5 Causes of Hypoxemia Every Respiratory Therapy Student Must Know

picture of the lungs

Hypoxemia is one of the most common conditions that a respiratory care practitioner will come across in their career.  In fact, some days it might seem like everybody in the hospital has it.  It is almost as if all the patients held a secret meeting before your shift started and decreed “At 07:30 today, we shall all drops our sats to 78%.  It will be great!”  Okay, maybe I am exaggerating, but you get my point.

So what is a poor lil’ old respiratory therapist to do?  You can’t just go hog wild and run around slapping nonrebreather masks on everybody.  On the other hand, you just can’t sit there and ignore the low satuation pulse oximeter alarms, and screams from distraught nurses caring for the increasingly blue patients. It can be a little overwhelming, especially to a rookie therapist. 

I think one thing to keep in mind is this simple truth: patients do not become hypoxemic out of the blue (see what I did there with the whole blue-cyanotic-hypoxemia thing? Genius.).

That is to say, there is always a reason or cause for the hypoxemia.  Yes, sometimes the cause is not obvious, and it may require some teamwork and deeper investigation with nursing and physicians, but that is what medicine is all about.  This paper will examine five of the most common causes of hypoxemia and what it means to the practicing respiratory therapist.

What is hypoxemia?

Before we discuss the causes of the menace that is hypoxemia, we need to define what it is.  It is an abnormal finding; bottom line is there are low oxygen (O2) levels in the blood.  This can, and will likely, lead to low oxygen levels in the tissues of the body, which is called hypoxia.  Typically, hypoxemia is defined in adults as a partial pressure of oxygen in arterial blood (PaO2) of < 80 mmHg on room air, at sea level.  There usually is a drop in arterial blood oxygen saturation (SaO2 < 90%) that goes hand in hand with this, and the patient may be showing some subtle signs of cyanosis (turning blue).  Other signs of hypoxemia in patients include: tachycardia, increased work of breathing with a deeper and/or faster breathing pattern, confusion and irritability, changes in blood pressure, and even loss of consciousness.  Note:  Confusion and irritability seen in coworkers is often a sign of lack of coffee, not hypoxemia.  Both under-caffeinated coworkers and hypoxemic patient should be approached with caution.  And now, the most common causes of hypoxemia…

1. V/Q Mismatch

This is by far the most common cause of hypoxemia.  There is a big difference between the pressure that O2 is exerting within the alveoli (PAO2) and what it is able to exert in the arterial blood (PaO2).  There is always a small difference between these two measurements, but if the difference is more than 15mmHg, that is considered a big, or high A-a gradient.  Remember that for adequate gas exchange (respiration) to occur in an area, you need good air to reach the alveoli (that is the ventilation, or V, part), and you need good blood flow (that is the Q part) to reach the same area.  The ventilation and perfusion must match up pretty well.  So anything that messes up either ventilation or perfusion, and causes a mismatch, will cause hypoxemia to occur.  The problem can lie up in the airways (asthma, COPD, mucous plugging), in the alveoli (full of pneumonia crud, full of pulmonary edema fluid), or in the vascular system (a big, fat pulmonary embolism, or something else that alters blood flow like severe shock).  How you treat the hypoxemia is obviously dictated by what you believe the cause is.

2. Hypoventilation

This is a cause of hypoxemia that sometimes gets overlooked.  If a patient starts to hypoventilate, either due to low respiratory rate, low tidal volume or both, their PaCO2 will rise to abnormally high levels.  Proper ventilation is not only needed to maintain normal PaCO2 levels, but is also needed to maintain proper oxygenation in the alveoli (PAO2) and in the blood (PaO2).  I could go into a detailed discussion about Dalton’s Law, but I think a simpler way to look at this is that there is only so much room for both CO2 and O2 in our body.  If there is too much CO2 present, it just leaves less room for O2- O2 sort of gets crowded out, and as a result PAO2 and PaO2 both drop.  Patients at risk for hypoventilation as a cause for their hypoxemia often have a central neurological problem (overdosed on painkillers, sedatives, heroin, or comatose).  They might also just have severe COPD with terrible lung mechanics, or they could be morbidly obese.

3. Right-to-left shunt

Another possible cause of hypoxemia is a right-to-left shunt.  This basically means that deoxygenated blood from the right heart has somehow reached over and mixed together with normally oxygenated blood associated with the left heart.  The resultant mixture of this blood will cause an overall drop in PaO2 and SaO2.  This shunting can be caused by a problem in the heart itself (septal wall defects) where the blood literally goes through a hole that should not be there.  More commonly in adults, the problem is actually in the lungs, in the form of a severe blockage in ventilation, causing a complete absence of ventilation to an area, but blood flow to that area is okay.  Major atelectasis, or severe multilobar pneumonia are examples of this.  Still another form of shunt would be the presence of an arteriovenous malformation within the vascular system in the lungs.

4. Thickened Alveolar-Capillary Membrane

Oxygen has to travel a long way to reach the alveoli.  Like you arriving at your hotel after a long car ride, oxygen just wants to relax a bit once it reaches its destination, it does not want to work hard, but rather wants to relax and easily diffuse through a thin membrane and get into the blood of the pulmonary capillaries.  Imagine how disappointed oxygen must be when it gets to the alveoli, and is ready to diffuse into the bloodstream, but suddenly gets met with a huge roadblock.  Gases like oxygen do not like thickened membranes- they despise them.  Patients with pulmonary fibrosis have scar tissue that thickens the membrane, likewise people that have pulmonary vascular disease that thicken the vessel walls will have thick membranes.  Anything that causes a thickened alveolar capillary membrane will result in hypoxemia, especially worsening with patient exertion.

5. Low inspired oxygen pressure

In this situation, both PAO2 and PaO2 are lower than normal.  The most common situation where this is seen is high altitudes.  There is simply not enough O2 at a normal pressure available to reach the alveoli; “the air is thin”.  The PaCO2 is often normal in this situation, so it is not a question of ventilation.  This is not a problem for those of us in the Midwest (writing this from Ohio), but is definitely something to keep in mind in mountainous regions, or in flying aircraft.

Tying things together

So now that we discussed some common causes of hypoxemia, what does this mean as far as treating the patient at the bedside?  It really goes back to trying to identify and then treating the cause.  If the suspected cause of hypoxemia is a mild to moderate V/Q mismatch, hitting the patient with 100% oxygen should help.  At least it will buy you time while you investigate further.  On the other hand, 100% oxygen will not help a patient with hypoxemia caused by a right-to-left heart shunt.  If the patient is hypoventilating, they need that situation corrected and ventilation supported, before we can do anything further. 

Like most things in respiratory care, you really need to start with a thorough review of the patient’s medical records and history to search for clues.  From there, you need to follow up with a thorough physical exam of the patient and start to put the pieces of the puzzle together.  Of course, you should be looking at labs as well (ABG’s, CBC, etc.), and gathering any other useful information from other testing/procedures (cardiac echocardiograms, critical care monitors and Swan Ganz catheters, etc.).  The key is to know when to escalate your interventions, or when to stop what you are doing with Plan A and go to Plan B.

Maybe I read too many comic books as a young lad, but I sort of viewed hypoxemia as my arch enemy when I was working in the hospital, and me being Super Therapist, felt it was my sworn duty to stamp out hypoxemia wherever it reared its evil head.  Pretty dramatic stuff, right?  Now that I just wrote that out and read it, I would definitely say I read too many comics and maybe have some issues, but hey, it helped me get through many a shift.  I do not think using your imagination to better care for your patients is a bad thing.

Your Respiratory Pal,

Chuck Mulholland MEd, RRT

Assistant Professor, Clinical Education Coordinator
Rhodes State College


Hess, D., MacIntyre, N., et al.  Respiratory Care Principles and Practice, 3rd Ed. Burlington, MA: Jones and Bartlett Learning; 2016.  Chapter 48 pp: 1137-1141.


Beachey, W.  Respiratory Care Anatomy and Physiology:  Foundations for Clinical Practice, 3rd Ed.  St. Louis, MI:  Elsevier; 2017.  Chapter 13 pp 231-234.


Sarkar M., Niranjan N., Banyal PK.  Mechanisms of hypoxemia.  Lung India 2017; 34:47-60.


PDF:  5 Causes of Hypoxemia.  Retrieved 10/7/2019 from:  http://www.yanyu.ca/mednotes/Course%203/5MechanismsOfHypoxemia.pdf