Presyncope caused by central hypovolaemia is not preceded by evoked potential alterations

Naloxone decreases tolerance to hypotensive, hypovolemic stress healthy humans

OBJECTIVE

In animal studies, naloxone, an opioid receptor antagonist, improves tolerance to hemorrhagic shock. The purpose of this study was to determine whether naloxone would augment tolerance to hypotensive hypovolemic stress (lower body negative pressure [LBNP]) in healthy human males.

DESIGN

This study was a repeated measures design.

SETTING

The experiments were conducted in a laboratory setting.

SUBJECTS

Eight healthy male subjects were tested. The subjects' ages were 30 +/- 4.0 yrs, height = 177 +/- 7.0 cm, and weight = 75.5 +/- 3.5 kg (mean +/- SEM).

INTERVENTIONS

Subjects underwent two LBNP exposures terminated by the onset of vasodepression. At each of the exposures, using a double-blind procedure, the subjects received an intravenous injection of either saline placebo or naloxone in a dosage totaling 0.4 mg/kg.

Simultaneous cerebrovascular and cardiovascular responses during presyncope

BACKGROUND AND PURPOSE

Presyncope, characterized by symptoms and signs indicative of imminent syncope, can be aborted in many situations before loss of consciousness occurs. The plasticity of cerebral autoregulation in healthy humans and its behavior during this syncopal prodrome are unclear, although systemic hemodynamic instability has been suggested as a key factor in the precipitation of syncope. Using lower body negative pressure (LBNP) to simulate central hypovolemia, we previously observed falling mean flow velocities (MFVs) with maintained mean arterial blood pressure (MABP). These findings, and recent reports suggesting increased vascular tone within the cerebral vasculature at presyncope, cannot be explained by the classic static cerebral autoregulation curve; neither can they be totally explained by a recent suggestion of a rightward shift in this curve.

METHODS

Four male and five female healthy volunteers were exposed to presyncopal LBNP to evaluate their cerebrovascular and cardiovascular responses by use of continuous acquisition of MFV from the right middle cerebral artery with transcranial Doppler sonography, MABP (Finapres), and heart rate (ECG).

RESULTS

At presyncope, MFV dropped on average by 27.3 +/- 14% of its baseline value (P < .05), while MABP remained at 2.0 +/- 27% above its baseline level. Estimated cerebrovascular resistance increased during LBNP. The percentage change from baseline to presyncope in MFV and MABP revealed consistent decreases in MFV before MABP.

CONCLUSIONS

Increased estimated cerebrovascular resistance, falling MFV, and constant MABP are evidence of an increase in cerebral vascular tone with falling flow, suggesting a downward shift in the cerebral autoregulation curve. Cerebral vessels may have a differential sensitivity to sympathetic drive or more than one type of sympathetic innervation. Future work to induce dynamic changes in MABP during LBNP may help in assessing the plasticity of the cerebral autoregulation mechanism.

Cerebral versus systemic hemodynamics during graded orthostatic stress in humans

BACKGROUND

Orthostatic syncope is usually attributed to cerebral hypoperfusion secondary to systemic hemodynamic collapse. Recent research in patients with neurocardiogenic syncope has suggested that cerebral vasoconstriction may occur during orthostatic hypotension, compromising cerebral autoregulation and possibly contributing to the loss of consciousness. However, the regulation of cerebral blood flow (CBF) in such patients may be quite different from that of healthy individuals, particularly when assessed during the rapidly changing hemodynamic conditions associated with neurocardiogenic syncope. To be able to interpret the pathophysiological significance of these observations, a clear understanding of the normal responses of the cerebral circulation to orthostatic stress must be obtained, particularly in the context of the known changes in systemic and regional distributions of blood flow and vascular resistance during orthostasis. Therefore, the specific aim of this study was to examine the changes that occur in the cerebral circulation during graded reductions in central blood volume in the absence of systemic hypotension in healthy humans. We hypothesized that cerebral vasoconstriction would occur and CBF would decrease due to activation of the sympathetic nervous system. We further hypothesized, however, that the magnitude of this change would be small compared with changes in systemic or skeletal muscle vascular resistance in healthy subjects with intact autoregulation and would be unlikely to cause syncope without concomitant hypotension.

METHODS AND RESULTS

To test this hypothesis, we studied 13 healthy men (age, 27 +/- 7 years) during progressive lower body negative pressure (LBNP). We measured systemic flow (Qc is cardiac output; C2H2 rebreathing), regional forearm flow (FBF; venous occlusion plethysmography), and blood pressure (BP; Finapres) and calculated systemic (SVR) and forearm (FVR) vascular resistances. Changes in brain blood flow were estimated from changes in the blood flow velocity in the middle cerebral artery (VMCA) using transcranial Doppler. Pulsatility (systolic minus diastolic/mean velocity) normalized for systemic arterial pressure pulsatility was used as an index of distal cerebral vascular resistance. End-tidal PACO2 was closely monitored during LBNP. From rest to maximal LBNP before the onset of symptoms or systemic hypotension, Qc and FBF decreased by 29.9% and 34.4%, respectively. VMCA decreased less, by 15.5% consistent with a smaller decrease in CBF. Similarly, SVR and FVR increased by 62.8% and 69.8%, respectively, whereas pulsatility increased by 17.2%, suggestive of a mild degree of small-vessel cerebral vasoconstriction. Seven of 13 subjects had presyncope during LBNP, all associated with a sudden drop in BP (29 +/- 9%). By comparison, hyperventilation alone caused greater changes in VMCA (42 +/- 2%) and pulsatility but never caused presyncope. In a separate group of 3 subjects, superimposition of hyperventilation during highlevel LBNP caused a further decrease in VMCA (31 +/- 7%) but no change in BP or level of consciousness.

CONCLUSIONS

We conclude that cerebral vasoconstriction occurs in healthy humans during graded reductions in central blood volume caused by LBNP. However, the magnitude of this response is small compared with changes in SVR or FVR during LBNP or other stimuli known to induce cerebral vasoconstriction (hypocapnia). We speculate that this degree of cerebral vasoconstriction is not by itself sufficient to cause syncope during orthostatic stress. However, it may exacerbate the decrease in CBF associated with hypotension if hemodynamic instability develops.

Middle cerebral artery velocity during head-up tilt induced hypovolaemic shock in humans

Middle cerebral artery mean velocity (Vmean) and pulsatility index (PI) were followed during head-up tilt induced hypovolaemic shock in nine subjects. Mean arterial pressure (MAP), heart rate (HR), central venous pressure (CVP) and thoracic (TI) electrical impedance were also recorded. Vmean, PI, and CVP did not change during head-up tilt to 50 degrees, while MAP increased from 92 (81-106) (median and range) to 100 (97-112) mmHg, HR from 63 (53-74) to 84 (68-89) beats min-1 and TI100kHz from 30 (27-36) to 32 (30-39) Ohm (P < 0.01) (n = 8). During maintained tilt, Vmean decreased from 52 (32-72) to 34 (16-59) cm s-1, whereas HR increased to 87 (52-108) beats min-1 and TI100 kHz to 33 (31-39) Ohm (P < 0.01). Presyncopal symptoms appeared after 33 (3-46) min and were associated with a MAP of 65 (32-84) mmHg (P < 0.01) and a HR of 58 (52-71) beats min-1 (P < 0.05). Vmean decreased to 25 (16-36) cm s-1, and cerebral conductance index (Vmean/MAPbrain) and PI increased (P < 0.01). Arterial collapse was observed (diastolic velocity of zero) in one subject at a brain (diastolic) blood pressure of 21 mmHg and he developed tachycardia (131 beats min-1) during presyncope. PaCO2 did not change. Maintained tilt resulted in central volume depletion reflected by changes in MAP, HR, and thoracic electrical impedance but not in CVP. Transcranial Doppler derived indices of cerebral perfusion demonstrated critically low values despite marked increase in conductance index.