Occlusion Sensors in Medical Devices: An In-Depth Overview
What is an Occlusion?
The word “occlusion” can have several meanings but in the world of medical devices, an occlusion is a blockage or closing of a flow passage, usually referring to an impediment in a liquid filled disposable tube. Occlusions are problematic because they restrict flow and prevent a device from operating as it was designed to; the magnitude of the problem depends on the type of device in question. Occlusions can occur in medical tubing for a variety of reasons including an obstruction at the end of a catheter tip, a patient in bed rolling onto a flow tube, or a pinch valve malfunctioning. In many applications it is extremely important to detect occlusions so that an operator can be alerted and the pump can be shut down. Undetected occlusions can cause problems including dangerous pressure buildup in a tube, insufficient drug delivery in an infusion pump or overheating in the case that a tube containing a cooling liquid is occluded.
Occlusions that occur in different sections of disposable tubing sets can have different effects:
- Blockages that occur in the fluid after it has passed through the pump are known as downstream occlusions. These occlusions result in a lack of liquid flow and, when using a positive displacement pump such as a peristaltic pump, they can cause dangerous pressure buildups that can result in burst tubing or, if the tube is only momentarily occluded, it can send high-pressure liquid into a patient.
- Blockages that occur in the fluid before it has passed through the pump are known as upstream occlusions. These occlusions are characterized by a decrease in pressure (vacuum) in the upstream tubing and restrict flow through the pump. Note that an IV bag running dry has the same effect as an upstream occlusion and can also be detected with an occlusion sensor.
A thorough evaluation and risk analysis of the overall medical device design should be conducted in order to determine how many sensors are necessary and optimal sensor placement.
How Are Occlusions Detected?
There are several ways to detect occlusions in medical disposable sets including the use of noninvasive pressure sensors, ultrasonic occlusion sensors, flowmeters, or, most commonly, load cell-based occlusion sensors.
Noninvasive Pressure Sensors are generally the most reliable method of detecting occlusions because they can accurately measure the pressure inside of a disposable set noninvasively. If the pressure inside of the disposable set can be accurately measured, it’s possible to determine the maximum normal operating pressure and set an alarm threshold at any pressures above that level or pressures deemed unsafe. Noninvasive pressure sensors require the use of a disposable polycarbonate pressure module that has a thin silicone diaphragm incorporated into it. One side of the silicone diaphragm touches the liquid while the other side of the diaphragm touches the sensor. Because the diaphragm is so thin (typically .010” to .020”), forces are transferred through the diaphragm to the load cell with minimal losses or error. These sensors are noninvasive in that the sensor itself does not have to touch the fluid but they do require the use of a custom disposable module.
There are two primary downsides to noninvasive pressure sensors compared to other methods of detecting occlusions:
- Noninvasive pressure sensors require adding more components to the disposable set. These components are low cost, however in many cases medical device manufacturers make the majority of their profits on the disposable set and even the modest cost of a disposable pressure module can be prohibitively expensive.
- The thin diaphragm in the disposable set may burst at relatively low pressures unless the disposable set is properly installed against the sensor. This means that special considerations must be made to ensure that the disposable set cannot be pressurized while it is not locked in place.
Load cell occlusion sensors (often referred to simply as “occlusion sensors”) are the most commonly used occlusion sensor and they function by mounting a load cell against the outside wall of a tube. The pressure in the tubing exerts a force on the inside wall of the tube, which is transferred through the tube walls and measured by a load cell on the outside surface. Conceptually, they are very similar to noninvasive pressure sensors except instead of adding an additional thin silicone diaphragm to the disposable set, they measure forces exerted through the walls of the tube. In a load cell occlusion sensor, the force sensor accurately measures the force applied to it, however, due to the viscoelastic properties of the tubing (mainly creep and stress relaxation) the zero pressure output of the sensor will move continuously over the course of many hours and therefore it’s impossible to get an accurate pressure measurement. When a tube is clamped in a sensor, the output can drift from stress relaxation by as much as the entire span of the sensor. The magnitude of the drift will vary significantly depending on the tubing material and wall thickness but it is always high. This drift isn’t easy to “subtract out” from the reading because it’s not only a function of time but temperature and pressure as well. At higher pressures, instead of relaxing, the force that the tubing exerts on the load cell will creep in the opposite direction of stress relaxation, meaning there can be an extremely high degree of hysteresis in the tubing as well. See below for an example of data taken with a load cell occlusion sensor.
Pressure cannot be measured accurately using a load cell occlusion sensor unless the tubing wall is extremely thin but depending on the system, a custom occlusion algorithm can be developed to detect occlusions, for example:
- If there’s an opportunity to re-zero the sensor often with no pressure in the tube (e.g. right before turning a pump on), effects from stress relaxation and creep of the viscoelastic tubing can be minimized and a reasonably accurate pressure measurement may be achieved.
- An algorithm can look at the time derivative of the output signal rather than the absolute sensor output. An alarm can be triggered when it detects sharp unexpected changes in the output of the sensor. This may not work for occlusions caused by gradual buildups in pressure caused by, e.g. partial blockages or tube fouling.
- If a peristaltic pump is being used, the pressure in the tubing will fluctuate sinusoidally with the rollers of the pump. The amplitude of the peak to peak signal from the peristaltic pumping, rather than the absolute output of the occlusion sensor can be analyzed and correlated with occlusions.
SMD Sensors will work closely with our customers to design an ideal load cell occlusion sensor for any application but ultimately it is up to our customers to analyze how their system functions under normal and occlusion conditions and develop the algorithm that works best under that specific set of circumstances.
Ultrasonic Occlusion Sensors work similarly to load cell occlusion sensors but instead of accurately measuring a force exerted by the tubing wall, they use the acoustic coupling between the tube and sensor as a stand-in for a force measurement. A viscoelastic tube will allow more ultrasound to pass into it the harder it is pressed against an ultrasonic transducer and therefore a signal that passes through the tubing will be slightly stronger at higher pressures than at lower pressures in the tube. Ultrasonic occlusion sensors are sometimes used due to their low cost but in our experience that is usually a mistake. These sensors suffer all of the same problems with the tubing that load cell occlusion sensors do such as creep and stress relaxation, plus the signal received doesn’t always linearly correlate with the force that the tubing is exerting on it and the output can be affected by other factors including a dirty tube, a moist tube, or microbubbles inside of the liquid. SMD Sensors does not offer this technology despite having the capabilities to do so. In fact, we’ve worked with various medical device companies to replace their ultrasonic occlusion sensors with load cell occlusion sensors after a device has been released to the market due to complaints and field failures of ultrasonic occlusion sensors.
Flow Meters are the final type of sensor that can be used to detect occlusions. In many applications, an occluded tube will manifest itself as a drop in flow rate and therefore a sensor that will noninvasively measure flow can also alert the user of an occluded tube. The most common type of flowmeter used in medical devices is an ultrasonic flowmeter and there are generally two types of ultrasonic flow meters used in medical devices:
- A disposable in-line flowmeter uses a custom disposable polycarbonate flow channel that interfaces with a permanent flow meter. This configuration results in a slightly costlier disposable set but achieves the best accuracy.
- A clamp on flow meter clamps on the outside wall of a tube and can measure flow while being completely noninvasive. It’s slightly less accurate than a disposable in-line flowmeter but with a typical accuracy of 3% it is more than adequate for many applications.
Flowmeters tend to be significantly more expensive than other types of occlusion sensors so they are rarely used solely to detect occlusions. However, if your application requires an accurate flow measurement anyway, detecting occlusions can be an added benefit.
Occlusion Sensor Placement
Occlusion sensors only detect changes in the part of the disposable set that they interface with so it’s important to locate your occlusion sensors appropriately. Generally, occlusion sensors should be located as close to the thing generating pressure (usually a pump) as possible. An occlusion between the sensor and the pump will not be detected because only the section of tubing between the pump and the occlusion will experience a change in pressure. As discussed above, occlusions can occur upstream or downstream of the pump and only an occlusion sensor located in the part of the tubing that’s being occluded will detect it. Occlusion sensors located upstream of the pump will not detect occlusions downstream and vice-versa. A risk analysis should be conducted for any application to determine the most likely locations for an occlusion to occur and where occlusions will cause problems.
What Is The Occlusion Sensor Design Process Like?
If you’re interested in a load cell occlusion sensor, you have two options – buy an appropriate load cell and incorporate it into your device yourself or work with a sensor manufacturer such as SMD Sensors to design a full custom load cell occlusion sensor assembly. The advantage of this process is that we are able to offer a design that integrates a lid for clamping the tube and holding it in place repeatably, a sealing diaphragm that offers liquid ingress protection, mounting features, and a PCB that offers a calibrated digitized signal. We can also combine our occlusion sensors with other complementary technology that we offer such as ultrasonic bubble sensors.
Due to the highly variable nature of each application, every load cell occlusion sensor we manufacture is a custom design. However we’ve recognized that it’s not always ideal to spend significant amounts of time and money developing a sensor that you’re not even sure will work for your application, which is why we are developing our semi-custom occlusion sensor product line. With this product line, the general design is fixed but it is parameterized such that critical parameters relating to the application such as tube diameter and maximum pressure can be easily adjusted in no time by our engineering team. These sensors are designed for one-off rapid-CNC machining to get custom parts in as little as two to three weeks. The semi-custom occlusion sensor product line is designed only for proof of concept testing. If the prototype sensor works for your application then it’s time to kick off the custom design process with our engineering team, which will most likely involve non-recurring engineering and custom injection mold tooling charges.
For occlusion detection using a non-invasive pressure sensor or ultrasonic flowmeter, there is typically an off the shelf option that will allow proof-of-concept testing in your lab and initial prototyping. If modifications are required to work with your application don’t hesitate to contact us to get started working with our experienced engineering team.