Abstract
Objective:
To present a novel urethral catheter design with a pilot balloon to reduce intraurethral retention balloon inflation pressures and to provide a visual alert during catheter placement.
Methods:
We manufactured our pilot balloon prototype from both molded and extruded silicone components. Various pilot balloon thicknesses were tested in order to determine the ideal compliance. We studied the filling pressures of the retention balloon of our prototype in a mechanical urethral model. The prototype catheter was then tested in ex-vivo human penis specimens obtained from gender affirming surgery and changes in the size of the retention balloon were measured under fluoroscopy.
Results:
The thickness of the pilot balloon was directly related to the inflation pressure of the retention balloon in the mechanical urethral model. The thickness chosen for the pilot balloon in our prototype was based on a retention balloon pressure of 70 kPa. In the ex vivo human penis model, the presence of the pilot balloon resulted in a 40% reduction in the cross-sectional area of the retention balloon compared to a standard urinary catheter.
Conclusions:
The prototype urinary catheter appears to decrease the filling pressure and size of an improperly positioned retention balloon inside a urethra. This can potentially reduce the risk of IUCIs. In addition, the prototype urinary catheter may act as a visual warning sign for the healthcare practitioner.
Keywords: urethral catheter, urethral injury, indwelling catheter, foley injury
Introduction:
Transurethral insertion of an indwelling urinary bladder catheter is a routine task performed by a variety of health-care providers in various settings. (1) Up to 20% of hospitalized patients undergo placement of an indwelling urinary catheter. (2, 3) Iatrogenic urethral catheter injuries (IUCIs) can occur by several mechanisms, including inflation of the retention balloon in the urethra, creation of a false passage, and removal of the catheter with the balloon still inflated. (4, 5) The incidence of IUCI is 6.7 per 1,000 catheter placements, although this is likely underestimated.(6) Recent prospective studies have found the actual rate of IUCI’s to be many times higher, with an incidence at least as high as catheter associated urinary tract infections (CAUTI).(7, 8) These injuries also represent a significant financial burden and cause of patient morbidity. (6) (9, 10)
When the retention balloon is properly inflated in the bladder, the filling pressures are low. However, when the retention balloon is improperly positioned and inflated in the urethra, the high balloon pressures stretch the urethral mucosa and corpus spongiosum and can cause one or both to rupture. Previous mechanical studies have demonstrated that syringe inflation of the retention balloon can generate over five atmospheres of pressure when inflated in a confined space.(11) Such injury can cause significant pain, infection, hemorrhage, urinary retention, and complex stricture disease. (8, 12, 13)
Aside from the introduction of silicone material, the design of the contemporary urethral catheter has not changed significantly since the introduction of the rubber balloon-based indwelling urethral catheter by Frederic Foley in 1937.(14) Here we report the experimental process followed to design and manufacture this new catheter that utilizes a pilot balloon, including how it performs with a model for urethral injury using ex-vivo human whole penis tissue.
Methods:
Design of a New Catheter
To mitigate urethral injury during inflation of the retention balloon within the urethra, we designed a silicone urethral catheter as shown in Figure 1. The catheter is secured in the bladder via a retention balloon in an identical manner to existing dual-lumen urinary catheters. However, the filling port of our catheter has an additional balloon sleeve that serves as a signal to the operator. Our “signal” balloon only inflates when the retention balloon is subjected to higher than normal filling pressures. Here, we describe the experimental process that yielded the present design.
Figure 1:
Schematic of the catheter with our “signal” pilot balloon and a retention balloon. (A) The retention balloon (green circle) is inflated within the bladder. (B) When the retention balloon is inflated within the urethra, the pilot balloon (red circle) is activated and serves as an alert “signal” to the operator.
I. Manufacture of early catheter prototypes using Latex
Our first proof of concept prototype was made by simply attenuating the solid wall of the filling-port shaft of a Bard 16-Fr. latex coude-tip catheter with a drill bit. (11) Doing so allowed the attenuated or weakened area to “balloon outwards” whenever the retention balloon was constrained during filling.
II. Assessment for variability of catheter retention-balloon filling-pressures within the bladder based on patient sex or BMI
Next, we determined the extent that we could attenuate the filling-port shaft, so that the filling port would balloon only when filled within the urinary tract outside of the bladder, but never when filled inside of the bladder. Furthermore, it was clear that we would need to account for any potential physical differences between patients which might cause filling pressures within the bladder to vary.
We measured the pressure within the retention balloon when the balloon of a standard latex 16 Fr. Bard™ coude-tip urinary catheter was filled within the bladder in 24 men and 24 women of varying BMI. To measure the retention-balloon filling pressure, we connected a digital fluid-pressure meter (Dwyer Instruments, Michigan City, IN) to one of the three ports of a 3-way stop-cock. The other two stop-cock ports were connected to a water-filled syringe and to the catheter’s filling-port. As an assay control, we measured these pressures before catheter insertion into the patient (i.e. in air), and after insertion. We obtained IRB approval and all patients gave written consent to participate in this research effort. Retention balloons were filled at 2 cc / seconds for all the experiments.
III. Manufacture and calibration of an all-silicone version of our catheter design
We decided to manufacture our catheter in silicone instead of latex because of the rising prevalence of latex allergies and the greater manufacturing flexibility available with silicone. (15) Our goal to develop a universal, allergy free catheter for both women and men. We determined that to create the pilot balloon in a silicone catheter model, instead of attenuating the wall of the filling port, we could create a balloon by adding a thin sleeve around the filling port while still maintaining communication with the filling port. This design was created to fit the specifications for urethral catheters in ASTM F623–99 and EN 1616:1997.
IV. Calibration of our catheter’s silicone pilot balloon design: sensitivity and specificity of the pilot balloon
The thickness of the pilot-balloon wall determines the pressure threshold that inflates the pilot balloon. If the pilot balloon activated when the retention balloon was located correctly within the bladder and not the urethra (false positive), the reliability of our catheter to alert the user that the retention balloon was being inflated within the urethra would decline. (Supplementary Figure 2) Similarly, if the pilot balloon wall thickness was too thick, the pilot balloon would not activate even when the retention balloon is filled within the urethra (false negative). We manufactured a range of catheter prototypes that varied only in pilot balloon wall thickness to optimize the pilot balloon’s sensitivity and specificity. We performed additional experiments sing these prototypes as described below.
V. Testing of pilot balloon wall thickness in two urethral models.
We tested 10 catheter prototype samples which varied only in pilot-balloon wall thickness in two urethra balloon-injury models.
1. Ambient air and artificial model of a rigid 30 Fr. urethra:
We placed the retention balloon of our catheter prototypes into either ambient air or an artificial model of a rigid 30 Fr. human urethra. Previous work by our group has shown that the filling pressure of a latex or silicone catheter retention balloon filled in ambient air is similar to those when filled inside the human bladder. (11) We assumed that 30 Fr. is the maximum stretched-state diameter for an adult human urethra that does not result in trauma. A 30 Fr. diameter metal tube was used as the model. We tested various thicknesses of the balloon, which increased by 0.1 mm across batches A to E. As we inflated the balloon with 10 mL of water, we measured the filling pressure while we observed whether or not our pilot balloon had activated.
2. Ex-vivo human whole penis/urethra model
Ex-vivo human penis specimens were harvested following penectomy during genital gender affirming vaginoplasty surgery. Discarded fresh penis specimens were collected from 12 different trans-women undergoing vaginoplasty surgery. These included the intact penile urethra, but not the bulbar urethra or penile shaft skin, as these are preserved for use with the vaginoplasty reconstructive surgery). The whole-penis specimens were kept in chilled saline, and used for our experiments within 3 hours of penectomy. Specimens lacked surrounding penile shaft skin and the dorsal neurovascular bundle but were otherwise completely intact from penis tip to the base of the penis shaft at the pubic symphysis. The retention balloon of 6 catheter prototypes and 6 BARD™ catheters were filled in the penile urethra of the 12 penis specimens using 10 ml of saline/contrast in 1 ml increments. A single Fluoroscopic image was captured of the balloon at each 1 ml increment, and balloon width and length were measured (Software: Preview, v. 9.0 (909.18); Apple, Inc.) Cross-sectional area was derived from these measurements as well.
Results:
I. Manufacture of early catheter prototypes using Latex.
We attenuated a segment of the filling port of a 16 Fr. Bard latex urinary catheter to manufacture our first working model. We colored the attenuated area with red dye to maximize visibility. (Supplementary Figure 1) Filling the retention balloon in ambient air does not activate the pilot balloon. Upon filling in a constrained environment, the pilot balloon activates and is visible to the operator.
II. Assessment for variability of catheter retention-balloon filling-pressures within the bladder based on patient sex or BMI
There was no statistically significant difference (p<0.05) in peak filling or steady-state retention balloon pressure when these were compared across the three different categories for patient age or BMI, or by sex. (Supplementary Table 1)
III. Manufacture and calibration of an all-silicone version of our catheter design
We measured the retention-balloon filling-pressure within the bladder across a number of different leading manufacturers’ silicone catheters. We then contracted an international manufacturer to provide samples of a silicone catheter whose retention balloon pressure during filling approximately equaled the average for commercially available silicone catheters to ensure that our all silicone catheter would perform similarly regarding traction and retention under normal use, and so meet International Organization of Standards and ASTM specifications for silicone catheters.
The mean peak and steady-state filling pressure within our prototype silicone catheter’s retention balloon during filling in an ambient-air environment (10 independent trials) was 251 kPa (SD 15.2) and 55 (SD 5.9) kPa, respectively.
IV. & V1. Calibration of our catheter’s silicone pilot balloon design (sensitivity and specificity of the pilot balloon) using an artificial model of a 30 Fr. urethra
Durometers measure a material’s hardness and elasticity. Lower measurements (less than 50 ShoreA) are preferred in silicone catheters due to the increased elasticity at lower filling pressures (less than 100 kPA). We manufactured the pilot balloon using material whose hardness is 25% less than that of our retention balloon so that to make fine adjustments to the filling (activation) pressure of the pilot-balloon we could simply vary the wall-thickness of the pilot-balloon.
When we filled the retention balloon of different batches of our catheter prototype in ambient air, the peak filling pressure was around 70 kPA across all pilot balloon wall thicknesses (A-E). (Figure 2) The pilot balloon, whose wall thickness is slightly higher than that of the retention balloon, activated at lower wall thicknesses (A, and slightly at thickness B). When we placed samples of each batch of catheters inside the artificial 30 Fr. rigid model of the urethra, the peak filling pressures rose with increasing pilot balloon wall thickness and the pilot balloon activated at all wall thicknesses (A-E).
Figure 2: Test of different pilot balloon wall thicknesses in an artificial urethra balloon injury model.
The retention balloon of our catheter prototypes was placed into either ambient air (blue) or into an artificial model of a rigid 30 Fr. human urethra (orange) and filled with 10 ml. water. We recorded the peak filling pressure (large orange and blue dots) and if the pilot balloon activated upon filling (*). The x-axis represents each batch of catheters (A-E) that differed only with respect to the wall-thickness of the pilot balloon; across A-E, wall thickness increased by 0.1 mm for each batch. Batch C had the pilot balloon wall thickness that most optimized sensitivity and specificity to distinguish between filling in ambient air (to represent the bladder) versus filling inside the 30 Fr. urethra model.
Pilot balloon thickness “C” was the thinnest balloon thickness at which the pilot balloon did not inflate when the retention balloon was in ambient air but did fill when the retention balloon was filled inside the urethra model. As such, pilot balloon wall thickness “C” appeared to optimize the sensitivity and specificity for activation of the pilot balloon to distinguish between filling of the retention balloon in the bladder versus inside the urethra.
We manufactured our final catheter prototype with the pilot balloon wall thickness equal to “C”, and tested it against an otherwise identical standard BARD™ silicone catheter. Based upon previously published data, the threshold at which urethral trauma appears to occur is 120 kPa.(16) As shown in Figure 3, all three of our prototype pilot balloon wall thicknesses (A, B, & C from Figure 2) do not surpass this threshold. However, the BARD™ catheter does surpass the 120 kPA threshold beginning at between 4–5 ml intraurethral filling volume.
Figure 3: Catheter retention balloon pressures during filling in 1 ml. increments, using two different 16 Fr. silicone catheters: commercially available ones made by BardTM versus our catheter prototype.
The blue line (Trauma threshold) is at ~120 kPA, and represents the threshold catheter retention balloon pressure above which urethral trauma occurs during urethral balloon inflation. The red line (Bard Foley catheter) represents the commercially available standard. The solid green line (prototype “C”) represents our catheter prototype with pilot balloon thickness “C” from Figure 2. The purple hatched line represents prototype “B” from Figure 2. The orange hatched line represents prototype “D” from Figure 2. Note that while our three catheter prototypes approach the trauma threshold at between 4–5 ml. filling-volume, none cross the threshold. All three prototypes remain below the threshold of urethral trauma across all filling volumes.
V 2. Comparison of the finalized prototype versus a standard catheter in an ex-vivo biologic model of the human urethra:
Fluoroscopic images of the ex-vivo fresh severed human penis model are shown in Figure 4. The cross-sectional area of both the BARD and our prototype retention balloons is shown in Supplementary Table 2. When the retention balloon is filled with 10 ml. fluid, our catheter prototype retention balloon had a 40% smaller area as compared to the corresponding BARD™ urinary catheter. This difference is equal to an approximate 14Fr reduction in the circumference of the retention balloon. In addition, when filled with 10 ml fluid, our catheter prototype retention balloon was 2 cm shorter than the BARD ™ catheter. The difference of smaller retention balloon area and length of the catheter prototype corresponded to approximately 6 ml. fluid displacement into the pilot balloon. Lastly, gross disruption of the urethral outline was seen when the BARD™ catheter retention balloon was filled to 10ml. Our catheter prototype showed no evidence of urethral disruption at similar volumes.
Figure 4:
A. Fluoroscopic images (AP and lateral) of a standard Bard silicone urinary catheter (left) and our prototype catheter inflated within the urethra of an ex-vivo human penis specimen.
Discussion:
Here, we demonstrate a novel catheter prototype with the addition of a pilot balloon connected to the same channel as the inflation port of the retention balloon. The pilot balloon is designed to offload the high inflation pressure of the retention balloon when the catheter is improperly positioned within the urethra and act as a warning signal to the person placing the catheter.
Our testing using mechanical and human urethra models demonstrates that there is an optimal thickness for our prototype pilot balloon. A thicker pilot balloon requires more pressure to inflate allowing more potential damage to the urethra, whereas a thinner pilot balloon will inflate at similar pressures to the retention balloon and may lead to more false positives. Ex vivo testing on penectomy specimens showed that the pilot balloon catheter, compared to a standard urinary catheter, decreased the diameter and length of the retention balloon when inflated in the urethra. This reduces the strain upon the urethral tissue and minimizes the risk for urethral injury and rupture.(16)
Previous attempts at minimizing the risk of IUCI have included training and educational workshops for providers with varying levels of success.(17),(5) (18) While proper instruction on catheter insertion is critical, it does not eliminate the risk of IUCI, and it can be quite difficult to ensure competency. Davis et al. have recently reported on a novel syringe with a pressure relief valve.(19) This one-way valve allows pressurized fluid to flow out of the system when activated. In a pilot trial on 100 patients, there were no IUCIs, but the safety mechanism was activated 7 times, which may suggest a low specificity. While the results are promising, it does not resolve the inherent risk of current catheter designs. Furthermore, there may be confusion on how to drain the balloon when the safety valve does activate.
The conventional assumption is that it is balloon pressure that causes urethral balloon-injury trauma. (16, 20) Instead, we believe that injury (mucosa stretch injury/rupture) is most directly caused by balloon volume (a function of both balloon pressure and balloon-material). Instead of focusing on the retention balloon (which is always out of view to the operator), our catheter design leverages balloon pressure and balloon durometer to reduce intraurethral retention balloon volume by incorporating a pilot balloon. The pilot balloon offloads retention balloon pressure and volume and while alerting the operator. By incorporating this pilot balloon into the common catheter design as a molded component, there is no change to how the catheter looks or functions when placed correctly, and no additional devices or accessories are required.
A limitation of this work is that the intact human penis/urethras we used included only penile urethra and not the bulbar urethra, where most urethral balloon injuries presumably occur. Further testing is needed to compare the safety and efficacy of this device to the current standard practice. Finally, it is important to note that genital gender affirming vaginoplasty surgery afforded our use of intact penis/urethra tissues very early in the development of our catheter.(21) This serves as a good example of how gender affirming surgery programs contribute not only to the care environment in the hospital setting, but also, as others have also noted, to an academic medical center’s research environment. (20)
Conclusion:
Iatrogenic urethral catheter injuries can cause significant patient morbidity. Even though the incidence of ICUIs appears to be similar to the incidence of CAUTI, efforts to decrease ICUIs are not as well implemented. Here we demonstrate the design and efficacy of a novel catheter with a pilot balloon to prevent the risk of ICUIs.
Supplementary Material
Disclosures:
DSA and MMG are cofounders of Safe Medical Design LLC. DSA and MMG are co-inventers and hold U.S. and international patents for this catheter design.
Footnotes
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