Surface structure on diamond foils generated by spatially nonuniform laser irradiation.

Here we report on the effects of material strength factors on the generation of surface structure due to nonuniform laser irradiation. The influence of material strength on the generation of perturbation on a diamond surface subjected to nonuniform laser irradiation was experimentally investigated. Our previous investigations suggested that stiffer and denser materials reduce surface perturbation due to spatially nonuniform laser irradiation, which was reproduced well by calculations with multi-dimensional hydrodynamic simulation code. In this work, we found that local fractures due to yield strength failure are generated by high degrees of irradiation non-uniformity. A characteristic crack-like surface structure was observed, which was not reproduced by the 2D simulation code calculations at all. The 2D simulations showed that the pressure at the diamond surface locally exceeds the Hugoniot elastic limit due to nonuniform irradiation, implying the potential for development of surface perturbations. We also measured the areal-density distribution of perturbations for single-crystal diamond and diamond with a thin high atomic number (high-Z) coating on its surface. The experimental results imply that the combination of a stiff material and thin high-Z coating can suppress the solid-strength effects caused by large irradiation non-uniformity. The knowledge given here is applicable to inertial confinement fusion target design, laser material processing, and universal problems involving solids and high-energy-density plasmas.

Effect of sulfur on sound velocity of liquid iron under Martian core conditions.

Sulfur has been considered to be a predominant light element in the Martian core, and thus the sound velocity of Fe-S alloys at relevant high pressure and temperature is of great importance to interpret its seismological data. Here we measured the compressional sound velocity (VP) of liquid Fe, Fe80S20 and Fe57S43 using ultrasonic pulse-echo overlap method combined with a Kawai-type multi-anvil apparatus up to 20 GPa, likely corresponding to the condition at the uppermost core of Mars. The results demonstrate that the VP of liquid iron is least sensitive to its sulfur concentration in the Mars' whole core pressure range. The comparison of seismic wave speeds of Fe-S liquids with future observations will therefore tell whether the Martian core is molten and contains impurity elements other than sulfur.

Interstitial hydrogen atoms in face-centered cubic iron in the Earth's core.

Hydrogen is likely one of the light elements in the Earth's core. Despite its importance, no direct observation has been made of hydrogen in an iron lattice at high pressure. We made the first direct determination of site occupancy and volume of interstitial hydrogen in a face-centered cubic (fcc) iron lattice up to 12 GPa and 1200 K using the in situ neutron diffraction method. The transition temperatures from the body-centered cubic and the double-hexagonal close-packed phases to the fcc phase were higher than reported previously. At pressures <5 GPa, the hydrogen content in the fcc iron hydride lattice (x) was small at x < 0.3, but increased to x > 0.8 with increasing pressure. Hydrogen atoms occupy both octahedral (O) and tetrahedral (T) sites; typically 0.870(±0.047) in O-sites and 0.057(±0.035) in T-sites at 12 GPa and 1200 K. The fcc lattice expanded approximately linearly at a rate of 2.22(±0.36) Å3 per hydrogen atom, which is higher than previously estimated (1.9 Å3/H). The lattice expansion by hydrogen dissolution was negligibly dependent on pressure. The large lattice expansion by interstitial hydrogen reduced the estimated hydrogen content in the Earth's core that accounted for the density deficit of the core. The revised analyses indicate that whole core may contain hydrogen of 80(±31) times of the ocean mass with 79(±30) and 0.8(±0.3) ocean mass for the outer and inner cores, respectively.

[New use of pulse-oximeter as a prophylactic Stimulator to the wearer and a lifesaving tools for prevention of hypoxic mishaps].

Pulse-oximeter has been widely used for the clinical assessment of physical status of a patient and as an alarming tool of hypoxia to medical personnel at the bedside or in the observation center. However, it has never been used for direct stimulation of the wearer. We considered innovation of pulse-oximeter as a prophylactic alarm-oximeter for the wearer. If SPO2 goes down to unfavorable level, the alarm-oximeter starts to send signal through a control box to a stimulator, such as an electrical nerve stimulator, a cold thermal tip, or mechanical device like a vibrator or compressor. The dermal stimulator is usually fixed to the right or left wrist with a Velcro band. The control box is affixed to the wristband by using Velcro. The alarm may be sent to an earphone or speaker with a verbal command like "take a deep breath". Alarm-oximeter will be combined to an oxygen inhaler or mechanical ventilatory assist device, or a drug administration system through electric line or wireless transmitter to start or change its function before the arrival of medical personnel. It will prevent hypoxic mishaps during medical intervention or sleep apnea syndrome. It will be also applicable to stop snoring.

Development of a comb needle with five needles for securing access to large blood vessels during emergency resuscitation.

PURPOSE: Animal tests have indicated that providing venous-arterial (V-A) bypass extracorporeal circulation immediately after cardiac arrest is a useful resuscitation technique for achieving resumption of a normal cardiac function and brain resuscitation. However, pulsation of the femoral artery cannot be felt in the case of cardiac arrest, and it takes a long time to puncture the femoral artery and vein. We developed a comb needle that has five 18-gauge metallic needles fixed in parallel on a plastic board. In this study, we investigated whether the comb needle would achieve puncturing of the femoral artery and vein in cadavers. METHODS: The comb needle was used to puncture the femoral artery and the femoral vein in 45 donated bodies. We placed the center needle of the comb needle 2 cm perpendicularly caudal to a point approximately one quarter the distance along a straight line connecting the anterior superior iliac spine and the pubic tubercle. An autopsy was performed following puncturing, and it was determined whether needles of the comb needle punctured the femoral artery and/or the femoral vein. RESULTS: Puncturing of both the femoral artery and the femoral vein was achieved in 35 cases (78%). In the left groin, both the femoral artery and the femoral vein were punctured in 16 cases (94%), and in the right groin, both the femoral artery and the femoral vein were punctured in 19 cases (68%). CONCLUSION: Using a comb needle, one insertion can achieve simultaneous puncturing of the femoral artery and the femoral vein with a high success rate in cadavers.

Morphological and crystallographic evolution of bainite transformation in Fe-0.15C binary alloy.

In this article, an in situ observation method, combining laser scanning confocal microscopy and electron backscattering diffraction, was used to investigate the morphological and crystallographic evolution of bainite transformation in a Fe-0.15C binary alloy. The nucleation at a grain boundary and inclusions, sympathetic nucleation, and impingement event of bainitic ferrite were directly shown in real time. The variant evolution during bainite transformation and misorientation between bainitic ferrites were clarified. Strong variant selection was observed during sympathetic nucleation.

Stress measurement under high pressure using Kawai-type multi-anvil apparatus combined with synchrotron radiation.

A system for stress measurement under high pressure has been developed at beamline BL04B1, SPring-8, Japan. A Kawai-type multi-anvil apparatus, SPEED-1500, was used to pressurize polycrystalline KCl to 9.9 GPa in a mechanically anisotropic cell assembly with the KCl sample sandwiched between dense Al(2)O(3) pistons. The variation of deviatoric stress was determined from the lattice distortion measured using two-dimensional X-ray diffraction with monochromatic synchrotron X-rays. The low-pressure B1 phase transformed to the high-pressure polymorph B2 during compression. The deviatoric stress increased with increasing pressure in both the B1 and B2 phases except for the two-phase-coexisting region at a pressure of 2-3 GPa. This new system provides one of the technical foundations for conducting precise rheological measurements at conditions of the Earth's lower mantle.

[Well leg compartment syndrome during prolonged surgery in the lithotomy position].

We experienced a case of the well leg compartment syndrome (WLCS) during total pelvic exenteration in a 54-year-old woman. She was placed in the head down-lithotomy position and her both lower legs were attached with elastic stocking and intermittent pneumatic compression for prevention of deep vein thrombosis. The surgery lasted for 13 hr and 15 min. Her vital signs stayed stable during the procedure. After emergence from anesthesia, she complained of severe bilateral crural pain. We found that her calves were swollen and rigid. Creatinin kinase increased to 40120 U x l(-1) the following morning. She was diagnosed as WLCS, and the left fibula paralysis remained as legacy of WLCS. WLCS during surgery is caused by inappropriate positioning of the lower limbs, in contrast to a compartment syndrome caused by trauma or injury. Its etiology consists of multi-factors e.g., prolonged surgery in the lithotomy position and hypo-perfusion. We emphasize the importance of both prevention and early treatment of WLCS. All anesthesiologists should pay attention to WLCS.

Rapidly induced hypothermia with extracorporeal lung and heart assist (ECLHA) improves the neurological outcome after prolonged cardiac arrest in dogs.

PURPOSE: We reported previously that therapeutic hypothermia with extracorporeal lung and heart assist (ECLHA) improved neurological outcome after 15 min cardiac arrest (CA) in dogs, although 45 min was needed to achieve hypothermia. We now investigate whether rapidly induced hypothermia with ECLHA (RHE) would result in a better outcome than slowly induced hypothermia with ECLHA (SHE) in dogs. METHODS: Fifteen mongrel female dogs were divided into two groups: an RHE (n = 7) and an SHE (n = 8) group. Normothermic ventricular fibrillation was induced for 15 min and the animals were resuscitated by ECLHA. Rapid hypothermia was induced with a heat exchanger added to the ECLHA circuit in the RHE group, and by immersing the drainage tube of the ECLHA circuit in an ice water bath in the SHE group. Hypothermia (33 degrees C) was maintained for 20 h. The dogs were weaned from ECLHA at 24 h after resuscitation and treated for 96 h; neurological deficit scores (NDS) were measured throughout this period. RESULTS: It took 1.6+/-0.8 min to reach 33 degrees C in the RHE group and 49.5+/-12.1 min to reach 33 degrees C in the SHE group. There was no difference in survival rate between the two groups. The NDS at 96 h in the RHE group was better than that in the SHE group (26% (range: 10-28%) versus 32% (26-37%); p < 0.05) although there was no significant difference in NDS between the two groups until 72 h. CONCLUSION: Rapid hypothermic induction might be an important factor to improve neurological outcomes in prolonged CA models.

The efficacy of epidural analgesia after video-assisted thoracoscopic surgery: a randomized control study.

PURPOSE: Video-assisted thoracoscopic surgery (VATS) is known to reduce the severity of pain after surgery. However, it has not yet been established whether epidural anesthesia/analgesia (EA) is necessary after VATS. We therefore conducted a randomized control study to examine whether or not EA is necessary for pain control after VATS. PATIENTS AND METHODS: Forty-six patients undergoing VATS were randomly allocated to one of 2 groups: 24 who were given EA after the procedure (EA group) and 22 who were not (NEA group). Patients in the EA group received a continuous infusion of fentanyl and bupivacaine via an epidural catheter for 2 days after VATS. The degree of postoperative pain was assessed on the total dose of additional analgesics administered, a visual analog scale (VAS), a verbal pain score at rest (VPS-R) and on movement (VPS-M), from the day of surgery to the 2nd postoperative day (2 POD). RESULTS: Additional use of rectal diclofenac sodium and intramuscular pentazocine was more frequent in the NEA group than in the EA group (p<0.05). The VAS, VPS-R, and VPS-M scores were significantly lower in the EA group than in the NEA group at 0 POD, from 0 to 1 POD, and from 0 to 2 POD, respectively (p<0.0001-0.05). Stepwise regression analysis revealed that EA was a significant independent variable of VPS-R and VPS-M from 0 to 1 POD (p<0.05). However, the incidence of nausea/vomiting in the EA group was 29%, which was more frequent than in the NEA group (5%) (p<0.05). CONCLUSION: While EA causes nausea/vomiting in some patients, it is effective for pain control until 1 POD after VATS, especially for pain on movements.

Two cases of transient left ventricular apical ballooning syndrome associated with subarachnoid hemorrhage.

Electrocardiogram (ECG) abnormalities secondary to subarachnoid hemorrhage are well known, but the etiology remains unclear. Transient left ventricular apical ballooning syndrome is characterized by acute onset myocardial infarction-like symptoms, transient (reversible) cardiac dysfunction, and shapes resembling ampulla on left ventriculography. We managed general anesthesia for two patients with transient left ventricular apical ballooning and ECG abnormalities associated with subarachnoid hemorrhage. During anesthesia, their hemodynamic status was almost stable although their cardiac performance analyzed by transthoracic echocardiography and transesophageal cardiography was poor. Anesthetic management of this syndrome may be simplified if less cardiosuppressive anesthetic management is used. We recommend evaluating cardiac function with transthoracic echocardiography or transesophageal cardiography when an subarachnoid hemorrhage patient has ECG abnormalities.

A moderate dose of propofol and rapidly induced mild hypothermia with extracorporeal lung and heart assist (ECLHA) improve the neurological outcome after prolonged cardiac arrest in dogs.

BACKGROUND AND PURPOSE: Propofol has been shown to protect against neuronal damage induced by brain ischaemia in small animal models. We reported previously that mild hypothermia (33 degrees C) in combination with extracorporeal lung and heart assist (ECLHA) improved the neurological outcome in dogs with cardiac arrest (CA) of 15 min induced during normothermia. In the present study, we investigated the neuroprotective effect of propofol infusion under mild hypothermia with ECLHA in this model. METHODS: Twenty-one female dogs (15 mongrel dogs and 6 beagles) were divided into three groups: Midazolam 0.1 mg/(kg h) infusion group (M, n=7), Propofol 2 mg/(kg h) infusion group (P2, n=7), Propofol 4 mg/(kg h) infusion group (P4, n=7). Normothermic ventricular fibrillation (VF) was induced in all dogs for 15 min, followed by brief ECLHA and 168 h of intensive care. The drug infusion was initiated at a constant rate after the restoration of spontaneous circulation (ROSC) to 24 h. Mild hypothermia (33 degrees C) was maintained for 20 h. Neurological deficit scores (NDS: 0%=normal, 100%=brain death) were evaluated for neurological function from 33 to 168 h. RESULTS: One dog in the M group died, and the remaining dogs survived for 168 h. The P4 group showed better neurological recovery compared with the M group (48 h, 21+/-16% versus 32+/-15%; 72 h, 7+/-6% versus 25+/-11%; 96 h, 6+/-6% versus 21+/-6%; 120 h, 5+/-5% versus 20+/-6%; 144 h, 4+/-4% versus 20+/-6%; 168 h, 4+/-4% versus 20+/-6%, p<0.05). One dog in the P2 and three dogs in the P4 group achieved full neurological recovery (NDS: 0%). The number of intact pyramidal cells in the hippocampal CA1 was greater in the propofol groups than midazolam group (p<0.05). CONCLUSION: The combination of propofol infusion at a rate of 4 mg/(kg h), 24h and rapidly induced mild hypothermia (33 degrees C) with ECLHA might provide a successful means of cerebral resuscitation from CA.

[A Kumamoto model of clinical training in tracheal intubation for paramedics].

BACKGROUND: Since July 2004, the Japanese Ministry of Health, Labor and Welfare approved certified paramedics to perform emergency prehospital tracheal intubation. A specialized training system in tracheal intubation has been established in Kumamoto Prefecture. METHODS: The Kumamoto Prefectural Medical Control Organization, a tracheal intubation task force was established: consisting of the departments of Anesthesiology at Kumamoto University and 11 other major hospitals, along with Kumamoto Prefecture and the 14 prefectural fire-departments. This group published the Kumamoto training guidelines and a training system for paramedics. RESULTS: Kumamoto Prefecture appealed for support of paramedic activity on television and in newspapers as public education. The prefectural governor officially asked hospitals to train paramedics. Because 9 of the 14 fire-departments had no regional teaching hospital, trainees were matched with other hospitals by the task force. The task force published a pamphlet to inform surgical patients about tracheal intubation training and to recruit patients as practice volunteers. Anesthesiologists undertook significant roles in making arrangements to facilitate the project at their hospitals, in addition to teaching paramedics prior to surgical procedures. CONCLUSIONS: The Medical Control Organization, Kumamoto Prefecture, anesthesiologists and emergency response personnel worked together successfully to promote the training program for clinical tracheal intubation by paramedics.

The effects of pre-arrest heparin administration dose for cardiac arrest model using extracorporeal lung and a heart assist (ECLHA) in dogs.

BACKGROUND AND PURPOSE: Clinical and experimental studies have shown that marked activation of blood coagulation occurs in cardiac arrest (CA) and cardiopulmonary resuscitation (CPR). Extracorporeal lung and heart assist (ECLHA) is applied in CA patients who cannot be rescued using conventional therapies. We hypothesized that the dose of heparin administered during the pre-arrest period would influence the outcome in a canine model of CA induced by 15 min of normothermia followed by ECLHA, which consists of heparin coating membrane lung and tubing. We therefore investigated the effects of two dose regimes of the pre-arrest heparin for this model. METHODS: Twelve mongrel female dogs were divided into two groups: a group given 200 U/kg heparin (H200, n=6) and a group given 700 U/kg heparin (H700, n=6), group during pre-arrest period. Normothermic ventricular fibrillation (VF) was induced in all dogs for 15 min, followed by 24h of ECLHA with rapidly induced mild hypothermia (33 degrees C) and 120 h of intensive care. Outcome evaluations included: (1) activated coagulation time (ACT); (2) catecholamine dose; (3) hematocrit (Hct) and platelet count; (4) survival rate; (5) neurological deficit scores (NDS); (6) postmortal macroscopic examination with the exception of the brain. RESULTS: In the H200 group, four dogs died of cardiogenic shock within 28 h. The autopsy revealed extensive patchy hemorrhages in the heart and intestine. In the H700 group, the amount of dopamine was significantly lower (6+/-10mg versus 75+/-41 mg, p<0.05) and the survival rate was significantly higher (100% versus 17%, p<0.05) than in the H200 group. The NDS at 120 h in the H700 group was 18+/-8% and the autopsy revealed an almost normal external appearance of the vital organs. There were no significant differences between groups in either the Hct and platelet count during the 24h of resuscitation, and no bleeding complications were observed. CONCLUSION: The use of ECLHA to resuscitate animals in prolonged CA may require a large dose of systemic heparin during the pre-arrest period even if ECLHA circuit was coated with heparin.

[Anesthetic managements of a patient with congenital myotonia (Becker type)].

A 55-year-old woman with congenital myotonia (Becker type), diagnosed by muscle biopsy and gene examination, underwent a right lower lobectomy assisted with thoracoscopy for lung cancer. After epidural tube replacement at T 9-10, general anesthesia was introduced with propofol 2.5 mg x kg(-1) and fentanyl 2.5 mcg x kg(-1). Vecuronium was administered prudently for muscle relaxation assessed with T 1 response (%) and train-of-four (TOF) ratio (%). T 1 response decreased to 50% 3 min and disappeared 4 min after vecuronium administration. Then she was intubated with a double lumen endobronchial tube. T 1 increased 25% within 27 min and 75% in 40 min. Surgery was uneventful and completed in 180 min. At the end of the operation, there was neither fading of twitch responses nor tetanic responses, and TOF ratio returned to 100%. An acetylcholinesterase inhibitor was not given. She was extubated when normal spontaneous breathing, clear consciousness and adequate pharyngeal reflex were present 25 min after discontinuing propofol. There were no perioperative adverse events including hyperthermia and myoglobinuria related to malignant hyperthermia. In conclusion, we managed the anesthesia for a patient with congenital myotonia (Becker type) with thoracic epidural anesthesia and total intravenous anesthesia. Non-depolarizing muscle relaxant could be used safely at the same dose as that used in non-myotonic patients, and did not cause malignant hyperthermia.

Comparison of a new heparin-coated dense membrane lung with nonheparin-coated dense membrane lung for prolonged extracorporeal lung assist in goats.

Thrombosis and bleeding are major complications in cases of prolonged extracorporeal lung assist (ECLA) with an artificial-membrane lung. Antithrombogenic treatment of the artificial-membrane oxygenator and circuits is indispensable for safe ECLA. The efficacy of a new heparin-coated membrane lung with minimal systemic heparinization was evaluated for 7 days and compared with a nonheparin-coated membrane lung in goats. The animals were randomly assigned to either the heparin-coated membrane group (HM group, n = 5) or nonheparin-coated membrane group (NHM group, n = 5). Activated coagulation time (ACT) during ECLA was controlled to below 150 s in the HM group, and to near 200 s in the NHM group. All goats in the HM group were sustained on ECLA for 7 days, but two goats in the NHM group died on the 4th and 6th days, respectively. The mean systemic administration rate of heparin during ECLA was 22.4 +/- 4.4 U/kg/h in the HM group and 39.0 +/- 10.0 U/kg/h in the NHM group. There was a significant difference between the two groups (P < 0.05). The oxygen transfer rate, the Pco(2) difference, the perfusion resistance, and platelet counts showed no significant changes. There was no plasma leakage from the artificial lung. Although several clots were observed in the stagnant areas of the artificial lung, they did not lead to deterioration of the function of the artificial lung. The excellent antithrombogenicity, gas exchange ability, and durability of this new artificial lung with circuits might contribute to successful prolonged ECLA with minimal systemic heparinization.

A comparison of postoperative sore throat after use of laryngeal mask airway and tracheal tube.

PURPOSE: We compared the degree of postoperative sore throat (PST) after use of a laryngeal mask airway (LMA; by two insertion techniques) and a tracheal tube (TT) in adult patients. METHODS: Eighty-six adult patients undergoing surgery of an extremity were randomized into three groups. The LMAs (size 4 for men, 3 for women) and TTs were lubricated with 2% lidocaine gel. After the induction of anesthesia, an LMA with the cuff deflated was inserted and then the cuff was inflated in group A, an LMA with the cuff inflated was inserted in group B, and the trachea was intubated using vecuronium in group C; staff anesthesiologists performed all these methods. LMA cuffs were inflated with the maximum recommended volume of air. TT cuffs were inflated with the minimum volume of air without gas leakage at 20 cm H(2)O pressure. The mode of ventilation depended on the individual anesthesiologists. Blood traces on the devices were examined after their removal. PST was rated immediately after anesthesia and on the first postoperative day, using a three-point score and a 100-mm visual analog scale, respectively. RESULTS: Most of the patients receiving an LMA breathed spontaneously and those receiving a TT underwent controlled ventilation. The ratio of positive blood traces on devices, as well as the degree of PST immediately after anesthesia, was similar in the three groups; however, on the first postoperative day, the severity of PST was greater in the LMA groups than in the TT group ( P = 0.016). The severity of PST was similar with the two LMA insertion techniques. CONCLUSION: In the conditions of our study, LMAs inserted with the cuff either fully inflated or deflated worsened PST compared with TTs.

[A case of intraoperative hyperkalemia probably due to preoperative feeding with a liquid enteral formulation].

A 67-year-old man with no remarkable preanesthetic complications underwent esophagectomy. The patient was fed only with a liquid enteral formulation (Ensure Liquid) for 5 days immediately before the operative day. His serum potassium level before Ensure Liquid administration was 4.1 mEq x l(-1). The first blood analysis at 30 min after the initiation of the surgery revealed an increase in serum potassium level (5.9-6.1 mEq x l(-1)) without any conceivable cause during the anesthetic management. A glucose-insulin infusion treatment lowered the serum potassium level to the normal range. The serum potassium level re-increased to 6.4 mEq x l(-1) 4 days after the initiation of jejunal feeding with Ensure Liquid and then returned to the normal range after the termination of Ensure Liquid administration. The patient's perioperative course suggests that the preoperative Ensure Liquid administration is a probable cause of the intraoperative hyperkalemia in this case.

[Anesthetic management for a patient with platypnea-orthodeoxia syndrome].

We experienced anesthetic management for a patient with platypnea-orthodeoxia syndrome. This syndrome is relatively uncommon and accompanies dyspnea and hypoxemia on changing to a sitting or standing from recumbent position. A 75-year-old man with the syndrome underwent atrial septal defect closure on cardiopulmonary bypass. General anesthesia was induced and maintained with midazolam, propofol, fentanyl and vecuronium bromide. During the induction, Spo2 decreased suddenly from 100% to 70%, Spo2, however, recovered to 97% immediately after changing to Trendelenburg position. The perioperative and postoperative course was uneventful, except for hypoxemia during induction. Although the exact mechanisms of platypneaorthodeoxia remains to be solved, right-to-left shunt by an anatomical abnormality and by change of the atrial septum is considered one of the hypoxic mechanisms. We suggest that it is necessary to prevent right-to-left shunt and hypoxemia in anesthetic management of a patient with platypneaorthodeoxia syndrome.

[Tracheal intubation using a guide wire for a 5-month-old baby with epiglottic cyst].

We report anesthetic management for a 5-month-old baby with an epiglottic cyst causing stenosis of the upper airway and growth failure. A flexible guide wire was first inserted into the trachea through the forceps port of the fiberoptic bronchoscope (O.D. 3.5 mm) nasally. After removal of the bronchoscope, the trachea was then successfully intubated with a trachea tube (I.D. 3.5 mm) passed over it. Her symptom improved after removal of the cyst. A flexible guide wire combined with fiberoptic bronchoscope is useful in tracheal intubation for a baby with a difficult airway.