Circulating catecholamines and systolic time intervals in normotensive and hypertensive patients with and without left ventricular hypertrophy

Higher exercise intensity delays postexercise recovery of impedance-derived cardiac sympathetic activity

Systolic time intervals (STIs) provide noninvasive insights into cardiac sympathetic neural activity (cSNA). As the effect of exercise intensity on postexercise STI recovery is unclear, this study investigated the STI recovery profile after different exercise intensities. Eleven healthy males cycled for 8 min at 3 separate intensities: LOW (40%-45%), MOD (75%-80%), and HIGH (90%-95%) of heart-rate (HR) reserve. Bio-impedance cardiography was used to assess STIs - primarily pre-ejection period (PEP; inversely correlated with cSNA), as well as left ventricular ejection time (LVET) and PEP:LVET - during 10 min seated recovery immediately postexercise. Heart-rate variability (HRV), i.e., natural-logarithm of root mean square of successive differences (Ln-RMSSD), was calculated as an index of cardiac parasympathetic neural activity (cPNA). Higher preceding exercise intensity elicited a slower recovery of HR and Ln-RMSSD (p < 0.001), and these measures did not return to baseline by 10 min following any intensity (p ≤ 0.009). Recovery of STIs was also slower following higher intensity exercise (p ≤ 0.002). By 30 s postexercise, higher preceding intensity resulted in a lower PEP (98 ± 14 ms, 75 ± 6 ms, 66 ± 5 ms for LOW, MOD, and HIGH, respectively, p < 0.001). PEP recovered to baseline (143 ± 11 ms) by 5 min following LOW (139 ± 13 ms, p = 0.590) and by 10 min following MOD (145 ± 17 ms, p = 0.602), but was still suppressed at 10 min following HIGH (123 ± 21 ms, p = 0.012). Higher preceding exercise intensity attenuated the recovery of indices for cSNA (from STIs) and cPNA (from HRV) in a graded dose-response fashion. While exercise intensity must be considered, acute recovery may be a valuable period during which to concurrently monitor these noninvasive indices, to identify potentially abnormal cardiac autonomic responses.

Cardiac Autonomic Responses during Exercise and Post-exercise Recovery Using Heart Rate Variability and Systolic Time Intervals-A Review

Cardiac parasympathetic activity may be non-invasively investigated using heart rate variability (HRV), although HRV is not widely accepted to reflect sympathetic activity. Instead, cardiac sympathetic activity may be investigated using systolic time intervals (STI), such as the pre-ejection period. Although these autonomic indices are typically measured during rest, the “reactivity hypothesis” suggests that investigating responses to a stressor (e.g., exercise) may be a valuable monitoring approach in clinical and high-performance settings. However, when interpreting these indices it is important to consider how the exercise dose itself (i.e., intensity, duration, and modality) may influence the response. Therefore, the purpose of this investigation was to review the literature regarding how the exercise dosage influences these autonomic indices during exercise and acute post-exercise recovery. There are substantial methodological variations throughout the literature regarding HRV responses to exercise, in terms of exercise protocols and HRV analysis techniques. Exercise intensity is the primary factor influencing HRV, with a greater intensity eliciting a lower HRV during exercise up to moderate-high intensity, with minimal change observed as intensity is increased further. Post-exercise, a greater preceding intensity is associated with a slower HRV recovery, although the dose-response remains unclear. A longer exercise duration has been reported to elicit a lower HRV only during low-moderate intensity and when accompanied by cardiovascular drift, while a small number of studies have reported conflicting results regarding whether a longer duration delays HRV recovery. “Modality” has been defined multiple ways, with limited evidence suggesting exercise of a greater muscle mass and/or energy expenditure may delay HRV recovery. STI responses during exercise and recovery have seldom been reported, although limited data suggests that intensity is a key determining factor. Concurrent monitoring of HRV and STI may be a valuable non-invasive approach to investigate autonomic stress reactivity; however, this integrative approach has not yet been applied with regards to exercise stressors.

Autonomic cardiac control. II. Noninvasive indices and basal response as revealed by autonomic blockades

Heart period, systolic time intervals, low and high frequency heart period variability, blood pressure, and respiration were measured in female subjects under three drug conditions (saline, atropine sulfate, metoprolol) while sitting and standing on three consecutive days. Following preinfusion baseline recordings, saline, metoprolol (14 mg), or atropine sulfate (2 mg) was infused for 15 min (by using a double-blind procedure). Recordings were taken during a postinfusion baseline and in response to an orthostatic stressor (standing versus sitting postures). At the end of the metoprolol session, atropine sulfate was infused and responses were monitored during the postinfusion (i.e., double blockade) baseline and during orthostatic stressor. Analyses of the blockade data revealed that the preejection period (PEP) reflected sympathetic but not vagal influences on the heart, and high frequency (HF, 0.12-0.40 Hz) heart rate variability (respiratory sinus arrhythmia) reflected vagal but not sympathetic influences on the heart. No other measure provided a specific index of the tonic sympathetic or vagal activation of the heart. Postinfusion PEP under saline predicted individual differences in postinfusion cardiac sympathetic activation, whereas postinfusion heart period (but not HF variability) under saline predicted individual differences in postinfusion cardiac vagal activation.

Autonomic cardiac control. III. Psychological stress and cardiac response in autonomic space as revealed by pharmacological blockades

Behavioral contexts can evoke a variety of autonomic modes of response, characterized by reciprocal, coactive, or independent changes in the autonomic divisions. In the present study, we investigated the reactive autonomic control of the heart in response to psychological stressors, using quantitative methods for analyzing single and double autonomic blockades, and through the use of noninvasive indices based on heart period variability and systolic time intervals. Analysis of the effects of pharmacological blockades revealed an overall pattern of increased sympathetic and decreased parasympathetic control of the heart during speech stress, mental arithmetic, and a reaction-time task. Unlike the classical reciprocal sympathetic-parasympathetic response to orthostatic challenge, however, the responses of the autonomic branches to stress were uncorrelated. This reflected notable individual differences in the mode of autonomic response to stress, which had considerable stability across stress tasks. The putative noninvasive indices of sympathetic (preejection period) and parasympathetic (respiratory sinus arrhythmia) control changed in accord with the results of pharmacological blockades. Together, these results emphasize the substantial individual differences in the mode of autonomic response to stress, the advantages of a quantitative approach to analyzing blockade data, and the importance of validity estimates of blockade data.

Reduced cardiovascular responsiveness to exercise-induced sympathoadrenergic stimulation in patients with cirrhosis

Cardiovascular responsiveness to sympathoadrenergic activation obtained by muscle exercise in the supine position was evaluated in 22 patients with cirrhosis (11 alcoholic, 11 postnecrotic/cryptogenic; 14 with ascites) and 10 controls of comparable age. Plasma norepinephrine, heart rate, diastolic arterial pressure and cardiac function, as evaluated by systolic time intervals, were monitored. At rest, cirrhotics had higher norepinephrine (154 +/- 19 S.E.M. ng/l) and heart rate (79 +/- 2 beats per min) than controls (71 +/- 3 ng/l, p less than 0.01; 67 +/- 2 beats per min, p less than 0.001), whereas diastolic arterial pressure was similar. Among systolic time intervals, electromechanical systole, pre-ejection period, electromechanical delay and pre-ejection period to left ventricular ejection time ratios were prolonged (p less than 0.05 or less). Exercise led to significant increases in plasma norepinephrine, heart rate and diastolic arterial pressure in both controls and patients. In the latter, however, whereas the increase in norepinephrine was greater (p less than 0.001), those in heart rate and diastolic arterial pressure were less (p less than 0.005). As expected, most systolic time intervals shortened, but the decrease in pre-ejection period (p less than 0.05), isometric contraction time (p less than 0.02) and pre-ejection period to left ventricular ejection time ratio (p = 0.06) was less in patients than in controls. Direct correlations between exercise-induced changes in norepinephrine and both diastolic arterial pressure (r = 0.81; p less than 0.005) and heart rate (r = 0.85; p less than 0.002) were observed in controls, while inverse correlations (r = -0.67, p less than 0.001 and r = -0.44; p less than 0.05) were found in cirrhotics. These results suggest that cardiovascular reactivity to the sympathetic drive is impaired in cirrhotics. The impairment of cardiac contractility may be due to altered electromechanical coupling.

Validation of a simple method of assessing cardiac preejection period: a potential index of sympathetic nervous system activity

Traditional noninvasive assessment of sympathetic nervous system (SNS) activity in cardiovascular functioning has been confounded by concurrent parasympathetic influences. Analyses of specific intervals of the cardiac cycle have indicated that the systolic preejection period (PEP) may serve as a reliable index of SNS activity independent of parasympathetic inhibition. In the present study, PEP values derived from a technique employing peripheral pulse wave tracings were compared to values obtained from simultaneous impedance cardiograph recordings. Recordings were made on 15 male subjects who were instructed to rest quietly sitting in an upright position. Results indicated that values obtained from both methods were highly correlated and not significantly different when measurement adjustments on total systole were taken into account. These findings support the validity of the fingertip peripheral-pulse method in obtaining measures of systolic time intervals under resting conditions.

Systolic time intervals in evaluation of the negative inotropic effect after single oral doses of mexiletine and disopyramide

A placebo-controlled, single blind, crossover study was done to evaluate the inotropic effects of single oral doses of mexiletine and disopyramide assessed by the measurement of Systolic Time Intervals (STI). Each of 8 healthy volunteers received five treatments in random order: 200 and 400 mg mexiletine, 100 and 200 mg disopyramide, and placebo. There was a significant increase in cumulated PEP after 400 mg mexiletine and 200 mg disopyramide. There was no significant change in LVET and QS2. Peak plasma levels were in the lower range of the reputed antiarrythmic levels. Plasma concentration-effect relationships are discussed. Although the study revealed large inter- and intrasubject variability in the measured STIs, it is concluded that a negative inotropic effect was detected despite the low plasma levels of the minor negative inotropic drugs.

Positive inotropic effects of an ACTH analogue (ACTH 1-17) on myocardial performance

The aim of the present investigation was to study the effects of a single 100-micrograms i.v. administration of the synthetic heptadecapeptide [beta-Ala1-Lys17]ACTH1-17-4-amino-N-butylamide (ACTH 1-17) on the left ventricular performance. The systolic time intervals (STI) were recorded in 20 healthy adult young subjects (10 treated with ACTH 1-17 and 10 receiving placebo) before as well as 20, 40, 60 and 80 min after the i.v. ACTH 1-17 or placebo infusion. The STI were recorded immediately after blood withdrawal for measuring cortisol, aldosterone, adrenaline and noradrenaline plasma levels. A highly significant statistical difference was demonstrated for preejection period (PEP) and preejection period/left ventricular ejection time (PEP/LVET) ratio between subjects treated with ACTH 1-17 and subjects receiving placebo. As expected, a significant increase of cortisol and aldosterone plasma levels was observed in subjects treated with ACTH 1-17. The difference of adrenaline and noradrenaline plasma levels was statistically highly significant between subjects treated with ACTH 1-17 and those receiving placebo. The lack of increase in PEP and PEP/LVET ratio recorded in subjects treated with ACTH 1-17 is consistent with an increased left ventricular contractile performance. An increased plasma catecholamine release is postulated as the mechanism of this improvement.

Systolic time intervals reconsidered. Reevaluation of the preejection period: absence of relation to heart rate

Within limits, systolic time intervals are reliable reflectors of cardiac status and responses to physiologic and pharmacologic challenges, with various functional correlates. That heart rate (HR) is an important determinant of the duration of systole is well accepted, owing to its effect on left ventricular ejection time (LVET). An independent rate effect on preejection period (PEP) is disputed. Some studies in pooled normal subjects at rest showed some degree of HR-PEP covariance, leading to widespread rate correction in practical use. However, although right atrial pacing showed the expected HR-LVET relation, it consistently failed to show an HR-PEP relation. Systolic time intervals were examined from echocardiograms of a deliberately heterogeneous group comprising 50 consecutively appearing persons with sinus rhythm. There was no HR-PEP covariance (r = 0.23; p = not significant). However, our subjects were otherwise comparable to those of other investigators, in that all other relations in these subjects were as expected from studies in both pooled and paced subjects: HR with LVET (r = -0.74; p less than 0.001), PEP/LVET with ejection fraction (r = -0.85; p less than 0.001), and PEP/LVET with velocity of circumferential fiber shortening (r = -0.65, p less than 0.001). Thus, HR correction of PEP is inappropriate. All other relations are substantiated in routinely encountered, unselected subjects.

Evaluation of plasma catecholamines in humans. Correlation of resting levels with cardiac responses to beta-blocking and sympatholytic drugs

The changes in systolic time intervals (STI) following reduction of adrenergic activity was used to validate supine resting plasma catecholamines (CATs) as an index of sympathetic activity. Blockade of sympathetic activity was achieved by two means in two groups: propranolol (10 mg i.v.) and clonidine (0.3 mg p.o.). The diminished sympathetic effect was evidenced by slowing (p less than 0.01) of heart rate with both drugs and the reduction (p less than 0.01) of blood pressure with clonidine. There was no correlation in our study between resting plasma CATs (norepinephrine alone or total), and changes in heart rate and preejection period (PEP). Moreover, to avoid changes in PEP that could be related to differences in blood pressure levels (clonidine-reduced blood pressure while propranolol did not), the changes in PEP were corrected for the change of mean arterial pressure (MAP) in the same patients (delta PEP/(delta MAP and % delta PEP/% delta MAP). No correlation could be found, still, between resting supine plasma CATs and these ratios. The difficulty in demonstrating a correlation between resting plasma CATs and the immediate cardiac response to adrenolytic agents can be explained by the number of factors influencing plasma levels. Circulating plasma CATs represent the spillover from adrenergic nerve endings, and, therefore, their level would depend on several factors including sympathetic nervous system activity, rate of reuptake, and rate of degradation.

Alpha-adrenoceptors, adrenaline, and exaggerated vasoconstrictor response to stress in essential hypertension

Stressful sympathetic stimulation by cold pressor test in patients with essential hypertension results in an exaggerated response of the already elevated plasma adrenaline, heart rate, blood pressure, and alpha-adrenoceptor-mediated vasoconstriction when compared with normotensive subjects. The stress-induced increase in adrenaline was correlated with the attendant increase in blood pressure. The stress-induced reduction in forearm flow was reversed during infusion of the postjunctional alpha 1-adrenoceptor blocker prazosin. Therefore, enhanced responses to sympathetic stress, as reflected and perhaps caused by an exaggerated rise in plasma adrenaline, may contribute to an increased alpha 1-adrenoceptor-mediated vasoconstriction in essential hypertension.

Systolic time intervals as possible predictors of pressure response to sustained beta-adrenergic blockade in arterial hypertension. A within-patient, placebo-controlled study

Systolic time intervals (STI) were recorded at rest and during isometric exercise (IHG) in 20 hypertensive outpatients, WHO Stage 1 or 2. In a double-blind crossover study, slow-release metoprolol 200 mg once daily and matched placebo were given for 4 weeks each, at the end of a 2-week placebo washout. Blood pressure and STI were taken in the last day of washout and of either crossover period. Treatment decreased blood pressure and heart rate values at rest and on peak IHG; it didn't modify preejection period index (PEPI), left ventricular ejection time index (LVETI), and their ratio at rest, but decreased the ratio between diastolic blood pressure and PEPI (DBP/PEPI ratio) at rest and on peak IHG and lengthened the PEPI at peak IHG. Resting PEPI values on placebo treatment showed a negative correlation with systolic (r = -0.72) as well as diastolic (r = -0.80) pressure reduction on slow-release metoprolol as compared with placebo treatment. The PEP/LVET ratio at rest on placebo treatment showed a negative correlation with systolic (r = -0.78) as well as diastolic (r = -0.82) pressure reduction at rest on metoprolol compared with placebo treatment. Patients with a resting PEP/LVET ratio less than 0.43 showed a reduction in both systolic and diastolic pressure approximating or exceeding 20 mm Hg, whereas patients with a PEP/LVET ratio greater than 0.47 showed a decrease in systolic and diastolic blood pressure of less than 10 mm Hg. In patients with a PEP/LVET ratio of 0.43 to 0.47 (50% of the trial population), STI didn't show any correlation with the pressure response to beta-blockade. A positive correlation was found between the DBP/PEPI ratio at rest on placebo treatment and systolic (r = 0.56) as well as diastolic (r = 0.76) pressure reduction at rest on slow-release metoprolol compared with placebo treatment. Thus, STI appeared as promising predictors of the magnitude of blood pressure response to sustained beta-blocking therapy in mild-to-moderate essential hypertension, mostly in patients with a resting PEP/LVET ratio less then 0.43 or greater then 0.47.

Plasma catecholamines and essential hypertension. An analytical review

Of 78 comparative studies of plasma catecholamines in patients with essential hypertension and in normotensive controls, most reported higher catecholamine levels in the hypertensives, although only about 40% of the studies were positive (reporting statistically significant hypertensive-normotensive differences). Although there was dramatic variability in catecholamine values within and across studies, virtually all studies of norepinephrine in young, consistently hypertensive patients were positive. The likelihood that a study was positive with respect to norepinephrine was independent of the likelihood with respect to epinephrine, so that total catecholamine values, or else the sum of norepinephrine plus epinephrine, differentiated hypertensives from normotensives to a greater extent than levels of either substance alone. The preponderance of literature on the subject supports the hypothesis that increased plasma catecholamine concentrations occur in some patients with essential hypertension. Elevated plasma norepinephrine in relatively young, established hypertensive patients is consistent with a pathophysiologic role for increased sympathetic neural activity in this subgroup.

Plasma norepinephrine in stroke

Cardiac arrhythmias, myocardial necrosis and ECG abnormalities in stroke may result from abnormally high levels of sympathetic activity. To examine this possibility, plasma norepinephrine, epinephrine and dopamine were measured in 74 patients with cerebral infarction, 18 with transient ischemic attacks and 33 non-stroke controls. Mean norepinephrine, epinephrine and dopamine values (pg/ml) in cerebral infarction (433.2, 81.6, 75.6 were higher (p less than 0.01) than in controls (281.1, 60.1, 40.5, respectively). Transient ischemic attacks produced values intermediate to these two groups (391.3, 80.9, 54.9). The elevated catecholamine concentrations in cerebral infarction could not be explained by differences in age, blood pressure, heart rate, stress, type or severity of stroke. The high plasma norepinephrine in the stroke group is consistent with an increase in peripheral sympathetic activity which could produce the cardiac abnormalities of cerebral infarction.

Evidences supporting an increased sympathetic tone and reactivity in a subgroup of patients with essential hypertension

Several experimental evidences have shown that, under standarized conditions, circulating catecholamines (CA) or norepinephrine (NE) levels can be used as a valid index of the sympatho-adrenal activity in animal and man. This approach in the study of hypertensive patients has permitted to uncover that about 50% of patients with labile hypertension and about 30% of patients with stable hypertension had elevated CA levels at rest for 20 minutes in the supine position. The increased CA levels were mainly due to a rise in NE in stable hypertension and to a rise in epinephrine (E) in labile hypertension. On the basis of circulating CA levels, the hypertensive patients were divided into hyperadrenergic (CA levels above normal range) and normoadrenergic (CA levels within the normal range) subgroups. The hyperadrenergic labile or stable hypertensive subgroups were found to be also characterized by an enhanced CA or NE increase in response to change in position from supine to standing, by a faster heart rate and by an increased myocardial contractility, while these parameters were normal in the normoadrenergic subgroups. These findings support therefore the existence of an increased sympathetic tone and reactivity in association with hyperkinetic cardiac functions in an important population of hypertensive patients. In response to two weeks treatment with beta-blockers (either propranolol or metoprolol) hyperadrenergic stable hypertensive patients were found to be more responsive to this therapy than normoadrenergic patients although both groups had the same initial blood pressure. Moreover, this treatment lowered basal NE or CA levels and restored the enhanced CA or NE response to change in position toward normal in hyperadrenergic patients while it did not modify significantly circulating supine or standing CA and NE in normoadrenergic patients. These findings strongly support a participation of the sympathetic system in the maintenance of an elevated blood pressure in hyperadrenergic patients and raise the possibility of using a more rational approach in the therapy of hypertension.