Journal of Vascular Surgery
Volume 50, Issue 1 , Pages 48-53, July 2009

Carotid endarterectomy performed in the morning is associated with increased cerebral microembolization

  • David Bowden, MB, BChir

      Affiliations

    • Cambridge Vascular Unit, Addenbrooke's Hospital, Cambridge, United Kingdom
    • ,
  • Natalie Hayes, MD, FRCS

      Affiliations

    • Department of Vascular Surgery, Leicester Royal Infirmary, Leicester, United Kingdom
    • ,
  • Nicholas London, MD, FRCS

      Affiliations

    • Department of Vascular Surgery, Leicester Royal Infirmary, Leicester, United Kingdom
    • ,
  • Peter Bell, MD, FRCS

      Affiliations

    • Department of Vascular Surgery, Leicester Royal Infirmary, Leicester, United Kingdom
    • ,
  • A. Ross Naylor, MD, FRCS

      Affiliations

    • Department of Vascular Surgery, Leicester Royal Infirmary, Leicester, United Kingdom
    • ,
  • Paul Hayes, MD, FRCS

      Affiliations

    • Cambridge Vascular Unit, Addenbrooke's Hospital, Cambridge, United Kingdom
    • Corresponding Author InformationCorrespondence: Paul Hayes, MD, FRCS, Consultant Vascular Surgeon, Box 201, Cambridge Vascular Unit, Addenbrooke's Hospital, Hills Road, Cambridge, CB2 0QQ

Received 4 November 2008; accepted 3 January 2009. published online 17 February 2009.

Article Outline

Objectives

Platelet function exhibits circadian variation with highest levels of activity in the morning and plays a central role in arterial thrombotic events, including thrombotic stroke following carotid endarterectomy (CEA). Prior to the platelet-rich thrombus occluding the carotid artery, multiple embolic signals are detected in the middle cerebral artery using transcranial Doppler ultrasound. We hypothesized that patients undergoing CEA early in the day may be at an increased stroke risk and this would manifest as an increased postoperative embolic count.

Methods

Data were collected prospectively on 235 patients undergoing primary CEA. Accurate start and finish times were recorded in addition to the number of postoperative emboli detected in the first three hours after CEA using transcranial Doppler (TCD) monitoring.

Results

For operations finishing before midday, there was a 3.6-fold increase in the number of emboli detected relative to afternoon finishes (53.2 vs 14.8, P = .002) with similar results for starts before 10:30am (48.1 vs 14.7, P =.002). There was also a significant correlation between start time and emboli count (P = .02). Of the 55 patients with no postoperative emboli, only 19 had a morning start (relative risk 0.63, P = .011). Patients were 6.9 times more likely to require treatment with Dextran-40 to prevent progression onto a thrombotic stroke if their CEA finished before midday (P = .008).

Conclusion

There is a significantly increased rate of postoperative embolization for operations begun earlier in the day. Carotid endarterectomies performed in the afternoon may be at less risk of developing postoperative thrombotic stroke.

 

Although carotid endarterectomy is of proven benefit in the management of symptomatic carotid artery disease, postoperative stroke continues to complicate a significant number of cases.1, 2 In one series, 2.7% of patients suffered a postoperative thromboembolic stroke despite the introduction of routine intraoperative transcranial Doppler (TCD) monitoring and completion angioscopy.3 Platelet-rich thromboemboli originating from the endarterectomy zone are the most common cause of such strokes 4 and the detection of high numbers of microemboli in the early postoperative period has been shown to be predictive of cerebral ischemia.5 Despite this, conflicting evidence exists regarding the effectiveness of antiplatelet therapy in reducing cerebral microemboli with some studies reporting dual antiplatelet therapy to be ineffective while others showed a significant beneficial effect.6, 7

Variation in patients' responses to both aspirin and clopidogrel has become increasingly recognized and may be a contributing factor in the occurrence of postoperative arterial thrombosis despite antiplatelet therapy.8 However, other factors are to be considered as coagulation involves a complex interplay of numerous hematologic factors in which platelets play an integral role, both mechanically and biochemically. It has been postulated that a circadian variation in the levels and activity of platelets are in part responsible for the morning peak seen in the incidence of thrombotic stroke 9 and a peak in platelet adherence and aggregability in the morning has been identified by numerous studies.10, 11, 12, 13 Several explanations for this diurnal variation in activity have been proposed including an increased sensitivity of platelets to catecholamines, the morning rise in systolic blood pressure, a rise in catecholamine levels in association with adopting the upright posture, and platelet sensitization via activation of the renin-angiotensin-aldosterone axis.12, 13, 14

Since this period of heightened platelet activity represents a time of significantly greater risk of stroke in patients with carotid artery disease, we hypothesized that performing carotid endarterectomy (CEA) in the morning may place patients at increased risk of postoperative thromboembolic stroke and that this would manifest as an increased postoperative embolic count using TCD.

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Materials and methods 

Study design 

Two hundred and thirty-five consecutive patients undergoing CEA over a 23-month period at the Leicester Royal Infirmary were recruited with informed consent and entered into this prospective study.

Preoperative assessment 

Patients were assessed in a single visit clinic where risk factor management and Duplex assessment was performed. Angiography was undertaken in less than 5% of patients.15 Aspirin (75-150 mg daily) and/or dipyridamole (300-600 mg daily) was continued throughout the operative period, with patients receiving their usual antiplatelet therapy early in the mornings, prior to CEA. Duplex scanning was repeated within 24 hours of surgery, and all patients had their TCD window marked.

Operative technique 

The basic technique of CEA (general anesthesia, systemic heparinization, loupe magnification, routine shunting [Pruitt-Inahara], patching [collagen impregnated Dacron graft] or groin saphenous vein and distal intimal tacking [for all patients]) has remained unchanged since 1988. Antiplatelet agents were given in the morning as routine. All patients were given 5000 u of unfractionated heparin in theater and no patient received an agent to reverse this at the end of the operation. All 235 procedures were performed by a consultant or a vascular higher surgical trainee under consultant supervision.

Intraoperative TCD monitoring 

TCD monitoring was commenced after induction of anesthesia using a fixed 2 MHz head probe (Scimed PC2-64B; Fishponds, Bristol, UK), which was protected by a semi-circular flat plate headguard. The surgeon and anesthetist aimed to ensure that mean blood flow velocity in the middle cerebral artery (MCAV) was >15 cm/s at all times. The threshold of 15 cm/s was chosen because Halsey has shown this to correlate with loss of cerebral electrical activity.16 If the MCAV was <15 cm/s following shunt insertion, the shunt was repositioned to exclude abutment against the distal ICA lumen. If the MCAV was still <15 cm/s, the blood pressure was therapeutically elevated by the anesthetist. Prior to complete patch closure, a 5 mm space was retained adjacent to the orifice of the external carotid artery. The shunt was removed and all vessels back vented and irrigated with heparinized saline. The lumen of the endarterectomy zone was then inspected with a flexible hysteroscope (Olympus 1070-48, Watford, UK). All intimal flaps >3 mm were repaired using 7-0 Prolene and any residual thrombi removed from the lumen. Finally, the start and stop time of surgery was noted for each patient.

Postoperative TCD monitoring 

Following recovery from anesthesia, the patient was transferred to the recovery area of theater or the high dependency unit for a subsequent 3-hour period of TCD monitoring. All TCD data were recorded onto digital audio tape (DAT) for subsequent off-line quantification of microemboli signals. Previous work has demonstrated that emboli detected in the postoperative period are exclusively particulate17 and that 3 hours monitoring was as effective as 6 hours.18 Dextran 40 (Pharmacia Ltd, Milton Keynes, England) was administered to any patient who had (1) ≥25 emboli in any 10-minute period or (2) emboli that distorted the MCA waveform which suggested that they were large. The threshold of ≥25 emboli per 10-minute period was based on the findings of an original pilot study.3 Intravenous Dextran was administered as a 20 mL bolus and then at 20 mL/h, increased stepwise every 10 minutes to a maximum of 40 mL/h if there was no reduction in the rate of embolization. Once the rate of embolization stabilized or reduced, Dextran was continued at that dose for a further 12 hours.

Off-line TCD data analysis 

Postoperative off-line quantification of microembolic signals (MES) using DAT recordings was performed by a highly experienced technician using standardized consensus criteria (Consensus Committee),19 MES being characterized by an amplitude of at least 3 dB greater than that of background blood flow and lasting less than 300 ms. The observer was blinded to the time of operation and the number of emboli occurring during the three hours postoperatively was quantified for each patient by postoperative analysis of DAT recordings.

Statistical analysis 

In order to enable examination of the effect of time of day on the thromboembolic potential of the patients, the data were analyzed first, according to whether surgery started before 10:30AM and second, according to whether it finished after midday. Data were analyzed using StatsDirect version 2.6.8. Continuous nonparametric data were analyzed with the Mann-Whitney U-test. Discrete data were analyzed using Fisher exact test or χ2 as appropriate. A P value of < 0.05 was taken as significant.

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Results 

Epidemiology 

Over a 23-month period, 276 patients were initially included in the study. Two hundred and forty six (89%) presented with TIA/amaurosis fugax or stroke, while thirty (11%) were asymptomatic. Sixty four patients (23%) had severe carotid stenosis (>85%) on the contralateral side. Fifty-nine patients (21%) had a history of at least one previous myocardial infarction; 72 (26%) were on antiangina therapy and 38 (14%) were diabetic.

Our primary analysis was based around whether CEA began before or after 10:30AM and the patients were divided into two groups on the basis of this. Patients were well matched in both groups, with no significant differences in age, presenting complaint or atherosclerotic risk factors (Table). No patient suffered an intraoperative neurologic event and accordingly, intraoperative emboli numbers were not studied.

Table. Operation start time and demographics
VariableBefore 10.30 (n = 93)After 10.30 (n =142)P value
>70 years age, n (%)44(47)58(41).33
Symptomatic, n (%)83(89)124(87).66
Diabetic, n (%)13(14)25(18).46
Previous MI, n (%)21(23)30(21).79
Angina, n (%)31(33)41(29).47
Ipsilateral stenosis ≥85%, n (%)43(46)62(44).69
Contralateral stenosis ≥70%, n (%)37(40)63(44).49
Aspirin use87(94)132(93).89
Dual antiplatelet agent use17(18)26(18).94

MI, myocardial infarction.

Postoperative embolization 

Of the 276 patients studied, 21 had no adequate temporal window to allow TCD monitoring to take place. There were four patients who underwent CEA when the TCD was unavailable as it was being used for other patients. This left data on 251 CEAs with data on embolization, but of these 16 operation times were not accurately recorded leaving a total of 235 cases for analysis. The data were then analyzed in two ways, comparing the numbers of postoperative emboli that were seen depending on whether the operation began before or after 10:30AM, or whether the operation finished before or after midday.

The mean number of emboli detected in those patients whose operations began before 10:30AM was 48.1 (95%CI 19.6-76.6). This was a 3.3-fold increase relative to the mean of 14.7 (95% CI 9.1-20.4) emboli for those patients whose operation began after 10:30AM (P = .002). In only 13.3% of patients whose operation began before 10:30AM were no emboli detected compared with 28.7% of patients whose operations began later (relative risk [RR] 0.49 [0.29-0.81], P =.003). Whether operating time was greater or less than 2 hours did not influence the number of emboli detected (P =.47), nor did whether the surgery was performed by a trainee or consultant surgeon (P = .29).

The observed differences in embolization rates were even greater if the operation finished before, rather than after midday. The mean number of emboli detected in those patients whose operations finished before midday was 53.2 (95%CI 20.6-85.8). This was a 3.6-fold increase relative to the mean of 14.8 (95% CI 9.2-20.4) emboli for those patients whose operation finished after midday (P = .002) (Fig 1). In only 12.8% of patients whose operation finished before midday were no emboli detected, compared with 28.8% of patients whose operations began later (OR 0.36 [0.17-0.77], P = .01) (Fig 2).

  • View full-size image.
  • Fig 1. 

    A comparison of the numbers of postoperative emboli recorded after carotid endarterectomy (CEA), split into groups according to whether the operation finished before or after midday. The data has been log(n) transformed to enable graphic representation of the emboli numbers.

There was a significant correlation between the time an operation was started and the magnitude of postoperative embolization (correlation coefficient −.15 [95% CI −0.27 to −0.03], P = .02) (Fig 3). The finish time and embolization rates displayed a similar trend but did not reach statistical significance (P =.11).

In addition, of the 55 patients in whom no postoperative emboli were detected, only 19 had operations that started in the morning (RR 0.63, P = .011). The patients with the five highest postoperative emboli counts all had operations that began before 9:30AM. Finally, if a CEA finished before midday patients were 6.9 times more likely to require Dextran-40 treatment to prevent progression onto a thrombotic stroke (9/84 patients vs 2/142 patients P = .008, Fisher exact test) (Fig 4).

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Discussion 

Numerous studies have revealed a morning peak in the incidence of thromboembolic stroke,9, 20, 21 with one meta-analysis demonstrating a 55% increase in ischemic strokes between the hours of 6AM and noon.22 Recent work has also identified an apparent circadian variation in the risk of coronary stent thrombosis following bare metal stent implantation, with a peak again being observed in the morning.23 Several theories have been proposed for this period of excess risk, including a circadian variation in platelet activity.10, 11 Since it has been suggested that rather than technical error it is the formation of platelet-rich thrombus around the endarterectomy zone that determines the risk of postoperative thrombotic stroke,24 we investigated the possibility that diurnal variations in platelet activity may represent a significant risk factor for patients undergoing CEA.

Our results suggest that performing CEA during the morning hours may place a patient at increased risk of complications arising from cerebral thromboemboli compared with undergoing surgery in the afternoons. Patients operated on in the morning shed significantly more microemboli than those undergoing surgery later in the day. High rates of postoperative microemboli have been shown to be associated with early cerebrovascular complications25 and are predictive of cerebral ischemia with up to 60% of patients with sustained embolization progressing onto a stroke in one study.5 In addition, Spencer et al reported a significantly higher number of MES in patients who suffered cerebrovascular complications26 and the development of carotid artery thrombosis and subsequent neurologic deficits has been associated with persistent MES.27

Although antiplatelet drugs such as clopidogrel and aspirin are routinely used perioperatively in order to reduce the risk of such complications, the concept of relative resistance to these agents is increasingly recognized and may be associated with certain genotypes as well as patient body weight and a variation in platelet response to adenosine diphosphate (ADP).28, 29 A circadian variation in platelet sensitivity to antiplatelet drugs has to our knowledge not been previously described, although the bioavailability of aspirin is reported to be twice as high in the mornings compared with the afternoon, which would tend to support our practice of early morning dosing prior to surgery.30 Up to 55% of patients in heart failure have been reported to show a degree of aspirin resistance31 and it is evident from the continued occurrence of further thrombotic events in those patients on prophylactic doses of antiplatelet drugs that such resistance is not confined to laboratory assessments of platelet function.32 Lev et al has demonstrated that patients exhibiting aspirin resistance are also more likely to be resistant to clopidogrel and may be at higher risk of thrombotic events during percutaneous coronary intervention (PCI),33 and interestingly, it has been reported that the use of unfractionated heparin as used routinely during CEA may substantially reduce the antiplatelet effect of aspirin.34 Despite dual antiplatelet therapy, it has been shown that some patients with carotid atheroma will therefore continue to embolize35 and although a recent study demonstrated the effectiveness of the GPIIb/IIIa receptor antagonist tirofiban in reducing microemboli its routine use for TCD-directed antiplatelet therapy has yet to be established.36 Clearly, the etiology of aspirin and clopidogrel resistance is complex and as yet unresolved, with numerous interacting relevant factors. A full discussion of this phenomenon has been reviewed elsewhere, however, and is beyond the scope of this article.32

It is therefore apparent that in addition to antiplatelet therapy additional factors should be considered when performing CEA in order to further reduce the risk of postoperative thromboembolic stroke. Although high preoperative platelet counts may intuitively appear to be a risk factor, studies of patients with essential thrombocythemia have suggested that platelet counts per se are not predictive of thrombosis risk and that function rather than absolute number is more clinically relevant37 Based on the hypothesis that timing of surgery may also be important, our observation of increased microemboli following surgery performed in the mornings may be explained in part by circadian variation in platelet activity levels in addition to other factors involved in coagulation.

Circadian rhythms of physiologic processes enable adaptation to environmental and physiologic challenges and are primarily controlled by the suprachiasmatic nucleus (SCN).38 However, peripheral circadian oscillators also exist and have been identified within non-neural tissues, including vascular smooth muscle and endothelial cells. A complex interaction between peripheral oscillators within such tissues in addition to SCN-dependent variation in autonomic tone appears to contribute to the observed diurnal variation in cardiovascular events. It has been hypothesized that the increased cardiovascular morbidity associated with shift workers39 may be a consequence of desynchronization between the SCN and peripheral clocks, since it takes several days longer for visceral clocks to entrain to a new day-night cycle.38 Diurnal variation in platelet function also appears to play a role, and although this phenomenon has long been recognized with peak aggregability occurring in the mornings, its etiology has not been fully elucidated.10, 11 Several mechanisms have been proposed, with some studies suggesting that the observed increase of up to 71% in aggregability is associated with the adoption of the upright posture.10, 12, 40, 41 Standing upright has been likened physiologically to acute hemorrhage, with activation of the sympathetic nervous system and release of catecholamines in an attempt to maintain adequate perfusion pressure.12 Catecholamines, in addition to causing vasoconstriction as part of this physiologic response, have been shown not only to potentiate platelet activation but also to result in the release of larger, more aggregable sequestered platelets from the spleen.40, 42 Evidence that a2-adrenoceptor antagonists may attenuate the observed morning increase in aggregability provides further evidence for the importance of catecholamines in this phenomenon. Epinephrine has also been shown to increase platelet-neutrophil adhesion, a key event in thrombosis and inflammation, via the increased expression of P-selectin and glycoprotein IIb/IIIa (GPIIb/IIIa) on platelets and to induce platelet-fibrinogen binding via the expression of fibrinogen receptors.43 In addition to activation of the sympathetic nervous system, assumption of the upright posture also activates the renin-angiotensin-aldosterone axis, which may also be significant since angiotensin II has been demonstrated to potentiate epinephrine-induced platelet activity.14

Despite these observations that adopting an upright posture promotes a prothrombotic state, requiring patients to remain supine prior to undergoing CEA in an attempt to reduce the risk of postoperative thrombosis is likely to be impractical. In addition, circadian variations in epinephrine levels and in platelet activity are not purely posture related; catecholamine levels vary depending on the time of day as well being influenced by stressful events such as surgery.44 Recent studies have also suggested regulation of diurnal platelet function at a molecular level, with genes such as circadian locomotors output cycles kaput protein (CLOCK) that are normally involved in the regulation of mammalian circadian rhythms influencing platelet activity directly.45 It should also be recognized that additional hemostatic factors showing diurnal variation in activity are likely to be of some importance in the observed morning peak of microemboli. Activity levels of endogenous tissue plasminogen activator (tPA) and plasminogen activator inhibitor type 1 (PAI-1) show marked circadian variation, with a resultant trough in tPA activity occurring before midday.46 High fibrinogen levels in addition to an increase in factor VII and factor VIII activity that may predispose to thrombus formation have also been observed in the mornings, as well as a relative resistance to the effects of both low-molecular weight and unfractionated heparin during this period.47

There are some limitations to this study. First, assessment of platelet function in each patient both prior to CEA and in the postoperative period to allow subsequent correlation with the observed frequency of MES would have provided additional data to support our hypothesis. However, our patient group was also enrolled in a separate study into platelet function and as a result activity was assessed only once, early in the morning. As a result, we did not also have immediate preoperative platelet activity data on those patients undergoing CEA in the afternoon to confirm that aggregability was lower. Despite this, much work has already demonstrated a clear diurnal variation in platelet activity and therefore we feel this does not invalidate our results.9, 10, 11, 12, 13 Second, although we hypothesize that increased platelet activity is likely to be one of the most important (and manageable) factors accounting for our observed data, this study was not designed to quantify the activity of each of the individual factors involved in the formation of a platelet-rich thrombus, some of which are also recognized to exhibit circadian periodicity as discussed above. It would be of interest to investigate whether an increased dose of heparin in CEAs performed in the morning is able to abolish the effect observed in our study.

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Conclusion 

In conclusion, it is apparent that a number of factors, including circadian variability in platelet aggregability, contribute to an overall pro-coagulant state in the mornings. The increase in postoperative microemboli seen in our study raises the interesting possibility that performing CEA in the afternoons represents a further potential reduction in the risk of postoperative thrombotic stroke. Whether this is likely to lead to a significant effect on patient outcome requires further investigation.

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Author contributions 


Conception and design: PH, AN, PB, NL

Analysis and interpretation: DB, PH, NH

Data collection: PH, AN, PB, NL, NH

Writing the article: DB, PH, NH

Critical revision of the article: AN, PB, NL

Final approval of the article: DB, PH, AN, NH, NL, PB

Statistical analysis: DB, PH, NH

Obtained funding: PH, AN

Overall responsibility: PH

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 Competition of interest: none.

PII: S0741-5214(09)00013-5

doi:10.1016/j.jvs.2009.01.011

Journal of Vascular Surgery
Volume 50, Issue 1 , Pages 48-53, July 2009

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