Impairment of the arterial baroreflex during symptomatic and silent myocardial ischemia in humans

Acute responses and chronic adaptations to exercise in humans: a look from the autonomic nervous system window

The objective of this review was to give an overview on the current knowledge on the neural mechanisms of cardiovascular regulation during acute exercise and the autonomic adaptations brought about by chronic exercise, that is, exercise training. Evidence derived mainly from human studies, which supports the contribution of the different control mechanisms, namely the centralcommand, the reflex drive from active muscles and the arterial baroreflex, with the attendant modifications in autonomic nervous system activity, in determining the acute cardiovascular responses to exercise are discussed, along with some controversial issues and evolving concepts in exercise physiology. In particular, data that show how the various neural mechanisms involved in cardiovascular regulation during exercise are differently modulated by factors related to the muscular activity being performed, such as the type and intensity of exercise and the size of the active muscle masses are presented, stressing the plasticity of the neural network. Thereafter, the clinical implications pertaining neural cardiovascular adaptations to exercise training are presented and discussed, in the context of cardiac diseases. In particular, I will summarize a series of investigations performed in our laboratory that utilized a new training methodology and different exercise formats to quantify the training load in cardiac patients. The way by which individualized exercise training doses affects the autonomic nervous system and the cardiorespiratory adaptations is highlighted.

Influence of cardiopulmonary reflex on the sympathetic activity during myocardial infarction

The time-course of changes in renal sympathetic nerve activity (RSNA), arterial and cardiopulmonary baroreflexes sensitivities was evaluated in conscious rats eight hours (8 h) and ten days (10 day) after myocardial infarction (MI), induced by coronary artery ligation. RSNA was recorded by a platinum electrode implanted in left renal nerve. Arterial and cardiopulmonary baroreflexes sensitivities were evaluated by changes in blood pressure and serotonin administration, respectively. Both 8 h and 10 day groups presented hypotension (103+/-4 vs. 102+/-2 vs. 115+/-4 mm Hg), but only 8 h showed tachycardia (422+/-22 vs. 378+/-11 vs. 384+/-9 bpm) when compared to Control rats. RSNA was depressed 8 h after MI and increased in 10 day group (12+/-2 vs. 39+/-8 vs. 22+/-2 mV/cycle). Although arterial baroreflex control of heart rate was similar in all groups, the arterial baroreflex control of RSNA in 8 h group was impaired during reductions (-0.35+/-0.10 vs. -1.66+/-0.23 vs. -0.09+/-0.14 mV/cycle/mm Hg) or increases (-0.77+/-0.17 vs. -1.63+/-0.58 vs. -1.66+/-0.17 mV/cycle/mm Hg) in blood pressure when compared to Control animals. Moreover, cardiopulmonary baroreflex bradycardic response was increased in 8 h rats and normalized in 10 day group. The results suggest that the increased cardiopulmonary baroreflex sensitivity in 8 h may contribute to the reduction in the tonic level of RSNA as well as in the impairment of the baroreflex control of RSNA in the presence of hypotension.

Effect of lignocaine injection in carotid sinus on baroreceptor sensitivity during carotid endarterectomy

OBJECTIVES

This study was undertaken to test the hypothesis that there is a neural basis for baroreceptor deterioration during carotid endarterectomy (CEA), by investigating intraoperative hemodynamic changes induced by intraluminal carotid stretch stimulation, before and after application of local anesthetic to the adventitial layer of the carotid sinus region.

METHODS

This was a prospective study of 20 patients undergoing elective CEA. During CEA, before removal of the atheroma, intraluminal stretch simulation of the carotid baroreceptors (rub test) was performed before and after injection of 1% lignocaine into adventitial tissue of the artery in the region of the carotid sinus. Continuous measurements of mean arterial blood pressure (MAP), electrocardiographic r-r intervals (R-R), heart rate, cardiac vagal tone, and carotid sinus baroreflex were recorded to determine alterations in baroreceptor function.

RESULTS

Rub test before injection of lignocaine was associated with a decrease in MAP and heart rate and an increase in R-R, cardiac vagal tone, and carotid baroreflex response, indicating a functioning baroreflex. After lignocaine injection and repetition of the rub test, no significant change was seen in MAP, heart rate, R-R, cardiac vagal tone, or carotid baroreflex response, indicating a nonfunctioning baroreflex. Comparing the peak responses to the rub test stimulus before and after lignocaine injection showed significant differences for all variables (P <.05), with carotid baroreflex response and heart rate being highly significant (P <.0005).

CONCLUSIONS

The baroreflex response to intraluminal stretch stimulation of the carotid sinus area is operational in patients undergoing CEA, and this response is abolished by infiltration of local anesthetic into the periadventitial tissue around the carotid sinus.

Impairment of coronary microvascular function in patients with neurally mediated syncope

Recent evidence suggests that myocardial ischemia may occur in patients with neurally mediated syncope and normal coronary angiograms. This study was conducted to evaluate if coronary microvascular function is impaired in such patients. Coronary hemodynamic studies and head-up tilt table tests (HUTs) were performed on 30 consecutive patients with normal coronary angiograms and recurrent syncope. Another ten subjects with atypical chest pain and no evidence of myocardial ischemia or syncope served as a control. Great cardiac vein flow (GCVF) and coronary sinus flow (CSF) were measured by the thermodilution method at baseline and after dipyridamole infusion (0.56 mg/kg i.v. for 4 minutes). Coronary flow reserve (CFR), derived from CSF and GCVF, was significantly lower in the 15 patients with positive HUT than in the other 15 patients with negative HUT (1.75 +/- 0.48 vs 2.64 +/- 0.8, P < 0.01 and 2.29 +/- 0.45 vs 3.07 +/- 0.63, P < 0.01, respectively). Ischemic-like ECG was noted during treadmill exercise test in 40% of the former and in 7% of the latter group (P = 0.01). There was no significant difference in CFR between patients with negative HUT and control subjects. Coronary microvascular function was impaired in syncopal patients with positive HUT and relatively preserved in those with negative HUT, suggesting the possible linkage between coronary microvascular dysfunction and the development of neurally mediated syncope.

Greater intima-media thickness in the carotid bulb is associated with reduced baroreflex sensitivity

The aim of this study was to evaluate the association between resting baroreflex sensitivity (BRS) and carotid intima-media thickness (IMT), a putative marker of subclinical atherosclerosis. Participants were 64 men and 18 women (median age, 57 years; range, 40 to 70 years), who did not have a previous history of coronary artery disease or treatment for hypertension. Resting BRS was measured during a 9-min baseline period using the noninvasive sequence technique; carotid IMT was subsequently determined using ultrasonography. Hierarchical multiple regression analyses showed that greater IMT in the carotid bulb (an area with a high density of baroreceptors) was associated with reduced BRS. These findings remained after adjusting BRS for resting mean arterial pressure, age, body mass index, gender, and smoking history, R2 = 0.06, P = .03. In contrast, IMT in the common and internal carotid regions (areas with presumably lower baroreceptor densities) did not account for a significant proportion of the variance in BRS. These results suggest that subclinical atherosclerosis, specifically in a region with high baroreceptor density, is associated with a reduced sensitivity of the baroreflex.

Autonomic nervous system activity before and during episodes of myocardial ischemia in patients with stable coronary artery disease during daily life

Spectral analysis of heart rate variability (HRV) was used to assess changes in the autonomic nervous system (ANS) 10 minutes before, during, and 10 minutes after 110 ischemic episodes (IEs) in 38 patients (25 men, age 61 +/- 10 years) with stable coronary artery disease. In 26 of 77 diurnal IEs (07:00-22:59) there were no changes in the spectral indexes (LF and HF) during the study period. In the remainder there was an increase in the LF:HF ratio due to HF withdrawal that started before the onset of the IE. All 33 nocturnal episodes also showed an increase in the LF:HF ratio, which was due not only to HF withdrawal, but also to a simultaneous increase in LF. Although it is not the only cause, the ANS plays a significant role in triggering IEs during daily life in patients with stable coronary artery disease. The common factor in all such episodes is a gradual withdrawal of parasympathetic tone.

Effects of a residential exercise training on baroreflex sensitivity and heart rate variability in patients with coronary artery disease: A randomized, controlled study

BACKGROUND

Myocardial ischemia and infarction impair baroreflex sensitivity (BRS), which when depressed is predictive of future cardiac events after myocardial infarction (MI). The main objective of this study was to determine whether exercise training improves BRS in patients with coronary artery disease.

METHODS AND RESULTS

Ninety-seven male patients with and without a previous MI were recruited after myocardial revascularization surgery and randomized into trained (TR) or untrained (UTR) groups. TR patients underwent a residential exercise program at 85% of maximum heart rate (HRmax) consisting of 2 daily sessions 6 times a week for 2 weeks. Eighty-six patients (45 TR and 41 UTR) completed the study. BRS was assessed at baseline and at the end of the protocol by the spontaneous baroreflex method. The standard deviation of mean R-R interval (RRSD) was also assessed as a measure of heart rate variability. At baseline, there were no significant differences between TR and UTR patients in any variable. In TR patients, BRS increased from 3.0+/-0.3 to 5.3+/-0.7 ms/mm Hg (P:<0.001), RRSD from 18.7+/-1.4 to 23.6+/-1.6 ms (P:<0.01), and R-R interval from 792. 0+/-15.5 to 851.3+/-20.5 ms (P:<0.001). No significant changes occurred in UTR patients. Increases in BRS and RRSD were significant in patients either with or without a previous MI.

CONCLUSIONS

Exercise training increases BRS and heart rate variability in patients with coronary artery disease. Improved cardiac autonomic function might add to the other benefits of exercise training in secondary prevention of ischemic heart disease.

Arterial baroreflex impairment in patients during acute coronary occlusion

OBJECTIVES

We tested whether acute coronary occlusion interferes with arterial baroreceptor control of heart rate in humans.

BACKGROUND

Subnormal baroreflex sensitivity (BRS) is an important risk indicator for sudden death. Animal research indicates that both chronic myocardial infarction and acute coronary occlusion impair baroreflex modulation of heart rate.

METHODS

We measured RR interval prolongation after phenylephrine-induced systolic pressure increases before and during 2-min coronary occlusions in 47 patients (27 men) undergoing clinically indicated single-vessel coronary angioplasty for stenoses in the proximal or midportion of the vessel causing >50% reduction in the arterial diameter, with normal antegrade flow (33 anterior descending, 10 circumflex, 4 right coronary artery). A control group of 11 patients treated for chronic total occlusion of a coronary artery was assessed to evaluate nonspecific changes in baroreflex function during a 2-min balloon inflation in the occluded artery.

RESULTS

The BRS decreased from 5.2+/-3.8 (mean+/-SD) to 4.1+/-3.5 ms x mm Hg(-1) (p=0.01) during the coronary occlusion in the 28 patients with preserved arterial baroreceptor control of heart rate-that is, adequate blood pressure responses and correlation coefficients of the slopes both in baseline and during coronary occlusion. The same phenylephrine dose increased systolic pressure less during than before coronary artery occlusion (21+/-21 versus 36+/-16 mm Hg, p < 0.0001), and in 6 patients it failed to prevent systolic pressure reduction during occlusion. Correlation coefficients of the baroreflex regressions decreased from 0.81+/-0.27 to 0.47+/-0.44 (p < 0.0001) during coronary artery occlusion in the 41 patients with adequate systolic pressure rises in both phenylephrine tests, and the association between RR intervals and rising systolic pressures was lost in 13 patients during coronary occlusion. Balloon inflation in a chronic total occlusion of a coronary artery did not cause significant changes in BRS (from 5.3+/-4.0 to 5.2+/-3.7 ms x mm Hg(-1)), correlation coefficient of the slope or phenylephrine-induced pressure rise.

CONCLUSIONS

Our study shows that abrupt coronary occlusion impairs baroreflex modulation of vagal and sympathetic nervous outflow in humans.

Assessment of time domain and spectral components of heart rate variability immediately before ischemic ST segment depression episodes

In an attempt to study autonomic function during the 5-minute period preceding ischemic ST segment depression (decreases ST) episodes, we selected 138 decreases ST episodes, without preceding decreases ST during the last 15 minutes before each episode, from the Holter tapes of 35 patients with multivessel coronary artery disease. For the 5-minute period preceding each decreases ST episode, we calculated the following heart rate variability (HRV) indices; the mean RR interval (RR5), the standard deviation of all RR intervals (SD Index5), the corresponding coefficient of variation (CV5), and the natural log (Ln) of the spectral components, total power at 0.000 to 0.400 Hz (TP5), low frequency power at 0.040 to 0.150 Hz (LF5), high frequency power at 0.150 to 0.400 Hz (HF5), and the ratio of the low to high frequency power (LF5/HF5). As HRV indices of the 24-hour period, we calculated the respective RR, SD Index, CV, LnTP, LnLF, LnHF, and Ln LF/HF. RR5, SD Index5, CV5, and Ln TP5 were all significantly lower than RR (t = -5.343, p = 3.7 x 10(-7)), SD Index (t = -19.091, p = 1.99 x 10(-40)), CV (t = -15.780, p = 1.28 x 10(-32)), and LnTP (t = -3.210, p = 0.0016), respectively. LnHF5 was inversely correlated with the magnitude of the decreases ST; r = -0.174, P < 0.05, and CV5 was inversely correlated with the natural log (Ln) of the ischemic event duration; r = -0.183, P < 0.05. Analogous results were obtained for both the painful and silent decreases ST episodes. It is concluded that HRV is decreased during the 5-minute period preceding decreases ST episodes, and is inversely related with the magnitude and the duration of the *ST.

Platelet-induced suppression of baroreceptor activity is mediated by a stable diffusible factor

We have demonstrated recently that platelets aggregating in the carotid sinus decrease baroreceptor sensitivity. The goals of the present study were to determine whether platelet-induced suppression of baroreceptor activity is mediated by a diffusible, transferable factor and, if true, whether the factor is short-lived or stable. Baroreceptor activity was recorded from the isolated carotid sinus during slow ramp increases in nonpulsatile pressure in rabbits anesthetized with sodium pentobarbital. Intraluminal exposure of the carotid sinus to washed rabbit platelets resuspended in Krebs buffer (3-5 x 10(8) cells/ml) and activated by thrombin decreased baroreceptor activity significantly (n = 7, P < 0.05). Maximum baroreceptor activity recorded at a pressure of 140 mmHg was reduced to 81 +/- 7% of the control maximum. Injection of cell-free supernatant obtained from filtered thrombin-activated platelets also suppressed baroreceptor activity to a similar extent after 10 min (n = 7) and after 2 h (n = 5) of incubation when maximum baroreceptor activity was reduced to 84 +/- 5 and 82 +/- 5% of the control maximum, respectively. The inhibitory influence of activated platelets and platelet supernatant on baroreceptor activity was still apparent after 10-60 min of heating (95 degrees C) (n = 5) and was reversible upon removal of platelets and supernatant from the sinus. The results indicate that activated platelets release a stable diffusible factor that suppresses baroreceptor activity. We speculate that this 'inhibitory factor' may contribute to impairment of the baroreceptor reflex and neurally-mediated increases in arterial pressure in atherosclerotic and thrombotic states.

Baroreceptor control of the circulation in man. An update

In recent years assessment of baroreflex cardiovascular control in humans has received growing attention for two major reasons, i.e. 1) the availability of new techniques to evaluate this reflex function and 2) the possibility that a baroreflex alteration might play a role in the clinical evolution of cardiovascular diseases. This paper will briefly examine the alterations in baroreflex function in a variety of diseases affecting the circulation. In particular it will be focused on 1) the baroreflex control of heart rate and microneurographically measured muscle sympathetic nerve traffic in human hypertension, 2) the impairment of this reflex homeostatic function occurring in congestive heart failure of different clinical severity and finally 3) the transient alterations in baroreflex-heart rate control occurring in man in the acute post-myocardial infarction phase and during episodes of spontaneous myocardial ischemia.