Correlations among critical closing pressure, pulsatility index and cerebrovascular resistance

We attempted to explore the relationships among critical closing pressure (CrCP), resistance-area product (RAP) and traditional resistance indices of cerebral hemodynamics. Twenty healthy volunteers were studied. Blood pressure was obtained with servo-controlled plethysmography. Cerebral blood flow velocity (CBFV) was monitored by transcranial Doppler. Hemodynamic changes were induced by hyperventilation and by 5% CO(2) inhalation. Beat-to-beat CrCP and RAP values were extracted by linear regression analysis of instantaneous arterial blood pressure (ABP) and CBFV tracings. Gosling's pulsatility index (PI) and cerebrovascular resistance (CVR) were calculated. RAP correlated well with CVR at rest and during provocative tests (p = 0.006 approximately <0.001). There was no correlation among CrCP, CVR and PI. The changes in CVR correlated with those in RAP (p = 0.008 for the 5% CO(2) test and p = 0.014 for the hyperventilation test). The changes in PI and CrCP showed significant correlation (p = 0.004 for the 5% CO(2) test and p = 0.003 for the hyperventilation test). RAP reliably reflected cerebrovascular resistance. The changes in CrCP were valuable in assessing cerebrovascular regulation. Estimating changes in CrCP and RAP provided better understanding of the nature of cerebrovascular regulation.     

Near-infrared spectroscopy in evaluation of cerebral oxygenation during vasovagal syncope

Near-infrared spectroscopy (NIRS) offers a non-invasive, real-time monitoring of cerebral oxygenation. This method is based on the oxygenation and the light wavelength dependent absorption of near-infrared light by tissue chromophores, e.g. oxyhaemoglobin and deoxyhaemoglobin. The objective of the present study was the application of NIRS for evaluation of the brain function during vasovagal syncope (VVS). The VVS is a clinical syndrome affecting ca 3.5% of the population and for which the widely used diagnostic examination in this disease entity is the head-up tilt table test (HUT). In this study 69 patients with a history of VVS were examined using HUT. In 42 patients VVS was provoked. Results of the examination have shown that the changes in cerebral oxygenation measured by the NIRS technique are distinctly visible before the syncope. A gradual decrease of oxyhaemoglobin followed by its sudden drop was observed in all the VVS patients. Changes in the oxyhaemoglobin concentration measured by NIRS were observed on average 3.3 min before the syncope. They preceded the presyncope symptoms about 1.3 min (p < 0.005), the blood pressure and heart rate drop 2.2 min (p < 0.0001) and the arterial blood saturation 2.6 min (p < 0.00001).     

Nitrate‐Potentiated Head‐Up Tilt Testing in Older Patients: Outcomes,Hemodynamic Responses and Prodrome Recognition

Background: To compare head‐up tilt testing (HUT) outcomes and hemodynamic responses, and the prevalence and correlates of prodromes, in elderly and younger patients with suspected vasovagal syncope (VVS). Methods: Consecutive outpatients with a history of recurrent unexplained syncope underwent HUT by being tilted to 70°; the test was potentiated by the administration of 300 μg of nitroglycerine after 20 minutes. Occurrence of VVS and hemodynamic responses during passive and nitroglycerine phases of HUT were evaluated; symptoms preceding HUT‐induced syncope were recorded, together with heart rate and arterial blood pressure values. Results: Four hundred and sixty of the 743 patients were HUT positive: 156 fainted during the unmedicated phase and 304 after nitroglycerine administration. The patients aged ≥65 years (n = 102) experienced VVS more frequently during the pharmacological stage of HUT; the overall rate of positive results was similar to that observed in the patients aged 36–64 years (n = 329) and only slightly lower than that observed in those aged ≤35 years (n = 312). In the older patients, who experienced fewer and mainly prodrome‐free spontaneous syncopal episodes, HUT increased the number of premonitory symptoms, and there were no significant age‐related differences in symptom prevalence or timing or the patients’ hemodynamic characteristics. Conclusions: The rate of VVS induced by nitroglycerine‐potentiated HUT is similar in elderly and younger patients. In the former, nitroglycerine‐potentiated HUT significantly increases the prevalence of prodromes in comparison with spontaneous episodes, which suggests that it may be useful not only for diagnosis but also for patient counseling. (PACE 2010; 33:1210–1216)     

Reliability of dynamic cerebral autoregulation measurement using spontaneous fluctuations in blood pressure

Spontaneous fluctuations in BP (blood pressure) and subsequent change in CBFV (cerebral blood flow velocity) in the MCA (middle cerebral artery) can be used to assess dynamic cerebral autoregulation using transfer function analysis; however, the reliability of this technique has not been assessed, in particular the contribution of intra-subject variability relative to inter-subject variability. Three bilateral CBFV, BP and RR interval recordings were performed in ten healthy volunteers on four separate occasions over a 2-week period. Data were analysed to provide the ARI (autoregulatory index), CBFV, RAP (resistance-area product) and CrCP (critical closing pressure). We also measured systolic and diastolic BP, and resting HR (heart rate). We calculated the SEM (standard error of measurement) and the ICC (intra-class correlation coefficient) and their 95% CIs (confidence intervals) for each parameter to assess their absolute (intra-subject) and relative (inter-subject) reliability. The CV (coefficient of variation) of SEM ranged from 1.7% (for CBFV) to 100.0% (for RAP), whereas the ICC was <0.5 for ARI, rising to >0.8 for CBFV and diastolic BP. These data demonstrate excellent absolute and relative reliability of CBFV, whereas ARI is of comparable reliability with the measurement of HR. Using these results it is possible to determine the sample size required to demonstrate a change in ARI, with a sample of 45 subjects in each group required to show a change in ARI of 1, whereas to detect a change in ARI >2 would require only 11 subjects per group. The results of the present study could be valuable to the future planning of cerebral autoregulation studies, but more work is needed to understand the determinants of intra-subject variability in autoregulatory parameters.     

Cerebrovascular regulation and vasoneuronal coupling

Maintenance of cerebral perfusion pressure is a prerequisite for the prevention of cerebral ischemia. Physiological fluctuations in systemic perfusion pressure are compensated by cerebrovascular autoregulation. Cerebral hypoperfusion could result from (1) systemic hemodynamic failure (eg, distal to severe arterial stenosis), overcharging the vasoregulatory capacity; (2) dysfunction and exhaustion of cerebrovascular autoregulation; or (3) both. Ultrasound offers an excellent temporal resolution, is noninvasive, and is easily applicable for follow-up investigations. Despite its poor spatial resolution, transcranial Doppler sonography has been used for determination of cerebral perfusion reserve studies measuring cerebral blood flow velocity (CBFV) during hypercapnia or application of vasoactive agents (eg, acetazolamide). This approach evaluates vasomotor regulation in patients with hemodynamic compromise distal to severe stenosis or occlusion of the brain supplying arteries. Monitoring CBFV during tilt table examinations directly measures cerebral autoregulation. In patients with systemic orthostatic hypotension, maintainance or failure of cerebrovascular compensation and, even more importantly, cerebrovascular dysautoregulation, despite normal systemic blood pressure regulation, may be demonstrated. Vasoneuronal coupling is reflected by CBFV variations during appropriate neuronal stimulation. Neuronal dysfunction is associated with CBFV abnormalities as exemplified by preconditions of focal cerebral dysfunction in the posterior cerebral artery (PCA) in migraineurs with aura, where massive alteration of vasoneuronal coupling and ischemia is threatening during spreading depression. A highly significant asymmetric gain of vasoneuronal coupling in the interictal state may act as a trigger mechanism in these patients. Testing for vasoneuronal coupling within the middle cerebral artery (MCA) territory is more difficult due to the poor spatial resolution with various neuronal stimuli (eg, motorsensory or cognitive paradigms), only eliciting local neuronal areas underrepresented in the MCA CBFV global changes. However, motor stimulation evoked CBFV may be used to indicate dysintegation of vasoneuronal coupling in the course of acute cerebral ischemia with sensorimotor hemiparesis and, moreover, seems to be of prognostic value regarding the motor deficit. © 1995 John Wiley & Sons, Inc.     

Effects of incremental levels of continuous positive airway pressure on cerebral blood flow velocity in healthy adult humans

Sleep disordered breathing is common in patients with cerebrovascular disease, and could exacerbate the cerebral damage in acute stroke. Data about the effects of continuous positive airway pressure (CPAP) upon cerebral perfusion are conflicting. We investigated whether increasing levels of CPAP may affect cerebral haemodynamics, assessed by transcranial Doppler (TCD) in normal humans. A group of 25 healthy young volunteers were evaluated before (CPAP0-pre), during (CPAP5, CPAP10 and CPAP15, denoting CPAP at 5, 10 and 15 cmH(2)O respectively) and after (CPAP0-post) application of incremental levels of CPAP delivered through a mouthpiece. The mean cerebral blood flow velocity (CBFV) and the pulsatility index (PI; an indirect measure of cerebrovascular resistance) in the middle cerebral artery were measured with TCD. Respiratory rate, heart rate, end-tidal carbon dioxide pressure (PETCO(2)), transcutaneous haemoglobin oxygen saturation (SpO(2)), mean arterial blood pressure and anxiety score were also recorded. Compared with CPAP0-pre, CBFV was significantly decreased as higher levels of CPAP were applied (P <0.0001). CPAP15 increased PI (P <0.05), ETCO(2) was reduced by CPAP10 and CPAP15 (P <0.0001), and anxiety score and SpO(2) increased at all levels of CPAP (P <0.05). Heart rate, respiratory rate and mean arterial pressure did not change. The decrease in CBFV was correlated with the fall in P ETCO(2) (CPAP15) and the increase in PI (CPAP10, CPAP15) (P <0.05). In conclusion, even low levels of CPAP delivered through a mouthpiece in awake, young volunteers led to a decrease in CBFV, measured by TCD. This fall in CBFV was associated with hypocapnia and with an increase in both cerebrovascular resistance and anxiety due to breathing against positive pressure. As the negative consequences of a fall in CBFV may outweigh the therapeutic effects of CPAP in the post-stroke setting, further studies of the cerebrovascular effects of CPAP with different interfaces in elderly patients with and without stroke are needed before intervention trials can be performed safely.     

The role of tilt training in preventing recurrent syncope in patients with vasovagal syncope: a prospective and randomized study

Background: Recurrent vasovagal syncope (VVS) can be a severely disabling disorder that may lead to an important deterioration of quality of life because of the severity and recurrence of episodes. This study sought to investigate the effectiveness of repeated orthostatic self-training in preventing syncope in patients with recurrent VVS.
Methods: Eighty-two consecutive patients (mean age 41 ± 4 years, 37 males) with recurrent VVS episodes and positive head-up tilt testing (HUT) were enrolled in this study. The patients were then randomized (1:1) to conventional therapy or conventional therapy plus additional tilt training sessions. The patients were followed for spontaneous syncope for one year. Primary end-points were the recurrence of syncope, the number of episodes, and the interval of time to the first recurrence.
Results: There were no significant differences of baseline clinical characteristics and parameters of HUT between the tilt training and control groups. The patients had 4 ± 2/year syncopal episodes prior to the HUT. The mean follow-up after randomization was 12 ± 2 months. Spontaneous syncope recurrence during follow-up was 56% (23 patients) versus 37% (15 patients) in the control and tilt training groups, respectively (P = 0.1). Time to first recurrence was also similar in both groups (70 ± 20 days vs 50 ± 15 days, P = 0.09). The frequency of recurrent syncopes was similar in all types of VVSs while the rate of episodes was significantly higher in control group in patients with vasodepressor type during follow-up period (32% vs 10%, P = 0.04). The mean number of recurrent syncope episodes was also similar in both groups (3 ± 1 vs 2 ± 1, P = 0.4).
Conclusions: Tilt training was unable to influence the spontaneous syncope recurrence for recurrent VVS except for vasodepressor type.     

Transcranial Doppler Monitoring in Parkinson’s Disease: Cerebrovascular Compensation of Orthostatic Hypotension

Despite of precipitous blood pressure falls in Parkinson’s Disease (PD) patients, they may not experience syncope or postural complaints. Can cerebral blood flow regulation explain why orthostatic hypotension (OH) has often no accompanying symptoms? In patients with PD and OH (18 asymptomatic; 8 symptomatic), arterial blood pressure (ABP) as well as Doppler-detected cerebral blood flow velocity (CBFV) in middle and posterior cerebral arteries (MCA and PCA) were monitored during head-up tilt and compared with 25 controls and eight non-PD-OH patients. Analysis included the transfer function between slow spontaneous pressure and flow-oscillations. ABP and CBFV were maintained at significantly higher levels in asymptomatic than symptomatic PD-OH (ABP: 85.7 ± 10.5 vs. 66.9 ± 12.5%; MCA-FV: 83.3 ± 9.3 vs. 66.1 ± 6.8%; PCA-FV: 84.4 ± 12.2 vs. 65.9 ± 9.3% of supine). When orthostatic complaints occurred, CBFV depended directly on ABP changes (MCA r² = 0.64; PCA r² = 0.62; both p < 0.05). Despite of a tilt-induced blood pressure instability in PD-OH, the transfer function parameters did not differ from normal [phase: MCA: 46.6 ± 20.5°; PCA 39.2 ± 28.8°, gain: MCA 2.0 ± 0.7; PCA 2.9 ± 1.6)]. Results showed a normal autoregulatory response to downward blood pressure shifts in PD. Moreover, orthostatic blood pressure instability is compensated equally sufficient in anterior and posterior parts of cerebral circulation. Whether in PD patients, OH becomes symptomatic rather seems to depend on blood pressure falling below the autoregulated range. (E-mail: Christina.Haubrich@t-online.de)     

Circadian and Infradian Rhythms of Vasovagal Syncope in Young and Middle‐Aged Subjects

Background: The most cardiovascular physiological and pathophysiological events show a circadian rhythm. It is thought that the autonomic nervous system and biologic factors play a key role in the pathogenesis of vasovagal syncope (VVS). In this study, we investigated the circadian and infradian variation of VVS. Methods: A prospective consecutive series of 246 patients (142 women; mean age 36 ± 7 years) with recurrent syncope and with a positive head‐up tilt testing (HUT) were included in this study. The daily and weekly distributions of the syncopal episodes were investigated. The assessments of episodes were done according to the days of the week and three time periods of the day (6 a.m.–12 noon, 12 noon–6 p.m., 6 pm–12 midnight). Results: According to the results of HUT, 76 patients (31%) had cardioinhibitory, 62 patients (25%) had vasodepressor, and 108 patients (44%) had mixed type of VVS. During the initial passive phase, 40% of patients (99/246) showed positive response. Time to syncope was 20 ± 2 minutes during HUT. The mean number of syncopal episodes was 4 ± 2/years. In all, 1,070 episodes were evaluated. The distribution of the episodes in 6‐hour intervals was significantly different from uniform occurrence (38%, 33%, and 29%, respectively) (P = 0.02). The frequency of episodes was higher in the morning (P = 0.045) and in the middle of the week (P = 0.046). A significant difference was found between week and weekend days in terms of the frequency of episodes (75.5% vs 24.5%, P = 0.01). Conclusion: VVS may show a circadian and infradian rhythm.     

直立倾斜试验诊断血管迷走性晕厥的价值

目的研究直立倾斜试验（ＨＵＴ）诊断血管迷走性晕厥的价值。方法６７例原因不明的晕厥患者、２０例正常对照组行ＨＵＴ时,并采用特异性及敏感性较高的静滴异丙肾上腺素重复试验。观察心率、收缩压、舒张压变化。结果４３例阳性患者中,血管抑制性晕厥３１例（３１／６７）,混合性晕厥１２例。患者组,基础早搏时与阳性时的心率、收缩压、舒张压之间有高度显著性差异（Ｐ＜０００１）。静滴异丙肾上腺素敏感性６４％,特异性８５％。结论静滴异丙肾上腺素重复倾斜试验对诊断血管迷走性晕厥具有重要价值。     

Comparison of transcranial Doppler flow velocity and cerebral blood flow during focal ischemia in rabbits.

The predictive value of transcranial Doppler (TCD) cerebral blood flow velocity (CBFV) measurements for cerebral blood flow (CBF) calculations in humans is still controversial, and experimental correlative studies are lacking. The aim of the present study was to validate TCD signals of CBFV during focal cerebral ischemia. Therefore, CBFV determined in the middle cerebral artery (MCA) was compared with values of CBF obtained from autoradiograms of ischemic brain areas. To determine CBFV, a transcranial Doppler ultrasound probe (TCD) adapted to small sample volumes was used in 9 rabbits. CBF was quantified after a final infusion of [14C]-iodoantipyrine in the same animals. For focal cerebral ischemia induction, two threads were flushed upward simultaneously into the internal carotid artery, resulting in a flow reduction in the ipsilateral MCA. After thread occlusion, mean systolic CBFV in the MCA decreased from 49 +/- 9 cm/s to 22 +/- 3 cm/s. CBF in the caudate nucleus was reduced (19 +/- 8 mL/100 g/min) compared to the contralateral nonischemic side (52 +/- 18 mL/100 g/min). The decrease in hemispheric CBF correlated well with the decrease in both mean systolic (r = 0.97) and diastolic (r = 0.94) CBFV in the MCA (p < 0.01). The decrease in CBFV determined by transcranial Doppler ultrasound in the MCA appears to reflect the reduction in CBF in the affected brain hemisphere and can be used as a quantitative in vivo parameter for tissue perfusion.     

Myocardial contractility and cardiac filling measured by impedance cardiography in patients with nitroglycerine-induced vasovagal syncope

OBJECTIVE: Increased myocardial contractility and inadequate cardiac filling leading to activation of the Bezold-Jarisch reflex were proposed as possible triggering mechanisms of vasovagal syncope (VVS). In the present study noninvasive hemodynamic measurements were performed in order to examine the role of myocardial contractility and cardiac filling in pathogenesis of VVS. METHODS: Hemodynamic parameters were measured during head-up tilt test (HUT) by impedance cardiography in 46 patients with unexplained syncope. Myocardial contractility was measured as index of contractility (IC), acceleration index (ACI), and ejection fraction (EF). Afterload was measured as systemic vascular resistance index (SVRI) and preload was expressed as end-diastolic index (EDI). Serial measurements were done 1 minute before HUT, during HUT at 1-minute intervals, and 1 minute after completion of HUT. RESULTS: HUT was positive in 30 patients (10 men, 20 women, mean age 36 +/- 16 years) and negative in 16 patients (8 men, 8 women, mean age 31 +/- 14 years). No significant differences were observed between HUT(+) and HUT(-) groups in hemodynamic parameters at supine rest and during HUT until the development of syncope. SVRI was lower in HUT(+) than in HUT(-) group at syncope (122.7 + 66.3 vs 185.6 + 51.4 dyn sec cm(-5)/m2, P = 0.002) and after syncope (117.0 + 61.1 vs 198.0 + 95.7 dyn sec cm(-5)/m2, P = 0.007). ACI, IC, EF, and EDI did not differ between groups at syncope. After syncope EF was higher in HUT(+) group compared to HUT(-) group (59.2 + 6.1 vs 52.7 + 9.4%, P = 0.02). CONCLUSION: The role of increased myocardial contractility and decreased cardiac filling is not confirmed in the present study.     

Altered cerebrovascular response pattern in interictal migraine during visual stimulation

A dysbalance of the cerebrovascular response during functional activation of the brain has been postulated as a factor in the pathophysiology of migraine. To determine the dynamic pattern of the cerebrovascular response in migraineurs compared with a control group, changes of the cerebral perfusion during cerebral activation were studied with high temporal resolution by functional transcranial Doppler sonography (fTCD). The cerebral blood flow velocity (CBFV) in the right posterior cerebral artery (PCA) and the left middle cerebral artery (MCA) was measured simultaneously during visual stimulation in 19 interictal migraineurs and in 19 age- and sex-matched control subjects. Data were analysed with a previously validated technique based on automated stimulus-related averaging of the CBFV. The MCA migraineurs exhibited a steady increase of CBFV during the stimulation, while normal subjects showed a habituation of the CBFV response. The lack of habituation in migraineurs was significantly (P < or = 0.05) more pronounced across patients with a high attack frequency (> or = 4 per month) compared with migraineurs with a low attack frequency (< 4 per month). In the PCA, compared with normal subjects, migraineurs showed significantly (P < or = 0.05) stronger CBFV changes at the beginning and after the end of stimulation, with a slower decline to baseline. Data are in accordance with electrophysiological findings in migraineurs. It is assumed that a lack of habituation of the cerebrovascular response in migraineurs might contribute to a disturbance of the metabolic homeostasis of the brain that might induce migraine attacks.     

Cerebral haemodynamics during the Mueller manoeuvre in humans

Voluntary negative intra‐thoracic pressure (Mueller manoeuvre) is known to reduce arterial blood pressure (ABP). To investigate changes in cerebral blood flow velocity (CBFV) during 15 s Mueller manoeuvres at –30 mmHg intra‐thoracic pressure, 27 young (aged 21–31 years, group A) and 11 older (52–64 years, group B) healthy adults were studied using transcranial Doppler and non‐invasive ABP measurement (Finapres). After closely following the initial ABP drop, CBFV showed an overshoot during temporary recovery of ABP. Then ABP and CBFV decreased significantly to below baseline. While ABP declined further until the end of the manoeuvre, CBFV increased in group A 4·7 s (2·4–8·5) (median and range) and in group B 5·7 s (4·1–7·2) after the onset of the CBFV decrease. Critical closing pressure (CCP), calculated for each cardiac cycle from the dynamic pressure–flow relationship (DPFR), indicated a reduction of intra‐cranial pressure during the first half of the strain. DPFR‐related estimation of cerebrovascular resistance provided a more physiological response than the conventional cerebrovascular resistance quotient ABP/CBFV, and decreased about 1·5 s before the observed CBFV increase. A modification of the previously described dynamic auto‐regulation index ROR correlated significantly with CO₂ reactivity values (r=0·61, P=0·001). In conclusion, changes in CBFV during Mueller manoeuvres are likely to reflect dynamic cerebral auto‐regulation and may provide an estimate of dynamic cerebral auto‐regulation capacity. In older adults, the maximal dynamic auto‐regulatory response seems to be unchanged, but the onset of reaction is slightly delayed.     

Short-term variability of cerebral blood flow velocity responses to arterial blood pressure transients

The time course of mean beat-to-beat changes in cerebral blood flow velocity changes induced by spontaneous transients in mean arterial blood pressure was studied in a group of 39 healthy subjects, ages 40 +/- 15 (SD) years. Continuous 10-min noninvasive recordings of cerebral blood flow velocity (CBFV) from both middle cerebral arteries (MCA) with Doppler ultrasound (US) and simultaneous beat-to-beat arterial blood pressure (ABP) were made. A total of 522 spontaneous positive transients of ABP and CBFV were extracted with a maximum of 15 transients for each subject. The CBFV transient amplitude was normalized by the corresponding ABP change and the area-under-the-curve (AUC) of the falling phase was used to classify the CBFV regulatory response as either weak, moderate or strong. The coherent average of ABP and CBFV of each category confirmed the consistency of this classification, reinforced by the agreement of separate averages for recordings from the right and left MCA. All 39 subjects showed at least two categories of transients, with all three categories present in 33 subjects (right MCA) and 29 subjects (left MCA), respectively. These results indicate a significant short-term variability of CBFV responses in healthy subjects whose origin remains unexplained.     

Cerebral critical closing pressure estimation from Finapres and arterial blood pressure measurements in the aorta

Estimates of cerebral critical closing pressure (CrCP) and resistance-area product (RAP) are often derived using noninvasive measurements of arterial blood pressure (ABP) in the finger, but the errors introduced by this approach, in relation to intra-vascular measurements of ABP, are not known. Continuous recordings of ABP (Finapres and solid-state catheter-tip transducer in the ascending aorta), cerebral blood flow velocity (CBFV, bilateral Doppler), ECG and transcutaneous CO(2) were performed following coronary catheterization. CrCP and RAP were calculated for each of 12,784 cardiac cycles from 27 subjects using the classical linear regression (LR) of the instantaneous CBFV-ABP relationship and also the first harmonic (H(1)) of the Fourier transform. There was a better agreement between LR and H(1) for the aortic measurements than for the Finapres (p < 0.000,01). For LR there were no significant differences for either CrCP or RAP due to the source of ABP measurement, but for H(1) the differences were highly significant (p < 0.000,03). The coherence functions between either CrCP or RAP values calculated with aortic pressure (input) or the Finapres (output) were significantly higher for H(1) than for LR for most harmonics below 0.2 Hz. When using the Finapres to estimate CrCP and RAP values, the LR method produces similar results to intra-arterial measurements of ABP for time-averaged values, but H(1) should be preferred in applications analysing beat-to-beat changes in these parameters.     

Carbondioxide reactivity of the blood flow in human basilar artery estimated by the transcranial Doppler method in normal men: a comparison with that of the middle cerebral artery

Using the transcranial Doppler technique, the carbondioxide reactivity of the blood flow velocity in the human basilar artery (BA) was investigated in healthy volunteers for comparison with that in the middle cerebral artery (MCA). An exponential curve with an exponent of 0.044 mmHg-1 was found to be a good fit to the BA data and 0.040 mmHg-1 to the MCA data, respectively. While the response of both arteries showed a good correlation to the end-tidal carbondioxide partial pressure (PETCO2), there was no significant difference observed between the carbondioxide (CO2) reactivity of each artery. In both arteries, the end-diastolic flow velocity decreased significantly with the reduction of PETCO2, which may represent the increasing resistance of cerebral microvasculature in hypocapnia. Because of the reliability and noninvasiveness of this method, this normal value should be quite helpful for the bed-side evaluation of an altered CO2 response in the patients with cerebrovascular disease.     

Cerebral Syncope: Loss of Consciousness Associated with Cerebral Vasoconstriction in the Absence of Systemic Hypotension

Transcranial Doppler (TCD) ultrasonography done during headupright tilt induced neurocardiogenic syncope has demonstrated that cerebral Vasoconstriction occurs concomitant with (or precedes) loss of consciousness. This article demonstrates evidence that cerebral blood flow changes alone (vasoconstriction), in the absence of systemic hypotension, may result in syncope. Five patients (4 men, 1 woman; mean age 41 ± 17 years) with recurrent unexplained syncope were evaluated by use of an upright tilt table test for 45 minutes with or without an infusion of low dose isoproterenol. TCDoppler ultrasonography was used to assess middle cerebral artery systolic velocity (Vs); diastolic velocity (Vd); mean velocity (Vm); and pulsatility index (PI = Vs = Vd/Vmean). Syncope occurred in five patients during the baseline tilt and in one patient during isoproterenol infusion. During tilt induced syncope, at an average mean arterial pressure of 89 ± 16 mmHg, TCD sonography showed a 2%± 10% increase in systolic velocity; a 51%± 27% decrease in diastolic velocity; and a 131 %± 87% increase in pulsatility index. One patient underwent continuous electroencephalographic recording during tilt, which demonstrated diffuse slow wave activity (indicating cerebral hypoxia) at the time of syncope concomitant with the aforementioned TCD changes in the absence of systemic hypotension. These fndings reflect an increase in cerebrovascular resistance secondary to arteriolar vasoconstriction distal to the insonation point of the middle cerebral artery, that occurred concomitant with loss of consciousness and in the absence of systemic hypotension. We conclude that in some individuals abnormal baroreceptor responses triggered during orthostatic stress may result in a derangement of cerebral autoregulation leading to cerebral vasoconstriction with resultant cerebral hypoxia in the absence of systemic hypotension.     

Sensitivity and Specificity of the Tilt Table Test in Young Patients with Unexplained Syncope

The usefulness of the head-up tilt testing (HUT) has heen previously addressed in diagnosing vasovagal neuroregulatory syncope in the teenage population. However, data concerning sensitivity and specificity is deficient due to the lack of control groups. We compared the response to HUT in young patients referred because of syncope or near syncope (n = 44, mean age 16 ± 3 years SD) to healthy young volunteers with a normal physical examination and no previous history of syncope (n = 18, mean age 16 ± 2 years) and io determine the sensitivity and specificity of HUT. The graded tilt protocol was performed at 15°, 30°, and 45° (each for 2 min), and then 60° for 20 minutes. Cuff blood pressure was measured every minute and lead IIECG was continuously monitored. Results; 25 of the 44 patients (57%) developed a vasovagaJ response or became symptomatic after 13.8 ± 5.7 minutes of HUT. Three of the 18 volunteers (17%) had a vasovagal response and became symptomatic after 9 ± 3 minutes of HUT. There was no statistical difference among the four groups (with and without tilt induced vasovagal response) in terms of age and baseline hemodynamic data. The sensitivity of 20 minutes HUT was 57% and its specificity was 83%. The presyncopal hemodynamic response in patients with history of syncope that was characterized by a significant decrease in systolic blood pressure and lack of increase of diastolic blood pressure as compared with baseline and with other groups. Gonclusions: 20 minutes at 60° HUT has a high specificity for the diagnosis of vasovagal syncope. Its limited sensitivity is counterbalanced by the advantage of limiting the incidence of false-positive results in patients without the vasovagal syndrome.     

The effect of vasospasm on cerebral perfusion: a colour duplex study of the extra- and intracranial cerebral arteries

To assess whether middle cerebral artery (MCA) vasospasm reduces the flow volume in the corresponding extracranial internal carotid artery (ICA) or global cerebral blood flow volume (CBFV) in subarachnoid haemorrhage (SAH) patients, a colour duplex ultrasound study of the intra- and extracranial cerebral arteries was performed. MCA vasospasm was defined as a time-averaged maximum flow velocity (TAMX) exceeding 120 cm/s. ICA flow volumes and CBFV, were compared in each patient at maximum TAMX recorded in one MCA ("maximum-vasospasm") and when TAMX in the same vessel was closest to mean reference values ("no-vasospasm"). Additionally, the CBFV course during the first 3 weeks after SAH was evaluated longitudinally. Data from age- and gender-matched healthy test persons served as control. In 28 patients with MCA vasospasm, 337 measurements were completed. Global CBFV was significantly reduced starting from day 3 after SAH. ICA flow volumes and CBFV were not different when comparing at "maximum-vasospasm" and "no-vasospasm". Compared with the control group, both were lower at either condition. Thus, in SAH patients, vasospasm even severe, in general does not further diminish ICA flow volumes and global CBFV, which are reduced already before the onset of vasospasm.