Gas Exchange

Oxygen Prescription

Total annual expenditures for supplemental oxygen in the U.S. are over $2.8 billion, with Medicare covering the majority of costs for long-term home oxygen therapy. This makes it the most expensive non-surgical treatment for Durable Medical Equipment (DME) under Medicare. While there is no restriction on writing a prescription for supplemental oxygen, insurance will only cover such a prescription if a patient meets certain criteria.

Criteria for oxygen therapy:

• Resting SaO2 <= 88%
• Resting SaO2 89% if evidence of cor pulmonale
• Exercise drop <= 88%
• Nocturnal drop to <= 88%

Dyspnea without desaturation is NOT a criteria for supplemental oxygen.

Over the years the specifics for each criteria has evolved. Resting SaO2 <= 88% has been an accepted criteria for continuous oxygen therapy for decades. Ideally it should be combined with an exercise assessment to determine the needs with activity. But this is only required if the need is >= 4lpm

Up until 10 years ago, oxygen desaturation with activity only qualified the patient for a portable tank for use with activity that has never been show to impact outcomes. Beginning 10 years ago, patients meeting the exercise criteria were equated to those with resting low saturation levels and qualified for continuous oxygen.

Up until April of this 2023, the criteria for nocturnal oxygen involved having 5 minutes of SaO2 <=88% during the monitoring period. Now the criteria is ANY desaturation to 88% or less regardless of the duration if the patient has symptoms of dyspnea.

For all the above criteria the patient must be medically stable, meaning witin 48 hours of hospital discharge. In addition everyone placed on supplemental oxygen must requalify after 3 months of therapy. In the hospital setting 40-50% of those patient started on oxygen at discharge are able to discontinue within 3 months.

Prior to the advent of affordable pulse oximetry, oxygen prescriptions were quite rigid and based on a single qualifying oxygen measurement. Single oxygen is essential for life, there is a counter myth that the presence of dyspnea must be associated with hypoxemia. As was discussed in the section on pulmonary function testing, dynamic air trapping and not the FEV1 tends to correltate best with the sensation of dyspnea. There is what I feel the be a uniquely American perspective that is 'more' of something is good, then 'too much' must be better. You will often hear in the context of supplemental oxygen the dangers of CO2 retention with oxygen supplementation. While there are situations where this is a concern, even in the face of chronic hypercapnia, the risk of further CO2 retention with supplemental oxygen is overstated. I feel the primary reason to avoid excessive oxygen supplementaltion is the development of oxygen toxicity.

I remember during ICU rounds msny years ago, the question of posed to the team - 'what is the safe level of oxygen supplementation?'. The student of the service answered with a truly inciteful response of 'room air'. While the danger of oxygen toxicity in the ICU has been well recognized for decades, it is only recently that the risk of oxygen toxicity in the ambulatory setting has been appreciated. There is even a feeling that oxygen supplementation may not be providing the a benefit. Given the undeniable link of the necessity of oxygen for life, it is impossible for many to imagine my supplementing oxygen could if criteria are met or to study this question in the lung disease population.

Due to concern over the potential harm of supplemental oxygen, I have taylored my approach to oxygen prescription. First is that I innsist that any patient on supplemental oxygen have a pulse oximeter. Even with the concerns about non-FDA approved devices, I feel strongly that oxygen should be titrated based on saturation levels. I also encourage patients when they are experiencing increase dyspnea to first check their oxygen saturation levels before increasing the oxygen flow rates.

Second, I ask to patient to titrate the oxygen flow rates based on three broad categories - rest, activity and sleep. For rest which would include ADL's and light household activity - I recommend a target saturation of 90 - 94%. Since we all experience an average desaturation of 3% with sleep, I suggest a target saturation of 94 - 96% when the patient is settled in bed and ready for sleep. Anything more that lightly strenous activity, is a challenge to titrate for there are a host of factors that can impact saturation. In addition the patient's are generally on a portable source for this level of activity. Finally when compared to rest and sleep, this intensity of activity only constitutes less than 10% of the day, I do allow the patiet to titrate the flow rate to effect rather than a specific saturation level.

What is a physiologic argument one can make for limiting flow rates to those achieving a saturation of no more than 94%?

In cases where the patient has known CO2 retention, I reduce the target saturation by 2-3% for each of the above categories.

One final consideration in the delivery of supplemental oxygen is the method of administration for activity. Thanks to direct to consumer advertising, every patient wants a portable concentrator. I have come to dread this conversation since the decision regarding a portable oxygen source is made at the corporate level by DME suppliers. While the prices of portable concentrators have come down significantly, they still remain much more expensive than portable tanks. Questionable advertising techniques also have patients convinced that portable concentrators are much lighter than tanks. While there are portable concentrators that weigh less than 3lbs, they are no lighter than a similarly capable portable tank. The primary limitation of any 'lightweight' portable system is that oxygen is delivered as an inspiratory pulse rather than continuous flow.

With continuous flow, oyygen delivered during the expiratory phase is wasted. By delivering oxygen as an inspiratory pulse, the life of a conventional oxygen cylinder can be extended 6-8 times that of continuous flow. The decrease demands allow for the minituration of portable concentrators. The downside of this delivery system is the negative inspiratory force that must be generated to trigger the system. The can be a significant problem for patients with severe obstructive lung disease who may have difficulty generating the necessary negative pressure. In addition, during periods of increase demand or stress patients may resort to mouth breathing and not trigger the pulse system at all.

When watching commericals for portable concentrators, another potential downside of these devices in minimized - noise. Most portable concentrators generate noise levels in the 40-50 decibel range which can be quite bothersome in quiet settings. I often equate the sound level of a portable concentrator to that of a fish tank air pump.

What limitation do you feel impacts the use of oxygen supplementation for air travel?