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Exposure to acceleration forces (+Gz) and anti-G protective manoeuvres causes changes in cardiac preload and afterload. These changes can result in cardiac hypertrophy or enlargement. Previous studies regarding the effect of acceleration in high-G aviators (HGAs) are few and inconclusive. An echocardiographic study was initiated to determine whether there are structural or functional cardiac differences between HGAs and low-G aviators (LGAs).
The present study was a cross-sectional study in which echocardiographic parameters in HGAs were compared with those in LGAs. Both retrospective and prospective data were collected. Fifty aviators were included in each group. The aviators who participated in the present study were randomly chosen from a cohort with similar demographic characteristics and flying hours. No major differences were found in cardiac dimensions and function between HGAs and LGAs. The authors speculate that the reason why no major differences were found was due to the short period of total exposure to very high +Gz forces and anti-G measures.
Exposure to both acceleration forces (+Gz) and anti-G protective manoeuvres cause changes in cardiac preload and afterload. These changes can hypothetically cause pathological changes in cardiac dimensions and function. An analogous phenomenon was previously observed in body builders and heavyweight lifters, who strain for short periods against a closed glottis (1).
Previous studies regarding the effect of acceleration on cardiac dimensions and function in high-G aviators (HGAs) are few and inconclusive. An increase in right ventricular dimensions in HGAs, compared with transport aviators, was reported in one study (2). This finding was concordant with elevated right ventricular pressure in miniature swine exposed to acceleration stresses (3). Contrary to these findings, a study comparing cardiac dimensions in 289 HGAs and 254 low-G aviators (LGAs) – the largest study to date – found no significant changes in cardiac dimensions (4).
The aim of the present study, conducted among Israeli Airforce aviators, was to determine whether there are structural or functional cardiac differences between HGAs and LGAs.
In a cross-sectional study, echocardiographic parameters of HGAs and LGAs (transport or rotary wing aviators) were compared. All subjects were active male Caucasian military aviators. Female aviators were not included because of their few numbers. Both retrospective and prospective data were collected.
A total of 50 aviators were included in each study group. The aviators participating in the present study were randomly chosen from a cohort with similar ages and flying hours. All aviators underwent an initial echocardiographic study as a routine screening of all military aviator cadets. The HGA group underwent follow-up echocardiography as a routine periodic screening. LGAs had a follow-up examination due to an abnormality in the routine physical examination or due to the present study. Only aviators who had two or more echocardiographic examinations at an interval of at least five years were included. Only pathological findings that were absent in the initial echocardiographic study and that could not be related to an established cardiac pathology (eg, valvular defect due to rheumatic heart disease) were included in the present study.
Participants were excluded for the following reasons: known cardiovascular or pulmonary disease, concurrent high blood pressure and low blood pressure, and lack of information regarding exercise or smoking habits.
Demographic and biographical details included age, body mass index, blood pressure, smoking status, active flying years and exercise habits.
Transthoracic echocardiography was performed with an ATL 5000 machine (Advanced Technology Laboratories, USA) using a 2.5 MHz to 4.0 MHz transducer. All echocardiography studies were performed by the same cardiac sonographer. Two-dimensional M-mode guided measurements, Doppler echocardiography quantification and evaluation of the degree of valvular regurgitation were performed according to the recommendations of the American Society of Echocardiography (5,6). The echocardiographic studies were interpreted by two experienced cardiologists.
Statistical analysis was performed with SAS spreadsheet statistical software (SAS Institute Inc, USA), using the Student’s t test to compare the different diagnostic techniques. A value of P<0.05 was considered to be statistically significant.
One hundred aviators were included in the present study: 50 were HGAs and 50 were LGAs. All of the study participants were Caucasian men. Both groups had similar demographic and clinical characteristics. Table 1 summarizes the demographic characteristics of our study groups.
No major differences between HGAs and LGAs were found in left ventricular mass and function (Table 2). Right ventricular size and mass did not significantly differ between both groups. Because univarite analysis did not demonstrate significant differences between both groups, multivariate analysis was not performed.
Significant pathological echocardiographic findings were not found in either group, most probably due to the meticulous screening and medical follow-up programs that the aviators undergo (7).
The present study was not able to detect any significant cardiac structural or functional defects in aviators in general, and showed no echocardiographic difference between HGAs and LGAs. The results of the present study agree with those of the largest study published to date (4), in which no major differences were detected in the echocardiographic examination results of 289 high-G and 254 low-G NATO pilots. The results of the NATO study and the present study support the hypothesis that cardiac changes do not occur in aviators exposed to significant +Gz forces and anti-G manoeuvres.
HGAs are exposed to significant and repetitive +Gz forces. To counterbalance its effects on performance and consciousness, aviators perform anti-G measures, which include respiratory anti-G straining manoeuvres, positive pressure breathing and use of anti-G suits (8). Both exposures to +Gz and anti-G measures result in repetitive intrathoracic hydrostatic changes, which can cause significant changes in cardiac preload and afterload (9). These changes can be compared with those of heavyweight lifters and body builders, who perform isometric exercise, with maximal voluntary muscular contraction combined with forced exhalation against a closed glottis. Echocardiographic studies in weight lifters detected increased sum of wall thickness (septum plus posterior wall), relative wall thickness and left ventricular end-systolic stress (10). Because HGAs perform similar respiratory measures, changes in cardiac structure can be expected; however, our study and the NATO study (4) did not detect such changes.
Our study did not address why there was a lack of changes in cardiac structure in HGAs. We speculate that this result was due to the short period of total exposure to very high +Gz forces and anti-G measures, which does not accumulate to cause sufficient hydrostatic cardiac changes that result in cardiac hypertrophy or enlargement.
Our medical policy includes an echocardiographic examination every five years in all HGAs. The present study results, as well as the NATO study results, contradict the need for periodic echocardiographic examinations, and may save time and money spent on these examinations.
Our study has a few limitations. First, our study design was partially retrospective. Second, some of the LGAs included in our study underwent echocardiographic examinations because of an abnormal physical examination. Although aviators with known cardiac defects were not included in the present study, and none were found to have significant cardiac defects, a selection bias is possible.
The present study shows no evidence of structural or functional cardiac changes in aviators exposed to high +Gz forces. The results of our study concern only the current aircrafts used in the Israeli Airforce. Development of the next generation of fighter aircrafts with better performances, and their possible effect on aviators’ health and welfare call for further studies and follow-up.