Near-infrared spectroscopy in the assessment of cerebral oxygenation at high altitude

Hypoxia plays a key role in the pathogenesis of acute mountain sickness (AMS), but individual susceptibility is variable and cerebral symptoms do not always correlate with PaO2 measurements. Cerebral hypoxia may be more relevant than PaO2. We studied trends in cerebral regional oxygen saturation by the technique of near-infrared spectroscopy in 20 subjects ascending rapidly to 4680 m. Subjects were enrolled in a placebo-controlled, double-blind trial of medroxyprogesterone for the prevention of AMS. The fall in cerebral oxygen saturation was less than in the periphery. At 4680 m, cerebral oxygenation correlated with peripheral saturation but not with PaCO2 or with cerebral symptoms scores. At 4680 m, subjects on medroxyprogesterone had higher cerebral and peripheral saturation compared with those on a placebo. We conclude that cerebral oxygenation monitored with the Critikon 2020 system provided important information on the complex relationship of hypoxia to AMS and that other factors, such as changes in blood flow or capillary permeability, may be equally important.

Carbon dioxide contributes to the beneficial effect of pressurization in a portable hyperbaric chamber at high altitude

Regional cerebral oxygenation (rSO2) and peripheral oxygen saturation (SpO2) have been studied in subjects inside a portable hyperbaric chamber at altitude during pressurization. The effects of the accumulation of carbon dioxide within the chamber on rSO2 and SpO2 have also been investigated. Three studies of cerebral regional oxygenation were undertaken, using near-IR spectroscopy, in subjects who had ascended to 3475 m in the Alps, 4680 m in the Andes or 5005 m in the Himalayas. At 3475 m and 5005 m the effects of the removal of inspired carbon dioxide by a soda lime scavenger were also studied. On pressurization of the chamber to 19.95 kPa, inspired carbon dioxide rose within the chamber from 0.03% (0.06 kPa) ambient to over 1% (1.3 kPa). At 5005 m, SpO2 rose from a baseline of 79.5% (S.D. 4.5%) to 95.9% (2.0%) (P<0.0001), and cerebral rSO2 rose from 64.6% (3.4%) to 69.4% (3.6%) (P<0.0001). The introduction of a soda lime CO2 scavenger into the breathing circuit resulted in a drop in SpO2 from 95.9% (2.03%) to 93.6% (2.07%) (P<0.001) and a fall in rSO2 from 69.4% (3.64%) to 68.5% (3.5%) (P<0.01). Chamber pressure was maintained throughout at 19.95 kPa. Similar changes were seen at the other altitudes. Cerebral rSO2 increased rapidly following pressurization at all three altitudes. Scavenging of inspired carbon dioxide was associated with a significant fall in cerebral rSO2 and SpO2, and we estimate that the contribution of carbon dioxide may account for up to one-third of the beneficial effect of the portable hyperbaric chamber.

Cerebral oxygenation at high altitude and the response to carbon dioxide, hyperventilation and oxygen. The Birmingham Medical Research Expeditionary Society

Cerebral oxygenation is likely to be of critical importance in determining function at high altitude. The present study has used the technique of near-IR spectroscopy to monitor changes in cerebral regional oxygenation in response to inhaled carbon dioxide, hyperventilation and supplementary oxygen on ascent to 4680 m over 3 days. At sea level, inhaled CO(2) resulted in a significant rise in cerebral regional oxygenation [from mean 69.6% (S.D. 2.4% to 71. 1+/-2.3%; means+/-S.D.; P<0.001). At 4680 m, CO(2) increased regional cerebral oxygenation (63.8+/-2.5% to 65.9+/-2.2%; P<0.001) and also increased peripheral oxygen saturation (75.1+/-6.1% to 83. 6+/-4.0%; P<0.001). Voluntary hyperventilation resulted in improved peripheral oxygen saturation at 2770 m, 3650 m and 4680 m, whereas cerebral regional oxygenation was reduced at sea level and at 2770 m, unchanged at 3650 m and increased at 4680 m. Supplementary oxygen (6 1itres/min) at 4680 m resulted in greater improvements in peripheral oxygen saturation (76.7+/-7.9% to 98.1+/-1.5%; P<0.001) and cerebral regional oxygenation (64.6+/-3.3% to 70.6+/-2.9%; P<0. 001) than were found with CO(2) or hyperventilation. We conclude that attempts to increase CO(2) inhalation or ventilation at high altitude are likely to be beneficial for cerebral oxygenation in the short term.

Patellar tendon bearing orthosis--application as adjunctive treatment in healing of lower-limb tissue loss

OBJECTIVES

To assess the efficacy of the patellar tendon bearing orthosis (PTBO) as adjunctive treatment of patients with significant lower-limb ischaemia and tissue loss complicated by neuropathy.

DESIGN, MATERIALS AND METHODS

Twelve consecutive patients (14 legs) with a variety of underlying conditions causing extensive lower limb tissue loss are described. Seven patients were diabetic. Six patients (seven legs) underwent vascular reconstruction procedures.

CHIEF OUTCOME MEASURES

Wound healing and amputation rate.

RESULTS

Eight patients had ulcer healing within 5 months, one at 18 months (patient with alcoholic neuropathy who was poorly compliant with treatment), one stopped using the PTBO at 3 months and was converted to protective footwear with healing, one died of a myocardial infarct 1 month after the PTBO was fitted (the ulcer was showing signs of healing), and one has just had surgery and been fitted with a PTBO.

CONCLUSIONS

Early results from this anecdotal series are encouraging and a prospective study to determine the application of the PTBO in patients with neuropathic and neuropathic/ischaemic tissue loss is planned.

Intracranial pressure at high altitude and acute mountain sickness

1. Raised intracranial pressure has been noted in severe forms of acute mountain sickness and high-altitude cerebral oedema, but the role of intracranial pressure in the pathogenesis of mild to moderate acute mountain sickness is unknown. 2. Serial measurements of intracranial pressure were made indirectly by assessing changes in tympanic membrane displacement in 24 healthy subjects on rapid ascent to 5200 m. 3. Acute hypoxia at 3440 m was associated with a rise in intracranial pressure, but no difference was found in pressure changes at 4120 or 5200 m in subjects with or without symptoms of acute mountain sickness. 4. Raised intracranial pressure, though temporarily associated with acute hypoxia, is not a feature of acute mountain sickness with mild or moderate symptoms.

Unconjugated faecal bile acids in familial adenomatous polyposis analysed by gas-liquid chromatography and mass spectrometry

Previous studies have suggested reduced formation of secondary bile acids in patients with familial adenomatous polyposis (FAP). Developments in the collection, extraction and analysis of faecal bile acids as well as in the accurate diagnosis of FAP by DNA markers prompted reinvestigation of this hypothesis. The median (interquartile range (i.q.r.)) faecal bile acid concentration (3.69 (1.66-5.36) mumol per g dry weight) and daily excretion rate (60.5 (29-149) mumol per g per 24 h) in ten patients with FAP were similar to those of nine control subjects (3.31 (0.65-8.38) mumol per g dry weight and 30.1 (7.9-228) mumol per g per 24 h). Although the median (i.q.r.) concentration of only one bile acid (12-oxo-lithocholic acid) was significantly different between patients with FAP and controls (49 (34-70) versus 0 (0-20) nmol per g dry weight, P = 0.006), the derivatives of chenodeoxycholic acid (3.35 (1.76-5.32) versus 0.51 (0.13-2.37) mumol per g dry weight, P = 0.02) and cholic acid (1.63 (0.42-2.34) versus 0.80 (0.13-3.57) mumol per g dry weight, P = 0.006) were increased in those with polyposis. These results show increased bacterial biotransformation of faecal bile acids in patients with FAP.

Faecal unconjugated bile acids in patients with colorectal cancer or polyps

The unconjugated faecal bile acid profiles of 14 patients with colorectal cancer, nine patients with polyps and 10 controls were compared using gas liquid chromatography, controlling for such confounding variables as cholecystectomy, gall stones and hepatic function. Patients with adenomatous polyps had a higher concentration of faecal bile acids (5.23 mumol/g, 2.16-13.67 (median, range) v 1.96, 0.91-6.97; p = 0.016) lithocholic acid (2.41, 0.88-3.22 v 1.07, 0.38-2.03; p = 0.013) and total secondary bile acids (5.23, 2.16-13.4 v 1.96, 0.73-6.63; p = 0.02) compared with control subjects. Patients with colorectal cancer had an increased (p = 0.029) proportion of secondary faecal bile acids (mol%) compared with controls (100, 96.5-100 v 95.19, 81.73-100) and the ratios of the primary bile acids, cholic and chenodeoxycholic acid, to their respective derivatives (secondary bile acids) were significantly lower in cancer patients compared with control and patients with polyps (p = 0.034 to 0.004). This study lends further support to the theory that bile acids may play a role in the development of polyps and colorectal cancer.

Induction of cholangiocarcinoma in the Golden Syrian hamster using methylazoxymethyl acetate

The Golden Syrian hamster, which has a bile acid profile similar to that of humans, is often used as an experimental model for cholangiocarcinoma. Unfortunately, most chemical carcinogens result in other tumour types also being induced which can be disadvantageous. In the present study, we describe the development of cholangiocarcinoma associated with precursor lesions of bile duct proliferation and dysplasia which occurred following a single i.v. dose of methylazoxymethyl acetate (10, 20 or 40 mg/kg body weight). Moreover, no other tumours were induced. This model may, therefore, prove useful in investigating the steps involved in the carcinogenesis of cholangiocarcinoma.

Comparability of hamster with human faecal unconjugated bile acids in a model of colorectal cancer

The relationship between experimental colorectal carcinogenesis and bile acids has usually been investigated in the rat, a species with a markedly different bile acid profile from man. In this study, we show that the hamster faecal bile acid profile is similar to that in man. Rectal cancer was induced in hamsters using twice weekly instillations of N-methyl-N-nitro-N-nitrosoguanidine (MNNG) for 4 weeks at doses of 1-8 mg kg-1. The medcian (range) faecal bile acid concentrations of tumour-bearing hamsters (0.52, 0.46-0.84 mumoles g-1 faeces) was reduced compared to controls (1.08, 0.95-1.65, mumoles g-1) and non-tumour bearing MNNG treated hamsters (1.18, 0.64-1.42 mumoles g-1), largely due to a decrease in cholic acid derivatives (all p less than 0.05) at least). This model may be more suitable for studying the relationship between colorectal cancer and bile acids.