methods is provided in Chapter 8.
4.3.2.6 Data Analysis. The differences between means of dependent samples were tested using the paired t-test, and a comparison between group means was made by the two sample t-test for all continuous variables apart from BTG and serum TXB2, which had skewed distributions. Linear correlations were measured by Pearson’s correlation coefficient. For plasma BTG and serum TXB2, differences between paired samples were tested with the Wilcoxon matched pairs signed-ranks test, and correlations were measured using the Spearman rank correlation coeffi cient.
4.3.3 Results
4.3.3.1 Patient Profile (Table 4.2). Those with a positive exercise test were older and had a significantly lower left ventricular score (LVS) indicating fewer segmental wall motion abnormalities of the left ventricle. Those with a negative exercise test had a higher coronary stenosis score (CSS) due to a greater number having total coronary artery occlusions, but for the number of patients studied this difference did not attain statistical significance. There were no differences between the two groups in the amount of cigarette smoking, levels of the blood lipids, body weight and the extent of coronary atherosclerosis expressed as the coronary athero matous score (CAS) and the mean CAS (mCAS). No patient had leg claudication, and all had palpable peripheral pulses.
4.3.3.2 Plasma BTG and TXB2. There were no differences in the resting plasma TXB2 levels between the two groups, nor was there a detectable change in plasma TXB2 levels following maximum exercise. In 54% of the samples, the plasma TXB2 levels were below the sensitivity of the RIA of 40 pg per ml. The measurement of small amounts of plasma TXB2 (between 40 and 100 pg per ml) did not distinguish the ischaemic from the non-ischaemic group, nor the pre and post exercise samples.
The resting BTG was the same in both groups. Following exercise, there was a small but significant increase in the BTG level. This occurred in both groups in the second treadmill test, although only in the negative test group with the first exercise test (Fig 4.1).
4.3.3.3 Serum TXB2. There was a small but significant increase in the formation of TXB2 in serum following exercise. However, this increase only occurred in the ischaemic group, and was not observed in non-ischaemic group (Fig 4.2). The rest ing serum TXB2 levels in the ischaemic group was significantly lower than the level in non-ischaemic subjects (Fig 4.2).
4.3.3.4 Platelet count ratio (Table 4.3). The PCR was consistently lower in the non-ischaemic group at rest, but this difference was not statistically significant. A decrease in the PCR following the first treadmill test, was observed in the non- ischaemic group but not in the ischaemic group. This change was not reproduced in the subsequent treadmill test.
4.3.3.5 Blood Cell Indices (Table 4.4). There was a significant increase follow ing exercise in the Hb, the He, the platelet count (PC) and the WCC, in both study groups. The values were the same for the ischaemic and non-ischaemic groups at rest and after exercise.
4.3.4 Discussion
For the appropriate conduct of a case-control study to evaluate platelet function in patients with stable CHD, it was necessary to determine if symptomatic or inducible ischaemia influenced platelet function since this could be a potential source of bias. A clinically important role has been clearly demonstrated for the involvement of platelets and thrombus in the acute occlusive ischaemic syndromes of unstable angi na, AMI and sudden ischaemic death.24,26,27 However, numerous studies of the role of platelets in effort induced myocardial ischaemia have produced conflicting results. Not only does a cause-effect relationship between platelet activation and exercise induced ischaemia remain unresolved, but indeed, even the presence of measurable changes has not been consistently demonstrated.
In addition to the requirements of conducting the case-control study, this study allows an evaluation of in vivo platelet function with exercise in patients with exer tional ischaemia and therefore may contribute to an understanding of mechanisms resulting in the increased risk of sudden death associated with exercise in individuals with CHD. Indeed, the results of this study indicate that during exercise-induced myocardial ischaemia in a stable CHD population, platelets are not activated as assessed by the parameters measured. In particular, measures of in vivo platelet function were unaffected by the presence or absence of exertional myocardial ischaemia. The results support previous work showing no relationship between exer tional ischaemia and plasma TXB2488 or plasma BTG levels.481
An increase, however, in plasma BTG following exercise has been previously documented.495 In the present study, the plasma BTG clearly increased in those individuals undergoing maximal exercise and the increase in the ischaemic group
was attenuated. Those with negative exercise tests were able to exercise to work loads which were maximal or near maximal for their age. In contrast, those with evidence of myocardial ischaemia attained lower levels of exercise, were limited by their cardiac response to exercise, and were below their predicted achievable work load. If the increase in BTG is a non-specific response to exercise itself, those with limitations due to ischaemia may not perform sufficient work to elicit an increase in BTG, even though the workload was sufficient to cause an increase in the platelet count, Hb and He.
The effect of exercise on the PCR in patients with CHD remains unresolved. A significant overall decrease in the PCR following exertion in the first treadmill test could not be reproduced in the third test under similar conditions. The reason for this is not apparent, but may be due to a training effect occurring between the two treadmill tests, or alternatively, to variability in platelet reactivity over time. The resting PCR in patients with CAD was previously shown to be significantly lower than in normal subjects.496 In that same study, following short-term strenuous exer cise, the PCR decreased in the CAD patients irrespective of the presence of ECG changes of myocardial ischaemia.496 In our study, the decrease in the PCR in the first treadmill test predominantly occurred in the non-ischaemic group. The PCR did not decline in those with exertional ischaemia, indicating that platelets do not become more reactive following exertional ischaemia, as measured by this method. It is possible, however, that platelet aggregation measured by whole blood aggrega tion methods may be influenced by pacing induced tachycardia,476 although this is controversial.44,497'501
Measurement of TXB2 is used as an estimate of the activity of platelet prostaglandin metabolism. Serum TXB2 provides a measure of the platelet response to endogenous agonists during ex vivo clotting. These agonists may be present before the clot is formed and/or produced during its formation. In this study, those individuals who had objective evidence of ischaemia, produced less TXB2 in serum, but had a modest increase following exercise. This increase was not apparent in the non- ischaemic group, who also had significantly higher resting levels of serum TXB2. Reuben and colleagues did not find an increase in serum TXB2 after exercise in a group of patients with CAD who had a positive exercise test for myocardial ischae mia.489 However, their exercise test was a symptom limited treadmill test, exercise being ceased with the first onset of angina. All patients were receiving conventional anti-anginal medications which were continued throughout the study, and it is possi ble that these drugs prevented any increase in serum TXB2.502 The plasma TXB2 levels did not relate to exercise or exercise-induced ischaemia. The plasma TXB2 levels however may not reflect true circulating TXB2 levels, its presence being due to technical factors, such as release during blood collection.503
An explanation of the changes observed in the serum TXB2 levels cannot be deter mined from this study. One possibility, however, is that increased substrates are formed during exercise induced ischaemia which will increase prostaglandin metabo lism by those cells involved in the clotting process. A similar increase in prostaglan din activity may occur during reversible platelet aggregation and/or non-obstructive thrombus formation on ruptured atheromatous plaque, and so increase localised thrombus formation.
A lower serum TXB2 was evident in the ischaemic group. This reduced production of TXB2 with clotting in patients with known inducible myocardial ischaemia has also been observed by others.489'504 It is possible that intermittent platelet activation may be occurring in patients with active myocardial ischaemia, depleting the poten tial for serum TXB2 production. Nevertheless, there is no evidence to support this suggestion from those measures of in vivo platelet activation in this population with exertional ischaemia. There was no significant difference between platelet counts or cholesterol levels in the two groups to account for the difference.504,505 The differ ences found in platelet function cannot be attributed to the severity of CAD. There was no significant difference in the extent of coronary atheroma in the two study groups as measured by the mean CAS. Even though there was a tendency in the group with a negative exercise test to have a higher stenosis severity score (mean CSS), in vivo platelet reactivity was the same as for the ischaemic group. The group with positive exercise tests, who were older, may have more extensive peripheral vascular disease to account for the differences in serum TXB2. However, there were no clinically detectable differences, and again, measures of in vivo platelet activation were the same in both groups. The higher serum TXB2 in the non-ischaemic group may be age related, given the significantly younger age of the non-ischaemic group. The haemoconcentration, increase in white cell numbers and rise in peripheral plate let numbers which occurred with exercise, have been previously well documented and substantiated.347,506 The changes in the blood cell indices occurred in both groups, with greater changes occurring in those attaining higher workloads.
4.3.5 Summary
1. In light of the foregoing observations, in order to detect a measurable difference in in vivo platelet function between young males with CHD and a normal control group, the exclusion of subjects with ongoing measurable myocardial ischaemia is not required.
2. Short-term maximally strenuous exertion may cause activation of plate lets irrespective of the presence of ischaemia. The avoidance of such exertion in rou tine unmonitored activity by CHD patients is advised for many important cardiologi cal reasons, and possible platelet activation may be an additional factor. Whether antiplatelet drugs will help to reduce the incidence of adverse consequences associat ed with such exercise remains to be determined.
3. Exercise-induced myocardial ischaemia does not appear to activate circulating platelets, nor are previously activated circulating platelets associated with exertional ischaemia.
4. It is possible, however, that including subjects with exertional ischae mia will reduce the possibility of detecting a difference in serum TXB2, since those with exertional ischaemia may produce less TXB2. Therefore, if a difference is demonstrated between the case and control group to be evaluated, then it is more likely to be a true difference.
4.4 PLATELET FUNCTION AND ANTI-ISCHAEMIC MEDICATIONS