On a night flight from Boston to San Francisco a voice came over the loudspeaker. “If there’s a doctor on board, would you please identify yourself by pressing your call bell?”

I reached up to hit the button, shrugging nonchalantly in response to my seat mate’s questioning look. “Duty calls,” my shrug was meant to convey.

But I didn’t feel nonchalant. What I felt was approximately two parts excitement and one part dread. There was the adrenaline rush of coming to the rescue, but also the fear of not being up to the task. This is how I always feel when called upon to provide emergency medical care outside the clinic or hospital, lacking the usual equipment and personnel, and especially when dealing with a problem outside my expertise.

A bystander who assists someone in an emergency is called a Good Samaritan. This term derives from the parable in the New Testament in which a Jewish traveler is beaten by robbers and left on the side of the road to die. His own people pass by without assisting the man, but a non-Jew, a native of Samaria, stops to help. The parable emphasizes the virtue of aiding a stranger.

Good Samaritan laws may require bystanders to aid an injured person in an emergency or protect an intervening bystander from a lawsuit should the victim fare poorly after being helped. These laws vary widely from state to state. In Massachusetts, bystanders have no obligation to help, but Good Samaritans, whether or not they have medical training, are immune from litigation if they have acted in good faith.

Even with legal immunity, though, these situations are never entirely comfortable — at least for me.

A few years ago, on a rainy Labor Day weekend, my neighbor, David, phoned and asked if I made house calls. “Sure,” I said. “What’s up?” David told me he didn’t think anything major was going on, but he just didn’t feel right. I grabbed my stethoscope, the only piece of medical equipment I keep at home, and walked the few yards from my house to his.

David was in his early 50s then, and very fit — the kind of neighbor you might see scaling his roof to make a repair. When I arrived, David greeted me at the door looking pretty well. Again I asked what was wrong and again he couldn’t quite articulate it, but perhaps unconsciously, he kept passing his hand over his chest.

I asked him to take off his shirt and lie down on the couch. I took his pulse, which was strong but irregular. When I listened to his heart, the diagnosis seemed clear. “I think you have atrial fibrillation.” That’s when the smaller chambers of the heart, the atria, quiver instead of beating normally. It’s very common, usually not too serious, and treatable. When someone looks as well as David did, “a fib” isn’t an emergency, and yet some sixth sense told me he needed to go to the ER. David told me he had a few things to do around the house first. His wife and I exchanged looks.

“No,” I said, “I want you to go now.” And, after a little more coaxing, David allowed his wife to drive him to our local community hospital, just five minutes down the road.

Many hours went by and I was surprised not to hear from David or his wife. People with atrial fibrillation who are stable are rarely hospitalized. They usually receive medication, sometimes blood thinners to prevent the tiny blood clots than can form in the quivering atria, and are sent home.

Finally, I called David’s wife and she told me that David had been given lidocaine intravenously and transferred to a larger medical center for a cardiac catheterization. Lidocaine? I thought. Catheterization? “What exactly did they say was wrong with him?” I asked. “Ventricular tachycardia,” she answered.

I had a wave of nausea.

“V-tach,” unlike a fib, is a very dangerous heart rhythm in which the large chambers of the heart, the ventricles, beat rapidly and blood pressure drops. In some cases, ventricular tachycardia leads to ventricular fibrillation, when the ventricles quiver and blood flow to the brain stops. It’s usually fatal if not treated immediately.

Of course if I’d known that’s what David had, I would have called 911. But I didn’t know and, really, how could I have, standing there in his cozy living room, with David looking so well?

When I made my way up the aisle on that plane to San Francisco, a passenger who had been lying on the floor was starting to get up. A flight attendant told me that everything was under control. My services were not needed. My emotions: one part disappointment, two parts relief.

As for David, he had a procedure to destroy the tiny area on his ventricle that was causing the v-tach and he’s been fine since. Now in his 60s, he’s still very fit, still climbing on his roof.

When someone new moves into the neighborhood, he introduces me as “The woman who saved my life.” It’s true. But it’s just as true that I’m the woman who could have been responsible for his death.

Suzanne Koven is an internal medicine physician who blogs at In Practice at Boston.com, where this article originally appeared. She is the author of Say Hello To A Better Body: Weight Loss and Fitness For Women Over 50.