Brain and muscle oxygen saturation during head-up-tilt-induced central hypovolaemia in humans

Shoulder surgery in beach chair position causing perioperative stroke: Four cases and a review of the literature

Perioperative stroke is a rare, but serious complication in shoulder-arthroscopy or arthroplasty. Recent literature suggests the beach chair position, widely used during shoulder surgery, might induce severe perioperative hypotension, and with this cerebral desaturation and ultimately perioperative stroke. In this article we report four cases of perioperative stroke in shoulder surgery. In all cases, patients underwent brief periods of hypotension, which might have caused perioperative stroke. Based on an analysis in our hospital, we think the prevalence of perioperative stroke during shoulder surgery is higher than reported. Surgeons should be aware of this risk when using the beach chair position.

Muscle oxygen saturation increases during head-up tilt-induced (pre)syncope

AIM

To evaluate whether muscle vasodilatation plays a role for hypotension developed during central hypovolaemia, muscle oxygenation (S_m O₂ ) was examined during (pre)syncope induced by head-up tilt (HUT). Skin blood flow (SkBF) and oxygenation (S_skin O₂ ) were determined because evaluation of S_m O₂ may be affected by superficial tissue oxygenation. Furthermore, we evaluated cerebral oxygenation (S_c O₂ ) and middle cerebral artery mean blood flow velocity (MCAv_mean ).

METHODS

Twenty healthy male volunteers (median age 24 years; range 19-38) were subjected to passive 50° HUT for 1 h or until (pre)syncope. S_c O₂ and S_m O₂ (near-infrared spectroscopy), MCAv_mean (transcranial Doppler) along with mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), cardiac output (CO) and total peripheral resistance (TPR) (Modelflow^® ) were determined.

RESULTS

(Pre)syncopal symptoms appeared in 17 subjects after 11 min (median; range 2-34) accompanied by a decrease in MAP, SV, CO and TPR, while HR remained elevated. During (pre)syncope, S_c O₂ decreased [73% (71-76; mean and 95% CI) to 68% (65-71), P < 0.0001] along with MCAv_mean [40 (37-43) to 32 (29-35) cm s^-1 , P < 0.0001]. In contrast, S_m O₂ increased [63 (56-69)% to 71% (65-78), P < 0.0001], while S_skin O₂ [64% (58-69) to 53% (47-58), P < 0.0001] and SkBF [71 (44-98) compared to a baseline of 99 (72-125) units, P = 0.020] were reduced.

CONCLUSION

We confirm that the decrease in MAP during HUT is associated with a reduction in indices of cerebral perfusion. (Pre)syncope was associated with an increase in S_m O₂ despite reduced S_skin O₂ and SkBF, supporting that muscle vasodilation plays an important role in the circulatory events leading to hypotension during HUT.

Hemodynamic Response Alterations in Sensorimotor Areas as a Function of Barbell Load Levels during Squatting: An fNIRS Study

Functional near-infrared spectroscopy (fNIRS) serves as a promising tool to examine hemodynamic response alterations in a sports-scientific context. The present study aimed to investigate how brain activity within the human motor system changes its processing in dependency of different barbell load conditions while executing a barbell squat (BS). Additionally, we used different fNIRS probe configurations to identify and subsequently eliminate potential exercise induced systemic confounders such as increases in extracerebral blood flow. Ten healthy, male participants were enrolled in a crossover design. Participants performed a BS task with random barbell load levels (0% 1RM (1 repetition maximum), 20% 1RM and 40% 1RM for a BS) during fNIRS recordings. Initially, we observed global hemodynamic response alterations within and outside the human motor system. However, short distance channel regression of fNIRS data revealed a focalized hemodynamic response alteration within bilateral superior parietal lobe (SPL) for oxygenated hemoglobin (HbO₂) and not for deoxygenated hemoglobin (HHb) when comparing different load levels. These findings indicate that the previously observed load/force-brain relationship for simple and isolated movements is also present in complex multi-joint movements such as the BS. Altogether, our results show the feasibility of fNIRS to investigate brain processing in a sports-related context. We suggest for future studies to incorporate short distance channel regression of fNIRS data to reduce the likelihood of false-positive hemodynamic response alterations during complex whole movements.

Organ perfusion during voluntary pulmonary hyperinflation; a magnetic resonance imaging study

Pulmonary hyperinflation is used by competitive breath-hold divers and is accomplished by glossopharyngeal insufflation (GPI), which is known to compress the heart and pulmonary vessels, increasing sympathetic activity and lowering cardiac output (CO) without known consequence for organ perfusion. Myocardial, pulmonary, skeletal muscle, kidney, and liver perfusion were evaluated by magnetic resonance imaging in 10 elite breath-hold divers at rest and during moderate GPI. Cardiac chamber volumes, stroke volume, and thus CO were determined from cardiac short-axis cine images. Organ volumes were assessed from gradient echo sequences, and organ perfusion was evaluated from first-pass images after gadolinium injection. During GPI, lung volume increased by 5.2 ± 1.5 liters (mean ± SD; P < 0.001), while spleen and liver volume decreased by 46 ± 39 and 210 ± 160 ml, respectively (P < 0.05), and inferior caval vein diameter by 4 ± 3 mm (P < 0.05). Heart rate tended to increase (67 ± 10 to 86 ± 20 beats/min; P = 0.052) as right and left ventricular volumes were reduced (P < 0.05). Stroke volume (107 ± 21 to 53 ± 15 ml) and CO (7.2 ± 1.6 to 4.2 ± 0.8 l/min) decreased as assessed after 1 min of GPI (P < 0.01). Left ventricular myocardial perfusion maximum upslope and its perfusion index decreased by 1.52 ± 0.15 s(-1) (P < 0.001) and 0.02 ± 0.01 s(-1) (P < 0.05), respectively, without transmural differences. Pulmonary tissue, spleen, kidney, and pectoral-muscle perfusion also decreased (P < 0.05), and yet liver perfusion was maintained. Thus, during pulmonary hyperinflation by GPI, CO and organ perfusion, including the myocardium, as well as perfusion of skeletal muscles, are reduced, and yet perfusion of the liver is maintained. Liver perfusion seems to be prioritized when CO decreases during GPI.

Coupling between arterial pressure, cerebral blood velocity, and cerebral tissue oxygenation with spontaneous and forced oscillations

We tested the hypothesis that transmission of arterial pressure to brain tissue oxygenation is low under conditions of arterial pressure instability. Two experimental models of hemodynamic instability were used in healthy human volunteers; (1) oscillatory lower body negative pressure (OLBNP) (N = 8; 5 male, 3 female), and; (2) maximal LBNP to presyncope (N = 21; 13 male, 8 female). Mean arterial pressure (MAP), middle cerebral artery velocity (MCAv), and cerebral tissue oxygen saturation (ScO2) were measured non-invasively. For the OLBNP protocol, between 0 and -60 mmHg negative pressure was applied for 20 cycles at 0.05 Hz, then 20 cycles at 0.1 Hz. For the maximal LBNP protocol, progressive 5 min stages of chamber decompression were applied until the onset of presyncope. Spectral power of MAP, mean MCAv, and ScO2 were calculated within the VLF (0.04-0.07 Hz), and LF (0.07-0.2 Hz) ranges, and cross-spectral coherence was calculated for MAP-mean MCAv, MAP-ScO2, and mean MCAv-ScO2 at baseline, during each OLBNP protocol, and at the level prior to pre-syncope during maximal LBNP (sub-max). The key findings are (1) both 0.1 Hz OLBNP and sub-max LBNP elicited increases in LF power for MAP, mean MCAv, and ScO2 (p ≤ 0.08); (2) 0.05 Hz OLBNP increased VLF power in MAP and ScO2 only (p ≤ 0.06); (3) coherence between MAP-mean MCAv was consistently higher (≥0.71) compared with MAP-ScO2, and mean MCAv-ScO2 (≤0.43) during both OLBNP protocols, and sub-max LBNP (p ≤ 0.04). These data indicate high linearity between pressure and cerebral blood flow variations, but reduced linearity between cerebral tissue oxygenation and both arterial pressure and cerebral blood flow. Measuring arterial pressure variability may not always provide adequate information about the downstream effects on cerebral tissue oxygenation, the key end-point of interest for neuronal viability.

Systematic review of near-infrared spectroscopy determined cerebral oxygenation during non-cardiac surgery

Near-infrared spectroscopy (NIRS) is used to monitor regional cerebral oxygenation (rScO₂) during cardiac surgery but is less established during non-cardiac surgery. This systematic review aimed (i) to determine the non-cardiac surgical procedures that provoke a reduction in rScO₂ and (ii) to evaluate whether an intraoperative reduction in rScO₂ influences postoperative outcome. The PubMed and Embase database were searched from inception until April 30, 2013 and inclusion criteria were intraoperative NIRS determined rScO₂ in adult patients undergoing non-cardiac surgery. The type of surgery and number of patients included were recorded. There was included 113 articles and evidence suggests that rScO₂ is reduced during thoracic surgery involving single lung ventilation, major abdominal surgery, hip surgery, and laparoscopic surgery with the patient placed in anti-Tredelenburg's position. Shoulder arthroscopy in the beach chair and carotid endarterectomy with clamped internal carotid artery (ICA) also cause pronounced cerebral desaturation. A >20% reduction in rScO₂ coincides with indices of regional and global cerebral ischemia during carotid endarterectomy. Following thoracic surgery, major orthopedic, and abdominal surgery the occurrence of postoperative cognitive dysfunction (POCD) might be related to intraoperative cerebral desaturation. In conclusion, certain non-cardiac surgical procedures is associated with an increased risk for the occurrence of rScO₂. Evidence for an association between cerebral desaturation and postoperative outcome parameters other than cognitive dysfunction needs to be established.

O2 supplementation to secure the near-infrared spectroscopy determined brain and muscle oxygenation in vascular surgical patients: a presentation of 100 cases

This study addresses three questions for securing tissue oxygenation in brain (rScO2) and muscle (SmO2) for 100 patients (age 71 ± 6 years; mean ± SD) undergoing vascular surgery: (i) Does preoxygenation (inhaling 100% oxygen before anesthesia) increase tissue oxygenation, (ii) Does inhalation of 70% oxygen during surgery prevent a critical reduction in rScO2 (<50%), and (iii) is a decrease in rScO2 and/or SmO2 related to reduced blood pressure and/or cardiac output?Intravenous anesthesia was provided to all patients and the intraoperative inspired oxygen fraction was set to 0.70 while tissue oxygenation was determined by INVOS 5100C. Preoxygenation increased rScO2 (from 65 ± 8 to 72 ± 9%; P < 0.05) and SmO2 (from 75 ± 9 to 78 ± 9%; P < 0.05) and during surgery rScO2 and SmO2 were maintained at the baseline level in most patients. Following anesthesia and tracheal intubation an eventual change in rScO2 correlated to cardiac output and cardiac stroke volume (coefficient of contingence = 0.36; P = 0.0003) rather to a change in mean arterial pressure and for five patients rScO2 was reduced to below 50%. We conclude that (i) increased oxygen delivery enhances tissue oxygenation, (ii) oxygen supports tissue oxygenation but does not prevent a critical reduction in cerebral oxygenation sufficiently, and (iii) an eventual decrease in tissue oxygenation seems related to a reduction in cardiac output rather than to hypotension.

Hyperventilation, cerebral perfusion, and syncope

This review summarizes evidence in humans for an association between hyperventilation (HV)-induced hypocapnia and a reduction in cerebral perfusion leading to syncope defined as transient loss of consciousness (TLOC). The cerebral vasculature is sensitive to changes in both the arterial carbon dioxide (PaCO2) and oxygen (PaO2) partial pressures so that hypercapnia/hypoxia increases and hypocapnia/hyperoxia reduces global cerebral blood flow. Cerebral hypoperfusion and TLOC have been associated with hypocapnia related to HV. Notwithstanding pronounced cerebrovascular effects of PaCO2 the contribution of a low PaCO2 to the early postural reduction in middle cerebral artery blood velocity is transient. HV together with postural stress does not reduce cerebral perfusion to such an extent that TLOC develops. However when HV is combined with cardiovascular stressors like cold immersion or reduced cardiac output brain perfusion becomes jeopardized. Whether, in patients with cardiovascular disease and/or defect, cerebral blood flow cerebral control HV-induced hypocapnia elicits cerebral hypoperfusion, leading to TLOC, remains to be established.

Cerebral hypoperfusion modifies the respiratory chemoreflex during orthostatic stress

The respiratory chemoreflex is known to be modified during orthostatic stress although the underlying mechanisms remain to be established. To determine the potential role of cerebral hypoperfusion, we examined the relationship between changes in MCA V(mean) (middle cerebral artery mean blood velocity) and ˙VE (pulmonary minute ventilation) from supine control to LBNP (lower body negative pressure; −45mmHg) at different CO(2) levels (0, 3.5 and 5% CO(2)). The regression line of the linear relationship between ˙V(E) and PETCO(2) (end-tidal CO(2)) shifted leftwards during orthostatic stress without any change in sensitivity (1.36+− 0.27 l/min per mmHg at supine to 1.06+− 0.21 l/min per mmHg during LBNP; P=0.087). In contrast, the relationship between MCA V(mean) and PETCO(2) was not shifted by LBNP-induced changes in PETCO2. However, changes in ˙V(E) from rest to LBNP were more related to changes in MCA V(mean) than changes in PETCO(2). These findings demonstrate for the first time that postural reductions in CBF (cerebral blood flow) modified the central respiratory chemoreflex by moving its operating point. An orthostatically induced decrease in CBF probably attenuated the ‘washout’ of CO(2) from the brain causing hyperpnoea following activation of the central chemoreflex.

Cerebral hypoperfusion modifies the respiratory chemoreflex during orthostatic stress

The respiratory chemoreflex is known to be modified during orthostatic stress although the underlying mechanisms remain to be established. To determine the potential role of cerebral hypoperfusion, we examined the relationship between changes in MCA Vmean (middle cerebral artery mean blood velocity) and V̇E (pulmonary minute ventilation) from supine control to LBNP (lower body negative pressure; −45mmHg) at different CO2 levels (0, 3.5 and 5% CO2). The regression line of the linear relationship between V̇E and PETCO2 (end-tidal CO2) shifted leftwards during orthostatic stress without any change in sensitivity (1.36±0.27 l/min per mmHg at supine to 1.06±0.21 l/min per mmHg during LBNP; P=0.087). In contrast, the relationship between MCA Vmean and PETCO2 was not shifted by LBNP-induced changes in PETCO2. However, changes in V̇E from rest to LBNP were more related to changes in MCA Vmean than changes in PETCO2. These findings demonstrate for the first time that postural reductions in CBF (cerebral blood flow) modified the central respiratory chemoreflex by moving its operating point. An orthostatically induced decrease in CBF probably attenuated the ‘washout’ of CO2 from the brain causing hyperpnoea following activation of the central chemoreflex.

Severe cerebral desaturation during shoulder arthroscopy in the beach-chair position

During shoulder arthroscopy in the beach-chair position, cerebral ischemia may be a serious complication because prolonged hypotension may affect regional cerebral oxygen supply. We present the cases of 2 patients in whom a reduction in mean arterial pressure after anesthesia provoked a decrease in frontal lobe oxygenation to below the level that causes presyncopal symptoms in the awake subject. In the healthy middle-aged patient, cerebral oxygenation decreased by approximately 40%, indicating that cerebral blood flow was markedly reduced, and intravenous administration of ephedrine rapidly restored cerebral oxygenation. During surgery in the beach-chair position, hypotension must be avoided, and in all patients regional, cerebral oxygenation should be monitored and optimized.

Influence of age on syncope following prolonged exercise: differential responses but similar orthostatic intolerance

Orthostatic tolerance is reduced with increasing age and following prolonged exercise. The aim of this study was to determine the effect of age on cardiovascular and cerebrovascular responses to orthostatic stress following prolonged exercise. Measurements were obtained before, and within 45 min after, 4 h of continuous running at 70-80% of maximal heart rate in nine young (Y; 27 +/- 4 years; V(O(2)max)) 59 +/- 10 ml kg(1) min(1)) and twelve older (O; 65 +/- 5 years; V(O(2)max)) 46 +/- 8 ml kg(1) min(1)) athletes. Middle cerebral artery blood flow velocity (MCAv; transcranial Doppler ultrasound), blood pressure (BP; Finometer) and stroke volume (SV) were measured continuously whilst supine and during 60 deg head-up tilt for 15 min or to pre-syncope. Orthostatic tolerance was reduced post-exercise (tilt completed (min:s, mean +/- s.d.): Pre, 14:39 +/- 0:55; Post, 5:59 +/- 4:53; P < 0.05), but did not differ with age (P > 0.05). Despite a 25% higher supine MCAv in the young, MCAv at syncope was the same in both groups (Y: 34 +/- 10 cm s(1); O: 32 +/- 13; P > 0.05). Although the hypotensive response to syncope did not differ with age, the components of BP did; SV was lowered more in the young (Y: -57 +/- 16%; O: -34 +/- 13%; P < 0.05); and total peripheral resistance was lowered in the older athletes but was unchanged in the young (Y: +8 +/- 10%; O: -21 +/- 12%; (at 10 s pre-syncope) P < 0.05). Despite a lower MCAv in the older athletes, time to syncope was similar between groups; however, the integrative mechanisms responsible for syncope did differ with age. The similar MCAv at pre-syncope indicates there is an age-independent critical cerebral blood flow threshold at which syncope occurs.

Cerebral Autoregulation and Syncope

Whatever the pathogenesis of syncope is, the ultimate common cause leading to loss of consciousness is insufficient cerebral perfusion with a critical reduction of blood flow to the reticular activating system. Brain circulation has an autoregulation system that keeps cerebral blood flow constant over a wide range of systemic blood pressures. Normally, if blood pressure decreases, autoregulation reacts with a reduction in cerebral vascular resistance, in an attempt to prevent cerebral hypoperfusion. However, in some cases, particularly in neurally mediated syncope, it can also be harmful, being actively implicated in a paradox reflex that induces an increase in cerebrovascular resistance and contributes to the critical reduction of cerebral blood flow. This review outlines the anatomic structures involved in cerebral autoregulation, its mechanisms, in normal and pathologic conditions, and the noninvasive neuroimaging techniques used in the study of cerebral circulation and autoregulation. An emphasis is placed on the description of autoregulation pathophysiology in orthostatic and neurally mediated syncope.

Capillary-oxygenation-level-dependent near-infrared spectrometry in frontal lobe of humans

Brain function requires oxygen and maintenance of brain capillary oxygenation is important. We evaluated how faithfully frontal lobe near-infrared spectroscopy (NIRS) follows haemoglobin saturation (SCap) and how calculated mitochondrial oxygen tension (PMitoO2) influences motor performance. Twelve healthy subjects (20 to 29 years), supine and seated, inhaled O2 air-mixtures (10% to 100%) with and without added 5% carbon dioxide and during hyperventilation. Two measures of frontal lobe oxygenation by NIRS (NIRO-200 and INVOS) were compared with capillary oxygen saturation (SCap) as calculated from the O2 content of brachial arterial and right internal jugular venous blood. At control SCap (78%+/-4%; mean+/-s.d.) was halfway between the arterial (98%+/-1%) and jugular venous oxygenation (SvO2; 61%+/-6%). Both NIRS devices monitored SCap (P<0.001) within approximately 5% as SvO2 increased from 39%+/-5% to 79%+/-7% with an increase in the transcranial ultrasound Doppler determined middle cerebral artery flow velocity from 29+/-8 to 65+/-15 cm/sec. When SCap fell below approximately 70% with reduced flow and inspired oxygen tension, PMitoO2 decreased (P<0.001) and brain lactate release increased concomitantly (P<0.001). Handgrip strength correlated with the measured (NIRS) and calculated capillary oxygenation values as well as with PMitoO2 (r>0.74; P<0.05). These results show that NIRS is an adequate cerebral capillary-oxygenation-level-dependent (COLD) measure during manipulation of cerebral blood flow or inspired oxygen tension, or both, and suggest that motor performance correlates with the frontal lobe COLD signal.

The postural reduction in middle cerebral artery blood velocity is not explained by PaCO2

In the normocapnic range, middle cerebral artery mean velocity (MCA Vmean) changes approximately 3.5% per mmHg carbon-dioxide tension in arterial blood (PaCO2) and a decrease in PaCO2 will reduce the cerebral blood flow by vasoconstriction (the CO2 reactivity of the brain). When standing up MCA Vmean and the end-tidal carbon-dioxide tension (PETCO2) decrease, suggesting that PaCO2 contributes to the reduction in MCA Vmean. In a fixed body position, PETCO2 tracks changes in the PaCO2 but when assuming the upright position, cardiac output (Q) decreases and its distribution over the lung changes, while ventilation (VE) increases suggesting that PETCO2 decreases more than PaCO2. This study evaluated whether the postural reduction in PaCO2 accounts for the postural decline in MCA Vmean). From the supine to the upright position, VE, Q, PETCO2, PaCO2, MCA Vmean, and the near-infrared spectrophotometry determined cerebral tissue oxygenation (CO2Hb) were followed in seven subjects. When standing up, MCA Vmean (from 65.3+/-3.8 to 54.6+/-3.3 cm s(-1) ; mean +/- SEM; P<0.05) and cO2Hb (-7.2+/-2.2 micromol l(-1) ; P<0.05) decreased. At the same time, the VE/Q ratio increased 49+/-14% (P<0.05) with the postural reduction in PETCO2 overestimating the decline in PaCO2 (-4.8+/-0.9 mmHg vs. -3.0+/-1.1 mmHg; P<0.05). When assuming the upright position, the postural decrease in MCA Vmean seems to be explained by the reduction in PETCO2 but the small decrease in PaCO2 makes it unlikely that the postural decrease in MCA Vmean can be accounted for by the cerebral CO2 reactivity alone.

Maintained cerebral and skeletal muscle oxygenation during maximal exercise in patients with liver cirrhosis

BACKGROUND/AIMS

In cirrhotic patients, insufficient redistribution of blood from splanchnic organs to the central circulation could limit blood supply to skeletal muscles and the brain during exercise.

METHODS

Eight cirrhotic patients performed incremental cycling to exhaustion (74 (49-123) W; median with range).

RESULTS

Heart rate increased from 68 (62-88)beats/min at rest to 142 (116-163)beats/min, cardiac output from 5.1 (3.3-7.2) to 12.9 (8.5-15.9)l/min, and mean arterial pressure from 89 (75-104) to 115 (92-129)mmHg (P<0.05), while the indocyanine green elimination determined hepatosplanchnic blood flow declined from 0.97 (0.55-1.46) to 0.62 (0.36-1.06)l/min (P<0.05). As assessed by near-infrared spectrophotometry, cerebral oxygenation (NIRS) was 61% (48-85%) and increased to 72% (57-86%) during exercise (P<0.05). The NIRS determined oxygenation of the vastus lateralis muscle also increased: the concentrations of oxygenated haemoglobin by 5.9 (0.57-9.47)micromol/l, deoxygenated haemoglobin by 7.2 (1.8-12.0)micromol/l, and thus total haemoglobin by 12.1 (3.6-21.5)micromol/l (P<0.05).

CONCLUSIONS

In patients with cirrhosis, exercise reduces hepatosplanchnic blood flow, while O(2) supply to muscle and brain appears to increase indicating that blood redistribution from splanchnic organs does not limit blood flow to working muscles and the brain.

Effects of hydroxyurea treatment on cerebral oxygenation in adult patients with sickle cell disease: an open-label pilot study

BACKGROUND

In patients with sickle cell disease (SCD), cerebral oxygen saturation (rSO(2)) has been reported to be below normal and to increase after red blood cell transfusion.

OBJECTIVE

This study was designed to determine the effects of long-term and short-term hydroxyurea (HU) treatment on cerebral oxygenation in patients with SCD.

METHODS

This open-label pilot study was conducted at the Department of Anesthesiology and the Center for Sickle Cell Disease, College of Medicine, Howard University, Washington, DC. Adult African American outpatients with SCD and hemoglobin (Hb) genotype HbSS (homozygous sickle Hb) who were receiving long-term (>6 months) HU treatment (15-30 mg/kg . d PO) or who had never received this treatment (control group) were enrolled. Patients in the treated and control groups were matched for age, sex, race, and Hb genotype. Cerebral oximetry (near-infrared spectroscopy) was performed to determine rSO(2) index. In a separate analysis to determine the effects of short-term HU treatment on cerebral oxygenation, hospitalized patients with SCD and vaso-occlusive crisis (VOC)receiving long-term therapy with HU were enrolled. We performed cerebral and pulse (fingernail) oximetry to determine rSO (2)index and arterial oxygen saturation (SpO(2)) after the administration of a single oral dose of HU (500-mg tablet) alone and again after dosing concomitantly with inhaled oxygen.

RESULTS

The study enrolled 11 patients in the HU group (6 women, 5 men; mean [SD] age, 37 [8] years) and 20 controls (8 women, 12 men; mean [SD] age, 35 [6] years). Mean (SD) rSO(2) index was significantly increased (but still low) in patients receiving long-term HU treatment compared with controls (46.1% [6.6%] vs 41.2% [7.6%]; P< 0.025). Hb concentration (9.6 [1.4] g/dL vs 8.5 [1.2] g/dL; P< 0.027), hematocrit (28% [3%] vs 24% [4%]; P < 0.028), and mean corpuscular volume (102% [7%] vs 89% [8%]; P < 0.027) also were significantly higher in the HU group compared with controls. In 8 patients with SCD and VOC (6 men, 2 women; mean [SD] age, 28 [5] years), single-dose HU, either alone or in combination with inhaled oxygen, did not significantly affect cerebral oxygenation, and SpO(2) failed to correlate with rSO(2) index in these patients.

CONCLUSIONS

The results of this open-label pilot study in patients with SCD suggest that the low cerebral oxygenation in these patients is significantly improved but not normalized with long-term HU treatment. A single dose of HU, either alone or in combination with inhaled oxygen, did not appear to influence cerebral oxygenation in patients with VOC.

Cerebral oxygenation during intermittent supramaximal exercise

This study examined cerebral deoxygenation during intermittent supramaximal exercise in six healthy male subjects (age: 27.2 +/- 0.6 years (mean +/- S.E.). The subjects performed seven times exercise at an intensity corresponding to 150% of maximal oxygen uptake (VO2max) on cycle ergometer (30 s exercise/15 s rest). Cerebral oxygenation was measured by near-infrared spectroscopy (NIRS). The peak blood lactate concentration after exercise was 15.3 +/- 0.2 mmol/l. Cerebral oxygenation increased in first repetition compared with at rest (+ 5.7 +/- 0.6 microM; P < 0.05), but then decreased with time. Thus, in the last repetition cerebral oxygenation was - 8.5 +/- 0.4 microM (P < 0.05). There was no significant change in arterial oxygen saturation (99.6 +/- at rest, 98.4 +/- 0.2 at the final set of intermittent exercise), and there was no correlated change in end-tidal CO2 concentration with cerebral oxygenation (P > 0.05). These findings suggest that the fatigue resulting from dynamic severe exercise related to a decrease in the cerebral oxygenation level.

Cerebral oximetry in patients with sickle cell disease

BACKGROUND

There is limited information concerning the brain's oxygen supply and demand in patients with sickle cell disease.

DESIGN

We measured near-infrared spectroscopy of brain oxygenation in 27 patients with sickle cell disease regardless of vaso-occlusive crisis, 14 normal healthy controls, and five anaemic patients without sickle cell disease. We also measured pre- and post-transfusion cerebral oximetry in 14 additional sickle cell disease patients who were on transfusion programmes.

RESULTS

The mean cerebral oxygen saturation in the combined steady-state and vaso-occlusive crisis population was found to be significantly lower than that in the controls and in anaemic patients without sickle cell disease (47.7% vs. 61.3%, 59.8%, P < 0.0001). Cerebral oxygen saturation failed to correlate with the haemoglobin concentration (r = 0.51, P > 0.5). However, cerebral oxygen saturation increased from 40.4% to 49.6% (P = 0.01) and correlated significantly with the haemoglobin level (r = 0.553, P = 0.003) in 14 subjects studied before and after transfusions. In seven subjects who received simple transfusions, cerebral oxygen saturation correlated strongly and positively with the haemoglobin level (r = 0.811, P = 0.001) and with percent normal haemoglobin (r = 0.786, P = 0.002), and negatively with abnormal sickle haemoglobin (r = -0.775, P = 0.003). None of these correlations was found to be statistically significant in the seven subjects given exchange transfusions. Cerebral oxygen saturation measured in the sickle cell disease subjects after transfusions was still significantly lower than in the anaemic subjects without sickle cell disease and in the normal controls (49.6% vs. 59.8% and 61.3%, P = 0.001).

CONCLUSIONS

We found that patients with sickle cell disease have subnormal values of cerebral oxygen saturation. Red cell transfusions significantly increased the brain oxygenation in these patients. Cerebral oximetry may be a useful, noninvasive method for assessing the effect of circulating normal red cells in sickle cell patients after transfusions.

Arterial desaturation during exercise in man: implication for O2 uptake and work capacity

Exercise-induced arterial hypoxaemia is defined as a reduction in the arterial O2 pressure (PaO2) by more than 1 kPa and/or a haemoglobin O2 saturation (SaO2) below 95%. With blood gas analyses ideally reported at the actual body temperature, desaturation is a consistent finding during maximal ergometer rowing. Arterial desaturation is most pronounced at the end of a maximal exercise bout, whereas the reduction in PaO2 is established from the onset of exercise. Exercise-induced arterial hypoxaemia is multifactorial. The ability to maintain a high alveolar O2 pressure (PAO2) is critical for blood oxygenation and this appears to be difficult in large individuals. A large lung capacity and, in turn, diffusion capacity seem to protect PaO2. A widening of the PAO2-PaO2 difference does indicate that a diffusion limitation, a ventilation-perfusion mismatch and/or a shunt influence the transport of O2 from alveoli to the pulmonary capillaries. An inspired O2 fraction of 0.30 reduces the widened PAO2-PaO2 difference by 75% and prevents a reduction of PaO2 and SaO2. With a marked increase in cardiac output, diffusion limitation combined with a fast transit time dominates the O2 transport problem. Furthermore, a postexercise reduction in pulmonary diffusion capacity suggests that the alveolo-capillary membrane is affected. An antioxidant attenuates oxidative burst by neutrophilic granulocytes, but it does not affect PaO2, SaO2 or O2 uptake (VO2), and the ventilatory response to maximal exercise also remains the same. It is proposed, though, that increased concentration of certain cytokines correlates to exercise-induced hypoxaemia as cytokines stimulate mast cells and basophilic granulocytes to degranulate histamine. The basophil count increases during maximal rowing. Equally, histamine release is associated with hypoxaemia and when the release of histamine is prevented, the reduction in PaO2 is attenuated. During maximal exercise, an extreme lactate spill-over to blood allows pH decrease to below 7.1 and according to the O2 dissociation curve this is critical for SaO2. When infusion of sodium bicarbonate maintains a stable blood buffer capacity, acidosis is attenuated and SaO2 increases from 89% to 95%. This enables exercise capacity to increase, an effect also seen when O2 supplementation to inspired air restores arterial oxygenation. In that case, exercise capacity increases less than can be explained by VO2 and CaO2. Furthermore, the change in muscle oxygenation during maximal exercise is not affected when hyperoxia and sodium bicarbonate attenuate desaturation. It is proposed that other organs benefit from enhanced O2 availability, and especially the brain appears to increase its oxygenation during maximal exercise with hyperoxia.

Cerebral oxygenation monitor during head-up and -down tilt using near-infrared spatially resolved spectroscopy

Reflectance near-infrared spectroscopy (NIRS) has become a suitable and easily manageable method to monitor cerebral oxygenation changes in presyncopal and syncopal symptoms caused by postural changing or standing. A new clinical tissue oxygenation monitor has been recently developed which measures absolute tissue haemoglobin saturation (Tissue Oxygenation Index, TOI) utilizing spatially resolved spectroscopy (SRS). The present study examined the effects of postural changes on cerebral oxygenation as reflected in SRS-NIRS findings. Cerebral oxyhaemoglobin (O2Hb), deoxyhaemoglobin (HHb), and the TOI were recorded from both sides of the forehead in five healthy male subjects (age range, 28-40 years) during 90 degrees head-up tilt (HUT) and -6 degrees head-down tilt (HDT). Three series of measurements were carried out on separate days. O2Hb was decreased during HUT. TOI was significantly lower in HUT than in the supine position (SUP). There was no significant change in TOI during HDT. A significant session effect was observed in the left forehead TOI during SUP, but not in the right. SRS-NIRS measurements confirmed sub-clinical alterations of cortical oxygenation during HUT. NIRS data from the left side of the forehead, which may vary with cognitive or emotional activation, were more variable than those from the right side.

Syncope, cerebral perfusion, and oxygenation

During standing, both the position of the cerebral circulation and the reductions in mean arterial pressure (MAP) and cardiac output challenge cerebral autoregulatory (CA) mechanisms. Syncope is most often associated with the upright position and can be provoked by any condition that jeopardizes cerebral blood flow (CBF) and regional cerebral tissue oxygenation (cO(2)Hb). Reflex (vasovagal) responses, cardiac arrhythmias, and autonomic failure are common causes. An important defense against a critical reduction in the central blood volume is that of muscle activity ("the muscle pump"), and if it is not applied even normal humans faint. Continuous tracking of CBF by transcranial Doppler-determined cerebral blood velocity (V(mean)) and near-infrared spectroscopy-determined cO(2)Hb contribute to understanding the cerebrovascular adjustments to postural stress; e.g., MAP does not necessarily reflect the cerebrovascular phenomena associated with (pre)syncope. CA may be interpreted as a frequency-dependent phenomenon with attenuated transfer of oscillations in MAP to V(mean) at low frequencies. The clinical implication is that CA does not respond to rapid changes in MAP; e.g., there is a transient fall in V(mean) on standing up and therefore a feeling of lightheadedness that even healthy humans sometimes experience. In subjects with recurrent vasovagal syncope, dynamic CA seems not different from that of healthy controls even during the last minutes before the syncope. Redistribution of cardiac output may affect cerebral perfusion by increased cerebral vascular resistance, supporting the view that cerebral perfusion depends on arterial inflow pressure provided that there is a sufficient cardiac output.

Cerebral oxygenation determined by near-infrared spectrophotometry in patients with fulminant hepatic failure

BACKGROUND/AIMS

In severe cases of acute liver failure (ALF), cerebral hyperperfusion may result in high intracranial pressure and brain damage. The aim of this study was to determine if near-infrared spectrophotometry (NIRS) could detect a raise in cerebral blood flow and oxygenation induced by noradrenaline (NA) infusion.

METHODS

In seven ALF patients (five females and two males; median age 49 years (range 20-70)) changes in cerebral concentration of oxy-(deltaHbO(2)) and total-haemoglobin (deltaHbT) were compared to the jugular bulb saturation (SvjO(2)) and cerebral blood flow velocity (Vmean) during NA infusion.

RESULTS

Mean arterial pressure increased from 68 (64-86) to 103 (87-118) mmHg and the cerebral perfusion pressure from 61 (53-79) to 95 (74-110) mmHg (P<0.05), while the intracranial pressure (7 (6-15) mmHg) was not significantly changed. In six patients cerebral deltaHbO(2) and deltaHbT increased 2.7 (0.3-9.6) and 2.0 (0.3-14.8) micromol l(-1), respectively, but cerebral oxygenation decreased in one patient. SvjO(2) increased from 68 (55-76) to 74 (64-78) % (P<0.05) concomitant with an increase in Vmean from 47 (34-65) to 68 (50-86) cm s(-1) (P<0.05). deltaHbO(2) covariated with changes in SvjO(2) during NA in all but one patient.

CONCLUSIONS

In ALF patients, a change in cerebral perfusion was detected by NIRS. The combination of NIRS and transcranial Doppler sonography may be valuable non-invasive techniques to detect cerebral hyperperfusion before intracranial hypertension becomes manifest.

Cerebral oxygenation during exercise in patients with terminal lung disease

STUDY OBJECTIVES

In patients with terminal lung disease who were exercising, we assessed whether improved arterial O2 saturation with an increased fraction of inspired oxygen (FIO2) affects cerebral oxygenation.

DESIGN

Randomized, crossover.

PATIENTS AND METHODS

The cerebral changes in oxyhemoglobin (DeltaHbO2) and changes in deoxyhemoglobin (DeltaHb) levels were evaluated using near-infrared spectrophotometry and the middle cerebral artery (MCA) mean velocity (V(mean)) was determined by transcranial Doppler ultrasonography in 13 patients with terminal lung disease (New York Heart Association class III-IV). Patients were allocated to an FIO2 of either 0.21 or 0.35 during incremental exercise with 15 min between trials.

RESULTS

Peak exercise intensity (mean [+/- SE], 26 +/- 4 W) reduced the arterial O2 pressure (at rest, 64 +/- 3 mm Hg; during exercise, 56 +/- 3 mm Hg) and the arterial oxygen saturation (SaO2) [at rest, 92 +/- 2%; 87 +/- 2%; p < 0.05], while the arterial CO2 pressure was not significantly affected. The MCA V(mean) increased from 49 +/- 5 to 63 +/- 7 cm/s (p < 0.05) as did the DeltaHb, while the DeltaHbO2 remained unaffected by exercise. With an elevated FIO2, the SaO2 level (at rest, 95.8 +/- 0.7%; during exercise, 96.0 +/- 1.0%) and arterial O2 pressure (at rest, 102 +/- 11 mm Hg; during exercise, 100 +/- 8 mm Hg) were not significantly affected by exercise, and the levels of blood oxygenation remained higher than the values established at normoxia (p < 0.05). The MCA V(mean) increased to a level similar to that achieved during control exercise (ie, to 70 +/- 11 cm/s). In contrast to control exercise, DeltaHb decreased while DeltaHbO2 increased during exercise with 35% O2 (p < 0.05).

CONCLUSION

An O2-enriched atmosphere enabled patients with terminal lung disease to maintain arterial O2 saturation during exercise. An exercise-induced increase in cerebral perfusion was not affected by hyperoxia, whereby the enhanced availability of oxygenated hemoglobin increases cerebral oxygenation. The clinical implication of the study is that during physical activity patients with terminal lung disease are recommended to use an elevated FIO2 to protect cerebral oxygenation.

Near-infrared spectroscopy determined brain and muscle oxygenation during exercise with normal and resistive breathing

To elevate effects of carbon dioxide (CO2) retention by way of an increased respiratory load during submaximal exercise (150 W), the concentration changes of oxy- (DeltaHbO2) and deoxy-haemoglobin (DeltaHb) of active muscles and the brain were determined by near-infrared spectroscopy (NIRS) in eight healthy males. During exercise, pulmonary ventilation increased to 33 (28-40) L min-1 (median with range) with no effect of a moderate breathing resistance (reduction of the pneumotach diameter from 30 to 14 and 10 mm). The end-tidal CO2 pressure (PETCO2) increased from 45 (42-48) to 48 (46-58) mmHg with a reduction of only 1% in the arterial haemoglobin O2 saturation (SaO2). During control exercise (normal breathing resistance), muscle and brain DeltaHbO2 were not different from the resting levels, and only the leg muscle DeltaHb increased (4 (-2-10) microM, P < 0.05). Moderate resistive breathing increased DeltaHbO2 of the intercostal and vastus lateralis muscles to 6 +/- (-5-14) and 1 (-7-9) microM(P < 0.05), respectively, while muscle DeltaHb was not affected. Cerebral DeltaHbO2 and DeltaHb became elevated to 6 (1-15) and 1 (-1-6) microM by resistive breathing (P < 0.05). Resistive breathing caused an increased concentration of oxygenated haemoglobin in active muscles and in the brain. The results indicate that CO2 influences blood flow to active skeletal muscle although its effect appears to be smaller than for the brain.

Tissue oximetry for the diagnosis of neurally mediated syncope

This study examined the potential clinical contribution of noninvasive brain and skeletal muscle oximetry as a diagnostic aid in neurally mediated syncope. Tilt table testing was performed in 15 patients with a history of syncope and 9 healthy volunteers. Spatially resolved reflectance near-infrared spectroscopy was used to examine regional oxyhemoglobin saturation in the frontal cerebral cortex and pectoral muscle. During upright tilt, syncope occurred in four patients. Each episode was associated with bradycardia, hypotension, and brain oxygen desaturation of > 20% from the supine reference baseline, while the largest desaturation in nonsyncopal patients was 13%. In two syncopal patients, a sudden increase in pectoral oxygen saturation preceded cerebral oxygen desaturation and unconsciousness, suggesting a sudden loss of peripheral sympathetic tone. Simultaneous desaturation in both tissues in the other two patients appeared to be in response to a diminished cardiac output. The muscle and brain oxygen saturation ratio increased by 75% with apparent sympathetic dysfunction, but never changed by > 25% in the other patients, and varied by < 15% in the volunteers. These results suggest that during tilt table testing, simultaneous assessment of brain and skeletal muscle oxygenation may provide a simple, objective aid for the identification of contributory sympathetic dysfunction.

Postoperative confusion preceded by decreased frontal lobe haemoglobin oxygen saturation

We describe a 58-year-old male patient with confusion and prolonged recovery after liver transplantation. A cause was not apparent for the confusion, but during surgery, monitoring of the frontal lobe cerebral haemoglobin oxygen saturation by near-infrared spectrophotometry showed cerebral hypo-oxygenation despite optimization of conventional cardiovascular parameters. It is possible that intraoperative cerebral ischaemia is the cause of postsurgical confusion and with near-infrared spectrophotometry this hypothesis may be tested clinically.

Well-being and cerebral oxygen saturation during acute heart failure in humans

Cerebral symptoms and near-infrared spectrophotometry-determined cerebral oxygen saturation (ScO2) were followed in patients treated for normotensive acute congestive heart failure. The reproducibility and normal range for ScO2 were established from 39 resting subjects without cardio-respiratory disease: the ScO2 ranged from 55 to 78% with a coefficient of variation for triple determination of 6%. Patients rated cerebral symptoms on a scale with end-points of 0 (best) and 10 (worst). In eight patients with acute heart failure, arterial oxygen tension increased during decongestive treatment, from 9.1 (4.9-10) to 10.4 kPa (7.3-17); median with range, as did arterial oxygen saturation, from 94 (48-97) to 97% (87-99) (P<0.02), whereas the mean arterial pressure, heart rate and arterial carbon dioxide tension remained unchanged. The cerebral symptom score improved from 8 (3-10) to 1 (1-9) and the ScO2 increased from 34 (20-58) to 50% (19-91) (P<0.02). A ninth patient presented with a silent but massive myocardial infarction: she was cerebrally obtunded with a ScO2 of 18% and soon died. In patients with normotensive acute heart failure and cerebral symptoms, cerebral oxygen saturation is low, and during successful treatment ScO2 increases with the well-being of the patient.

Interference of cerebral near-infrared oximetry in patients with icterus

Near-infrared spectrophotometry assesses cerebral oxygen saturation (ScO(2)) based on the absorption spectra of oxygenated and deoxygenated hemoglobin and the translucency of biological tissue in the near-infrared band. In patients with icterus, however, bilirubin can potentially hinder cerebral oximetry. In 48 patients undergoing orthotopic liver transplantation, we related total plasma bilirubin to ScO(2) as determined from spectrophotometry with wavelengths of 733 and 809 nm. Before surgery, ScO(2) was 59% (15%-78%) (median with range) and bilirubin was 71 (6-619) micromol/L with a negative correlation (r = -0.72; P < 0.05). The 95% prediction interval included the lowest measurable ScO(2) of 15% at a bilirubin level of 370 micromol/L. During reperfusion of the grafted liver, the ScO(2) increased by 7% (-8% to 17%) (P < 0.05), and bilirubin did not influence this increase. In one patient, the ScO(2) remained below 15% despite a decrease in bilirubin from 619 to 125 micromol/L, suggesting that tissue pigmentation deposits also absorb light. In conclusion, bilirubin dampens the spectrophotometry-determined cerebral oxygen saturation at 733 and 809 nm. A bilirubin level of 370 micromol/L, tissue pigment deposits, or both, may render determination of cerebral oxygen saturation impossible. Even at high bilirubin values, changes in cerebral perfusion may be visible.

IMPLICATIONS

In 48 patients undergoing liver transplantation, the interference of icterus on cerebral oximetry by near-infrared light was investigated. Bilirubin absorbed the near-infrared light and lowered the measured cerebral oxygen saturation. Even at high bilirubin values, changes in cerebral oxygenation, as seen during reperfusion of the grafted liver, may be visible.

Cerebral desaturation during exercise reversed by O2 supplementation

The combined effects of hyperventilation and arterial desaturation on cerebral oxygenation (ScO2) were determined using near-infrared spectroscopy. Eleven competitive oarsmen were evaluated during a 6-min maximal ergometer row. The study was randomized in a double-blind fashion with an inspired O2 fraction of 0.21 or 0.30 in a crossover design. During exercise with an inspired O2 fraction of 0.21, the arterial CO2 pressure (35 +/- 1 mmHg; mean +/- SE) and O2 pressure (77 +/- 2 mmHg) as well as the hemoglobin saturation (91.9 +/- 0.7%) were reduced (P < 0.05). ScO2 was reduced from 80 +/- 2 to 63 +/- 2% (P < 0.05), and the near-infrared spectroscopy-determined concentration changes in deoxy- (DeltaHb) and oxyhemoglobin (DeltaHbO2) of the vastus lateralis muscle increased 22 +/- 3 microM and decreased 14 +/- 3 microM, respectively (P < 0.05). Increasing the inspired O2 fraction to 0.30 did not affect ventilation (174 +/- 4 l/min), but arterial CO2 pressure (37 +/- 2 mmHg), O2 pressure (165 +/- 5 mmHg), and hemoglobin O2 saturation (99 +/- 0.1%) increased (P < 0. 05). ScO2 remained close to the resting level during exercise (79 +/- 2 vs. 81 +/- 2%), and although the muscle DeltaHb (18 +/- 2 microM) and DeltaHbO2 (-12 +/- 3 microM) were similar to those established without O2 supplementation, work capacity increased from 389 +/- 11 to 413 +/- 10 W (P < 0.05). These results indicate that an elevated inspiratory O2 fraction increases exercise performance related to maintained cerebral oxygenation rather than to an effect on the working muscles.

Near-infrared oximetry of the brain

Near-infrared (IR) light easily penetrates biological tissue, and the information offered by in vivo spectroscopy of cerebral oxygenation is detailed and comes with a high temporal resolution. Near-IR light spectroscopy (NIRS) reflects cerebral oxygenation during arterial hypotension, hypoxic hypoxaemia and hypo- and hypercapnia. As determined by dual-wavelength NIRS, the cerebral O2 saturation integrates the arterial O2 content and the cerebral perfusion, and as established for skeletal muscle, NIRS obtains information on tissue oxygenation and metabolism beyond that obtained by venous blood sampling. Caveats of cerebral NIRS include insufficient light shielding, optode displacement and a sample volume including muscle or the frontal sinus mucous membrane. The relative influence from the extracranial tissue is minimized by optode separation and correction for an extracranial sample volume, or both. The natural pigment melatonin and also water are of little influence to spectroscopic analysis of cerebral oxygenation, whereas bilirubin systematically lowers ScO2 and attenuates the detection of changes in cerebral oxygenation. By NIRS, reduction of cytochrome oxidase is demonstrated during hypoxic hypoxaemia and head-up tilt-induced arterial hypotension, but the changes are small. In the clinical setting, NIRS offers useful information in patients with both systemic and local cerebral circulatory impairment, for example, during cranial trauma, surgery on the cerebral arteries, orthostasis and acute heart failure. Whereas mapping of the brain circulation is needed for jugular venous sampling to reflect either global or local oxygenation, the determination of cerebral oxygenation by NIRS has the advantage of localized monitoring of the cerebral cortex.

Reproducibility of the alterations in circulation and cerebral oxygenation from supine rest to head-up tilt

Cardiovascular stability, as affected by several diseases, may be assessed by head-up tilt testing. Follow-up studies are essential in both evaluating interventions and assessing progression. However, data on the reproducibility of the changes in circulatory status and cerebral oxygenation provoked by head-up tilt testing are fundamental to follow-up studies. The aim of this study was, therefore, to assess the reproducibility of the alterations in stroke volume (SV), mean arterial pressure (MAP), as well as oxygenated ([O2Hb]) and deoxygenated haemoglobin ([HHb]) concentration in cerebral tissue from supine rest (SUP) to head-up tilt (HUT). SV was calculated by Modelflow, a pulse contour method, from the finger arterial pressure wave measured by Portapres, the portable version of Finapres. [O2Hb] and [HHb] were measured using near-infrared spectroscopy (NIRS). Ten healthy individuals visited the laboratory on two different days. On both days, they underwent 10 min SUP followed by 10 min 70 degrees HUT twice. SV decreased, which was (in part) compensated for by an increased heart rate, while MAP increased slightly during HUT compared with SUP. Although [HHb] increased during HUT, no presyncope symptoms were experienced. The circulatory variables (SV, HR and MAP) as well as [HHb] showed an acceptably small systematic and random error as well as reproducibility error compared with the observed difference between HUT and SUP and were similar between and within visits. Therefore, it is concluded that MAP measured by Portapres and SV calculated by Modelflow as well as HHb measured by NIRS seem to be reproducible and may therefore be used in follow-up studies.

Near-infrared spectrophotometry determined brain oxygenation during fainting

During orthostatic hypotension we evaluated whether presyncopal symptoms relate to a reduced brain oxygenation. Nine subjects performed 50 degrees head-up tilt for 1 h and eight subjects were followed during 2 h of supine rest and during 1 h of 10 degrees head-down tilt. Cerebral perfusion was assessed by transcranial Doppler determined middle cerebral artery blood velocity (MCA vmean), while brain blood oxygenation was assessed by near-infrared spectrophotometry determined concentration changes for oxygenated (delta HbO2) and deoxygenated haemoglobin and brain cell oxygenation by the oxidized cytochrome c concentration (delta CytO2). During head-up tilt, six volunteers developed presyncopal symptoms and mean arterial pressure (88 (78-103) to 68 (57-79) mmHg; median and range), heart rate (96 (72-111) to 65 (50-107) beats min-1), MCA vmean (59 (51-82) to 41 (29-56) cm s-1), delta HbO2 (by -5.3 (-3.0 to -14.8) mumol l-1) and delta CytO2 were reduced (by -0.2 (-0.1 to -0.4) mumol l-1; P < 0.05). During tilt down the cardiovascular variables recovered immediately and delta HbO2 increased to 2.2 (-0.9-12.0) mmol L-1 above the resting value and also delta CytO2 recovered. In the nonsyncopal head-up tilted subjects as in the controls, blood pressure, heart rate, MCA vmean and brain oxygenation indices remained stable. The results suggest that during orthostasis, presyncopal symptoms relate not only to cerebral hypoperfusion but also to reduced brain oxygenation.

Middle cerebral artery blood velocity depends on cardiac output during exercise with a large muscle mass

We tested the hypothesis that pharmacological reduction of the increase in cardiac output during dynamic exercise with a large muscle mass would influence the cerebral blood velocity/perfusion. We studied the relationship between changes in cerebral blood velocity (transcranial Doppler), rectus femoris blood oxygenation (near-infrared spectroscopy) and systemic blood flow (cardiac output from model flow analysis of the arterial pressure wave) as induced by dynamic exercise of large (cycling) vs. small muscle groups (rhythmic handgrip) before and after cardioselective beta 1 adrenergic blockade (0.15 mg kg-1 metoprolol i.v.). During rhythmic handgrip, the increments in systemic haemodynamic variables as in middle cerebral artery mean blood velocity were not influenced significantly by metoprolol. In contrast, during cycling (e.g. 113 W), metoprolol reduced the increase in cardiac output (222 +/- 13 vs. 260 +/- 16%), heart rate (114 +/- 3 vs. 135 +/- 7 beats min-1) and mean arterial pressure (103 +/- 3 vs. 112 +/- 4 mmHg), and the increase in cerebral artery mean blood velocity also became lower (from 59 +/- 3 to 66 +/- 3 vs. 60 +/- 2 to 72 +/- 3 cm s-1; P < 0.05). Likewise, during cycling with metoprolol, oxyhaemoglobin in the rectus femoris muscle became reduced (compared to rest: -4.8 +/- 1.8 vs. 1.2 +/- 1.7 mumol L-1, P < 0.05). Neither during rhythmic handgrip nor during cycling was the arterial carbon dioxide tension affected significantly by metoprolol. The results suggest that as for the muscle blood flow, the cerebral circulation is also affected by a reduced cardiac output during exercise with a large muscle mass.

Cerebrovascular and cardiovascular measurements during neurally mediated syncope induced by head-up tilt

BACKGROUND AND PURPOSE

This study examines changes in systemic hemodynamics and in cerebral blood velocity that occur during neurally mediated syncope (NMS) to determine whether cerebral autoregulation is intact or impaired in patients with recurrent NMS.

METHODS

Beat-to-beat recordings of heart rate, blood pressure (volume clamp photoplethysmography), stroke volume (impedance cardiography), and right middle cerebral artery blood velocity (transcranial Doppler sonography) were performed at rest and during 80 degrees head-up tilt. Twelve patients with NMS and 10 healthy control subjects were studied.

RESULTS

Baseline values and the initial response to head-up tilt of control subjects and patients with NMS were similar. The mean latency to onset of syncope was 11.8 +/- 11.1 minutes. At syncope, heart rate, systolic and diastolic blood pressure, and diastolic cerebral blood velocity decreased significantly, whereas systolic cerebral blood velocity did not change. Calculated cerebrovascular resistance was significantly reduced from 1.85 +/- 0.60 to 1.32 +/- 0.27 mm Hg/cm per second, whereas the pulsatility index increased from 0.92 +/- 0.16 to 1.52 +/- 0.21. We never observed a change in cerebral blood velocity before the rapid decline in blood pressure, nor did we observe any significant change in respiratory pattern.

CONCLUSIONS

The decrease in cerebrovascular resistance during NMS indicates that the integrity of cerebrovascular autoregulation is maintained even when syncope is imminent. The selective loss of diastolic flow during syncope and the increase in pulsatility index are likely caused by collapse of downstream vessels as diastolic blood pressure decreases below the critical closing pressure of cerebral vessels.

Cerebral oxygen saturation and blood flow during liver transplantation

After reperfusion of a liver graft, transcranial Doppler determined middle cerebral artery flow velocity, increases more than expected from the arterial carbon dioxide tension (PaCO2). We evaluated if this indication of cerebral hyperperfusion is reflected in the jugular oxygen saturation (SjO2) (n = 31) and oxygen saturation (ScO2) (near-infrared spectrophotometry, n = 22). From the dissection phase to the anhepatic phase SjO2 71.0% (range 62.3%-78.5%), ScO2 70% (range 65%-77%), and PaCO2 34.9 mm Hg (range 30.8-38.3) remained statistically unchanged. In contrast, during the early reperfusion phase, SjO2 increased to 77.0% (71.4%-83.0%) (P < 0.01) and the ScO2 to 75% (68%-80%) (P < 0.05) as PaCO2 increased to 37.5 mm Hg (34.8-41.9) (P < 0.001). Notably, SjO2 also increased at reperfusion from 71.6% (66.5%-78.0% mm Hg) to 80.0% (76.8%-84.8%) in the four patients in whom PaCO2 decreased at reperfusion from 37.6 mm Hg (36.8-39.5) to 34.0 mm Hg (32.3-36.8). If the changes in SjO2 after reperfusion of the grafted liver should be explained as a reflection of changes in cerebral blood flow, in response to PaCO2, it would indicate a highly accentuated CO2 reactivity of 13%/mm Hg. The results support that cerebral blood flow and, in turn, oxygenation increase after reperfusion because the grafted liver liberates a vasodilating substance(s).

Dissociated cerebral vasoparalysis in acute liver failure. A hypothesis of gradual cerebral hyperaemia

BACKGROUND/AIMS

Normally, cerebral blood flow responds to changes in the arterial carbon dioxide tension (PaCO2) but not to changes in mean arterial pressure, commonly referred to as the cerebral CO2-reactivity and autoregulation. In patients with fulminant hepatic failure and in the rat with thioacetamide-induced liver failure, autoregulation is absent, presumably due to cerebral vasoparalysis. Since also CO2-reactivity may then be compromised, it was studied in patients with fulminant hepatic failure and rats with thioacetamide-induced liver failure.

METHODS

In ten patients (median age 32 (range 20-48) years)) and in ten age-matched volunteers, cerebral perfusion was elevated by transcranial Doppler assessed mean flow velocity (V(mean)) in the middle cerebral artery during hypo- and hyper-capnia. In six rats with liver failure and in six control rats, cerebral blood flow was measured repeatedly by the intracarotid 133 Xenon injection technique.

RESULTS

In the patients and volunteers, PaCO2 was lowered from 33 (23-44) to 28 (23-39) mmHg by hypocapnia and raised to 40 (34-48) mmHg by hypercapnia or 5% CO2 inhalation. During hypocapnia, the CO2-reactivity did not differ significantly between patients and volunteers, 4.0 (1.1-7.4) vs. 3.0 (1.7-5.0)% mmHg(-1), while it was reduced during hypercapnia in the patients, 2.2 (1.8-5.2) vs. 4.6 (3.0-8.0)% mmHg(-1) (p < 0.05). In the rats, PaCO2 was reduced from 39 (37-40) to 30 (29-31) mmHg and then raised to 51 (41-55) mmHg. During hypocapnia, CO2-reactivity was similar in rats with liver failure and in control rats, 2.3 vs 2.7% mmhg(21), respectively. In all rats with liver failure CO2-reactivity was abolished during hypercapnia, while it was 1.5% mmHg(-1) in the control rats (p < 0.01).

CONCLUSIONS

The finding that cerebral CO2 reactivity is reduced in hypercapnia, while it is preserved in hypocapnia, suggests that gradual dilation of the cerebral resistance vessels develops in fulminant hepatic failure and connects previous morphological studies with changes in the regulation of cerebral blood flow, i.e. impaired cerebral autoregulation and blunted CO2-reactivity.