment was performed with a crossover, randomized-to-order design.

The study was approved by the National Cheng Kung University Hospital’s human ethics committee. All subjects provided written informed consent.

Seventeen emergency medical professionals, composed of 9 emergency medical-technician firefighters and 8 emergency department registered nurses from a university medical center, participated voluntarily in this study. All subjects had current certifications in basic life support and have experienced in prehospital or In-hospital CPR.

No participant had any muscular skeletal injury, sprain, or pain. Participants were not allowed to eat within 30 minutes of the tests. The consumption of alcohol, tea, and coffee was prohibited on the days of the test. The study was conducted at the university’s motion analysis laboratory, Department of Medical Engineering. The participants, who were all experienced emergency health care providers, practiced on manikins before they began the tests.

Each participant performed CPR using 3 C/V ratios, namely, 15:2, 30:2, and 50:5. To avoid stress coordinated with ventilation efforts that might affect the quality of the chest compression efforts, they performed chest compres- sions with breaks stipulated for ventilations. A 5-second pause was incorporated between each set of 15 and 30 ECCs to simulate CPR in actual practice, according to the 2000 and 2005 basic life support (BLS) guidelines, respectively [5,18]. A 12-second pause was incorporated between each group of 50 ECCs. The order of compression ratios was randomized for each study participant. The duration of ECC was 5 minutes for each C/V ratio, with a rest period of 50 minutes in between sessions.

Equipment

The Resusci Anne SkillReporter manikin (Laerdal Medical, Wappingers Falls, NY) was equipped with a 6-axis force transducer (AMTI MC3A-6-1000, Advanced Mechanical Technology, Inc, Watertown, Mass) to measure the compres- sion forces at a sampling rate of 1000 Hz. Quality of chest compressions (compression rate, compression depth, and percent correct compressions) at various compression ratios were recorded by the SkillReporter. Percent correct compres- sions were defined as compression depth of 4 to 5 cm, compression rate of 100 per minute or higher, correct hand position, and complete release of each chest compression.

Response of rescuers and difficulty of performing ECC

physiologic parameters, namely, heart rate, blood pressure, and pulse oxygen saturation, were measured for each study participant before and after each ECC session. Two subjective scales, the perceived exertion (modified Borg scale) and the 10-cm Visual analogue scale (VAS), were used to rate the intensity of the exercise associated with chest compression. Participants were asked to rate how

they felt after each ECC session. The VAS and Borg scales were good subjective scales for reproducibly measuring symptoms in the steady state [18,19]. The ratings of perceived exertion (RPE) were recorded with a modified Borg scale from 0 to 10, with 0 for no exertion at all, 3 for moderate exertion, 4 for somewhat heavy, 5 for heavy, and 10 for extremely strong or almost maximal exertion. The participants also ranked their discomfort in different regions of the body using 10-cm VAS after each ECC. These areas were the neck, wrist, back of hand, palm, fingers, hands, waist, knee, and shoulder. The Visual Analogue Scale was also applied to rate the extent of discomfort in the compression and decompression phases.

Motion analysis

Because joint angles during exercise may affect joint force and moment, we measured angular motions of the joints by 3-dimensional motion analysis. Similar 3-dimen- sional motion analysis has been used to define patterns of joint motion in the studies of various Musculoskeletal problems, especially those related to repetitive motion [20-22]. To record such motions, various passively reflective markers were placed on selected anatomical landmarks on the bodies of each subject. Thirty-seven of these markers were placed on selected anatomical land- marks on the right upper extremity, the head, the trunk, and the bilateral lower extremities of each subject to locate the embedded axis segments [23]. The marker motions were recorded by a video-based motion system with 8 Motion Analysis HiRes cameras (Motion Analysis Corp, Santa Rosa, Calif). This configuration supports high resolution and enables complex motions to be captured. The presented data cover both the compression and decompression phases, and the results are average angles from measurements made during a 20-second period. The positions of the light- reflecting markers in 3-dimensional space were captured by a charge-coupled device and were computed by Motion Analysis EVA software (Motion Analysis Corp), which collected the 2-dimensional camera images and processed the data to 2- or 3-dimensional assessments.

Data analysis

During the 5-minute CPR session, the angle data for the joints and the compression forces were recorded at 1 minute 20 seconds to 1 minute 40 seconds and in the final period (4 minutes 20 seconds to 4 minutes 40 seconds). Motion data (mean) collection in 20-second periods of compression cycles were derived. Descriptive statistics and the repeated- measures analysis of variance (RM ANOVA) were applied to evaluate the variations among compression forces. An estimated sample size of 17 study subjects had an ? of .05 and a power of 0.80, with an SD of 55 N and an expected difference between means of 35 N. The data were analyzed

by SPSS version 13 (SPSS, Chicago, Ill). Statistical significance was set to P b .05 throughout the experiment.

Results

The median age of the participants was 35 +- 5.7 years, and 50% were female. The median weight was 64 +- 14.3 kg, and the median height was 164 +- 7.9 cm. The mean compression forces (in Newtons), delivered at 1 minute 20 seconds to

1 minute 40 seconds and at 4 minutes 20 seconds to

4 minutes 40 seconds, were 494.65 +- 53.58 and 478.64 +- 50.29, respectively (P = .047), for C/V ratio of 15:2; 473.57 +-

49.69 and 435.59 +- 56.79, respectively (P b .001), for 30:2,

and 468.44 +- 38.05 and 442.18 +- 43.40, respectively (P =

.012), for 50:5 (Fig. 1). A pairwise comparison indicates that the ratios 30:2 and 15:2 produced equally good results. However, the forced delivered by the 50:5 group was inferior to that delivered at 15:2 (mean difference, 26.21; P = .024; 95% confidence interval [CI], 3.87-48.56) after 1 minute 20 seconds to 1 minute 40 seconds of ECC. The forces delivered at 15:2, 30:2, and 50:5 varied significantly at 4 minutes 20 seconds to 4 minutes 40 seconds (P = .008, RM ANOVA; observed power, 0.849). Pairwise comparisons of the compression forces at 30:2 and 50:5 indicate that they were inferior to that at 15:2 after 4 minutes 20 seconds of ECC. The 50:5 ratio produced a reduced compression force 2 minutes before the ECC was completed. The force at 30:2 was diminished at 4 minutes 20 seconds but not at 1 minute 20 seconds. A comparison of physiologic variables (Table 1) before and after ECC indicates that heart rate and systolic blood pressure (SBP) increased significantly during each period of ECC. In addition, participants had a higher post-

Fig. 1 Compression force delivered at 1 minute 20 seconds and at

4 minutes 20 seconds for C/V ratios of 15:2, 30:2, and 50:5 (in Newtons).

SBP (mm Hg)

15:2 113.00 +- 18.41

121.53

+- 19.22

8.52

2.25

14.80

.011 ?

30:2 115.29 +- 19.71

124.52

+- 16.33

9.23

2.84

15.62

.007 ?

50:5 113.64 +- 17.81

127.58

+- 18.87 ??

13.94

6.94

20.93

.001 ?

Diastolic blood pressure (mm Hg)

15:2 75.53 +- 0.86

75.35

+- 10.93

-0.17

-4.68

4.33

.935

30:2 75.17 +- 10.12

75.35

+- 11.07

0.17

-4.43

4.78

.936

50:5 77.52 +- 8.93

75.00

+- 8.31

-2.52

-6.19

1.13

.163

Mean arterial pressure (mm Hg)

15:2 88.01 +- 11.50

90.74

+- 12.82

2.72

-1.37

6.82

.178

30:2 88.54 +- 12.44

91.74

+- 11.53

3.20

-0.33

6.73

.073

50:5 89.56 +- 10.93

92.52

+- 10.39

2.96

-0.53

6.45

.091

Heart rate (beats per min)

15:2 76.71 +- 12.82

88.12

+- 16.04

11.41

5.39

17.43

.001 ?

30:2 75.88 +- 12.71

91.94

+- 17.51

16.05

10.98

21.13

b.001 ?

50:5 76.11 +- 13.51

88.64

+- 17.24

12.52

6.04

19.01

.001 ?

Oxygen saturation (%)

15:2 97.35 +- 1.11

97.41

+- 0.50

0.05

-0.52

0.64

.835

30:2 97.31 +- 1.01

97.50

+- 0.51

0.18

-0.51

0.89

.580

50:5 97.06 +- 0.92

97.18

+- 0.91

0.12

-0.34

0.59

.580

* P b .05, paired t test (significant).

?? P b .05, significantly different from group 15:2 (paired t test).

ECC change in SBP at 50:5 than at 15:2 (P = .026, pairwise comparison; 95% CI for mean difference, -11.29 to -0.83). However, diastolic blood pressure, mean arterial blood pressure, and SaO2 did not change significantly during ECC.

Table 1 Rescuers’ physiologic parameters before and after performing ECC

Before ECC After ECC Mean Difference

P ?

Mean +- SD

Mean +- SD

95% CI of the difference

Lower Upper

Table 2 Subjective RPE, level of fatigue, and discomfort data at 3 C/V ratios

Shoulder

1.54 +- 1.70

1.75 +- 1.78

1.61 +- 1.80

.819

* P b .05, significantly different from group 15:2 (paired t test).

?? P b .05, significantly different from group 30:2 (paired t test).

??? P b .05, significantly different by RM ANOVA test.

Repeated-measures analysis of variance revealed differ- ences between the perceived exertion (3.38 +- 1.64 at 15:2;

4.06 +- 1.43 at 30:2; 4.35 +- 1.54 at 50:5; P = .045; observed

power, 0.605) associated with the 3 ratios. The median

Group 15:2

Group 30:2

Group 50:5

P

RPE

Mean, modified Borg scale

3.38 +- 1.64

4.06

+- 1.43

4.35 +- 1.54 ?, ??

.045 ???

Median, modified Borg scale

3

4

4

Level of fatigue, VAS

Overall

2.20 +- 1.740

2.76

+- 1.73 ?

3.67 +- 2.04 ?

.008 ???

ECC motion

Decompression

1.20 +- 1.89

1.50

+- 1.84

1.25 +- 1.28

.443

Compression

1.85 +- 1.68

2.81

+- 2.04

2.50 +- 1.86

.226

Discomfort of body areas, VAS

Neck

0.95 +- 0.92

1.14

+- 1.35

1.31 +- 1.13

.207

Wrist

1.68 +- 2.00

1.87

+- 1.59

2.16 +- 1.89

.177

Back of hand

1.94 +- 2.01

1.98

+- 2.05

2.20 +- 1.97

.829

Palm

1.87 +- 2.11

2.25

+- 2.18

2.28 +- 2.42

.663

Fingers

1.01 +- 1.37

1.25

+- 1.65

1.21 +- 1.35

.243

Hand

1.35 +- 1.70

1.42

+- 1.67

1.37 +- 1.81

.944

Waist

1.40 +- 1.63

1.56

+- 1.28

1.83 +- 1.79 ?

.024 ???

Knee

1.66 +- 1.66

1.77

+- 1.35

1.78 +- 1.20

.939

1 min 20 s

4 min 20 s

Paired

Mean, 95% CI

P

Mean

SEM

Mean

SEM

Difference

Lower

Upper

Head 15:2

Shoulder

Elbow

1.252

6.191

-5.476

5.922

6.728

-1.835

15.293

.115

30:2

-4.149

6.661

-6.148

5.066

1.999

-12.063

16.061

.767

50:5

-1.848

6.086

2.431

3.687

-4.279

-17.031

8.471

.487

Upper trunk

15:2

0.211

6.687

4.012

6.89

-3.801

-14.073

6.47

.444

30:2

5.528

11.269

12.444

10.912

-6.916

-29.097

15.265

.856

50:5

5.395

8.014

9.247

5.57

-3.852

-17.436

9.732

.358

Lower trunk

15:2 27.224

9.57

23.136

10.191

4.088

-4.214

12.389

.312

30:2 21.528

11.269

22.444

10.912

-0.916

-14.174

12.344

.885

50:5 22.304

11.377

23.46

10.037

-1.156

-16.796

14.482

.877

15:2 53.822

6.347

56.394

5.922

-2.572

-14.841

9.697

.537

30:2 51.345

6.634

56.179

5.066

-4.834

-16.534

6.866

.829

50:5 55.355

6.067

52.642

3.687

2.713

-7.041

12.467

.638

15:2 5.389

4.927

4.23

5.387

1.159

-9.155

11.473

.738

30:2 6.489

5.829

6.243

2.486

0.246

-8.069

8.561

.847

50:5 9.826

8.36

8.385

6.394

1.441

-13.313

16.195

.589

Wrist

15:2

-41.78

12.189

-59.803

12.619

18.023

-6.785

42.831

.299

30:2

-46.673

13.091

-66.427

9.372

19.754

-2.709

42.217

.318

50:5

-45.235

13.271

-64.251

9.114

19.016

-3.369

41.401

.347

Right hip

15:2

56.553

7.44

55.218

7.533

1.335

-13.638

16.308

.258

30:2

61.162

8.855

57.267

7.981

3.895

-12.941

20.731

.425

50:5

60.644

9.116

55.12

6.812

5.524

-10.404

21.452

.412

Left hip

15:2

56.135

7.633

53.858

8.395

2.277

-13.751

18.305

.193

30:2

61.463

9.677

57.044

8.998

4.419

-14.256

23.094

.305

50:5

60.322

9.333

53.563

7.59

6.759

-10.164

23.682

.335

perceived exertions at 15:2, 30:2, and 50:5 were 3, 4, and 4, respectively. Five minutes of ECC was considered a “moderate to somewhat heavy” exercise at each ratio. The fatigue, measured by VAS, was 2.20 +- 1.74, 2.76 +- 1.73, and

Table 3 Mean joint angles (?) measured at 1 minute 20 seconds and 4 minutes 20 seconds at 3 different C/V ratios

3.67 +- 2.04 at 15:2, 30:2, and 50:5, respectively (P = .008, RM ANOVA; observed power, 0.848). These results indicate that the ratio of 50:5 represented heavier exercise and greater fatigue after 4 minutes 20 seconds of ECC. The ratio of 30:2 also involves greater fatigue than the ratio of 15:2 after

4 minutes 20 seconds of ECC. Table 2 summarizes the subjective discomfort data from the 3 C/V ratios. Waist discomfort, indicated by a high VAS, was noted in the 50:5 group after 4 minutes 20 seconds of ECC.

Joint angles will affect joint force and moment. However, the measurements of angles revealed no significant differ- ences in the head, shoulder, lower trunk, hip, and knee angles between 1 minute 20 seconds and 4 minutes 20 seconds (Table 3) for any group. The clinical and statistical implications of these findings were inconclusive because of the small number of participants.

Table 4 shows the output records for manikin Annie at 1 minute 20 seconds, 2 minutes 50 seconds, and 4 minutes 20seconds for each group. These data indicate that experienced providers can maintain an adequate compres- sion rate and compression depth throughout the 5-minute ECC session. The percentages of correct compression rates were between 74.7% and 83.2%, and the medians were between 80% and 90%. This percentage of correct chest compression is similar to the correct compression rates in previous investigations [17,25,26]. In summary, most volunteers could perform quality ECC for 5 minutes at any ratio.

Discussion

Increasing the number of consecutive compressions may affect the quality of CPR. Rescuer fatigue also plays an important role, and so, this study helps to evaluate that

Mean

113.9

112.8

113.5

108.2

107.4

108.2

112.7

110.3

110.5

Median

114.0

111.0

110.0

106.0

104.0

104.0

107.5

106.0

105.0

Percentiles

25

107.5

105.0

105.7

101.0

99.5

99.0

103.2

100.0

100.0

50

114.0

111.0

110.0

106.0

104.0

104.0

107.5

106.0

105.0

75

119.0

120.2

123.2

116.0

114.0

117.5

119.0

118.0

119.0

Compression depth (mm)

Mean

47.8

46.4

44.7

44.6

43.9

41.8

44.3

43.2

41.8

Median

48.0

46.5

44.5

44.0

43.5

42.5

44.5

43.0

40.0

Percentiles

25

45.5

44.0

41.7

42.5

41.0

38.5

42.2

40.0

39.0

50

48.0

46.5

44.5

44.0

43.5

42.5

44.5

43.0

40.0

75

50.0

49.2

48.0

47.0

46.2

44.2

46.0

46.0

44.0

Percent correct compression (%)

Mean

74.7

75.4

82.2

82.4

78.2

78.3

83.2

79.8

81.9

Median

82.5

90.0

95.0

92.0

93.0

94.0

93.5

90.0

80.0

Percentiles

25

54.0

54.5

72.0

74.2

63.5

58.0

77.2

74.2

74.2

50

82.5

90.0

95.0

92.0

93.0

94.0

93.5

80.0

80.0

75

96.0

97.0

100.0

98.0

98.0

97.7

97.5

98.0

98.0

aspect. This study demonstrated that an experienced rescuer performing ECC on a standard manikin can deliver a similar force for 2 minutes at C/V ratios of 30:2 and 15:2 but deliver a lower compression force when performing ECC at a 30:2 ratio for 5 minutes. The C/V ratio 50:5 was used herein to simulate more Continuous chest compressions and resulted in a reduced compression force in 2 minutes. The ECC at a ratio of 30:2 or 50:5 for 5 minutes represented a “somewhat heavy” exercise. The ratios 50:5 and 30:2 are more fatiguing than 15:2 when performing ECC for 5 minutes. Although there is a significant decline in performance over time for all 3 compression ratios, the quality of chest compressions did not suggest a significant decline in percent proper compres- sions after 5 minutes.

Table 4

Ratio

Time to measure

Quality of chest compressions at various compression ratios

15:2

30:2

1 min 20 s 2 min 59 s 4 min 20 s 1 min 20 s 2 min 59 s

50:5

4 min 20 s 1 min 20 s 2 min 59 s 4 min 20 s

Compression rate (per min)

Chest compressions are important for cardiac arrest patients, with or without ventilation. Literature has focused on raising the C/Vratio or chest compression only during CPR. Some investigations have found that compression-only CPR resulted in a higher proportion of patients with Favorable neurologic outcomes than the conventional CPR [24,25]. However, rescuer fatigue must be considered in determining the effectiveness of chest compression, especially when the compression period before return of spontaneous circulation (ROSC) is prolonged or CPR has to be continued. More consecutive compression in CPR affects rescuer fatigue.

The current American Heart Association (AHA) CPR guidelines recommend a C/V ratio of 30:2 [5]. Deschilder et al [17] found that although the 30:2 ratio is rated as more exhausting than the 15:2 ratio, it still delivers 5 minutes of quality ECC. Yannopoulos et al [26] also found that the quality of 30:2 CPR or fatigue did not change significantly

within 5 minutes of ECC. In this study, although the force delivered by 30:2 was similar to that delivered at 15:2 for 2 minutes, it was lower after 5 minutes of ECC. In this study, the measured outputs for manikin Annie (compres- sion rate per minute, compression depth, and percent correct compression at 5 minutes) satisfied the AHA standard. However, rescuer fatigue occurred, and the compression force fell after 5 minutes at a ratio of 30:2. The AHA recommendation of changing the rescuer after every 1 to 2 minutes of ECC should be followed where possible [5,11].

An earlier investigation by Riera et al [15] found that health professionals trained in CPR can easily tolerate uninterrupted ECC for 2 minutes. Other studies have found that the quality of continuous ECC is better during the first 2 minutes than that after 3 minutes [16] and that 5 minutes of continuous ECC lowers Compression quality [27]. Bjorshol et al [28] found that the depth and rate of chest compression did not fall within 10 minutes of BLS at any of the 3 chest compression ventilation ratios (15:2, 30:2, and 50:2) when experienced paramedics performed ECC. Our result on the quality of chest compressions validated the work of Bjorshol et al. However, the findings of this study also indicate that increasing the continuous compression ratio of ECC reduces the compression force after 2 minutes, confirming the recommendation that rescuers should take turns in providing ECC to avoid fatigue [4]. This alternation is particularly important when performing ECC at a high continuous compression session.

The hip was used as a fulcrum in ECC, and the mean measured angles of the hips and lower trunk remained

constant throughout this study. In our work, only waist discomfort was noted at the 50:5 ratio as opposed to other joint areas after 5 minutes of ECC. However, lumbar fatigue is known to delay the responses of the lumbar muscle to sudden loads [29], and repeated flexion may be related to an increased risk of Spinal injury [30]. The rhythm of the body motion or the synchronization of the bones is optimized during repetitive exercise to maximize the efficiency of the exercise, and repetitive motion may cause muscular fatigue [22]. Further large-scale studies must be undertaken to measure the impact of increasing the C/V ratio or of performing continuous ECC on the potential for back injury in health care providers.

There is a significant decline in performance over time for all 3 compression ratios; in fact, it appears most pronounced for the 30:2 ratio. However, the quality of chest compressions from the SkillReporter did not suggest a significant decline in percent proper compressions after

5 minutes. It is possible that there is a threshold compression force above which a proper compression is delivered, and although we have demonstrated a decline in the force over time, each participant was still able to exceed the “threshold” force.

This study has some limitations. Small sample size and lack of blinding may have affected the results of this study. Although the observed differences of compression force and fatigue were significant, they were generally small and were measured only for experienced rescuers. Further study should evaluate how an appropriate force would be and how this force relates to compression depth. The study should be repeated with larger samples and different populations. It had to have been obvious to the participants what the goal of the study was, and the rescuers were not blinded to the ratio, bringing the possibility of bias to the study results. Although joint angles will affect joint force and moment, the relevance of the angles of joint movements in this study is not immediately apparent and needs further investigation. With regard to the findings in mean joint angles, it is possible that the sample size was the limiting step in identifying differences because estimations of possible sample size was based on the expected difference of compression force.

In conclusion, although the evidence that it is beneficial to patients is growing, rescuer fatigue is an issue increasing consecutive compression during CPR. The compression force may decrease after 4 minutes of CPR at a C/V ratio of 30:2 and after 2 minutes of CPR at 50:5 ratio, affecting the strength of the compression. The current AHA recommendation that rescuers switch every 2 minutes during chest compression is reasonable and should be followed. Our results suggest that changing every 2 minutes may prevent fatigue at higher compression ratios, but further study (looking at effects of 2-minute intervals of CPR by 2 rescuers) would need to be conducted to validate this recommendation.

Acknowledgments

The authors thank Ted Knoy for his editorial assistance.

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