3. RESULTS
3.1 Characteristics of the
studies
A total of 200 articles were identified by the systematic search (Figure
1), of which 3 randomised controlled trials and 15 observational cohort
studies were included in the systematic review. The details of the
included studies are summarised in Table 1. All 15 observational studies
had a score of more than 6 with NOS, and therefore, are determined as
high-quality observational studies. Three randomised controlled trials
met at least 6 of the 12 criteria set in the 2015 Updated Method
Guidelines for Systematic Reviews in the Cochrane Back and Neck
Group10, and they are recognised as high-quality
studies.
3.2 Assessment of antiplatelet
resistance
The assessment of antiplatelet resistance widely varies using several
different platelet function tests and by using downstream arachidonic
acid breakdown products such as serum TxB2 or its metabolite in the
urine, 11-dehydroTxB2, reflecting the effect of aspirin on platelets.
3.2.1 Light transmission
aggregometry:
Platelet-rich plasma (PRP) is prepared by centrifuging the 5ml of
anticoagulated blood sample at 150g for 10 minutes at room temperature.
PRP is then adjusted to a platelet count of a minimum of 150 000 - 300
000 µl. The samples are then assayed using light transmission
aggregometry by adding 0.05 ml of arachidonic acid. However, it can also
be performed without adjustment of platelet count, and type I collagen
and ADP could also be used instead of arachidonic
acid.11 The aggregation was plotted against time and
reported as total aggregation % at 5 minutes.12,13
3.2.2 Impedance platelet
aggregometry :
It measures platelet aggregation by continuously monitoring electrical
impedance changes due to activation of platelets and adhesion to the
metal sensor electrodes in 3-5 separate channels in multiple electrode
aggregometry (MEA).14–21 Whole blood sample is used
in each channel with the addition of arachidonic acid to assess the
effect of aspirin (ASPItest ), ADP for platelet P2Y12 inhibitor
effect (ADPtest ) or thrombin receptor agonist peptide to measure
glycoprotein IIb/IIIa inhibitor effect
(TRAPtest ).14,19–21 Collagen can also be used
as a substitute for arachidonic acid to assess the effect of
aspirin.17 The aggregation result is obtained as an
arbitrary area under the curve (AUC) or presented as an aggregation unit
against time (AU x min).
3.2.3 Platelet function assay (PFA):
PFA-100 (Dade Behring, Germany) is a commercially available
point-of-care platelet function assay, which assesses platelet
activation under high shear stress through aspiration of whole blood
through collagen/epinephrine coated apertured cartridges (CEPI) or
collagen/ADP coated apertured cartridges (CADP).11,22The assessment is reported as aperture closure time (CT), the time taken
for subsequent activation of platelets to obstruct the apertures in the
CEPI and CADP cartridges.
3.2.4 VerifyNow assay :
VerifyNow system (Accumetrics, San Diego, CA, USA) is a cartridge-based
rapid assay system, which assesses the effect of aspirin on platelet
reactivity in VerifyNow Aspirin Test using arachidonic acid as an
agonist and VerifyNow P2Y12 Test measures the direct inhibition of
clopidogrel on the P2Y12 receptors.23,24 Aspirin test
results are expressed as aspirin reaction units (ARU) and P2Y12 test
results as P2Y12 reaction units (PRUs).
3.2.5 Thromboelastogram (TEG):
Heparinised whole blood is used in the TEG assay (Haemoscope Corp,
Niles, IL, USA) to evaluate the platelet function in terms of clot
maximum amplitude with added arachidonic acid (MAAA) or
without a platelet agonist (MA0), which is compared with
kaolin-activated TEG assay (MAKH) to derive percent of
platelet aggregation using the formula: %MAAA =
[(MAAA - MA0) / (MAKH- MA0)] x 100%.17 The result is
reported as a percent aggregation of platelets.
3.2.6 Whole-blood flow cytometry:
The antiplatelet resistance can be measured by incubating blood with or
without arachidonic acid (1.0 mmol/L) for 2 minutes, adding
radiolabeled antibodies to CD41a or CD62P receptors on platelets, fixing
the samples with 1% paraformaldehyde and analysing with a fluorescent
cell sorter (Becton-Dickinson FACScan; BD Immunocytometry Systems, San
Jose, CA, USA).17 The result is described as the
percent increase in the expression of the CD62P receptor after
activation.
3.2.7 Thromboxane B2 (TxB2):
The urinary 11-dehydroTxB2 is the excretory form of TxB2 and its
concentration is usually measured using enzyme immunoassay kits (Cayman
Chemical, MI, USA).11–13 The results are normalised
for urinary creatinine concentration. Serum TxB2 level reflects
cyclooxygenase-2-dependent thromboxane biosynthesis, and the level is
measured in the plasma after whole blood is cultured at 37 °C for 24
hours and centrifuged at 700 x g for 15 minutes.11Serum TxB2 level can also be measured by using
immunoassay18 (Neogen, Lexington, KY, USA) or
radioimmunoassay.25 Alternatively, centrifuged plasma
could be used to measure serum 11-dehydroTxB2 level with an enzyme
immunoassay kit (Assay Designs Inc, Ann Arbor, MI,
USA).17
3.3 Definition of antiplatelet
resistance
There is no uniform definition of antiplatelet resistance in literature,
and it varies hugely with each method of assessment. Light transmission
aggregometry-derived platelet aggregation of ≥20% with arachidonic acid
is regarded as aspirin resistant,13 and in some
studies, it is an aggregation of
>30%.26,27 Aspirin resistance is
determined by impedance aggregometry if AUC ≥30
units15,16,21,28 or AUCASPI>300 units (APSItest value >75
percentile).17,18,20
Using the VerifyNow system, ARU >550 and PRU
>230 are considered aspirin resistance and clopidogrel
resistance respectively,23,29 although, PRU cut-off
point could be as low as 188 for clopidogrel to be
resistant.24 Aspirin resistance is determined as
having a collagen and/or epinephrine (CEPI) closure time <193
seconds in the PFA-100.22
Serum TxB2 inhibition <90%18 and increase
in serum 11-dehydro TxB2 >25% from
baseline,17 urinary 11-dehydro TxB2 levels higher than
67.9ng/mmol of creatinine is illustrated as aspirin
resistance.12 Platelet aggregation >50%
on TEG30,31 and 25% increase in expression of CD62P
receptor following simulation in whole-blood flow cytometry are other
definitions of aspirin resistance.17
3.4 Antiplatelets used in the
studies
All studies used aspirin as primary antiplatelet therapy, and
clopidogrel is added to aspirin to constitute dual antiplatelet therapy
in some studies.13,18,24,28 While clopidogrel 75 mg is
always used, the dosage for aspirin varies from 80-325 mg with 100 mg
being the most commonly used dosage. A postoperative loading dose of
intravenous aspirin 500 mg was used in one study.27
3.5 Incidence of antiplatelet
resistance
The incidence of overall aspirin resistance ranged from
11-51.5%13,15,17,20–23,26–28 and the incidence of
clopidogrel resistance was reported as 22%.23 12.6%
of patients on dual antiplatelet therapy were found to be resistant to
both aspirin and clopidogrel, however, it was reduced to 10.6% after
30-day treatment.23 Preoperative aspirin resistance
was found to be 13-29%20,26,27.
In terms of TxB2 measurements, inhibition >90% was not
achieved until postoperative day 5 and only 34% of patients reached the
effective platelet inhibition by then.12,18Insufficient inhibition of TxB2 is observed with aspirin 100mg, but not
with a higher dose of 325mg.11
The aspirin resistance had disappeared in all previously perioperative
resistant patients when retested at 6-month13 and
12-month follow-ups.26,27
3.6 Effect of Cardiopulmonary Bypass
(CPB)
The effect of CPB on aspirin resistance is not clear. Platelet
aggregation and thromboxane are significantly inhibited after off-pump
CABG, but not after the on-pump CABG.25 The
cardiopulmonary bypass time is described as an independent predictor of
an ASA non-response in one study.21 Nonetheless, it
was also demonstrated that CPB has no significant effect on aspirin
resistance in other studies.26,27
3.7
Outcomes
3.7.1 Vein graft
occlusion
Antiplatelet resistance is a predictor of graft
occlusion.23 Aspirin resistance, together with
compromised endothelial integrity in vein grafts, leads to graft
thrombosis and failure within a few days after CABG.17Moreover, the risk of late occlusion of vein grafts is increased by 13
folds (odds ratio) in patients with aspirin
resistance.22 Dual antiplatelet therapy with aspirin
and clopidogrel is a strong predictor of vein graft patency and is
associated with the reduced vein graft occlusion
rate.23
3.7.2 Mortality, Myocardial
Infarction (MI) and
Stroke
There was no overall difference in mortality, MI or stroke in patients
with aspirin resistance and those without it at 6-month and 12-month
follow-up.20,21,28 Whereas, all patients who died
during the follow-up period exhibited aspirin resistance
previously.26,27
The addition of clopidogrel to aspirin does not reduce adverse outcomes
or increase bleeding episodes. However, dual antiplatelet therapy leads
to a lower rate of adverse events in younger (age <65 years)
obese patients with body mass index (BMI)
>30.28 In patients with clopidogrel
resistance undergoing off-pump CABG, high residual platelet reactivity
is associated with higher mortality, MI and target vessel
revascularisations.24
3.7.3 Postoperative immediate blood
loss
Postoperative 12-hour blood loss was higher in preoperative
aspirin-sensitive patients compared to the patients with preoperative
aspirin resistance (mean volume of 555 ml vs 406ml).29Although the chest drain output was comparable within the first hour
after surgery, the aspirin-sensitive group had more blood loss at 6 and
12 hours. In addition, they are more likely to require allogenic blood
transfusion postoperatively.14