Acute Abdomen in the 17th Week of Twin Pregnancy due to Ovarian Torsion - A Late Complication of IVF.

Background: A 32-year-old woman with tubal factor infertility due to bilateral laparoscopic salpingectomy conceived twins with in vitro fertilization (IVF). She developed moderate ovarian hyperstimulation syndrome which was treated with anticoagulant therapy. The subsequent course of the twin pregnancy was normal until the 17th week of gestation when she presented to hospital because of a sharp pain in the right lower abdomen which ceased after admission. Case: Except for a single incident of vomiting, patient had no other subjective symptoms. The clinical examination showed tenderness of the lower right abdominal segment on palpation. The surgeon and the urologist found no signs of an acute surgical or urologic condition, and laboratory findings were within normal reference ranges for pregnant women. Two days after admission the pain reappeared; it was now much stronger and colic-like. The pain was initially located supraumbilically but subsequently spread diffusely across the lower abdomen. Abdominal guarding was present and laboratory findings showed an increase in inflammatory parameters. An enlarged and edematous right ovary was found on transvaginal ultrasound. Conclusion: Exploratory laparotomy via a vertical midline abdominal transection revealed a torqued necrotic right ovary with elements of inflammation and inflammatory adhesions involving the entire pelvis. The patient underwent right-sided ovariectomy and adhesiolysis. Recovered was normal and the patient was delivered of healthy twins in the 37th week of gestation.

Ex vivo detection of mesiobuccal canals in maxillary molars using CBCT at four different isotropic voxel dimensions.

AIM: To study observers' ability to detect mesiobuccal (MB) canals in maxillary molars using iCAT cone-beam computed tomography (CBCT) at different voxel dimensions and to assess the impact of clinical experience on accuracy of detection. METHODOLOGY: Using 12 experimental models with two molars each, CBCT scans were acquired at four different voxel dimensions. From the cross-section view of these scans, 96 videos were generated. Five endodontic postgraduate students and two endodontic staff watched the videos and counted the MB canals in each root. Horizontal sections of the roots were evaluated under magnification to determine the true canal numbers. The detection of MB canals within the four resolutions was compared by odds ratio, and the weighted χ(2) test compared detection accuracy to raters' clinical experience. Rater agreement was measured by kappa statistics. RESULTS: Overall, 92% of the maxillary molars had two MB canals upon analysis of horizontal cross-sections. The CBCT detection increased from 60.1% at 0.4 mm voxel size to 93.3% at 0.125 mm voxel size. Significant differences (P < 0.01) were observed between the different resolutions except for the 0.2 and the 0.125 voxel scans. Second-year trainees were significantly (CI = 0.2929-0.712) more accurate than first-year trainees and endodontic staff at MB canal detection (87.9% against 77.1% and 76.8%). Intra-rater reliability increased with higher-resolution scans (41.1% to 96.4%). CONCLUSIONS: The reliability of detection of maxillary molar MB2 canals in CBCT scans increased as the resolution improved. Accuracy of MB2 canal detection among observer groups could not be correlated with the observers' level of clinical experience.

Mefloquine induces dose-related neurological effects in a rat model.

Mefloquine is one of the drugs approved by the FDA for malaria chemoprophylaxis. Mefloquine is also approved for the treatment of malaria and is widely used for this purpose in combination with artesunate. However, the clinical utility of the compound has been compromised by reports of adverse neurological effects in some patients. In the present study, the potential neurological effects of mefloquine were investigated with six 7-week-old female rats given a single oral dose of the compound. Potential mefloquine-induced neurological effects were monitored using a standard functional observational battery, automated open field tests, automated spontaneous activity monitoring, a beam traverse task, and histopathology. Plasma mefloquine concentrations were determined 72 h after dosing by using liquid chromatography-mass spectrometry. Mefloquine induced dose-related changes in endpoints associated with spontaneous activity and impairment of motor function and caused degeneration of specific brain stem nuclei (nucleus gracilis). Increased spontaneous motor activity was observed only during the rats' normal sleeping phase, suggesting a correlate to mefloquine-induced sleep disorders. The threshold dose for many of these effects was 187 mg/kg of body weight. This dose yielded plasma mefloquine concentrations after 72 h that are similar to those observed in humans after the treatment dose. Collectively, these data suggest that there may be a biological basis for some of the clinical neurological effects associated with mefloquine.

The combined effects of pyridostigmine and chronic stress on brain cortical and blood acetylcholinesterase, corticosterone, prolactin and alternation performance in rats.

Thousands of soldiers who served in the Gulf War have symptoms that have been collectively termed Gulf War Illness (GWI). It has been suggested that a combination of operational stress and pyridostigmine, a drug given as a pretreatment to protect soldiers against the effects of exposure to nerve agents, might have had unexpected adverse health effects causing these symptoms. Our laboratory has previously modeled operational stress in rats using a paradigm of around-the-clock intermittent signalled footshock. In the present studies, this model was used to investigate the potential synergistic effects of chronic stress and pyridostigmine on physiology and behavior. Seventy-two rats were trained to perform an alternation lever pressing task to earn their entire daily food intake. The rats were then implanted with osmotic minipumps containing vehicle, pyridostigmine (25 mg/ml pyridostigmine bromide) or physostigmine (20 mg/ml eserine hemisulfate). The pumps delivered 1 microl/h, which resulted in a cumulative dosing of approximately 1.5 mg/kg/day of pyridostigmine or 1.2 mg/kg/day of physostigmine, equimolar doses of the two drugs. The rats were then returned to their home cages where performance continued to be measured 24 h/day. After 4 days, 24 of the 72 rats were trained to escape signalled footshock (avoidance-escape group) and 24 other rats (yoked-stressed group) were each paired to a rat in the avoidance-escape group. The remaining 24 rats were not subjected to footshock (unstressed group). Shock trials were intermittently presented in the home cage 24 h/day for 3 days, while alternation performance continued to be measured. Since only 12 test cages were available, each condition was repeated to achieve a final n of six rats per group. Pyridostigmine and physostigmine each decreased blood acetylcholinesterase levels by approximately 50%. Physostigmine also decreased brain cortical acetylcholinesterase levels by approximately 50%, while pyridostigmine had no effect on cortical acetylcholinesterase activity. Alternation performance was impaired on the first day of stress and then recovered. Neither pyridostigmine nor physostigmine affected performance in the absence of stress or increased the effects of stress alone. Corticosterone was significantly increased in the yoked stress group compared to unstressed controls. These data suggest that pyridostigmine does not exacerbate the effects of stress on performance or levels of stress hormones. Furthermore, these data do not suggest that stress enables pyridostigmine to cross the blood brain barrier.

The reality of picture archiving and communication systems (PACS): a survey.

Toward the end of 1997 vendors were succeeding in installing picture, archiving and communication systems (PACS) in larger numbers. It was hard to separate fact from fiction at times. This survey was undertaken by 2 members of the academic community to confirm what was operational, how well the installed systems worked, and what they were doing. Fax questionnaires were sent to nearly 1,000 facilities worldwide to identify PACS of any size in clinical operation on a date certain, February 1, 1998. A total of 177 PACS were identified. Facilities furnished responses during the first survey period from May 1 to November 1, 1998. A second survey, done in February and March of 2000, sought long-term follow-up data. Many systems operated 5 or more types of modalities. Computed tomography (CT) was the most common modality type at 83%, but the distribution of the rest held surprises. There also was an unexpectedly large use of soft copy reading and filmless operation in 1998. Clear trends toward increased use of computed radiography and digital radiography and a significant expansion on the percentage of all of a facility's examinations on the PACS were seen over the 2 years. Satisfaction with original PACS vendors was relatively high. Eighty-nine percent remained with their original vendor. Only 10 sites reported they changed vendors, and 4 facilities said they abandoned their system. The users reported their expectations of the PACS had been met in 81% of cases. Some (65%) declared their systems were cost effective. The most striking response was that 97% of the users would recommend PACS to others.

Secondary hypoxemia exacerbates the reduction of visual discrimination accuracy and neuronal cell density in the dorsal lateral geniculate nucleus resulting from fluid percussion injury.

The purpose of this study was to determine the impact of secondary hypoxemia on visual discrimination accuracy after parasagittal fluid percussion injury (FPI). Rats lived singly in test cages, where they were trained to repeatedly execute a flicker-frequency visual discrimination for food. After learning was complete, all rats were surgically prepared and then retested over the following 4-5 days to ensure recovery to presurgery levels of performance. Rats were then assigned to one of three groups [FPI + Hypoxia (IH), FPI + Normoxia (IN), or Sham Injury + Hypoxia (SH)] and were anesthetized with halothane delivered by compressed air. Immediately after injury or sham injury, rats in groups IH and SH were switched to a 13% O2 source to continue halothane anesthesia for 30 min before being returned to their test cages. Anesthesia for rats in group IN was maintained using compressed air for 30 min after injury. FPI significantly reduced visual discrimination accuracy and food intake, and increased incorrect choices. Thirty minutes of immediate posttraumatic hypoxemia significantly (1) exacerbated the FPI-induced reductions of visual discrimination accuracy and food intake, (2) further increased numbers of incorrect choices, and (3) delayed the progressive recovery of visual discrimination accuracy. Thionine stains of midbrain coronal sections revealed that, in addition to the loss of neurons seen in several thalamic nuclei following FPI, cell loss in the ipsilateral dorsal lateral geniculate nucleus (dLG) was significantly greater after FPI and hypoxemia than after FPI alone. In contrast, neuropathological changes were not evident following hypoxemia alone. These results show that, although hypoxemia alone was without effect, posttraumatic hypoxemia exacerbates FPI-induced reductions in visual discrimination accuracy and secondary hypoxemia interferes with control of the rat's choices by flicker frequency, perhaps in part as a result of neuronal loss and fiber degeneration in the dLG. These results additionally confirm the utility of this visual discrimination procedure as a sensitive, noninvasive means of assessing behavioral function after experimental traumatic brain injury.

Behavioral effects Of 8-OH-DPAT in chronically stressed male and female rats.

The present study tested the hypothesis that chronic stress desensitizes serotonergic 5-HT(1A) receptors and alters behavioral changes following 5-HT(1A) agonist administration. Eating, acoustic startle response (ASR), and locomotor activity were measured in stressed and nonstressed male and female rats after 8-OH-DPAT administration. Stressed rats were paired and stressed by around-the-clock intermittent foot shock. Controllable stress (CS) rats could avoid/terminate shock for themselves and their yoked partners by pulling a ceiling chain, whereas their partners, the uncontrollable stress (UCS) rats, could not. Rats earned their entire daily ration of food by pressing a lever. In previous experiments, this paradigm was stressful, but not debilitating and rats continued to eat, groom, sleep, and avoid/escape greater than 99% of shock trials. Locomotor activity and ASR were measured in the present study after saline and 8-OH-DPAT administration (0.25 mg/kg, IP) before, 24 h, and 72 h after shock onset. 8-OH-DPAT only decreased food intake significantly in male and female rats after the first administration. Stress decreased food intake in both the CS and UCS rats, with UCS rats eating the least. However, the effects of stress and 8-OH-DPAT were not additive. 8-OH-DPAT significantly increased peak startle amplitude at 100 and 120 dB, and decreased latency to peak startle amplitude at 100 dB in male and female rats. In contrast, 8-OH-DPAT did not alter percent prepulse inhibition (%PPI) at 100 dB, but significantly decreased %PPI in males but not females at 120 dB. Stress did not have a consistent effect on ASR, but reduced %PPI in males, but not females. Neither stress nor 8-OH-DPAT significantly altered locomotor activity. Although the results do not show an increased sensitivity to 8-OH-DPAT in stressed rats, the unexpectedly weak effects of 8-OH-DPAT alone on the behavioral measures chosen limits the conclusions that can be drawn.

Arteether-induced brain injury in Macaca mulatta. I. The precerebellar nuclei: the lateral reticular nuclei, paramedian reticular nuclei, and perihypoglossal nuclei.

Malaria poses a threat across several continents: Eurasia (Asia and parts of Eastern Europe), Africa, Central and South America. Bradley (1991) estimates human exposure at 2,073,000,000 with infection rates at 270,000,000, illnesses at 110,000,000, and deaths at 1,000,000. Significant mortality rates are attributed to infection by the parasite Plasmodium falciparum, with an estimated 90% among African children. A worldwide effort is ongoing to chemically and pharmacologically characterize a class of artemisinin compounds that might be promising antimalarial drugs. The U.S. Army is studying the efficacy and toxicity of several artemisinin semi-synthetic compounds: arteether, artemether, artelinic acid, and artesunate. The World Health Organization and the U.S. Army selected arteether for drug development and possible use in the emergency therapy of acute, severe malaria. Male Rhesus monkeys (Macaca mulatta) were administered different daily doses of arteether, or the vehicle alone (sesame oil), for a period of either 14 days, or 7 days. Neuropathological lesions were found in 14-day arteether treated monkeys in the precerebellar nuclei of the medulla oblongata, namely: (1) the lateral reticular nuclei (subnuclei magnocellularis, parvicellularis, and subtrigeminalis), (2) the paramedian reticular nuclei (subnuclei accessorius, dorsalis, and ventralis), and the perihypoglossal nuclei (n. intercalatus of Staderini, n. of Roller, n. prepositus hypoglossi). The data demonstrate that the simina meduallry precerebellar nuclei have a high degree of vulnerability when arteether is given for 14 days at dose levels between 8mg/kg per day and 24 mg/kg per day. The neurological consequences of this treatment regimen could profoundly impair posture, gait, and autonomic regulation, while eye movement disorders might also be anticipated.

Chronic sustained stress increases levels of anterior pituitary prolactin mRNA.

Our laboratory is investigating the effects of chronic stress on physiological, endocrine and behavioral measures, in order to elucidate the neuronal substrates for the pathophysiological consequences of stress in humans. In these studies, we have employed a rodent model of sustained stress in which rats are exposed to around-the-clock intermittent footshock that can be avoided or escaped by rats in the controllable stress group, but not by rats in the uncontrollable stress group. Each rat in the uncontrollable stress group is paired (yoked) to a rat in the controllable stress group such that the controllable stress group rat avoids or escapes shock for both rats. A third group of rats receives no shock (controls). We have previously reported that in male rats, plasma prolactin levels were elevated after 3 days of stress in both stress groups. In the present experiments we determined whether the increases in plasma prolactin were correlated with increases in anterior pituitary prolactin mRNA. In addition, we measured hormones and mRNA at three time points and we examined these responses in female as well as male rats. Adult male and female Sprague-Dawley rats were exposed to chronic stress for 1, 3 or 14 days. In unstressed control rats, levels of anterior pituitary prolactin mRNA were fivefold higher in female as compared to male rats. However, stress increased levels of anterior pituitary prolactin mRNA over baseline in both genders. After 1 day of stress, anterior pituitary prolactin mRNA levels increased in male and female rats belonging to both stress groups, with no significant difference seen between the uncontrollable vs. controllable stress groups. After 3 days of stress, anterior pituitary prolactin mRNA levels were even more elevated, and rats in the uncontrollable stress group had higher anterior pituitary prolactin mRNA levels than those in the controllable stress group. After 14 days of stress, there were no significant differences in control and stressed groups with respect to anterior pituitary prolactin mRNA. These data suggest that chronic sustained stress increases the synthesis of anterior pituitary prolactin mRNA during the first days of stress, and that levels return to prestress values sometime between 3 and 14 days of stress.

Coronary artery restenosis after atherectomy is primarily due to negative remodeling.

The primary cause of restenosis following directional coronary atherectomy (DCA) remains obscure. "Negative remodeling," a decrease in vessel area, is believed to be more causative than is increase in plaque area. The DCA technique used in these patients, designed to facilitate the removal of plaque, should allow a more precise evaluation of the relative roles of these two mechanisms. Twenty-five patients underwent DCA. In 17, complete angiographic and intravascular ultrasound (IVUS) images were obtained before and after DCA and at follow-up (6 to 9 months). Internal elastic lamina (IEL), lumen, and plaque areas were calculated at preatherectomy, postatherectomy, and follow-up. Postatherectomy, the mean IEL area increased by 32% and the mean plaque area decreased by 51%, resulting in a significant mean increase in lumen area, 500%. At follow-up when compared to postatherectomy, the change in IEL area was variable; however, the mean did not change significantly (p = 0.58). Plaque area change, when standardized for initial vessel size, was small (mean increase 2.8 +/- 3.5%). The mean lumen area did not decrease significantly at follow-up (p = 0.43). A highly significant correlation (r = 0.96) was noted between IEL area change and lumen area at follow-up. In contrast, the correlation between plaque area change and lumen area change over the same period was much less significant (r = 0.64). These data indicate that decrease in IEL area primarily is responsible for restenosis.

Sustained stress disrupts the performance and acquisition of delayed alternation in rats.

The effects of sustained stress on acquisition and performance of a delayed alternation task were studied in male rats. Rats lived 24 h per day in operant cages where they earned all of their food via lever pressing. During the stress portion of each experiment, one group of rats was able to avoid or escape signaled intermittent footshock (Avoidance/Escape group), a second group (Yoked) did not have control over shock termination, a third group never received shock (Control). Shock trials were presented around-the-clock at approximately 5-min intervals and the stress portion of each study lasted 1 week. We have previously reported that rats tolerate this paradigm well and avoid/escape 99% of the shock trials. Three experiments were conducted. In Experiment 1, rats learned the delayed alternation task prior to stress onset; in Experiment 2, rats were exposed to stress and the alternation task concurrently; in Experiment 3, rats were stressed for 14 days prior to being required to perform the delayed alternation task. In the first experiment, stress decreased both food intake and the accuracy of responding during the first days of stress. In the second experiment (acquisition), stressed rats required more days to reach asymptotic performance on the alternation task. In Experiment 3, rats stressed for 14 days prior to acquisition of the delayed alternation task performed similarly to controls.

Are interatrial band myocytes maximally hypertrophied in normal canine hearts?

In canine right atrial hypertrophy, the cross-sectional area (Axs) of right atrial myocytes increases, whereas the Axs of the broader interatrial band myocytes does not. In the current study, myocyte reconstructions showed that right atrial myocyte length increased in proportion to Axs in right atrial hypertrophy. On the other hand, mean interatrial band myocyte length in both normal and right atrial hypertrophy dogs was roughly inversely proportional to mean Axs, as expected if interatrial band myocyte volume was constant. Plotting mean Axs vs. myocyte length for individual interatrial band myocytes revealed a distribution whose border defined a maximal volume curve; many myocytes were well beneath that curve. Mononuclear myocytes (generally diploid) were limited by a 65,000-micrometer 3 curve, which many binuclear myocytes (generally tetraploid) surpassed; myocyte ploidy thus constrained myocyte volume. However, because many mononuclear and binuclear myocytes had lower volumes, their failure to hypertrophy cannot be attributed to attainment of the maximal volume possible for their ploidy.

Comparison by contact lens cytology and clinical tests of three contact lens types.

PURPOSE: To determine if hydrogel lens materials from different FDA groups have different effects on the human ocular surface. METHODS: Contact lenses from Groups I and II, and Group IV were tested on 10 healthy adapted hydrogel contact lens wearers. For each comparison, the subject wore 2 new contact lenses for approximately 45 min. Lens movement was measured, and comfort evaluated using a subjective scale. After removal of the lenses, fluorescein staining and tear break-up time were measured. Cells were collected by contact lens cytology (CLC), and the size and number of cells harvested from each lens were determined. RESULTS: There were differences between lens groups in lens movement and comfort, but no differences in fluorescein staining or tear break-up time. CLC showed no differences in cell counts or cell size. There was no correlation between the results of CLC and the clinical tests for lens movement, lens comfort, tear break-up time, and fluorescein staining. CONCLUSIONS: With short durations of lens wear, differences in ionicity and water content have no effect upon the size and number of cells collected from a contact lens. This suggests that, in the short-term, the shedding of cells beneath a hydrogel lens is not affected by the choices of lens material.

Fitting characteristics of Dailies daily disposable hydrogel contact lenses.

PURPOSE: We evaluated the fitting characteristics of nelfilcon A contact lenses to determine the optimal time for judging lens performance during trial fitting. METHODS: Twenty subjects wore a pair of DAILIES (daily disposable soft lenses) for 8 hours. A slit lamp camera was used to videotape lens fit every 5 minutes during the first 30 minutes and at 8 hours. Lens movement and centration were measured in 0.1 mm increments by a single observer. RESULTS: Lens movement decreased significantly during the first 30 minutes of wearing time (P < 0.001). Mean lens movement measured 0.31 mm initially, decreased to 0.17 mm at 25 minutes of wear, and then increased to 0.33 mm at 8 hours (P < 0.001). At 5 minutes, 93% of eyes showed lens movement within 0.2 mm of the value at 8 hours. Lens centration did not change during the first 30 minutes of wear and was comparable to centration measured at 8 hours. CONCLUSIONS: During trial fitting, the optimal time for evaluating movement of nelfilcon A contact lenses is during the first 5 minutes of wear. Lens movement measured at 8 hours will likely be greater than lens movement measured 10 to 30 minutes after insertion.

Myocardial blood flow in awake dogs with chronic tricuspid regurgitation.

The primary purpose of this study was to define regional blood flow in dogs with chronic tricuspid regurgitation (TR) in order to determine if the marked hypertrophy of the right atria resulted in compromised myocardial perfusion. Myocardial blood flow (ml/min/gm) was measured with radiolabeled microspheres in eight dogs with TR during rest, moderate exercise (5 dogs), and infusion of adenosine (1 mg/kg/min), an index of minimal vascular resistance. Similar measurements were obtained in eight normal dogs. In TR, the ratio of right atrium (RA) and right ventricle (RV) to body weight was greater than in normal dogs, 77% and 30%, respectively. During rest, flow in the RA appendage was less than in nonappendage region in the normal dogs; no differences were noted in TR dogs, indicating an augmented hemodynamic role of the appendage in TR. Both RA and RV blood flow was greater in TR during rest but no other differences in flow were found between the two groups. Minimum vascular resistance in RV but not RA was slightly increased in TR versus normal. During marked myocyte hypertrophy, the vasculature of RA develops sufficiently to provide the same flow capacity as in the normal heart.

Myonecrosis after revascularization procedures.

The detection of elevated cardiac enzyme levels and the occurrence of electrocardiographic (ECG) abnormalities after revascularization procedures have been the subject of recent controversy. This report represents an effort to achieve a consensus among a group of researchers with data on this subject. Creatine kinase (CK) or CK-MB isoenzyme (CK-MB) elevations occur in 5% to 30% of patients after a percutaneous intervention and commonly during coronary artery bypass graft surgery (CABG). Although Q wave formation is rare, other ECG changes are common. The rate of detection is highly dependent on the intensity of enzyme and ECG measurement. Because most events occur without the development of a Q wave, the ECG will not definitively diagnose them; even the ECG criteria for Q wave formation signifying an important clinical event have been variable. At least 10 studies evaluating > 10,000 patients undergoing percutaneous intervention have demonstrated that elevation of CK or CK-MB is associated not only with a higher mortality, but also with a higher risk of subsequent cardiac events and higher cost. Efforts to identify a specific cutoff value below which the prognosis is not impaired have not been successful. Rather, the risk of adverse outcomes increases with any elevation of CK or CK-MB and increases further in proportion to the level of intervention. This information complements similar previous data on CABG. Obtaining preprocedural and postprocedural ECGs and measurement of serial cardiac enzymes after revascularization are recommended. Patients with enzyme levels elevated more than threefold above the upper limit of normal or with ECG changes diagnostic for Q wave myocardial infarction (MI) should be treated as patients with an MI. Patients with more modest elevations should be observed carefully. Clinical trials should ensure systematic evaluation for myocardial necrosis, with attention paid to multivariable analysis of risk factors for poor long-term outcome, to determine the extent to which enzyme elevation is an independent risk factor after considering clinical history, coronary anatomy, left ventricular function and clinical evidence of ischemia. In addition, tracking of enzyme levels in clinical trials is needed to determine whether interventions that reduce periprocedural enzyme elevation also improve mortality.

Myocardial oxygenation in dogs during reactive hyperemia.

The mechanisms of myocardial oxygenation during reactive hyperemia were studied in the beating heart using continuous near infrared (NIR) spectroscopy. In open chest dogs, NIR spectroscopy was used to monitor brief occlusions of the left anterior descending artery. These occlusions produced a precipitous drop in tissue oxygen stores (tHbO2+MbO2), tissue blood volume, and the oxidation level of mitochondrial cytochrome a,a3. Reperfusion produced a rapid increase in the NIR signals to supranormal levels, followed by gradual return to baseline. When the duration of occlusion was increased from 20 to 120 s, an essentially linear increase was produced in the overshoot areas defined by the NIR signals. Near infrared spectroscopy (NIRS) separated reactive hyperemia into two phases according to the tissue level of deoxyhemoglobin and deoxymyoglobin (tHb+Mb): (1) an early phase during which the tHb+Mb level was supranormal, reflecting enhanced O2 extraction; and (2) a late phase during which the tHb+Mb level was below baseline, reflecting decreased O2 extraction and increased tissue O2 availability. During reactive hyperemia, when O2 availability was maximal by NIR spectroscopy, O2 consumption was elevated but submaximal, indicating that MVO2 was not limited by O2 availability. Cytochrome a,a3 oxidation state also was restored fully. Thus, myocardial oxygenation is highly regulated during reactive hyperemia. Cellular O2 supply and mitochondrial oxidation state are restored early during reactive hyperemia by increased O2 delivery, increases in tissue blood volume and enhanced O2 extraction. © 1998 Society of Photo-Optical Instrumentation Engineers.