Gender‐Based Differences in the Biomechanical Behavior of the Thorax During CPR Maneuvers

ABSTRACT

In this study, 18 rib cages (8 males and 10 females) were segmented from computer tomography (CT) images. In order to analyze the potential differences in thoracic biomechanics during cardiopulmonary resuscitation (CPR), a set of numerical experiments was conducted using finite elements (FE). Compression forces were applied at different points on the rib cage. Results indicated that the optimal compression area for both sexes is the sternum at the 5th rib level, requiring the least force to achieve the desired compression depth. Males required greater force than females. Among females, those with lower width/depth ratios (more rounded thoracic shape) required less force compared to those with higher ratios (more oval‐shaped thorax).

Cardiopulmonary resuscitation (CPR) is a common emergency procedure for cardiac arrest. This study presents numerical experiments simulating this maneuver in both genders to determine the optimal location and force for chest compressions. By analyzing deformation and stress, this investigation demonstrates that the ideal compression point is lower on the sternum, with men requiring more force than women. Among women, those with a rounded thoracic shape need less force compared to those with an oval‐shaped chest morphology.

Abbreviations

CPR

cardiopulmonary resuscitation

CT

computer tomography

FE

finite elements

1. Introduction

Cardiopulmonary resuscitation (CPR) is an emergency maneuver that combines the performance of a series of chest compressions to maintain blood circulation and mouth‐to‐mouth breathing to oxygenate the lungs of a person who has experienced cardiac arrest. Thus, this maneuver is of vital importance to ensure adequate oxygenation until cardiac and respiratory activity are restored or until medical assistance arrives [1].

Currently, it is estimated that between 4 and 5 million people die each year from sudden cardiac death worldwide [2], and performing this maneuver can increase survival chances by three times [3]. Nevertheless, there are few studies that delve into the possible differences between men and women in response to such a maneuver, which could significantly reduce the effectiveness of CPR.

When performing CPR, it is important to be aware of the associated risks, as it is sometimes challenging to strike a balance between compressions deep enough to ensure adequate blood circulation and avoid injuries to the patient. Around 40%–60% of cases in which this maneuver is performed result in serious damage to the rib cage [4]. The severity of these injuries can range from tissue or organ hematomas to life‐threatening complications such as myocardial rupture. However, the most common injuries are rib fractures, which occur due to the significant stresses applied to the rib cage and typically do not jeopardize the patient's life [5]. Several studies have associated a higher risk of generating these secondary injuries with a series of factors, such as the duration of the maneuver, female sex, age, and the depth of compressions [4].

At the anatomical level, there are some differences between both genders, primarily in the size and shape of the rib cage, which results in certain functional differences in its biomechanics. In a study focused on analyzing the morphological and functional implications of sexual dimorphism in the chest [6], it was concluded that men's rib cages are 12.4% larger than women's. Regarding morphology, it has also been observed that male chests are wider and shorter than those of women; likewise, women's ribs exhibit a greater inclination, and their sternums are located higher than in men [7, 8, 9]. Another characteristic to consider is that, proportionally, women have longer ribs [8]. Additionally, there are differences in the sternum, which is considerably thinner in women than in men [10].

On the one hand, women show a higher incidence of sternal fractures, as previously mentioned, due to the thinner sternum in women compared to men. Additionally, as women age, they face a higher risk of developing degenerative bone diseases like osteoporosis, which leads to a decline in bone quality and increases the likelihood of fractures or bone failure. This same issue can affect the ribs, thereby increasing the probability of fractures following CPR [4, 11]. It is important to note that women requiring this maneuver are often older, which could explain the increased incidents associated with chest compressions due to poorer bone quality [4, 12, 13].

Another reason that might explain the higher incidence of injuries in women is that they tend to receive deeper compressions than men. This could be because the same forces are applied to both genders without considering the anatomical differences that make women's rib cages more prone to greater deformities than men's under the same force. The reason for this is that the distensibility of the thoracic wall varies between genders due to existing dimorphisms. First, women have lower maximum rib flexion moments than men, as well as lower flexion slopes in moment‐angle curves, and generally thinner cortical bone in the ribs. All of this, combined with the previously mentioned greater antero‐posterior inclination of the ribs, can increase their distensibility compared to men's under‐compression forces [14].

Numerical simulations are undeniably a valuable complement to experimental studies on CPR, which are frequently conducted on human cadavers. The application to these methods to the CPR recommendations has been little explored to date. However, the finite element (FE) simulations offer a cost‐effective method for replicating various conditions in the model. They are able to predict the impact of modifications of the model parameters, such as thorax geometry, elastic properties, applied forces and their localization, and so forth.

Recently, Suazo et al. developed an FE model based on a single adult rib cage to simulate the effects of different locations for performing CPR compressions [15]. Moradicheghamahi et al. conducted FE simulations of CPR compressions on a range of rib cage models of different sizes, constructed by varying the dimensions of a geometric model obtained from a computer tomography (CT) image. Their objective was to study the effect of thoracic dimensions on the compression depth during CPR maneuvers [16]. Finally, Jeon et al. studied the influence on fatigue life during simulated manual CPR of four variables: the properties of the trabecular bone, the site of application of the compression force on the breastbone, the magnitude of applied compression force, and the rate of application of the compression force. For that, they used three age‐specific rib cage models, each differentiated by trabecular bone properties [17].

In the present study, the potential differences in the biomechanical behavior of the thorax between men and women during CPR maneuvers are for the first time numerically analyzed. The analysis of these differences is carried out based on the rib cage geometries of real patients and on patient‐specific elastic properties. The goal is to optimize chest compressions and thus to reduce associated risks for both genders.

2. Methods

To conduct the present study, 8 thoracic CT images of male patients and 10 of female patients were selected from a database on the Grand Challenge website https://grand‐challenge.org/algorithms/rib‐segmentation/, where automatic rib segmentation was proposed. These files were fully anonymized, thus no clinical information about the patients was available. Both male and female patients were randomly selected, choosing those with good image quality to facilitate the process of segmentation and ensuring that the rib cages were fully visible in the CT scans. In Table 1, the chest dimensions of each individual, as well as the group average, can be found. Figure 1 shows a male thoracic model developed for the study, in which the four floating ribs were omitted as they do not affect the response to CPR.

TABLE 1.

Dimensions of the selected rib cages and the averages and deviation for each gender.

Dimensions (mm)
Patient	Gender	Width	Height	Depth
1	Male	284	295	213
2	Male	286	319	159
3	Male	304	286	160
4	Male	271	270	174
5	Male	299	309	168
6	Male	281	291	188
7	Male	310	281	168
8	Male	313	291	191
Men average		$293.29\pm 15.23$	$292.73\pm 15.31$	$177.56\pm 18.60$
9	Female	290	257	163
10	Female	280	237	156
11	Female	277	256	130
12	Female	307	267	171
13	Female	265	290	157
Women subgroup 1 average		$283.70\pm 15.73$	$261.34\pm 19.02$	$155.26\pm 15.34$
14	Female	283	293	172
15	Female	279	280	148
16	Female	247	272	152
17	Female	216	255	172
18	Female	250	271	135
Women subgroup 2 average		$254.90\pm 27.40$	$274.10\pm 13.80$	$155.92\pm 15.79$

Note: The width measurement is taken at the level of the 5th rib, which usually corresponds to the maximum width of the chest. The height is measured from the highest point of the sternum to the plane of the 10th ribs. The depth of the rib cage is taken from the most caudal point of the sternum to the articular facets of the vertebra. All measurements were performed using 3D Slicer software.

FIGURE 1.

Example of a male thoracic cage model used in the present study. (A) Sagittal view of the rib cage. (B) Anterior view, showing the sternum and the 10 pairs of ribs considered for the numerical experiments. (C) Detail of the five compression areas considered for the compression force application: P1 and P2 located in the sternum and P3, P4, and P5 in the costal cartilages of the left part of the rib cage.

Given the thoracic dimensions of every individual, it is interesting to calculate some ratios to be able to compare the geometry among males and females. In Table 2, the three ratios calculated for the geometry analysis can be noted: height/width, width/depth, and height/depth. The presented results are the averages and deviations for male patients and female patients separately. For further analysis in the discussion, the data for women are differentiated into two subgroups.

TABLE 2.

Average ratios and deviations of the rib cages for each group based on the previously presented dimensions.

Ratios
Group	Height/width	Width/depth	Height/depth
Men	$1.00\pm 0.07$	$1.67\pm 0.18$	$1.66\pm 0.20$
Women subgroup 1	$0.93\pm 0.09$	$1.84\pm 0.17$	$1.69\pm 0.20$
Women subgroup 2	$1.08\pm 0.07$	$1.65\pm 0.25$	$1.77\pm 0.19$

The models were segmented with 3D Slicer [18] and meshed using the ScanIP software with tetrahedral elements. The minimum mesh edge length was set to 3 mm to effectively capture the complexity of the thoracic geometry. This approach prevents the creation of excessively fine meshes, which result in higher computational costs, especially given the large number of tests to be conducted. Approximately, meshes with 25,000 nodes and 60,000 elements were generated (Figure 2). It is important to note that not all patients had the same exact number of elements, as the meshing depends on the geometry and dimensions of each case.

FIGURE 2.

Illustration of the meshing process. (A) Mesh of a male thorax created with ScanIP. (B) Anterior view of the superposition of the mesh onto the CT scan in the Bonemat interface for material properties assignment. (C) Mapping of the Young's modulus on the bone tissue obtained by Bonemat.

Bone exhibits heterogeneous material properties; thus, accurately describing these properties is essential to creating representative finite element (FE) models that yield meaningful results. The apparent density of bone tissue can be determined from a Computed Tomography (CT) scan. Once the density is known, it can be used to calculate Young's modulus, typically using a power equation [19]. Researchers at the Instituto Ortopedico Rizzoli in Bologna, Italy, created a software program called Bonemat to help with accurate material assignment for a FE mesh [20]. To assign the mechanical properties to the bone tissue of this study, this application was used, which provided the elastic modulus for each element of the FE mesh based on the gray level of the original CT image.

To ensure that the software can accurately assign the bone's material properties, a series of parameters must be input. The assignment is based on Hounsfield attenuation, which varies according to the intensities detected in the CT scan image. With this information and the parameters, it is possible to obtain the bone mineral density for each mesh element, where higher values correspond to a higher Young's modulus. These parameters consist of calibration factors that the software uses when computing the elastic modulus. In this study, the calibration file provided by the software itself was used, as CT imaging is considered a highly standardized process. This calibration file is based on three equations, and the default parameters provided by the software were used. These equations are the calibration of the bone mineral density, ${\rho }^{\mathrm{QCT}}$, from gray level, $\mathrm{HU}$, (Equation 1); the calibration of the bone mineral density of the ashes, ${\rho }^{\mathrm{Ash}}$, from ${\rho }^{\mathrm{QCT}}$ (Equation 2); and the calibration of Young's modulus from ${\rho }^{\mathrm{Ash}}$ (Equation 3).

$${\rho }^{\mathrm{QCT}}=-0.00393573+0.000791701\cdot \mathrm{HU}$$ (1)

$${\rho }^{\mathrm{Ash}}=0.079+0.877\cdot {\rho }^{\mathrm{QCT}}$$ (2)

$$E=14664\cdot {\left({\rho }^{\mathrm{Ash}}\right)}^{1.49}$$ (3)

Young's modulus of the cortical rib cage bone normally ranges from 7 to 24 GPa as Katzenbergeret et al. in [21] have recently reported. This study aimed to provide a more accurate characterization of the heterogeneity of the rib bone, so a single value was not assigned to each patient. However, it was observed that the maximum Young's modulus values ranged from 12.8 to 22.8 GPa.

As it was not completely visible in the CT scan, the mechanical properties of the costal cartilage were defined in ANSYS APDL, the software chosen to perform the FE simulations. The corresponding area of the cartilage was manually selected, and the material properties (E = 27 MPa and $\nu$ = 0.45) were assigned. These properties are the same as assigned to the costal cartilague in the studies of Suazo et al. in [15] and of Moradicheghamahi et al. in [16], which are well‐justified by previous studies [22, 23, 24, 25, 26].

FE simulations were conducted using ANSYS based on the procedure described by Suazo et al. in [15]. They chose five points to test the thoracic response to the CPR compression forces for a single patient. Based on this study, our research analyzed the biomechanical behavior of the rib cage for 18 patients (8 males and 10 females), where the compression forces were applied to the same five zones of interest (Figure 1C). On one hand, loads are tested on the sternum: P1 refers to the center of the sternum, while P2 is located more caudally. The remaining test areas are located on the costal cartilage of the left side: P3, P4, and P5 are positioned on the 4th, 5th, and 6th ribs, respectively. Forces between 100 and 600 N were applied in increments of 50/100 N depending on the patient group, as will be seen later. Similarly, it was assumed that the spine was fixed (since the patient lies in the supine position during the CPR maneuver), limiting its displacement in all directions.

In each model, a series of nodes were preselected to monitor the behavior of the thoracic cage and extract results. These included the nodes of the surface corresponding to the five zones where the compression was applied in order to control the achieved resulting depth, and the nodes of each rib to record the maximum Von Mises stresses individually. Therefore, for each patient, the depth of compression achieved in the five areas of force application was measured, as these are the zones where an effective target compression is sought. Additionally, for every rib, the maximum Von Mises stresses were recorded. Regarding the stress results, the maximum value was decided to be taken as the 95th percentile of the stresses, in order to avoid the effect of possible stress concentrators.

Thus, to obtain the results for both males and females, the average depths and stresses, along with their deviations, were calculated separately for each gender. The objective is to achieve the desired depths, that is, effective compressions (between 4 and 6 cm [27]) without reaching stresses higher than 50 MPa, as beyond this value, there is a high risk of rib fractures as pointed out in [15, 16]. Figure 3 shows an example of the deformation obtained for a male patient when 600 N is applied at P1.

FIGURE 3.

Example of the deformation experienced by the rib cage of a male patient when a compression force was applied at P1. (A) Original morphology. (B) Deformation when 200 N is applied. (C) 300 N. (D) 400 N. (E) 500 N. (F) 600 N. It can be noted that the thorax is compressed homogeneously providing an effective CPR compression.

3. Results

FE simulations were conducted for the male group with forces ranging from 200 to 600 N, with a step of 100 N. Figures 4, 5, 6, 7, 8 show the average results of this group as well as the deviation in each case; the depths reached at the different points of interest after applying the forces in each zone, and the maximum Von Mises stresses recorded at the ribs in every simulation.

FIGURE 4.

Results of the male group when a force ranged between 200 and 600 N is applied in the compression location P1. Left: Average maximum compression depth measured in each area of interest and its deviation as a function of the amplitude of the force applied. Right: Average maximum Von Mises stress on each of the ribs and the group deviations for all forces. Right (left) ribs are denoted by R (L) followed by their position number.

FIGURE 5.

Results of the male group when a force ranged between 200 and 600 N is applied in the compression location P2.

FIGURE 6.

Results of the male group when a force ranged between 200 and 600 N is applied in the compression location P3.

FIGURE 7.

Results of the male group when a force ranged between 200 and 600 N is applied in the compression location P4.

FIGURE 8.

Results of the male group when a force ranged between 200 and 600 N is applied in the compression location P5.

Similarly, the results for the 10 women are presented. In this case, originally the same magnitudes of force ranging from 100 to 500 N were going to be applied to all the women. However, when the FEM tests began, it became evident that the behavior of the rib cages under the same stresses varied significantly among different women, unlike the men, whose results showed reduced variance. As a result of these preliminary findings, it was decided to divide the women's database into two subgroups (five patients in each) based on the observed deformations: for subgroup 1, which endured higher loads, forces from 100 to 500 N were applied with a step of 100 N; for subgroup 2, which endured lower force magnitudes, forces from 100 to 300 N were applied with a step of 50 N. These force scales were selected due to the fact that greater efforts exceeded the target depth. Thus, the results presented for both subgroups of women display different force scales to be considered in the discussion of the results. Hence, the results of both subgroups are shown in Figures 9, 10, 11, 12, 13.

FIGURE 9.

Results of the female group when a force ranged between 100 and 500 N for subgroup 1, and 100 and 300 N for subgroup 2, are applied in the compression location P1. Right: subgroup 1 results. Left: subgroup 2 results.

FIGURE 10.

Results of the female group when a force ranged between 100 and 500 N for subgroup 1, and 100 and 300 N for subgroup 2, are applied in the compression location P2.

FIGURE 11.

Results of the female group when a force ranged between 100 and 500 N for subgroup 1, and 100 and 300 N for subgroup 2, are applied in the compression location P3.

FIGURE 12.

Results of the female group when a force ranged between 100 and 500 N for subgroup 1, and 100 and 300 N for subgroup 2, are applied in the compression location P4.

FIGURE 13.

Results of the female group when a force ranged between 100 and 500 N for subgroup 1, and 100 and 300 N for subgroup 2, are applied in the compression location P5.

4. Discussion

The simulations conducted on men show results consistent with those observed in previous studies [15]. Firstly, the forces applied on the sternum, at P1 and P2, manage to generate a homogeneous deformation of the rib cage at the control points. This indicates that the rib cage deforms as a whole rather than exhibiting localized deformations, which would be ineffective during cardiac massage. Figure 3 shows the compressions of a male rib cage subjected to forces at P1, demonstrating that the force applied to the sternum produces a consistent deformation throughout the entire thorax. It behaved similarly when tested at P2. At P1 (Figure 4, left), it can be observed that higher efforts (600 N) are required to achieve the optimal compression, whereas when the force is applied more caudally, as at P2 (Figure 5, left), lower forces (500 N) are needed to achieve the optimal compression. Regarding the maximum stresses generated in each case, these are excessive when 600 N is applied at P1, which would jeopardize the integrity of ribs R1 and L1 (Figure 4, right). By applying 500 N at P2, the Von Mises stresses will not exceed 50 MPa in any rib (Figure 5, right), making it a more efficient compression zone in terms of required efforts and generated stresses.

Moving forward to the compression zones on the costal cartilage (P3, P4, and P5), satisfactory results are not obtained in any case. Regarding P3 (Figure 6), complete results are not available due to excessive distortions of the elements causing ANSYS to be unable to solve all simulations. Nevertheless, high stresses can already be observed on L4, where compression is applied, without having reached the target depth, thus ruling it out as an effective compression zone. At P4 (Figure 7), it is observed that efforts of around 400 N would be necessary to achieve the desired deformation, exceeding the Von Mises stresses occurring in L5 (60 MPa). Lastly, at P5 (Figure 8), an unbalanced compression of the rib cage was observed, indicating excessive deformation in specific areas with the risk of fracture associated with it.

Regarding the deviation observed in the deformation depth graph, the results are consistent across different individuals, with slightly greater variability noticeable when higher forces are applied. In the Von Mises stresses graph, a greater deviation can be observed, but the safety range is maintained in the compressions applied in P2, which proves to be the most efficient.

Therefore, for the studied male group, it is concluded that P2 would be the optimal point of force application, as it is the most efficient since the bone structure is not subject to excessive‐high stresses. The same conclusion was reached by Suazo et al. [15] It is coherent with recommendations found in the literature.

Moving on to the results of the women's group, at points P1 and P2 (Figures 9 and 10), a similar behavior to men's can be observed. At P1, for the subgroup requiring higher efforts (subgroup 1) 400 N of force would be needed to achieve the required depth, whereas for subgroup 2, it would be around 250 N; a difference worth noting. On the other hand, at P2, 300 N would be sufficient for subgroup 1, and 200 N for subgroup 2. In no case, it is observed that the 50 MPa stress threshold is exceeded, beyond which there is a certain risk of fracture.

Continuing with the tests on the costal cartilage, the conclusions that can be reached are the same as in the case of men. First, at P3 (Figure 11), the results are incomplete for the same reason mentioned previously, but considerable stresses can already be observed making this area unsuitable. At P4 (Figure 12), the bone integrity of the rib cage would be compromised by exceeding the recommended stresses; so, once again, this area of force application is not efficient. Finally, in Figure 13, showing the results of the tests with the application at P5, the previously mentioned unbalanced deformation behavior is observed again, risking fractures of rib L6.

Thus, the most efficient zone for the force application during a CPR maneuver would be at point P2 for both men and women, but requiring lower force magnitudes for women than for men. In essence, the optimal loads would be 500 N for men, 300 N for the women of subgroup 1, and 200 N for the women of subgroup 2. These differences in magnitude could explain why there is a higher risk of generating secondary injuries during CPR in women, as higher loads than necessary are applied, which excessively deform their rib cages and compromise their thoracic integrity.

4.1. Differences in the Mechanical Properties and Geometry of the Rib Cages

As discussed earlier, differences can be noted between the results obtained in men and women, with higher forces being necessary in the case of the former to achieve the required deformation for proper chest compressions. Additionally, within the group of women, different behaviors were observed, with Subgroup 1 requiring greater forces than Subgroup 2 to achieve the same compression depth. This variability may arise for various reasons, as already mentioned in the introduction of this study, there are morphological and physiological differences between men and women that may explain the increased risk of injury in the latter.

The following information regarding the mechanical properties of the rib cages and their geometry will be presented in an attempt to explain the variability in the magnitude of the required efforts in each case, especially between both subgroups of women. Beginning with the mechanical properties, the weighted average of the elastic modulus for each patient was calculated from the meshes with the material properties already assigned using the software Bonemat. In order to do so, the elastic modulus of each generated material was considered and weighted based on the number of elements assigned to each material. The results obtained can be consulted in Table 3.

TABLE 3.

Weighted average of the Young's modulus of the bone for all groups.

Mean elastic modulus (MPa)
Group	Men	Women subgroup 1	Women subgroup 2
Mean	$2713\pm 302$	$2768\pm 565$	$2438\pm 434$

Note: Since it is a heterogeneous material, the calculation was performed by weighting the modulus based on the number of elements with that value. As a result, a representative value for each patient is obtained, taking into account the differences between trabecular and cortical bone.

It can be observed that the subgroup of women who endured lower forces presents the lowest average elastic modulus (2438 MPa) compared to the other group of women (2768 MPa) and men (2713 MPa), whose elastic modulus was more similar. These differences might have occurred sporadically or perhaps indicate lower bone quality in this group, which has proven to be the most vulnerable. Without clinical information about the patients of this study, it cannot be stated that the differences in the test results are only due to the mechanical properties of the bone. Another characteristic that could explain the observed differences is the geometry of the rib cages. Therefore, the height, maximum width, and maximum depth of all of them were calculated, and a series of ratios were computed to facilitate the comparison of the results.

As shown in Table 1 and Table 2 provided in the methodology, there are indeed certain differences in the rib cages' geometry between groups that should be discussed. In Table 1, which displays the information on the average dimensions of the rib cages for all the groups, it can be observed that the men's rib cages are wider than the women's, being the rib cages of subgroup 1 wider than the ones of subgroup 2. Besides, men's rib cages are longer and deeper than women's. This is because, in general, men's rib cages are bigger. Table 2 shows that the value of the average length/width ratio for men and both subgroups of women is approximately 1.00, indicating that their rib cages are approximately as wide as long.

On the other hand, the ratios of the two subgroups of women can be noted. The main differences are as follows: in the height/width ratio, the women who required higher forces to achieve the same deformation (subgroup 1), present a lower average value (0.93 compared to 1.08), indicating that they have wider rib cages in relation to their length, and therefore, are less slender; and in the width/depth ratio, women in the subgroup 2 show a lower average value (1.65 compared to 1.84). The latter indicates that this group of women has more rounded rib cages, that is, less wide and deeper, which would explain why they deform to a greater extent under the same forces than the other subgroup.

4.2. Comparison With the Literature

As mentioned in the Section 1, Moradicheghamahi et al. conducted FE simulations of CPR compressions on a range of rib cage models of different sizes (216). They constructed these geometries by varying the dimensions (X, Y, and Z) of a geometrical model obtained from a CT image of a man's chest. Their objective was to study the effect of thoracic dimensions on compression depth during CPR maneuvers [16]. The authors used the Haller index of the rib cage, defined as the ratio of the transverse diameter and the antero‐posterior diameter at P1. Their results suggested that, with a fixed level of compression force, performing CPR on rib cages with a low Haller index leads to compression depths below the average. Alternatively, if a target compression depth is set for CPR, in general, a lower compression force would be required for individuals with a higher Haller index.

In our case, the Haller index can be comparable to the width/depth ratio, since maximum width and maximum depth are approximately located at P1. Men and women of subgroup 2 presented a similar value for this index (1.67 and 1.65, respectively), but lower than that obtained for the women of subgroup 1 (1.84). If we compare the results obtained at P2 for men and women of subgroup 1 in Figure 14, for a fixed level of compression force (250 N, for example; marked with a red line in the graphs), performing CPR on rib cages leads to compression depths smaller for men than for the women of subgroup 1. Alternatively, for a target compression depth set for CPR (40 mm, e.g., marked with a blue line in the graphs), a lower compression force is required for the women of subgroup 1 compared with that required for men; this is coherent with the results obtained in [16]. However, when comparing results obtained at P2 for the women of subgroup 1 with those of subgroup 2, for a fixed level of force, compression depths are higher for the women of subgroup 2 which present a lower value of the Haller index. Additionally, for a fixed value of compression depth, less force is required for the women of subgroup 2 to achieve this depth.

FIGURE 14.

Detail of the deformation depth graphs when compression is applied at P2. (A) Men's group. (B) Women subgroup 1. (C) Women subgroup 2. The graphs include two reference lines: The red one indicates the deformation achieved with an applied force of 250 N and the blue line which represents the force required to reach the recommended minimum depth for CPR compressions.

Although the explanation for these contradictory results can be found in the lower values of Young's modulus of the bone tissue of these women, the most probable causes are the differences in the anatomy previously discussed in the Section 1, since the men's and the women of subgroup 1's bone presents similar Young's modulus. These differences were: the men's rib cages are 12.4% larger than women's; male chests are wider and shorter than women's; women's ribs exhibit a greater inclination, and their sternums are located higher than those in men; women have longer ribs; and women's sternum is considerably thinner than men's [7, 8, 9]. Despite the great work carried out in [16], differences such as the rib inclination, sternum location, or the thinness of the ribs cannot be modeled just by varying the dimensions X, Y, and Z of a male rib cage. Therefore, the use of actual images of the male and female thorax is totally needed to analyze the biomechanical behavior of the thorax during CPR maneuvers in order to establish protocols that can guarantee the survival of the patient causing less harm.

4.3. Limitations

In this study, certain limitations have been encountered that should be taken into account for future projects on gender differences in CPR. First, the main limitation of the present study is that the internal organs and soft tissues of the rib cage have not been modeled, nor has the pectoralis muscle that covers the cage. Even though the mechanical stiffness of the rib cage structure is responsible for most of the resistance to compression during a CPR maneuver, the contribution of internal organs and tissues, and pectoralis muscle might also be significant [22].

Furthermore, there was a lack of patient medical history information. This could have included factors such as the patient's age, the presence of bone pathologies, or other relevant information. Such data would have allowed for a more in‐depth analysis of potential causes of differences and the establishment of more personalized protocols for each situation. Additionally, expanding the study database would have been beneficial to obtain more robust and representative results for each group.

Finally, we have not considered the effect of applying repeatedly applied forces on the rib cage, which may contribute to the accumulated fatigue of the tissues as the real CPR advances in time.

It is important to highlight that even if the results of this study allow the establishment of personalized protocols or protocols based on gender differences for CPR maneuvers, these do not guarantee the success of the maneuver. The reality is much more complex, and the possibility of the return of blood circulation and survival of these patients is influenced by multiple factors, many of which are beyond the scope of this study.

5. Conclusions

In this study, differences in the biomechanical behavior of the thorax between men and women during CPR maneuvers have been found. First, we have verified that the optimal compression area for both sexes is the sternum at the 5th rib level, requiring the least force to achieve the desired compression depth. Males require greater force than females and among females, those with lower width/depth ratios (more rounded thoracic shape) require less force compared to those with higher ratios (more oval‐shaped thorax). To date, these differences are not taken into account in the current protocols established for CPR maneuvers. The forces applied during a CPR maneuver are difficult to control since this is a manual process that is performed in an emergency situation. Therefore, emergency teams or other volunteers might be trained in these protocols if finally these differences are taken into account. Nevertheless, more investigation is needed for the special design of gender‐based protocols for CPR maneuvers.

Ethics Statement

This study was conducted in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available in Grand Challenge at https://grand‐challenge.org. These data were derived from the following resources available in the public domain: Rib segmentation, https://grand‐challenge.org/algorithms/rib‐segmentation/.