Urine output (UO) is usually measured manually each hour in acutely ill patients. This task consumes a substantial amount of time. Furthermore, in the literature there is evidence that more frequent (minute-by-minute) UO measurement could impact clinical decision making and improve patient outcomes. However, it is not feasible to manually take minute-by-minute UO measurements. A device capable of automatically monitoring UO could save precious time of the healthcare staff and improve patient outcomes through a more precise and continuous monitoring of this parameter. This paper presents a device capable of automatically monitoring UO. It provides minute by minute measures and it can generate alarms that warn of deviations from therapeutic goals. It uses a capacitive sensor for the measurement of the UO collected within a rigid container. When the container is full, it automatically empties without requiring any internal or external power supply or any intervention by the nursing staff. In vitro tests have been conducted to verify the proper operation and accuracy in the measures of the device. These tests confirm the viability of the device to automate the monitoring of UO.
Critical care unit staff has the support of multiple monitoring devices capable of measuring most of the patient’s physiological parameters. More often than not, these devices also check whether these parameters remain within acceptable values, and they alert the healthcare staff (usually via audible warnings) when the parameters take values
Critical patients’ urine is collected in a graduated container which is often divided into several chambers with an overall capacity of approximately 500 mL. This container is connected to a 1500–3000 mL plastic bag. Every hour, the reading of the container of every patient must be manually recorded. This requires walking to the patient’s bed, taking the measure of UO production visually, writing it down on the nursing documentation sheet, opening the valve that releases urine from the graduated container to the plastic bag, waiting for the urine to drain, closing the valve, checking that the valve is properly sealed, and checking if the plastic bag needs to be emptied. This entire process can take up to two minutes [
This paper presents a patent pending device [
Our device has a container with a single 90 mL chamber. This container receives the urine through a flexible tube, which in turn is connected to a Foley catheter. In its outer wall it has a capacitive sensor (see Figure
Device design.
When full, the container that collects the urine will be emptied automatically without requiring electrical power. To achieve this, the device uses magnetic forces to prevent the activation of the emptying mechanism of the container until it is nearly full of urine. The drain orifice is closed by a stopper placed at the end of a moving rod to which two floats are coupled. In the absence of magnetic force, a small amount of liquid in the container would be sufficient for the floats to pull up the rod and the stopper and therefore to trigger the emptying. But the magnetic force prevents this from happening until the container is almost full; only at this point does the buoyancy force overcome the magnetic force. Then the magnetic force disappears because the magnet moves away from the metal surface. Before the magnetic force can reappear, most of the liquid in the container has to be drained.
We shall describe now the balance of the different forces that come into play at different points during filling. When the container is empty the exit hole is sealed by the stopper that is on the end of a rod. The rod has a float located in its lower part and a motion limiter at the top (see Figure
Location of the magnets and metal pieces in the upper float and on the shoulder of the hollow cylindrical guide.
When the container starts filling, two new forces come into play. There is the buoyancy force that equals the weight of the volume of liquid displaced by the rod, the lower float, and the stopper (see Figure
Functioning of the device. Each subfigure shows the forces that come into play in each of the states. The solid black downwards pointing arrows correspond to forces caused by gravity; the solid blue upwards pointing arrows to buoyancy forces; the dashed red downwards pointing arrows to magnetic forces; and the green inclined arrows to forces caused by the pressure of the column of liquid.
Empty
Filling
Completely full
Starts draining
Draining
Closes again
When the liquid reaches the second float, three different forces are acting on it: the weight of the float (
In our prototype (see Figure
On the left we show lateral and bottom views of our prototype. On the right we show the corresponding parts in the design diagram.
The capacitive sensor we used was manufactured by Sensortechnics GmbH. An interface circuit was built to enable communication between the sensor and a serial port to Bluetooth adapter, which sends the readings to and receives the commands from the central PC (see Figure
For the correct operation of the stopper, it must have a highly polished smooth surface, and it must be made of a corrosion resistant material. The patient’s urine may contain hard inorganic and soft organic sediments that may cause the stopper not to close properly. In order to avoid this, either the stopper or the walls of the hole, or both, must be made of a soft material such as silicone, so that the stopper can block the outflow of liquid even in the presence of sediments. To prevent the rod from adhering to the hollow guide, the rod should not have flat surfaces. It must have a conical or spherical surface. The top of the second float and the top of the motion limiter must not have a flat surface, since in this case fluid could accumulate on them and would change the balance of forces, which may cause the device to malfunction. To avoid this, the top of the second float and of the motion limiter must have a pyramidal or conical shape.
To verify the proper operation of the prototype we have used a saline solution with similar properties to urine. A dropper was used to simulate the urine flow into the device (see Figure
Setup used in the validation of the device.
This setup permits the automation of the process of carrying out multiple measures of the fluid drained by the device each time its content is released. Given that the PGW 4502e scale has an accuracy guaranteed by the manufacturer of 0.01 g, we shall consider that measures obtained from the scale are the ground truth which we shall use to determine how reliable the device is; that is, the release of fluid from the device always occurs when the liquid reaches the same level and always drains the same amount of fluid before starting to accumulate liquid again.
Using the in vitro testing setup described in Section
Automating the monitoring of UO can provide the same benefits that the automation of the monitoring of many other physiological parameters has already brought to critical care units: decreasing the workload of the health care staff, simplifying the construction of digital records of the patient, and providing more frequent measures of the parameter. As we have already argued, in a critical care unit with 10 patients, up to 8 hours of health staff time a day are used in tasks related to the monitoring of UO. If these tasks were automated, there may be an improvement in patient outcomes equivalent to an increase in the staffing of the unit proportional to the saved time [
There are a few papers in the scientific literature describing solutions to automate some of the steps involved in the UO monitoring process. At the end of the 1980s several automatic urine meters based on ultrasound sensors were proposed [
Hersch et al. developed a device capable of measuring urine output every minute using a photoelectric cell that counts drops of urine [
A disadvantage of the device presented in this paper when compared with the manual urine meters currently used in critical care units is its use of metals and magnets. Manual urine meters can be made entirely of plastic and therefore are MRI compatible. Our device should be disconnected from the patient before performing an MRI.
We have designed and built a device capable of automatically monitoring the UO of critical care patients. This device automates all tasks related to the monitoring of UO, with the exception of emptying the plastic bag that collects the urine. Currently this parameter is measured and monitored manually by nursing staff, which requires at least one supervisory visit to the patient’s bedside every hour. We estimate that this device could save up to 7 hours of nursing staff work per day per 10 patients. Furthermore, with this device more frequent urine production measures can be taken (up to one per minute). In the literature there is evidence that indicates that more frequent UO measurement can impact clinical decision making and improve patient outcomes [
None of the authors of this paper has received any money or any grants, owns shares, or has some other kinds of financial conflict of interests with the commercial entities mentioned in this paper.
This work was supported by the Spanish MEC and the European FEDER under Grant TIN2009-14372-C03-03.