Dependency of blood flow velocity in the middle cerebral artery on end-tidal carbon dioxide partial pressure--a transcranial ultrasound Doppler study

Multi-modal monitoring of acute brain injury

OBJECTIVE

To review the scientific basis for and utility of the traditional cerebral monitors used currently in neurointensive care, together with research techniques that are soon likely to become used in managing severe head injury and subarachnoid haemorrhage.

DESIGN AND CONTENT

Firstly, the pathophysiology of acute brain injury including cerebral haemodynamics, oxygen and metabolism and the role of secondary insults are discussed. Secondly, the importance of assessment of cerebrovascular autoregulation and reactivity is reviewed together with methods for its continuous non-invasive measurement using transcranial Doppler and intracranial pressure/arterial pressure recordings. Thirdly, the respective roles of jugular venous oxygen and brain tissue oxygen monitoring are analysed. Fourthly, the use of cerebral microdialysis is described, together with an overview of its utility.

CONCLUSION

Cerebral multimodal monitoring can be helpful for the optimal management of acute brain injury and essential for future exploratory trials of neuroprotective drugs.

Effect of sympathetic muscle vasoconstrictor activity on capsaicin-induced muscle pain

Neuropathic pain syndromes with sympathetically maintained pain are often associated with a deep somatic pain component. An adrenergic interaction between sympathetic vasoconstrictor neurons and cutaneous afferents has been demonstrated. To determine whether a sympathetic-afferent interaction exists in deep somatic tissues, we investigated the effect of sympathetic muscle vasoconstrictor activity on experimentally induced pain. In 12 healthy volunteers, capsaicin was infused into the anterior tibial muscle. Intensity and quality of muscle and referred pain were assessed. The analyses were performed during the presence of low sympathetic muscle vasoconstrictor activity induced by breathing 100% O(2) gas (normocapnia), and during high activity induced by inspiration of 95% O(2) and 5% CO(2) (hypercapnia). The degree of sympathetic muscle vasoconstrictor discharge was monitored indirectly by measuring systemic blood pressure and end-expiratory CO(2) and by performing duplex sonography of muscle resistance vessels. The intensity, quality, and spatial distribution of muscle and referred pain were not significantly different during resting and increased sympathetic muscle vasoconstrictor discharge, indicating that such activity does not influence pain after intramuscular infusion of capsaicin.

Effects of cerebrovascular challenges on plasma endothelin

Plasma endothelin elevations have been associated with cerebrovascular pathology. Mechanisms of stimulation, however, are unknown. Therefore, in healthy subjects a marked physiological cerebrovascular response was experimentally provoked by hypercapnia, hypocapnia, and alternating capneic conditions. During these challenges plasma immunoreactive-endothelin-1 (ir-ET-1) concentrations were determined using a radioimmunassay. Physiological effects were continuously recorded for pCO(2), cerebral blood flow velocity, pulse frequency, and arterial blood pressure. No alterations in plasma ET-1 levels were found upon any of the cerebrovascular stimuli. We conclude that massive cerebrovascular challenges in healthy individuals do not lead to high circulating ET-1 levels.

Twenty-four-hour non-invasive monitoring of systemic haemodynamics and cerebral blood flow velocity in healthy humans

Acute short-term changes in blood pressure (BP) and cardiac output (CO) affect cerebral blood flow (CBF) in healthy subjects. As yet, however, we do not know how spontaneous fluctuations in BP and CO influence cerebral circulation throughout 24 h. We performed simultaneous monitoring of BP, systemic haemodynamic parameters and blood flow velocity in the middle cerebral artery (MCAV) in seven healthy subjects during a 24-h period. Finger BP was recorded continuously during 24 h by Portapres and bilateral MCAV was measured by transcranial Doppler (TCD) during the first 15 min of every hour. The subjects remained supine during TCD recordings and during the night, otherwise they were seated upright in bed. Stroke volume (SV), CO and total peripheral resistance (TPR) were determined by Modelflow analysis. The 15 min mean value of each parameter was assumed to represent the mean of the corresponding hour. There were no significant differences between right vs. left, nor between mean daytime vs. night time MCAV. Intrasubject comparison of the twenty-four 15-min MCAV recordings showed marked variations (P < 0.001). Within each single 15-min recording period, however, MCAV was stable whereas BP showed significant short-term variations (P < 0.01). A day-night difference in BP was only observed when daytime BP was evaluated from recordings in the seated position (P < 0.02), not in supine recordings. Throughout 24 h, MCAV was associated with SV and CO (P < 0.001), to a lesser extent with mean arterial pressure (MAP; P < 0.005), not with heart rate (HR) or TPR. These results indicate that in healthy subjects MCAV remains stable when measured under constant supine conditions but shows significant variations throughout 24 h because of activity. Moreover, changes in SV and CO, and to a lesser extent BP variations, affect MCAV throughout 24 h.

Exaggerated interictal cerebrovascular reactivity but normal blood flow velocities in migraine without aura

Interictal cerebrovascular reactivity and blood flow velocities were tested in 23 patients with migraine without aura and 10 age- and sex-matched healthy controls by using the breath holding index (BHI). The mean systolic, diastolic and mean velocities and pulsatility indices were not different in the controls and patients. The BHI was found to be significantly greater (P=0011) in the patients (1.64 +/- 0.33) compared with the controls (1.26 +/- 0.37), showing an exaggerated reactivity to hypercapnia in migraineurs. Reactivity to pCO2 theoretically depends on pre-existing arteriolar tone and thereby on baseline velocity. Our finding of similar blood flow velocities in controls and patients suggests that the underlying cause for this high reactivity may not be an increased vasotonus but an increased sensitivity to changes in blood CO2 levels.

Middle cerebral artery flow velocity correlates with common carotid artery volume flow rate after CO2 inhalation

Cerebral vasoreactivity can be studied with transcranial Doppler (TCD) by monitoring CO2-induced middle cerebral artery (MCA) velocity changes. Expected MCA mean velocity (Vm) changes due to changes in end-expiratory CO2 (EE-CO2) are established, but reactivity of common carotid artery (CCA) volume flow rate (VFR) has not been extensively reported. The authors assess the relationship between MCA Vm, CCA VFR, and EE-CO2. Ten normal individuals without cerebrovascular disease and with CCA diameters of more than 3.0 mm were studied. CCA VFR was obtained by Color Velocity Imaging Quantification and Ipsilateral MCA Vm by standard TCD methods. Each side was studied before, during, and after inhalation of 5% CO2. EE-CO2, blood pressure, and pulse rate were monitored. Four women and 6 men with mean age of 36 years were included. Significant correlations between MCA Vm and EE-CO2, CCA VFR and EE-CO2, and MCA Vm and CCA VFR were found. MCA Vm and CCA VFR increased 5.2% and 4.3% per mm Hg increase in EE-CO2, respectively. MCA Vm increased 0.3 cm/s for each ml/min increase in CCA VFR. In normal individuals, there is a direct correlation between MCA Vm, CCA VFR, and EE-CO2. Measurement of CCA VFR changes during CO2 inhalation may be an alternative method to estimate cerebral vasoreactivity when the MCA velocity cannot be obtained because of inadequate acoustic temporal windows.

Hypertension as a risk factor for cerebral injury during cardiopulmonary bypass. Protein S100B and transcranial Doppler findings

We studied 22 patients aged 53-78 years scheduled for cardiac surgery under cardiopulmonary bypass. Blood pressure, cardiac output, transcranial Doppler blood flow velocity, arterial blood gases, body temperature and protein S100B, as a marker for cerebral integrity, were evaluated in normotensive and hypertensive patients. Pre-operative mean (SD) arterial blood pressure was 93 (11) mmHg in the normotensive group compared with 116 (15) mmHg in the hypertensive group. We found an increase in protein S100B levels in both groups. Serum protein S100B concentrations in the hypertensive group were significantly higher than in the normotensive group (p < 0.001). The highest mean (SD) values were 2.04 (0.65) micromol x l(-1) in the normotensive group and 7.02 (4.55) micromol x l(-1) in the hypertensive group. These results suggest that cardiopulmonary bypass is associated with a significantly higher rate of cerebral injury in hypertensive patients than in normotensive patients. This may be due to altered autoregulation and insufficient cerebral perfusion. Modifications of cardiopulmonary bypass management for hypertensive patients might be made to decrease the risk of cerebral injury.

[Clonidine combined with flunitrazepam before carotid endarterectomy decreases cerebrovascular CO2 reactivity]

OBJECTIVE

Assess cerebrovascular CO2 reactivity changes using transcranial Doppler sonography (TCD) after oral premedication associating clonidine (2 micrograms.kg-1) and flunitrazepam (70 micrograms.kg-1) in patients scheduled for carotid stenosis surgery.

STUDY DESIGN

Prospective study, not randomized, the patient being his own "control".

PATIENTS AND METHODS

Thirteen patients undergoing carotid endarterectomy under cervical plexus block were included. The monitoring included: automated arterial pressure cuff, ECG, radial artery catheter, TCD with probe secured in temporal window. The study of the cerebrovascular CO2 reactivity was performed with TCD recording on the side of operation, on the day before, and on the day of carotid endarterectomy, 90 min after the premedication, immediately before surgery. To change PaCO2, four ventilatory states were successively performed: (1) normoventilation, (2) hyperventilation, (3) hypoventilation, (4) "breath-holding test". At each state, it was noted: HR, MAP, PaCO2, mean blood flow velocity in the middle cerebral artery (Vm-MCA), resistance index of Pourcelot (RI), cerebrovascular reactivity (slope Vm-MCA/PaCO2). The results (+/- SEM) were analyzed by Wilcoxon test or t test.

RESULTS

After premedication, cerebrovascular CO2 reactivity decreased (0.043 +/- 0.019 vs 0.034 +/- 0.013; p < 0.05) without modification of RI (0.578 +/- 0.291 vs 0.612 +/- 0.025; NS). No complication during carotid clamping was reported.

CONCLUSION

Inclusion of clonidine in premedication before carotid stenosis surgery must be questioned because a decrease of cerebrovascular CO2 reactivity could be deleterious in case of intraoperative stroke.

Effects of the non-selective phosphodiesterase inhibitor pentoxifylline on regional cerebral blood flow and large arteries in healthy subjects

The vasodilating properties of the non-selective phosphodiesterase (PDE) inhibitor pentoxifylline were evaluated. Pentoxifylline has been reported to increase cerebral blood flow (CBF) and improve recovery rate of stroke patients. Whether these results are due to a dilating effect on arteries or to other mechanisms is not clear. In the present double-blind crossover study, 10 healthy subjects received pentoxifylline 300 mg or placebo intravenously on separate days. Blood flow velocity in the middle cerebral artery (V(mca)) was recorded by transcranial Doppler and rCBF was measured using (133)Xenon-inhalation SPECT. High-frequency ultrasound was used for measurements of temporal and radial artery diameter. Cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) concentrations were assessed in plasma. Except for increased heart rate (P < 0.05), systolic blood pressure (P < 0.05) and plasma cAMP (P < 0.001), no significant differences in CBF, rCBF(mca) or plasma cGMP were seen between placebo and pentoxifylline infusion. During pentoxifylline infusion, V(mca) decreased 7.2% (SD 12.0; P < 0.05) and temporal artery diameter increased 9.0% (SD 7.0; P < 0.001), suggesting minor dilatation of the large arteries. However, this change was not significantly different from placebo. In conclusion, pentoxifylline 300 mg had no effect on rCBF. A possible minor dilatation of the middle cerebral artery and the temporal artery cannot be excluded. Any potential clinical effect of pentoxifylline is most likely mediated through non-vascular mechanisms.

Dipyridamole dilates large cerebral arteries concomitant to headache induction in healthy subjects

Dipyridamole is used for secondary prophylaxis in ischemic stroke and as a vasodilator agent in myocardial scintigraphy. An important side effect to administering dipyridamole is headache. The aim of the current study was to investigate the effects of dipyridamole on cerebral blood flow, large artery diameter, and headache induction. Twelve healthy subjects were included in this single-blind placebo-controlled study in which placebo (0.9% NaCl) and dipyridamole 0.142 mg/kg x min were administered intravenously over 4 minutes 1 hour apart. Blood flow velocity in the middle cerebral artery (Vmax) was recorded by transcranial Doppler and regional cerebral blood flow in the middle cerebral artery (rCBFmca) was measured using single photon emission computed tomography and 133Xenon-inhalation. Blood pressure, heart rate, and pCO2 were measured repeatedly. Headache response was scored every 10 minutes on a verbal scale from 0 to 10 (10 = worst). Dipyridamole caused a decrease in pCO2 (P < 0.001). pCO2 corrected rCBFmca was 41.7 +/- 6.9 mL/100 g x min after placebo versus 41.2 +/- 6.9 after dipyridamole (P > or = 0.05). pCO2 corrected Vmca decreased 8.4% +/- 11.7 (P < 0.001) after dipyridamole, indicating a mean 5.6% +/- 6.7 (P = 0.005) relative increase of the arterial diameter. After dipyridamole the median peak headache score was 2 (range 0 to 7) compared with 0 (range 0 to 3) after placebo (P = 0.02). Dilatation of the middle cerebral artery outlasted the headache response. In conclusion, dipyridamole causes a modest pCO2 independent dilatation of the MCA, which is time-linked to the onset, but not to the cessation, of headache.

Estimating normal and pathological dynamic responses in cerebral blood flow velocity to step changes in end-tidal pCO2

The regulation of cerebral blood flow (CBF) following changes in arterial blood pressure (ABP) and end-tidal pCO2 (EtCO2) are of clinical interest in assessing cerebrovascular reserve capacity. Linear finite-impulse-response modelling is applied to ABP, EtCO2 and CBF velocity (CBFV, from transcranial Doppler measurements), which allows the CBFV response to ideal step changes in EtCO2 to be estimated from clinical data showing more sluggish, and additional random variations. The confounding effects of ABP changes provoked by hypercapnia on the CBFV are also corrected for. Data from 56 patients suffering from stenosis of the carotid arteries (with normal or diminished cerebrovascular reactivity to EtCO2 changes--CVRCO2) were analysed. The results show the expected significant differences (p < 0.05) between EtCO2 steps up and down, the significant contribution from ABP variation, and also differences in the dynamic responses of patients with reduced CVRCO2 (p < 0.01 after 10 s). For the latter the CBFV response appears exhausted after about 15 s, whereas for normals CBFV continues to increase. While dispersion of individual step responses remains large, the method gives encouraging results for the non-invasive study of compromised haemodynamics in different patient groups.

Effects of nitroglycerine on cerebral blood flow velocity, quantitative electroencephalogram and cognitive performance

BACKGROUND

Despite the widespread use of nitroglycerine, little is known of its cerebral haemodynamic effects. The present study was undertaken to evaluate the effects of nitroglycerine on cerebral circulation and cognitive function in normal volunteers.

DESIGN

Twelve healthy male volunteers (age 25 +/- 3 years; mean +/- SD) received continuous intravenous infusions of nitroglycerine at 1 mg h-1 and 2 mg h-1 rates as well as placebo (0.9% saline) under randomized, double-blind cross-over conditions on three different occasions. A 10-min quantitative EEG (qEEG) was recorded before, during and repeatedly after infusion. Blood flow velocity in the left arteria cerebri media (MCA) was measured simultaneously by means of transcranial Doppler sonography. Cognitive performance was assessed with a computerized test battery.

RESULTS

Administration of nitroglycerine caused a significant reduction of mean blood flow velocity in the left MCA. In parallel a significant increase of absolute power density < 4 Hz was noted. The latter effect was dose dependent. Cognitive functions, such as attention and memory, were not altered.

CONCLUSIONS

The decrease in cerebral blood flow velocity in the left MCA may be due to vasodilatation and/or reduction in perfusion. The concomitant increase of slow wave power in the qEEG may indicate an impairment of brain function.

Transcranial doppler sonography-ergometer test for the non-invasive assessment of cerebrovascular autoregulation in humans

Cerebrovascular hemodynamics during physical stress have been sparsely investigated, mostly through risky invasive techniques. The aim of this study was to determine the effect of ergometer stress on cerebrovascular hemodynamics in humans using the non-invasive and thus clinically-applicable method of transcranial Doppler sonography (TCD) combined with simultaneous non-invasive measurements of cardiovascular parameters. In eighteen healthy subjects (six women, twelve men; 29.3+/-4.6 years old) left midcerebral artery blood flow velocities (CBFVs) were continuously monitored using TCD during 3 min at rest, 3 min during ergometry and 3 min recovery. Simultaneously, systolic, diastolic, mean CBFVs, pulsatility index (PI), heart rate, beat-to-beat blood pressure (BP) and transcutaneous p(CO(2)) were measured. The subjects were supine with elevated trunk. Ergometry was performed by pedalling a Mühe-ergometer. In eight volunteers, the procedure was repeated within the next day to test the repeatability of the results. Heart rate increased significantly during ergometry (from 65.2+/-11 to 105. 3+/-12.3/min; P<0.05). The systolic BP increased significantly slightly later during ergometry (from 118.9+/-8.6 to 141.6+17.9 mmHg; P<0.05). Transcutaneous p(CO(2)) was initially within physiological ranges, but increased significantly after a delay during the 3rd min of cycling (from 39.7+/-3.7 to 41.1+/-4.7 mmHg; P<0.05). MFV started to rise significantly after 1 min of the exercise period (from 59.6+10.9 to 68.3+13.9 cm/s; P<0.05). PI increased immediately and significantly at the start of exercise (PI at rest 0.93+0.11; PI ergometry 1.1+0.13; P<0.05). The results were found to be reproducible in the eight volunteers. The cerebrovascular changes during ergometer exercise may reflect the combined activation of the cerebrovascular autoregulative mechanisms (neurogenic, myogenic and metabolic). The TCD-ergometer test presented here is non-invasive and would seem to present a low risk for patients who are judged fit enough for mild exercise. The test may contribute to the detection of cerebrovascular abnormalities in various diseases.

Effect of hyperventilation on brain tissue oxygenation and cerebrovenous PO2 in rats

Previous studies have shown that cortical tissue oxygenation is impaired during hyperventilation. However, it is important to quantify the effect of hyperventilation on brain tissue PO(2) and cerebrovenous PO(2) simultaneously especially since cerebral venous oxygenation is often used to assess brain tissue oxygenation. The present study was designed to measure the sagittal sinus PO(2) (PvO(2)), brain tissue PO(2) in the thalamus (PtO(2)), and brain temperature (Bt) simultaneously during acute hyperventilation. Isoflurane-anesthetized rats were hyperventilated for 10 min during which time the arterial carbon dioxide tension (PaCO(2)) dropped from 40.3+4.9 mmHg to 23.5+2.8 mmHg. PtO(2) declined from 26.0+/-4.2 mmHg to 14.8+/-5.2 mmHg (P=0.004) while brain temperature decreased from 36.5+0.3 degrees C to 36.2+0.3 degrees C (P=0.02). However, PvO(2) and arterial blood pressure (BP) did not change during hyperventilation. The maintenance of PvO(2) when perfusion is thought to decline and PtO(2) decreases suggests that there may be a diffusion limitation, possibly due to selective perfusion. Therefore, cerebrovenous PO(2) may not give a good assessment of brain tissue oxygenation especially in conditions of acute hyperventilation, and deeper brain regions other than the cortex also show impaired tissue oxygenation following hyperventilation.

Multivariate dynamic analysis of cerebral blood flow regulation in humans

The contributions of beat-to-beat changes in mean arterial blood pressure (MABP) and breath-by-breath fluctuations in end-tidal CO2 (EtCO2) as determinants of the spontaneous variability of cerebral blood flow velocity (CBFV) were studied in 16 normal subjects at rest. The two input variables (MABP and EtCO2) had significant cross-correlations with CBFV but not between them. Transfer functions were estimated as the multivariate least mean square finite impulse response causal filters. MABP showed a very significant effect in explaining CBFV variability (p < 10(-11), Fisher's aggregated-p test) and the model mean square error was significantly reduced (p < 0.001) by also including the contribution of EtCO2. The estimated mean CBFV step response to MABP displayed the characteristic return to baseline caused by the cerebral autoregulatory response. The corresponding response to EtCO2 showed a gradual rise taking approximately 10 s to reach a plateau of 2.5%/mmHg. This study demonstrated that spontaneous fluctuations in EtCO2 can help to explain the CBFV variability at rest if appropriate signal processing techniques are employed to address the limited power and bandwith of the breath-by-breath EtCO2 signal.

Cerebrovascular carbon dioxide reactivity during general anesthesia: a comparison between sevoflurane and isoflurane

We compared cerebrovascular carbon dioxide reactivity during the administration of sevoflurane and isoflurane anesthesia by measuring cerebral blood flow velocity (CBFV) as an indirect measurement of cerebral blood flow. Thirty patients, 20-70 yr old, undergoing lower abdominal surgery and without known cerebral or cardiovascular system disease, were randomly assigned to either sevoflurane or isoflurane treatment groups. Anesthesia was induced with thiopental 5 mg/kg IV and maintained with either sevoflurane or isoflurane in 67% nitrous oxide and oxygen. The CBFV and pulsatility index (PI) of the left middle cerebral artery were monitored with transcranial Doppler. The P(ETCO)2 was increased stepwise from 20 to 50 mm Hg by changing the respiratory rate with a constant tidal volume. At every 5-mm Hg stepwise change in P(ETCO)2, CBFV and PI were recorded. CBFV increased with increasing P(ETCO)2. CBFV was significantly smaller in the isoflurane group at P(ETCO)2 = 20-40 mm Hg than in the sevoflurane group. The rate of change of CBFV with changes in CO2 was larger in the isoflurane group than in the sevoflurane group. PI was constant over time and was not different between groups. In conclusion, hypocapnia-induced reduction of intracranial pressure might be more effective during the administration of isoflurane than sevoflurane.

IMPLICATIONS

Changes in cerebral blood flow caused by the changes of carbon dioxide tension are greater during the administration of isoflurane anesthesia compared with sevoflurane anesthesia. Attempts to decrease intracranial pressure by decreasing carbon dioxide tension may be more successful during isoflurane than sevoflurane anesthesia administration.

Effects of nicardipine-, nitroglycerin-, and prostaglandin E1-induced hypotension on human cerebrovascular carbon dioxide reactivity during propofol-fentanyl anesthesia

STUDY OBJECTIVE

To investigate the effects of nicardipine-, nitroglycerin-, and prostaglandine E1-induced hypotension on cerebrovascular carbon dioxide (CO2) reactivity over a wide range of arterial CO2 tension (PaCO2) (PaCO2; range 25 to 50 mmHg).

DESIGN

Prospective, randomized study.

SETTING

Operating room of a university-affiliated hospital.

PATIENTS

36 ASA physical status I and II patients without cerebrovascular disease, hypertension, or diabetes mellitus, undergoing an elective abdominal surgery.

INTERVENTIONS

Patients were randomly allocated to one of three groups (nicardipine-, nitroglycerin-, or prostaglandin E1-induced hypotension group; 12 in each group). Anesthesia was induced and maintained with a bolus dose, followed by a continuous infusion of propofol (6.7 +/- 1.5 mg/kg/hr) and fentanyl (1.68 +/- 0.4 micrograms/kg/hr). Deliberate hypotension of mean arterial pressure 55 to 60 mmHg was induced and maintained with a bolus dose, followed by a continuous infusion of nicardipine (6.80 +/- 0.75 micrograms/kg/min), nitroglycerin (3.20 +/- 1.10 micrograms/kg/min), or prostaglandin E1 (0.103 +/- 0.052 microgram/kg/min).

MEASUREMENTS AND MAIN RESULTS

Time-averaged mean red blood cell velocity in the right middle cerebral artery (Vmca) at PaCO2 ranging from 25 to 50 mmHg was measured with transcranial Doppler ultrasonography. A minimum of six simultaneous measurements of Vmca and PaCO2 were obtained during baseline and deliberate hypotension in each patient. Absolute slope between Vmca and PaCO2 during baseline and deliberate hypotension was determined individually by linear regression analysis. Absolute slope was treated as the variable, because it yielded a significant close correlation coefficient (r > 0.95; p < 0.05). Comparisons between baseline and deliberate hypotension were made by analysis of variance for repeated measures. Mean absolute slope was significantly reduced from 1.88 +/- 0.57 cm/sec/mmHg (mean +/- SD) to 1.21 +/- 0.46 in the nicardipine group (p < 0.05), from 1.75 +/- 0.69 to 1.35 +/- 0.47 in the nitroglycerin group (p < 0.05), and from 1.95 +/- 0.89 to 1.33 +/- 0.70 (p < 0.05) in the prostaglandin E1 group, respectively.

CONCLUSION

Nicardipine-, nitroglycerin-, and prostaglandin E1-induced hypotension attenuate the human cerebrovascular CO2 reactivity during propofol-fentanyl anesthesia.

The influence of increasing concentrations of nitrous oxide on cerebral blood flow velocity in hypocapnic patients with brain tumours

An increase of more than 50% in cerebral blood flow velocity in the middle cerebral artery was recently reported in hypocapnic volunteers, while inhaling 50% nitrous oxide. We measured cerebral blood flow velocity in the middle cerebral artery in 10 anaesthetized hypocapnic (ETCO2 = 25 mmHg) patients with brain tumours while administering increasing concentrations of nitrous oxide. At an end-tidal concentration of 50% and 70% nitrous oxide in oxygen, neither mean arterial pressure (base-line: 84 +/- 8 mmHg vs. (50% nitrous oxide): 82 +/- 9 mmHg and (70% nitrous oxide): 80 +/- 8 mmHg) nor cerebral blood flow velocity in the middle cerebral artery (base-line: 32 +/- 7 cm s-1 vs. (50% nitrous oxide): 34 +/- 8 cm s-1 and (70% nitrous oxide): 34 +/- 9 cm s-1) changed significantly. The data from our clinical investigation indicate that administration of increasing concentrations of nitrous oxide to already anaesthetized and hypocapnic patients does not change cerebral blood flow velocity in the middle cerebral artery.

Vestibular evoked blood flow response in the basilar artery

BACKGROUND AND PURPOSE

Monitoring of the basilar artery (BA) is difficult and has been sparsely performed. The aim of this study was to present physiological data of functional transcranial Doppler sonography (TCD) of the BA during caloric vestibular stimulation in healthy volunteers.

METHODS

TCD of the BA was performed in 26 healthy volunteers (14 women, 12 men, age 25.1+/-3 years) during caloric vestibular stimulation. Vertigo was documented using electronystagmography (ENG) and a subjective vertigo scale ranging from 0 to 10 points. Simultaneously, capnogpraphy was performed.

RESULTS

All subjects experienced vertigo, nausea and oszillopsia during vestibular irrigation. The average subjective vertigo was for a period of 106 s (+/-65.4); the average subjective estimated degree of vertigo was 6.7 points (+/-1.5). In all subjects, ENG demonstrated horizontal nystagm to the left non-irrigated side. In 14 subjects the subjective vertigo was rated by the individuals as extreme (point score > or =7) and in 12 subjects as low (point score <7). Mean flow velocity (MFV) in the BA increased significantly during vestibular irrigation, being more prominent in the initial irrigation and vertigo phase (5.8+/-5.9%, P<0.05) than in the second vertigo phase (2.2+/-8.8%, P<0.05). The calculated pulsatility index (PI), which indicates the condition of the small resistance vessels, decreased significantly (-4.9+/-8.1%; 4.3+/-8.9%, P<0.05) during both phases of vestibular activation. End tidal pCO2 did not change significantly (constant 5.4+/-0.4 Vol%), but respiration frequency was significantly increased during vestibular stimulation (12.3+/-3.8 min(-1) to 16.4+/-5.3 min(-1) and 16.3+/-4.8 min(-1), P<0.05) probably as a vegetative sign of vertigo. The observed MFV- and PI-changes were more prominent, although not quite significant, in the subgroup of subjects who experienced extreme subjective vertigo than in the subgroup who experienced low subjective vertigo.

CONCLUSION

These observations indicate that MFV increase in the posterior circulation is due to activation of the vestibulocerebellum. In addition, it is possible that the previously elaborated MFV increase in the MCA might contribute to MFV increase in the BA via the posterior communicating artery. The difference in the 2 subgroups (extreme vertigo vs. low vertigo) may reflect the great variety of anatomical and physiological conditions of the peripheral vestibular organ, the brainstem anatomy and the corresponding blood supply. For clinical purposes this TCD-test may contribute to the investigation of the vasomotor reserve of the posterior circulation, e.g. in patients with vertebrobasilar ischemia, bilateral vestibular loss or local neurodegenerative disease.

Transcranial Doppler evaluation of cerebral hemodynamics in migraineurs during prophylactic treatment with flunarizine

Transcranial Doppler (TCD) recording was used to evaluate the mean flow velocity (MFV) and cerebrovascular reactivity to CO2 in 21 migraineurs during the interictal phase. Nine were affected by migraine with aura (MwA) and 12 by migraine without aura (MwoA). During each session the middle cerebral artery (MCA) flow velocity was examined in basal conditions, in hypocapnia after a 3-min period of hyperventilation, in basal conditions a second time, and in hypercapnia after breath-holding. The same procedure was followed in a group of 21 age- and sex-matched volunteers. Recordings were performed before (T1), during (T2), and after (T3) prophylactic treatment with flunarizine (10 mg/day for 2 months) to assess the possible effect of this drug on cerebral hemodynamics. In basal condition, increased MFV values were found in both MwA and MwoA patients. In MwA patients the reactivity index (RI) to hypocapnia was significantly increased in T1 (p < 0.05). This abnormal cerebrovascular reactivity disappeared during flunarizine treatment (T2) and in the post-therapy period (T3).

Acute mountain sickness is not related to cerebral blood flow: a decompression chamber study

To evaluate the pathogenetic role of cerebral blood flow (CBF) changes occurring before and during the development of acute mountain sickness (AMS), peak mean middle cerebral artery flow velocities () were assessed by transcranial Doppler sonography in 10 subjects at 490-m altitude, and during three 12-min periods immediately (SA1), 3 (SA2), and 6 (SA3) h after decompression to a simulated altitude of 4,559 m. AMS cerebral scores increased from 0. 16 +/- 0.14 at baseline to 0.44 +/- 0.31 at SA1, 1.11 +/- 0.88 at SA2 (P < 0.05), and 1.43 +/- 1.03 at SA3 (P < 0.01); correspondingly, three, seven, and eight subjects had AMS. Absolute and relative at simulated altitude, expressed as percentages of low-altitude values (%), did not correlate with AMS cerebral scores. Average % remained unchanged, because % increased in three and remained unchanged or decreased in seven subjects at SA2 and SA3. These results suggest that CBF is not important in the pathogenesis of AMS and shows substantial interindividual differences during the first hours at simulated altitude.

Mapping of cerebrovascular reactivity using BOLD magnetic resonance imaging

Blood oxygen level-dependent (BOLD) contrast MRI is a simple non-invasive method of estimating "perfusion," and combined with a vasodilatory stimulus, may allow estimation of cerebral vascular reserve. We compared BOLD carbon dioxide (CO2) reactivity in the middle cerebral artery (MCA) perfusion territory to MCA flow velocity reactivity determined using transcranial Doppler ultrasound (TCD) in 16 patients with unilateral carotid artery stenosis or occlusion. Both BOLD and TCD reactivities were calculated from measurements acquired when the subjects were breathing air, and again when breathing a 6% CO2/air mixture, and were normalized by dividing by the difference in end tidal (ET) CO2. There was a significant correlation between interhemispheric MCA reactivity difference (contralateral-ipsilateral to the stenosis or occlusion) determined by BOLD MRI and TCD (r = 0.75, p < 0.001). In contrast, treating each hemisphere individually, there was no correlation between the absolute BOLD and TCD MCA CO2 reactivities (r = 0.08, p = 0.670). This appeared to be due to a variable BOLD signal change in the non-stenosed hemisphere between subjects, with little change in the normal hemisphere of a few subjects. In one patient, focal regions of reduced reactivity were seen in non-infarcted regions of the stenosed hemisphere, in the borderzones between arterial territories. BOLD reactivity maps provide information on the whole MCA territory reactivity, and may identify small regions of impaired reactivity which are not detected using TCD. However, BOLD reactivity maps only appear to provide semi-quantitative rather than quantitative data.

Middle cerebral artery flow velocity and cerebral oxygenation during abdominal aortic surgery

Cerebral perfusion was evaluated in twelve patients undergoing elective infra-renal abdominal aortic aneurysmectomy by transcranial Doppler ultrasonography-determined middle cerebral artery mean flow velocity, near-infrared spectroscopy-assessed cerebral oxygen saturation and systemic haemodynamic variables. The middle cerebral artery mean flow velocity and cerebral oxygen saturation decreased during cross-clamping of the aorta, and both increased upon declamping of the aorta with the oxygen saturation change lagging behind the change in the flow velocity. The changes in cerebral flow velocity and oxygen saturation paralleled the deviations in cardiac output and end-tidal carbon dioxide tension.

Vasomotor response to CO2 and L-Arginine in patients with severe internal carotid artery stenosis; pre- and post-surgical evaluation with transcranial Doppler

BACKGROUND AND AIM

Carotid artery disease may cause both thromboembolism and cerebral blood flow disturbances, particularly in subjects with impaired hemodynamic compensatory mechanisms. The aim of this study was to evaluate by transcranial Doppler (TCD) the hemodynamic changes induced by CO2 and L-Arginine stimulation in a selected population with severe unilateral carotid stenosis (70-80%), before and after carotid endarterectomy, in order to determine the effect of surgery in the vascular hemodynamics of these patients.

METHODS

We studied 20 subjects (mean age 66.4 years) consecutively admitted to our institute with ischemia and unilateral severe internal carotid artery stenosis (70-80%) detected by Color Doppler. All patients underwent arterial digital subtraction angiography to confirm the ultrasonographic evaluation. TCD was performed bilaterally; blood flow velocity was monitored during CO2 and L-Arginine stimulation both in basal conditions and three months after surgery.

RESULTS

After endarterectomy, mean velocity increased in response to both stimuli with a trend toward statistical significance. A significantly lower reactivity to L-Arginine on the stenotic side was found in the pre-operative phase: this asymmetrical reactivity was no longer observable after carotid endarterectomy.

CONCLUSIONS

We found a statistically significant difference in L-Arginine reactivity in the stenotic side of patients with severe unilateral internal carotid stenosis. This is probably related to an alteration of the endothelium function due to the carotid pathology, since the abnormalities disappeared three months after endarterectomy.

CO2 reactivity testing without blood pressure monitoring?

BACKGROUND AND PURPOSE

Responsiveness to CO2 is an established test of cerebrovascular reserve capacity. Arterial partial pressure of CO2 (PCO2) and arterial blood pressure (BP) are key parameters for cerebral blood flow. To investigate the interaction between PCO2 and BP, we performed a study with simultaneous measurement of CO2 and BP during CO2 reactivity testing with transcranial Doppler sonography.

METHODS

Eighty-one healthy volunteers, aged 19 to 74 years, underwent examination defined by a protocol with multimodality monitoring of BP, heart rate (HR), PCO2, and Doppler frequencies (DFs) of the left middle cerebral artery (MCA). Reproducibility was tested in a subgroup of 14 volunteers >/=65 years of age by CO2 reactivity testing on different days.

RESULTS

Increase of PCO2 was accompanied by a parallel increase of mean+/-SD time values of DF (3. 6+/-1.6%/mm Hg CO2). BP levels were significantly elevated after 60-second hypercapnia (mean values, 0.5+/-0.55 mm Hg/mm Hg CO2). A significant decrease over time was seen only for pulsatility in DF but not in BP. Analysis of variance and covariance with repeated measures revealed a highly significant effect of CO2 on MCA Doppler shift. A less-pronounced effect on DF was seen for BP. Correlation analysis showed no significance for CO2 reactivity, but a significant correlation between test and retest was seen in BP-related CO2 reactivity.

CONCLUSIONS

The CO2 response curve showed the known linear increase of DF. The parallel significant increase in BP most likely results from activation of the central sympathetic nervous system. The poor reproducibility for Doppler CO2 reactivity is to some extent explainable by variability of BP. CO2-induced increases in BP can have relevant influence on MCA Doppler shift and lead to misinterpretation of Doppler CO2 test results.

Intraoperative changes of transcranial Doppler velocity: relation to arterial oxygen content and whole-blood viscosity

The association of arterial oxygen content (CaO2) and viscosity with transcranial Doppler (TCD) blood flow velocity in the middle cerebral artery was studied in 20 adults without cerebrovascular disease undergoing abdominal surgery associated with significant fluctuations in hematology. TCD measurements and arterial blood samples were obtained before and directly after surgery but before blood transfusion. There was an inverse association between baseline mean velocity and CaO2 (r = -0.56), hematocrit (r = -0.50), hemoglobin (r = -0.51), and high-shear viscosity (r = -0.46). After intraoperative blood loss, intraindividual fluctuations of TCD measurements, blood oxygenation, and rheologic factors were studied. In multiple regression analysis, changes in CaO2 had the strongest association with changes in TCD values, accounting for 55% of the variation in mean velocity. Addition of hematocrit and viscosity could not account for more variation in mean velocity than CaO2 alone.

Analysis of CO2 vasomotor reactivity and vessel diameter changes by simultaneous venous and arterial Doppler recordings

BACKGROUND AND PURPOSE

The use of flow velocity changes in the middle cerebral artery (MCA) measured by Doppler techniques as an index of corresponding cerebral blood flow (CBF) changes is based on the assumption that the insonated arterial diameter remains stable. The postulate of unchanging vessel calibers during CBF changes, however, is still under debate. We performed simultaneous measurements of arterial and venous blood flow velocities by transcranial Doppler ultrasound during various stages of hypercapnia to analyze diameter changes in the insonated vessels by comparing differences in the vasomotor reactivity.

METHODS

Simultaneous Doppler recordings of 1 MCA and of a contralateral venous vessel thought to represent the sphenoparietal sinus (SPS) were carried out with a pair of 2-MHz range-gated transducers in 16 young healthy subjects during variations of end-tidal PaCO2.

RESULTS

During hypercapnia the mean blood flow velocity of the MCA rose from 62. 5+/-10.2 to a maximum of 99+/-12.2 cm/s (vasomotor reactivity of 60. 1+/-17.3%). The corresponding values in the SPS were significantly higher (P<0.001), revealing a rise from 17.8+/-5.7 to 34.9+/-14.3 cm/s (vasomotor reactivity of 91.4+/-25.9%). Exponential and linear regression analyses revealed an identical high correlation (r2=0.97 and 0.98 for the MCA and SPS, respectively). Slopes were 0.034+/-0. 01 on the arterial and 0.048+/-0.01 on the venous side. The CO2 reactivity (percentage per mm Hg, EtCO2) was found to be 4.5+/-1%/mm Hg in the MCA and 6.8+/-1.5%/mm Hg in the SPS. This difference indicates a vasodilation of the MCA in comparison to the venous vessel.

CONCLUSIONS

We have demonstrated a different reaction pattern between intracranial venous and arterial vessels related to end-tidal CO2. Relating the flow velocities to the square of the vessel diameter and assuming a global rise of CBF and not extensible sinus walls, our results indicate that the MCA undergoes a vasodilation of 9.5+/-7% in maximal hypercapnia.

Lack of effect of CO2 on cerebral arterial diameter in man

STUDY OBJECTIVE

To establish the quantitative effects on the diameter of cerebral arteries following controlled changes in arterial carbon dioxide tension (PaCO2).

DESIGN

Nonrandomized interventional study.

SETTING

Angiography suite of a tertiary referral hospital.

PATIENTS

12 anesthetized patients suffering from a cerebral arteriovenous malformation undergoing endovascular treatment.

INTERVENTION

Induced hypocapnia by hyperventilation and induced graded hypercapnia by the administration of carbon dioxide to the anesthetized patient's breathing circuit.

MEASUREMENTS AND MAIN RESULTS

A digital angiography computer was used to make computerized measurements and calculations of the diameter of deep and small cortical arteries outside the vascular territory of cerebral arteriovenous malformations following controlled and standardized changes in PaCO2. Cardiovascular parameters were simultaneously measured and cardiac output (CO) calculated. No statistically significant changes in the diameter of cerebral arteries down to a size of 0.57 mm, which was the smallest artery studied, could be observed following changes in PaCO2 in the range between 28 +/- 4 mmHg and 74 +/- 4 mmHg. However, there was a 64% change in cardiac index following the above change in PaCO2.

CONCLUSION

Deep cortical cerebral arteries down to a diameter of 0.57 mm seem to act merely as conductance vessels. The observed dramatic increase in CO following an increase in PaCO2 may offer an explanation for the changes in cerebral blood flow and cerebral flow velocity recorded by others and usually attributed to cerebral vasodilatation, which we were unable to demonstrate in this study.

Uncoupling of blood flow and metabolism in focal epilepsy

PURPOSE

Interictal measurements of cerebral blood flow are less helpful in localizing epileptic foci than are measurements of brain metabolism. This may be related to an uncoupling of blood flow and metabolism. In this study, brain metabolism and blood flow were compared in an acute experimental model of focal interictal epilepsy.

METHODS

Interictal epileptic foci were induced by an epicortical application of penicillin in rats. After 1 h, stereotyped interictal activity was initiated, lasting until the end of the experiment. Brain metabolism was determined with [14C]deoxyglucose, and cerebral blood flow with [14C]iodoan-tipyrine autoradiography.

RESULTS

In control experiments, metabolism and blood flow were coupled. In animals with focal interictal epileptic activity, the metabolism was strongly increased in the focus and reduced in areas lateral to the focus. In contralateral brain areas, blood flow and metabolism varied in a parallel fashion. Ipsilateral to the focus, however, blood flow and metabolism were altered disproportionately. In the focus, the increase of blood flow was less marked than the increase of metabolism, and the area with increased blood flow was larger than the area with increased metabolism. Lateral to the focus, in the area with a hypometabolism, blood flow was not concomitantly reduced.

CONCLUSIONS

The experiments show that blood flow and metabolism in focal epilepsy may be uncoupled in widespread regions. This is due neither to structural abnormalities nor to the duration or discharge pattern of epileptic activity. The results explain why interictal metabolic investigations have a higher predictive value in presurgical epilepsy evaluation than do interictal measurements of blood flow.

Reliability of the transient hyperemic response test in detecting changes in cerebral autoregulation induced by the graded variations in end-tidal carbon dioxide

The transient hyperemic response (THR) in the middle cerebral artery (MCA) after the release of brief compression of the ipsilateral common carotid artery has been used to study cerebral autoregulation. We conducted the present study to evaluate the reliability of THR to detect changes in cerebral autoregulation induced by graded variations in PETCO2. Seven healthy adult volunteers were recruited. Fifteen THR tests were performed on every volunteer: three at baseline PETCO2, three each at PETCO2 of 7.5 mm Hg and 15 mm Hg above the baseline, and then three each at PETCO2 of 7.5 mm Hg and 15 mm Hg below the baseline. Transient hyperemic response ratio (THRR) and strength of autoregulation (SA) were calculated using established formulae. Both THRR and SA were highly sensitive (96%) in detecting the changes in cerebral autoregulation induced by graded changes in PETCO2. The within-individual variability of SA was significantly smaller than that of THRR at all levels of PETCO2.

IMPLICATIONS

This study demonstrates the reliability of the THR test, when used for repetitive measurements, in detecting changes in cerebral autoregulation induced by graded changes in PETCO2. This test may provide a simple and noninvasive method of evaluating changes in cerebral autoregulation within an individual.

-The effect of desflurane on cerebral blood flow velocity and cerebrovascular reactivity to CO2 in children-

OBJECTIVE

To assess in children with a transcranial Doppler the effect on cerebral blood flow velocities of desflurane, whose cerebral vasodilator effects have been studied in animals and in adults with intracranial lesions.

STUDY DESIGN

Prospective clinical study.

PATIENTS

Ten healthy children, mean age: 3.4 yr, ASA physical class 1, undergoing minor urologic surgery, were included in this study.

METHOD

Induction was obtained with atropine 10 micrograms.kg-1, fentanyl 3 micrograms.kg-1 and propofol 3 mg.kg-1. Endotracheal intubation was facilitated by atracurium 0.3 mg.kg-1. Mechanical ventilation, with a 50% air/oxygen mixture was adjusted to achieve an end-tidal CO2 (PETCO2) level of 38 +/- 2 mmHg. Monitoring included measurement of mean arterial blood pressure (MAP), heart rate, PETCO2, SpO2 and end-tidal desflurane concentrations (FETDes). Mean blood flow velocities (Vmean) were measured in the middle cerebral artery using a bi-directional 2 MHz TCD system (EME-TC 2000 S). A first TCD measurement followed intubation (T1). Thereafter, desflurane was adjusted to 1 MAC. Six other TCDs were recorded each minute until FETDes reached the inspired fraction (T2-T7). Thereafter, CO2 reactivity was assessed with a hypocapnia test, induced by hyperventilation. Measures were done at T8 (PETCO2: 33 +/- 1 mmHg), T9 (PETCO2: 29 +/- 1 mmHg), and T10 (initial PETCO2: 38 +/- 1 mmHg). All these measurements were made before starting surgery. Analysis of variance (ANOVA) was used to analyse the data (P < 0.05 was considered as significant).

RESULTS

The Vmean and heart rate increased significantly with increasing concentrations of desflurane (Vmean from 68 +/- 27 to 106 +/- 30 cm.s-1 and heart rate from 109 +/- 17 to 136 +/- 15 b.min-1 between T1 and T7). During hypocapnia, Vmean decreased to 68 +/- 23 cm.s-1 at T9, and returned to normal values with PETCO2 at 38 mmHg at T10. SpO2 remained unchanged. Mean arterial pressure was stable from T1 to T7, but decreased significantly at T9 and T10.

CONCLUSION

Desflurane elicits a dose-dependent increase in cerebral blood flow velocities and heart rate, but does not change mean arterial pressure, suggesting that its cerebrovascular action is independent of its systemic vascular action. CO2 reactivity is maintained at one MAC. The results in children are similar to those seen in adults.

Assessment of cerebral pressure autoregulation in humans--a review of measurement methods

Assessment of cerebral autoregulation is an important adjunct to measurement of cerebral blood flow for diagnosis, monitoring or prognosis of cerebrovascular disease. The most common approach tests the effects of changes in mean arterial blood pressure on cerebral blood flow, known as pressure autoregulation. A 'gold standard' for this purpose is not available and the literature shows considerable disparity of methods and criteria. This is understandable because cerebral autoregulation is more a concept rather than a physically measurable entity. Static methods utilize steady-state values to test for changes in cerebral blood flow (or velocity) when mean arterial pressure is changed significantly. This is usually achieved with the use of drugs, shifts in blood volume or by observing spontaneous changes. The long time interval between measurements is a particular concern in many of the studies reviewed. Parallel changes in other critical variables, such as pCO2, haematocrit, brain activation and sympathetic tone, are rarely controlled for. Proposed indices of static autoregulation are based on changes in cerebrovascular resistance, on parameters of the linear regression of flow/velocity versus pressure changes, or only on the absolute changes in flow. The limitations of studies which assess patient groups rather than individual cases are highlighted. Newer methods of dynamic assessment are based on transient changes in cerebral blood flow (or velocity) induced by the deflation of thigh cuffs, Valsalva manoeuvres, tilting and induced or spontaneous oscillations in mean arterial blood pressure. Dynamic testing overcomes several limitations of static methods but it is not clear whether the two approaches are interchangeable. Classification of autoregulation performance using dynamic methods has been based on mathematical modelling, coherent averaging, transfer function analysis, crosscorrelation function or impulse response analysis. More research on reproducibility and inter-method comparisons is urgently needed, particularly involving the assessment of pressure autoregulation in individuals rather than patient groups.

Cerebral hemodynamics in young hypertensive subjects and effects of atenolol treatment

The aim of this study was to evaluate changes in cerebral hemodynamics in young patients with uncomplicated hypertension before and after effective antihypertensive treatment with a beta-blocker drug. Changes in mean flow velocity in the middle cerebral artery from normal condition to hypercapnia were evaluated by means of a transcranial Doppler in 42 hypertensive patients and 21 healthy subjects comparable for age and sex distribution. We obtained hypercapnia with breath-holding and evaluated cerebrovascular reactivity with the breath-holding index (BHI). After a baseline evaluation (time 0), patients were randomly assigned to a placebo (group 1) or atenolol (group 2) therapy. The evaluation was repeated after 30 (time 1) and 60 (time 2) days of treatment. Before treatment, hypertensive patients had significantly lower BHI values (0.96 +/- 0.1 group 1 and 0.85 +/- 0.3 group 2) than controls (1.69 +/- 0.4) (P < 0.0001). During treatment, mean blood pressure significantly decreased in group 2 patients. In the same group, BHI values significantly increased with respect to the pre-treatment evaluation: 1.39 +/- 0.2 at time 1 and 1.44 +/- 0.2 at time 2 (P < 0.0001). On the contrary, mean blood pressure and BHI values remained unchanged in the placebo group. Furthermore, BHI values were significantly higher in group 2 than in group 1 patients at times 1 (P < 0.001) and 2 (P < 0.0001). These findings suggest that hypertension causes reduced capability of cerebral vessels to adapt to functional changes. This condition, which is reversible after treatment, could be implicated in the increased susceptibility to ischemic stroke in hypertension.

Changes in cerebral blood flow during PaCO2 variations in patients with severe closed head injury: comparison between the Fick and transcranial Doppler methods

OBJECT

The aim of this study was to reassess whether middle cerebral artery blood flow velocity (MCAv) variations measured by transcranial Doppler ultrasonography during acute PaCO2 manipulation adequately reflect cerebral blood flow (CBF) changes in patients with severe closed head injury.

METHODS

The study was performed by comparing MCAv variations to changes in CBF as assessed by measurements of the difference in the arteriovenous content in oxygen (AVDO2). The authors initiated 35 CO2 challenges in 12 patients with severe closed head injury during the acute stage. By simultaneous recording of systemic and cerebral hemodynamic parameters, 105 AVDO2 measurements were obtained. Patients were stratified into two groups, "high" and "low," with respect to whether their resting values of MCAv were greater than 100 cm/second during moderate hyperventilation. Four patients displayed an elevated MCAv, which was related to vasospasm in three cases and to hyperemia in one case. The PaCO2 and intracranial pressure levels were not different between the two groups. The slope of the regression line between 1 divided by the change in (delta)AVDO2 and deltaMCAv was not different from identity in the low group (1/deltaAVDO2 = 1.08 x deltaMCAv - 0.07, r = 0.93, p < 0.001) and significantly differed (p < 0.05) from the slope of the high group (1/deltaAVDO2 = 1.46 x deltaMCAv - 0.4, r = 0.83, p < 0.001).

CONCLUSIONS

In patients with severe closed head injury, MCAv variations adequately reflect CBF changes as assessed by AVDO2 measurements in the absence of a baseline increase in MCAv. These observations indicate that both moderate variations in PaCO2 and variations in cerebral perfusion pressure do not act noticeably on the diameter of the MCA. The divergence from the expected relationship in the high group seems to be due to the heterogeneity of CO2-induced changes in cerebrovascular resistance between differing arterial territories.

Interaction among autoregulation, CO2 reactivity, and intracranial pressure: a mathematical model

The relationships among cerebral blood flow, cerebral blood volume, intracranial pressure (ICP), and the action of cerebrovascular regulatory mechanisms (autoregulation and CO2 reactivity) were investigated by means of a mathematical model. The model incorporates the cerebrospinal fluid (CSF) circulation, the intracranial pressure-volume relationship, and cerebral hemodynamics. The latter is based on the following main assumptions: the middle cerebral arteries behave passively following transmural pressure changes; the pial arterial circulation includes two segments (large and small pial arteries) subject to different autoregulation mechanisms; and the venous cerebrovascular bed behaves as a Starling resistor. A new aspect of the model exists in the description of CO2 reactivity in the pial arterial circulation and in the analysis of its nonlinear interaction with autoregulation. Simulation results, obtained at constant ICP using various combinations of mean arterial pressure and CO2 pressure, substantially support data on cerebral blood flow and velocity reported in the physiological literature concerning both the separate effects of CO2 and autoregulation and their nonlinear interaction. Simulations performed in dynamic conditions with varying ICP underline the existence of a significant correlation between ICP dynamics and cerebral hemodynamics in response to CO2 changes. This correlation may significantly increase in pathological subjects with poor intracranial compliance and reduced CSF outflow. In perspective, the model can be used to study ICP and blood velocity time patterns in neurosurgical patients in order to gain a deeper insight into the pathophysiological mechanisms leading to intracranial hypertension and secondary brain damage.

Can cerebrovascular reactivity be assessed without measuring blood pressure in patients with carotid artery disease?

BACKGROUND AND PURPOSE

Conventional methods of assessing cerebrovascular reactivity (CVR) omit the influence of blood pressure (BP). This study demonstrates the significant influence of BP during the assessment of CVR in patients with carotid artery disease.

METHODS

In 56 subjects the CVR was bilaterally assessed by measurement of cerebral blood flow velocity change in response to inhalation of 5% CO2 in air while BP was continuously monitored. Three methods of calculating the CVR were used: the conventional ratio between relative cerebral blood flow velocity and end-tidal CO2, simple linear regression, and multiple linear regression analysis (MLRA). The clinical significance of the difference in CVR indices was evaluated. The Bland-Altman test was applied to quantify the comparability and bias between measurements. The magnitude and significance of a change in BP during the CVR assessment were calculated in conjunction with an estimate of the velocity change attributed to the BP. The statistical significance of the data segment length on the variability and magnitude of the CVR index was computed.

RESULTS

The value of the CVR index was reduced by 20% and 6% in comparison to the conventional ratio approach when MLRA and linear regression were applied, respectively. With the use of MLRA, in 96% of cases the value of the BP coefficient was statistically significant, and in four patients the increase in velocity was primarily attributed to the increase in BP.

CONCLUSIONS

The influence of BP is significant and requires consideration when the CVR index is calculated in patients with carotid artery disease.

Myogenic cerebrovascular autoregulation in migraine measured by stress transcranial Doppler sonography

BACKGROUND AND PURPOSE

Transcranial Doppler sonography (TCD) studies may help to elucidate the nature and role of vascular abnormalities in migraine. Our aim in this study was to evaluate cerebrovascular autoregulative response in migraine patients with and without aura to blood pressure increase using stress TCD.

PATIENTS AND METHODS

Using transcranial Doppler ultrasound at rest and during ergometer stress (stress TCD), we studied the changes in mean flow velocities and resistance index (RI) in relation to physical stress in the middle cerebral artery. Fifteen migraine patients without aura, 15 migraine patients with aura, and 15 healthy control subjects were examined. Patients suffered from predominantly unilateral headache and were studied during an attack-free period. The Pourcelot's RI as a measure of cerebrovascular reactivity was calculated by dividing the difference between systolic and diastolic velocity by the systolic velocity.

RESULTS

None of the subgroups showed any difference during ergometer exercise with regard to blood pressure, endtidal CO2, heart rate, or mean flow velocity. In all subgroups, sufficient physical stress was achieved. With respect to RI change, migraine patients without aura and healthy controls did not differ (p > 0.05). However, the RI change of migraine patients with aura was significantly lower than the RI change of migraine patients without aura or healthy subjects (p > 0.05). The discrimination analysis showed in addition that RI change (absolute and as a percentage) and mean flow velocity change (as a percentage) could be used as diagnostic variables to detect patients with aura symptoms.

CONCLUSION

Differences exist in cerebrovascular reactivity in migraine patients with aura that may contribute to the neurologic disturbances in these patients during attack. We propose that there is disorder of myogenic cerebrovascular autoregulation in migraine patients with aura during headache-free intervals.

CO2 reactivity in patients after subarachnoid haemorrhage

CO2 reactivity was tested in patients with transcranial Doppler sonography (TCD) and endtidal CO2 measurements after an average time interval of ten months after subarachnoid haemorrhage (SAH). After deliberately changing breathing there was a significant change in endtidal CO2 and in flow velocities in all three examination groups. Comparing 27 patients with SAH and 5 patients treated for incidental aneurysms and 20 patients without cerebrovascular disease there were no significant differences in CO2 reactivity. Furthermore, there were no right to left differences. In 12 patients with vasospasm, two of them treated by percutaneous transluminal angioplasty for delayed ischaemic deficits, CO2 reactivity was normal at the time of investigation. Furthermore, normal CO2 reactivity was found in patients after SAH and surgery for ruptured aneurysms regardless of the severity of the SAH.

Sex dependency of cerebrovascular CO2 reactivity in normal subjects

BACKGROUND AND PURPOSE

Cerebrovascular CO2 reactivity can be assessed easily and reliably by transcranial Doppler sonography. The objectives of the present study were to evaluate sex differences in cerebral CO2 reactivity and to specify the relation between CO2 and cerebral blood flow velocity.

METHODS

CO2 reactivity of the circulation of both middle cerebral arteries was measured by bilateral transcranial Doppler sonography in 60 healthy volunteers (30 men, 30 women) aged 21 to 58 years. End-tidal carbon dioxide tensions (PETCO2) were elevated with the use of carbogene gas (95% O2, 5% CO2). In each subject the mean blood flow velocity (Vmean) was plotted as a function of PETCO2.

RESULTS

The best-fit curves for the relation of Vmean/PETCO2 were exponential functions, with the following basic equation: Vmean (cm/s) = aebx, where a is a theoretical quantity representing Vmean at a PCO2 of 0 mm Hg, b is the relative slope of the curve (slope divided by the value of the function) corresponding to the definition of reactivity, and x is the PETCO2 (mm Hg). The mean value of b was 0.037 +/- 0.008 in women and 0.030 +/- 0.010 in men. ANOVA demonstrated a significant difference between men and women (P < .001).

CONCLUSIONS

This study demonstrates a highly significant sex-related difference in CO2-induced cerebral vasomotor reactivity. The relation between altered carbon dioxide tensions and blood flow velocities of both middle cerebral arteries in 60 healthy volunteers was found to be exponential.

Evaluation of hemodynamic responses in head injury patients with transcranial Doppler monitoring

Transcranial Doppler (TCD) can monitor middle cerebral artery (MCA) velocity which can be recorded simultaneously with other physiologic parameters such as end tidal (Et) CO2, arterial blood pressure and intracranial pressure (ICP), in head injured patients. Relative changes in MCA velocity can be used to reflect relative MCA blood flow changes during ICP waves, and also to evaluate cerebral autoregulation, CO2 reactivity and hemodynamic responses to mannitol and barbiturates. The utility and practicality of short intervals of TCD monitoring to evaluate hemodynamic responses, was evaluated in a group of 22 head injured patients (average Glasgow coma score 6). During ICP A waves, MCA velocity always decreased during the peak of the wave, and during ICP B waves, fluctuated synchronously with the ICP. Dynamic cerebral autoregulation, and reactivity to CO2, were reduced within 48 hours of admission. Impaired cerebral autoregulation within 48 hours of admission did not correlate with outcome at 1 month. Mannitol infusion caused an increase in MCA velocity (15.4 +/- 7.9%) which was significantly correlated to the impairment of dynamic autoregulation (r = 0.54, p < 0.0001). The MCA velocity response to a test dose of barbiturates was significantly correlated to the ICP (r = 0.61, p < 0.01) response as well as to the CO2 reactivity (r = 0.37, p < 0.05). Continuous MCA velocity monitoring using TCD may be useful in evaluating a variety of hemodynamic responses in head injury patients and may replace more cumbersome cerebral blood flow techniques which have been used in the past for these purposes.

Cerebrovascular and cardiovascular responses to graded tilt in patients with autonomic failure

BACKGROUND AND PURPOSE

Patients with autonomic nervous system failure often experience symptoms of orthostatic intolerance while standing. It is not known whether these episodes are caused primarily by a reduced ability to regulate arterial blood pressure or whether changes in cerebral autoregulation may also be implicated.

METHODS

Eleven patients and eight healthy age- and sex-matched control subjects were studied during a graded-tilt protocol. Changes in their steady state middle cerebral artery mean flow velocities (MFV), measured by transcranial Doppler, brain-level mean arterial blood pressures (MABPbrain), and the relationship between the two were assessed.

RESULTS

Significant differences between patients and control subjects (P < .05) were found in both their MFV and MABPbrain responses to tilt. Patients' MFV dropped from 60 +/- 10.2 cm/s in the supine position to 44 +/- 14.0 cm/s at 60 degrees head-up tilt, whereas MABPbrain fell from 109 +/- 11.7 to 42 +/- 16.9 mm Hg. By comparison, controls' MFV dropped from 54 +/- 7.8 cm/s supine to 51 +/- 8.8 cm/s at 60 degrees, whereas MABPbrain went from 90 +/- 11.2 to 67 +/- 8.2 mm Hg. Linear regression showed no significant difference in the MFV-MABPbrain relationship between patients and control subjects, with slopes of 0.228 +/- 0.09 cm.s-1.mm Hg-1 for patients and 0.136 +/- 0.16 cm.s-1.mm Hg-1 for control subjects.

CONCLUSIONS

The present study found significant differences between patients and control subjects in their MFV and MABPbrain responses to tilt but no difference in the autoregulatory MFV-MABPbrain relationship. These results suggest that patients' decreased orthostatic tolerance may primarily be the result of impaired blood pressure regulation rather than a deficiency in cerebral autoregulation.

AVERAGE: a Windows program for automated analysis of event related cerebral blood flow

Functional transcranial Doppler sonography (fTCD) is used to measure changes in brain perfusion during different states of brain activity by evaluating flow velocities within the major brain arteries. We developed a computer program called AVERAGE which can be used with various TCD devices and allows for subtle quantitative off-line analysis of Doppler flow signals. AVERAGE supports data transformation, heart beat analysis, noise reduction, trigger signal and marker modification, artifact analysis and artifact rejection, as well as data reduction. Perfusion differences and their time course within two different arteries can be analyzed by parametric and non parametric statistical methods. This is important, for instance, in studies on hemispherical dominance during mental task processing. Each data processing step is supported by graphical output. If investigated bilaterally with interhemispheric analysis even minimal activations can reliably be detected and quantified with a sensitivity known from positron emission tomography approaches.

Application of interhemispheric index for transcranial Doppler sonography velocity measurements and evaluation of recording time

BACKGROUND AND PURPOSE

To validate the reliability of transcranial Doppler sonography velocity measurements in clinical settings, assessment of the reproducibility of repeated bilateral simultaneous measurements and the optimal recording time is needed. Our hypothesis was that interhemispheric indices would prove more valid than the absolute velocity measurements usually applied. The potential interference between ultrasound beams in bilateral samplings also needs evaluation.

METHODS

Thirty healthy volunteers were studied at rest within hours or with a 2-month interval between measurements. Absolute blood flow velocities and side-to-side indices between velocities obtained in the middle cerebral arteries were measured over a 30-second period by two independent examiners. The correlation coefficient (r) and the coefficients of variation of the difference between either absolute velocities (CV) or indices (CVi) were calculated. The beat-to-beat variation of the diastolic, systolic, and mean velocities was also recorded. For evaluation of ultrasound beam interference, measurements were performed with and without one probe unplugged.

RESULTS

In the interobserver study in which measurements were repeated within hours, r = .92, CV = 8.8%, and CVi = 4.1%. In the intraobserver study in which measurements were repeated with a 2-month interval, r = .8, CV = 13.0%, and CVi = 7.3%. A recording time of 30 seconds reduced CVi to 2.6%, whereas for absolute velocities 5-second recordings produced an acceptable variation. There was no significant interference between bilaterally placed probes.

CONCLUSIONS

The introduction of interhemispheric indices improve interobserver and intraobserver reproducibility by approximately 50%. We recommend use of the index in clinical settings in which unilateral velocity changes are expected. For measurement of an interhemispheric index, a recording time of 30 seconds is recommended, whereas 5-second measurement periods yield a sufficient estimate of absolute velocities.

Cerebrovascular CO2 reactivity in elderly and younger adult patients during sevoflurane anaesthesia

PURPOSE

General anaesthetic agents and aging affect cerebrovascular CO2 reactivity (CCO2R). The purpose of this study was to investigate the effect of aging on CCO2R in patients during sevoflurane anaesthesia.

METHODS

Twenty-four patients were divided into two groups of 12 according to age; 20-40 yr and 50-70 yr. Anaesthesia was induced with 5 mg.kg-1 thiopentone and maintained with sevoflurane 1.0 to 1.5% (end-tidal) and nitrous oxide 66% in oxygen to maintain anaesthesia. End-tidal CO2 tension (PETCO2) was altered from 20 to 50 mmHg in 5 mmHg steps by changing the respiratory rate. Middle cerebral blood flow velocity (CBFV) and pulsatility index (PI) were measured by transcranial Doppler (TCD) at each step change in PETCO2. The CCO2R was calculated as the change of CBFV per mmHg at each 5 mmHg interval.

RESULTS

In each group, there were no changes in blood pressure, heart rate, end-tidal sevoflurane concentration, or PI as the PETCO2 was increased from 20 to 50 mmHg. The CCO2R at PETCO2 of 35 to 50 mmHg in the younger group (0.80 +/- 0.27 (SD) cm.sec-1.mmHg-1) was larger than that in the elderly group (0.31 +/- 0.16 cm.sec-1.mmHg-1) (P < 0.01).

CONCLUSION

It is concluded that, during sevoflurane anaesthesia, CCO2R is well preserved, and that the CCO2R at PETCO2 of 35 to 50 mmHg in the 20 to 40 yr age group is greater than that in the 50 to 70 yr age group.

Inter-examiner variability of transcranial Doppler procedure and reports: a multicenter survey. Italian Transcranial Doppler Group

OBJECTIVE

To assess the clinical use of Transcranial Doppler (TCD) and the reproducibility of TCD reports.

METHODS

A multicenter survey involving 45 Italian laboratories (the Italian Transcranial Doppler Group-ITDG) was carried out by investigating the examination procedure, the parameters and normative data, and the interpretation criteria applied to reach diagnostic conclusions. The inter-examiner agreement on 10 TCD reports was computed using Kappa statistics. Results. Investigators consider both qualitative (flow direction, signal detectability) and quantitative measurements (mean flow velocity and pulsatility index values), applying their own normative criteria reference which may differ slightly between centers. TCD reports appear to be highly reproducible when based upon the interpretation of qualitative data (Kappa index: 0.95-1.00), whereas a moderate agreement is obtained when considering alterations in quantitative parameters (Kappa: 0.44-0.81).

CONCLUSIONS

The application of the same criteria in different laboratories may facilitate the standardization of TCD examinations and support the reproducibility of clinical reports based on TCD parameters.

Changes in cerebral blood flow velocity in healthy young men during overnight sleep and while awake

Using transcranial Doppler ultrasonography, we measured in 6 healthy young men cerebral blood flow velocity (CBFV) in the middle cerebral artery as well as oxygen saturation by a non-invasive method. Continuous recordings were taken starting from a point before the onset of sleep, throughout the duration of normal nighttime sleep, ending after awakening. During stages 2, 3 and 4, CBFV was approximately 15% lower than during the waking period preceding sleep. CBFV during rapid eye movement sleep did not differ from the presleep waking value, whereas the postsleep waking value was 6.6% lower. In 5 subjects CBFV showed a transient rise after sleep onset. Oxygen saturation was lower during sleep than during waking.

Transcranial Doppler ultrasonography in the neurocritical care unit

TCD ultrasonography is a noninvasive means to study the cerebral vasculature. By varying the depth and angle of insonation of a pulsed sound wave, the direction and velocity profile of the cerebral arteries can be ascertained. This can be used to identify areas of focal stenosis and increased resistance and to estimate the adequacy of cerebral flow. TCD ultrasonography commonly is used in SAH to detect cerebral vasospasm. Many centers interpret rising velocities as increasing vessel narrowing and initiate medical strategies based on these values. TCD use in head trauma is less clearly defined. TCD ultrasonography is considered an acceptable confirmatory test for the determination of brain death. TCD ultrasonography is capable of studying dynamic cerebrovascular processes. By being able to determine vessel patency, TCD may become a useful adjuvant to thrombolytic therapy. Continuous monitoring of flow velocities and profiles along with testing to cerebrovascular reserve promises to be a future active area of research.