hypoxemia
Meaning of hypoxemia: A pulse oximeter reports how full the hemoglobin is, not how much oxygen the blood holds, and the two can part company.
Definition of hypoxemia
Hypoxemia is a lower than normal level of oxygen in arterial blood, the blood leaving the lungs for the rest of the body. It is confirmed by measuring the partial pressure of oxygen in an arterial sample, and estimated at the bedside by a pulse oximeter clipped to a finger or earlobe. It is a measurement, not a diagnosis.
What causes it?
| Mechanism | What goes wrong | Typical settings |
|---|---|---|
| Mismatch of air and blood | Some parts of the lung receive air but little blood, others blood but little air | Asthma, chronic obstructive lung disease, pneumonia, a clot in the lung |
| Shunt | Blood passes lung tissue that is getting no air at all | Collapsed lung regions, air sacs filled with fluid or pus |
| Underbreathing | Too little air moves in and out overall | Sedative or opioid overdose, weak breathing muscles, severe obesity |
| Thickened barrier | Oxygen crosses slowly from air sac into blood | Scarring of the lung tissue |
| Thin air | Less oxygen available to breathe in | High altitude |
Mismatch between airflow and blood flow accounts for most cases met in practice. The distinction between the mechanisms is not academic, because giving extra oxygen corrects some of them readily and barely touches a large shunt. How fast the level falls matters as much as how far: a gradual decline over months is tolerated far better than the same value reached in an hour.
How is hypoxemia different from hypoxia?
Hypoxemia is about the blood. Hypoxia is about the tissue, and means cells are not receiving the oxygen they need. Usually one leads to the other, but they separate more often than the words suggest. Severe anemia leaves too few red cells to carry oxygen even though the blood is fully saturated, so the measured value is normal while tissue starves. A blocked artery starves the tissue beyond it while every other organ is supplied normally. Carbon monoxide poisoning and cyanide poisoning both produce tissue hypoxia with unremarkable oxygen readings.
Why can a pulse oximeter mislead?
It works by shining light through tissue and comparing how much is absorbed by oxygen carrying hemoglobin against hemoglobin that is empty. That design has known limits. Carbon monoxide bound to hemoglobin looks almost identical to oxygen, so the reading stays reassuring during poisoning. Cold fingers, poor circulation, movement and dark nail polish all disturb the signal. Readings tend to overstate the true value in people with darker skin, so a normal number carries less reassurance than it appears to. An arterial blood sample settles the question when the reading and the person do not agree.
Used in a sentence
Pulse oximetry revealed hypoxemia that prompted supplemental oxygen during the hospital stay.
Good to know
Hypoxemia describes low oxygen in the blood, while hypoxia describes inadequate oxygen delivery to tissue, a distinction that matters because the two do not always occur together.
Word origin
Where “hypoxemia” comes from
Formed from hypo-, Greek for under, and -emia, from haima, meaning blood, combined with the root for oxygen.