Although the pathophysiology of compartment syndrome is well understood by clinicians, there is less certainty as to the different techniques for measuring intra-compartmental pressure (ICP), the gold standard for diagnosing compartment syndrome. Previous studies have reported that the use of a simple 18-gauge needle is advised against because of erroneously high measurements. Inaccurate measurements could result in errors in diagnosis and significant clinical consequences. The current study was designed to determine the most accurate means of measuring ICP. We conclude that the Compass® Compartment Pressure and Stryker® Intra-compartmental Pressure Monitor System are both accurate pressure transducers. Also, we show that there is little difference between the simple 18-gauge needle, 18-gauge side-port needle, slit catheter, and a new tetrahedron needle (with spiral holes arranged axially) in their ability to accurately measure pressure when used with either transducer.
Compartment syndrome is the result of swelling within a muscle that is enclosed by fascia, a layer of inflexible fibrous tissue. Because of the swelling within the fascial compartment and the lack of flexibility that the fascia provides, the perfusion pressure to the compartment decreases and the muscles and nerves do not receive sufficient amounts of blood/oxygen. If left untreated, or without a timely diagnosis, compartment syndrome can lead to muscle ischemia, necrosis, loss of the afflicted limb, and, in the most severe of cases, renal failure, and death (1, 2). Treatment consists of a fasciotomy and decompression of all the affected tissues within the compartment (2).
Although the pathophysiology of compartment syndrome is well known among clinicians, the diagnosis is often difficult to make; clinical observations are not a sufficient means to diagnosing compartment syndrome (2). ICP measurement has been shown to provide objective data and identify patients who are in need of a fasciotomy (3). It is particularly useful in patients who are unconscious or impaired, young children, and with patients who suffer from regional nerve blocks, as part of the diagnosis is to assess the level of pain of the patient. Measuring ICP has no significant risks, while not monitoring ICP can lead to missed diagnosis. In recent studies, it has been reported that certain ICP measurement devices and methods are neither accurate nor reproducible (4, 5). Inaccuracies of large magnitudes would be expected to result in inappropriate diagnoses.
When Moed and Thorderson (5) first compared the slit catheter, side-ported needle, and simple needle in a canine model in 1993 they found that the simple needle provided consistently higher measurements than the other two terminal devices. Both Rorabeck et al.(6) and McQueen et al.(2) state that the slit catheter is the most accurate terminal device for measurement. In 2001, Uliasz et al.(4) used a test chamber to compare the Stryker with the IV pump and Whitesides’ (7) method. They reported that the Whitesides’ (7) method failed to produce reliable results. In 2005, Boody and Wongworawot (8) measured the slit catheter, side-ported needle, and simple needle in a model similar to Uliasz et al. (4). Their findings were comparable to Moed and Thorderson (5); the simple 18-gauge needle provided inaccurate measurements. Most recently, in 2012, Hammerberg et al.(9) reported that the slit catheter, side-ported needle, and simple needle can all be used clinically with confidence. Their report states that the Whitesides’ (7) method, contrary to Uliasz et al. (4), is an accurate alternative to electronic means.
Therefore, the purpose of this study is to compare the reliability of the different needles and catheters available with two different commercially available transducers with the hypothesis that, when using the appropriate technique, the majority of the variability in the measured pressure is due to the location of the needle tip within the tissue rather than the differences in the commercially available needles and/or pressure measuring equipment.
For our first experiment we created a model of compartment syndrome that consisted of a clear cylinder with a waterproof bottom and a side port (Fig. 2). We filled the cylinder with water. We labeled the cylinder like a graduated cylinder, allowing visualization of the fluid meniscus against the markings on the cylinder to determine fluid level and thus applied pressure.
We obtained fresh pig muscle and cut it into 2-cm cubes that we placed at the bottom of the cylinder for testing. We attached needles and catheters to two different handheld compartment measurement devices: the Stryker Intra-compartmental Pressure Monitor System (Stryker Corporation; Kalamazoo, MI) and the Compass Compartment Pressure (Mirador Biomedical; Seattle, WA). We then inserted the needles into the center of the muscle. We tested each transducer with an 18-gauge needle, an 18-gauge side-port needle, slit catheter, tetrahedron needle point with holes arranged axially in a spiral (Fig. 1). We tested a total of eight combinations, that is, the Stryker device with the 18-gauge needle, the 18-gauge side-port needle, the slit catheter, and the tetrahedron needle point with holes arranged axially in a spiral and the Compass device with the same four terminal device types. All measurements were taken from the same depth within the muscle to eliminate tissue variability. We tested each of the eight combinations at each fluid level by measuring the pressure according to manufacturer instructions for use (see Appendix).
Our initial level of fluid was 2.0 inches above the tip of the needle or catheter insertion. We then incrementally filled the water column with 21ºC tap water. We obtained pressure measurements after each 2-inch addition to the water column height. The final fluid height was 24.0 inches. This process was repeated for a total of five trails for each of the eight measurement combinations. We used a different cube of pig muscle for each trial.
We then converted the heights of the water column at each testing point into pressure data with the standard known values for the densities of mercury, and water at given temperatures (10). We used the translated pressures, expressed in mm Hg, as the control measures of pressure within the muscle tissue at each level of the water column.
In a second experimental setup, and in order to provide a more accurate representation of a compartment, we obtained a fresh pig leg with skin intact from the knee to the most distal part of the hoof (Fig. 3). The purpose of this second model was to see how the measurement devices responded to going through layers of fascia, skin, and other tissue types. We outlined the tibia and then identified the different compartments. We introduced a slit catheter through the skin and the fascia and into the anterior compartment. We connected the catheter to a liter of tap water for continual infusion into the compartment. We allowed the compartment to equilibrate with the applied water pressure. As in the first setup, we tested each of the eight combinations of needle or catheter and electronic transducer by inserting the terminal device into the same puncture hole at the same depth. We increased the ICP by altering the height of the water column above the level of the catheter tip. We obtained pressure measurements after each 2-inch addition of water to the column height. The final fluid height was 24.0 inches. We repeated this process for a total of four trials for each of the eight combinations while using a different compartment in the same pig leg for each trial.
The experimental design for each of the two model experiments was of a mixed design. The between-subjects factor was the tested transducer combination and the within-subject factor was the pressure. To estimate the effects of each of the factors defined by the sub-experiments, we performed a repeated-measures analysis of variance (level of significance, alpha = 0.05) using SPSS Statistics (IBM Software; Armonk, NY)
We tested a total of eight combinations of the two measurement devices (Compass v. Stryker) and four terminal device configurations (simple 18-gauge, 18-gauge with side-port, slit catheter, spiral) to determine if there was a difference in measurements between the two transducers and terminal devices. We found no differences in measurements taken with the Compass or Stryker devices across the different depths (and pressure) of the water column (two-way ANOVA, no main effect of device, p = 0.999; Fig. 4A.). There was no difference in measurements across the needle configurations (two-way ANOVA, no main effect of device, p = 0.759; Fig. 4B.) and no interaction effect between the device type and needle configuration (three-way ANOVA, no interaction effect, p = 0.999). That is, we found no differences in these devices and needle configurations in our tests using the water column.
We confirmed that the relationship between the measured pressures and actual water column pressures was linear for all configurations. In all eight configurations the measured pressure was nearly identical to the controlled water pressure of the column as seen with linear regression analysis (r2 > 0.99, p <0.001 for all cases).
When infusing water into the pig leg, we tested a total of eight combinations of the two pressure transducers (Compass v. Stryker) and four terminal device configurations (simple 18-gauge, 18-gauge with side-port, slit catheter, spiral) to determine if there was a difference in measurements between the two transducers and terminal devices. We found there to be no difference in measurements taken with the Compass or the Stryker devices across the different levels (and pressure) of the water bag (two–way ANOVA, no main effect of device, p= .901; Fig. 5A.). There was no difference in measurements across the needle configurations when the pig leg model was used (two-way ANOVA, no main effect of device, p = 0.662; Fig. 5B.). There was also no interaction effect between the device type and needle configuration (three-way ANOVA, no interaction effect, p = 0.997; Fig.). This means that we found no differences in these devices and needle configurations in our tests using the pig leg as a model for compartment syndrome.
We were able to determine that the relationship between the measured and actual applied pressures was linear for all eight configurations. In all configurations the measured pressure was nearly identical to the controlled water pressure of the bag height as seen with linear regression analysis (r2 > 0.98, p <0.001 for all cases).
The goal of this study was to measure the pressure in two different models while comparing commercially available electronic transducers, needles and catheters. Our results can be used to assist in the determination of the need for a fasciotomy. In the first model, a graduated cylinder with a known height of fluid, we were able to compare the measurements of the two devices against a standard known pressure (directly calculated from the volume of the water column). In this model, we assumed that the pig muscle tissue has characteristics similar to human muscle under physiological and pathophysiological conditions. We believe that any influence on the measured pressure is not due to the use of pig tissue because of their similar physiology. We also believe that the use of tap water as the infused fluid, instead of saline, did not influence the measured pressure because there was no relevant drift throughout the experiment. Therefore, it is likely that the measured pressure is due primarily to the interaction of the tissue with the terminal device tip and the static occlusive properties, or the electronic handheld transducer.
Previous studies have used the graduated cylinder model before; however, to our knowledge, there has not been a study that has used another compartment pressure model. Therefore, the second model, a fresh pig leg with skin intact, was tested to determine if there is a difference in the measured pressures in a model that better represents the actual anatomy. For example, placing the needle tip into an intramuscular tendon can cause inaccurate pressures (11). This observation would not be accounted for in the water column model where the pig cube is of near uniform consistency. Secondly, since we were testing a new needle that has multiple holes, we wanted to determine if the needle holes could span the thin layer of fascia in between compartments, thus measuring multiple compartments at once and providing inaccurate pressure measurements.
One assumption that we made by using the pig leg model was that the ICP adequately equilibrated to the height of the infusion bag. Previous studies have shown that a constant ICP can be maintained by a given height of a column of saline in a steady state of inflow versus fluid loss in live canine legs (5). Since our pig leg model was relatively intact, but not living tissue, we were able to assume that a constant pressure could be adequately maintained within the closed compartment during the testing. It was found that negligible amounts of water perfused out of the compartment; the bag was continually checked to ensure the water level was maintained at the determined height (applied pressure).
We used standard conversion factors for both models to convert the measured height of the column to mm Hg so that our data could be easily compared to the experimental data. Our range of 2-24 inches of H2O corresponded to 4-45 mm Hg. We chose this range of measurements because it covers both normal pressure in the muscle compartment (10-12 mm Hg) and the elevated pressures (30-45 mm Hg) that prompt surgeons to perform a fasciotomy (12, 13). Pressure measurements were recorded from the same point of insertion and same depth to eliminate variation between recordings because previous studies report that ICP varies with depth, position of the limb, and distance from the injury (14, 2)
We tested the Compass against the Stryker because in all of the previous studies, the Stryker has been found to be an accurate transducer (the “gold standard”) for measuring ICP (8, 9). In our study, the Stryker and Compass were both comparable in accuracy to each other and to the controlled pressure in both models. We thus conclude that either of the devices can be used with confidence in practice. It is important to note that the Stryker costs about ten times as much as the Compass because the Stryker can be reused while the Compass is a one-time use device. It should be noted, however, that the Stryker also requires a single-use Diaphragm Chamber that costs more than an entire Compass unit. Although both devices are easy to use, the Compass is much smaller than the Stryker and may be better for chronic compartment syndrome where measurement is needed to be taken over a longer period of time with a slit catheter.
All of the tested terminal devices were comparable in measurements; any differences were not statistically significant. The simple 18-gauge needle is typically available in most hospitals. However, there has been significant criticism due to the notion that the need for a saline flush causes disequilibrium within the tissue, resulting in inaccurate measurements (15). The simple 18-gauge needle can also produce high measurements when the needle tip is placed within a tendon or if the needle tip becomes blocked from subcutaneous tissue. Previous research has shown that the simple 18-gauge needle can overestimate the pressure by as much as 22 mm Hg (5, 8). However, in our experiments, when the needles were flushed with 0.3 cc of water, as strongly suggested by both device manufacturers, no such inaccuracies were measured. It is therefore important to follow the procedure outlined by both device companies. The simple 18-gauge needle, the side-ported needle, and the spiral needle cannot be used for continuous ICP measurements. The slit catheter can be used for continuous measurement and it has small cuts at the tip of the catheter in order to reduce the likelihood of catheter occlusion. Because the slit catheter is designed for continuous measurement of the compartment; continual infusion of saline is required to produce accurate measurements. The increase in pressure due to the infusion of saline is small and in a large compartment is unlikely to affect the ultimate diagnosis (16). The spiral needle is new and has never been examined experimentally (e.g., with water column testing). This spiral needle is of importance to measuring ICP because of its unique design. The needle point is not open; rather there are holes arranged axially in a spiral down to 1.3 cm from just after the tip of the needle. The idea is that, since there is no open tip, the needle should not be able to be blocked by subcutaneous tissue. One possible concern is that, since the multiple holes of the needle span a length of 1.2 cm, one hole could be in one compartment while another may be taking reading from a second compartment, possibly leading to inaccurate measurements. We found no such inaccuracies using either the Stryker or the Compass in situ. However, it is possible that with smaller compartments, the needle holes could span the distance of two compartments and measure inaccurate pressures. Therefore, this needle should be tested within smaller compartments.
It is of our opinion that all terminal devices can be used with confidence in the clinic when diagnosing compartment syndrome. The slit catheter is ideal for continuous monitoring of compartments while the simple 18-gauge needle, side-port 18-gauge needle, and spiral needle are all appropriate for measuring compartment pressure where multiple measurements are necessary. Any of these terminal devices can be used with either the Stryker or Compass to provide the clinician with accurate data for an appropriate diagnosis.
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