A 55-year-old man presents to the Emergency Room with
crushing chest pain. He is diagnosed with an acute heart attack. The nurse, following the American Heart
Association Advanced Cardiovascular Life Support guidelines and out of habit,
puts the patient on oxygen. A football
player comes off the field after a particularly grueling set of plays. He sits
on the bench and puts on an oxygen mask. A patient with stable chest pain asks
his doctor for oxygen to be used at home. Would oxygen be helpful in any of
these scenarios? When is oxygen therapy useful and necessary?
Oxygen was discovered by Joseph Priestly in 1774 and is the
second most abundant element in the air (after nitrogen). Oxygen takes up 21% of the Earth’s atmosphere.
Oxygen is necessary for all of the
functions of the body. As we breathe, the oxygen in the air is brought to tiny
sacs in the lungs. In these lung sacs, oxygen is absorbed by the blood stream.
In the blood stream, oxygen binds to the red blood cells. The oxygen, attached
to red blood cells, travels through the lungs to the heart where it is pumped
to the rest of the body. Oxygen is then extracted by the muscles and organs of
the body to aid in metabolism.
Since oxygen is so essential to the body, how can we be sure
we are getting enough? In medicine, the most common way to measure oxygen is
with a pulse oximeter, an infrared device that provides the oxygen saturation
in the blood. An oxygen saturation between 90% and 100% is normal while a
saturation below 90% is considered low. Low oxygen saturation, or hypoxemia,
causes shortness of breath, an increased heart rate and it can be very
dangerous. To treat hypoxemia, oxygen therapy is delivered in a variety of
ways. A nasal cannula has two prongs, which are placed in the nose, and gives
extra oxygen. A facemask can provide higher dosages of oxygen, with
concentrations up to 100% (ie, the gas in the mask is 100% oxygen versus the 21%
oxygen present in the air). Lastly, in
extreme cases of hypoxemia and breathing distress, a tube is placed in the
throat and the patient is placed on a respirator to help aid in process of
breathing.
Oxygen has been a key treatment in cardiology for more than
100 years. In 1900, it was shown that oxygen could relieve chest pain, a
finding that led to the routine use of oxygen in all heart patients. Over the
years it has been found that oxygen therapy in patients with hypoxemia is
undeniable but in patients with normal oxygen saturation the benefit is not as
clear. In patients who are hypoxemic and have an acute heart attack or who are
in congestive heart failure (CHF), with fluid filling their lungs, oxygen can
be life saving. In cases with hypoxemia, oxygen therapy increases the oxygen
saturation in the blood, decreases the resistance to blood flow in the lungs
and eases the work of the heart by lowering the heart rate and blood pressure,
thus reversing the body’s adaptation to a low saturation.
While the body has mechanisms to counter act low levels of
oxygen in the blood, high levels of oxygen saturation is a man-made phenomenon
caused by oxygen therapy. The body has no mechanism to handle this situation
and in fact oxygen therapy can be damaging. There are two possible reasons for
this. First, the red blood cells are nearly saturated with oxygen. Adding extra
oxygen doesn’t increase the amount of oxygen delivered to the cells of the
body. Secondly, excess oxygen can cause spasm or narrowing of the heart
arteries, reducing blood flow to the heart. In CHF patients with normal oxygen
saturation, high flow supplemental oxygen caused a decrease in the heart’s
pumping capacity and increased the pressure in the lungs, causing worse
outcomes. In heart attack patients with normal oxygen saturation, excess oxygen
therapy can cause a reduction in flow to the heart arteries. Patients with
oxygen therapy had larger heart attacks than those not given extra oxygen. In a recent study of over 6000 patients with
a heart attack and normal oxygen saturation, there was no change in the rate of
death or recurrent heart attack with oxygen therapy.
Can oxygen be beneficial for the athlete? Can it be
considered a performance-enhancing drug?
When supplemental oxygen was given to soccer players and other athletes,
there was no demonstrated improvement in performance and no reduction in
recovery time. On the athletic field, it is felt that oxygen may provide a
placebo effect (an expectation of doing something good) without a real
physiologic benefit.
Oxygen clearly has a role and is life saving for those heart
patients who have low oxygen saturation. For heart patients with normal oxygen
levels, as well as football players and other athletes, supplemental oxygen
likely has no benefit and can be harmful. Based on these findings, none of the
patients in the scenarios would be candidates for supplemental oxygen.
