We convene a team of doctors whose role it is to decide. They weigh the risks and benefits for each patient. As with transplant, we have clear rules for who gets started on ECMO. You must be sick enough to benefit, but not so sick — with other comorbidities like metastatic cancer or multi-organ failure — that you will most likely die regardless. That night, the man’s lungs are more fragile and the calculus is that he is the one who gets the machine.

I grasp at these memories because I am stuck trying to understand what it might feel like for doctors like me to come face-to-face with the reality of a limited supply of ventilators. But the truth is, nothing in my experience could possibly ready me for this. I turn to the literature.

In the wake of Hurricane Katrina, clinicians and ethicists began to have these tough discussions, and several states advanced frameworks for how to allocate scarce resources during disasters. These guidelines prioritize the likelihood of a patient’s surviving to hospital discharge and beyond, and in some cases factor in age — meaning that a younger patient would get a ventilator rather than an older one.

They also suggest that we consider withdrawing the ventilator from a patient who has had a trial of critical care but is not improving, to make it available for someone else. These concepts all make sense to me in the abstract. And I suppose I should be reassured that if this moment comes to pass, there will be protocols.

But I think of explaining this to a patient or over the phone to a loved one I will never meet, and my heart breaks. As a critical-care doctor, I am comfortable with end-of-life conversations and I am familiar with death. But I have learned to initiate these conversations based on a patient’s goals and medical realities — not because of a shortage of machines that can be manufactured in a warehouse. Recently, a colleague told me about a patient who said he was willing to give up his ventilator to someone younger and healthier, who might benefit more. This is the world we live in now. The story gutted me.

But in Boston, we are not there. I hang up the phone and return to the buzz of the unit, to check on my patient. Sepsis from her pneumonia, coupled with the immune compromise of chemotherapy, threatens to overwhelm her. Though the ventilator is helping to buy her time, she still might not make it.

But I know that if she dies, I will be able to tell her husband that we did everything we could. I will be able to tell myself that too.

Daniela J. Lamas is a critical care doctor at Brigham and Women’s Hospital.

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