Scaling to produce size-independent indices of echocardiographic derived aortic root dimensions in elite Rugby Football League players
Show less
Simon A Oates, Lynsey Forsythe, John D Somauroo, Keith P George, Michael Papadakis, David Oxborough First Published January 17, 2019 Research Article
https://doi.org/10.1177/1742271X18818607
Article information
Article has an altmetric score of 7 Free Access
Abstract
The assessment of aortic root dimensions is important in cardiac pre-participation screening. Scaling of cardiac dimensions removes the impact of body size allowing meaningful inter/intra group comparisons. Developing appropriate scaling approaches, scaling variables and extending the application to major vessels is warranted so underlying pathology can be detected and managed appropriately. The study aims to define relationships between aortic root dimensions and body surface area/height. Two hundred and twenty elite Rugby Football League athletes were recruited. All participants completed anthropometric assessments, a 12-lead ECG and echocardiogram. Aortic root was measured at the aortic annulus, sinus of valsalva, sinotubular junction and the proximal ascending aorta. Linear and allometric scaling were performed on the relationship between aortic measurements and body surface area/height. Absolute aortic root measurements fell within normal population data (mean ± standard deviation (range): aortic annulus: 22 ± 2 (17–28) mm, sinus of valsalva: 28 ± 3 (20–38) mm, sinotubular junction: 22 ± 3 (14–33) mm, proximal ascending aorta: 22 ± 3 (15–31) mm). Linear scaling to height produced size-independent indices at all aortic measurement sites (P < 0.05). Conversely, linear scaling using body surface area did not produce size-independent indices at any site (P > 0.05). Allometric scaling, using both body surface area and height, produced size-independent indices at all sites (P < 0.05). We recommend linearly scaling aortic root dimensions to height in elite Rugby Football League athletes and discourage the use of body surface area as a linear scaling quantity. Allometric scaling is also effective when using both body surface area and height.
Keywords Echocardiography, aortic root, scaling, allometry, pre-participation screening
Introduction
The athlete’s heart is a term used to describe the morphological, functional and electrical cardiac adaptations that occur in response to chronic exercise training.1,2 Athletes exhibit increased left ventricular wall thickness and enlargement of all cardiac chambers3 which occasionally overlap with mild phenotypes of cardiac disease. The differentiation between physiology and pathology is crucial to ensure that individuals at potential risk of sudden cardiac death are identified and managed appropriately.4 Aortic dissection accounts for a small but significant (0.8%) proportion of sudden cardiac deaths in athletes4 and accurate assessment of aortic root dimensions is important to aid the identification of at-risk individuals during cardiac pre-participation screening.
Unlike the cardiac chambers, it is not clear whether the major vessels adapt structurally to intense training. In a mixed cohort of athletes, the absolute and linearly scaled to body surface area (BSA) aortic root dimensions were found to be within published guidelines for the general population.5–7 To simplify clinical assessment an absolute diameter of 40 mm has been proposed as the upper limit of an acceptable aortic dimension, irrespective of body habitus or athletic activity. Such practice may, however, falsely reassure smaller individuals with aortopathy or, conversely, cause unnecessary concerns and further investigation in larger individuals such as basketball players in whom up to 4.6% have aortic root dimensions > 40 mm.8,9
It is understood that aortic root dimensions are related to body size.10 Scaling (or indexing) of vessel dimensions is important for comparison between individual patients and for generating size-independent reference values.11 The process of scaling and the choice of the scaling variable are vital to the validity of any indexing process.12 Currently, recommendations suggest that aortic root dimensions are indexed linearly by BSA.7 This provides a simplistic approach, which has been adopted unchallenged in clinical practice. It is, however, potentially fallacious, as simply dividing aortic root dimensions (one-dimensional measure) by BSA (a two-dimensional area) assumes linearity between the two variables and ignores dimensionality theory.12 The retention of linear scaling with BSA in clinical evaluation is one of mathematical ease rather than scientific merit. Previous work has demonstrated the non-linear relationship of aortic root size to BSA in a non-athletic population but also highlighted a close to linear relationship to height.13 There is a growing evidence base in clinical cardiology to recommend the use of allometric scaling,11–13 which produces size-independent indices.11
Rugby Football League is a team sport that utilises a mixed training load and is classified as a moderate static/moderate dynamic sport.14 Due to the nature of the sport, the body size of individuals can vary significantly. In view of this, Rugby Football League athletes serve as an ideal model to assess the impact of body size on aortic root dimensions. The aim of this study was to establish the range of aortic root dimensions in a large cohort of elite Rugby Football League athletes and define the nature of the relationship between body size (BSA and height) and aortic root dimensions using both linear (ratiometric) and allometric models.
Methods
Two hundred and twenty male Rugby Football League players (mean ± standard deviation (range) age: 21 ± 5 (14–34) years) were prospectively recruited from three English Super League teams during their annual cardiac pre-participation screening. All players were non-smokers, free from cardiovascular disease and diabetes and not taking any prescribed medication. Players were asked to refrain from training and drinking caffeine or alcohol up to 24 hours prior to data collection. The pre-participation screening involved completion of a health questionnaire, the assessment of height and body mass, a measurement of brachial artery blood pressure, a resting 12-lead electrocardiogram and a transthoracic echocardiogram. Screening results were reported by a Sports Cardiologist (JDS) with clinical referrals made for any participant requiring further cardiac evaluation. After screening and further evaluation, if necessary, all participants were included in the study and determined to be free of underlying cardiac disease. All players provided written informed consent.
Anthropometric assessment
A routine standard anthropometric assessment included measurements of body mass (Seca supra 719, Hannover, Germany) and height (Seca 217, Hannover, Germany) with BSA calculated as previously described.15
12-Lead electrocardiogram
A resting 12-lead electrocardiogram was undertaken in accordance with American Heart Association guidelines.16 All electrocardiograms were acquired using commercially available equipment (Schiller, Cardiovit ms-2010, Doral, US).
Transthoracic echocardiogram
Echocardiograms were performed using a commercially available ultrasound system with multi-frequency-phased array transducer (2.5–4 MHz) (Vivid Q, GE Medical, Horton, Norway), by two experienced sonographers adhering to guidelines by the American Society of Echocardiography.7 Standard or high parasternal long axis views were utilised to assess the aortic root and all measurements were made offline using dedicated analysis software (EchoPac Version 110.0.2 GE Medical, Horton, Norway). The aortic root was assessed at four levels: the aortic annulus, defined as the level of the hinge point of the aortic cusps; the sinus of valsalva, at the level of the coronary ostia; the sinotubular junction, defined as the level where the sinus bulge terminates and the proximal ascending aorta, defined as 1 cm distal to the sinotubular junction. All sites were measured at end diastole as defined by the onset of the QRS complex using the inner edge-to-inner edge method.7,13 Measurements were averaged over three cardiac cycles. A previous study from our laboratory demonstrated excellent inter and intra-observer variability data for all measurement sites with intraclass correlation coefficients ranging from 0.84 to 0.97.13
Statistical analysis
Significance was set at P < 0.05. All absolute and scaled aortic root dimensions are presented as mean ± standard deviation. Absolute values were compared to the American Society of Echocardiography guidelines.7 First, each aortic parameter was assessed to establish whether it met Tanners special circumstance.17 Tanners special circumstance is met when the coefficient of variation for the body size variable divided by the coefficient of variation of the aortic dimension is equal to the Pearson’s correlation between the two variables, with the fit line passing through the origin.17 When met it indicates size independence has been achieved. For scaling, initially all aortic root dimensions (Y) were linearly scaled to BSA and height (X) such that the index produced was Y/X. These scaled values were then correlated, via bivariate correlation analysis, to BSA and height to establish whether the linear scaling approach resulted in size-independent indices. Subsequently, all aortic root dimensions were scaled allometrically to BSA and height. β exponents were determined using allometric scaling of the order y = a.xb via a non-linear iterative method to generate allometrically scaled values. The allometrically scaled values were then correlated against BSA or height to determine size independence. To assess the impact of age on aortic root size independent of body size, a covariate analysis was undertaken using the model y = a:xb*exp(c * age). The subsequent β was compared with that obtained using the general model y = a.xb and the c value was calculated.
Results
All demographics are presented in Table 1. Four of the 220 players were referred for further evaluation due to meeting specific non-training-related electrocardiography criteria18 or inconclusive echocardiography but were subsequently deemed to have a normal heart.
Table
Table 1. Participant demographics
Table 1. Participant demographics
View larger version
Absolute and scaled aortic root dimensions are presented in Table 2. None of the participants presented with absolute aortic root dimensions > 40 mm. None of the aortic dimensions met Tanners special circumstance with linear scaling to BSA; however, this condition was met for all aortic root dimensions when assessed relative to height. Linear scaling of aortic root dimensions to BSA did not produce size independence for any of the aortic root dimensions (see Figure 1). Linear scaling to height produced size-independent indices for all aortic root dimensions (see Figure 2).
figure
Figure 1. Measurement sites of the aortic root using inner edge method at end diastole. A –aortic annulus, B – sinus of valsalva, C – sinotubular junction and D – proximal ascending aorta.
figure
Figure 2. Non-linear regression of the order y=a.xb for body surface area (BSA) to (a) aortic annulus, (b) sinus of valsalva, (c) sinotubular junction and (d) proximal ascending aorta.
figure
Figure 3. Non-linear regression of the order y=a.xb for height to (a) aortic annulus, (b) sinus of valsalva, (c) sinotubular junction and (d) proximal ascending aorta.
Table
Table 2. Absolute and linearly/allometrically scaled aortic root measurements to BSA and height. Cut-offs provided (+2 standard deviations) for linearly scaled to height measurements
Table 2. Absolute and linearly/allometrically scaled aortic root measurements to BSA and height. Cut-offs provided (+2 standard deviations) for linearly scaled to height measurements
View larger version
Allometric scaling of aortic root dimensions to BSA produced size independence for all aortic root dimensions with β exponent values of 0.574, 0.571, 0.742 and 0.543 for aortic annulus, sinus of valsalva, sinotubular junction and proximal ascending aorta, respectively. Allometric scaling of aortic root dimensions to height also produced size-independent indices with all β exponents close to 1 (Figure 3). Following assessment for the impact of age, the β exponents were similar to those obtained from the single variant assessment with c values close to 0 suggesting the derived exponent can be used across the age range of this population.
Discussion
The key findings of this study were (1) linearly scaling aortic root dimensions to height produced size-independent indices in elite Rugby Football League players, (2) allometric scaling using both height and BSA also produces size-independent aortic root dimensions, (3) BSA was found not to produce size-independent aortic root dimensions when using linear scaling methods and (4) absolute and linearly scaled aortic root dimensions were found to be within normal population limits.
The current study demonstrates a linear relationship of aortic dimensions to the athlete’s height. This finding has also been seen in a non-athletic population.14 The reason for this consistency likely reflects dimensionality theory in that height as well as all aortic root dimensions are one-dimensional measures and are thus likely linearly related. In view of this we can suggest that adopting this approach will likely produce size-independent indices and support more accurate clinical decision making. The technique is simple, quicker and more intuitive to clinicians than allometric scaling, so is applicable to busy clinical environments with time pressures. Linear scaling of aortic root dimensions to height would potentially reduce the number of unnecessary further investigations in sports with athletes of large body size such as basketball, where up to 4.6% of players may demonstrate an SOV measurement above the cut-off value of 40 mm.6,8 Based on the results of our study we would recommend that linear scaling to height is applied in the clinical setting. Values of up to 14 mm/m for the aortic annulus, 20 mm/m for the sinus of valsalva, 16 mm/m for the sinotubular junction and 16 mm/m for the proximal ascending aorta should be accepted as the upper limits of normal in elite male Rugby Football League players.
In a large cohort of elite Rugby Football League athletes, aortic root dimensions were all < 40 mm thus falling within the accepted normal range.7 Similar to existing data, allometric scaling of aortic root dimensions to BSA and height produced size-independent indices.14 This method does not rely on a linear relationship between the two variables. This approach has not been widely adopted in the context of daily clinical practice.10 This is likely due to the perceived mathematical complexities and hence linear scaling remains the primary scaling method for most measures of cardiac and vessel size.11 The impact of age was also assessed in our non-linear model and supports the use of this exponent when scaling for BSA across this age range (14–34) of elite Rugby Football League players.
Linear scaling using BSA, the process suggested in current clinical guidelines,7 did not result in size-independent aortic root dimensions. Similar to our study, current literature has consistently shown that linear scaling to BSA does not produce size-independent indices for a range of cardiac variables.11 Linear scaling assumes a linear relationship between the cardiac dimension and the body size parameter which is often not the case in biological relationships.12 In addition, BSA is open to measurement estimation error and cannot differentiate the impact of excess fat or lean tissue.12
Limitations
The participants in this study were a homogenous group of athletes suggesting that the results of this study can only be used to guide cardiac pre-screening for aortic root dilation in this population. The study only included male participants aged 14–34 so the results cannot be generalised to very young/older athletes or females. Ethnic differences were not considered and it has been noted that differential cardiac adaptations have been observed in varied ethnicities.19 The study employed a cross-sectional design so a cause and effect relationship between aortic root dimensions and body size cannot be fully established.
Conclusions
This paper adds to a growing evidence base that advocates the use of evidence-based scaling of cardiovascular variables. The paper also discourages the use of BSA as a linear scaling quantity as it does not produce size-independent measures of the aortic root, despite it being the current accepted practice in the clinical setting. The paper provides absolute and scaled values of the aortic root in an elite Rugby Football League population. This will help to guide practitioners in pre-screening programmes within this sport and may help prevent cardiac events in this population. The findings of this paper contribute to a growing database on normative aortic root measurements. Further work should be carried out to define normal values within other diverse sporting populations. These comparisons are made possible through the use of scaling to remove the effect of body size. Linearly scaling to height or allometric scaling is receiving growing attention from the scientific community as an appropriate method. Further research should aim to advocate this further.
Acknowledgements
Not applicable.
Contributors
SO, DO, LF, KG and JS – contribution to conception and design. SO, DO, LF – contribution to acquisition and analysis. All authors – contribution to interpretation. SO, DO – drafted manuscript. All authors – critically revised manuscript and gave final approval.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Ethics Approval
Ethical approval was obtained from the Ethics Committee at Liverpool John Moores University.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was undertaken as part of a student PhD fellowship funded through Cardiac Risk in the Young.
Guarantor
DO.
References
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5. Iskandar, A, Thompson, P. A meta-analysis of aortic root size in elite athletes. Circulation 2013; 127: 791–798.
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7. Lang, R, Badano, L, Mor-Avi, V Recommendations for cardiac chamber quantification by echocardiography in adults: an update from the American Society of Echocardiography and the European Association of Cardiovascular Imaging. J Am Soc Echocardiogr 2015; 28: 1–39.
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14. Levine, B, Baggish, A, Kovacs, R Eligibility and disqualification recommendations for competitive athletes with cardiovascular abnormalities: task force 1: classification of sports: dynamic, static, and impact. Circulation 2015; 66: 2350–2355.
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View Abstract
Show less
Simon A Oates, Lynsey Forsythe, John D Somauroo, Keith P George, Michael Papadakis, David Oxborough First Published January 17, 2019 Research Article
https://doi.org/10.1177/1742271X18818607
Article information
Article has an altmetric score of 7 Free Access
Abstract
The assessment of aortic root dimensions is important in cardiac pre-participation screening. Scaling of cardiac dimensions removes the impact of body size allowing meaningful inter/intra group comparisons. Developing appropriate scaling approaches, scaling variables and extending the application to major vessels is warranted so underlying pathology can be detected and managed appropriately. The study aims to define relationships between aortic root dimensions and body surface area/height. Two hundred and twenty elite Rugby Football League athletes were recruited. All participants completed anthropometric assessments, a 12-lead ECG and echocardiogram. Aortic root was measured at the aortic annulus, sinus of valsalva, sinotubular junction and the proximal ascending aorta. Linear and allometric scaling were performed on the relationship between aortic measurements and body surface area/height. Absolute aortic root measurements fell within normal population data (mean ± standard deviation (range): aortic annulus: 22 ± 2 (17–28) mm, sinus of valsalva: 28 ± 3 (20–38) mm, sinotubular junction: 22 ± 3 (14–33) mm, proximal ascending aorta: 22 ± 3 (15–31) mm). Linear scaling to height produced size-independent indices at all aortic measurement sites (P < 0.05). Conversely, linear scaling using body surface area did not produce size-independent indices at any site (P > 0.05). Allometric scaling, using both body surface area and height, produced size-independent indices at all sites (P < 0.05). We recommend linearly scaling aortic root dimensions to height in elite Rugby Football League athletes and discourage the use of body surface area as a linear scaling quantity. Allometric scaling is also effective when using both body surface area and height.
Keywords Echocardiography, aortic root, scaling, allometry, pre-participation screening
Introduction
The athlete’s heart is a term used to describe the morphological, functional and electrical cardiac adaptations that occur in response to chronic exercise training.1,2 Athletes exhibit increased left ventricular wall thickness and enlargement of all cardiac chambers3 which occasionally overlap with mild phenotypes of cardiac disease. The differentiation between physiology and pathology is crucial to ensure that individuals at potential risk of sudden cardiac death are identified and managed appropriately.4 Aortic dissection accounts for a small but significant (0.8%) proportion of sudden cardiac deaths in athletes4 and accurate assessment of aortic root dimensions is important to aid the identification of at-risk individuals during cardiac pre-participation screening.
Unlike the cardiac chambers, it is not clear whether the major vessels adapt structurally to intense training. In a mixed cohort of athletes, the absolute and linearly scaled to body surface area (BSA) aortic root dimensions were found to be within published guidelines for the general population.5–7 To simplify clinical assessment an absolute diameter of 40 mm has been proposed as the upper limit of an acceptable aortic dimension, irrespective of body habitus or athletic activity. Such practice may, however, falsely reassure smaller individuals with aortopathy or, conversely, cause unnecessary concerns and further investigation in larger individuals such as basketball players in whom up to 4.6% have aortic root dimensions > 40 mm.8,9
It is understood that aortic root dimensions are related to body size.10 Scaling (or indexing) of vessel dimensions is important for comparison between individual patients and for generating size-independent reference values.11 The process of scaling and the choice of the scaling variable are vital to the validity of any indexing process.12 Currently, recommendations suggest that aortic root dimensions are indexed linearly by BSA.7 This provides a simplistic approach, which has been adopted unchallenged in clinical practice. It is, however, potentially fallacious, as simply dividing aortic root dimensions (one-dimensional measure) by BSA (a two-dimensional area) assumes linearity between the two variables and ignores dimensionality theory.12 The retention of linear scaling with BSA in clinical evaluation is one of mathematical ease rather than scientific merit. Previous work has demonstrated the non-linear relationship of aortic root size to BSA in a non-athletic population but also highlighted a close to linear relationship to height.13 There is a growing evidence base in clinical cardiology to recommend the use of allometric scaling,11–13 which produces size-independent indices.11
Rugby Football League is a team sport that utilises a mixed training load and is classified as a moderate static/moderate dynamic sport.14 Due to the nature of the sport, the body size of individuals can vary significantly. In view of this, Rugby Football League athletes serve as an ideal model to assess the impact of body size on aortic root dimensions. The aim of this study was to establish the range of aortic root dimensions in a large cohort of elite Rugby Football League athletes and define the nature of the relationship between body size (BSA and height) and aortic root dimensions using both linear (ratiometric) and allometric models.
Methods
Two hundred and twenty male Rugby Football League players (mean ± standard deviation (range) age: 21 ± 5 (14–34) years) were prospectively recruited from three English Super League teams during their annual cardiac pre-participation screening. All players were non-smokers, free from cardiovascular disease and diabetes and not taking any prescribed medication. Players were asked to refrain from training and drinking caffeine or alcohol up to 24 hours prior to data collection. The pre-participation screening involved completion of a health questionnaire, the assessment of height and body mass, a measurement of brachial artery blood pressure, a resting 12-lead electrocardiogram and a transthoracic echocardiogram. Screening results were reported by a Sports Cardiologist (JDS) with clinical referrals made for any participant requiring further cardiac evaluation. After screening and further evaluation, if necessary, all participants were included in the study and determined to be free of underlying cardiac disease. All players provided written informed consent.
Anthropometric assessment
A routine standard anthropometric assessment included measurements of body mass (Seca supra 719, Hannover, Germany) and height (Seca 217, Hannover, Germany) with BSA calculated as previously described.15
12-Lead electrocardiogram
A resting 12-lead electrocardiogram was undertaken in accordance with American Heart Association guidelines.16 All electrocardiograms were acquired using commercially available equipment (Schiller, Cardiovit ms-2010, Doral, US).
Transthoracic echocardiogram
Echocardiograms were performed using a commercially available ultrasound system with multi-frequency-phased array transducer (2.5–4 MHz) (Vivid Q, GE Medical, Horton, Norway), by two experienced sonographers adhering to guidelines by the American Society of Echocardiography.7 Standard or high parasternal long axis views were utilised to assess the aortic root and all measurements were made offline using dedicated analysis software (EchoPac Version 110.0.2 GE Medical, Horton, Norway). The aortic root was assessed at four levels: the aortic annulus, defined as the level of the hinge point of the aortic cusps; the sinus of valsalva, at the level of the coronary ostia; the sinotubular junction, defined as the level where the sinus bulge terminates and the proximal ascending aorta, defined as 1 cm distal to the sinotubular junction. All sites were measured at end diastole as defined by the onset of the QRS complex using the inner edge-to-inner edge method.7,13 Measurements were averaged over three cardiac cycles. A previous study from our laboratory demonstrated excellent inter and intra-observer variability data for all measurement sites with intraclass correlation coefficients ranging from 0.84 to 0.97.13
Statistical analysis
Significance was set at P < 0.05. All absolute and scaled aortic root dimensions are presented as mean ± standard deviation. Absolute values were compared to the American Society of Echocardiography guidelines.7 First, each aortic parameter was assessed to establish whether it met Tanners special circumstance.17 Tanners special circumstance is met when the coefficient of variation for the body size variable divided by the coefficient of variation of the aortic dimension is equal to the Pearson’s correlation between the two variables, with the fit line passing through the origin.17 When met it indicates size independence has been achieved. For scaling, initially all aortic root dimensions (Y) were linearly scaled to BSA and height (X) such that the index produced was Y/X. These scaled values were then correlated, via bivariate correlation analysis, to BSA and height to establish whether the linear scaling approach resulted in size-independent indices. Subsequently, all aortic root dimensions were scaled allometrically to BSA and height. β exponents were determined using allometric scaling of the order y = a.xb via a non-linear iterative method to generate allometrically scaled values. The allometrically scaled values were then correlated against BSA or height to determine size independence. To assess the impact of age on aortic root size independent of body size, a covariate analysis was undertaken using the model y = a:xb*exp(c * age). The subsequent β was compared with that obtained using the general model y = a.xb and the c value was calculated.
Results
All demographics are presented in Table 1. Four of the 220 players were referred for further evaluation due to meeting specific non-training-related electrocardiography criteria18 or inconclusive echocardiography but were subsequently deemed to have a normal heart.
Table
Table 1. Participant demographics
Table 1. Participant demographics
View larger version
Absolute and scaled aortic root dimensions are presented in Table 2. None of the participants presented with absolute aortic root dimensions > 40 mm. None of the aortic dimensions met Tanners special circumstance with linear scaling to BSA; however, this condition was met for all aortic root dimensions when assessed relative to height. Linear scaling of aortic root dimensions to BSA did not produce size independence for any of the aortic root dimensions (see Figure 1). Linear scaling to height produced size-independent indices for all aortic root dimensions (see Figure 2).
figure
Figure 1. Measurement sites of the aortic root using inner edge method at end diastole. A –aortic annulus, B – sinus of valsalva, C – sinotubular junction and D – proximal ascending aorta.
figure
Figure 2. Non-linear regression of the order y=a.xb for body surface area (BSA) to (a) aortic annulus, (b) sinus of valsalva, (c) sinotubular junction and (d) proximal ascending aorta.
figure
Figure 3. Non-linear regression of the order y=a.xb for height to (a) aortic annulus, (b) sinus of valsalva, (c) sinotubular junction and (d) proximal ascending aorta.
Table
Table 2. Absolute and linearly/allometrically scaled aortic root measurements to BSA and height. Cut-offs provided (+2 standard deviations) for linearly scaled to height measurements
Table 2. Absolute and linearly/allometrically scaled aortic root measurements to BSA and height. Cut-offs provided (+2 standard deviations) for linearly scaled to height measurements
View larger version
Allometric scaling of aortic root dimensions to BSA produced size independence for all aortic root dimensions with β exponent values of 0.574, 0.571, 0.742 and 0.543 for aortic annulus, sinus of valsalva, sinotubular junction and proximal ascending aorta, respectively. Allometric scaling of aortic root dimensions to height also produced size-independent indices with all β exponents close to 1 (Figure 3). Following assessment for the impact of age, the β exponents were similar to those obtained from the single variant assessment with c values close to 0 suggesting the derived exponent can be used across the age range of this population.
Discussion
The key findings of this study were (1) linearly scaling aortic root dimensions to height produced size-independent indices in elite Rugby Football League players, (2) allometric scaling using both height and BSA also produces size-independent aortic root dimensions, (3) BSA was found not to produce size-independent aortic root dimensions when using linear scaling methods and (4) absolute and linearly scaled aortic root dimensions were found to be within normal population limits.
The current study demonstrates a linear relationship of aortic dimensions to the athlete’s height. This finding has also been seen in a non-athletic population.14 The reason for this consistency likely reflects dimensionality theory in that height as well as all aortic root dimensions are one-dimensional measures and are thus likely linearly related. In view of this we can suggest that adopting this approach will likely produce size-independent indices and support more accurate clinical decision making. The technique is simple, quicker and more intuitive to clinicians than allometric scaling, so is applicable to busy clinical environments with time pressures. Linear scaling of aortic root dimensions to height would potentially reduce the number of unnecessary further investigations in sports with athletes of large body size such as basketball, where up to 4.6% of players may demonstrate an SOV measurement above the cut-off value of 40 mm.6,8 Based on the results of our study we would recommend that linear scaling to height is applied in the clinical setting. Values of up to 14 mm/m for the aortic annulus, 20 mm/m for the sinus of valsalva, 16 mm/m for the sinotubular junction and 16 mm/m for the proximal ascending aorta should be accepted as the upper limits of normal in elite male Rugby Football League players.
In a large cohort of elite Rugby Football League athletes, aortic root dimensions were all < 40 mm thus falling within the accepted normal range.7 Similar to existing data, allometric scaling of aortic root dimensions to BSA and height produced size-independent indices.14 This method does not rely on a linear relationship between the two variables. This approach has not been widely adopted in the context of daily clinical practice.10 This is likely due to the perceived mathematical complexities and hence linear scaling remains the primary scaling method for most measures of cardiac and vessel size.11 The impact of age was also assessed in our non-linear model and supports the use of this exponent when scaling for BSA across this age range (14–34) of elite Rugby Football League players.
Linear scaling using BSA, the process suggested in current clinical guidelines,7 did not result in size-independent aortic root dimensions. Similar to our study, current literature has consistently shown that linear scaling to BSA does not produce size-independent indices for a range of cardiac variables.11 Linear scaling assumes a linear relationship between the cardiac dimension and the body size parameter which is often not the case in biological relationships.12 In addition, BSA is open to measurement estimation error and cannot differentiate the impact of excess fat or lean tissue.12
Limitations
The participants in this study were a homogenous group of athletes suggesting that the results of this study can only be used to guide cardiac pre-screening for aortic root dilation in this population. The study only included male participants aged 14–34 so the results cannot be generalised to very young/older athletes or females. Ethnic differences were not considered and it has been noted that differential cardiac adaptations have been observed in varied ethnicities.19 The study employed a cross-sectional design so a cause and effect relationship between aortic root dimensions and body size cannot be fully established.
Conclusions
This paper adds to a growing evidence base that advocates the use of evidence-based scaling of cardiovascular variables. The paper also discourages the use of BSA as a linear scaling quantity as it does not produce size-independent measures of the aortic root, despite it being the current accepted practice in the clinical setting. The paper provides absolute and scaled values of the aortic root in an elite Rugby Football League population. This will help to guide practitioners in pre-screening programmes within this sport and may help prevent cardiac events in this population. The findings of this paper contribute to a growing database on normative aortic root measurements. Further work should be carried out to define normal values within other diverse sporting populations. These comparisons are made possible through the use of scaling to remove the effect of body size. Linearly scaling to height or allometric scaling is receiving growing attention from the scientific community as an appropriate method. Further research should aim to advocate this further.
Acknowledgements
Not applicable.
Contributors
SO, DO, LF, KG and JS – contribution to conception and design. SO, DO, LF – contribution to acquisition and analysis. All authors – contribution to interpretation. SO, DO – drafted manuscript. All authors – critically revised manuscript and gave final approval.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Ethics Approval
Ethical approval was obtained from the Ethics Committee at Liverpool John Moores University.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was undertaken as part of a student PhD fellowship funded through Cardiac Risk in the Young.
Guarantor
DO.
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