High on Aspirin Platelet Reactivity in Pediatric Patients Undergoing the Fontan Procedure

Current guidelines recommend routine aspirin thromboprophylaxis following each stage of palliation for children with single ventricle heart disease (1). In the only randomized trial to evaluate efficacy of aspirin prophylaxis after the Fontan procedure, 21% of patients still had a thromboembolism (TE) (2). One possible mechanism is suboptimal inhibition of ex-vivo platelet function in response to aspirin referred to as High on-Aspirin Platelet Reactivity (HAPR). Increased platelet turnover, platelet activation, inflammation, endothelial dysfunction due to hypoxemia, cardiopulmonary bypass, and Fontan conduit surfaces may lead to impaired aspirin responsiveness in this population. We evaluated laboratory markers of aspirin responsiveness in the early post-operative period after Fontan procedure.

Twenty patients undergoing the Fontan procedure (13 males, median age 33 months) were enrolled between August of 2014 and June of 2015. This study was approved by the Institutional Review Board at Children's National Health System and all families provided informed consent for participation. Aspirin was discontinued in all patients five days prior to their surgical procedure. Following induction of anesthesia, blood was obtained for thromboelastography with platelet mapping (TEG-PM) and a urine sample was collected for urine 11-dehydro-thromboxane B2 (11-dTXB2) testing. These studies were repeated five days after re-initiation of aspirin (5mg/kg/day started after tolerating enteral intake). TEG-PM was also obtained from a control population of 9 healthy patients (4 males, median age 55 months) not on aspirin prior to percutaneous cardiac interventions or tonsil/adenoidectomy. HAPR was defined as <30% arachidonic acid induced platelet aggregation inhibition (%AA inhibition).

There were no significant demographic differences between Fontan patients and controls. Of the twenty Fontan patients, 50% (10/20) demonstrated HAPR by TEG-PM of which 30% (3/10) were noted to have no response to aspirin (%AA inhibition = 0%). One of these 3 patients had an embolic stroke. Four patients in this cohort had a previous TE and 3 of those patients were found to have HAPR after Fontan completion. Those with HAPR had a median %AA inhibition of 11% [0-25] as compared to those without HAPR who had a median %AA inhibition of 36% [32-42] (P <0.01). While urine 11-dTXB2 did not significantly decrease with aspirin therapy in patients with HAPR, Fontan patients without HAPR demonstrated a significant 50% median decrease in urine 11-dTXB2 from 6840 pg/mg Cr to 3433 pg/mg Cr (P=0.008) after receiving aspirin. Other pertinent laboratory data are summarized in Table 1.

Table 1.

Hemostatic Measurements^*

	Controls (n=9)	Pre-Op Fontan (n=20)†	Post-Op Fontan (n=20)†	HAPR (n=10)†	No HAPR (n=10)†
Hemoglobin (g/dL)	11.5 [11.2-12.1]	15.7 [14.8-16.4]‡	13.2 [11.7-13.9]§	15.7 [14.9-17]	15.6 [14.5-16]
PT (s)	N/A	14.2 [13.5-14.8]	17.9 [16.5-20.1]§	14.1 [13.2-14.5]	14.6 [13.7-15.2]
PTT (s)	N/A	31 [29-32]	39 [32-45]§	30.6 [28.5-32.5]	30.5 [29-32.4]
INR	N/A	1.1 [1.0-1.2]	1.5 [1.3-1.7]§	1.0 [0.96-1.1]	1.1 [1.0-1.2]
K (min)	1.3 [1.2-1.6]	1.7 [1.5-2.2]‡	1.3 [1.1-2.1]	1.6 [1.4-2.4]	1.9 [1.5-2.3]
Alpha Angle (deg)	71 [67-74]	67 [61-70]‡	71 [64-73]	68 [60-71]	65 [60-69]
MA (mm)	62 [60-67]	61[58-66]	69 [64-73]§	63[59-66]	60 [58-67]
G (kd/s2)	8.3 [7.4-10.4]	7.8 [6.9-9.8]	11.0 [9.0-13.7]§	8.4 [7.2-9.8]	7.4 [6.9-10.1]
ADP% aggregation	90 [77-100]	89 [71-95]	72 [45-88]§	93 [85-100]	75 [63-91]∥
AA% inhibition	0 [0-9.8]	14 [9-33]‡	29 [10-36]	11 [0-25]	36 [32-42]∥
MRTG (dynes/cm2/s)	15.6 [12.1-17.5]	11.6 [8.5-14.1‡	15.9 [12.9-19.5]§	16.4 [14.5-20.3]	15.5 [11.7-18.8]
TMRTG (min)	7.3 [6.4-8.7]	6.9 [6.0-8.7]	5.8 [5.5-6.8]§	5.6 [5.4-7.0]	6.1 [5.5-6.8]
TG (dynes/cm2)	749 [727-808]	730 (699-807]	842 [793-882]§	861 [825-904]	821 [773-862]

^*Data are reported as medians with interquartile ranges. Two-tailed values of P < 0.05 were considered statistically significant.

^†All patients underwent cardiopulmonary bypass for their Fontan completion, 14 of which were with a fenestrated intra/extra-cardiac conduit with a limited atriotomy and 6 with an extracardiac conduit. Underlying cardiac anatomy in the study population included double outlet right ventricle (7 patients), tricuspid atresia (4 patients), hypoplastic left heart syndrome (3 patients), double inlet left ventricle (3 patients), unbalanced atrioventricular canal (2 patients), and other (1 patient).†

^‡Statistically significant difference between control and Pre-Op Fontan; Mann-Whitney U-test

^§Statistically significant difference between Pre-Op and Post-Op Fontan; Wilcoxon signed-ranks test

^∥Statistically significant difference between patients with HAPR and those with no HAPR; Mann-Whitney U-test

HAPR = high on aspirin platelet reactivity; Pre-Op = pre-operative; Post-Op = post-operative, g = grams; dL = deciliter; PT = prothrombin time; s = seconds; PTT = partial thromboplastin time; INR = international normalization ratio; K = clot kinetics; min = minutes; deg = degree; MA = maximum amplitude; mm = millimeters; G = clot stability; kd = kilodynes; ADP = adenosine diphosphate; AA = arachidonic acid; MRTG = maximum rate of thrombin generation; cm = centimeters TMRTG = time to maximum rate of thrombin generation; TG = thrombin generation;

The results of this study are the first to characterize the laboratory profile of aspirin response and define the frequency of HAPR in pediatric patients undergoing the Fontan procedure. Aspirin thromboprophylaxis appears to be the best strategy for preventing TE after Fontan procedure (1); however, TE events continue to be encountered in a substantial proportion of patients, particularly in the early post-Fontan period (2). Although a number of platelet function assays have been used to measure aspirin response and detect HAPR in children with cardiac disease, there is no gold standard and reported cut-off values to differentiate ‘responders’ from ‘non-responders’ are arbitrary (3). We utilized TEG-PM for measuring aspirin responsiveness because it involves using AA which is a specific activator of the cyclooxygenase-1 enzyme, it is readily available at most pediatric cardiac centers, results are available without lengthy processing times to inform clinical decisions, and because it has demonstrated good correlation with light-transmission aggregometry and urine 11-dTXB2 (4, 5). We chose a more stringent threshold (<30% AA-induced platelet aggregation inhibition) to define HAPR to identify patients who were clearly exhibiting poor aspirin response and to account for higher values of AA-induced platelet aggregation that are expected in the TEG-PM assay since it also includes fibrin generation and the interaction of platelets with fibrin (4).

Additionally, we observed hypercoagulable changes that were identified by comparing TEG dynamic parameters (Thrombin generation [TG], Maximum rate of thrombin generation [MRTG] and time to maximum rate of thrombin generation [TMRTG]) measured before and after the Fontan procedure. Conventional coagulation tests (prothrombin time and partial thromboplastin time) suggested a hypocoagulable state after Fontan procedure; however, TEG maximum amplitude, G value, TG and MRTG increased significantly in post-operative Fontan patients whereas TMRTG was significantly decreased. These results suggest the development of platelet and plasmatic hypercoagulability with evidence of enhanced thrombin generation and indicate that aspirin in combination with anticoagulation may be more effective strategy for preventing TE in this population and warrants further evaluation.

Our study was limited by small sample size. Moreover, the utility of TEG-PM for assessing aspirin response has not been adequately studied in pediatrics and the cut-offs to define HAPR in this study have not been validated in a larger cohort.

HAPR is common in post-operative Fontan patients and may contribute to aspirin prophylaxis failure. Future studies are needed to determine if HAPR is a risk factor for TE and to evaluate changes in aspirin response from birth through the staged palliation of single ventricle patients.