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author:("cerussite, A. E.")
1.  Impact of stepwise hyperventilation on cerebral tissue oxygen saturation in anesthetized patients: a mechanistic study 
While the decrease in blood carbon dioxide (CO2) secondary to hyperventilation is generally accepted to play a major role in the decrease of cerebral tissue oxygen saturation (SctO2), it remains unclear if the associated systemic hemodynamic changes are also accountable.
Twenty-six patients (American Society of Anesthesiologists I–II) undergoing nonneurosurgical procedures were anesthetized with either propofol-remifentanil (n = 13) or sevoflurane (n = 13). During a stable intraoperative period, ventilation was adjusted stepwise from hypoventilation to hyper-ventilation to achieve a progressive change in end-tidal CO2 (ETCO2) from 55 to 25 mmHg. Minute ventilation, SctO2, ETCO2, mean arterial pressure (MAP), and cardiac output (CO) were recorded.
Hyperventilation led to a SctO2 decrease from 78 ± 4% to 69 ± 5% (Δ = −9 ± 4%, P < 0.001) in the propofol-remifentanil group and from 81 ± 5% to 71 ± 7% (Δ = −10 ± 3%, P < 0.001) in the sevoflurane group. The decreases in SctO2 were not statistically different between these two groups (P = 0.5). SctO2 correlated significantly with ETCO2 in both groups (P < 0.001). SctO2 also correlated significantly with MAP (P < 0.001) and CO (P < 0.001) during propofol-remifentanil, but not sevoflurane (P = 0.4 and 0.5), anesthesia.
The main mechanism responsible for the hyperventilation-induced decrease in SctO2 is hypocapnia during both propofol-remifentanil and sevoflurane anesthesia. Hyperventilation-associated increase in MAP and decrease in CO during propofol-remifentanil, but not sevoflurane, anesthesia may also contribute to the decrease in SctO2 but to a much smaller degree.
PMCID: PMC3992996  PMID: 23278596
2.  Impact of phenylephrine administration on cerebral tissue oxygen saturation and blood volume is modulated by carbon dioxide in anaesthetized patients† 
BJA: British Journal of Anaesthesia  2012;108(5):815-822.
Multiple studies have shown that cerebral tissue oxygen saturation () is decreased after phenylephrine treatment. We hypothesized that the negative impact of phenylephrine administration on is affected by arterial blood carbon dioxide partial pressure () because CO2 is a powerful modulator of cerebrovascular tone.
In 14 anaesthetized healthy patients, i.v. phenylephrine bolus was administered to increase the mean arterial pressure ∼20–30% during hypocapnia, normocapnia, and hypercapnia. and cerebral blood volume (CBV) were measured using frequency domain near-infrared spectroscopy, a quantitative technology. Data collection occurred before and after each treatment.
Phenylephrine caused a significant decrease in during hypocapnia [=−3.4 (1.5)%, P<0.001], normocapnia [=−2.4 (1.5)%, P<0.001], and hypercapnia [=−1.4 (1.5)%, P<0.01]. Decreases in were significantly different between hypocapnia, normocapnia, and hypercapnia (P<0.001). Phenylephrine also caused a significant decrease in CBV during hypocapnia (P<0.01), but not during normocapnia or hypercapnia.
The negative impact of phenylephrine treatment on and CBV is intensified during hypocapnia while blunted during hypercapnia.
PMCID: PMC3325051  PMID: 22391890
carbon dioxide; cerebral blood volume; cerebral tissue oxygen saturation; modulation; phenylephrine
3.  Effect of phenylephrine and ephedrine bolus treatment on cerebral oxygenation in anaesthetized patients 
BJA: British Journal of Anaesthesia  2011;107(2):209-217.
How phenylephrine and ephedrine treatments affect global and regional haemodynamics is of major clinical relevance. Cerebral tissue oxygen saturation ()-guided management may improve postoperative outcome. The physiological variables responsible for changes induced by phenylephrine and ephedrine bolus treatment in anaesthetized patients need to be defined.
A randomized two-treatment cross-over trial was conducted: one bolus dose of phenylephrine (100–200 µg) and one bolus dose of ephedrine (5–20 mg) were given to 29 ASA I–III patients anaesthetized with propofol and remifentanil. , mean arterial pressure (MAP), cardiac output (CO), and other physiological variables were recorded before and after treatments. The associations of changes were analysed using linear-mixed models.
The CO decreased significantly after phenylephrine treatment [▵CO=−2.1 (1.4) litre min−1, P<0.001], but was preserved after ephedrine treatment [▵CO=0.5 (1.4) litre min−1, P>0.05]. The was significantly decreased after phenylephrine treatment [▵=−3.2 (3.0)%, P<0.01] but preserved after ephedrine treatment [▵=0.04 (1.9)%, P>0.05]. CO was identified to have the most significant association with (P<0.001). After taking CO into consideration, the other physiological variables, including MAP, were not significantly associated with (P>0.05).
Associated with changes in CO, decreased after phenylephrine treatment, but remained unchanged after ephedrine treatment. The significant correlation between CO and implies a cause–effect relationship between global and regional haemodynamics.
PMCID: PMC3136202  PMID: 21642644
cardiac output; cerebral tissue oxygen saturation; ephedrine; mean arterial pressure; phenylephrine
4.  In vivo water state measurements in breast cancer using broadband diffuse optical spectroscopy 
Physics in medicine and biology  2008;53(23):6713-6727.
Structural changes in water molecules are related to physiological, anatomical and pathological properties of tissues. Near infrared (NIR) optical absorption methods are sensitive to water, however detailed characterization of water in thick tissues is difficult to achieve because subtle spectral shifts can be obscured by multiple light scattering. In the NIR, a water absorption peak is observed around 975nm. The precise NIR peak shape and position is highly sensitive to water molecular disposition. We introduce a Bound Water Index (BWI) that quantifies shifts observed in tissue water absorption spectra measured by broadband Diffuse Optical Spectroscopy (DOS). DOS quantitatively measures light absorption and scattering spectra and therefore reveals bound-water spectral shifts. BWI as a water state index was validated by comparing broadband DOS to Magnetic Resonance Spectroscopy, diffusion-weighted MRI and conductivity in bound water tissue phantoms. Non-invasive DOS measurements of malignant and normal breast tissues performed in 18 subjects showed a significantly higher fraction of free water in malignant tissues (p<0.0001) compared to normal tissues. BWI of breast cancer tissues inversely correlated with Nottingham-Bloom-Richardson histopathology scores. These results highlight broadband DOS sensitivity to molecular disposition of water, and demonstrate the potential of BWI as a non-invasive in-vivo index that correlates with tissue pathology.
PMCID: PMC2586905  PMID: 18997265

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