Laser phase noise to intensity noise conversion by lowest-order group-velocity dispersion in optical fiber: exact theory.

An exact result for the spectral density of intensity variations that occur after propagation of ergodic light in a medium having lowest-order-only group-velocity dispersion is obtained and applied to the problem of semiconductor laser phase noise to intensity noise conversion in a single-mode optical fiber. It is shown that the intensity spectrum after propagation formally approaches, for a large laser linewidth or a long (or high-dispersion) fiber, the intensity spectrum of a thermal source having the same line shape as the laser.

Statistical determination of the length dependence of high-order polarization mode dispersion.

We describe a method of characterizing high-order polarization mode dispersion (PMD). Using a new expansion to approximate the Jones matrix of a polarization-dispersive medium, we study the length dependence of high-order PMD to the fourth order. A simple rule for the asymptotic behavior of PMD for short and long fibers is found. It is also shown that, in long fibers (~1000 km), at 40 Gbits/s the third- and fourth-order PMD may become comparable to the second-order PMD.

Intraocular pressure fluctuations during scleral buckling surgery.

PURPOSE: The purposes of this study are to measure real-time intraocular pressure (IOP) during scleral buckling and to determine the effects of elevated IOPs on ocular perfusion. PATIENTS AND METHODS: A standard 4-mm, 20-gauge infusion cannula was inserted through the pars plana, connected to a standard hemodynamic monitoring unit with an electronic pressure transducer, and calibrated. The authors measured real-time IOP in 20 eyes undergoing scleral buckling surgery for primary rhegmatogenous retinal detachments and determined the IOP required to close the central retinal artery. Pressure measurements were read from the monitor videoscreen intraoperatively and from a continuous paper tracing postoperatively. RESULTS: The patients ranged in age from 24 to 88 years (mean, 59.7 years). The highest IOP elevations occurred during scleral depression and cryopexy, ranging up to 210 mmHg (mean, 116 mmHg). Pressures at which the central retinal artery closed ranged from 48 to 110 mmHg (mean, 79.2 mmHg). Manipulations of the globes caused IOPs greater than the central retinal artery perfusion pressures in 13 of the 20 patients. The duration of pressures in excess of the central retinal artery perfusion pressure ranged from 6 to 402 seconds (mean, 118.8 seconds). There were no intraoperative or postoperative complications from the infusion cannula. CONCLUSIONS: Conventional scleral buckling surgery causes wide fluctuations in IOP and may impair ocular perfusion. Additional studies are needed to determine the long-term consequences of these pressure elevations.

Delayed postoperative nonembolic neurologic deficit after left internal carotid endarterectomy in presence of known contralateral carotid artery occlusion.

Neurologic sequelae are known complications of carotid endarterectomy. The current overall perioperative stroke rate is 2-5% (1,2). The incidence of perioperative morbidity and mortality is increased in certain patient subgroups, including neurologically unstable patients and patients who have known contralateral common or internal carotid artery occlusion (3-9). We present the case of a patient who had known total right carotid artery occlusion and a known previous right cerebrovascular accident who sustained a contralateral deficit after a left internal carotid thromboendarterectomy with vein patch angioplasty.

Frequency control using a complex effective reflectivity in laterally coupled semiconductor laser arrays.

Frequency selectivity of a novel type of multielement, multisection laterally coupled semiconductor laser array is studied using the round-trip method. It is found that such a structure should lead to a strong frequency selectivity owing to a periodic dependency of the threshold gain on the frequency. A gain-guided two-coupledcavity device was fabricated. The experimental results show excellent agreement with the theoretical prediction.

Nerve blocks in the critical care environment.

The critical care patient population has much to gain from properly administered neural blockade. Effective analgesia alone may make the difference between a patient who is able to compensate for their acute insult and one who cannot. A good example is the patient with multiple fractured ribs, who, after intercostal nerve blocks, no longer requires intubation and mechanical ventilation. The authors believe that effective analgesia is just the beginning of the beneficial effects of neural blockade, because blockade of the afferent limb of sympathetic and sensory nerves may circumvent the neuroendocrine response to acute injury. There is evidence that the stress response is not beneficial in the hospital setting and in fact may be detrimental. Some of the effects include elevated plasma catecholamines, ADH, cortisol, and blood glucose, which contribute to tachycardia, hypertension, increased myocardial work and oxygen consumption, salt and water retention, and a catabolic state with negative nitrogen balance. Whether these changes result in reduced morbidity and mortality has been the subject of several studies, but more studies are needed. It would seem that critically ill patients with little physiologic reserve might be the best population to study because even a small improvement may improve survival. A small beneficial effect in healthy postoperative patients may not be clinically apparent. Most would agree that neural blockade used intraoperatively results in reduced blood loss and a lower incidence of postoperative thromboembolism. The continuation of these techniques into the postoperative period may reduce morbidity and mortality in high-risk patients. A word of caution is in order. The indiscriminate application of the techniques described in this article to critically ill patients would not be in the patients' best interest. Nerve blocks are only safe in the hands of those physicians specifically trained to perform them. In addition, local anesthetics have a low therapeutic ratio, and their administration requires continual observation. The use of epidural or intrathecal opioids alone or in combination with other agents also has potentially serious side effects, and requires continual patient monitoring. The proper performance and maintenance of these techniques requires a large commitment of time, manpower, equipment, and a multidisciplinary approach to include physicians, nursing, and support staff. Nerve blocks and other sophisticated techniques started in the operating room or critical care unit should not necessarily be discontinued when the patient is transferred to a ward bed because the full benefit of this therapy may not have been fully realized.(ABSTRACT TRUNCATED AT 400 WORDS)

Saphenous nerve entrapment at the adductor canal.

A retrospective study of 30 patients who met the clinical criteria for saphenous nerve entrapment at the adductor canal is described. Patients experienced symptoms, usually anterior knee pain, for an average of 36 +/- 7 months. Each patient received an average of 1.9 +/- 0.4 saphenous nerve blocks at the adductor canal during treatment. Baseline pain level (measured by the visual analog scale) was 6.4 +/- 0.3. Final pain level at followup was significantly decreased (2.8 +/- 0.5, P less than 0.001). Eighty percent of patients had improved after a series of blocks. Age, medications taken, number of blocks performed, and length of followup were unrelated to outcome. Length of symptoms did significantly correlate with final pain level (r = 0.39, P less than 0.05). The diagnosis of this syndrome, description of the saphenous nerve block at the adductor canal, and the possible etiology are presented.

An evaluation of analgesia associated with the immobility response in laboratory rabbits.

The immobility response (IR) was studied in rabbits to evaluate its analgesic properties and reliability as a method of restraint. The participation of the endogenous opioid system in IR was studied indirectly by evaluating the effects of the narcotic antagonist naloxone on this phenomenon. Twenty-four adult New Zealand White rabbits were subjected to six noxious stimuli while restrained by IR and while restrained under control conditions. Testing on each animal was repeated under both conditions following the administration of naloxone. The noxious stimuli consisted of three levels of electric shock (10 volts, 30 volts, and 50 volts) applied to the shaved forearm, and mechanical pressure applied to the pinna, front toe, and hind toe. Withdrawal and changes in blood pressure, heart rate, and respiration were used as indicators of pain perception. Distress associated with noxious electrical and pressure stimulation was significantly reduced by IR, which suggested that the phenomenon does have a significant analgesic component. However, the rabbits showed wide variability in their susceptibility to IR induction, and even animals which did not withdraw in response to noxious stimulation under IR sometimes exhibited physiological changes suggestive of distress. Therefore, IR should not be considered as a reliable or humane alternative to analgesic/anesthetic drugs for laboratory rabbits. Naloxone had little effect on IR or IR-associated analgesia.

The effects of pentobarbital, fentanyl-droperidol, ketamine-xylazine and ketamine-diazepam on arterial blood pH, blood gases, mean arterial blood pressure and heart rate in adult male rats.

Although anesthetics are known to cause respiratory and cardiovascular depression in humans, these adverse effects rarely have been investigated in laboratory rodents. This study evaluated the effects of four different injectable drugs, pentobarbital, fentanyl-droperidol (Innovar-Vet), ketamine-xylazine and ketamine-diazepam on the respiratory and cardiovascular systems of rats. Results showed marked acidosis, hypercarbia and hypoxia with high doses of Innovar-Vet, moderate respiratory depression with all dosages of pentobarbital and minimal respiratory depression with ketamine-xylazine and ketamine-diazepam. Innovar-Vet, ketamine-xylazine and pentobarbital caused profound hypotension, particularly at high dosages, while ketamine-diazepam caused the least depression in mean arterial blood pressure of all drugs evaluated. None of the drugs studied produced significant alterations in heart rate. Throughout all dosages investigated, the ketamine-diazepam combination showed the least overall effects on ventilation and perfusion of the four parenteral drug combinations studied.

A comparison of pentobarbital, fentanyl-droperidol, ketamine-xylazine and ketamine-diazepam anesthesia in adult male rats.

Proper use of anesthetics is of paramount importance for humane animal care. Current research trends show a greater reliance on rats for laboratory investigations. This study compared several dosages for four different drugs, (pentobarbital, fentanyl-droperidol, ketamine-xylazine and ketamine-diazepam) for use in the laboratory rat. Each drug was evaluated in respect to its onset, duration of effect, recovery, adverse effects and mortality. A quantitative assessment of the depth of anesthesia also was obtained for all dosages of each drug. Results showed that all tested dosages of pentobarbital, ketamine-xylazine and ketamine-diazepam were suitable anesthetics for use in the laboratory rat. Low dosages of fentanyl-droperidol (Innovar-Vet), however, appeared to produce a state known as neuroleptanalgesia as opposed to anesthesia seen with the other agents.

The effects of pentobarbital, fentanyl-droperidol, ketamine-xylazine and ketamine-diazepam on noxious stimulus perception in adult male rats.

The increased use of rats for laboratory studies has increased the urgency for a better understanding of their perception of painful or noxious stimuli and the means of obtunding such stimuli. Four different injectable drugs, pentobarbital, fentanyl-droperidol (Innovar-Vet), ketamine-xylazine and ketamine-diazepam, were evaluated in this study. Unlike previous studies in rodents, this study has quantitated noxious stimulus perception following a temporal sequence of observations. Results showed the greatest inhibition of noxious stimulus perception with Innovar-Vet, lesser inhibition with ketamine-xylazine and ketamine-diazepam, and the least obtunding of nociception with pentobarbital. Results of this study also suggested that a spatial orientation, similar to that present in man, exists within the CNS of rats for receipt of noxious stimulation.

The effects of pentobarbital, fentanyl-droperidol, ketamine-xylazine and ketamine-diazepam on core and surface body temperature regulation in adult male rats.

Many commonly used anesthetics cause hypothermia by inhibiting central and peripheral thermoregulatory mechanisms. Although it is probable that a loss of thermal homeostasis contributes directly to the high mortality frequently reported following anesthesia of laboratory rodents, this adverse effect has been investigated rarely in the past. This study compared the effects of three parenteral anesthetics (pentobarbital, ketamine-xylazine and ketamine-diazepam) and a neuroleptanalgesic (fentanyl-droperidol) on core and surface body temperature regulation in rats. Results showed a profound hypothermia with all dosages of pentobarbital, while ketamine-xylazine and ketamine-diazepam caused a dose-dependent depression in core and surface body temperature. All dosages of fentanyl-droperidol (Innovar-Vet) caused minimal depression in thermoregulation, suggesting that it is the drug which requires the least external thermal support. Results of this study also suggested that inability to compensate for heat loss, particularly from the body core, may profoundly influence anesthetic toxicity and the safety of anesthetic procedures.

Cardiovascular response to endotracheal intubation during four anesthetic techniques.

In order to investigate the impact of four different anesthetic regimens on the cardiovascular response to endotracheal intubation, 24 patients were anesthetized to approximately the same depth of anesthesia (MAC-fraction) using one of four supplements to N2O-O2 (60:40) anesthesia: Group 1 = morphine 0.5 mg/kg i.v., Group 2 = Innovar 0.1 ml/kg i.v., Group 3 = halothane, 0.4% end-tidal, Group 4 = enflurane 0.7% end-tidal. Cardiovascular variables were recorded from radial arterial and thermistor-tipped pulmonary artery catheters and were determined at the following times: (1) while the patients were awake prior to induction of anesthesia; (2) after a stable level of anesthesia had been induced; and (3) within the first minute after endotracheal intubation. Only the groups receiving morphine and halothane sustained significant increases in heart rate, rate pressure product and pulmonary capillary wedge pressure after intubation. It is concluded that N2O-O2 anesthesia supplemented with either morphine or halothane at the 1 MAC dose level does not adequately prevent cardiovascular response after endotracheal intubation, and that these techniques require additional anesthetic depth or other adjuvant drugs in order to prevent cardiovascular complications.

Cardiac catheters for diagnosis and treatment of venous air embolism: a prospective study in man.

One hundred consecutive patients undergoing neurosurgical procedures in the seated position were monitored for venous air embolism with a Swan-Ganz pulmonary artery (PA) catheter, precordial Doppler ultrasound device, and continuous end-tidal CO2 (FETCO2) analysis. Simultaneous determinations of right atrial and pulmonary capillary wedge pressures were also performed during each operation. Although 80 episodes of air embolism were detected by changes in Doppler sounds, only 36 were associated with increased PA pressure, and only 30 developed a decrease in FETCO2. Changes in PA pressure and FETCO2 agreed closely (r = 0.86), and only marked changes were associated with systemic hypotension. Air was recovered from the right atrium and PA only in small amounts (2 to 20 ml) during air embolism, although it was possible to aspirate large quantities of blood. Twenty-nine patients were found to have right atrial pressures that were higher than pulmonary capillary wedge pressures. Paradoxical air embolism from a probe-patent foramen ovale was possible in these patients, and one developed signs and symptoms of systemic air embolism postoperatively. We conclude that noninvasive monitoring with the combination of a precordial Doppler device and end-tidal CO2 analysis is satisfactory for rapid detection of clinically significant venous air embolism. The unique advantage of Swan-Ganz monitoring, however, is that it permits identification of patients who may sustain paradoxical air embolism, and that it differentiates the hemodynamic effects of brain-stem manipulation from those caused by air embolism.