I recommend that you watch Dr. Farkas lecture on YouTube [link is to the YouTube video] because, on the YouTube video, you can follow along with the excellent autogenerated transcript.
All that follows is from the above resource.
Here is a 10-minute video I [Dr. Josh Farkas] made for the Cooper Critical Care Conference. It explores some basic & useful concepts about hypoxemia physiology, including how to apply this at the bedside. The algorithms in the video aren’t intended to be strictly followed, but rather merely as general conceptual schemas.
What follows are some screenshots from Dr. Farkas’ YouTube video with accompanying text from the autogenerated YouTube transcript.
00:22-00:28There are only really three mechanismsof hypoxemia that you need to worryabout: hypoventilation, vq mismatch, andshunting.
00:28-02:38Shunting in hypoventilation the alveoliand the bronchi are working perfectlyand the problem is simply there’s notenough ventilation getting to the lungso the alveoli are burning throughoxygen faster than they’re gettingoxygen probably the most commonlyencountered cause of hypoventilation isa central brain drive problem where thebrain is simply not stimulating the body00:48to breathe and we’re most familiar with00:50this probably in the context of opioid00:52intoxication so folks come in their00:53respiratory rate is low they’re00:55substantially altered they’re not00:56breathing this is relatively easy to00:58diagnose other causes of central01:00hypoventilation are folks with brain01:02stem strokes or substantial brain01:04pathology and once again relatively01:05straightforward to diagnose on the basis01:07of substantially altered mental status01:09so the second type of hypoventilation is01:11a respiratory mechanical problem where01:13the brain is stimulating the body to01:15breathe but the body is unable to01:16breathe either due to a tracheal01:18obstruction or upper airway obstruction01:20or neuromuscular problem involving the01:22diaphragm in the chest ball for patients01:24with acute respiratory mechanical01:25problem they will compensate for this01:27with tachypnea and they will only01:29develop hypercapnia and hypoxemia when01:32they’re really on the brink of death so01:34most of these patients will present with01:35tachypnea and they’ll be struggling to01:37breathe but they won’t necessarily be01:39substantially hypercapnic nor hypoxemic01:41and things of this nature would include01:42epiglottitis acute transverse myelitis01:45gumbray syndrome neuromuscular problems01:47things like periodic hypokalemic01:49paralysis so some sort of acute01:50neuromuscular or airway catastrophe and01:53once again these patients are not really01:55going to present with hypoxemia usually01:57they will present with almost like an01:59exfixial problem so the last cause of01:59exfixial problem so the last cause of02:01hypoventilation is a chronic respiratory02:03mechanical problem and this can be very02:04difficult to diagnose so these are folks02:06with for example chronic muscular02:08dystrophy or chronic obesity02:10hypoventilation syndrome and what02:11happens here is that the brain adapts to02:14a higher level of paco2 so the brain is02:16kind of okay with this and it doesn’t02:18really stimulate the patient to drive02:20their respiratory rate too too high so02:22patients may show up on a couple liters02:24of oxygen and they’re not an extremist02:26they don’t look too bad most of these02:27patients can be diagnosed because they02:29may be carrying some sort of chronic or02:31obvious diagnosis of a neuromuscular02:34problem but occasionally very rarely you02:36might encounter this for the first timeit can be a little tricky to find this.