Evolving Concepts in Aerospace and United States Air Force Cardiology

Case Presentation #1: A 49-year-old male United States Air Force (USAF) pilot who has 25 years of service in the military, and over 3,000 hours in fighter jets (F-16), presented with complaints of decreased exertional capacity. He is an avid bicyclist and triathlete, with a new onset of dyspnea and chest pressure after completing a half Ironman triathlon the week prior. To evaluate these symptoms, the patient undergoes an exercise stress echocardiogram. He exercises for 24 minutes on a standard Bruce treadmill protocol and achieves 100% of his maximum predicted heart rate with no symptoms or ischemic electrocardiogram (ECG) changes. Resting echocardiographic images demonstrate normal left ventricular (LV) size and thickness with normal LV contractility. Exercise images reveal a moderate decrease in contractility of the mid to distal anterior wall and apex suggestive of ischemia. Coronary angiography demonstrates a 99% mid left anterior descending stenosis, which is treated with a drug-eluting stent. Six months later, the patient returns to running half-marathons and performing triathlons without symptoms. He now asks his physician if he can return to his flying duty. How should the Air Force flight surgeon proceed?

Case Presentation #2: A 22-year-old Air Force Academy cadet is applying for flight training in order to fulfill his lifelong dream of being a pilot. During the medical evaluation, ventricular pre-excitation is noted on his screening ECG. In order to evaluate a systolic murmur, an echocardiogram is performed, revealing hyperdynamic systolic function and a bicuspid aortic valve with mild regurgitation. Should he be allowed to proceed to pilot training?

Discussion

The goal of aerospace medicine is to protect the life and health of pilots, aircraft passengers, and individuals on the ground. Thus, the risk of sudden incapacitation to the pilot is of great concern. The aerospace environment may involve perturbations to normal physiology resulting from decreased oxygen content, as well as changes in temperature and barometric pressure. Furthermore, any condition that takes the pilot's undivided attention away from flying (i.e., palpitations, chest pressure, or dizziness) can have devastating effects. For high performance aircraft there are also added demands on the body, such as increased gravitational forces ("pulling G's"), which can reduce preload, and thus decrease cardiac output up to 40%. Therefore, Air Force aviators must be highly physically fit and avoid medications which could impair the normal adaptive cardiovascular mechanisms to these stresses. The USAF uses not only published data, but internal data collected over decades to develop policies.

Since the inception of the Air Force in 1946, all cardiac studies done on aviators have been maintained in a cardiac data repository library. As of 2015, this cardiac data library has over 1.2 million digitized studies on over 280,000 aviators and includes ECGs, echocardiograms, treadmill testing, nuclear stress testing, Holter/event monitors, angiography, cardiac CT and cardiac MRIs. This data, along with the airman's medical records, are studied extensively to develop policies regarding the ability to safely return to flying duty.

The prevalence of known coronary artery disease (CAD) in actively serving Air Force aviators is 0.5%, which corresponds closely to data published in young civilian aviators.1 Screening by most non-invasive modalities is limited by high false-positive test rates.2 For secondary prevention and major adverse cardiovascular event rates for CAD (such as cardiac death, myocardial infarction (MI), or revascularization as in case #1 above), an otherwise healthy population with CAD must be studied. The Aerospace Medical Service has continued to perform follow-up clinical evaluations on nearly all Air Force aviators with CAD since 1954. This includes former aviators who were disqualified from flight duties, as well as those who either retired or separated from the military. In 2008, a retrospective review of 30 aircrew members with prior revascularization, which totaled 95 person years with annual non-invasive ischemic evaluations, was performed to determine outcomes. In this cohort, there were no MIs, no deaths, and only two instances of recurrent obstructive disease found by non-invasive stress testing, which required repeat revascularization. This endpoint differed slightly from a prior similar study on 122 aviators with revascularization, (in which 50% were CABG) with combined endpoint of cardiovascular death, MI, or revascularization of 3.6% annual risk at five years.3 Based on this data, aviators with known obstructive CAD were allowed to return to flying based on reassuring findings on coronary angiography, echocardiograms, and stress testing.

Since 2008, 172 aviators with obstructive CAD (with and without revascularization) have undergone serial follow-up. There have been no deaths, no MIs, and no incapacitating events. The revascularization rate is 2% annually, all driven by recurrence of symptoms and/or concerning findings on non-invasive testing. Therefore, aviators with diagnosed obstructive CAD, either with or without revascularization, with favorable prognostic indicators, can return to flying and without physical restrictions. The only duty limitation is that patients with coronary revascularization are restricted to non-high-performance aircrafts, and must fly with another qualified pilot.

Bicuspid aortic valve (BAV) (as in case #2 above) was a disqualifying condition for pilots according to Air Force policy prior to 2008. The USAF cardiac database contains a total of 249 cases of BAV in aviators age 17-29 years-old, which have been followed for a mean of 10 years (the largest cohort of asymptomatic BAV aviators to date). These records revealed an 8.5% progression rate to severe aortic stenosis, regurgitation and/or valve replacements over a 10 year period.4 There were no deaths or instances of sudden incapacitation while flying. All cases of severe disease that went on to valve replacement were identified during routing testing. Given this excellent prognosis from an aviation perspective, the policy was changed in late 2008 to allow airmen with BAV to enter USAF flight training programs. Anecdotally, a pilot with BAV and severe aortic valve insufficiency underwent valve replacement in 2003 with a bioprosthetic porcine aortic valve. This patient had excellent functional status after five years with continued optimal functioning of his valve by echocardiograms. This prompted a literature review on bioprosthetic valves showing excellent outcomes out to 10 years, and less than 1% annual sudden incapacitation rates.5 In 2008, he became the first attack pilot returned to flight status with aortic valve replacement. He receives annual follow-up and currently is still flying for the USAF.6

Unlike the low prevalence of obstructive CAD or valvular disease in aviators, arrhythmias and cardiac electrical abnormalities are much more common. The authors of this analysis recently completed a review of their cardiac database, which yielded 682 aviators with ventricular pre-excitation on ECG with no history of Wolff-Parkinson-White (WPW) syndrome who were followed for over 10 years to evaluate for onset of symptoms. In this cohort, 64 (9.4%) developed signs or symptoms consistent with WPW syndrome. The only statistically significant differences in those who progressed were younger age, lower total cholesterol and low-density lipoprotein, lower diastolic blood pressure, and higher physical fitness test scores. The annual event rate was 0.93%. In those with ablation, return of ventricular pre-excitation or symptoms was <1% annually with no sudden incapacitation. Based on this data, young asymptomatic pilots with WPW pattern do require an electrophysiology (EP) study, with subsequent ablation if high risk findings are present; however, pilots who have already completed training and have reassuring findings on history and physical along with non-invasive studies are not required to undergo an EP study.

The prevailing concept in Aerospace cardiology in the modern era is that of evidence-based medicine. In the distant past, policy was derived from expert opinion without clear data supporting all decisions. The USAF is on the forefront of changing long-held medical standards in Aerospace cardiology and continues to collaborate closely with the US Army, Navy, NASA, and NATOs to keep the aerospace environment safe throughout the world. With regards to case #1 above; this Colonel with CAD s/p revascularization was granted a flying waiver and cross trained from a fighter, (high-performance single seat F-16) to a cargo aircraft (multi-passenger C-130). Regarding case #2 above, this cadet underwent EP study and ablation of a left posterior-lateral pathway. His BAV has remained stable with only mild aortic insufficiency. Recently, he finished pilot training and currently is an F-16 fighter pilot in the USAF.

References

  1. Rayman RB, Davenport ED, Kruyer WB, et.al. In Rayman's Clinical Aviation Medicine. 5th ed. New York, NY: Castle Connolly; 2013:21-35.
  2. Fizsimmons P, Palm-Leis A, Thompson W, Kruyer W. Comparison of noninvasive cardiac testing in 759 military aviators: Angiographic correlation and clinical follow-up. Aviat Space Environ Med 2001; 72:229.
  3. Barnett SL, Fitzsimmons PJ, Kruyer WB. Coronary artery revascularization in aviators: outcomes in 122 former military aviators. Aviat Space Environ Med 2003;74:389.
  4. Davenport ED, Kruyer WB. Clinical and Aeromedical Guidelines for Bicuspid Aortic Valve. Presented at the Aerospace Medical Association 83rd Annual Scientific Meeting, May 2012. Aviat Space Environ Med 2012;83:307.
  5. Doty JR, Salazar JD, Liddicoat JR, Flores JK, Doty DB. Aortic valve replacement with cryopreserved aortic allograft: Ten-year experience. J Thorac Cardiovasc Surg 1998;115:371-80.
  6. Hardy JC, Leonard F. Aortic valve replacement in a United States Air Force pilot: Case report and literature review. Aviat Space Environ Med 1997;68:221-4.

Clinical Topics: Arrhythmias and Clinical EP, Congenital Heart Disease and Pediatric Cardiology, Diabetes and Cardiometabolic Disease, Clinical Topic Collection: Dyslipidemia, Heart Failure and Cardiomyopathies, Invasive Cardiovascular Angiography and Intervention, Noninvasive Imaging, Prevention, Sports and Exercise Cardiology, Valvular Heart Disease, Implantable Devices, EP Basic Science, SCD/Ventricular Arrhythmias, Atrial Fibrillation/Supraventricular Arrhythmias, Congenital Heart Disease, CHD & Pediatrics and Arrhythmias, CHD & Pediatrics and Imaging, CHD & Pediatrics and Interventions, CHD & Pediatrics and Prevention, CHD & Pediatrics and Quality Improvement, Lipid Metabolism, Nonstatins, Interventions and Coronary Artery Disease, Interventions and Imaging, Interventions and Structural Heart Disease, Angiography, Computed Tomography, Echocardiography/Ultrasound, Nuclear Imaging, Exercise, Sports & Exercise and Congenital Heart Disease & Pediatric Cardiology, Sports & Exercise and ECG & Stress Testing, Sports & Exercise and Imaging

Keywords: Aerospace Medicine, Aortic Valve, Aortic Valve Insufficiency, Aortic Valve Stenosis, Aviation, Blood Pressure, Cardiac Output, Cholesterol, Constriction, Pathologic, Coronary Angiography, Coronary Artery Disease, Dizziness, Drug-Eluting Stents, Dyspnea, Echocardiography, Electrocardiography, Electrophysiology, Evidence-Based Medicine, Exercise Test, Expert Testimony, Follow-Up Studies, Heart Rate, Heart Valve Diseases, Lipoproteins, LDL, Medical Records, Military Personnel, Myocardial Infarction, Oxygen, Physical Fitness, Prevalence, Prognosis, Retrospective Studies, Running, Secondary Prevention, Surgeons, Systolic Murmurs, Tomography, X-Ray Computed, Wolff-Parkinson-White Syndrome, Sports


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