Autonomic nervous stimulation affects left ventricular relaxation more than left ventricular contraction

Effects of Thoracic Epidural Anesthesia on Neuronal Cardiac Regulation and Cardiac Function

Cardiac sympathetic blockade with high-thoracic epidural anesthesia is considered beneficial in patients undergoing major surgery because it offers protection in ischemic heart disease. Major outcome studies have failed to confirm such a benefit, however. In fact, there is growing concern about potential harm associated with the use of thoracic epidural anesthesia in high-risk patients, although underlying mechanisms have not been identified. Since the latest review on this subject, a number of clinical and experimental studies have provided new information on the complex interaction between thoracic epidural anesthesia–induced sympatholysis and cardiovascular control mechanisms. Perhaps these new insights may help identify conditions in which benefits of thoracic epidural anesthesia may not outweigh potential risks. For example, cardiac sympathectomy with high-thoracic epidural anesthesia decreases right ventricular function and attenuates its capacity to cope with increased right ventricular afterload. Although the clinical significance of this pathophysiologic interaction is unknown at present, it identifies a subgroup of patients with established or pending pulmonary hypertension for whom outcome studies are needed. Other new areas of interest include the impact of thoracic epidural anesthesia–induced sympatholysis on cardiovascular control in conditions associated with increased sympathetic tone, surgical stress, and hemodynamic disruption. It was considered appropriate to collect and analyze all recent scientific information on this subject to provide a comprehensive update on the cardiovascular effects of high-thoracic epidural anesthesia and cardiac sympathectomy in healthy and diseased patients.

Control of cardiac rate, contractility, and atrioventricular conduction by medullary raphe neurons in anesthetized rats

The sympathetic actions of medullary raphé neurons on heart rate (HR), atrioventricular conduction, ventricular contractility, and rate of relaxation were examined in nine urethane-anesthetized (1-1.5 g/kg iv), artificially ventilated rats that had been adrenalectomized and given atropine methylnitrate (1 mg/kg iv). Mean arterial pressure (MAP), ECG, and left ventricular pressure were recorded. The peak rates of rise and fall in the first derivative of left ventricular (LV) pressure (dP/dtmax and dP/dtmin, respectively) and the stimulus-R ($-R) interval were measured during brief periods of atrial pacing at 8.5 Hz before and after ventral medullary raphé neurons were activated by dl-homocysteic acid (DLH, 0.1 M) or inhibited by GABA (0.3 M) in local microinjections (90 nl). LV dP/dtmax values were corrected for the confounding effect of MAP, determined at the end of the experiments after giving propranolol (1 mg/kg iv) to block sympathetic actions on the heart. DLH microinjections into the ventral medullary raphé region increased HR by 44 +/- 2 beats/min, LV dP/dtmax by 1,055 +/- 156 mmHg/s, and the negative value of LV dP/dtmin by 729 +/- 204 mmHg/s (all, P < 0.001) while shortening the $-R interval by 2.8 +/- 0.8 ms (P < 0.01). GABA microinjections caused no significant change in HR, LV dP/dtmax, or $-R interval but reduced LV dP/dtmin from -5,974 +/- 93 to -5,548 +/- 171 mmHg/s and MAP from 115 +/- 4 to 105 +/- 5 mmHg (both, P < 0.01). Rises in tail skin temperature confirmed that GABA injections effectively inhibited raphé neurons. When activated, the neurons in the ventral medullary raphé region thus enhance atrioventricular conduction, ventricular contractility, and relaxation in parallel with HR, but they provide little or no tonic sympathetic drive to the heart.

Postinfarct sympathetic hyperactivity differentially stimulates expression of tyrosine hydroxylase and norepinephrine transporter

The balance between norepinephrine (NE) synthesis, release, and reuptake is disrupted after acute myocardial infarction, resulting in elevated extracellular NE. Stimulation of sympathetic neurons in vitro increases NE synthesis and the synthetic enzyme tyrosine hydroxylase (TH) to a greater extent than it increases NE reuptake and the NE transporter (NET), which removes NE from the extracellular space. We used TGR(ASrAOGEN) transgenic rats, which lack postinfarct sympathetic hyperactivity, to test the hypothesis that increased cardiac sympathetic nerve activity accounts for the imbalance in TH and NET expression in these neurons after myocardial infarction. TH and NET mRNA levels were identical in the stellate ganglia of unoperated TGR(ASrAOGEN) rats compared with Sprague Dawley (SD) controls, but the threefold increase in TH and twofold increase in NET mRNA seen in the stellate ganglia of SD rats 1 wk after ischemia-reperfusion was absent in TGR(ASrAOGEN) rats. Similarly, the increase in TH and NET protein observed in the base of the SD ventricle was absent in the base of the TGR (ASrAOGEN) ventricle. Neuronal TH content was depleted in the left ventricle of both genotypes, whereas NET was unchanged. Basal heart rate and cardiac function were similar in both genotypes, but TGR(ASrAOGEN) hearts were more sensitive to the β-agonist dobutamine. Tyramine-induced release of endogenous NE generated similar changes in ventricular pressure and contractility in both genotypes, but postinfarct relaxation was enhanced in TGR(ASrAOGEN) hearts. These data support the hypothesis that postinfarct sympathetic hyperactivity is the major stimulus increasing TH and NET expression in cardiac neurons.

New arguments for positive inotropic vagal influence on cardiac contractility in cats

Vagal influences on the heart were studied in narcotized cats, in which the cardiotropic effects of sympathetic nerves, heart rate, and pre- and afterload were excluded. Under these conditions vagus nerve exerted only a positive inotropic effect probably mediated by an unknown humoral factor.

Dynamic sympathetic regulation of left ventricular contractility studied in the isolated canine heart

We investigated the dynamic sympathetic regulation of left ventricular end-systolic elastance (Ees) using an isolated canine ventricular preparation with functioning sympathetic nerves intact. We estimated the transfer function from both stellate ganglion stimulation to Ees and ganglion stimulation to heart rate (HR) for both left and right ganglia by means of the white noise approach and transformed those transfer functions into corresponding step responses. The HR response was much larger with right sympathetic stimulation than with left sympathetic stimulation (4.3 +/- 1.4 vs. 0.7 +/- 0.6 beats . min-1 . Hz-1, P < 0.01). In contrast, the Ees responses without pacing were not significantly different between left and right sympathetic stimulation (0.72 +/- 0.34 vs. 0.76 +/- 0. 42 mmHg . ml-1 . Hz-1). Fixed-rate pacing significantly decreased the Ees response to right sympathetic stimulation (0.53 +/- 0.43 mmHg . ml-1 . Hz-1, P < 0.01), but not to left sympathetic stimulation (0.67 +/- 0.32 mmHg . ml-1 . Hz-1, not significant). Although the mechanism by which the sympathetic nervous system regulates cardiac contractility is different depending on whether the left or right sympathetic nerves are activated, this difference does not affect the apparent response of Ees to dynamic sympathetic stimulation.

Noninvasive measurement of the time constant of left ventricular relaxation using the continuous-wave Doppler velocity profile of mitral regurgitation

BACKGROUND

The time constant of isovolumic relaxation (tau) is an important parameter of ventricular diastolic function, but the need for invasive measurement with high-fidelity catheters has limited its use in general clinical cardiology. The Doppler mitral regurgitant velocity spectrum can be used to estimate left ventricular (LV) pressure throughout systole and may provide a new noninvasive method for estimating tau.

METHODS AND RESULTS

Mitral regurgitation was produced in nine dogs, and ventricular relaxation was adjusted pharmacologically and with hypothermia. High-fidelity ventricular pressures were recorded, and tau was calculated from these hemodynamic data (tau H) assuming a zero-pressure asymptote. Continuous-wave mitral regurgitant velocity profiles were obtained, and the ventriculo-atrial (VA) pressure gradient was calculated by the simplified Bernoulli equation; tau was calculated from the Doppler data from the time of maximal negative dP/dt until LV-LA pressure crossover. Three methods were used to correct the Doppler VA gradient to better approximate the LV pressure before calculating tau: 1) adding actual LA V wave pressure (to yield tau LA); 2) adding 10 mm Hg (tau 10); and 3) no adjustment at all (actual VA gradient used to calculate tau 0). The agreement between tau H and the three Doppler estimates of tau was assessed by linear regression and by the mean and standard deviation of the error between the measurements (delta tau). the measurements (delta tau). tau H ranged from 29 to 135 msec. Without correction for LA pressure, the Doppler estimate of tau seriously underestimated tau H: tau 0 = 0.30 tau H + 9.4, r = 0.79, delta tau = -35 +/- 18 msec. This error was almost completely eliminated by adding actual LA pressure to the VA pressure gradient: tau LA = 0.92 tau H + 7.6, r = 0.95, delta tau = 2 +/- 7 msec. Addition of a fixed LA pressure estimate of 10 mm Hg to the VA gradient yielded an estimate that was almost as good: tau 10 = 0.89 tau H + 4.9, r = 0.88, delta tau = -2 +/- 12 msec. In general, tau was overestimated when actual LA pressure was below this assumed value, and vice versa. Numerical analysis demonstrated that assuming LA pressure to be 10 mm Hg should yield estimates of tau accurate to +/- 15% between true LA pressures of 5 and 20 mm Hg.

CONCLUSIONS

This study demonstrates that the Doppler mitral regurgitant velocity profile can be used to provide a direct and noninvasive measurement of tau. Because mitral regurgitation is very common in cardiac patients, this method may allow more routine assessment of tau in clinical and research settings, leading to a better understanding of the role of impaired ventricular relaxation in diastolic dysfunction and the effect of therapeutic interventions.