Head-down Position Research Articles

Cerebellar infarction after uncomplicated laparoscopic hysterectomy

Objective To review a case of cerebellar infarction after uncomplicated laparoscopic hysterectomy. Methods The clinical course and radiological findings in a 45-year-old previously healthy woman are reviewed. Results The patient suffered from headache and dizziness from the first postoperative day after a surgically uncomplicated laparoscopic hysterectomy. At 1 week after the operation she was readmitted because of continuing symptoms. Computer tomographic scan results of the brain were normal, while magnetic resonance imaging revealed two small infarctions in the right cerebellar hemisphere. At 3 years after the operation she still has mild neurological symptoms and is not capable of her previous work. Conclusion Haemodynamic changes related to head-down tilt position combined with pneumoperitoneum may have hazardous effects on the brain even in previously clinically healthy patients.

Pediatric foreign body aspiration.

1. Joshua D. Rovin, MD* 2. Bradley M. Rodgers, MD† 1. 2. *Division of Pediatric Surgery, Children’s Medical Center, University of Virginia Health Sciences Center, Charlottesville, VA. 3. 4. †Editorial Board. After completing this article, readers should be able to: 1. Delineate the signs and symptoms of foreign body aspiration. 2. Explain the process of evaluating for suspected foreign body aspiration. 3. Describe the possible radiographic manifestations of foreign body aspiration. 4. Explain the management of foreign body aspiration in children. 5. Delineate the potential late complications of foreign body aspiration. Aspiration of foreign bodies by children can lead to serious illness and sometimes even death. According to National Safety Council statistics, in 1995, mechanical suffocation accounted for 5% (167) of all unintentional deaths among children in the United States younger than 4 years of age. Most of these deaths occurred in children younger than 1 year of age (81 deaths, accounting for 10% of all unintentional deaths in children younger than 1 year of age). Fortunately, the number of deaths has decreased over the past decade, which has been attributed to better education of the public and stricter guidelines for toy manufacturers. Nevertheless, foreign body aspiration continues to account for a significant number of preventable childhood deaths. Younger children are at the highest risk for accidental foreign body aspiration. This increased incidence has been attributed to several factors among younger children, including that they: 1) have the tendency to put small objects into their mouths; 2) often cry, shout, run, and play with objects in their mouths; and 3) do not have molars to chew certain foods adequately. Most studies show that fewer than 15% of foreign body aspirations occur among children older than 5 years of age. Boys comprise more than 50% of all cases of foreign body aspiration. By far, the objects aspirated most frequently are organic or food matter. In North America, peanuts are most common (Table 1⇓ ). View this table: Table 1. Commonly Aspirated Foreign Bodies Unlike aspiration in adults, there is only a …

Histochemical responses of human soleus muscle fibers to long-term bedrest with or without countermeasures.

Effects of 2- or 4-month bedrest in -6 degrees head-down tilt position with or without countermeasures on the histochemical properties of fiber phenotype and cross-sectional area (CSA) were studied in human soleus. The CSAs in slow fibers decreased approximately 32% during 4-month bedrest. This reduction was normalized after 1-month recovery. Although the reduction of percent slow fibers was not significant statistically, the percent intermediate fibers was significantly elevated 4 months after bedrest. Such shift in fiber type was not normalized following 1-month recovery. Effects of wearing an anti-g Penguin suit which has a modest, but continuous resistance at the knee and ankle (Penguin-1) or with knee resistance without loading on the ankle (Penguin-2) for 10 consecutive hours daily were also investigated during approximately 2 months of bedrest. The subjects performed knee extension and flexion for the last 15 min of each hour while in a supine position in bed. Bedrest-induced fiber atrophy was prevented in the Penguin-1 group but not the Penguin-2 group. Transformation of fiber type was not prevented in either Penguin suit group. It is suggested that long-term bedrest causes an atrophy and a shift of fiber phenotype toward fast-twitch type in human soleus. Data also indicated that loading on the muscle is an effective countermeasure for prevention of fiber atrophy but not fiber-type transformation.

Observations in energy and macronutrient intake during prolonged bed-rest in a head-down tilt position

Objective: to report observations in energy and macronutrient intakes, and body weight duringprolonged bed-rest in a head down tilt (HDT) position. Design: open study, each subject was his own control, and was studied during 14 days of baseline, 42 days of −6° HDT bed-rest, and 12 days of recovery. Subjects: eight healthy young man were recruited, one dropped out. Methods: energy and macronutrientcontent of the diet were calculated from weighed amounts of food consumed and French food composition tables. Results: body weight declined during HDT (74.0±3.2 to 71.8±3.2 kg, P < 0.001) andincreased during recovery (72.7±3.2 kg, P < 0.001). Energy intake decreased during HDT (by 17% after 4–5 weeks) and increased during recovery but remained lower than during baseline ( P < 0.001). During HDT fat intake, expressed by a percentage of energy, decreased ( P < 0.01) while carbohydrate increased ( P = 0.04); protein intake did not change ( P = 0.08). The reverse trends were observed during recovery. Conclusions: the present study reports a spontaneous reduction in energy and relative fat intake duringprolonged HDT bed rest. We believe that these findings have implications for the clinical setting.

Beyond the Trendelenburg position: Friedrich Trendelenburg's life and surgical contributions

Friedrich Trendelenburg (1844-1924) was a giant figure in the formative years of modern surgical practice. His name lives on in the Trendelenburg position, a pelvis-up, head-down position that is of great use in surgical practice. That position, however, was certainly well known before Trendelenburg and the linkage of his name was by no means the greatest of Trendelenburg's achievements. Trendelenburg was a world class surgeon, innovator, and educator who made novel advances that spanned the entire range of surgical practice. (Surgery 1999;126:78-82.)

Brain Norepinephrine Changes With Simulated Weightlessness and Relation to Exercise Training

Maintenance of nervous system function during periods of a deconditioning syndrome is important to prevent diminished psychological/behavioral, and physiological function observed during periods of bed rest, physical inactivity, and weightlessness. A main neurotransmitter is norepinephrine (NE), and its regulation yields insight into nervous system function. This research tested the hypotheses that, 1) deconditioning syndrome induced by simulated weightlessness of 9 days via the head-down tilt (HDT) model results in a blunted noradrenergic turnover rate in selected brain tissue and, 2) that exercise training acts as a countermeasure for these changes in noradrenergic activity. Male Sprague–Dawley rats (3 months, n = 60) were divided into either a HDT (HDT, n = 20), cage control (CAGE-CN, n = 20) or an exercise trained HDT (HDT-EX, n = 20) group. Each group was further subdivided into a saline ( n = 10) or alpha-methyl-tyrosine (AM, n = 10) (200 mg/kg) injected subgroup. Animals in the HDT groups were tail suspended in a 30° head-down tilt position for 9 days. Norepinephrine turnover was determined 3 h following administration of saline or alpha-methyl-para-tyrosine. The NE turnover rate (ng gm −1·h −1) for the CN, HDT, and HDT-EX groups, respectively, were as follows: locus coeruleus, 63 ± 33, *134 ± 65, 85 ± 61; hypothalamus, 195 ± 50, *47 ± 47; *93 ± 34; cerebellum, 10 ± 18, *65 ± 15, *53 ± 19; cerebral cortex, 6 ± 20, *28 ± 15, *68 ± 22. (*Denotes significant difference from the control group at the p ⩽ 0.05 level of significance; +denotes significant difference from the HDT group at the p ⩽ 0.05 level of significance.) These findings suggest that: 1) norepinephrine turnover rate adapts in a tissue-specific manner following a 9-day tail suspension, 2) increased norepinephrine turnover rates and norepinephrine tissue content in the HDT group are consistent with neural adaptation to a chronic stress response.

An experimental study of death in a reverse suspension.

To investigate the course of respiration and circulation in a head-down position, 14 rabbits were set in reverse suspension. The respiratory rate increased a little, but the amplitude of the respiratory movements did not change in the beginning of the experiment. The amplitude of respiratory movements then began to reduce gradually, and toward the end of the experiment, it reduced suddenly. PaO2 increased in the beginning of the experiment and then began to decrease in accordance with the reduction of the amplitude of the respiratory movements. The blood pressure decreased with relation to the decrease of PaO2 resulting from the reduction of the amplitude of the respiratory movements. All rabbits died in 17 to 44 hours (average, 26 hours). The results suggested that the cause of death in a head-down position is due to postural asphyxia resulting from hindered respiratory movements, and that it is possible to survive for at least half a day in a head-down position.

THE USE OF SMALL NEEDLE REDUCES HEMATOMA FORMATION DURING INTERNAL JUGULAR VEIN PUNCTURE

S202 INTRODUCTION: The internal jugular vein (IJV) is commonly used as an access to the central venous system. However, there are no reports on the relationship between needle size and complications. In this report, we examined whether smaller needles reduce hematoma formation as one of the complications of IJV cannulation. METHOD: With informed consent and institutional approval, a 16-gauge catheter (Argyle, Japan) was inserted in the IJV of 32 adult patients, who were randomly divided into three groups according to the needle diameter used for the initial puncture: group A; 22-gauge, group B; 18-gauge, and group C; 14-gauge. After induction of anesthesia with thiamylal and fentanyl and tracheal intubation with vecuronium, patients were mechanically ventilated and placed in a 10?head-down (Trendelenberg) position [1,2]. The occiput was placed on a 2 cm-thin pad and the head was rotated 30?to the left. The IJV were punctured under ultrasonic guidance using a 7.5-MHz linear transducer (Model SSA-260A, Toshiba, Japan). Ultrasonic images and physician's voice were recorded in the video tape recorder for later analysis. Control diameter of the IJV before puncture (IJ), the narrowest diameter of the IJV during puncture (IJ punct), the number of patients in which the needle was penetrated from anterior wall to posterior wall of the IJV (Penet), the number of puncture times (Punct), and the incidence of hematoma formation diagnosed by ultrasound (Hem), were recorded for individual patient. Student's t test, Fisher's probability test, and Mann Whitney's test were used for the statistical analysis, with P< 0.05 considered to be significant. RESULTS: (Table 1)Table 1DISCUSSION: We demonstrated that the use of a small needle for IJV cannulation resulted in vessel puncture with less vessel distortion, decreased the puncture of the posterior wall, and reduced the incidence of hematoma formation. This is likely due to less vessel trauma with the smaller needles.

Effects on respiratory function of the head-down position and the complete covering of the face by drapes during insertion of the monitoring catheters in the cardiosurgical patient.

BACKGROUND: We evaluated the effect on the respiratory gas exchange of the 30 degrees head-down position and the complete covering of the face by sterile drapes. These are used to cannulate the internal jugular vein and position the pulmonary artery catheter in the cardiosurgical patient. During the two manoeuvres, 20 coronary patients and 10 patients with end-stage heart disease were supplied with oxygen (FiO2 =0.4) by a Venturi mask, while 20 coronary patients breathed room air. The arterial blood samples to measure oxygen (PaO2) and carbon dioxide (PaCO2) tension and oxygen saturation (SaO2) were analysed by a blood gas system. RESULTS: The contemporary application of the head-down position and the drapes over the face significantly increased PaO2 and SaO2 in all the patientssupplied with oxygen. Without the head-down position, leaving the drapes over the face, did not significantly change the two parameters in the coronary patients supplied with oxygen, but induced a significant increase in PaO2 and SaO2 in the patients with end-stage heart disease. In the coronary patients that were breathing room air, PaO2 and SaO2 were stable throughout the study. CONCLUSIONS: We conclude that the 30 degrees head-down position and the complete covering of the face by drapes does not interfere with respiratory gas exchange and can be safely performed in coronary patients supplied with oxygen or breathing room air and in patients with end-stage heart disease supplied with oxygen (FiO2 of 0.4).

Interpersonal relationships in isolation and confinement: Long-term bed rest in head-down tilt position

The long-term bed-rest was organized by ESA and CNES, in order to simulate the physiological effects of weightlessness: eight volunteers had to stay 42 days in bed, in a head down tilt position (−6 °). There were two subjects in a room, they could not be alone and it was difficult for them to have their own personal space and intimacy. In these circumstances, as in outer space, interpersonal relationships were of prime importance. This situation enabled us, through systematic observation, to analyze the evolution of the relational behavior in dyads, and to quote some social indicators of adaptation. Results show significant withdrawal, and the time spent alone was marked by the emergence, during the experiment, of specific preferential activities. Behavioral contagion was observed in each dyad (people engaged in the same activities at the same time), except in the one case of abandon. Moreover, the highest rates of inactivity and withdrawal were noted in this case. Verbal indicators were useful to comment these results and showed that, for all the dyads, one of the two subjects always played a regulating role by expressing a very positive perception of the situation. These results emphasize the importance of psycho-sociological factors in isolation and confinement. Thus, it appears that different modalities of interpersonal relationships, and not only verbal interactions, play a significant role in adaptation to stress situations.

Energy and substrate metabolism during a 42-day bed-rest in a head-down tilt position in humans.

Microgravity-induced changes in body composition (decrease in muscle mass and increase in fat mass) and energy metabolism were studied in seven healthy male subjects during a 42-day bed-rest in a head-down tilt (HDT) position. Resting energy expenditure (REE), fat and glucose oxidation were estimated by indirect calorimetry on days 0, +8 and +40 of the HDT period. Assessments were performed both in post-absorptive conditions and following two identical test meals given at 3-h intervals. Body composition (dual x-ray absorptiometry) was measured on days 0, +27, +42. Mean post-absorptive lipid oxidation decreased from 53 (SEM 8) mg x min(-1) (day 0) to 32 (SEM 10) mg x min(-1) (day 8, P = 0.04) and 36 (SEM 8) mg x min(-1) (day 40, P = 0.06). Mean post-absorptive glucose oxidation rose from 126 (SEM 15) mg x min(-1) (day 0) to 164 (SEM 14) mg x min(-1) (day 8, P = 0.04) and 160 (SEM 20) mg x min(-1) (day 40, P = 0.07). Mean fat-free mass (FFM) decreased between days 0 and 42 [58.0 (SEM 1.8) kg and 55.3 (SEM 1.7) kg, P < 0.01] while fat mass increased without reaching statistical significance. The mean REE decreased from 1688 (SEM 50) kcal x day(-1) to 1589 (SEM 42) kcal x day(-1) (P = 0.056). Changes in REE were accounted for by changes in FFM. Mean energy intake decreased from 2532 (SEM 43) kcal x day(-1) to 2237 (SEM 50) kcal x day(-1) (day 40, P < 0.01) with only a minor decrease in the proportion of fat. We concluded that changes in fat oxidation at the whole body level can be found during HDT experiments. These changes were related to the decrease in FFM and could have promoted positive fat balance hence an increase in fat mass.

The effect of pneumoperitoneum on intraocular pressure in rabbits with alpha-chymotrypsin-induced glaucoma.

Increased intraperitoneal pressure is associated with physiological changes including alterations of intraocular pressure (IOP). We have previously shown that IOP is not adversely affected by increased intraperitoneal pressure up to 15 mm Hg in women with no preexisting eye disease. The aim of this study was to measure IOP changes associated with increased intraperitoneal pressure (up to 15 mm Hg) of 2 h duration in 12 rabbits with alpha-chymotrypsin-induced glaucoma. A reliable model of glaucoma was created by injecting alpha-chymotrypsin into the posterior chamber of the right eye in 12 rabbits. Thereafter, 5 of the 12 rabbits with glaucomatous eyes were treated with topical timolol. The left eye was used as a control. During pentobarbital general anesthesia, increased intraperitoneal pressure up to 15 mm Hg was created by intraperitoneal CO2 insufflation. Body temperature and expired CO2 were kept constant throughout the study. IOP measurements were made using an electronic pneumotonometer. IOP, mean arterial pressure, heart rate, and central venous pressure were recorded in head-up and head-down positions before, during, and after increased intraperitoneal pressure. The IOP of both eyes, in both treated and untreated rabbits, increased significantly from baseline only when increased intraperitoneal pressure associated with the head-down position resulted in a significant increase in central venous pressure. However, the IOP increase remained within the diurnal range. The major finding of this study is that, in a reliable model of glaucoma, CO2 pneumoperitoneum was associated with an increase in IOP when a head-down position was combined with pneumoperitoneum. In rabbits with alpha-chymotrypsin-induced glaucoma, increased intraperitoneal pressure (up to 15 mm Hg) resulted in a significant intraocular pressure increase when pneumoperitoneum was associated with the head-down position. However, the intraocular pressure increase remained within the diurnal range.

The Effect of Pneumoperitoneum on Intraocular Pressure in Rabbits with alpha-Chymotrypsin-Induced Glaucoma

Increased intraperitoneal pressure is associated with physiological changes including alterations of intraocular pressure (IOP).We have previously shown that IOP is not adversely affected by increased intraperitoneal pressure up to 15 mm Hg in women with no preexisting eye disease. The aim of this study was to measure IOP changes associated with increased intraperitoneal pressure (up to 15 mm Hg) of 2 h duration in 12 rabbits with alpha-chymotrypsin-induced glaucoma. A reliable model of glaucoma was created by injecting alpha-chymotrypsin into the posterior chamber of the right eye in 12 rabbits. Thereafter, 5 of the 12 rabbits with glaucomatous eyes were treated with topical timolol. The left eye was used as a control. During pentobarbital general anesthesia, increased intraperitoneal pressure up to 15 mm Hg was created by intraperitoneal CO2 insufflation. Body temperature and expired CO2 were kept constant throughout the study. IOP measurements were made using an electronic pneumotonometer. IOP, mean arterial pressure, heart rate, and central venous pressure were recorded in head-up and head-down positions before, during, and after increased intraperitoneal pressure. The IOP of both eyes, in both treated and untreated rabbits, increased significantly from baseline only when increased intraperitoneal pressure associated with the head-down position resulted in a significant increase in central venous pressure. However, the IOP increase remained within the diurnal range. The major finding of this study is that, in a reliable model of glaucoma, CO2 pneumoperitoneum was associated with an increase in IOP when a head-down position was combined with pneumoperitoneum. Implications: In rabbits with alpha-chymotrypsin-induced glaucoma, increased intraperitoneal pressure (up to 15 mm Hg) resulted in a significant intraocular pressure increase when pneumoperitoneum was associated with the head-down position. However, the intraocular pressure increase remained within the diurnal range. (Anesth Analg 1998;86:1283-8)

Pressure-diameter characteristics of the rat saphenous vein in vivo. Effects of using different technics to change intraluminal pressure

THE RAT SAPHENOUS VEIN IN VIVO. EFFECTS OF USING DIFFERENT TECHNICS TO CHANGE INTRALUMINAL PRESSURE G.L. Nadasy, M. Lorant, M. Szentivanyi Jr., M. Toth. E. Monos Exp Res Dept and 2nd Inst Physiol, Semmelweis Univ Med, Budqwst, Hunga~ Aim of study: To test whether the observed in vivo rigidity of the rat saphenous vein depends on the dynamics of intraluminal pressure change and the technics of its inducement. Methods: Intraluminal pressure in the saphenous vein of anesthetized rats was changed by (i) graded occlusion of the thigh with an elastic thread at different rates, (ii) infusion f . 1’ o same and withdrawal of blood through a side branch at different rates (iii) acute head-up and head-down tilt positions. Outer diameter of the exposed saphenous artery was measured by an in vivo microangiograph. Results: Independently of the method applied and of the rate of pressure change the vessels were showing substantial rigidity with the exception of lowest pressures (<I -2 mmHg). Conclusion: Diameter of the saphenous vein of rhe rat is hardly influenced by the intraluminal pressure changes commonly occurring in this vessel in vivo. Lumen volume will be rigidly set by smooth muscle contractlon-relaxation, only a moderate role can be attributed to distensibility. (OTKA TOOl77-95 and CO I94B)

SIMPLIFIED ECHO-GUIDED INTERNAL JUGULAR VEIN PUNCTURE

Introduction. Echo-guided cannulation of the right internal jugular vein (RIJV) provides a higher success rate and fewer complications than traditional landmark-guided techniques. [1,2] However, the echo-guided puncture needs a dedicated echo scanner (Site Rite[registered sign]) and/or sterile scanner manipulation, both of which may reduce the advantage of the echo-guided technique. We describe the "simplified" echo-guided puncture, in which echo images are only used to determine the site and direction of needle insertion, and IJV cannulation is performed in a usual manner without echo-guidance after sterile draping. This prospective randomized study was undertaken to compare the access rate and the incidence of carotid artery (CA) puncture by the simplified echo-guided technique with the landmark-guided technique. Methods. After institutional approval, 160 adult patients aged 27 to 89 undergoing general anesthesia and RIJV cannulation were prospectively studied. After induction of anesthesia and endotracheal intubation, patients were placed in 15[degree sign] head-down position with the head rotated 30[degree sign] to the left. Patients were randomly assigned to one of 2 groups. In Landmark group, RIJV puncture was performed using the external anatomical landmarks. In Echo group, the site and direction of needle insertion was decided and marked on the neck before sterile draping, referring to 2-D echo images of the RIJV and CA. Patients in Echo group were subdivided into 7.5-MHz group and 3.75-MHz group, in which a 7.5-MHz probe (PLF-703NT, Toshiba) and a 3.75-MHz probe (PSH-37LT, Toshiba) were used, respectively. In all groups, seeking puncture was performed with a 23G needle to localize the RIJV and to guide the following cannulation puncture. The number of finder needle passes for RIJV access and the frequency of CA puncture were compared among groups. Results. There were no significant differences among groups regarding age, height and weight. Although the 7.5-MHz probe allowed better visualization of the RIJV and CA than the 3.75-MHz probe, no significant differences were seen between these 2 groups regarding the number of attempts for RIJV access and the incidence of CA puncture. (Table 1)Table 1Discussion. The use of the echo for determination of the site and direction of needle insertion significantly decreased the number of needle passes for the RIJV access, regardless of the echo probe frequency. Most impressively, RIJV access was achieved on the first attempt in 92.5% (74/80) of patients in Echo group, which is no worse than the results reported in echo-guided cannulation (74-78%). [1,2] The incidence of CA puncture was also reduced in Echo group, although statistical significance was not reached. Along with the increased use of TEE in cardiac surgery, availability of echo imaging devices has also increased in cardiac anesthesia. The simplified echo-guided puncture could be used to facilitate IJV cannulation as an alternative technique of echo-guided cannulation when a dedicated echo scanner is not available.

Carbon Dioxide Embolism During Hip Arthography in an Infant

Carbon Dioxide Embolism During Hip Arthography in an Infant

An anesthetic curiosity in New York (1875-1900): a noted surgeon returns to "open drop" chloroform.

An anesthetic curiosity in New York (1875-1900): a noted surgeon returns to "open drop" chloroform.

Modulation of respiratory-related genioglossus motor unit activity associated with changes in head position in humans.

There is little information available regarding how respiratory-related activity of the genioglossus motor unit is modulated in various head positions. The present study was undertaken to investigate what type of genioglossus motor unit contributes to the respiratory-related activity of the genioglossus muscle and how its activity is modulated with various head positions in 8 awake adult males. Single genioglossus motor unit activity and nasal airflow were simultaneously recorded in the natural head position during nasal breathing. Forty-five genioglossus motor units were classified into three types: inspiratory motor unit, inspiratory-expiratory motor unit and non-respiratory motor unit. The duration and instantaneous firing frequency of the inspiratory motor unit during inspiration significantly increased in the 30° headup position while it decreased in the 30° head-down position (p<0.05). The intantaneous firing frequency of the inspiratory-expiratory motor unit also significantly increased in the 30° head-up position while it decreased in the 30° head-down position (p<0.05), however, there was no significant difference between respiratory phases. These findings suggest that there are two types of respiratory-related genioglossus motor unit, which show different responses to various head positions. These genioglossus motor units may alleviate tongue collapse in altered head positions by different mechanisms.

Effects of exaggerated lithotomy position on ventilation and hemodynamics during radical perineal prostatectomy.

Ventilation and haemodynamics were studied in nine anaesthetized male patients undergoing perineal prostatectomy in the exaggerated lithotomy position (flexed, head-down position). In the flexed head-down position, as compared to the supine position, there was a significant decrease in arterial oxygen tension (13.1 +/- 2.1 to 11.2 +/- 1.1 kPa, p < 0.001), a significant increase in carbon dioxide tension (4.6 +/- 5.7 to 5.7 +/- 0.2 kPa, p < 0.001) and shunt fraction (7.8 +/- 2.3 to 14.0 +/- 3.0%, p < 0.01). There were also increases in pulmonary wedge pressures (11.3 +/- 2.9 to 17.9 +/- 2.9 mmHg, p < 0.01), mean pulmonary artery pressures (17.6 +/- 3.8 to 25.2 +/- 3.5 mmHg, p < 0.001) and central venous pressures (11.9 +/- 2.1 to 14.7 +/- 2.8, p < 0.05). Mean arterial pressure fell from 92 +/- 14 to 75 +/- 10 mmHg (p < 0.05). Heart rate and cardiac output were unchanged. All these changes had already been seen but to a lesser extent when the patients were positioned in the flexed supine position. When the patients were returned to the supine position following surgery, arterial oxygen tension was returned to baseline values. Mean pulmonary artery, pulmonary capillary wedge and central venous pressures also fell significantly. It is concluded that the extreme exaggerated lithotomy used during surgery results in impaired oxygenation and increased cardiac filling pressures but to acceptable levels in healthy anaesthetized patients. All values returned to normal when patient position was normalized after surgery.

Superimposition, symbology, visual attention, and the head-up display.

In two experiments we examined a number of related factors postulated to influence head-up display (HUD) performance. We addressed the benefit of reduced scanning and the cost of increasing the number of elements in the visual field by comparing a superimposed HUD with an identical display in a head-down position in varying visibility conditions. We explored the extent to which the characteristics of HUD symbology support a division of attention by contrasting conformal symbology (which links elements of the display image to elements of the far domain) with traditional instrument landing system (ILS) symbology. Together the two experiments provide strong evidence that minimizing scanning between flight instruments and the far domain contributes substantially to the observed HUD performance advantage. Experiment 1 provides little evidence for a performance cost attributable to visual clutter. In Experiment 2 the pattern of differences in lateral tracking error between conformal and traditional ILS symbology supports the hypothesis that, to the extent that the symbology forms an object with the far domain, attention may be divided between the superimposed image and its counterpart in the far domain.