What is Limb Occlusion Pressure? (LOP)
Limb occlusion pressure (LOP) is a foundational concept to understand in blood flow restriction training. You may also hear it referred to as “AOP” or arterial occlusion pressure - both have the same meaning and can be used interchangeably.
In this article, we’ll provide a general breakdown of LOP and deliver a helpful analogy to illustrate the concept.
Remember, blood flows two directions in our body:
- From the heart to the periphery (I.E. limbs) - Arterial flow
- From the periphery to the heart - Venous flow
If blood flowing from our heart to our limbs is called arterial flow, and the term is called “arterial occlusion pressure” we can deduce logically what that’s referring to!
You guessed it - it’s the minimum pressure required to fully occlude or stop arterial blood flow to the limb.
If we’re getting technical, arterial or limb occlusion pressure is the minimum pressure required to fully occlude arterial flow to the limb distal to the cuff at a specific time. As we will discuss later on, this pressure can change from day to day and person to person due to a variety of factors. Cuff width also plays a role in required pressure, but more on that in a bit.
LOP is important in surgery - think about a tourniquet being applied to temporarily stop blood from reaching the arm. Surgeons want to ensure arterial flow is stopped, but they don’t want to arbitrarily apply too intense a level of pressure that could cause damage to the tissues or nerves.
They’ll measure LOP to find the appropriate pressure to apply.
Using Limb Occlusion to Measure Blood Pressure
Understanding how traditional blood pressure measurements work can be helpful in understanding the global concept of LOP in BFR training.
Most of us have had our blood pressure tested at some point. You might recall that the cuff inflates to a fairly tight pressure, then slowly deflates in small increments. What exactly is happening here?
Remember, your heart is like a big pump. When it pumps blood, this creates oscillations or “swings” in pressure. During a pump, pressure increases because blood is being pushed through the arteries.
Between pumps, pressure drops because less blood is moving through the arteries at that moment.
Here’s that analogy we referenced in the beginning of this article:
Picture a hose turning off and on in quick spurts. When it’s on, the pressure along the inner walls of the hose is high because a high volume of water is actively traveling through the hose. If you were asked to pinch the hose closed during this time, it would be difficult to fully close off due to the water pressure.
When the hose is off, the pressure along the inner walls is lower (making the hose easier to pinch closed).
The higher pressure when the water is turned on can be equated to blood flow during heart beats (contractions). The technical term is systolic blood pressure.
The lower pressure when the water is turned off can be equated to blood flow between heart beats. The technical term here is diastolic blood pressure.
This is why we’re given two numbers when our blood pressure is tested, (I.E. 120/80). The first number is your systolic pressure and the second is your diastolic pressure.
Back to how the actual blood pressure cuff works:
Blood pressure cuffs are able to detect and measure these oscillations or swings in pressure. When the cuff inflates, it inflates past the pressure required to occlude systolic blood flow (the higher pressure while the heart is contracting).
This means that the pressure in the cuff is high enough to fully occlude arterial flow, even during heart pumps when pressure is higher. At this point, the cuff detects that there are no longer any oscillations in pressure.
The cuff then begins to slowly deflate, waiting to detect the first sign of an oscillation. The pressure the cuff is at when it first detects an oscillation signal is your systolic blood pressure. Here, the pressure in the cuff is low enough to allow blood flow through during the systolic phase because the heart pump action creates a strong enough pressure.
During the diastolic phase however, blood cannot pass through until the cuff lowers pressure. Because of this, the cuff can only detect one signal. As the cuff deflates further, it can eventually detect a second oscillation, which is your diastolic blood pressure.
From this, the blood pressure machine can gather two numbers, (I.E. 136/90 or 120/84).
Now, this does not directly apply to blood flow restriction training since you don’t use a traditional blood pressure cuff to perform BFR, nor is a formal systolic/diastolic pressure taken.
However, this is a great foundational concept to begin to build a framework for hemodynamic assessment.
With that in mind, we can take a closer look at measuring LOP for BFR training.
Measuring Limb Occlusion Pressure (LOP) for BFR Training
Since a traditional blood pressure cuff is not used to measure LOP for BFR training, how exactly can you detect your or your patients’ LOP? There are two primary methods for accurately measuring limb occlusion pressure.
Using a Doppler Ultrasound to Measure Limb Occlusion Pressure
Doppler ultrasound devices provide an accurate, non-invasive detection of blood flow using high frequency sound waves. In the case of BFR, the doppler would be placed distal to the cuff (at your wrist for upper limb measurement). With the doppler against your wrist, it is able to detect as its sound waves bounce off moving blood cells in your vessels.
At this point, either you or a practitioner will begin to inflate the applied BFR cuff until the doppler can no longer detect blood flow into the limb (sounds similar to the blood pressure measurement discussed earlier, right?)
Once blood flow is no longer detected, you or your practitioner will slowly deflate the cuff, waiting for the doppler to detect the first sign of blood flow. As an example, if the cuff is at 140 mmHg when blood flow is first detected, we can be confident that the limb occlusion pressure (at that specific time, with that specific cuff, in that specific position) is just at 140 mmHg.
With that figure in mind, you can work backwards to determine your desired occlusion percentage. To train at 50% occlusion, you’d just cut 140 mmHg in half, inflating the cuff to 70 mmHg.
This is an effective means of calculating your LOP prior to BFR training. However, not everyone has access to a doppler, nor the expertise to properly utilize it for measurement.
Thankfully, the evolution of BFR technology has brought more seamless options to the market, democratizing blood flow restriction as a whole. Let’s take a look at option # 2 for measuring LOP.
Using a Specialized Cuff for Measuring Limb Occlusion Pressure (LOP)
Certain specialized blood flow restriction cuffs are capable of measuring occlusion automatically, eliminating the need for a doppler device. These cuffs can detect oscillations or swings in pressure as the cuff inflates, ultimately calculating limb occlusion pressure.
Advances in technology have taken BFR from a limited access modality to an extraordinarily feasible solution for anyone who is medically safe to engage in blood flow restriction.
If you’d like to take the guesswork out of finding your specific LOP, SAGA’s intelligent BFR cuffs auto-calibrate to precision with a single tap within our mobile app. You can find out more on our revolutionary BFR cuffs here.
Individual Factors that Determine Limb Occlusion Pressure
As mentioned earlier, several physiological and external factors play a role in one’s LOP at any given time. For the purposes of this article, we will focus on the primary movers:
- Blood Pressure - Higher blood pressure will require higher pressure to occlude arterial flow. Note the hose example discussed earlier to help understand this concept.
- Cuff width - A wider cuff will require less pressure to occlude arterial flow. Additionally, because the applied pressure is distributed across a larger surface area, it results in lower pressure gradients to the underlying soft tissues. This drastically reduces any risk of a nerve-related tissue injury.
- Limb size- Larger limbs require greater pressures to reach limb occlusion. In BFR literature, this has shown to be true across the range of studied cuff sizes.
By now, you should have a global understanding of what limb occlusion pressure is, how to measure it, and the primary factors that can influence it. We’ll be back soon for another educational article to help you maximize results with your BFR training.