As a doctor, I was not surprised the first question asked at the White House physician’s news conference on Saturday was about President Trump’s oxygen saturation level. That would be what any clinician would want to know to assess how sick a patient might be and whether supplemental oxygen or even a ventilator is needed. It is vital clinical data, and, fortunately, Sean Conley’s answer of 96 percent was reassuring. The number fell within the normal range, which is generally over 95 percent for patients who don’t have preexisting lung conditions.

But on Sunday, Conley admitted that the president’s blood oxygen level had dipped. When asked whether it had dropped to the “low 90s,” he responded it hadn’t gone down to the “low 80s.” The timeline is unclear, but worrisome drops in oxygen levels, in the aggregate, are clearly what prompted transfer to the Walter Reed National Military Medical Center, a brief course of supplemental oxygen, experimental monoclonal antibodies, remdesivir and, more recently, the addition of steroids. All these measures were a response to the desaturation of the oxygen in the president’s blood. If there is a single metric to watch in covid-19, it is a patient’s oxygen saturation level.

However, there is much more than clinical value to knowing the president’s oxygen saturation level in this case. The number may also be of critical value to American democracy. As the nation begins to contemplate presidential incapacity, the president’s oxygen saturation can help guide the timing of any transfer of presidential power under the 25th Amendment.

To maintain stability and continuity of government, it is essential to be prepared to invoke the 25th Amendment at the right time: Too early and it would be an overreaction and an abridgment of presidential power; too late and it could precipitate a crisis and a leadership vacuum.

The amendment offers two ways to effect a transfer. Section 3 lets the president voluntarily delegate power to the vice president when a loss of decision-making capacity is anticipated. It is the choice of the president and not forced upon him. This is what President Ronald Reagan did in 1985 when he had surgery. President George W. Bush did the same in 2002 and 2007 when he underwent colonoscopies.

Section 4 is more cumbersome. It allows the vice president and the majority of the Cabinet, or another body that Congress may appoint, to remove the president from the powers of the office because of incapacity. This provision has never been used.

The briefings on President Trump's health are a deliberate exercise in obfuscation, says physician and Post contributing columnist Leana S. Wen. (The Washington Post)

Section 3 is far preferable to Section 4. It is less contentious and more responsive to unpredictable medical circumstances. This was the stance of White House contingency plans drafted during the Reagan administration. Their view was to use Section 3, not Section 4, whenever possible. Those plans were passed on to the administrations of both Bushes and Bill Clinton. The contingency plans operative during President Barack Obama’s administration and now Trump’s are still not public.

Even though Section 3 was the preferred approach of past White Houses, the challenge remains: When should it be invoked? Presidents have generally invoked the Section 3 provision when they go under anesthesia, because anesthesia is a life-threatening risk under any circumstances. When the issue is a planned colonoscopy, the timing is obvious. But what of a novel disease like covid-19, whose course is less certain, when a low blood oxygen level can go very wrong very quickly? In both cases, a president would lose decision-making capacity. You can’t be in control when you are unconscious under anesthesia or when your mental status is affected by low oxygen. Given this, when should the voluntary transfer of presidential power occur?

There will be a natural reluctance for any president to let go of the reins, to hold on for as long as possible. Political calculations could easily distort good judgment and compromise continuity of government.

To depoliticize this decision — which has national security implications — I would suggest the president’s blood oxygen levels serve as a trigger to transfer power to the vice president. The White House physician, working with the White House counsel, should prospectively incorporate this metric into their planning to avoid potential infighting and chaos in response to a medical emergency.

Because of the way oxygen binds hemoglobin in the blood, deterioration from covid-19 can be swift. A decrease in oxygen levels of only 10 percent below normal can lead to a precipitous decline in a patient’s ability to breathe and to think clearly. At that point, it’s like cascading rapidly down a steep ski slope into dangerously low levels of oxygen that may require a ventilator.

Fortunately, as dramatic as this decline can be, during the course of the pandemic, medical experts have learned that it is predictable. With the use of a simple pulse oximeter, a device that fits over one’s finger, oxygen levels can be monitored. For less than $25, a pulse oximeter can track your pulse and measure the flow of oxygen. This is considered good clinical practice when keeping patients at home with mild cases of covid-19. When oxygen levels fall below 90 percent, patients are instructed to go to the hospital because they are in potential danger.

A similar standard should be used for the transfer of presidential power when monitoring the president at Walter Reed. Once the oxygen saturation dips below 90 percent, Section 3 should be invoked by the president as a precaution so the vice president can serve as the acting president. This would allow the president to make this choice while he is still able to do so and avoid any question of presidential succession, as happened during the Reagan assassination attempt.

While it is tempting to target a specific oxygen saturation level below 90, this is more of a warning signal than a bright-line distinction. Low levels could be reversed with supplemental oxygen or, more ominously, be the start of a steep and quick decline. In any case, when the president gets into that danger zone, serious discussions about Section 3 should be initiated because the situation could get out of hand quickly. Think of breaching 90 percent as an early indicator of very significant trouble.

The drafters of the 25th Amendment never clearly specified when a president was “unable” to discharge the office and powers of the presidency. They wisely left the criteria for presidential ability vague, neither anticipating covid-19 nor the diagnostic benefits of a pulse oximeter. While no single number can speak to all our values, a pulse oximeter reading can give an objective, evidence-based way to make a decision that would otherwise be politically fraught. Blood oxygen levels can also be monitored as a harbinger of recovery and the reversal of this designation.

I fervently hope that none of this comes to pass, that the president has a mild case of covid-19 and that he and the first lady recover fully. The odds are in the president’s favor. But when it comes to the health of the nation, we have to plan for the worst-case scenario while we hope for the best. A simple pulse oximeter may be just what the doctor ordered if we hope to avoid a constitutional crisis.