Including sheath effects in the interpretation of planar retarding potential analyzer's low-energy ion data.

The interpretation of planar retarding potential analyzers (RPA) during ionospheric sounding rocket missions requires modeling the thick 3D plasma sheath. This paper overviews the theory of RPAs with an emphasis placed on the impact of the sheath on current-voltage (I-V) curves. It then describes the Petite Ion Probe (PIP) which has been designed to function in this difficult regime. The data analysis procedure for this instrument is discussed in detail. Data analysis begins by modeling the sheath with the Spacecraft Plasma Interaction System (SPIS), a particle-in-cell code. Test particles are traced through the sheath and detector to determine the detector's response. A training set is constructed from these simulated curves for a support vector regression analysis which relates the properties of the I-V curve to the properties of the plasma. The first in situ use of the PIPs occurred during the MICA sounding rocket mission which launched from Poker Flat, Alaska in February of 2012. These data are presented as a case study, providing valuable cross-instrument comparisons. A heritage top-hat thermal ion electrostatic analyzer, called the HT, and a multi-needle Langmuir probe have been used to validate both the PIPs and the data analysis method. Compared to the HT, the PIP ion temperature measurements agree with a root-mean-square error of 0.023 eV. These two instruments agree on the parallel-to-B plasma flow velocity with a root-mean-square error of 130 m/s. The PIP with its field of view aligned perpendicular-to-B provided a density measurement with an 11% error compared to the multi-needle Langmuir Probe. Higher error in the other PIP's density measurement is likely due to simplifications in the SPIS model geometry.

Anesthesia breathing circuits protected by the DAR Barrierbac S breathing filter have a low bacterial contamination rate.

PURPOSE: In order to reuse the same anesthesia breathing circuit for more than one patient, it has been proposed to add a breathing filter between the Y-piece and the artificial airway. The purpose of this study was to evaluate the in vivo bacterial filtration efficacy of an anesthesia filter in a usual clinical anesthesia setting. METHODS: A sterile DAR Barrierbac S breathing filter was inserted at the Y-piece of a sterile single-use anesthesia breathing circuit before induction of general anesthesia. At the end of anesthesia, the breathing circuit connector of the filter and of the endotracheal tube connector were cultured separately on growth media (chocolate and blood agar). These were incubated for 48 hr and bacterial identification was conducted using standard methods. RESULTS: Bacterial cultures were negative on both sides of the filter membrane of 1842 of the 2001 filters studied. Cultures were positive on the patient side of 104 filters. In two of those, the same bacteria were found on both the circuit side and the patient side of the filter. Therefore these data indicate a clinical effectiveness of 99.9% (confidence interval, CI 95%, 99.6-99.998%), and an in vivo filtration efficacy of 98.08% (CI 95%, 92.54-99.67%). CONCLUSION: Using the upper limit of the CI, it can be assumed that the practice of using a sterile DAR Barrierbac S breathing filter for every patient while reusing the anesthesia breathing circuit would result in a cross contamination rate of the breathing circuit lower than once every 250 cases.

Another feature of TURP syndrome: hyperglycaemia and lactic acidosis caused by massive absorption of sorbitol.

Endoscopic transurethral resection of the prostate (TURP) can be complicated by absorption of a large volume of irrigation fluid. The clinical features of this complication are referred as the TURP syndrome. We report a case where hyperglycaemia and lactic acidosis complicated the TURP syndrome caused by the massive absorption (approximately 15 litres) of a sorbitol- mannitol irrigation solution. The proposed mechanism is a type B lactic acidosis related to the metabolism of sorbitol.

Submental endotracheal intubation: an alternative to tracheotomy in patients with midfacial and panfacial fractures.

BACKGROUND: The submental route for endotracheal intubation has been proposed as an alternative to tracheotomy in the surgical management of patients with maxillofacial trauma. The purpose of this study was to review our experience with this procedure. METHODS: Medical records of 25 patients who had surgical reduction of midfacial or panfacial fractures while securing their airway with submental intubation were reviewed. After standard orotracheal intubation, a passage was created by blunt dissection with a hemostat clamp through the floor of the mouth in the submental area. The proximal end of the orotracheal tube was pulled through the submental incision. Surgery was completed with minimal interference from the endotracheal tube. At the end of surgery, the tube was pulled back to the usual oral route. RESULTS: Mean duration of surgery was 7.9 hours (range, 2-16 hours). Mean duration of postoperative mechanical ventilation was 5.2 days (range, 1-24 days). Fourteen of these patients required prolonged (>24 hours) postoperative mechanical ventilation because of associated injuries. Two patients later required a tracheotomy because of prolonged respiratory failure. One patient died of multiple organ failure. One complication of the submental intubation was observed: a superficial infection of the submental wound. CONCLUSION: Submental intubation is a simple technique associated with a low morbidity. It is an attractive alternative to tracheotomy in the surgical management of selected cases of maxillofacial trauma.

Alfentanil causes less postoperative nausea and vomiting than equipotent doses of fentanyl or sufentanil in outpatients.

BACKGROUND: The relative potencies of alfentanil, fentanyl, and sufentanil as a risk factor for postoperative nausea and vomiting have not been determined. They were compared in a randomized study designed to obtain equipotent plasma concentrations of these three opioids at the beginning of the recovery period. METHODS: The study included 274 patients treated on an outpatient basis. The steady state opioid plasma concentration providing a predicted 50% reduction of the minimum alveolar concentration of isoflurane was used to determine the relative potency of the opioids. The opioids were prepared in equal volumes at concentrations of alfentanil 150 microg/ml, fentanyl 50 microg/ml, and sufentanil 5 microg/ml and were administered in vol/kg. Anesthesia was induced in a blinded fashion with a bolus of the study opioid (0.05 ml/kg) and 4-6 mg/kg thiopental and was maintained with isoflurane (0.6-1%) in a nitrous oxide-oxygen mixture with a continuous infusion of the study opioid (0.06 ml x kg(-1) x h(-1)). If necessary, up to five additional boluses of opioid (0.02 ml/kg) could be given. This opioid administration protocol was tested by pharmacokinetic simulations. RESULTS: The incidence of postoperative nausea and vomiting was not different in the postanesthesia care unit, but in the ambulatory surgery unit it was significantly lower for alfentanil compared with fentanyl and sufentanil (12, 34, and 35%, respectively P < 0.005). Pharmacokinetic modeling showed that the end-anesthesia opioid plasma concentrations were approximately equipotent in the three groups. However, modeling does not support that the difference between groups in the postoperative period can be explained by a more rapid disappearance of alfentanil from the plasma. CONCLUSIONS: Alfentanil, compared with approximately equipotent doses of fentanyl and sufentanil, is associated with a lower incidence of postoperative nausea and vomiting in outpatients.

Esophageal and tracheal distortion by the Esophageal-Tracheal Combitube: a cadaver study.

PURPOSE: To understand the anatomical relationships of the Esophageal-Tracheal Combitube (ETC) with the larynx, pharynx, esophagus and trachea. METHODS: An extensive dissection of the neck and thorax of a 70-yr-old caucasian male cadaver was done to expose the larynx, pharynx, trachea and esophagus. The ETC was inserted following the manufacturer's recommendations. Effects of the ETC on the surrounding structures were observed with the ETC first inserted in the esophagus and then, in the trachea. RESULTS: When inserted in the esophagus, the ETC produced marked bulging of the anterior wall of the esophagus and anterior displacement (4.5 cm) of the trachea. Inflation of the distal cuff of the ETC produced distension of the esophagus. When inserted in the trachea, the ETC also caused anterior protrusion. CONCLUSION: Protrusion of the anterior wall of the esophagus and distension resulting from inflation of the distal cuff could lead to esophageal injuries. These observations may explain the previously reported complications associated with the use of the ETC.

Complications associated with the use of the Esophageal-Tracheal Combitube.

PURPOSE: To report four cases of subcutaneous emphysema, pneumomediastinum and pneumoperitoneum associated with the use of the Esophageal-Tracheal Combitube (ETC) during prehospital management of cardiac arrest. CLINICAL FEATURES: Between September 1994 and April 1996, 1139 patients were resuscitated with the ETC and the semiautomated external defibrillator as part of the CPR protocol for prehospital management of cardiac arrest by basic emergency medical technicians. Eight of these patients presented with subcutaneous emphysema. Four of them, declared dead after arrival in the emergency room (ER), had autopsy studies. In two, autopsy revealed large (6 and 6.5 cm respectively) longitudinal transparietal lacerations of the anterior wall of the oesophagus. Multiple superficial lacerations of the oesophagus were also present in another patient, while no lesion of the airway or the oesophagus was found in the last patient. CONCLUSION: These cases suggest that subcutaneous emphysema, pneumomediastinum and pneumoperitoneum might be complications associated with the use of the ETC. At least in two cases, oesophageal laceration appears to be the mechanism by which these complications occurred.

Neostigmine requirements for reversal of neuromuscular blockade following an infusion of mivacurium.

PURPOSE: To study the efficacy of neostigmine compared with placebo for the antagonism of neuromuscular blockade at the end of a mivacurium infusion, and to determine its optimal dose. METHODS: One hundred adult patients undergoing an elective surgical procedure received a standardized anaesthetic with 20-30 micrograms.kg-1 alfentanil, a propofol infusion and nitrous oxide. Muscle relaxation was maintained at 90-95% T1 depression with 0.2 mg.kg-1 mivacurium followed by an infusion. Neuromuscular blockade was measured with an integrated evoked electromyogram in response to train-of-four (TOF) stimuli at the ulnar nerve every 20 sec. Patients were randomized into four groups. At the end of surgery, the mivacurium infusion was stopped and patients received, immediately, in a double-blind manner, neostigmine (10, 20, or 40 micrograms.kg-1) or placebo according to a random number table. The T1 and TOF ratio were recorded until adequate recovery of neuromuscular function (TOF ratio > 0.70). During the reversal period, non-invasive blood pressure and heart rate were recorded every minute. The incidence of postoperative nausea and vomiting (PONV) was recorded in the recovery room. RESULTS: Data from 94 patients who completed the protocol were analysed. Compared with placebo, neostigmine 10 micrograms.kg-1 did not reduce the time to TOF > 0.70 (17.0 +/- 5.1 vs 14.6 +/- 4.2 min respectively). However the time was decreased with neostigmine 20 micrograms.kg-1 and 40 micrograms.kg-1 (P < 0.001), but with no difference between these last two groups (11.4 +/- 3.0 and 11.4 +/- 3.5 min respectively). Changes in systolic blood pressure and heart rate were not different between the four groups. Very few PONV events were observed in all groups (global incidence 7.4%). CONCLUSION: Recovery of neuromuscular blockade following a mivacurium infusion is accelerated by neostigmine. A dose of neostigmine 20 micrograms.kg-1 appears optimal with no further reduction in recovery time obtained from a larger dose.

Weaning from ventilatory support.

Resumption of spontaneous unassisted breathing after an episode of acute respiratory failure often is achieved without major difficulty. In a significant number of patients however, weaning from mechanical ventilation is a long and difficult process that markedly increases the duration of mechanical ventilation and consumes a significant fraction of critical care resources. Some criteria have been suggested to predict early and more accurately the moment the patient is ready to be separated from the ventilator. At the present time, the f/VT ratio (rapid shallow breathing index) appears to yield the best predictive power. None of these indices, however, is powerful enough to be relied on solely, and their use should be limited to that of aids to the critical care physician. The inability to sustain spontaneous ventilation usually is the consequence of an imbalance between respiratory demand and respiratory muscle capacity. Increased elastic workload, increased resistive workload, and increased VE are the main causes of excessive demand imposed on the respiratory system. Respiratory muscle pump failure usually relates to peripheral nerve dysfunction or muscular dysfunction. Left ventricular dysfunction also is an important cause of weaning failure. The usual methods of weaning from mechanical ventilation are T-piece trials with abrupt definitive discontinuation of mechanical ventilation if tolerated or with progressive intermittent trials, IMV, and PSV. All have their advantages and disadvantages, and the method of weaning per sé is not the only critical factor. Although their conclusions were different regarding the best method of weaning, however, two recent clinical trials suggest that ventilatory management has a major influence on the outcome of weaning from mechanical ventilation in difficult-to-wean patients. The global management of such patients requires a systematic approach with consideration of all factors involved in the process of separation from the ventilator. New computer-assisted systems already are tested and, in the future, may provide a significant advantage in the management of weaning from mechanical ventilation.

Treatment of postoperative nausea and vomiting: comparison of propofol, droperidol and metoclopramide.

PURPOSE: To compare the efficacy of propofol in a subhypnotic dose (10 mg iv), droperidol (1.25 mg iv), or metoclopramide (10 mg iv) in the treatment of PONV in the post anaesthesia care unit (PACU). METHODS: In a prospective, randomized, double-blind protocol, over four months, all eligible inpatients and outpatients were asked to give their consent to be included in the study should they suffer PONV in the PACU. They received a standardized general anaesthetic without any prophylactic antiemetic drug. In the recovery room, patients complaining of persistent nausea (lasting more than ten minutes) and/or experiencing at least two episodes of retching or vomiting were given one of the three study drugs. Recurrence of retching or vomiting was recorded for 60 min after administration of the study drug and nausea severity was assessed on a visual analog scale. Patients still complaining of PONV 30 min after administration of the study drug received a rescue medication (dimenhydrinate). RESULTS: Seventy-eight patients received one of the study drugs. The recurrence of retching or vomiting was higher with propofol (58%) than with droperidol (4%) or metoclopramide (24%) (P < 0.001). More patients who received propofol needed the rescue medication (54%) than those who received droperidol (15%) or metoclopramide (28%) (P < 0.02). No difference was observed in nausea severity. CONCLUSION: A subhypnotic dose of propofol (10 mg iv) is less effective than the conventional antiemetic drugs droperidol and metoclopramide for the treatment of PONV in the PACU.

Expiratory muscle activity increases intrinsic positive end-expiratory pressure independently of dynamic hyperinflation in mechanically ventilated patients.

Intrinsic positive end-expiratory pressure (PEEPi) has usually been interpreted as suggesting dynamic hyperinflation, but expiratory muscle activity may also increase end-expiratory alveolar pressure without any additional increase in end-expiratory lung volume. The aim of this study was to assess the influence of expiratory muscle activity, which increases abdominal pressure during expiration and is followed by a sudden drop at end-expiration, on PEEPi measurement in mechanically ventilated patients. We studied eight tracheally intubated patients breathing in an assisted mode in whom expiratory muscle activity was present. PEEPi was measured from the fluctuations of esophageal pressure (Pes) while continuous recording of gastric pressure (Pga) and of changes in abdominal cross-sectional area assessed expiratory muscle activity. PEEPi was also measured by the airway occlusion method in one patient, and diaphragmatic electromyographic activity was recorded to determine the timing of inspiratory muscle activity in two patients. Varying the level of ventilatory support (pressure support level, peak flow rate, or PEEP level) induced increases in measured PEEPi from 6.7 +/- 3.4 to 13.2 +/- 5.9 cm H2O. Concomitantly, the expiratory rise in Pga increased from 3.1 +/- 2.7 to 8.6 +/- 5.0 cm H2O, and the abrupt decay in Pga observed at the end of expiration increased from 4.2 +/- 3.7 to 10.6 +/- 6.1 cm H2O. The drop in Pga and the drop in Pes at end-expiration were synchronous, and these changes, together with electromyographic measurements, were consistent with a concomitant relaxation of the expiratory muscles and activation of the inspiratory muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of pressure-controlled with different I:E ratios versus volume-controlled ventilation on respiratory mechanics, gas exchange, and hemodynamics in patients with adult respiratory distress syndrome.

BACKGROUND: Pressure-controlled (PCV) and pressure-controlled inverse ratio ventilation (PCIRV) have been proposed instead of volume-controlled conventional ratio ventilation (VC) with positive end-expiratory pressure (PEEP) for patients with adult respiratory distress syndrome (ARDS). The advantages advocated with the use of PCIRV are to decrease airway pressures and to improve gas exchange. However, most studies did not compare PCIRV and VC while keeping both the level of ventilation and end-expiratory alveolar pressure (total-PEEP) constant. METHODS: Nine patients with moderate to severe ARDS (lung injury score 2.83 +/- 0.18) had their lungs ventilated with VC, PCV with a conventional ratio (I:E 1:2; PC 1/2), and PCIRV (I:E 2:1 and 3:1; PC 2/1 and PC 3/1, respectively). Ventilator settings were adjusted to keep tidal volume, respiratory rate, FIo2, and total-PEEP constant in every mode. With each mode, a complete set of ventilatory, hemodynamic, and gas exchange parameters was obtained after 30 min. RESULTS: In PC 3/1, the data obtained could not be strictly compared to the other modes because total-PEEP was higher despite external-PEEP being set at zero. For the other modes (VC, PC 1/2, and PC 2/1), despite differences in peak airway pressures, no difference was noted for end-inspiratory and end-expiratory static airway pressures (which better reflect alveolar pressures) nor for lung and respiratory system compliance. Arterial oxygenation deteriorated slightly with PC 2/1 despite a higher mean airway pressure, whereas alveolar ventilation tended to be slightly, but not significantly, improved (lower PaCo2). A decrease in systolic and mean arterial pressure also was observed with PC 2/1 without other significant hemodynamic change. CONCLUSIONS: In this prospective controlled study, no short-term beneficial effect of PCV or PCIRV could be demonstrated over conventional VC with PEEP in patients with ARDS.

Renal function and hemodynamics during prolonged isoflurane-induced hypotension in humans.

The effect of isoflurane-induced hypotension on glomerular function and renal blood flow was investigated in 20 human subjects. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured by inulin and para-aminohippurate (PAH) clearance, respectively. Anesthesia was maintained with fentanyl, nitrous oxide, oxygen, and isoflurane. Hypotension was induced for 236.9 +/- 15.1 min by increasing the isoflurane inspired concentration to maintain a mean arterial pressure of 59.8 +/- 0.4 mmHg. GFR and ERPF decreased with the induction of anesthesia but not significantly more during hypotension. Postoperatively, ERPF returned to preoperative values, whereas GFR was higher than preoperative values. Renal vascular resistance increased during anesthesia but decreased when hypotension was induced, allowing the maintenance of renal blood flow. We conclude that renal compensatory mechanisms are preserved during isoflurane-induced hypotension and that renal function and hemodynamics quickly return to normal when normotension is resumed.

Risk of cross-infection related to the multiple use of disposable syringes.

The rate of blood contamination of IV tubings used in anaesthesia practice was investigated. Only IV tubings started in the operating room were studied. First, 300 tubings of three different types were tested at the three distal injection sites. The contamination rate was 3.3 per cent at the injection site closest to the IV catheter and 0.3 per cent at the furthest. The presence of a check-valve did not affect the contamination rate. Second, 300 third injection sites fixed at a level equal to or above the IV catheter were tested. None of them was contaminated. Finally, in order to evaluate whether changing the needle alone could prevent the contamination of syringes, injections were made into a tubing where blood was flowing. Thirty-four per cent of the syringes tested positive for blood. We conclude that IV tubings have a significant contamination rate in usual practice. This rate decreases as the distance from the IV catheter increases. The use of the third site fixed at a level equal to or above the IV catheter carries a lower risk of contamination. Changing the needle alone is a useless procedure to prevent cross-contamination.

Isoflurane-induced hypotension in orthognathic surgery.

The effect of isoflurane-induced hypotension on reduction of blood loss, improvement of surgical field, and postoperative edema was investigated in 52 patients undergoing combined maxillary and mandibular osteotomies. Anesthesia was maintained with fentanyl, N2O, O2, and isoflurane. Deliberate hypotension was induced by increasing isoflurane inspired concentration. Blood loss in the hypotensive group (MAP 55-65 mm Hg) was significantly less than that in the control group (MAP 75-85 mm Hg); 454.0 +/- 211.3 mL versus 755.3 +/- 334.6 mL (P less than 0.001). Fewer patients had to be transfused in the hypotensive group, 12.0% versus 44.4% (P less than 0.02). The surgical field was significantly improved by the hypotensive technique, but operative time was not shortened. Subjective and objective measurements of postoperative edema failed to show any effect of deliberate hypotension. Our data suggest that isoflurane-induced hypotension effectively reduces blood loss and the number of transfusions in orthognathic surgery.