Eli5: when giving CPR, wouldn’t the lungs be compressed and released, creating a vacuum and providing some oxygen? If so, how does brain hypoxia still occur?


Eli5: when giving CPR, wouldn’t the lungs be compressed and released, creating a vacuum and providing some oxygen? If so, how does brain hypoxia still occur?

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If your heart isn’t pumping, then blood isn’t transporting the oxygen you are getting from the lungs to the brain. CPR can only work for the heart so much.

The blood flow is what’s important. There’s some oxygen in the blood, but the body is designed with the idea of that supply constantly moving. The brain most notably doesn’t keep much of a reserve of oxygen because it always trusts blood to be coming and going with more oxygen.

Well that’s stopped, and the brain shuts down pretty quick. Damage and death follows soon behind.

The goal isn’t necessarily to keep the proverbial train well stocked, but to keep the train moving at all. An unconscious person doesn’t need all that much oxygen and there’s still some in the lungs. It’s good enough that you should just focus on keeping the heart “beating”.

And honestly, odds aren’t even all that great that the person will come around just from CPR. Call 911 (get someone else to do it if possible if you’re doing CPR) and get medical help on its way. Brain damage occurs in about 3-4 minutes without blood flow. The purpose of CPR is buy time on that clock while the ambulance is on its way.

CPR is intended to delay brain hypoxia as long as possible. But it is still not as good as when the person is breathing themselves. It is not just about getting air into the lungs but the oxygenated blood in the lungs needs to be pumped to the brain. The primary purpose of cheast compressions is to pump the heart manually in order to get some blood flow. The modern concensus taking into accunt all the studies on this subject is that the cheast compressions are by far the most important part of CPR and a lot of expects even suggest only to do cheast compressions. The reasons is as you say that there will be some airflow to the lungs when doing cheast compressions and this is enough to get oxygen to the brain. Even just the lungs and blood alone have enough oxygen to keep the brain alive until the ambulance arrives so even with the airways closed you could argue that cheast compressions will be more important then clearing the airways.

The expansion and contracting of your lungs is controlled by the diaphragm, a muscle that sits at the bottom of the ribcage; well below the area compressed by CPR thrusts. Without the movement of that muscle, we don’t get full breaths.The fact is, humans actually aren’t all that efficient with oxygen. We breathe out about 75% of the oxygen that we breathe in. So a single breath can provide enough oxygen to supply the heart and brain for a few minutes.

That time can be extended with CPR forcing the blood around to keep the brain well oxygenated. The thrusts are so important that often times it is better to do continual compressions without any rescue breaths. Many defibrillator units echo this by not instructing the user to give rescue breaths. Rather it goes through cycles to check for a rhythm, then asks the user to continue compressions if no rhythm or no shockable rhythm is found.

Cardio-Pulmonary Resuscitation (CPR) is poorly named. CPR’s goal is not to restore spontaneous sinus rhythm or breathing, but to delay brain death when these are absent.

External chest compressions impart just enough movement to the heart to cause its chambers and valves to shuffle some blood through the circulatory system. Any blood flow will allow gas exchange to occur in the brain, which can be sufficient to delay brain death.

Spontaneous heart function and breathing return only after whatever caused them to cease is resolved. CPR alone does not directly rectify the cause of cardiac or respiratory arrest.