A survey on air bubble detector placement in the CPB circuit: a 2011 cross-sectional analysis of the practice of Certified Clinical Perfusionists

Abstract

The ideal location of air bubble detector (ABD) placement on the cardiopulmonary bypass (CPB) circuit is debatable. There is, however, very little data characterizing the prevalence of specific ABD placement preferences by perfusionists. Therefore, the purpose of this study was to survey the perfusion community to collect data describing the primary locations of air bubble detector placement on the CPB circuit.

In June 2011, an 18-question on-line survey was conducted. Completed surveys were received from 627 participants. Of these, analysis of the responses from the 559 certified clinical perfusionists (CCP) was performed.

The routine use of ABD during CPB was reported by 96.8% of CCPs. Of this group, specific placement of the bubble detector is as follows: distal to the venous reservoir outlet (35.6%), between the arterial pump and oxygenator (3.8%), between the oxygenator and arterial line filter (35.1%), distal to the arterial line filter (ALF) (23.6%), and other (1.8%). Those placing the ABD distal to the venous reservoir predominately argued that an emptied venous reservoir was the most likely place to introduce air into the circuit. Those who placed the ABD between the oxygenator and the arterial line filter commonly reasoned that this placement protects against air exiting the membrane. Those placing the ABD distal to the ALF (23.6%) cited that this location protects from all possible entry points of air. A recent false alarm event from an ABD during a case was reported by 36.1% of CCPs.

This study demonstrates that the majority of CCPs use an ABD during the conduct of CPB. The placement of the ABD on the circuit, however, is highly variable across the perfusion community. A strong rationale for the various ABD placements suggests that the adoption of multiple ABD may offer the greatest comprehensive protection against air emboli.

Keywords

cardiopulmonary bypass safety air bubble detector air embolism safety devices perfusion

Introduction

Air bubble detectors (ABD) alert the perfusionist to the presence of air in the cardiopulmonary bypass (CPB) circuit and have become an important safety device used to guard against massive air embolism during open-heart surgery. Since their introduction over 30 years ago, the ABD (which can be attached to the tubing of the extracorporeal circuitry), has rapidly been adopted by practicing perfusionists in the United States of America.^1,2 Perfusion safety surveys indicate that the use of ABD during cardiopulmonary bypass (CPB) procedures has increased from 47.9% in 1986 to 87.8% in 2000.^3,4

Currently, the prudence of ABD use during the conduct of CPB has been collectively accepted and articulated in the Practice Guidelines by both the American Society of ExtraCorporeal Technology (AmSECT) and the American Academy of Cardiovascular Perfusion (AACP). Each organization recommends that a bubble detector be used during the conduct of CPB.^5,6

Despite this professional consensus on using the ABD during CPB, the ideal placement of the device on the extracorporeal circuit is unclear and can vary greatly.^7,8 Apart from a single unscientific poll in 1993, there is an absence of information that characterizes the specific location of ABD placement that perfusionists select during the conduct of CPB.⁹

To further develop best practice guidelines, it may be important to understand the current application of the use of this safety device by perfusionists. Therefore, the purpose of this study is to survey the perfusion community to: (i) collect data describing the primary locations of air bubble detector placement, and (ii) understand the rationale of this placement.

Methods

Institutional review

Following review by the SUNY Upstate Medical University Institution Review Board (IRB), this study was granted approval.

Survey time frame

The survey was conducted during June 2011 – August 2011.

Survey instrument

The survey contained 18 questions covering three core sections: (1) Demographics, (2) Perfusion Work Details, (3) Details of Bubble Detector Use. The questions were formatted on a web-based commercial survey site (www.surveymonkey.com). The survey data security and confidentiality of participants were ensured through physical protection, software devices and hardware features.

Target population and data collection

The target population was perfusionists certified by the American Board of Cardiovascular Perfusion (ABCP). To collect responses from this population, members of the American Society of Extracorporeal Technology (AmSECT) and members of Perfusion.com were asked to participate in the confidential voluntary survey via the online forums, PerfList and PerfMail, respectively. To increase participation, a second follow-up posting on these forums was performed four weeks later. Finally, for under-represented geographic regions in the survey, as compared to the ABCP database, direct emailing requests to perfusionists were made using the CTSnet database.

Selection criteria

Respondents who indicated that they were certified by the AACP were included in the analysis. Non-CCP and incomplete surveys where participants failed to answer questions on the details of bubble detector placement were excluded.

Statistical analysis

Confidence intervals of the survey were calculated using a confidence level of 95% and a population of the 2011 ABCP certified clinical perfusionists (n=3760). All other data are expressed as percentages (%) within the total number of respondents.

Results

Respondent rate and confidence limits

A total of six hundred and twenty-seven individuals submitted a survey. Of these, 559 surveys met the inclusion criteria of individuals with CCP status, for a response rate of 14.9% of ABCP certified perfusionists in the year 2011. The confidence interval of the survey is ± 3.8%.

Demographics

The respondent characteristics are shown in Table 1. Regarding the titles of respondents, the majority identified themselves as staff perfusionists (52.7%), with the second largest group being chief perfusionists (40.5%). With concern to the gender distribution, there is 71.9% male and 28.1% female. The respondent age range was diverse, with the largest group being between 36-45 years old (37.3%). Nearly half of the respondents (49.6%) had been practicing for >20 years. Most participants worked as either an employee of a hospital-based practice (36.6%) or in an academic/hospital-based (31.9%) institution.

Table 1.

Demographic data

Job Description
Chief Perfusionist/Manager	40.5% (226)
Staff Perfusionist	52.7% (294)
Perfusion Education Faculty	3.8% (21)
Locum Tenens	2.0% (11)
Retired	1.1% (6)
Gender
Female	28.1% (156)
Male	71.9% (399)
Age (years)
18–25	1.8 % (10)
26–35	15.5% (86)
36–45	20.9% (116)
46–55	37.3% (207)
56–65	23.8% (132)
> 65	0.7 % (4)
Years in Perfusion
0–5	9.5% (53)
6–10	12.6% (70)
11–15	9.9% (55)
16–20	18.3 % (102)
> 20	49.6% (276)
Employment Venue
Hospital-based/Academic	31.9% (173)
Hospital-based	36.6% (199)
Large contract group	9.8% (53)
Small contract group	18.8% (102)
Self-employed	2.9 % (16)

The geographic regions represented by the respondents were compared to the 2011 ABCP published data indicating CCP locations (Table 2).

Table 2.

Geographical location of respondents compared to ABCP population.

Geographic Location	Survey Respondents (n=559)	2011 CCP Population (n=3760)
Northeast USA	23.3%	19.3%
Southeast USA	22.0%	26%
Midwest USA	25.2%	24.9%
West USA	13.2%	14.3%
Southwest USA	10.0%	10.6%
Outside of USA	6.3%	3.8%

The patient population and circuitry details are shown in Table 3. Perfusionists who work primarily with adult patients represented 67.8% of the participants. In regards to circuitry, hard-shell venous reservoirs and centrifugal arterial pumps were used most often (87.4% and 45%) among participants, respectively. Oxygenator units with an integrated arterial line filter (ALF) were used by 19.3% of the respondents.

Table 3.

Perfusion work details

CPB Patient Population
Adult Only	67.8% (377)
Adult & Pediatric	24.1% (134)
Pediatric Only	8.1% (45)
Venous Reservoir Use
Hard-shell Reservoir	87.4% (485)
Bag Reservoir	6.8% (38)
Both	5.6% (31)
Arterial Pump Usage
Roller	38.5% (215)
Centrifugal	45.0% (251)
Both	16.5% (92)
Oxygenator with integrated ALF
Yes	19.3% (107)
No	80.7% (447)

Bubble detector use and placement

Of the 559 CCP participants, 541 (96.8%) routinely use an air bubble detector during the conduct of CPB (Table 4). For this group, 74.6% indicated that the air bubble detector is in modes that will automatically servoregulate the system (pump stop, coast, or clamp).

Table 4.

Air Bubble Detector Use & Mode

Routine Use of Bubble Detector
Yes	96.8%
	(541)
No	3.2%
	(18)
Mode of Bubble Detector Use
Audible Alarm	25.4%
	(138)
Audible alarm with pump interface	74.6%
	(406)

The specific placement of the bubble detector in the CPB circuit is shown in Figure 1. Briefly, 35.6% were placed distal to the venous reservoir outlet, 3.8% were between the pump outlet and the oxygenator, 35.1% between the oxygenator and the arterial line filter, and 23.6% placed the bubble detector distal to the ALF. Additionally, 1.8% of perfusionists stated that they use multiple bubble detectors in their primary circuit so a single location could not be verified.

Figure 1.

Air bubble detector placement locations by certified clinical perfusionists. Arrow shows the CPB circuit location indicated by survey respondents. (There was also a 1.8% “Other” response associated with the use of 2 bubble detectors).

By cross-tabbing the data and comparing the distribution of ABD locations between centrifugal pump to roller pump, users showed no significant differences between groups. Cross-tabbing data and comparing primarily pediatric CCPs with adult only CCPs revealed that the pediatric-only group placed the ABD distal to the arterial line filter more often than their adult-only counterparts (33.0% vs. 19.0%). Similarly, users of an oxygenator with an integrated ALF place the ABD in locations before the oxygenator inlet more when compared to CCPs who use a conventional oxygenator (51.9% vs. 36.1%, respectively).

Regarding the selected placement of the bubble detector, 89% indicated that this was a standardized location used by their department (40.1% written departmental protocol, 48.9% unwritten consensus agreement). The remaining 11.0% stated that it is not standardized at their institution and can vary from perfusionist to perfusionist.

Rationale for bubble detector placement

Selected comments provided by respondents on their rationale of the selected ABD placement is shown in Table 5. Briefly, those placing the ABD distal to the venous reservoir (35.6%) argued that an emptied venous reservoir was the most likely place to introduce air into the circuit and, therefore, was the best location. Those who placed the ABD between the oxygenator and arterial line filter (35.1%) commonly reasoned that this placement protects against air exiting the membrane while also providing a means for removal of air through the ALF purge. Those placing the ABD distal to the ALF (23.6%) most often cited that this protects the patient from all possible entry points of air. Some CCPs stated that the use of multiple ABD would be ideal to allow for more than one location.

Table 5.

Rationale of specific air bubble detector placement. (Selected representative open-ended responses)

Distal to Venous Reservoir (n=171)	Between Pump and Oxygenator (n=16)	Between Oxygenator and Art. Line Filter (n=154)	Distal to Arterial Line Filter (n=109)
“It really is just a backup level detector. We feel the risk of draining the reservoir is greater than finding bubbles in the circuit”	“catch bubbles being pumped to oxygenator early”	“I’m more concerned with what’s coming out of the oxygenator than going in. Placing between the Oxy and A-line filter allows for trapping and purging air if necessary. I would never do a case with a venous air detector only.”	“distal to anywhere air could be introduced, ie oxygenator; I am aware of a situation at another institution where bubble detector was placed in position A. Cardioplegia blood line was reversed during the case sending air to patient”
“First site of air entry before air really enters the circuit.”	“convenience… right in front of my seated position”	“To try to catch any bubbles that might come out of oxygenator”	“It will alert to air that can enter anywhere in the circuit. Some placements miss air entry from oxy or arterial / cardioplegia outlets.”
“the venous reservoir will be the first place and the most likely place to introduce air into the circuit. therefore in my opinion, the best place to put it. this way you have time and room to get the air out prior to the arterial filter.”	“The biohead is 1.5” below venous bag outlet, not enough room for it there; if you’ve deprimed your biohead, your flow will stop before you get massive air to bubble detector.”	“This is where we put in school.”	“We use low level detectors that will stop the pump if reservoir is drained. We want to catch inadvertant entry of air or any source distal to oxygenator, sampling line, etc.”
“earliest point for safety device to intervene should a bubble or volume problem occur”	“early detection without many false alarms.”	“…it’s the way I was taught for starters, but more importantly, when using VAVD I find having the detector in the C location is more beneficial if you pull air across your oxygenator membrane by not using the vacuum correctly or you forget to turn off the vacuum when your flows are down…”	“Last connection in circuit (other than the arterial cannula) before entering patient”
“Because it’s the way it’s always been done that way (unfortunately that’s the rationale). There are many other reasons, but last safety mechanism if the level alarm fails in addition to a bubble detector.”	“if your level alarm fails and you somehow missed your head filling with air your alarm will sound at the first site of a bubble before your oxy sees air - hopefully.”	“Post oxygenator, which is the last place air could enter circuit, and pre filter so that you can eliminate the air.”	“Beyond all possible sources of air entrainment.”
“Earliest and most likely site for air.”	“Use of a bio head provides some protection for air emboli as it will get trapped there. It is also prior to the oxygenator, which also will act as an air trap.”	“If air were to enter circuit it could still be purged out via the Art filter.”	“I want to be able to detect air through EVERY part of the circuit–right up until it is going up the arterial line.”
“Historical placement”	“We’d rather hear the alarm before it even gets to the oxygenator so it’s for patient safety.”

Air bubble detector false alarms

Of the respondents, 36.1% believed that they have experienced a bubble detector false alarm during the conduct of CPB. The majority (96.4%) indicating that this event had minimal impact and was only a minor inconvenience; however, the remaining 5.1% did believe that the false alarm had a significant impact on the case.

Open-ended comments

Participants were given the opportunity to add any comments on the survey and the topic of air bubble detectors. Of the 559 respondents, 118 (21%) entered comments. Of these, 42.4% were comments of support and/or encouragement to publish results. The remaining comments could be generally categorized into four groups: False Alarms (n=17), Criticisms and/or Recommendation (n=4), Multiple ABD (n=8) and Miscellaneous Comments (n=38). Selected comments are shown in Table 6.

Table 6.

Open-ended comments on survey and topic of air bubble detector

Comments on ABD False Alarms (n=17)	Criticisms and/or Recommendations (n=4)	Comments on using Multiple ABD (n=9)	Miscellaneous Comments (n=38)
“I’ve only had one false alarm from a bubble detector … when I kicked it …”	“Not sure what age, gender, and employment type matter other than to stereotype or say someone has an aversion to change.”	“… we have 2 air detectors – one between the reservoir and oxygenator and one distal to the arterial filter. Both servo-regulate the roller pump”	“You must still remember to turn on the alarm!”
“False alarm when density changed in reservoir without adequate time for homogenization”	“Large contract group is how many people? Small contract group is how many people? Questions about what type of bubble detector sensitivity. Sorin pump S5 allows small, medium, and large air detection.”	“If I had 2 bubble detectors I would place also place one distal to oxygenator on the arterial line …”	“I realize there are differing opinions as to best placement. There are also many different ways to pump air …”
“Most false interruptions occurred during RAP/VAP … change in viscosity may have caused this”	“Looking at air detectors used with level sensors. It may be interesting to see where people use their bubble detectors if they employ level pads at all and if they “alarm only”	“Perhaps the optimal system would have two ABDs: one distal to the VR, the other distal to AF.”	“Filtered oxygenators present a huge problem … as there is really no purge distal to the oxygenator”
“I’d rather deal with the rare false positive than to ever have to deal even one false negative when it comes to air in the circuit”	“ Regarding false alarms, it is hard to determine if a bubble alarm is a ‘false alarm’ … So take the false alarm answer with a grain of salt. A follow up question would be … ‘how did you know it was a false alarm’.”	The ideal, safest set-up would be to have multiple air detectors.”	“Rationalizations for all placements can be made. Which provides the best security is still debated by all with passionate arguments.”
False alarms are usually caused by placing the detector to close to the venous reservoir”	“[regarding false alarms] How do you quantify the last question? You won’t always see air pass through a bubble detector and causes it to alarm. So, if you don’t physically see air is that what you define as a false positive?”	“Some institutions run two bubble sensors on the patient circuit.”	“Most places I go it [ABD placement] is not standardized even if there is a written protocol. Each perfusionist tends to place it where they see fit.”
“The bubble detector will alarm with microbubbles, such as when adding albumin or crystalloid.”		“I think more than one detector in the circuit would be good too.”	“Would LOVE it if manufacturers made more sensitive bubble detectors.”
“Only had false alarms on our new S5’s while in pulse mode. Not on our Terumo 5000’s”		“Multiple bubble detectors would be ideal …”	“You may want to check what location the manufacturer suggests on their particular equipment.”
“Bubble alarm is occasionally activated with initiating bypass … due to rapid change in solution composition”		“If one had more than a single alarm, I could see placing a sensor in another place other than distal to the arterial filter.”	“We also use an air detector on the cardioplegia. Placed just after the heat exchanger.”
		“… it might be wise if the manufacturers allow for more than one servo air detector in the circuit in the future.”

Discussion

A principle finding of this study is that 96.8 (± 3.8%) of certified clinical perfusionists routinely employ the use of an air bubble detector during the conduct of CPB. This adds to previous surveys revealing the progressive adoption of air bubble detectors by perfusionists during the conduct of CPB.^3,4

Additionally, this study, for the first time, moves the discussion beyond simply whether an ABD is used or not, to where on the CPB circuit the perfusionist chooses to place this safety device. It is evident from this survey that the placement of the ABD is far from universally agreed upon by certified clinical perfusionists. Nearly 36% of CCPs will place the ABD distal to the venous reservoir. This placement is supported in a major textbook, Cardiopulmonary Bypass: Principles and Practice,⁸ that states, “… for most prompt air detection, the sensor should be placed between the CPB reservoir and the systemic pump.” This statement suggests that air will most likely enter the CPB system via the emptying of the venous reservoir and, therefore, having the ABD just distal to the reservoir will halt the infiltration of air. This rationale is strengthened by the Australian and New Zealand College of Perfusionists voluntary incident reporting system, Perfusion Incident Reporting System (PIRS), which suggests that the emptied venous reservoir is the dominant mechanism for air entering a CPB system (personal communication, T. Willcox C.C.P.). The majority of respondents (59%), however, place the ABD on the arterial line; ~35% between the oxygentaor and arterial line filter and ~24% distal to the arterial line filter. This arterial line placement is also supported by textbooks with statements that “… the ideal position is arguably close to the outlet of the oxygenator”⁷ and “it is usual practice to have the bubble detector on the arterial outlet of the circuit” and that “bubble detector[s] on the arterial line are desirable safety devices.”¹⁰ Furthermore, many of the manufacturers’ operator manuals recommend that the ABD be placed on the arterial line (Table 7). Additionally, there is often a recommendation to position the ABD a certain distance from the patient’s arterial cannula (presumably to allow adequate time to detect and stop the pump to avoid air embolism). The dominant rationale for arterial line placement is the notion that air can enter the system in other ways (besides an empty venous reservoir) and positioning the detector on the arterial line protects against other possible sources of air entry.

Table 7.

Manufacturers recommendations (HLM Operating Manuals)

Manufacturer	Console	Users Manual
Terumo	System 1	- It is recommended that an ABD system be used on the arterial line for every case.
		- Proper positioning of the air sensor is the choice of the medical professional … the air sensor must be positioned a minimum of 4 feet from the patient.
Sorin	S5	- The arterial line must be monitored with a bubble sensor.
		- The minimum distance between the bubble sensor & patient is: 1.0 meter where tubing diameter is constant over the entire length.
Maquet	HL-20	- Apply the sensor as horizontally as possible to the tube on a suitable place on the pump.
Medtronic	Performer	- Flow probe/ABD: It is recommended that the arterial ABD be used on every case.
		- It is recommended that the transducer be positioned a minimum of 4 feet from the patient to allow the air detection system sufficient time to respond before air can reach the patient.
Sarns	8000	- Sarns recommends that the air sensor be the last device in the extracorporeal circuit before the patient and be as far from the patient as possible.
		- However, proper positioning of the air sensor is up to the medical professional.

As seen by the open-ended comments in Table 5, it appears that there is valid rationale for the each of the various ABD placement positions indicated by certified clinical perfusionists. Perhaps the use of more than a single ABD would provide the greatest safety from air embolism. This notion of multiple ABD is not new;¹¹ however only a small percentage (< 2%) of respondents in our study use more than a single ABD on the primary CPB circuit. One limitation to more widespread adoption of the use of multiple bubble detectors (e.g. concurrently using one distal to the venous reservoir and one on the arterial line) is the availability of such a configuration. While cardioplegia ABD are becoming more common, only the Terumo System 1 (Terumo Cardiovascular Systems, Ann Arbor, MI) offers the user the option of multiple ABD for the primary circuit.

Approximately 36% of respondents indicated that, within the last 2 years, they have experienced an ABD false alarm during the performance of CPB. Often, the false alarm was associated with the initiation of retrograde autologous prime (RAP) procedures where the rapid change from crystalloid prime to blood was thought to trigger the false alarm. A small percentage (~5%) of those reporting ABD false alarms felt that the disruption had a significant impact on the case. Some respondents pointed out that the question was unclear and did not define what constituted a false alarm. In retrospect, this could have been better clarified.

There are other limitations within this study. The authors assume that this was a representative sample of the CCP population and constitutes the power of a scientific survey. The response rate was 559 (~15% of the CCP population) with a geographic, gender and age distribution that approximates the ABCP 2011 database, yet, since this was conducted via the internet, it could be open to some selection bias. Additionally, though unlikely, it could be possible for an individual to enter a duplicate survey and, thereby, skew the results.

In summary, this study establishes that the use of the ABD during the conduct of CPB is nearly universally practiced by CCPs. Additionally, this survey demonstrates that clear differences exist between perfusionists regarding the placement of ABD on the CBP circuit. The thoughtful and strong rationale for the different characterized site locations suggests that the use of multiple ABD on the primary CPB circuit should be considered.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflict of Interest Statement

None declared.

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