Abstract
Introduction
Air bubbles in the dialysis circuit are rarely visible after automatic priming; however, they are often visible after the needles are manually connected to the circuit. To prevent this issue, we thought to prime needles with a circuit at automatic priming by the hemodialysis machine. In order to achieve this idea, we designed and manufactured a novel capped needle to connect the needles to the extracorporeal circuit before the automatic priming of the hemodialysis machine. This study investigated the effectiveness of this novel capped needle and compared it with the conventional method for preventing air bubble contamination.
Methods
We tested novel capped needles ten times to evaluate whether the dialysis machine works appropriately and removes air bubbles even with the attached capped needle. Next, we performed 25 trials using the conventional method, in which skilled nurses manually connect the needle. In both methods, we thoroughly counted the air bubbles with our naked eyes. We predicted that the capped needle would leave few bubbles in the circuit. In order to evaluate fewer bubbles, we conducted an additional experiment using a microparticle counter to measure the size and number of the bubbles.
Results
We thoroughly searched for air bubbles during each of the ten tests but could not find any bubbles visible to the naked eye. In the conventional method, bubbles were visible in 29 out of 50 cases. The bubble count was significantly lower in the capped-needle method than in the conventional method (p < 0.0001, Pearson's χ<sup>2</sup> test). In the additional experiments using the microparticle counter, the average remaining air volume in the extracorporeal circuit was 0.0999 ± 0.2438 nL when the priming was performed using the novel capped needles.
Conclusion
The novel capped needle eliminated all visible bubbles efficiently and effectively; therefore, it could be a valuable device for hemodialysis treatment. The reduction of air from the dialysis circuit may improve patient prognosis.
Keywords: Microbubbles, Air contamination, Renal dialysis, Dialysis solutions, Artificial kidneys
Introduction
Air contamination during hemodialysis can negatively impact outcomes [1, 2, 3]. The presence of an intracardiac shunt, most frequently caused by a patent foramen ovale, may allow air bubbles to enter the body's circulation directly. More than 20% of patients undergoing hemodialysis have a patent foramen ovale [4]. Many cerebral air embolisms have been reported in relation to hemodialysis [4, 5, 6]. Furthermore, air contamination from hemodialysis can damage pulmonary vessels. Air bubbles in the circuit can obstruct the pulmonary vascular bed and cause ischemic damage, leading to pulmonary hypertension [7, 8]. Microbubbles were first described as microembolic signals detected by ultrasound about 20 years ago [9]. Microbubbles slip through the pulmonary blood vessels and circulate throughout the body, resulting in microembolisms in various organs of patients undergoing hemodialysis. The accumulation of microinfarct damage could be related to poor prognosis in patients undergoing hemodialysis [10]. In particular, the mean microbubble count was higher at the beginning of hemodialysis treatments than during the rest of the session [11]. In 2019, Forsberg et al. [8] reported the results of autopsies on 5 patients who died during or a short time after hemodialysis. In all 5 autopsy cases, patients had air embolisms in many organs, including the brain [8]. Thus, air contamination during hemodialysis is detrimental, and any remaining visible air bubbles in the circuit should be removed. Recently, automatic priming of hemodialysis machines has become widely used. Due to a large amount of dialysate being circulated, the liquid overflows from the circuit, leaving no air that can be seen with the naked eye. However, when the extracorporeal circuit is connected to the puncture needle, visible bubbles often remain in the needle connecting tube. If we connect the needle before the automatic priming process is initiated, air bubbles can be eliminated. However, the caps currently in use for metal needles are not designed to connect to the extracorporeal circuit. We tried to resolve these issues by developing a novel capped needle. Then, the hemodialysis circuit was primed together using this novel capped needle, and we checked the remaining air in the circuit. We presented a section of this research at the 2021 ERA-EDTA conference [12].
Materials and Methods
The novel capped needle was designed to enable connection to the extracorporeal circuit before automatic priming. Because this process applies intense negative pressure to the dialysate, the cap and needle had to be glued together. A groove was made at the base of the cap so that it could be easily removed with a slight twist. The cap had an open end, and a screw was attached to be firmly joined to the extracorporeal circuit (shown in Fig. 1). A connector was then placed between the two capped needles to create a closed circuit (shown in Fig. 2). With the needle and circuit closed and in one piece, the automatic priming of the hemodialysis machine would remove air from the needle as well as from the circuit. The medical staff could puncture the cap after twisting it off, applying very little force.
Fig. 1.
Structure of the novel capped needle.
Fig. 2.
Photographs of two capped needles and the connector to make a closed circuit between them.
Conventional Method
In most hemodialysis machines, it is necessary to connect the arterial and venous sides of the blood circuit to create a closed circuit before automatic priming can be performed. We used a DCS-100NX (Nikkiso Co., Ltd., Tokyo, Japan) to prime the extracorporeal circuit. Nikkiso, as well as other manufacturers, are currently using this general method for automatic priming. After the process of automatic priming, our expert nurses disconnected the arterial and venous sides of the extracorporeal circuit and connected both ends to the needles. After this, they operated the dialysis machine to pour dialysate into the circuit, attempting to remove the air bubbles by blowing them out of the needle. After this process, we thoroughly examined the extracorporeal circuit and needle-connecting tube for the presence of air bubbles. Due to the fact that bubbles on the arterial side of the extracorporeal circuit are likely to be supplemented by the air trap chamber, only bubbles from the chamber outlet on the venous side of the needle were checked.
The Novel Capped-Needle Method
The capped needles were connected to the closed extracorporeal circuit. Then, the circuit and capped needle were automatically primed together. Ten consecutive trials were performed. In each method, we thoroughly examined the air bubbles in the extracorporeal circuit and needle connecting tube.
Additional Experiments Using a Microparticle Counter
We predicted that the capped needle would leave a minuscule amount of bubbles in the circuit. Due to this, a microparticle counter had to be used to measure the size and number of the bubbles. We performed automatic priming 25 times using the novel capped-needle method. After that, we took 3 mL of dialysate samples. In this experiment, the number and diameter of air bubbles were measured using the RION particle counter KL-04A (RION Co., Ltd.).
Statistical Analyses
We calculated the volume of each bubble according to the formula for finding the volume of a sphere. Data are expressed as means ± standard deviations. The Pearson's χ2 test was used to test for differences in the number of bubbles, whereas the Student's t test was used to test for differences in the amount of bubbles. All statistical analyses were performed using Microsoft Excel (version 2019). A p value less than 0.05 was considered statistically significant.
Results
Results of the Conventional Method
We counted the remaining bubbles in 50 consecutive cases. All the bubbles were found in the connecting tube of the needle and not in the dialysis circuit. We found bubbles in 29 out of 50 cases. The diameter of the circuit was 3.5 mm. Considering that large bubbles would reach at least this diameter, the minimum volume of the bubbles reaching the tube wall was calculated to be 22.43 μL. As the small bubbles were of various sizes, their volumes could not be precisely calculated. The average air volume in 50 tests was 11.67 ± 17.28 μL (Table 1). Since most of the large bubbles were not spherical but elongated, the actual volume of the bubbles was probably several times larger than this value.
Table 1.
Results of the conventional method
| 1st | 2nd | 3rd | 4th | 5th | 6th | 7th | 8th | 9th | 10th | 11th | 12th | 13th | 14th | 15th | 16th | 17th | 18th | 19th | 20th | 21st | 22nd | 23rd | 24th | 25th | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Large | 1 | 1 | 3 | 1 | 1 | 3 | 1 | 1 | 2 | ||||||||||||||||
| Small | 3 | Multiple | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||||||||||||||||
| Volume of air bubble, mm2 | 0 | 0 | 0 | 22 | 0 | 22 | 0 | 67 | 0 | 0 | 0 | 0 | 22 | 0 | 22 | 0 | 67 | 22 | 0 | 0 | 0 | 22 | 0 | 0 | 45 |
| 26th | 27th | 28th | 29th | 30th | 31st | 32nd | 33rd | 34th | 35th | 36th | 37th | 38th | 39th | 40th | 41st | 42nd | 43rd | 44th | 45th | 46th | 47th | 48th | 49th | 50th | |
| Large | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 3 | |||||||||||||||
| Small | 1 | 1 | |||||||||||||||||||||||
| Volume of air bubble, mm2 | 22 | 22 | 22 | 22 | 0 | 45 | 0 | 0 | 0 | 0 | 0 | 22 | 0 | 0 | 22 | 0 | 22 | 22 | 0 | 0 | 67 | 0 | 0 | 0 | 0 |
We connected the needle by hand and then pumped dialysate through the dialysis machine in the usual manner until it overflowed the needle. We then carefully observed the dialysis circuit from the chamber outlet on the venous side to the needle. All the bubbles were found in the connecting tube of the needle, not in the dialysis circuit. The air bubbles larger and smaller than the diameter of the extracorporeal circuit were considered as large and small, respectively. The average air remaining in the needle tube using the new method was calculated as 11.67±17.28 µL.
Results of Novel Capped-Needle Method
For the capped needle method, ten consecutive automatic priming trials were performed with the novel capped needle. In all cases wherein automatic priming of the entire extracorporeal circuit with the capped needle was performed, there were no bubbles visible to the naked eye on the arterial as well as the venous sides of the circuit.
Result of Additional Experiments Using a Microparticle Counter
Twenty-five consecutive automatic priming trials for evaluating the capped-needle method were performed. A total of 361 microparticles were observed in the dialysate collected from the extracorporeal circuit. The average air remaining in the extracorporeal circuit using the capped-needle method was 0.0999 ± 0.2438 nL (Table 2).
Table 2.
Results of an additional experiment using a microparticle counter in the capped-needle method
| Diameter | 1st | 2nd | 3rd | 4th | 5th | 6th | 7th | 8th | 9th | 10th | 11th | 12th | 13th | 14th | 15th | 16th | 17th | 18th | 19th | 20th | 21st | 22nd 23rd | 24th | 25th | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 10 µm | 0 | 5 | 1 | 1 | 32 | 0 | 11 | 0 | 43 | 0 | 0 | 10 | 16 | 0 | 1 | 0 | 23 | 0 | 2 | 0 | 15 | 1 | 2 | 47 | 12 |
| 15 µm | 0 | 0 | 1 | 0 | 15 | 0 | 2 | 0 | 15 | 0 | 0 | 5 | 6 | 0 | 0 | 0 | 8 | 0 | 0 | 0 | 4 | 1 | 1 | 15 | 3 |
| 20 µm | 0 | 0 | 0 | 0 | 7 | 0 | 0 | 0 | 2 | 0 | 0 | 0 | 3 | 0 | 0 | 0 | 4 | 0 | 0 | 0 | 3 | 1 | 0 | 4 | 1 |
| 25 µm | 0 | 0 | 0 | 0 | 3 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 3 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 1 | 0 | 3 | 0 |
| 30 µm | 0 | 0 | 0 | 0 | 2 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 1 | 0 | 1 | 0 |
| 40 µm | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 |
| 50 µm | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 |
| 60 µm | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 |
| 70 µm | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 |
| 80 µm | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 |
| 90 µm | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 |
| Total amount of air, nL | 0 | 0 | 0 | 0 | 0.52 | 0 | 0.01 | 0 | 0.47 | 0 | 0 | 0.01 | 0.1 | 0 | 0 | 0 | 0.1 | 0 | 0 | 0 | 0.05 | 1.07 0 | 0.14 | 0.02 |
We performed 25 auto-priming trials using the new needle cap. A total of 3 mL of dialysate was collected from the tip of the needle and 361 microparticles were observed. The average air remaining in the needle tube using the new method was calculated as 0.0999±0.2438 nL.
Comparison of Residual Air Volume between the Conventional and Capped-Needle Methods
Air bubbles were observed in 29 out of 50 cases in the manual connection group and none of the 10 cases in the novel cap group. The visible bubble count was significantly lower than the conventional method when using the capped-needle method (p < 0.0001, Pearson's χ2 test). The volume of air bubbles in the conventional method was at least 11.67 ± 17.28 μL, whereas it was 0.0999 ± 0.2438 nL in the novel capped-needle method (p < 0.0001, Student's t test). This clearly shows that the volume of bubbles observed in the conventional method was significantly higher than in the capped-needle method. The new capped needle was able to reduce the amount of residual air bubbles by a hundred thousand times.
Discussion
Air has even been found in the blood vessels of hemodialysis patients [13, 14, 15, 16]. This air may have entered the body through the hemodialysis circuit. The air bubbles delivered to the patient from the hemodialysis circuit could be crushed by the turbulence flow of the vascular access and become the source of microbubbles. Some of them can then slip through the lungs and cause microinfarctions in organs throughout the body. Therefore, the air bubbles remaining in the dialysis circuit need to be reduced as much as possible.
On the other hand, medical staff must take time out from patient care to prepare the hemodialysis circuit in a renal dialysis facility. The nurses have to prepare circuits for many patients and attempt to get the air out of them within a limited time. The nurse who connected the circuit to the needle in this study was highly experienced and may have taken more care than usual. It was surprising to find that air bubbles remained in more than half of the cases even under these conditions. We have always thought that starting dialysis without leaving air bubbles in the circuit cannot be solved by effort alone. This novel capped needle can completely and reliably remove all visible air bubbles without burdening medical professionals. We were concerned that incorporating the needle into the closed circuit would create pressure resistance and prevent automatic priming, but there was no problem. Microbubbles were also measured in the dialysate solution for confirmation, but almost no microbubbles were recorded (0.0018 nL/3 mL). The amount of air bubbles remaining in the capped-needle method was significantly lower than in the conventional method. In this study, the conventional method permitted air contamination more than one hundred thousand-fold higher than the novel capped-needle method. This capped needle reduces the amount of air entering the patient from the dialysis circuit at the start of each dialysis session. The reduction of air from the dialysis circuit may improve patient prognosis.
This study has certain limitations. This cap cannot solve the creation of microbubbles due to suction pressure during hemodialysis, incomplete priming of dialyzers, or air contamination during hemodialysis from the connections of circuits. Although we report a novel means of resolving bubbles that are visible to the naked eye, the problem of microbubbles remains unresolved, and further studies on methods to tackle this issue are warranted. Also, capped needles do not prevent large-scale air accidents. However, dialysis staff do not have to worry about air contamination when connecting the needle to the circuit, so that they can do it calmly and securely. As a result, it may be effective in preventing accidents such as disconnection.
Conclusion
The novel capped needle eliminated all visible bubbles efficiently and effectively; therefore, it could be a valuable device for hemodialysis treatment. This easy-to-use and highly effective cap could be considered an essential device for hemodialysis treatment.
Statement of Ethics
This study was exempted from institutional review as only materials such as tubes, needles, and dialyzers were used, and human subjects were not involved in the study. This is not applicable as the study did not include human participants.
Conflict of Interest Statement
The author has been granted a patent for this structure by the Japanese Patent Office (registered on August 27, 2021, WO2018-186495).
Funding Sources
The authors did not receive any financial support for this article's research, authorship, and publication.
Author Contributions
The inspiration for the research began with the conception of the cap design by Kazuhiko Shibata. The research design was conducted by Takahiro Shinzato, Shigeru Nakai, Koichi Tamura, Tatsuo Hashimoto, and others. Data collection was mainly conducted by Kazuhiko Shibata. Analysis and interpretation of the data was conducted by Yusuke Kobayashi, Shigeki Toma, Shigeru Nakai, Takahiro Shinzato, and others. The paper was written by Kazuhiko Shibata, Shigeru Nakai, Takahiro Shinzato, Tatsuo Hashimoto, and Koichi Tamura, and important revisions of the paper were made by Shigeru Nakai, Takahiro Shinzato, Shigeki Toma, and Koichi Tamura. All the authors commented on previous versions of the manuscript. All the authors have read and approved the final manuscript.
Data Availability Statement
All data generated or analyzed during this study are included in this article.
Acknowledgments
We thank Mrs. Sachiko Hayashi, the head nurse at the Yokohama Minami Clinic, and other supporting staff for their immense help in this study. We would also like to thank Nipro Corporation (Osaka, Japan) for producing these capped needles free of charge.
Funding Statement
The authors did not receive any financial support for this article's research, authorship, and publication.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
All data generated or analyzed during this study are included in this article.
