JECT. 2010;42:71–74
The Journal of ExtraCorporeal Technology
Arterial Filter Bypass Loop: What Occurs in this Area
during Cardiopulmonary Bypass and Are There
Potential Patient Implications
Justin L. Hawkins, CCP, CPC; Gerard J. Myers, RRT, CCP; Jean-Francois Légaré, MD;
Wilfred Swyer, CCP
Queen Elizabeth II (QEII) and Izaak Walton Killam (IWK) Health Science Centers, Halifax, Nova Scotia, Canada
Abstract: The arterial filter is an integral part of bypass circuitry.
When introduced, manufacturers suggested a bypass loop for retrograde priming and de-airing, and for uninterrupted blood flow
in case of malfunction. Practice has shown antegrade priming and
de-airing is possible. This questions the necessity of the loop and
presents the question—what occurs in the loop during bypass?
After obtaining Human Research Ethics Board approval, eight
consecutive patients (n = 8) were chosen for this study. Exclusive
exclusion criterion was receiving any transfusions during cardiopulmonary bypass, as this could possibly influence results.
The choice of patient numbers was based simply on proof of
concept. Investigation involved isolation and collection of loop
contents after cardiopulmonary bypass was completed. Testing
included complete blood count, prothrombin time, international
normalized ratio, partial thromboplastin time, activated clotting
time, plasma free hemoglobin, slide photography with analysis for platelet clumping, and debris detection. One perfusionist
collected samples, providing uniform collection and isolation
technique. Regular blood samples were collected from the bypass
circuitry, and from patients’ pre-operative blood work. Analysis
of data revealed that platelet counts in the bypass loops were statistically lower than control. Evidence of platelet clumping was
present in 3 of 8 bypass loop samples, representing 37.5% of the
study population. There was no clumping detected in any of the
controls. In patients where platelet clumping was present, a positive correlation was noted between mean bypass time and size
of platelet clumps. Prothrombin time and international normalized ratio results were immeasurable. Hemoglobin levels were
higher in the loop samples. There was no evidence of debris or
fibrin monomer present in any of the samples analyzed. The study
results indicate that during “normal” cardiopulmonary bypass
with an arterial filter bypass loop, platelet aggregates can accumulate in the loop, therefore opening the arterial bypass loop in
any case may subject the patient to micro/macro emboli.
Keywords: arterial filter, bypass loop, platelet clumping. JECT.
2010;42:71–74
The majority of arterial line filters are made from a
hydrophilic polyester material, with an effective surface
area ranging from 400–800 cm2, enclosed in a polycarbonate housing (1–3). Arterial filters were first reported in the
early 1960s and are a standard of care (4). The primary
advantage noted with incorporating the filter in the cardiopulmonary bypass circuit is prevention of micro emboli,
either being particulate or micro air from reaching the
patients circulation (1,5). A landmark study from England
in 1994 compared neurological outcome in patients with
and without an arterial line filter. Patients without the arterial line filter had more emboli and a poorer outcome (6).
During the time of original introduction, manufacturers
suggested a bypass loop be incorporated for retrograde
priming and de-airing, and as a means for providing uninterrupted blood flow in the event of filter malfunction over
the course of bypass (7).
If the arterial filter becomes unusable during the course
of a bypass case, what is the cause? Also, if the main purpose of the filter was to increase patient safety, how prudent
is it to open the bypass loop if the filter has been “filtering
out” material with embolic potential, and now it is clogged
and rendered useless.
More recent filter designs on the market today, such as the
Sorin Synthesis® (Mirandola, Italy) integrated oxygenator/
arterial filter and the Maquet Quart® (Rastatt, Germany)
arterial filter, do not require retrograde filling for priming; neither design incorporates a bypass loop and both
are primed in an antegrade fashion (8,9). The Medtronic
Affinity® (Minneapolis, MN) arterial filter, which is widely
used in conventional cardiopulmonary bypass, is also
Received for publication March 25, 2008; accepted September 27, 2009.
Address correspondence to: Justin L. Hawkins, CCP, CPC, Cardiovascular
Perfusion Services, QEII Health Science Center, 1796 Summer Street,
Room 5512, Halifax, Nova Scotia, Canada, B3K 6A3. E-mail: jushawk72@
hotmail.com
The senior author acknowledges that the Sorin Group provided financial
support for laboratory analysis of data obtained in this study.
71
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J.L. HAWKINS ET AL.
incorporated in their “resting heart system,” and it does
not include a bypass loop in this application. This seriously
questions the functional need for the bypass loop, and the
patient safety aspect. The purpose of this investigation was
to determine what is actually taking place in the stagnant
pool of blood proximal to the clamp during bypass.
4/1 ratio. Samples for blood gas analysis were drawn every
30 minutes. If ACT dropped to below 480 seconds 10,000
units of heparin was administered and ACT was checked
again in 5 minutes. No adverse events, such as failure of
any circuit components or mechanical pump problems,
occurred during any of the cases, and no cases required
reinitiating cardiopulmonary bypass after termination.
MATERIALS AND METHODS
Patient Demographics
Study patients’ average age was 71 years, with an average weight of 79.3 kg; body surface area (BSA) averaged
1.89. The gender was split 62.5% males and 37.5% females
(Table 1).
After research ethics board approval was obtained, eight
consecutive patients (n = 8) undergoing cardiopulmonary
bypass were studied. The only exclusion criterion noted was
a patient receiving transfusions during cardiopulmonary
bypass. The investigation involved isolation of contents of
the bypass loop after bypass had been completed. Samples
were drawn from the bypass loop within 5 minutes after
termination of cardiopulmonary bypass. Laboratory testing using LH 755 hematology analyzer (Beckman-Coulter,
Miami, FL) included complete blood count (CBC), prothrombin time (Pt), international normalized ratio (INR),
partial thromboplastin time (Ptt), plasma free hemoglobin, and slide photography for analysis of platelet clumping and debris detection. The same perfusionist performed
the cardiopulmonary bypass, and subsequent sample collection for all study cases, providing for uniform collection
and isolation technique. All blood collection tubes were
from the same lot number for all samples.
Cardiopulmonary Bypass Circuitry
The cardiopulmonary bypass (CPB) circuit consisted of
Gish Vision Oxygenator (Irvine, CA), Dideco D734 arterial filter (Mirandola, Italy), and Sorin Phosphorylcholine
coated tubing incorporating a 5 µm pre-bypass filter in the
A-V loop (Mirandola, Italy). The arterial filter used was
coated with a biocompatible phosphorylcholine coating
called Mimesys™ (Sorin Biomedica, Mirandola, Italy). This
coating is phospholipid-based polymer that mimics the bilipid surface of cellular membranes. This in turn makes the
polyvinylchloride tubing relatively inert to the blood (9).
The circuit was first flushed with carbon dioxide for a minimum of 3 minutes. Priming solution consisted of 1800 mL
Normosol R® (Hospira, Saint-Laurent, Quebec, Canada)
and 10,000 units of porcine heparin (Pharmaceutical partners of Canada Inc., Ontario, Canada). All patients received
a dosage of 300–350 units/Kg of heparin prior to bypass.
Activated clotting time (ACT) was assessed using the ACT
II (Medtronic Inc, Minneapolis, MN). Target value prior to
commencing CPB was >480 seconds.
Perfusion technique involved no use of altered priming
techniques such as RAP (retrograde priming). Target flow
was calculated by formula body surface area (BSA) × 2.4
L/m2. During each case patient temperature was allowed to
drift to a low temperature of 32°C. Cold cardioplegia was
administered by both antegrade and retrograde routes at a
JECT. 2010;42:71–74
Procedure Demographics
Pump time for cases averaged 117 minutes, with an average clamp time of 87 minutes. The operative procedures were
75% coronary artery bypass grafts and 25% combination
coronary bypass grafting and valve replacement (Table 1).
Bypass Loop Modification and Isolation Technique
For this study the arterial filter bypass loop was modified
to incorporate two uncoated 3/8 connectors with leur ports
(Medtronic, Minneapolis, MN). The first Leur port was
for sample collection, and the other to alleviate negative
pressure, which could cause trauma to blood cells in the
collected sample. After termination of cardiopulmonary
bypass, and after consultation with attending surgeon, the
content of the loop was collected as shown in (Figure 1).
Table 1. Patient and procedure demographic.
Age (years)
Weight (kg)
BSA (m2)
Gender (M/F) (%)
Pump time (minutes)
Clamp time (minutes)
Procedures
CABG, coronary artery bypass graft.
Figure 1. Bypass loop modification.
71 ± 13
79.3 ± 31.7
1.89 ± .42
62.5/37.5
117 ± 53
87 ± 39
75% CABG
25% CABG/Valve
ARTERIAL FILTER BYPASS LOOP
Collection tubes for blood samples were from the same
lot number for all samples. Blood samples for control
hematology (plasma free hemoglobin, Pt, INR, Ptt, CBC,
ACT) were drawn prior to termination of CPB. Bypass
loop samples were collected after bypass was terminated
and after consulting attending surgeon and anesthetist to
ensure patient stability was established. The findings were
analyzed for statistical relevance.
73
Platelet drops from pre-bypass levels to circulating levels
during bypass were approximately 50%. The numbers precipitating into the bypass loop area during the bypass case
was approximately 30–50% of circulating platelet levels.
Slide smear photography noted presence of platelet clumps
in three of the eight bypass loop samples. Clumping occurred
in the three cases with longer bypass times (Table 3).
DISCUSSION
STATISTICS
All statistical analysis was performed using Microsoft
Excel 2000 Data Analysis tool pack (Microsoft, Redmond,
WA). Values are expressed as plus or minus standard deviation of the mean.
RESULTS
Laboratory analysis of data noted that Ptt, Pt, INR tests
were deemed immeasurable due impart to the high levels
of heparin used during cardiopulmonary bypass. There was
no evidence of fibrin monomer present in either the control samples or bypass loop samples.
Composition of the collected samples noted the hemoglobin in the bypass loop was higher as compared to baseline hemoglobin; which would be due to the weight of the
red cells settling out in the bypass loop. The difference in
plasma free hemoglobin in the control circulation and the
loop was not significant (Table 2).
Table 2. Blood composition in circuit/blood composition in bypass
loop.
Blood Composition Circuit
WBC (10E9/L) 8.6 ± 8.2
RBC (10E12/L) 2.96 ± .88
Hgb (g/L) 96.12 ± 33.88
Hct .276 ± .099
Plt (10E9/L) 134.87 ± 115.13
Plasma free Hgb 15.36 ± 4.06
ACT (seconds) 615 ± 105
Blood Composition Bypass Loop
WBC (10E9/L) 5.65 ± 3.05
RBC (10E9/L) 4.35 ± .75
Hgb (g/L) 137.62 ± 11.62
Hct .392 ± .053
Plt (10E9/L) 76 ± 61
Plasma free Hgb 15.12 ± 4.28
ACT (seconds) 969 ± 89
WBC, white blood cells; RBC, red blood cells; Hgb, hemoglobin; Hct,
hematocrit; Plt, platelet count.
Platelets contain many molecules which are thromboactive such as adenosine di-phosphate (ADP) thromboxane
(TxA2), serotonin and pro-thrombin, adenosine monophosphate (AMP), and platelet factor four (PF4). When
platelets are deposited on foreign surfaces these substances
are emitted, sending chemical messages to other platelets,
which subsequently leads to activation and deposition of
more platelets (10–12). Activated platelets adhere and
aggregate via GPIIb/IIIa receptors (13). From the results
of this investigation it is clear that during cardiopulmonary bypass platelets are precipitating in the arterial filter
bypass loop area. This creates an area where conditions are
perfect to produce platelet aggregation, as was evident in
this study. Li et al. (13) noted, platelets which lack GPIIb/
IIIa receptor complexes do not adhere upon activation,
which may explain the absence of aggregation in five of
the study population.
Bypass loops were incorporated more than 25 years ago
as a suggestion by filter manufacturers to facilitate retrograde priming of these devices (14,15). There is no documented evidence to support retrograde priming of modern
day arterial filters and antegrade priming is fully possible
and effective (15). Some clinicians argue that the filter
bypass loop is a viable means of diverting blood around a
filter that becomes obstructed with debris. However, if sufficient thrombus and other debris obstruct the filter media,
opening the bypass loop would allow such material to enter
the arterial circulation.
Physiology texts note that arterioles range from 5–30 µm,
and further branch leading to capillary beds in which
the vessel diameter here is in the range of 5–10 µm (16).
Table 3. Bypass pump time and platelet clump size.
Procedure
Bypass Time
Minutes
Platelets
Pre 10E9/ L
Platelets
Circuit 10E9/L
Platelets
Loop 10E9/L
ACT Circuit
Seconds
ACT Loop
Seconds
Clump Size,
Microns
Sex
AVR + CABG × 1
CABG × 3
CABG × 4
CABG × 3
CABG × 3
AVR + CABG × 2
CABG × 3
CABG × 2
158
110
130
91
138
147
96
64
169
199
475
126
192
400
150
360
46
145
250
97
65
187
99
190
20
117
73
66
41
126
60
110
687
592
560
611
700
680
580
510
999
900
880
999
999
999
889
999
23–71
None
14–36
None
None
38–64
None
None
Male
Male
Female
Female
Male
Male
Male
Female
AVR, aortic valve replacement; CABG, coronary artery bypass graft.
JECT. 2010;42:71–74
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J.L. HAWKINS ET AL.
Platelet aggregates in this study were found to be in the
range of 14–71 µm. Opening of the bypass loop may deliver
the accumulated aggregates directly to the brain and other
micro-vascular tissue beds. Since these aggregates do not
spontaneously dissipate when they enter the general circulation, they might incur a thrombolytic event with associated morbidity to the patient.
As noted earlier, during cardiopulmonary bypass the
stagnant pool of precipitated blood elements can contain
platelet clumps as large as 71 µm. As well, there is a positive
correlation pointing to increased size of clumps as cardiopulmonary bypass time increases. Therefore even during
“normal” cases the bypass loop can be accruing embolic
material. With this being the case, the bypass loop serves
no purpose because it is not necessary for priming and not
always a safe option if an emergency situation arises.
CONCLUSIONS
Cardiopulmonary bypass is a dynamic procedure that is
intimately connected to the patient’s circulatory system.
The statement in the medical oath “first, do no harm” is a
powerful reminder to us as clinicians that patient safety is
our utmost concern. A critical evaluation of the circuit and
techniques must occur particularly as technology changes.
If a component is incorporated “just because its always
been there” critical questions need to be asked. Sometimes
the evidence obtained through questioning and study challenges our current thoughts. The intended purpose noted
for the arterial filter bypass loop 25 years ago is not current with today’s modern filters. Newer filter designs are
fully capable of being antegrade primed and de-aired efficiently. The sole intended purpose noted for the bypass loop
through manufacturer instructions for use (IFU) is retrograde priming; this alone is not a valid reason for keeping
this loop in place. Some of the recent filter designs do not
require retrograde priming in their IFU (Sorin Synthesis®
and Quadrox®, Maquet) (3,8).
To conclude, from the results of this investigation, the
arterial filter bypass loop appears to serve no purpose
JECT. 2010;42:71–74
and may compromise patient safety under certain clinical
conditions.
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