The Physics of Proper CPR

When it comes to performing high quality CPR, the key thing that you want is consistency. To ensure that your patient has the highest possible chance of survival you need to make sure that you are compressing your patient's chest at least 5-6 cm, allowing the chest to fully recoil and then repeating at a rate of 100-120 beats to minute. While this might sound easy at first, in practice you will realize that your biggest enemy quickly becomes your own fatigue in performing this task. That is why one of the most important parts of performing high quality CPR is ensuring that you position yourself with your shoulders over top of the patient and are pushing straight down on them.

The amount of force that is required to compress the patient's chest is about 500 N. This means that each time that you push down on your patient's chest you are performing 25 N⋅m of work. While this may not seem like a lot at first, when you consider that you will need to do this at least 100 times every minute the amount of work that is required to perform CPR begins to add up. In fact, every minute that you are doing CPR, you are performing about 2500 N⋅m of work on your patient. As you could imagine this becomes very tiring, very quickly, and can lead you to inadvertently decreasing the rate, depth and or quality of your compressions. This is why it is recommended if there is more than one person present at the scene, that you switch off doing compressions about every 2 minutes. 

One of the most common mistakes that people who are first learning CPR often make is not positioning themselves with their shoulders directly over their patient. This is because when many people first start out learning CPR they think that they will be able to go for longer if they lean back and get themselves into a more comfortable position. However, by doing this they are actually likely to become tired more quickly because they are quite literally creating more work for themselves to do. This is because it requires significantly more force to compress the patient's chest 5-6 cm when pushing down at an angle than it would when pushing straight down on the chest. In the example depicted above, we can see that by leaning back 30 degrees the amount of force that is required to compress the patient's chest increases from 500 N to 577.35 N. This translates to the person performing CPR needing to exert 2886.75 N⋅m of work on their patient every minute, which is significantly higher than the 2500 N⋅m of work needed when CPR is performed properly. This is why it is so important to perform CPR with your shoulders directly over the patient.