Materials and methods

This prospective study was carried out at the Netaji Subhash Chandra Bose Medical College, Jabalpur (Department of Medicine and Pathology) from July 2007 to July 2008 on 180 patients. Our institution is a medical college and tertiary care hospital situated in central India, with 800 inpatient capacity. The hospital deals with more than 200 000 outpatients and 25 000 inpatients per year.

In total, 180 patients were included (60 patients with acute coronary syndrome (ACS), 60 with stable angina, and 60 with non-cardiac chest pain from those patients admitted to the intensive coronary care unit and the emergency ward) with complaints of chest pain. American College of Cardiology and the European Society of Cardiology diagnostic criteria were used for making the diagnosis.17Patients who satisfied the diagnostic criteria of ACS and stable angina were included in the first two groups. Patients with musculoskeletal pain and acid peptic disease with no evidence of IHD by investigations were included in the non-cardiac chest pain group. Patients who were receiving drugs, such as heparin, that can cause thrombocytopenia, and patients with bleeding disorders, pre-eclampsia and sepsis, and recent blood transfusion, were excluded from the study.

The study was approved by the Ethics Committee of Netaji Subash Chandra Bose Medical College, Jabalpur, India, in accordance with ethical standards of the Declaration of Helsinki. All patients gave their informed consent and full clinical investigations were performed. Baseline investigations included: complete blood count, blood sugar, urea, creatinine, lipid profile, troponin-T, creatine phosphokinase myocardial band (CPK-MB), ECG, echocardiography, tread mill test (TMT), colour Doppler (CD). Troponin T ⩾0.03 μg/l and CPK-MB ⩾7.0 μg/l were taken as positive. Estimation of platelet count, MPV, PDW and P-LCR were performed in all patients. Venous blood samples were drawn from all subjects after admission (within 30 min) before initiation of treatment. For sample collection, standardised EDTA sample tubes were used, and all samples were processed within 30 min after blood collection, using a Sysmex KX21-N autoanalyzer (Sysmex, Kobe, Japan).

Statistical analysis

The analysis was performed using SPSS version 11.5 for Windows (SPSS, Chicago, Illinois USA). ANOVA tests were applied for comparing the three groups, and Bonferroni’s post hoc test was used for comparison of individual groups. Pearson’s correlation coefficient for correlating attributes was applied. A p value of <0.05 was considered significant. Results are expressed as mean (SD).

Results

There were 114 (63.33%) men and 66 (36.67%) women with an age range from 40 to 70 years (mean 50 years).Other demographic and clinical details of patients are shown in table 1.

The patients in the ACS group (n = 60) had lower platelet counts compared with the other two patient groups; significant differences were observed between all three groups (p<0.001). The MPV was higher in the ACS group compared with the stable angina and non-cardiac chest pain groups (p<0.001). The ACS group had a high mean PDW compared with the stable angina and non-cardiac chest pain groups; the difference between the three groups was statistically significant (p<0.001). The ACS group had a high P-LCR compared with the stable angina and non-cardiac chest pain groups. There was a significant difference between all the groups in PLC-R (p<0.001). Table 2 displays platelet count and platelet volume values in each group of patients.

No relationship was demonstrated between platelet count and platelet volume indices with age and sex (p>0.05). No significant differences in platelet count were demonstrated with respect to smoking status, sex, age, hypertension and diabetes mellitus. Other factors such as hypertension did not showed any changes in platelet count.

There was a significant negative correlation between platelet count and MPV (r = −0.520; p<0.001). A significant positive correlation observed between MPV and PDW (r = 0.670; p<0.001) and P-LCR (r = 0.820; p<0.001). Past history of aspirin consumption did not significantly alter platelet count and platelet volumes (p>0.05). None of the patients in the non-cardiac chest pain group had a history of prior aspirin use (table 3).

Discussion

Platelets have been implicated in the pathogenesis of various disorders since their discovery in 1842 by Donne. They have an important role in the initiation of atherosclerotic lesions and coronary thrombus formation. The present study showed high MPV in ACS as compared with stable angina and non-cardiac chest pain patient groups. This finding is similar to previous studies demonstrating that MPV increases in unstable angina and myocardial infarction.7 8 9 13 14 16 18 19 20 21 Few studies have reported stable MPV during unstable angina and myocardial infarction.16 22

There are several theories that may explain the increase in MPV in ACS. One explanation is a physiological consequence of the hormone thrombopoietin.24 Experiments in animal models and observations in humans25 have demonstrated elevated MPV following platelet destruction. Platelet reduction occurs secondary to platelet destruction and in an attempt to maintain haemostasis newer platelets are released from the bone marrow. These younger platelets are RNA containing and larger, more haemostatically active than mature platelets.31 However our study demonstrated reduced platelet counts in ACS; this finding has also been detected in previous studies.13 14 Also, there was an inverse correlation between MPV and platelet count.

These findings may reflect the aforementioned physiological mechanism to explain MPV enhancement in ACS. Therefore in ACS, platelet count reduction and compensatory platelet volume enhancement suggest an ongoing process of platelet consumption underlying this condition. In addition, we know that platelets have a crucial role in the pathogenesis of ACS. Following rupture of an atherosclerotic plaque, increased platelet reactivity stimulates clot formation and partial coronary obstruction. Previous reports have demonstrated that reactive platelets are larger than inactive ones.6 Therefore, the MPV rise seen during ACS may be due to increased platelet reactivity, which in turn increases platelet surface expression of IIb/IIIa receptors and P selectin proteins.26 According to our study results, MPV was higher in patients with stable angina compared with the normal population. This finding has been demonstrated in previous studies.13 14 16 23 A possible explanation for increased platelet volume and MPV is increased platelet activity and activation of the coagulation cascade due to enhanced vasoconstriction substances. Previous evidence has indicated that recurrent periods of intra-coronary platelet aggregation and platelet consumption occurs in coronary artery disease patients.

The increase in MPV values may be a result of platelet swelling when EDTA is used as an anticoagulant, a finding reported by previous studies.23 However, a more recent study demonstrated that this increase of platelet size amounts to approximately <0.5 fl when the analysis is performed within 2 h after venipuncture.19 The reported platelet swelling with EDTA may have been due to varying concentrations of EDTA used in the blood tubes; therefore to minimise the effect of EDTA on platelet size, in the present study, standardised sample tubes were used, and all samples were processed within 30 min after blood collection.

Some studies have shown that decreases in platelet count may be a characteristic of the pre-thrombotic state in coronary heart disease.25 In our study, patients with ACS had significantly lower platelet counts compared with patients with stable angina and non-cardiac chest pain. This finding was similar to previous study results.13 14 This may be a result of platelet consumption in the acute phase of clot formation and subsequent thrombosis. None of the available literature showed significant differences in platelet counts between the stable angina and the non-cardiac chest pain groups. We are the first group to report a significant difference in platelet counts between patients with stable angina compared with the normal population. The present study also showed no relation between platelet count and age or sex (p>0.05) a finding similar to previous studies.16 However, conflicting with our results, some studies have shown that platelet count is gender dependent, being higher in women than in men.26 27

In our study, PDW in ACS was high compared with stable angina and non-cardiac chest pain groups, a finding similar to results of previous studies.12 16 18 We also demonstrated for the first time a noticeable increase of PDW in patients with stable angina as compared with the normal population. In respect of PDW rising in ACS, a study by Trowbridge et al found that patients with ACS and large platelet volumes also had significantly different volume distributions when assessed for volume dispersion, asymmetry and convexity. It is argued that this platelet volume distribution provides a signature for a prethrombotic state in IHD.12 Hence, considering that PDW is an index of platelet heterogeneity, this may explain the above-witnessed increase in ACS.

We also showed a high P-LCR in patients with ACS compared with stable angina and non-cardiac chest pain groups; this is consistent with previous studies.16 26 28 The present study also demonstrated, for what is thought to be the first time, a noticeable increase of PDW in patients with stable angina as compared with the normal population (non-cardiac chest pain).

Many studies have shown a negative correlation between platelet count and MPV, and positive correlations between MPV and PDW, and MPV and P-LCR.16 29 These results were replicated in our study. We also observed that past history of aspirin consumption did not make a significant change to the platelet count and platelet volume parameters in the ACS and stable angina groups of patients (p>0.05). These results were also demonstrated by Erhart S et al and Harrison P D et al.11 30 However a study by Mesbah Ardakani et al showed low MPV in patients with history of aspirin consumption in stable angina compared with the patients without such a history.16

The present study demonstrates that the platelet count and platelet volume indices vary between IHD patients and the normal population, and that these indices may be of value to detect patient at high risk for future cardiovascular events. With the support of further clinical studies, platelet volume indices can be used as a diagnostic or prognostic measurement in cardiovascular disease. However, there are some limitations in the sampling, calculation and comparison of these indices, with variation between different laboratories and instruments; these factors make these indices less applicable in clinical practice.

In conclusion, patients with larger platelets can easily be identified during routine haematological analysis, and these patients could possibly benefit from preventative treatment. Therefore, the readily available investigations such as platelet count and platelet volume indices could be used for predicting the development of acute coronary events.