Spectral analysis of blood pressure and heart rate, catecholamine and neuropeptide Y plasma levels in a new model of neurogenic orthostatic hypotension in dog

[Orthostatic hypotension: which initial cardiovascular and biological explorations? Which therapeutic solutions? The place of recommendations]

Orthostatic hypotension (OHT) is a common symptom that affects about 1% of the population. Its prevalence increases with age. In addition to the morbidity related to falls and syncope, OHT is a risk factor for cognitive decline and cerebrovascular morbidity and mortality. In most cases, its cause is easily found by the examination and some simple explorations. For severe and/or less obvious mechanism, some paraclinical methods of investigation can be proposed. These include head upright tilt-test and Valsalva maneuver. Both explore the entire baroreflex arc, and are realised by non-invasive measurements of heart rate and blood pressure. Other tests explore more specifically the parasympathetic outflow tract or sympathetic. In some situations cardiac MIBG scintigraphy and urinary catecholamines may be useful. The respective choice of these tests depends on the clinical setting and suspected type of dysautonomic lesion. Identifying the cause and assessment of functional impairment guide the therapeutic choices, which are primarily based on non-pharmacological approach.

Changes in vascular alpha 1- and alpha 2-adrenoceptor responsiveness by selegiline treatment

Pharmacoepidemiological studies have reported an excess of mortality with selegiline, a MAO B inhibitor used in the treatment of Parkinson's disease. The mechanism of this putative adverse effect remains unknown but an interaction with the sympathetic nervous system was suggested. The aim of the present study was to investigate the influence of selegiline (10 mg/daily, orally during one week) on vascular alpha1- and alpha2-adrenoceptor responsiveness in conscious unrestrained dogs. Selegiline significantly increased resting values of both systolic and diastolic blood pressures and noradrenaline plasma levels (HPLC) without changing heart rate. Moreover, spectral analysis of systolic blood pressure (Fast Fourier Transformation) showed that selegiline increased the relative energy of a low frequency band without modifying the total spectrum. ED 50 calculated from dose-pressor response curves with phenylephrine (after beta-blockade by propranolol), an index of alpha1-adrenoceptor response or with noradrenaline (after alpha1- and beta blockade by prazosin plus propranolol), an index of alpha2-adrenoceptor response, were significantly higher after selegiline. Selegiline failed to modify the number of platelet alpha2-adrenoceptors measured by [(3)H] RX 821002 binding. Yohimbine-induced increase in noradrenaline release was significantly more marked after selegiline. These results support the evidence that selegiline induces a vascular alpha1- and alpha2-adrenoceptor-hyposensitivity that can be explained by the increase in noradrenaline release elicited by the drug.

Changes in short-term variability of blood pressure and heart rate during the development of obesity-associated hypertension in high-fat fed dogs

OBJECTIVE

To investigate the nature and time course of autonomic nervous system changes elicited by a 21-week ad libitum high-fat diet (HFD) in dogs.

RESULTS

The HFD increased body weight (+22.0+/-2.8% at week 21) with an abdominal circumference gain significantly more elevated than the thoracic one. The increases in insulin and free fatty acid plasma levels were correlated with body weight changes. Systolic and diastolic blood pressures and heart rate significantly increased (+23+/-6, +28+/-5 and 19+/-9% respectively). Arterial hypertension was characterized by an increase in cardiac output (+22.3+/-7.7%), in left ventricular mass (+18.1+/-5.0% at week 21) and a decrease in spontaneous baroreflex efficiency (-55+/-6%). The time course of autonomic changes (using spectral analysis of systolic blood pressure and heart rate) showed the existence of time-dependent modifications, which were linked with food intake. The initial rise in arterial blood pressure during body weight increment (observed between the 1st and 8th week of HFD) was associated with a transient increase in the low frequency band of systolic blood pressure variability and noradrenaline plasma levels associated with a long-lasting decrease in the high frequency band of heart rate variability. Early changes in short-term variability were significantly correlated with free fatty acid plasma levels. In contrast, the steady-state of obesity-related hypertension was associated with a decreased high frequency band of heart rate variability, without significant changes in noradrenaline plasma levels.

CONCLUSIONS

This study shows that the HFD induces abdominal obesity, hyperinsulinaemia and arterial hypertension, with a left ventricular hypertrophy associated with a biphasic changes in autonomic activity: an early and long-lasting decrease in parasympathetic nervous system activity and an early but transient increase in sympathetic activity. The present data suggest that autonomic nervous system changes are dependent on the time course of obesity development.

Fluoxetine in orthostatic hypotension of Parkinson's disease: a clinical and experimental pilot study

Recent clinical studies have reported a beneficial effect of fluoxetine, a serotonin reuptake inhibitor, in patients with severe refractory orthostatic hypotension. The present study was undertaken to investigate the effect of fluoxetine in orthostatic hypotension occurring during Parkinson's disease on both blood pressure values and number of clinical symptoms during orthostatic procedure evaluated using a validated clinical rating scale. In a pilot study performed in fourteen patients with idiopathic Parkinson's disease plus orthostatic hypotension, fluoxetine hydrochloride (20 mg orally daily during one month) significantly reduced the fall in systolic blood pressure [-33 +/- 21 (SD) mmHg before fluoxetine vs -22 +/- 19 mmHg after fluoxetine, P = 0.03] elicited by standing without modifying heart rate. The drug also significantly reduced the number of postural symptoms occurring during the orthostatic procedure [2.9 +/- 1.5 (SD) before fluoxetine vs 1.2 +/- 1.3 after fluoxetine, P = 0.006]. A similar pattern of response was obtained in an experimental model of neurogenic orthostatic hypotension obtained in chronically sino-aortic denervated dogs submitted to an 80 degrees head-up tilt test procedure under chloralose anaesthesia. Fluoxetine did not change plasma noradrenaline levels. This pilot study suggests a slight but clinically significant effect of fluoxetine on both hemodynamic parameters and clinical symptoms in parkinsonian patients suffering from orthostatic hypotension.

Effects of yohimbine, an alpha 2-adrenoceptor antagonist, on experimental neurogenic orthostatic hypotension

Yohimbine has been proposed for the treatment of neurogenic orthostatic hypotension; however, no controlled trial has been performed in experimental models of orthostatic hypotension or in patients with autonomic failure. The aim of the present study was to compare the effects of yohimbine (0.05 mg/kg, intravenously [i.v.]) and placebo (saline) in a new model of neurogenic orthostatic hypotension obtained by sinoaortic denervation (SAD) in chloralose-anaesthetized dogs. Blood pressure, heart rate, noradrenaline plasma levels and systolic blood pressure and heart rate short-term variabilities (calculated on low frequency [40-50 MHz] and high frequency [390-490 MHz] bands) were measured in supine position and after a 10 min 80 degrees head-up tilting. The drugs were administered in a double-blind cross-over randomized fashion. The head-up tilting performed in normal animals increased diastolic blood pressure (+12 +/- 4 mmHg), heart rate (+39 +/- 12 beats per minute [bpm]), the low frequency band of systolic blood pressure and noradrenaline plasma level, without changing systolic blood pressure or heart rate variability. In SAD dogs, a marked fall in systolic (-80 +/- 11 mmHg) and diastolic (-43 +/- 4 mmHg) blood pressures was observed within 1 min after placebo, without modification in heart rate, systolic blood pressure and heart rate short-term variabilities and noradrenaline plasma levels. In SAD dogs, yohimbine (0.05 mg/kg, i.v.) delayed the blood pressure fall elicited by head-up tilting, but failed to modify its magnitude. These results show that, in the model of orthostatic hypotension obtained by SAD, yohimbine, at an alpha 2-adrenoceptor selective dose (0.05 mg/kg), delays the fall in blood pressure elicited by head-up tilting. The effect of yohimbine can be explained by an increase in sympathetic tone.

Effects of octreotide on experimental neurogenic orthostatic hypotension in anaesthetized dogs

This paper investigates the effects of octreotide (0.1 mg/kg, subcutaneous) on cardiovascular adaptation during head-up tilt test in an experimental model of neurogenic orthostatic hypotension (OH) obtained by chronic sinoaortic denervation in anaesthetized dogs. Blood pressure (BP), heart rate (HR), spectral variability (Fast Fourier transformation on 512 consecutive points, delta t: 2 Hz) and plasma catecholamine levels were measured in a double blind cross-over randomized study versus placebo, in supine position and during a head-up tilt test (80 degrees, 10 min) in six sinoaortic denervated and six control (normal) dogs. In normal dogs, head-up tilt test significantly increased HR and diastolic blood pressure (DBP). Plasma noradrenaline levels and energy of the low frequency band (40-150 mHz) of systolic blood pressure (SBP) significantly increased whereas the energy of the low frequency band of HR significantly decreased. Placebo and octreotide failed to modify supine and head-up tilt values of the measured parameters (except the value of low frequency band of SBP, which increased after octreotide). In sinoaortic denervated dogs, supine values of BP, HR and plasma noradrenaline levels were significantly higher than in controls whereas the energy of the low frequency spectral band of HR and SBP was similar to controls. Head-up tilt test induced a dramatic decrease in BP. HR, plasma noradrenaline levels and energy of the low frequency band of SBP and HR remained unchanged during head-up tilt tests. Neither supine nor head-up tilt values of these parameters were modified 45 min after octreotide or placebo administration. These results show that sinoaortic denervation is a reproducible model of OH characterized by a lack of activation of sympathetic efferent pathways during head-up tilt tests. Octreotide at the dose used remains ineffective to prevent the fall in BP under these experimental conditions.

Which drug for which orthostatic hypotension?

This review focuses on the actual limits of the clinical pharmacology of drugs used for the treatment of orthostatic hypotension. The evidences for heterogeneity of the pathophysiological mechanisms of primary orthostatic hypotension and autonomic failure are discussed. The available data on the efficacy of some drugs used in orthostatic hypotension are also discussed.

Effects of pentobarbital and etomidate on plasma catecholamine levels and spectral analysis of blood pressure and heart rate in dogs

The present paper investigates the effects of two anaesthetic drugs (pentobarbital and etomidate) on both short-term variabilities of systolic blood pressure (SBP) and heart rate (HR) using fast Fourier transformation and catecholamine plasma levels. HR and BP were continuously recorded through an arterial catheter and blood samples were taken from the jugular vein. Spectral analysis was performed first in the conscious state and six minutes after induction of anaesthesia on a series of 256 consecutive BP and HR values (delta t: 2 Hz). The area under the curve (AUC) was determined in the low-frequency component of the SBP (LF: 40-150 mHz), in the high-frequency band of HR (HF: respiratory frequency +/- 50 mHz) and in the total frequency spectra. Results were normalized by calculation of the ratio between AUC of LF and HF and the total AUC of the corresponding spectrum (TS). Etomidate (2 mg/kg i.v.) induced a significant increase in TS and LF without changing the LF/TS ratio or the HR variability when compared with the awake period. Catecholamine plasma levels did not change. In contrast, pentobarbital (30 mg/kg i.v.) decreased the values of LF, HF, LF/TS and HF/TS and catecholamine plasma levels. These results suggest that pentobarbital decreases the activity of the two components of the autonomic nervous system whereas etomidate induces only minimal changes.