Lack of effect of CO2 on cerebral arterial diameter in man

Sleep deprivation impairs neurovascular coupling and cerebral vasomotor reactivity

Sleep deficiency increases the risk of cerebrovascular diseases. However, the effects of sleep deprivation (SD) on cerebral blood flow have been poorly studied. We examined the effect of 24-h of SD on the resting posterior cerebral artery (PCA) and middle cerebral artery (MCA) flow velocities (FV), the visually evoked FV response in the PCA (neurovascular coupling), and the hypercapnia-induced FV response in the MCA (cerebral vasoreactivity). Visual evoked potential (VEP) and transcranial Doppler examinations were performed in 25 healthy adults before and after 24-h of SD. Cerebral vasoreactivity was measured by breath-holding test in left and right MCA. The visually evoked FV response was evaluated in left and right PCA. There was a tendency for increased resting mean FV in PCA (p = 0.08) and MCA (p = 0.07) after SD. Both the visually evoked FV response in the PCA and the hypercapnia-induced FV increase in the MCA were significantly lower after than before SD, however, no change in VEP amplitudes was found. Our study suggested that the impaired functional stimulation-evoked FV response after SD was not due to a reduced neuronal activation, but probably to a decreased vasodilatory response. Negative effects of SD on cerebral hemodynamics were also demonstrated by reduced cerebral vasoreactivity.

The effect of continuous positive airway pressure therapy on early atherosclerosis in patients with severe obstructive sleep apnea-hypopnea syndrome

PURPOSE

Obstructive sleep apnea-hypopnea syndrome (OSAHS) is the most common sleep-related breathing disorder. Longer term, repeated episodes of hypercapnia and hypoxemia during sleep are associated with inflammatory and atherosclerosis-related factors. The aim of this study was to explore the effect of continuous positive airway pressure (CPAP) therapy on cerebral vasoreactivity and early atherosclerosis in patients with severe OSAHS.

METHODS

Forty-one patients with severe OSAHS were enrolled. The mean follow-up time was 39.8 ± 9.1 months. Cardiovascular risk factors were assessed, and laboratory tests, carotid artery intima-media thickness (CIMT) measurement and cerebrovascular reserve capacity (CRC) measurement were performed. After the baseline examination, 28 patients received CPAP therapy (treated group), which was not available for 13 patients (untreated group). Parameters were compared before and after treatment, between treated and untreated patients.

RESULTS

Cardiovascular risk factors, baseline polysomnographic parameters, laboratory values, CIMT and CRC of the two groups were similar at baseline. At the follow-up, CRC did not differ between the two groups, but CIMT was significantly lower in the treated group than in the untreated group (0.73 ± 0.11 mm vs. 0.84 ± 0.21 mm, p = 0.027). The CIMT of both groups increased significantly during the follow-up period (from 0.65 ± 0.11 mm to 0.73 ± 0.11 mm in the treated group, and from 0.69 ± 0.11 mm to 0.84 ± 0.21 mm in the untreated group), but the increase in the treated group was smaller than in the untreated group (0.09 ± 0.09 mm vs. 0.15 ± 0.15 mm).

CONCLUSION

In patients with severe OSAHS, CPAP treatment significantly reduced the progression of CIMT.

Nitric oxide-sensitive guanylyl cyclase signaling affects CO2-dependent but not pressure-dependent regulation of cerebral blood flow

Cerebrovascular CO2 reactivity is affected by nitric oxide (NO). We tested the hypothesis that sildenafil selectively potentiates NO-cGMP signaling, which affects CO2 reactivity. Fourteen healthy males (34 ± 2 yr) were enrolled in the study. Blood pressure (BP), ECG, velocity of cerebral blood flow (CBF; measured by transcranial Doppler), and end-tidal CO2 (EtCO2) were assessed at baseline (CO2 ~39 mmHg), during hyperventilation (CO2 ~24 mmHg), during hypercapnia (CO2 ~46 mmHg), during boluses of phenylephrine (25–200 µg), and during graded head-up tilting (HUT). Measurements were repeated 1 h after 100 mg sildenafil were taken. Results showed that sildenafil did not affect resting BP, heart rate, CBF peak and mean velocities, estimated regional cerebrovascular resistance (eCVR; mean BP/mean CBF), breath/min, and EtCO2: 117 ± 2/67 ± 3 mmHg, 69 ± 3 beats/min, 84 ± 5 and 57 ± 4 cm/s, 1.56 ± 0.1 mmHg·cm−1·s−1, 14 ± 0.5 breaths/min, and 39 ± 0.9 mmHg, respectively. Sildenafil increased and decreased the hypercapnia induced in CBF and eCVR, respectively. Sildenafil also attenuated the decrease in peak velocity of CBF, 25 ± 2 vs. 20 ± 2% ( P < 0.05) and increased the eCVR, 2.5 ± 0.2 vs. 2 ± 0.2% ( P < 0.03) during hyperventilation. Sildenafil did not affect CBF despite significant increases in the eCVRs that were elicited by phenylephrine and HUT. This investigation suggests that sildenafil, which potentiates the NO-cGMP signaling, seems to affect the cerebrovascular CO2 reactivity without affecting the static and dynamic pressure-dependent mechanisms of cerebrovascular autoregulation.

Assessment of middle cerebral artery diameter during hypocapnia and hypercapnia in humans using ultra-high-field MRI

In the evaluation of cerebrovascular CO2 reactivity measurements, it is often assumed that the diameter of the large intracranial arteries insonated by transcranial Doppler remains unaffected by changes in arterial CO2 partial pressure. However, the strong cerebral vasodilatory capacity of CO2 challenges this assumption, suggesting that there should be some changes in diameter, even if very small. Data from previous studies on effects of CO2 on cerebral artery diameter [middle cerebral artery (MCA)] have been inconsistent. In this study, we examined 10 healthy subjects (5 women, 5 men, age 21-30 yr). High-resolution (0.2 mm in-plane) MRI scans at 7 Tesla were used for direct observation of the MCA diameter during hypocapnia, -1 kPa (-7.5 mmHg), normocapnia, 0 kPa (0 mmHg), and two levels of hypercapnia, +1 and +2 kPa (7.5 and 15 mmHg), with respect to baseline. The vessel lumen was manually delineated by two independent observers. The results showed that the MCA diameter increased by 6.8 ± 2.9% in response to 2 kPa end-tidal P(CO2) (PET(CO2)) above baseline. However, no significant changes in diameter were observed at the -1 kPa (-1.2 ± 2.4%), and +1 kPa (+1.4 ± 3.2%) levels relative to normocapnia. The nonlinear response of the MCA diameter to CO2 was fitted as a continuous calibration curve. Cerebral blood flow changes measured by transcranial Doppler could be corrected by this calibration curve using concomitant PET(CO2) measurements. In conclusion, the MCA diameter remains constant during small deviations of the PET(CO2) from normocapnia, but increases at higher PET(CO2) values.

Effect of ST36 Acupuncture on Hyperventilation-Induced CO 2 Reactivity of the Basilar and Middle Cerebral Arteries and Heart Rate Variability in Normal Subjects

This study was conducted to verify the effect of acupuncture on cerebral haemodynamics to provide evidence for the use of acupuncture treatment as a complementary therapy for the high-risk stroke population. The effect of ST36 acupuncture treatment on the hyperventilation-induced CO2 reactivity of the basilar and middle cerebral arteries was studied in 10 healthy male volunteers (mean age, 25.2 ± 1.5 years) using a transcranial Doppler sonography with an interval of 1 week between measurements, and a portable ECG monitoring system was used to obtain ECG data simultaneously. The CO2 reactivity of the basilar and middle cerebral arteries increased significantly after ST36 acupuncture treatment, whereas the mean arterial blood pressure and pulse rate did not change significantly. The high-frequency power significantly increased after ST36 acupuncture treatment, and the percentage increase of high-frequency power correlated significantly with the percentage increase in the CO2 reactivity of the contralateral middle cerebral artery. These data suggest that ST36 acupuncture treatment increases CO2 reactivity, indicating improvement of vasodilatory potential of the cerebral vasculature to compensate for fluctuations caused by changes in external conditions. The increase in parasympathetic tone by ST36 acupuncture treatment is responsible for this therapeutic effect.

Anxiety, pCO2 and cerebral blood flow

This study examined the effect of anxiety on cerebral blood flow at different levels of pCO2 in healthy participants (N=29). Three types of breathing were used to manipulate pCO2 in a within-subject threat-of-shock paradigm: spontaneous breathing, CO2-inhalation and hyperventilation resulting in normo-, hyper- and hypocapnia. Transcranial Doppler ultrasonography was used to measure CBF velocity (CBFv) in the right middle cerebral artery, while breathing behavior and end-tidal pCO2 were monitored. During normocapnia, elevated anxiety was clearly associated with increased CBFv. Consistent with the cerebral vasoconstrictive and vasodilating effects of, respectively, hypo- and hypercapnia, we observed a positive linear association between CBFv and pCO2. The slope of this association became steeper with increasing anxiety, indicating that anxiety enhances the sensitivity of CBFv to changes in pCO2. The findings may elucidate conflicting findings in the literature and are relevant for brain imaging relying on regional cerebral blood flow.

Effects of Hyul-Bu-Chuke-Tang on Erythrocyte Deformability and Cerebrovascular CO(2) Reactivity in Normal Subjects

Aim. Hyul-bu-chuke-tang (HCEt) is a well-known traditional herbal medicine that is used for the treatment of ischemic cerebrovascular disorders. We investigated the acute effects of HCEt on erythrocyte deformability and cerebrovascular CO₂ reactivity (CVR) in healthy male subjects. Materials and Methods. We examined erythrocyte deformability in an HCEt group (n = 14) and a control group (n = 10). CVR was measured using hyperventilation-induced CO₂ reactivity of the middle cerebral artery and transcranial Doppler (TCD) in the HCEt group (n = 11). A historical control group (n = 10) of CVR measurements was also created from our previous study. All measurements were performed prior to and 1, 2, and 3 hours after HCEt administration. Results. HCEt significantly improved erythrocyte deformability 1 hour after administration compared to the control group (2.9 ± 1.1% versus −0.6 ± 1.0%, P = 0.034). HCEt significantly improved the CVR 2 hours after administration compared to the historical control group (9.1 ± 4.0% versus −8.1 ± 4.1%, P = 0.007). The mean blood pressure and pulse rate did not vary from baseline values in either group. Conclusions. We demonstrated that HCEt improved erythrocyte deformability and CVR. Our findings suggest that an improvement in erythrocyte deformability contributes to HCEt's effect on cerebral microcirculation.

Cerebral and myocardial blood flow responses to hypercapnia and hypoxia in humans

In humans, cerebrovascular responses to alterations in arterial Pco2 and Po2 are well documented. However, few studies have investigated human coronary vascular responses to alterations in blood gases. This study investigated the extent to which the cerebral and coronary vasculatures differ in their responses to euoxic hypercapnia and isocapnic hypoxia in healthy volunteers. Participants ( n = 15) were tested at rest on two occasions. On the first visit, middle cerebral artery blood velocity ( V̄P) was assessed using transcranial Doppler ultrasound. On the second visit, coronary sinus blood flow (CSBF) was measured using cardiac MRI. For comparison with V̄P, CSBF was normalized to the rate pressure product [an index of myocardial oxygen consumption; normalized (n)CSBF]. Both testing sessions began with 5 min of euoxic [end-tidal Po2 (PetO2) = 88 Torr] isocapnia [end-tidal Pco2 (PetCO2) = +1 Torr above resting values]. PetO2 was next held at 88 Torr, and PetCO2 was increased to 40 and 45 Torr in 5-min increments. Participants were then returned to euoxic isocapnia for 5 min, after which PetO2 was decreased from 88 to 60, 52 and 45 Torr in 5-min decrements. Changes in V̄P and nCSBF were normalized to isocapnic euoxic conditions and indexed against PetCO2 and arterial oxyhemoglobin saturation. The V̄P gain for euoxic hypercapnia (%/Torr) was significantly higher than nCSBF ( P = 0.030). Conversely, the V̄P gain for isocapnic hypoxia (%/%desaturation) was not different from nCSBF ( P = 0.518). These findings demonstrate, compared with coronary circulation, that the cerebral circulation is more sensitive to hypercapnia but similarly sensitive to hypoxia.

Cerebral hemodynamic effects of acute hyperoxia and hyperventilation after severe traumatic brain injury

The purpose of this study was to examine the effects of hyperventilation or hyperoxia on cerebral hemodynamic parameters over time in patients with severe traumatic brain injury (TBI). We prospectively studied 186 patients with severe TBI. CO₂ and O₂ reactivity tests were conducted twice a day on days 1-5 and once daily on days 6-10 after injury. During hyperventilation there was a significant decrease in intracranial pressure (ICP), mean arterial pressure (MAP), jugular venous oxygen saturation (Sjvo₂), brain tissue Po₂ (Pbto₂), and flow velocity (FV). During hyperoxia there was an increase in Sjvo₂ and Pbto₂, and a small but consistent decrease in ICP, end-tidal carbon dioxide (etco₂), partial arterial carbon dioxide pressure (Paco₂), and FV. Brain tissue oxygen reactivity during the first 12 h after injury averaged 19.7 ± 3.0%, and slowly decreased over the next 7 days. The autoregulatory index (ARI; normal = 5.3 ± 1.3) averaged 2.2 ± 1.5 on day 1 post-injury, and gradually improved over the 10 days of monitoring. The ARI significantly improved during hyperoxia, by an average of 0.4 ± 1.8 on the left, and by 0.5 ± 1.8 on the right. However, the change in ARI with hyperoxia was much smaller than that observed with hyperventilation. Hyperventilation increased ARI by an average of 1.3 ± 1.9 on the left, and 1.5 ± 2.0 on the right. Pressure autoregulation, as assessed by dynamic testing, was impaired in these head-injured patients. Acute hyperoxia significantly improved pressure autoregulation, although the effect was smaller than that induced by hyperventilation. The very small change in Paco₂ induced by hyperoxia does not appear to explain this finding. Rather, the vasoconstriction induced by acute hyperoxia may allow the cerebral vessels to respond better to transient hypotension. Further studies are needed to define the clinical significance of these observations.

Feeling lightheaded: the role of cerebral blood flow

OBJECTIVE

The main aims of this study were a) to investigate the relationship between lightheadedness and cerebral blood flow velocity (CBFv) during hyperventilation-induced hypocapnia, and b) to investigate whether and why the relationship between lightheadedness and CBFv may change after several episodes of this sensation.

METHODS

Three hypocapnic and three normocapnic overbreathing trials were administered in a semirandomized order to healthy participants (N = 33). Each type of breathing trial was consistently paired with one odor. Afterward, participants were presented each odor once in two spontaneous breathing and in two normocapnic overbreathing trials. CBFv in the right middle cerebral artery was measured by transcranial Doppler ultrasonography (TCD). Also breathing behavior and self-reported lightheadedness were measured continuously. Each trial was followed by a symptom checklist.

RESULTS

Self-reported lightheadedness was closely related to changes in CBFv in the hypocapnic overbreathing trials. During the subsequent normocapnic trials, however, participants experienced more lightheadedness and "feeling unreal" to the odor that had previously been paired with hyperventilation-induced hypocapnia. These complaints were not accompanied by changes in end-tidal CO(2) nor in CBFv.

CONCLUSIONS

The results show that lightheadedness is associated with changes in CBFv but that after a few episodes, the underlying mechanism for this symptom may shift to perceptual-cognitive processes. These findings may help to understand why lightheadedness occurs during emotional distress and panic. In addition, altered cerebral blood flow is unlikely to play a primary precipitating role in recurrent symptoms of lightheadedness.

Increased phase synchronization and decreased cerebral autoregulation during fainting in the young

Vasovagal syncope may be due to a transient cerebral hypoperfusion that accompanies frequency entrainment between arterial pressure (AP) and cerebral blood flow velocity (CBFV). We hypothesized that cerebral autoregulation fails during fainting; a phase synchronization index (PhSI) between AP and CBFV was used as a nonlinear, nonstationary, time-dependent measurement of cerebral autoregulation. Twelve healthy control subjects and twelve subjects with a history of vasovagal syncope underwent 10-min tilt table testing with the continuous measurement of AP, CBFV, heart rate (HR), end-tidal CO2 (ETCO2), and respiratory frequency. Time intervals were defined to compare physiologically equivalent periods in fainters and control subjects. A PhSI value of 0 corresponds to an absence of phase synchronization and efficient cerebral autoregulation, whereas a PhSI value of 1 corresponds to complete phase synchronization and inefficient cerebral autoregulation. During supine baseline conditions, both control and syncope groups demonstrated similar oscillatory changes in phase, with mean PhSI values of 0.58+/-0.04 and 0.54+/-0.02, respectively. Throughout tilt, control subjects demonstrated similar PhSI values compared with supine conditions. Approximately 2 min before fainting, syncopal subjects demonstrated a sharp decrease in PhSI (0.23+/-0.06), representing efficient cerebral autoregulation. Immediately after this period, PhSI increased sharply, suggesting inefficient cerebral autoregulation, and remained elevated at the time of faint (0.92+/-0.02) and during the early recovery period (0.79+/-0.04) immediately after the return to the supine position. Our data demonstrate rapid, biphasic changes in cerebral autoregulation, which are temporally related to vasovagal syncope. Thus, a sudden period of highly efficient cerebral autoregulation precedes the virtual loss of autoregulation, which continued during and after the faint.

Decreased upright cerebral blood flow and cerebral autoregulation in normocapnic postural tachycardia syndrome

Postural tachycardia syndrome (POTS), a chronic form of orthostatic intolerance, has signs and symptoms of lightheadedness, loss of vision, headache, fatigue, and neurocognitive deficits consistent with reductions in cerebrovascular perfusion. We hypothesized that young, normocapnic POTS patients exhibit abnormal cerebral autoregulation (CA) that results in decreased static and dynamic cerebral blood flow (CBF) autoregulation. All subjects had continuous recordings of mean arterial pressure (MAP) and CBF velocity (CBFV) using transcranial Doppler sonography in both the supine supine position and during a 70 degrees head-up tilt. During tilt, POTS patients (n = 9) demonstrated a higher heart rate than controls (n = 7) (109 +/- 6 vs. 80 +/- 2 beats/min, P < 0.05), whereas controls demonstrated a higher MAP than POTS (87 +/- 2 vs. 77 +/- 3 mmHg, P < 0.05). Also during tilt, mean CBFV decreased 19.5 +/- 2.6% in POTS patients versus 10.3 +/- 2.0% in controls (P < 0.05). We then used a transfer function analysis of MAP and CFBV in the frequency domain to quantify these changes. The low-frequency (LF; 0.04-0.15 Hz) component of CBFV variability increased during tilt in POTS patients (supine: 3 +/- 0.9 vs. tilt: 9 +/- 2, P < 0.02). In POTS patients, there was an increase in LF and high-frequency coherence between MAP and CBFV, an increase in LF gain, and a lack of significant change in phase. Static CA may be less effective in POTS patients compared with controls, since immediately after tilt CBFV decreased more in POTS patients and was highly oscillatory and autoregulation did not restore CBFV to baseline values until the subjects became supine. Dynamic CA may be less effective in POTS patients because MAP and CBFV during tilt became almost perfectly synchronous. We conclude that dynamic and static autoregulation of CBF are less effective in POTS patients compared with control subjects during orthostatic challenge.

Anesthetic management in an angiographic suite: a retrospective review of 88 cases

BACKGROUND

Advances in the field of interventional and diagnostic radiology have resulted in anesthesiologists becoming involved in angiographic suites. In the present study, we evaluated the characteristics of patients and the anesthetic management in an angiographic suite, to determine what factors influenced the patient outcome.

METHODS

Data pertaining to patients that were anesthetized at an angiographic suite in a university hospital between 1 January 2007 and 31 December 2007 were evaluated retrospectively. Specifically, we evaluated the patient characteristics and the types of anesthesia administered, to determine which factors were related to patient outcome.

RESULTS

Sixty-four percent of the patients enrolled in this study were women. Cases involving coiling for unruptured and ruptured aneurysm, embolization for intracranial arteriovenous malformation and fistula, pediatric diagnostic angiography, embolization for extracranial arteriovenous malformation, and implantable cardioverter-defibrillator (ICD) implantation all required the involvement of anesthesiologists. Major postoperatve complications included pneumonia, atelectasis, and hydrocephalus. In addition, GCS, net fluid balance, and anesthesia time had influence on patient outcome.

CONCLUSIONS

We evaluated the characteristics of patient groups, procedures, and postoperative complications in an angiographic suite. The results of our analysis revealed that a through understanding of nervous and vascular pathology, as well as knowledge of current interventional radiology, neuroanesthesia and vascular anesthesia techniques is essential for development of safe and effective care.

Modelling vascular reactivity to investigate the basis of the relationship between cerebral blood volume and flow under CO2 manipulation

Changes in cerebral blood flow (f) and vascular volume (v) are of major interest in mapping cerebral activity and metabolism, but the relation between them currently lacks a sufficient theoretical basis. To address this we considered three models: a uniform reactive tube model (M1); an extension of M1 that includes passive arterial inflow and venous volume (M2); and a more anatomically plausible model (M3) consisting of 19 compartments representing the whole range of vascular sizes and respective CO2 reactivities, derived from literature data. We find that M2 cannot be described as the simple scaling of a tube law, but any divergence from a linear approximation is negligible within the narrow physiological range encountered experimentally. In order to represent correctly the empirically observed slope of the overall v-f relationship, the reactive bed should constitute about half of the total vascular volume, thus including a significant fraction of capillaries and/or veins. Model M3 demonstrates systematic variation of the slope of the v-f relationship between 0.16 and 1.0, depending on the vascular compartment under consideration. This is further complicated when other experimental approaches such as flow velocity are used as substitute measurements. The effect is particularly large in microvascular compartments, but when averaged with larger vessels the variations in slope are contained within 0.25 to 0.55 under conditions typical for imaging methods. We conclude that the v-f relationship is not a fixed function but that both the shape and slope depend on the composition of the reactive volume and the experimental methods used.

Effects of the nitric oxide synthase inhibitor L-NMMA on cerebrovascular and cardiovascular responses to hypoxia and hypercapnia in humans

Cerebral blood flow is highly sensitive to alterations in the partial pressures of O(2) and CO(2) (P(O(2)) and P(CO(2)), respectively) in the arterial blood. In humans, the extent to which nitric oxide (NO) is involved in this regulation is unclear. We hypothesized that the NO synthase (NOS) inhibitor N(G)-monomethyl-l-arginine (l-NMMA), attenuates the sensitivity of middle cerebral artery blood velocity (V(p)) to isocapnic hypoxia (end-tidal P(O(2)) = 50 Torr) and euoxic hypercapnia (end-tidal P(CO(2)) = +9 Torr above resting values) in 10 volunteers (age, 28.7 +/- 1.3 years; height, 179.2 +/- 2.4 cm; weight, 78.0 +/- 3.7 kg; mean +/- s.e.m.). The techniques of transcranial Doppler ultrasound and dynamic end-tidal forcing were used to measure(V(p)), and control end-tidal P(O(2)) and end-tidal P(CO(2)), respectively. At baseline (isocapnic euoxia), following intravenous administration of l-NMMA, mean arterial blood pressure (MAP) increased (76.3 +/- 7.3 to 86.2 +/- 9.4 mmHg) and heart rate (HR) decreased (59.5 +/- 9.0 to 55.2 +/- 9.5 beats min(-1)) but (V(p)) was unchanged. Hypoxia-induced increases in MAP, HR and were similar with and without l-NMMA (5.0 +/- 0.7 versus 7.1 +/- 1.0 mmHg, 11.5 +/- 1.4 versus 12.4 +/- 1.5 beats min(-1), 6.5 +/- 0.8 versus 6.6 +/- 0.8 cm s(-1) for DeltaMAP, DeltaHR and Delta , respectively). Hypercapnia-induced increases in MAP, HR and (V(p)) were similar with and without l-NMMA (7.4 +/- 3.1 versus 8.1 +/- 2.2 mmHg, 10.4 +/- 4.6 versus 10.0 +/- 4.2 beats min(-1), 16.5 +/- 1.5 versus 17.6 +/- 1.5 cm s(-1) for DeltaMAP, DeltaHR and Delta(V(p)) , respectively) but the sensitivity of the(V(p)) response at the removal of hypercapnia was attenuated with l-NMMA. In young healthy humans, pharmacological blockade of nitric oxide synthesis does not affect the increases in cerebral blood flow with hypoxia and hypercapnia, suggesting that nitric oxide is not required for the cerbrovascular responses to hypoxia and hypercapnia.

Impaired cerebral CO2 vasoreactivity: association with endothelial dysfunction

Conflicting data exist on the role of nitric oxide (NO) in cerebral blood flow (CBF) autoregulation. Previous studies involving human and animal subjects seem to indicate that NO involvement is limited to the CO(2)-dependent mechanism (chemoregulation) and not to the pressure-dependent autoregulation (mechanoregulation). We tested this hypothesis in patients with impaired endothelial function compared with healthy controls. Blood pressure, heart rate, end-tidal Pco(2), CBF velocities (CBFV), forearm blood flow, and reactive hyperemia were assessed in 16 patients with diabetes mellitus and/or hypertension and compared with 12 age- and sex-matched healthy controls. Pressure-dependent autoregulation was determined by escalating doses of phenylephrine. CO(2) vasoreactivity index was extrapolated from individual slopes of mean CBFV during normocapnia, hyperventilation, and CO(2) inhalation. Measurements were repeated after sodium nitroprusside infusion. Indexes of endothelial function, maximal and area under the curve (AUC) of forearm blood flow (FBF) changes, were significantly impaired in patients (maximal flow: 488 +/- 75 vs. 297 +/- 31%; P = 0.01, AUC DeltaFBF: 173 +/- 17 vs. 127 +/- 11; P = 0.03). Patients and controls showed similar changes in cerebrovascular resistance during blood pressure challenges (identical slopes). CO(2) vasoreactivity was impaired in patients compared with controls: 1.19 +/- 0.1 vs. 1.54 +/- 0.1 cm.s(-1).mmHg(-1); P = 0.04. NO donor (sodium nitroprusside) offsets this disparity. These results suggest that patients with endothelial dysfunction have impaired CO(2) vasoreactivity and preserved pressure-dependent autoregulation. This supports our hypothesis that NO is involved in CO(2)-dependent CBF regulation alone. CBFV chemoregulation could therefore be a surrogate of local cerebral endothelial function.

Two-photon imaging of cortical surface microvessels reveals a robust redistribution in blood flow after vascular occlusion

A highly interconnected network of arterioles overlies mammalian cortex to route blood to the cortical mantle. Here we test if this angioarchitecture can ensure that the supply of blood is redistributed after vascular occlusion. We use rodent parietal cortex as a model system and image the flow of red blood cells in individual microvessels. Changes in flow are quantified in response to photothrombotic occlusions to individual pial arterioles as well as to physical occlusions of the middle cerebral artery (MCA), the primary source of blood to this network. We observe that perfusion is rapidly reestablished at the first branch downstream from a photothrombotic occlusion through a reversal in flow in one vessel. More distal downstream arterioles also show reversals in flow. Further, occlusion of the MCA leads to reversals in flow through approximately half of the downstream but distant arterioles. Thus the cortical arteriolar network supports collateral flow that may mitigate the effects of vessel obstruction, as may occur secondary to neurovascular pathology.

The authors quantify changes in blood flow in the pial arteriolar network of rodent cortex following targeted occlusions to individual vessels.

Acute hyperoxaemia-induced effects on regional blood flow, oxygen consumption and central circulation in man

AIM

Despite numerous in vitro and animal studies, circulatory effects and mechanisms responsible for the vasoconstriction seen during hyperoxaemia are yet to be ascertained. The present study set out to: (i) set up a non-invasive human model for the study of hyperoxia-induced cardiovascular effects, (ii) describe the dynamics of this effect and (iii) determine whether hyperoxaemia also, by vasoconstriction alters oxygen consumption (O(2)).

METHODS

The study comprised four experiments (A, B, C and D) on healthy volunteers examined before, during and after 100% oxygen breathing. A: Blood flow (mL min(-1).100 mL(-1) tissue), venous occlusion plethysmography was assessed (n = 12). B: Blood flow was recorded with increasing transcutaneous oxygen tension (P(tc)O(2)) levels (dose-response) (n = 8). C: Heart rate (HR), stroke volume, cardiac output (CO) and systemic vascular resistance (SVR) was assessed using echocardiography (n = 8). D: O(2) was measured using an open circuit technique when breathing an air-O(2) mix (fraction of inhaled oxygen: F(i)O(2) = 0.58) (n = 8).

RESULTS

Calf blood flow decreased 30% during O(2) breathing. The decrease in calf blood flow was found to be oxygen dose dependent. A similar magnitude, as for the peripheral circulation, of the effect on central parameters (HR/CO and SVR) and in the time relationship was noted. Hyperoxia did not change O(2). An average of 207 (93) mL O(2) per subject was washed in during the experiments.

CONCLUSION

This model appears suitable for the investigation of O(2)-related effects on the central and peripheral circulation in man. Our findings, based on a more comprehensive (central/peripheral circulation examination) evaluation than earlier made, suggest significant circulatory effects of hyperoxia. Further studies are warranted to elucidate the underlying mechanisms.

The effects of carbon dioxide on oxygenation and systemic, cerebral, and pulmonary vascular hemodynamics after the bidirectional superior cavopulmonary anastomosis

OBJECTIVES

We investigated the effects of different CO(2) tensions on oxygenation, pulmonary blood flow (Qp), cerebral blood flow, and systemic blood flow (Qs) after the bidirectional superior cavopulmonary anastomosis (BCPA).

BACKGROUND

Hypoxemia refractory to management of a high pulmonary vascular resistance index (PVRI) may complicate recovery from the BCPA.

METHODS

After BCPA, CO(2) was added to the inspired gas of mechanically ventilated patients. The Qp, Qs, PVRI, and systemic vascular resistance index (SVRI) were calculated from oxygen consumption, intravascular pressures, and oxygen saturations. Cerebral blood flow was estimated by near infrared spectroscopy and transcranial Doppler.

RESULTS

In nine patients (median age 7.1, range 2 to 23 months), arterial oxygen tension increased significantly (p < 0.005) from 36 +/- 6 mm Hg to 44 +/- 6 to 50 +/- 7 mm Hg at arterial carbon dioxide tensions (PaCO(2)) of 35, 45, and 55 mm Hg, respectively and decreased to 40 +/- 8 mm Hg at PaCO(2) 40 mm Hg. At a PaCO(2) of 55 and 45 compared with 35 mm Hg, Qp, cerebral blood flow, and Qs increased significantly, PVRI, Qp/Qs, and the ratio of Qp to inferior vena caval blood flow were unchanged, but SVRI decreased.

CONCLUSIONS

We have demonstrated that after the BCPA, systemic oxygenation, Qp, Qs, and cerebral blood flow increased and SVRI decreased at CO(2) tensions of 45 and 55 mm Hg compared with 35 mm Hg. We suggest that hypoxemia after the BCPA is ameliorated by a higher PaCO(2) and that low PaCO(2) or alkalosis may be detrimental. Hypercarbic management strategies may allow earlier progression to the BCPA, which may contribute to reducing the interval morbidity in patients with a functional single ventricle.

Role of nitric oxide in the regulation of cerebral blood flow in humans: chemoregulation versus mechanoregulation

BACKGROUND

From animal studies it emerged that nitric oxide is important for the modulation of CO2-mediated cerebral blood flow (CBF chemoregulation) but not for the pressor-dependent mechanism (mechanoregulation). This hypothesis was tested in 18 healthy subjects.

METHODS AND RESULTS

Peak velocity (PV), diastolic velocity (DV), and mean velocity (MV) were measured by transcranial Doppler of the middle cerebral artery. Chemoregulation was assessed during normocapnia, hypocapnia, and after inhaled mixture of 95% O2+5% CO2. Mechanoregulation was evaluated by incremental doses of phenylephrine. Measurements were repeated during infusion of sodium nitroprusside (SNP). Regional cerebrovascular resistance (CVR) was calculated as mean blood pressure (BP)/MV. SNP infusion decreased mean BP by 7 mm Hg and CVR decreased from 1.38+/-0.08 to 1.29+/-0.09 mm Hg/cm x s(-1); P=0.01, resulting in unaffected CBF. Phenylephrine (25 to 250 microg) caused a similar increase in BP in a dose-response fashion before and during SNP infusion. Despite the increments in BP and CVR, CBF remained unaffected. During hyperventilation (end-tidal CO2 approximately 24 mm Hg), CVR increased by 75+/-3% and PV and DV decreased by 27+/-2% and 43+/-2%, respectively (P<0.001 for all). SNP infusion blunted the vasoconstrictive effect of hypocapnia; CVR increased only by 57+/-5%, and PV and DV decreased by 23+/-2% and 35+/-3%, respectively, (P<0.05 for all). Similarly, SNP augmented the vasodilatory effect of hypercapnia.

CONCLUSIONS

Exogenous nitric oxide donor affects the basal cerebral vascular tone without affecting the CBF mechanoregulation. However, it selectively affects only the chemoregulatory mechanism (CO2-dependent). Thus, the CO2-NO axis is a cardinal pathway for CBF regulation in humans.

Control system analysis of visually evoked blood flow regulation in humans under normocapnia and hypercapnia

OBJECTIVE

Among other factors, the cerebral blood flow (CBF) is regulated in accordance to the arterial CO(2) tension and the cortical activity. The CO(2) test is commonly used to measure the vascular reserve capacity. Most functional imaging studies rely on the activity-flow coupling (AFC). We aimed to combine both challenges in order to increase the insight into mechanisms of CBF regulation.

METHODS

Fifteen healthy students underwent a functional transcranial Doppler test using a visual stimulation paradigm: firstly under normocapnia and secondly under conditions of hypercapnia. Hypercapnia was induced by breathing a carbogene gas mixture of 5% CO(2) and 95% O(2). The entire time course of flow velocity adaptation in the posterior cerebral artery (PCA) was analyzed mathematically using a control system approach.

RESULTS

Resting CBF velocities increased by nearly 26% under conditions of hypercapnia, whereas the slight increase in arterial blood pressure (ABP) and the decrease in the Pourcelot-Pulsatility index (PI) were statistically not significant. From the control system parameters which were time delay, rate time, gain, attenuation and natural frequency, only the parameter rate time, indicative for the initial steepness of flow velocity increase, showed a statistically significant decrease, consistently for the peak systolic and enddiastolic flow velocity data. As concluded from the unchanged gain parameter the absolute amount of blood flow evoked by the same visual stimulus increased also by 26%.

CONCLUSION

Evaluated by Doppler measurements hypercapnia seems to influence the AFC in two ways: It decreases the steepness of the initial increase in blood flow velocity and enhances the absolute amount of blood flow evoked by the same stimulus.

Transcranial Doppler sonography and internal jugular bulb saturation during hyperventilation in patients with fulminant hepatic failure

Mechanical hyperventilation is often used to postpone or ameliorate intracranial hypertension in patients with fulminant hepatic failure (FHF). Because such treatment may critically reduce cerebral blood flow (CBF), bedside techniques to monitor CBF are warranted. In this study, we evaluated the efficacy of transcranial Doppler (TCD) sonography of the middle cerebral artery (MCA) and internal jugular bulb saturation (svJO(2)) to determine relative changes in CBF during mechanical hyperventilation in 8 patients with FHF (median age, 40 years; range, 20 to 54 years). We found that TCD and svJO(2) decreased during hyperventilation in parallel with CBF, determined by the xenon 133 ((133)Xe) washout technique. Quantitatively, the TCD method was less accurate to determine carbon dioxide (CO(2)) reactivity compared with svJO(2) and the (133)Xe technique. This indicates a slight change in MCA diameter during hyperventilation. We conclude that TCD and svJO(2) monitoring may give valuable information on relative changes in CBF during hyperventilation. However, the TCD method appears less accurate for quantitative estimation of CO(2) reactivity in patients with FHF.

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.

Anaesthesia for interventional neuroradiology

Neuroradiologists have extended their treatment modalities in the field of vascular neurosurgery. The rapidly emerging and re-engineered neuroradiological techniques confront the anaesthetist with an increasing number of patients with severe neurological disease. More of these patients will need general anaesthesia in order to facilitate the endovascular procedure, including catheter placement, deposition of embolic material, and improved imaging. Anaesthetists are challenged by additional anaesthesiological aspects previously not encountered in neuroanaesthesia. A safe anaesthetic management is based on a broad understanding of pathophysiological and technical issues that arise with the endovascular treatment of cerebral vasculopathy.

MRI measures of middle cerebral artery diameter in conscious humans during simulated orthostasis

BACKGROUND AND PURPOSE

The relationship between middle cerebral artery (MCA) flow velocity (CFV) and cerebral blood flow (CBF) is uncertain because of unknown vessel diameter response to physiological stimuli. The purpose of this study was to directly examine the effect of a simulated orthostatic stress (lower body negative pressure [LBNP]) as well as increased or decreased end-tidal carbon dioxide partial pressure (P(ET)CO(2)) on MCA diameter and CFV.

METHODS

Twelve subjects participated in a CO(2) manipulation protocol and/or an LBNP protocol. In the CO(2) manipulation protocol, subjects breathed room air (normocapnia) or 6% inspired CO(2) (hypercapnia), or they hyperventilated to approximately 25 mm Hg P(ET)CO(2) (hypocapnia). In the LBNP protocol, subjects experienced 10 minutes each of -20 and -40 mm Hg lower body suction. CFV and diameter of the MCA were measured by transcranial Doppler and MRI, respectively, during the experimental protocols.

RESULTS

Compared with normocapnia, hypercapnia produced increases in both P(ET)CO(2) (from 36+/-3 to 40+/-4 mm Hg, P<0.05) and CFV (from 63+/-4 to 80+/-6 cm/s, P<0.001) but did not change MCA diameters (from 2.9+/-0.3 to 2.8+/-0.3 mm). Hypocapnia produced decreases in both P(ET)CO(2) (24+/-2 mm Hg, P<0.005) and CFV (43+/-7 cm/s, P<0.001) compared with normocapnia, with no change in MCA diameters (from 2.9+/-0.3 to 2.9+/-0.4 mm). During -40 mm Hg LBNP, P(ET)CO(2) was not changed, but CFV (55+/-4 cm/s) was reduced from baseline (58+/-4 cm/s, P<0.05), with no change in MCA diameter.

CONCLUSIONS

Under the conditions of this study, changes in MCA diameter were not detected. Therefore, we conclude that relative changes in CFV were representative of changes in CBF during the physiological stimuli of moderate LBNP or changes in P(ET)CO(2).