The HRR increased significantly with increasing recovery time except for insignificant increase from 4^th to 5^th minute during recovery. This finding can be attributed to the normal interplay of the ANS after exercise [3,20,21]. The preoperative HRR in the patients in this study was found to be abnormal at 2^nd minute of recovery, which is in accordance with the finding of the study by Pedro Paulo S and Jurij- Matija Kalisnik [1,2,22], which suggests that the cardiac vagal modulation is reduced in patients with CAD or previous myocardial infarction. But, the recovery at 1^st minute which has also been supported as a measure to predict CV mortality was found normal preoperatively. The similar findings have been reported in a study by Ricrado, which shows that only 17% of their patient population showed abnormal HRR responses despite of the status that may have influenced the autonomic activity [10]. It may thus be inferred that the HRR at 2^nd minute is more sensitive to detect the impaired autonomic modulation in the CAD patients, which was similarly found in the study by Katerina Shetler [23]. Heart rate recovery, in this study, showed a significant improvement at all time points on all days. This finding is contradictory to the available literature which suggests that there is depression of the cardiac autonomic modulation following CABG [7,12,24] It has been found that the induction of anesthesia during surgery causes a decrease in HRV. Also, CABG itself causes reduction of the HRV indices. Also, the systemic inflammation that follows CABG has been found to decrease the autonomic modulation of heart.

Though, the results of the present study can be supported in the light of the following studies. A study by Matija et al. [22], reported an increase in the HF and nHF component of HRV spectral analysis on the fourth and seventh postoperative day as compared to the preoperative values [22]. The LF/HF ratio showed an increase in sympathetic discharge from preoperative to postoperative state. A study by Bauernschmitt also suggests that HRV parameters at the beginning of the ICU-course were comparable to the preoperative values and to values of healthy volunteers in their study. Thus, they concluded that the surgery (CABG) per se does not seem to have major impact on this parameter [25].

Since, it has been found that recovery from 30s-2minutes is vagally mediated and later recovery depends on sympathetic modulation [17], the improvement in HRR at 2^nd minute in this study, may suggest that the revascularization (CABG) may also improve HRR postoperatively. The improvement in the HRR in this study suggests that the ß- blockers do not affect HRR which may provide a step further in solving the prevailing conflicting evidence regarding the effect of ß- blockers on HRR. Also, the clinical administration of the cardiovascular drugs have been found to have no significant impact on the prognostic value of Heart Rate Recovery [19,23,26]. The improvement in HRR in the study may also be attributed to the exercise regime provided to the patients during their inpatient stay in the hospital. Although, this factor alone is less likely to cause any significant improvement. Thus, whether short term training provided during Phase I cardiac rehabilitation can positively effect the impaired autonomic modulation or not can not be certained at this moment due to lack of the available evidence and warrants further investigation into this matter. The increased respiratory rate during exercise may also have improved the HRR. The statistically significant increase in the respiratory rate has been found during ambulation [27] and a strong respiratory modulation has been observed in the cardiac vagal outflow in humans [28]. This interaction observed may be because the cell bodies of the efferent limb of parasympatheticfunction involving cardiacvagal motor neurons, reside in boththe dorsal motor nucleus of the vagus and the nucleus ambiguous, which are sensitive to both baroreceptor and the respiratory activity [29].

More recently, the mechanisms responsible for postexerciseHRR have been challenged, even contesting its prognostic value [13]. Thus, currently it seems that the HRR requires further investigation and refinement before the definitive criteria for its clinical use and interpretation can be adopted. The results of the study show a significant correlation among the chronotropic variables and Heart Rate Recovery which is in accordance to the available literature. These results underlie the assumption that, both the coefficients are dependent on the autonomic nervous system. The cardiac acceleration during exercise and deceleration after exercise is thought to involve an interplay between the two limbs of the autonomic nervous system, which points towards the possibility of the “accentuated antagonism” suggesting that the activity of one limb of the autonomic nervous system is directly related to the activity of the other limb [18]. Though, different physiologic mechanisms are responsible for these initial and final HR transients. The rapid initial HR transient is exclusively mediated by vagal inhibition. On the other hand, the physiological mechanisms involved in the postexercise transient are unclear. Though, the reduction in the adrenergic component seems to play a greater role in the subsequent HR decrease. Thus, it may be concluded that the information obtained from both the transients is complementary [7].

Although, the prognostic clinical usefulness of HRR presents strong epidemiologic evidence under different conditions and methods, the results from the previous studies on the methods of quantization the initial and final HR transients suggest that HRR measurements may be unreliable in men and women with or without symptoms. Conversely, the results of the intraday and interday calculations of the chronotropic heart rate responses have been found to be highly reliable in a large and heterogeneous sample under a variety of clinical conditions. It is thus possible that the chronotropicHR responses may also provide a relevant, complementary prognostic indicator, based on its more objective dependence on cardiac vagal integrity, high reliability, and modest association with the final transient. Also, the higher discriminatory power of the initial transient compared with HRR has been supported by various studies [17].

Therefore, in contrast to initial transient, the widespread use of HRR measurements may present an additional challenge (i.e., low reliability), potentially compromising the clinical analysis and implications.
The Rate Pressure Product (RPP) or the double product which signifies the myocardial oxygen consumption was evaluated for its variation

1. During recovery
2. Following CABG
3. With different recovery positions.

The results showed a significant reduction in the RPP during recovery (over 5 minutes). This is supported by the available literature which suggests that there is arterial vasodilatation immediately following the exercise and decrease in HR during recovery due to sympathetic withdrawal and parasympathetic reactivation, which would have caused the reduction in RPP during recovery [20]. The RPP was found to increase significantly from preoperative to postoperative period, though the increase was insignificant from fourth to seventh postoperative day due to progressive myocardial recovery. The increase in RPP following surgery may be attributed to the surgical stress due to acute injury to the myocardium during the surgery [3]. The increased metabolic demand of the acutely injured myocardium increases its oxygen requirement to perform the work of the myocardial contractility, thus justifying increased RPP following surgery.

The present study suggests that recovery in the supine position following the six minute walk test in the immediate phase following CABG is better than recovery in the sitting position. This is because of more HRR and lower RPP (myocardial oxygen consumption) in supine position when compared to the sitting recovery position. This may be because; there is transient decrease in the venous return immediately following exercise, which reduces coronary blood flow when the HR and the myocardial oxygen demands may still be high. Thus, when the patient assumes the supine position following exercise, the venous return is increased, which increases the stroke volume, thus reducing the need for HR to increase, to adequately augment the cardiac output to meet the increased metabolic requirements of the body. This is supported by the fact that the stroke volume and heart rate are inversely related to each other [30]. In contrast to the above situation, during recovery in the sitting position; the venous return is further reduced which requires the heart rate to increase proportionately, to enhance the cardiac output which in turn requires the increased myocardial contractility, thus increasing the myocardial oxygen consumption, i.e., RPP [20].

Also, in earlier research studies, the blood catecholamine levels have been found to be higher in the upright position than in the supine lying position [31,32]. These higher catecholamine levels correspond to the enhanced sympathetic activity in the sitting position than in supine position, thus favoring recovery in the supine position. The results also suggest that HRR varies with different recovery positions. This may help to reduce the current methodological variations prevailing to investigate the HRR after the exercise. The evidences thus suggest that the six minute walk test might be useful for evaluation of exercise tolerance in phase I and phase II of inpatient cardiac rehabilitation programs or to assess functional responses to selected interventions early after the acute event.

The six minute walk test in this study was well tolerated by all the patients with no need for pauses or interruptions due to symptoms except for two patients who stopped test at 4 minutes 10 seconds during the morning test on fourth postoperative day and another patient who also interrupted the test on the fourth postoperative day, during the afternoon test, at 5 minutes 21 seconds due fatigue. The significant improvement in the distance walked, from preoperative to postoperative period; during the six minute walk test has been found in the present study, which is in consistence with the results of the study by A. Hirschhorn et al. [33] The improvement would have been because of the exercises of the hospital based inpatient physiotherapy protocol administered similarly to all the patients of both the groups. Also, the drugs such as ß- blockers and ACE inhibitors have been found to improve exercise capacity [34]. Thus, there might be role of these drugs in improving the walk distance significantly. Thus, the increased six minute walk distance following surgery suggests that the surgery per se does not seem to affect the functional capacity of an individual following exercise.

Small sample size of the study, although the present study had the statistical power of > 85%, No females were included in the study, the present study included only low risk patients, Only postoperatively stable patients were included, More definitive measures of the cardiac autonomic modulation, i.e., Heart Rate Variability and Baroreflex Sensitivity were not used to assess the changes in the autonomic nervous system in the early phase following CABG, although HRR shows significant correlation with these definitive measures The findings of the present study may be concluded as follows:

The six minute walk test can be safely administered as early as fourth postoperative day following elective CABG, the improvement in the HR responses during and after exercise following surgery suggests that CABG may improve these autonomic measures early following the surgery, the improvement in the chronotropic variables and heart rate recovery in immediate post operative period also implies that these mortality predictors may be modified (improved) in the early phase (Phase I cardiac rehabilitation) following CABG, it may thus also be concluded that the adverse effects of CABG on the cardiac autonomic modulation are presently over-exaggerated. The chronotropic variables may better represent underlying cardiac autonomic dysfunction than the Heart Rate Recovery, the heart rate recovery varies with the recovery position and has been found to be better in the supine position when compared to the sitting position, the normal exercise responses (i.e., normally declining heart rate and RPP during recovery), following surgery suggests that cardiac surgery (CABG) per se does not cause any impairment in responses to the submaximal exercise.