Background
Military jet fighter pilots are routinely exposed to acceleration (+Gz) forces. This recurrent exposure may influence various cardiac parameters. A few previous studies have evaluated the impact of exposure to acceleration forces on cardiac morphology and function, but these studies were mostly based on small cohorts, and subjects did not undergo baseline echocardiographic examinations before +Gz exposure.
Methods
Ninety-six jet fighter pilots with high +Gz exposure underwent echocardiographic evaluation before and 7 to 12 years after repeated +Gz exposure. Echocardiographic parameters were recorded using M-mode echocardiography and included left ventricular diameter at end-systole and end-diastole, interventricular septal thickness, thickness of the posterior wall, aortic root diameter and aortic valve opening, diameter of the left atrium, and left ventricular mass. Medical records of the subjects identified were evaluated for the development of adverse events.
Results
The average age at the time of the initial echocardiographic examination was 19.2 years. All subjects were healthy, without cardiovascular risk factors, and had no prior exposure to acceleration forces. The average flying period on jet planes at the time of follow-up examination was 1,812 hours. The mean follow-up period was 9.13 years. All parameters evaluated by M-mode echocardiography were not significantly changed from the baseline examination. No adverse events occurred during the follow-up period.
Conclusions
Exposure to acceleration forces has no significant impact on cardiac and aortic morphology.
Exposure to both acceleration forces (+Gz) and anti-G protective maneuvers may have a deleterious effect on the heart by inducing changes in cardiac preload, afterload, and the maximal pressures generated in the aorta, left ventricle, right ventricle, and left atrium. These changes, which have been demonstrated in animal models, can hypothetically cause long-term pathologic changes in cardiac dimensions and function within the population of military and civil high-G jet fighter pilots. An analogous phenomenon was previously observed in bodybuilders and heavy weight lifters, who strain for short periods against a closed glottis, but these data are conflicting and cannot serve as the sole reference regarding aviators.
The impact of acceleration forces on the cardiovascular system has been a subject of extensive research, but results are inconclusive. An increase in right ventricular dimensions in high-G jet fighter pilots, compared with transport aviators, was reported in one study. This finding was concordant with elevated right ventricular pressures in miniature swine exposed to acceleration stresses. Contrary to these findings, a study comparing cardiac dimensions in 289 high-G pilots and 254 low-G pilots (the largest study to date) found no significant changes in cardiac dimensions. Other smaller studies support these findings.
Thus, much remains unknown about the impact of these forces on heart structure and function, and this is certainly the basis for some concerns regarding the long-term cardiovascular health of jet fighter pilots.
We performed the first study on jet fighter pilots in which baseline echocardiographic studies performed before any +Gz exposure were compared with follow-up studies to assess the impact of acceleration forces on various cardiac and aortic parameters evaluated by M-mode echocardiography.
Methods
Israeli Air Force (IAF) academy candidates are evaluated before their enlistment in the military at the IAF Aeromedical Center. All candidates undergo thorough histories and physical examinations, including anthropometric measures and blood pressure measurement, resting electrocardiography, pulmonary function testing, and routine baseline echocardiographic examinations. It is required that the results of all tests be normal before entering the air force academy.
Echocardiographic inclusion criteria for admission to the jet fighter track include normal cardiac dimensions, normal ejection fraction, and no valvular pathology except for insignificant regurgitations. All tests (except echocardiography) are performed yearly thereafter, so it may be assumed that all aviators in the IAF are generally healthy. Echocardiographic follow-up has been performed routinely for IAF jet fighter pilots since 2003 according to IAF Aeromedical Center policy. All echocardiographic examinations were performed after careful physical examinations and electrocardiography. Positive findings on physical examination or electrocardiography were recorded.
We retrieved all files of active high-performance jet fighter pilots who underwent echocardiographic examinations before their enlistment and thus before any +Gz exposure and excluded all candidates with abnormal findings, except those with minor valvular regurgitations. All pilots underwent routine second follow-up echocardiographic examinations after ≥7 years of +Gz exposure. Anthropometric measures, blood pressure, and heart rate were recorded at the time of the first and follow-up echocardiography. All subjects were male Caucasian military aviators who flew >1,600 hours on high-performance jet aircraft (F-16 or F-15) capable of performing up to 9 G. Female aviators were excluded because they did not present in adequate numbers for statistically meaningful conclusions. Baseline echocardiographic measurements results were collected retrospectively, whereas follow-up measurements were performed prospectively. Only subjects with normal baseline echocardiographic findings were included. Participants were excluded for known cardiovascular or pulmonary disease or elevated blood pressure above the cutoff of 140/90 mm Hg.
To assess the influence of “G dose” exposure variability on cardiac parameters, we divided our cohort into two groups. The first group was composed of 20 aviators from squadrons in which most missions are “air to air” ones, thus maintaining an average of approximately 8 min of high-G (>5 G) per flying hour. The second group contained 76 aviators from squadrons in which most mission are not air to air, who are thus exposed to high G forces for an average of 4 min/hour. The two groups were compared by means of flying hours and cardiac parameters.
All echocardiographic studies were performed at the IAF Aeromedical Center with one of three devices (HP 500 Sonos, ATL 5000HDI, and Philips HD 11 XE; Philips Medical Systems, Andover, MA). All studies were done by one of three experienced sonographers and interpreted by one of three cardiologists specializing in echocardiography. Most studies (>66%) were performed by one of the sonographers and interpreted by one of the cardiologists. All examinations were reinterpreted by a single experienced cardiologist. The intraclass coefficient of reliability for interexaminer variability was >0.95 for all absolute cardiac parameters ( Table 1 ). Transthoracic echocardiography was performed by M mode. The M-mode measurements were evaluated from the two-dimensional images. The measurements were performed by positioning the curser perpendicular to the parasternal long-axis view of the left ventricle, aorta, and left atrium. Cardiac parameters were measured according to the American Society of Echocardiography’s guidelines and standard committee recommendations for echocardiographic measurements. When abnormal findings were encountered, additional measurements were performed using two-dimensional images. Left ventricular and atrial volumes were measured only when abnormal dimensions were found. Ejection fraction was estimated visually, which is regarded as acceptable method in the context of normal examination. Left ventricular mass was calculated automatically from the M-mode measurements on the basis of a standard mathematical formula described originally by Troy et al. and modified by the American Society of Echocardiography. Measurements were corrected for body surface area.
| Variable | First echocardiography | Follow-up echocardiography | P | Intraclass coefficient |
|---|---|---|---|---|
| Aortic root (mm) | 28.4 ± 2.7 | 30 ± 3.4 | .41 | 0.95 |
| Left atrium (mm) | 33 ± 4.4 | 34.5 ± 4.1 | .38 | 0.96 |
| Left ventricular end-diastolic diameter (mm) | 50.5 ± 3.9 | 51.8 ± 3.7 | .62 | 0.95 |
| Left ventricular end-systolic diameter (mm) | 30.9 ± 3.7 | 31.8 ± 3.5 | .41 | 0.97 |
| Septum (mm) | 9 ± 1.3 | 9.2 ± 1 | .22 | 0.95 |
| Posterior wall (mm) | 8.8 ± 1.3 | 8.6 ± 0.9 | .15 | 0.96 |
| Left ventricular mass (g) | 202 ± 43 | 210 ± 37.5 | .52 | 0.95 |
The study was approved by the ethics committee of the Medical Corps of the Israeli Defense Forces.
Statistical analysis was performed using SAS (SAS Institute Inc, Cary, NC). Pearson’s correlation was used to determine the association between each related variable. One-sample t tests were used to compare continuous variables. P values ≤ .05 were considered significant for all statistical analyses.
Results
Ninety-six military jet fighter pilots who underwent baseline normal echocardiography before any +Gz exposure were identified. Their medical records and echocardiographic studies were reviewed. The second echocardiographic examination was performed 9.13 years (range, 7–12 years) after the initial examination. Anthropometric measures, blood pressure, and heart rate are presented in Table 2 . No significant changes in these parameters were noted. Echocardiographic measures are presented in Table 1 . M-mode parameters were all minimally elevated compared with the baseline echocardiographic evaluation, but no statistical significance was reached. No correlations between cardiac parameters, rate of minor regurgitations, and the number of flying hours or G exposure were found when comparing pilots who are routinely exposed to higher G doses with those who are exposed to “standard” G doses ( Table 3 ). Follow-up examinations demonstrated 84 nonsignificant valvular regurgitations (mitral, tricuspid, and pulmonary) in 42 subjects (44%) compared with 19 regurgitations in 19 subjects (20%) at baseline examinations. Significant valvular regurgitation was found neither at baseline nor on follow-up examinations.
| Variable | First echocardiography | Follow-up echocardiography | P |
|---|---|---|---|
| Age (y) | 19.2 ± 1.4 | 28.3 ± 3.6 | |
| Flying hours on high-+Gz fighter jet planes | 0 | 1,812 ± 118 | |
| Body surface area (m 2 ) | 1.8 ± 0.1 | 1.9 ± 0.1 | .85 |
| Weight (kg) | 69 ± 7.6 | 75 ± 10 | .79 |
| Systolic blood pressure (mm Hg) | 123 ± 11.6 | 123 ± 10.8 | .22 |
| Diastolic blood pressure (mm Hg) | 73 ± 8.3 | 74 ± 8.7 | .15 |
| Heart rate (beats/min) | 65.5 ± 11.7 | 63.5 ± 10.7 | .24 |
| Variable | Standard exposure ( n = 76) | High exposure ( n = 20) | P |
|---|---|---|---|
| Age (y) | 28.2 ± 3.5 | 28.9 ± 3.8 | .67 |
| Flying hours | 1796 ± 120 | 1840 ± 112 | .1 |
| Body surface area (m 2 ) | 1.9 ± 0.1 | 1.9 ± 0.2 | .41 |
| Weight (kg) | 75.1 ± 10.1 | 74.2 ± 9.6 | .47 |
| Systolic blood pressure (mm Hg) | 122.2 ± 11.3 | 124.8 ± 8.6 | .73 |
| Diastolic blood pressure (mm Hg) | 74.2 ± 8.9 | 73.9 ± 8.4 | .37 |
| Heart rate (beats/min) | 63.6 ± 11 | 62.9 ± 10.4 | .42 |
| Aortic root (mm) | 30 ± 3.7 | 30.1 ± 2 | .35 |
| Left atrium (mm) | 34.8 ± 4 | 33.1 ± 4 | .91 |
| Left ventricular end-diastolic diameter (mm) | 51.8 ± 4 | 51.6 ± 2.3 | .38 |
| Left ventricular end-systolic diameter (mm) | 31.6 ± 3.6 | 32.4 ± 3.4 | .70 |
| Septum (mm) | 9.3 ± 0.9 | 8.9 ± 1.4 | .96 |
| Posterior wall (mm) | 8.6 ± 0.9 | 8.6 ± 1 | .33 |
| Left ventricular mass (g) | 211 ± 37 | 202 ± 39 | .72 |
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