Article, Cardiology

Relationship between chest compression depth and novice rescuer body weight during cardiopulmonary resuscitation

a b s t r a c t

Objective: This study determined if rescuer body weight is a major determinant of chest compression depth (CCD) among novice rescuers by analyzing the results of cardiopulmonary resuscitation (CPR) skill tests among medical students and determined the body weight threshold for those unable to achieve adequate CCD. Methods: Retrospective analysis of CPR test results was performed. A total of 107 medical students completed the tests, which included 5 cycles of CPR. Data were collected using a ResusciAnne SkillReporter. Anthropometric data including participant body weight, body mass index, and height were also collected. The relationships between CCD and anthropometric data were evaluated by Pearson correlation coefficient. In addition, univariate Linear regression analysis was used to assess the association between body weight and CCD.

Results: The highest positive correlation was found between CCD and body weight (r = 0.636, P b .001). Body weight correlated with CCD by as much as 40.5% (R2 = 40.5, P b .001). Using a regression model, we described the association between body weight and CCD as follows: expected CCD (mm) = 0.46 x body weight + 17.59. The regression equation shows that rescuers weighing more than 70.5 kg could achieve a CCD of 50 mm.

Conclusions: Rescuer body weight is a major determinant of CCD in novice rescuers. Based on these findings, if 2 or more rescuers are available to perform CPR, chest compression should preferentially be performed by rescuers of healthy weight or overweight.

(C) 2016

Introduction

Chest compression force and depth are proportional to the provider’s upper body mass [1,2]. As a result, underweight rescuers may have trouble achieving and maintaining adequate chest compression depth during cardiopulmonary resuscitation (CPR) [3,4]. Many researchers have concluded that underweight rescuers require additional attention and specialized training [3,5,6].

It has been previously reported that some underweight rescuers do not achieve adequate CCD despite receiving specialized CPR training [6]. We hypothesized that rescuer body weight is a major determinant of CCD in novice rescuers. To test this hypothesis, we analyzed the results of CPR skill tests performed by medical students to determine the body weight threshold for lack of adequate CCD.

? Financial disclosure statement: The authors have no financial relationships relevant to this article to disclose.

?? Conflict of interest statement: The authors have no conflicts of interest relevant to this article to disclose.

* Corresponding author at: Department of Emergency Medicine, College of Medicine, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul, Republic of Korea, 06974. Tel.: +82 2 6299 1820; fax: +82 2 6299 2558.

E-mail addresses: [email protected], [email protected] (J.H. Oh).

Methods

Study design

We retrospectively reviewed the results of CPR skill tests performed by medical students. The study design was approved by the institutional review board of our university, who waived informed consent.

Data collection

CPR training is included in the regular curriculum of third- and fourth-year students in our medical college. The regular curriculum in- cludes 5 hours of theory lectures on adult and pediatric basic life sup- port as well as 8 hours of CPR training sessions using adult manikins during the first semester of the third year. A single instructor teaches 10 students on 5 manikins (4 Little Annes [Laerdal Medical, Stavanger, Norway] and 1 ResusciAnne SkillReporter [Laerdal Medical] manikins) during training sessions. The regular curriculum of the fourth-year medical students includes 20 minutes of online lectures and 2 hours of training sessions with adult (ResusciAnne SkillReporter [Laerdal Medical]), child (Resusci Junior Basic and SkillGuide Pediatric manikin [Laerdal Medical]), and infant (Resusci Baby QCPR Anni [Laerdal Medical]) manikins.

http://dx.doi.org/10.1016/j.ajem.2016.09.006

0735-6757/(C) 2016

2412 J.H. Oh, C.W. Kim / American Journal of Emergency Medicine 34 (2016) 24112413

The third- and fourth-year medical students performed CPR skill tests at the end of the first semester in 2016. The tests included 5 cycles of CPR (each cycle included 30 chest compressions and 2 ventilations) on simulated adult cardiac arrest patients using a ResusciAnne SkillReporter (Laerdal Medical). We analyzed data regarding the average Ventilation volume (mL), total number of ventilations, average chest compression depth (ACD, mm), average chest compression rate (ACR, numbers/min), total number of chest compressions, number of incorrect hand positions, and the number of incomplete recoils from the measured data. In addition, demographic (sex and age) and anthropometric data (height and body weight) were collected. Body mass index (BMI) was calculated using height and body weight.

Statistical analysis

All statistical analyses were performed using IBM SPSS Statistics, version 23.0 (IBM Corp, Armonk, NY). The variables were expressed as means (SD). Data were analyzed using Shapiro-Wilk tests to verify the normality of the distribution. For normally distributed data, 2-sided Student t tests were used; otherwise, Mann-Whitney U tests were used to compare CPR performances between third- and fourth-year students. The relationships between ACD and anthropometric data (body weight, BMI, height) were evaluated by Pearson correlation coef- ficient. In addition, univariate linear regression analysis was performed to describe the association between body weight and ACD. P values b .05 were considered statistically significant.

Results

A total of 82 third-year students and 36 fourth-year students completed the test. Test results of 11 students (8 and 3 third- and fourth-year students, respectively) were excluded from analysis for incomplete data. Thus, a total of 107 student test results were analyzed, including those from 74 third-year and 33 fourth-year students.

Demographic and anthropometric data

The 107 medical students included 71 (66%) men and 36 (34%) women. The mean age was 26.1 (2.5) years. The mean heights and body weights were 171.2 (8.2) cm and 64.3 (11.3) kg, respectively. The mean BMI was 21.8 (2.5) kg/m2.

Comparisons of CPR performances between third- and fourth-year students

CPR performances did not differ significantly between third- and fourth-year students (Table 1).

Table 1

Comparisons of cardiopulmonary resuscitation performances between third- and fourth- year students

Relationships between ACD and anthropometric data

The highest positive correlation was observed between ACD and body weight (r = 0.636, P b .001, Table 2). The Correlation coefficients were 0.550 between ACD and BMI and 0.544 between ACD and height. The association between ACD and body weight is shown as a scatter plot in Fig. 1.

Association of ACD with body weight by univariate linear regression analysis

The body weight contributed to the ACD by as much as 40.5% (coefficient of determination, R2 = 40.5, P b .001). A regression model described the association between body weight and ACD as the following equation:

Expected ACD = 0.46 x body weight + 17.59.

The regression equation indicates that rescuers with body weights greater than 70.5 kg could achieve 50-mm ACD.

Discussion

As was expected, body weight was major determinant of CCD. As a result, we could determine 70.5 kg to be the body weight threshold for inadequate CCD.

This value has several clinical implications. First, we could establish individualized CPR education plans based on this value. Second, if 2 or more rescuers are available to perform CPR, rescuers with body weights greater than 70.5 kg are preferable to perform chest compression.

Fourth-year medical students at our medical college had additional CPR education compared with the third-year students. Although the CPR performance data did not differ significantly between third- and fourth-year students, some of the third-year students did not achieve 50-mm ACD despite having body weights greater than 70.5 kg (Fig. 1). In contrast, all fourth-year students with body weights greater than

70.5 kg achieved 50-mm ACD. We consider this difference to be due to the effects of additional CPR education.

Exercise frequency and muscular fitness level have also recently been reported to be important predicting factors of high-quality CPR [7,8]. The specific muscles required for successful chest compressions and chest wall recoil have also been reported [9].

Based on this growing evidence, it appears that healthy-weight or overweight rescuers could improve ACD with repeated CPR education. However, repeated education alone might not improve CCD in under- weight rescuers. Instead, these rescuers should increase their muscle strength, especially the Pectoralis major, Erector spinae, and Rectus abdominis muscles [9]. If we draw 2 reference lines in the scatter plot of ACD and body weight, we could easily individualize the CPR educa- tion plans for each student (Fig. 2). For example, we could provide addi- tional CPR training for students in quadrant 4. In contrast, the students in quadrants 1 and 2 do not need additional training. However, retraining at certain time intervals is necessary to prevent deterioration of CPR skills [10]. We did not test the effects of repeated education alone

Third-year students (n = 74)

Fourth-year students

(n = 33)

P value

on ACD among the students in quadrant 3. Further studies are necessary to assess how students in quadrant 3 could improve their ACD, such as

Average ventilation volume (mL) 461.5 (141.4) 455.8 (173.3) .859a

Total ventilations (n) 8.0 (2.8) 7.3 (3.2) .431b

ACD (mm) 47.0 (8.7) 47.3 (6.5) .695b

ACR (n/min) 115.8 (11.8) 117.1 (12.9) .615a

Table 2

Relationships between ACD and anthropometric data

Data are presented as mean (SD).

BMI (kg/m2) 21.8 (2.5) 0.550 b.001

Height (cm) 171.2 (8.2) 0.544 b.001

Total chest compressions (n)

Incorrect hand position (n)

151.1 (3.3)

57.2 (53.0)

151.4 (2.40)

61.2 (56.9)

.871b

.836b

Variables

Data (N = 107)

Pearson correlation

coefficient P value

Incomplete recoil (n)

1.7 (7.6)

0.2 (0.6)

.202b

Body weight (kg)

64.3 (11.3)

0.636

b.001

a Statistical significances were tested by 2-sided Student t test.

b Statistical significances were tested by Mann-Whitney U test.

Data are presented as mean (SD).

J.H. Oh, C.W. Kim / American Journal of Emergency Medicine 34 (2016) 24112413 2413

Fig. 1. Scatter plot showing the distribution of ACD according to student body weight. Gray and black dots indicate third- and fourth-year students, respectively.

Fig. 2. Scatter plot with 2 reference lines. The horizontal line indicates 50-mm ACD, whereas the vertical line indicates 70.5-kg body weight. Gray and black dots indicate third- and fourth-year students, respectively.

repeated CPR education, increasing muscle strength, increasing body weight, or a combination of several methods.

Our study had several limitations. First, we regarded the medical stu- dents as novice rescuers. However, the Level of knowledge on CPR and the training experience were higher among the medical students than bystanders or lay rescuers. Therefore, the regression equation could not be applied to other types of rescuers. Second, many factors besides rescuer body weight may also influence ACD, including muscle strength, fitness level, and previous experience with performing CPR. Therefore, the regression equation will not result in an accurate ACD because it considered only body weight. Third, the study data could not be applied to patients experiencing cardiac arrest because it was produced using a mechanical model. Therefore, additional analysis of data obtained from cardiac arrest patients is required.

Conclusions

Rescuer body weight is a major determinant of CCD in novice rescuers. Based on the findings of the current study regarding the relationship between rescuer body weight and expected CCD, if 2 or more rescuers are available to perform CPR, chest compressions should preferentially be performed by those of healthy weight or overweight.

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