Those Pesky PR Intervals

There’s that old Chinese curse, “May you live in interesting times,” and we are sure living in some interesting times right now. We are both blessed and cursed by a deluge of social, societal, informational, political, and cultural changes. Some are for the better, plenty are for the worse, but we’ve gotten pretty far down into the weeds when the debate is over a few thousandths of a second. 

The issue that has prompted the current outburst is a change to acceptable pilot EKGs, increasing the top end limit of the “PR interval” by exactly 8/100 of a second. As everyone knows, EKGs are a measure of the electrical activity and timing of the cardiac cycle. ‘P’ waves measure the electrical activity of the atria, the two smaller upper chambers of the heart that collect venous blood returning from the body (right atrium) and oxygenated blood returning from the lungs (left atrium). The ‘R’ wave is the start of contraction of the ventricles, the major pumping chambers that send blood to the body (left) and lungs (right). The time the atria have to empty out before the ventricles start to contract and send the blood they get from the atria off to the body is that now famous PR interval, the timing between the ‘P’ waves and the ‘R’ waves. There’s no “settled science” on exactly how long that interval should be and exactly what is “normal,” as there’s so much “normal variation” in healthy adults. Still, it is important because the length of the PR interval, too short or too long, can indicate heart disease.

Traditionally that time interval has been set at up to 0.21 sec until it is considered “prolonged,” and beyond that is considered one of a number of different iterations of AV (atrio-ventricular) heart block. With the change the FAA made, an increase of up to 0.08 sec, which establishes a PR interval of <0.3 seconds in otherwise healthy and asymptomatic pilots, is now acceptable. The AOPA has already offered our support to Federal Air Surgeon Dr. Susan Northrup, who announced the changes in October of last year based on widely agreed and established medical and clinical data. All the fuss over these changes comes back to our curse of “interesting times” that have raised some questions: (1) Is this a reasonable change based on supportable clinical data? (2) Is it safe for pilots to fly with this condition? (3) Is there a side effect of the COVID virus or vaccine that injures the heart in a way that prolongs intracardiac conduction? And (4) did the FAA make the change based on some tacit acknowledgment that the virus and/or vaccine is wreaking such havoc on the heart that without accepting this condition there would be no planes in the sky?

Answering these questions is a bit complicated in our current times since we no longer have an agreement on much of anything and that includes what science actually is, let alone what “settled science” means. Science used to be a field of study defined by methodical inquiry and discovery that amplified our present knowledge with additional facts and knowledge breakthroughs leading to thoughtful, evidence-based reassessments of our currently held beliefs. In our current “interesting times,” traditional scientific methods have been preempted by emotionally based rabid, visceral assertions of “settled science” that are, by the very wording, totally the opposite of the core scientific principles. Much to our detriment, scientific research and medical publishing are now more often driven by sources of funding, political ideologies and grant requirements rather than a genuine search for fact-based discoveries.

Even with that background, every cardiologist and cardiology textbook has traditionally defined first-degree AV Block as a PR Interval between 0.21 and 0.29 seconds. For more than half a century, long predating our “interesting times,” a prolonged PR interval < 0.3 seconds has been known to be a common variant and generally asymptomatic. There is no clinical data that indicates this condition causes any significant complications or risks for the patient. As far back as 1944, a US Air Force study of 1,000 healthy, asymptomatic pilots found PR intervals up to 0.28 seconds and in a ten-year follow-up these aviators found no clinical complications or symptoms had developed. The American College of Cardiology published an extensive review of AV delay (they label it “AV block,” an overstated misnomer) in 2018 and doesn’t even consider asymptomatic AV intervals <0.3 seconds to be a pathological condition that requires any intervention.

Numerous recent clinical studies show that first-degree AV delay with a PR interval 0.21 to 0.29 seconds is a normal variant and for patients who have normal cardiac function, no treatment is necessary beyond routine observation for possible worsening conduction delay. A recent meta-analysis of over 400,000 patients who were incidentally found to have first-degree delay confirmed this and stated, “Current expert advice suggests that this poses little risk and is not associated with significant symptoms and no specific treatment is required.” Most published studies find some degree of AV delay on routine EKGs in 1.0% to 1.5% of normal adults under age 60. A recent study obtaining EKGs on over 2% of the entire general population in Finland confirmed this, documenting a 2.5% incidence of asymptomatic prolonged PR interval with no complications in over 30 years of follow-up. They found the prevalence of AV delay increases with age and after 60 it’s found on routine EKGs in about 6% of normal, asymptomatic adults. It’s more common in males by about a 2 to 1 ratio. The finding is present in more than 10% of young athletes due to increases in parasympathetic autonomic tone. The new FAA standard of <0.3 sec is still stricter than acceptable limits in sports organizations since in healthy athletes a PR interval less than 400 milliseconds is considered normal.

These data are consistent with the new FAA guidelines. An AME can still refer any pilot with AV delay for cardiac evaluation at their discretion, and the guidelines also stipulate that any pilot who presents with delayed PR interval and symptoms such as dizziness, fainting, and shortness of breath or palpitations must be deferred for cardiac evaluation. PR intervals of 0.3 seconds or longer (considered “marked delay”) is not accepted under the revised FAA guidelines that also require deferral for a complete cardiac work-up. This is in agreement with published studies that show patients with a marked PR interval delay (0.3 seconds or greater) are at an increased risk of developing atrial fibrillation, congestive heart failure, or higher degree AV delay leading to complete block and risks of syncope and even sudden death. So the long answer to questions (1) and (2) is, yes, changing the acceptable PR limits is reasonable and well supported by decades of research data and clinical experience.

Answering the last two questions is also complicated by our present “interesting times,” as any efforts to discuss COVID issues with an exchange of serious information have become almost impossible. Attempting to tiptoe around the sensitivity of others, the answer to question (3) is also yes. It’s clear from dozens of sources of clinically supportable data that there is a risk of myocardial inflammation from both the COVID virus and vaccine. The virus triggers a cascade of inflammatory responses that I’ve talked about in these pages for over a year and by their very nature, vaccines are supposed to set off an inflammatory response; that’s what builds immunity to the disease it’s intended to fight. There’s also published data that inflammation from COVID infection can rarely prolong PR intervals but, also, without clinical significance in most patients. Although the same degree of certainty is not possible with possible side effects of the vaccine, it’s reasonable to postulate that the mRNA shots can also lead to AV delay in rare cases.

Acknowledging these facts, we come to the hot-button item to answer question (4) and whether the FAA suruptitiously changed their regulations to accommodate COVID- or vaccine-related cardiac complications. Digging into the background of the FAA’s decision to increase acceptable PR interval limits, it’s clear that these changes were in the works at least 5 years ago and formally issued back in 2017 and 2018. This is obviously several years before the virus entered the population. Dr. Warren Silberman, former manager of the FAA Aerospace Medical Certification Division and former consultant to the AOPA Piot Protection Services, can be seen here 5 years ago documenting the changes in an FAA video for AME guidance that had been issued the year before. There have been a number of different internal and external documents dating back years that were used by physicians and medical reviewers to develop the new OneGuide that will incorporate all the current revisions into one single source document. Changes that reflect that 2018 policy were released back in October 2021 and prompted the current fuss. So the answer to question (4) is no, there’s no evidence this was done for any other reason than the FAA is simplifying pilot medical exams and issuances by making a mandatory work-up of asymptomatic pilots with normal cardiac function and this EKG variant unnecessary. The regulations preserve the ultimate issuance of medical clearance up to the discretion of the FAA to evaluate on a case-by-case basis.

I’ve quoted Mark Twain dozens of times in these pages, since his acerbic wit and incredible command of the language only require only a few words to get right to the “heart” of the matter. Borrowing again from his insight into human nature he once commented, “You just can’t reason a man out of an opinion he didn’t reason himself into.” Even though he said that over a hundred years ago it applies even more now in our “interesting times.” “Reason” no longer applies and things that used to be “true” now all depend on your political leanings, opinions, manners, and emotions. People’s “truths” are no longer arrived at by “reason,” instead by emotion, so there’s “just no reasoning” with many folks. No one will change their mind, and disagreeing with someone else’s “truth” brings a vitriolic barrage of social media barbs directed your way. But I’m an optimist and hopefully a few “reasonable” facts will bring some clarity to this discussion and help those who launched these attacks see that the changes were made long ago and will help expedite medical clearance to fly, and do not risk safety in the sky.

Kenneth Stahl, MD, FACS
Kenneth Stahl, MD, FACS is an expert in principles of aviation safety and has adapted those lessons to healthcare and industry for maximizing patient safety and minimizing human error. He also writes and teaches pilot and patient safety principles and error avoidance. He is triple board-certified in cardiac surgery, trauma surgery/surgical critical care and general surgery. Dr. Stahl holds an active ATP certification and a 25-year member of the AOPA with thousands of hours as pilot in command in multiple airframes. He serves on the AOPA Board of Aviation Medical Advisors and is a published author with numerous peer reviewed journal and medical textbook contributions. Dr. Stahl practices surgery and is active in writing and industry consulting. He can be reached at [email protected].

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