Cerebrovascular CO2 reactivity in normotensive and hypertensive man

Transcranial Doppler sonography: nitrous oxide and cerebral blood flow velocity in children

To determine the effect of nitrous oxide (N2O) on cerebral blood flow velocity (CBFV) and cerebrovascular resistance index (RI+) in children, ten ASA physical status I or II patients aged one to eight years old, scheduled for urological procedures, were studied. Anaesthesia was induced with thiopentone 2 mg.kg-1, fentanyl 5 micrograms.kg-1 and diazepam 0.3 mg.kg-1. Muscular relaxation was ensured by using vecuronium 0.1 mg.kg-1. After tracheal intubation, anaesthesia was randomly assigned to either a mixture of air in oxygen (N2/O2) or 70% N2O in oxygen (N2O/O2) producing an FIO2 of 30%. Three sets of measurements of CBFV and RI+ were made with both gas mixtures. The CBFV and RI+ were measured in the middle cerebral artery (MCA) with a transcranial Doppler monitor. Measurements were made while using the initial gas mixture, then the second gas mixture was administered, and finally, the patient again was given the initial gas mixture. A continuous caudal epidural or lumbar epidural block was performed before skin incision. Neuromuscular blockade was maintained with vecuronium 0.05 mg.kg-1. Temperature, heart rate, end-tidal CO2, arterial oxygen saturation, haematocrit and arterial blood pressure were maintained constant. Ventilation was adjusted to achieve normocapnia. The CBFV increased when 70% N2/O2 was replaced by 70% N2O/O2 (P less than 0.05) while the CBFV decreased when 70% N2/O2 was readministered (P less than 0.05). Likewise, the CBFV decreased when 70% N2O/O2 was replaced by 70% N2/O2 (P less than 0.05) while the CBFV increased when 70% N2O/O2 was readministered (P less than 0.05).

Cerebrovascular responses to carbon dioxide in children anaesthetized with halothane and isoflurane

To determine the effects of isoflurane and halothane on cerebrovascular reactivity to CO2, 30 children aged one to six years were anaesthetized with isoflurane or halothane in an air and oxygen mixture with an FIO2 of 0.3. The end-tidal concentrations (0.5 minimum alveolar concentration (MAC) or 1.0 MAC) of isoflurane or halothane were age-adjusted. After achieving a steady-state at both 0.5 MAC and 1.0 MAC isoflurane and halothane, the end-tidal carbon dioxide tension (PETCO2) was randomly adjusted to 20, 40, or 60 mmHg. Cerebral blood flow velocity (CBFV) and the cerebrovascular resistance index (RI+) in the middle cerebral artery (MCA) were measured by a transcranial Doppler monitor. Three measurements of CBFV and RI+ were obtained at each PETCO2 and isoflurane or halothane concentration. Any rise in the PETCO2 caused an increase in CBFV during both 0.5 MAC (r2 = 0.99 and 0.99) and 1.0 MAC (r2 = 0.96 and 0.95) isoflurane and halothane anaesthesia, respectively (P less than 0.05). The CBFV for isoflurane increased as PETCO2 increased from 20 to 60 mmHg for both 0.5 MAC and 1.0 MAC (P less than 0.05). The CBFV for halothane increased as PETCO2 increased from 20 to 40 mmHg for both 0.5 MAC and 1.0 MAC halothane (P less than 0.05), but did not change as PETCO2 increased from 40 to 60 mmHg for both 0.5 MAC and 1.0 MAC halothane. The RI+ showed an inverse relationship with CBFV at each PETCO2 for 0.5 MAC (r2 = 0.98 and 0.99) and 1.0 MAC (r2 = 0.76 and 0.53) isoflurane and halothane, respectively (P less than 0.05). The CBFV did not differ significantly between 0.5 and 1.0 MAC isoflurane and halothane at corresponding PETCO2 values. The cerebrovascular response to CO2 at 20 mmHg between 0.5 MAC and 1.0 MAC halothane was not significantly different. These data strongly suggest that isoflurane and halothane in doses up to 1.0 MAC do not affect the cerebrovascular reactivity of the MCA to CO2 in anaesthetized, healthy children.

Transcranial Doppler: response of cerebral blood-flow velocity to carbon dioxide in anaesthetized children

To determine the effect of carbon dioxide on the cerebral circulation in anaesthetized infants and children, 13 healthy children, ASA physical status I or II, between three months and seven years of age and scheduled for urologic surgery, were studied. Anaesthesia was induced with thiopentone and vecuronium. After tracheal intubation, anaesthesia was maintained with 70 per cent nitrous oxide in oxygen, fentanyl 2 micrograms.kg-1, vecuronium 0.05 mg.kg-1 and 0.8-1.0 per cent end-tidal isoflurane. A caudal block was performed before surgery. Systolic arterial pressure, heart rate, oxygen saturation, temperature, and end-tidal isoflurane were maintained constant. Ventilation was adjusted to achieve an end-tidal PCO2 (PETCO2) of 20 mmHg. The PETCO2 was then randomly adjusted between 20 and 80 mmHg by the addition of carbon dioxide from an exogenous source. Cerebral blood flow velocity increased logarithmically and directly with the PETCO2 (r2 = 0.56). There were no complications associated with the use of transcranial Doppler sonography. These data indicate that CO2 has a direct effect on the velocity of blood in the middle cerebral artery in infants and children anaesthetized with isoflurane.

Lateralisation of cortical function during cognitive tasks: regional cerebral blood flow studies of normal individuals and patients with schizophrenia

To assess cognitively-related regional asymmetries of brain function, regional cerebral blood flow (rCBF) was determined by the xenon inhalation method while normal subjects performed 10 different tasks and also while they were at rest. In addition to healthy subjects, patients with schizophrenia were also studied. A total of 447 rCBF studies were carried out during the following conditions: the Wisconsin Card Sort Test, a numbers matching test, a symbols matching test, Raven's Progressive Matrices, an auditory discrimination test, an auditory control task, two versions of a visual continuous performance task, line orientation, semantic classification, and resting. On the whole, those tasks that seem to require or allow for internal verbalisation resulted in the greatest activation of the left hemisphere compared with the right; right hemisphere activation predominated only in the two tasks primarily involving attention and vigilance. Furthermore, a consistent regional topography of normal cerebral functional laterality was seen: under most conditions left prefrontal cortical activity exceeded that of right prefrontal cortex; during all non-auditory tasks, parieto-occipital cortical activity had an opposite pattern-greater right than left. During most conditions the schizophrenic patients displayed the same pattern. While several cognitively specific between-group differences were found, no single cortical region was consistently implicated and no specific direction of abnormal asymmetry predominated. These data suggest that there is a predominant task-independent functional pattern of cortical activity emphasising relatively greater left anterior and right posterior activation. This pattern may reflect the verbal and attentional primacy of these areas, respectively.

Cerebral circulation in chronic arterial hypertension

Several new concepts have emerged recently regarding the effects of chronic hypertension on cerebral blood vessels. First, hypertrophy of large cerebral arteries in chronic hypertension attenuates increases in pressure of downstream vessels and protects the cerebral microvasculature. Second, in contrast to large cerebral arteries, which become less distensible during chronic hypertension, distensibility of cerebral arterioles increases during chronic hypertension despite hypertrophy of the arteriolar wall. Third, dilatation of cerebral blood vessels with disruption of the blood-brain barrier, and not vasospasm, appears to be the critical factor in the pathogenesis of hypertensive encephalopathy. This concept is supported by the finding that cerebral edema in stroke-prone spontaneously hypertensive rats is preceded by vasodilatation and disruption of the barrier. Fourth, alterations of endothelium-mediated dilatation may impair vasodilator responses in chronic hypertension and predispose to ischemia. Finally, chronic hypertension impairs dilatation of collateral blood vessels in the cerebral circulation. The implication of this finding is that increased susceptibility to cerebral infarction in chronic hypertension may be related in part to compromised responses of the collateral circulation.

Oxygen extraction fraction at maximally vasodilated tissue in the ischemic brain estimated from the regional CO2 responsiveness measured by positron emission tomography

The oxygen extraction fraction (OEF) at maximally vasodilated tissue in patients with chronic cerebrovascular disease was evaluated using positron emission tomography. The vascular responsiveness to changes in PaCO2 was measured by the H2(15)O autoradiographic method. It was correlated with the resting-state OEF, as estimated using the 15O steady-state method. The subjects comprised 15 patients with unilateral or bilateral occlusion and stenosis of the internal carotid artery or middle cerebral artery or moyamoya disease. In hypercapnia, the scattergram between the OEF and the vascular/responsiveness to changes in PaCO2 revealed a significant negative correlation in 11 of 19 studies on these patients, and the OEF at the zero cross point of the regression line with a vascular responsiveness of 0 was 0.53 +/- 0.08 (n = 11). This OEF in the resting state corresponds to exhaustion of the capacity for vasodilation. The vasodilatory capacity is discussed in relation to the lower limit of autoregulation.

Cortical "stress tests" in schizophrenia: regional cerebral blood flow studies

A total of 261 regional cerebral blood flow (rCBF) studies were carried out on 34 medication-free patients with chronic schizophrenia and 50 normal subjects. rCBF, an indicator of local cortical metabolism and activity, was measured during the resting state and also during four cognitive activation tasks or "cortical stress tests." The latter included the Wisconsin Card Sort (WCS), a test of prefrontal lobe function; a simple numbers matching task, and two versions of a visual Continuous Performance Task (CPT). Multivariate comparisons of the two subject groups were performed for each of the five testing conditions, and discriminant function analyses for each condition were carried out to define mathematical models that differentiated normal subjects from medication-free patients. The best such model was determined and was then applied to another group of patients who had diagnoses other than schizophrenia or for whom the diagnosis was unclear. This group included two patients with clinical "frontal lobe syndrome" and radiological evidence of frontal lobe damage. The most robust differences between the groups were seen in frontal rCBF during the WCS. In the discriminant function analysis, rCBF during the WCS was the best discriminator between the two groups, retrospectively classifying 85% of the subjects correctly. rCBF during the resting state and one of the CPTs correctly classified subjects at a rate only marginally better than chance. When the model derived from WCS rCBF was applied to a second group of patients, the two patients with known frontal lobe disease were classified as "schizophrenic" with 100% certainty. Three other patients with psychotic illnesses were also assigned to this group with greater than 80% certainty, whereas a patient with character disorder (rule-out affective disorder) was classified as "normal" with a high level of confidence. These data suggest (1) that schizophrenia is characterized by a deficit in prefrontal function that is revealed when regionally specific demand exceeds the physiological capacity, and (2) that functional brain imaging studies, such as rCBF, can best identify brain abnormalities during "cortical stress tests."

Carbon dioxide reactivity of cerebral cortical and pial arteries in spontaneously hypertensive and normotensive rats--a morphometric study

The responsiveness of cerebral arteries to the changes in arterial carbon dioxide tension (paCO2) was studied in spontaneously hypertensive rats (SHRs) and normotensive rats (NTRs). A freeze substitution method was applied for the preparation of pial and cortical arteries for morphometrical study. Hypercapnia was induced by giving 8% CO2, and hypocapnia was provided by hyperventilation. The ratios of internal (d) to external diameter (D) (d/D ratio) of both pial and cortical arteries in SHR were not different from those in NTRs during normocapnia. In hypercapnia, the ratios of pial and larger cortical arteries (D greater than or equal to 20 micron) in SHRs were 80.9 +/- 0.8% and 78.6 +/- 0.6%, respectively, being significantly smaller than 86.2 +/- 0.7% and 82.2 +/- 0.5% in NTR. In contrast, the d/D ratio of pial arteries in hypocapnia was 72.5 +/- 1.4% in SHRs, which was significantly larger than 67.5 +/- 1.4% in NTRs. The responsiveness of smaller cortical arteries (D less than 20 micron) to paCO2 was not different between SHRs and NTRs. The present results suggest that in SHRs cerebrovascular CO2 reactivity is decreased as compared to NTRs, especially in pial and larger cortical arteries.

Transcranial Doppler CO2 test for the detection of hemodynamically critical carotid artery stenoses and occlusions

Cerebral CO2-reactivity was tested by transcranial Doppler sonography (Doppler CO2 test) in 232 patients. Time averaged flow velocity in the middle cerebral artery at the 40 mm Hg blood pCO2 level was taken as a reference point, and the relative increase of flow in hypercapnia of 46.5 mm Hg pCO2 was defined as "Normalized Autoregulatory Response" (NAR). A total of 82 patients with no evidence of cerebrovascular disease gave "normal" values for NAR (23.2 +/- 5.2 SD). In 150 patients with 233 stenoses and occlusions of the internal carotid artery NAR was significantly decreased in higher-grade stenoses (P = 0.01 for 80% diameter reduction, P less than 10(-6) for 90% or more). In such stenoses, patients with NAR less than 14 had suffered more frequently (P less than 0.01) from ipsilateral transient ischemic attacks and/or stroke during the previous 6 months than patients with "normal" NAR. Preoperative NAR less than 14 always improved to "normal" values following carotid surgery, while preoperative NAR greater than 19 remained unchanged (60 cases). The transcranial Doppler CO2 test is thought to be a reliable noninvasive method to detect hemodynamically critical carotid stenoses and occlusions. This may be of interest in selecting patients for superficial temporal artery-middle cerebral artery bypass and carotid surgery. For practical use 4 categories of NAR are suggested.

Transcranial measurement of blood velocities in the basal cerebral arteries using pulsed Doppler ultrasound: velocity as an index of flow

Blood velocities have been measured transcranially, at small Doppler angles, in the middle cerebral artery of normal volunteers. Cerebral blood flow was changed by varying carbon dioxide tension. In four volunteers, the relationships between arterial pCO2 and percentage change in intensity weighted mean, median, and maximum Doppler-shifted frequencies in the internal carotid and middle cerebral arteries were linear with slopes of 2.5 and 2.8% per mm Hg change in pCO2. In 38 volunteers, the relationship between end-expiratory pCO2 and time-averaged maximum Doppler frequency was linear over the range of pCO2 20-60 mm Hg with slopes of 2.5 and 2.9 percentage change per mm Hg, for internal carotid and middle cerebral, respectively. These results are very similar to those reported using direct methods of measuring cerebral blood flow. As the transcranial Doppler method is reproducible, this indicates that changes in middle cerebral blood velocity may be used to monitor changes in flow.

Treatment of hypertensive emergencies with the calcium channel blocker nifedipine

The severe elevations in blood pressure that occur in hypertensive emergencies pose a serious threat to life or vital organ functions. However, use of antihypertensive agents to acutely reduce blood pressure during hypertensive emergencies may cause deficits in the perfusion of the central nervous system or the heart. Therefore, a knowledge of cerebral blood flow regulation during acute treatment of hypertensive emergencies is indispensable. Experience with the calcium channel blocker nifedipine in the acute treatment of patients with hypertensive emergencies has shown that this agent has a pronounced vasodilatory effect, especially in vessels with a high vasoconstrictor tone, and that it does not reduce cardiac output or cerebral blood flow. The drug is highly efficacious and safe, and reports of serious side effects are rare. However, nifedipine should be used with caution in patients with suspected or proved critical arteriosclerotic stenosis of the cerebral arteries, because a reduction in perfusion pressure with any drug places these patients at risk for development of ischemic symptoms. Nifedipine can be used as a first-line drug for acute reduction of blood pressure in patients with hypertensive emergencies.

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

The end-tidal carbon dioxide partial pressure (PCO2) response curves for the flow velocity in the middle cerebral artery were studied in 31 normal subjects with transcranial Doppler techniques. An exponential curve with an exponent of 0.034 mm Hg-1 was found to be a good fit to the recorded data. By means of this relationship, recordings of flow velocity in cerebral arteries can be normalized to a standard value of PCO2. Physiological aspects of cerebrovascular reactivity to PCO2 and the clinical implications of the PCO2 response curve are discussed. The normal material provides a reference for assessing pathological responses.

Local vascular response to change in carbon dioxide tension. Long term observation in the cat's brain by means of the hydrogen clearance technique

Thirty six small hydrogen sensitive electrodes were inserted into the brains of 6 cats to evaluate the local vascular response to change in PaCO2, of cortex, subcortical white matter, and caudate nucleus. Repeated measurements (617) of local cerebral blood flow (ICBF) were performed over a period of 12 weeks. Within a PaCO2 range from 19 to 96 mmHg the local response of CBF was linear in most of the regions measured. The absolute local CO2 reactivity (CO2-R) showed a positive correlation to ICBF at PaCO2 = 40 mmHg (ICBF40) with the regression line: absolute CO2-R = 0.02 ICBF40 + 0.22, r = 0.71 (p less than 0.01). Therefore relative ICBF change was calculated in relation to ICBF40 to make comparisons between the CO2 response of different measuring days and of different regions examined. No significant change in relative CO2-R was observed during the 12 weeks interval. Differences of relative CO2-R between investigated regions were insignificant. The uniformity of relative CO2 response might support the hypothesis of a direct effect of PaCO2 or pH on the vessel wall. For comparison of CBF, the individual determination of CBF40 and relative CO2-R would be necessary.

An evaluation of the plethysmographic method of measuring cranial blood flow in the new-born infant

Cranial blood flow values obtained plethysmographically in the human new-born infant have been very variable and frequently very low. We have used a doppler ultrasound velocitymeter and skull compliance measurements to investigate whether the technique itself affects the cranial arterial inflow and whether the methods of calculation used introduce bias. Blood velocities were measured in intracranial and extracranial arteries during plethysmography, i.e. before, during and after jugular vein compressions of short duration. Skull expansion after tilting was used to estimate skull compliance. In about 10% of jugular vein compressions, arterial velocities decreased immediately, due to direct pressure on the carotid artery. In the remaining compressions a fall in arterial velocities was seen four or five heartbeats after the onset of the compression. This was probably caused by a decrease in the cranial perfusion pressure consequent on rising venous pressure. Some babies demonstrated very poor skull compliance and these babies had correspondingly low estimates of cranial blood flow. We conclude that jugular venous occlusion plethysmography is not a suitable method for use in the clinical field and is only useful as a research tool in carefully selected situations.

Changes in cerebral blood flow during hyperventilation and CO2-breathing measured transcutaneously in humans by a bidirectional, pulsed, ultrasound Doppler blood velocitymeter

We have used a bidirectional pulsed ultrasound doppler system which measures the instantaneous mean velocity across the lumen of a blood vessel in order to determine the relationship between alveolar PCO2 (PACO2) and blood flow in the four arteries supplying the brain in humans. Both high and low PACO2-values were explored. Six subjects, 3 males and 3 females (22-40 years) were studied by use of this non-invasive technique. To increase the PACO2 the subjects were breathing 4, 6 and 8% CO2 in air. PACO2 was reduced by voluntary hyperventilation down to a chosen end-expiratory PCO2 value of about 2.2 kPa. We found a linear relationship between arterial blood flow expressed as a percentage of control level and PACO2 in the range from 3.3 to 7.3 kPa. At the very lowest PACO2 values a levelling off of the response, with flow values of 40 to 45%, was observed. The CO2-reactivities in the 6 persons varied between 28.1 and 30.0%/kPa. The time course and the magnitude of the flow response were similar in all four arteries.

Normal human aging and cerebral vasoconstrictive responses to hypocapnia

Cerebral vasoconstrictive capacitance was measured during voluntary hyperventilation hypocapnia in 22 healthly normal volunteers aged 21--65 years by serial 133Xe inhalation estimates of rCBF by the initial slope index method of Risberg (ISI2) in the steady state followed by the hypocapnic state. End-tidal PCO2 was monitored by a capnograph. There was significant linear correlation between reduction of PECO2 and the ISI2 values. Significant reduction of cerebral vasoconstrictive response to hypocapnia was found with normal advancing age which is attributed to (1) minor atherosclerosis or loss of elasticity of cerebral vessels with advancing age, (2) the presence of an ischemic threshold during hyperventilation at which CBF tends to stabilize.