CYANIDE, ACSSAVA, AND DIABETES

635 A raised cortisol/creatinine ratio may have other origins than nocturnal hypoglycsemia, but the patients we studied showed improvements in fasting blood-glucose and glycosuria after reduction in insulin dose, and the cortisol/creatinine ratio

returned to normal.

preliminary survey suggests that the maximum prevalence of nocturnal hypoglycsemia, and thus of chronic insulin overtreatment, is about 27% in Oxford outpatient insulin-treated diabetics. It may be less; for instance, the patient whose cortisol/creatinine ratio rose after a drinking bout almost certainly did not suffer chronic overtreatment because several other urines all had normal ratios and glycosuria was minimal or absent. In most of the patients who, we suggest, had unrecognised hypoglycaemia, consistently (i.e., in more than half the urine samples provided) raised cortisol/creatinine ratios and glycosuria were found. Transient hypoglycxmia due Thus

our

inattention to diet or to other events which cause a transient rise in cortisol levels overnight will also give false-positive results. We feel that the figure of 20% suggested by Schwandt et al. is likely to be close to the general incidence of chronic overtreatment in adult insulin-treated diabetics, which is far below the 70% reported by Rosenbloom and Giordanoin childhood diabetes. Schwandt et al. suggest that a detailed analysis of the patient’s complaints may help to diagnose chronic overtreatment in unstable diabetics. This may be so, but in our experience of outpatient diabetics, symptoms were not often reported, and the finding of consistently raised cortisol/creatinine ratios in sequential early-morning urine specimens proved of great value in detecting chronic insulin overtreatment. One occasionally useful clinical clue not mentioned in Gale and Tattersall’s excellent survey is restriction of the results of home urine testing to either 0 or 2% (or higher) glycosuria at a fixed time (e.g., early morning), especially if this time comes soon after hypoglycaemic symptoms. Use of a slightly smaller insulin dose should then give results varying between 0 and t%. Provided that excesses of food and drink are excluded, the extreme values are likely to be caused by variation between "good" and "excessively good" control, with consequent over-reaction to the latter. to

Nuffield Departments of Clinical and Medicine, Radcliffe Infirmary, Oxford OX2 6HE

Biochemistry

R. F. SMITH R. A. MOORE C. M. ASPLIN T. D. R. HOCKADAY

we

consulted knew of any toxic effect of cyanide on the panand we ascribed the frequency of diabetes to malnutri-

creas

tion. Department of Medicine, Rumaillah Hospital, P.O. Box 42, Doha, Quatar

J. C. DAVIDSON

ARTEFACTUAL HYPOXÆMIA DURING ESTIMATION OF PaO2 BY SKIN-ELECTRODE

SiR,—Miss Whitehead and her colleagues (July 21, p. 157) show

clearly that low transcutaneous oxygen tension (tc P02) readings can be obtained from rib pressure on the electrode. However, they do not tell us how low tcP02 resulting from pressure can be easily identified. The transcutaneous technique requires that the flow of oxygen from the arterialised capillaries is so large that the oxygen consumption of the tissues below the electrode, and the oxygen consumption of the electrode itself, are negligible. To achieve this, maximal or near-maximal vasodilatation is needed. This can only be achieved by thermal vasodilatation.I If the capillaries below the electrode are compressed blood-flow is decreased and the conditions for the tcP02 technique are lost. Pressure on the electrode itself has no influence on tcPO2 reading, but pressure on the electrode when positioned on the skin will compress the underlying capillaries. The force needed to achieve this varies with the tissue: if the underlying structure is soft, on the abdomen or forearm, tcP02 will not fall until a pressure of about 40 mm Hg (5.3 kPa) is exerted. A similar pressure on the head or over any bone may result in a zero reading as in Whitehead’s case. In our experience such pressure, with resulting low tcPO2 levels, is rare in the newborn except when they lie on the electrode, and we were surprised to read of it in 5 out of 34 infants in Whitehead’s series. Earlier investigations from the same neonatal unit did not mention these pressure tcP02 periods. There are three ways of identifying this pressure effect and thus avoiding misinterpreting them as serious hypoxxmia. (1) When pressure is exerted the tcPO2 starts to fall immediately and it always falls faster than after, for instance, complete apncea. (2) At the same time as tcP02 falls "flow" (i.e., the amount of energy needed to maintain a constant temperature of the electrode) also falls abruptly; this variable can be measured by the equipment manufactured by Dragerwerke (Lubeck) and Hellige (Freiburg) but is not shown in Whitehead’s figure. (3) Inspection of the electrode will reveal pressure, 1. Huch R, Huch A, Lübbers DW. Transcutaneous measurement of blood Po2 (tcPo;):- Method and application in perinatal medicine. J Perinat Med

1973; 1: 185.

CYANIDE, CASSAVA, AND DIABETES SiR,—I was interested to read your editorial on Diabetes, Cyanide, and Rat Poison (Aug. 18, p. 341). In 1967 McGlashan’ reported a high incidence of diabetes in an area straddling the Zaire-Zambian border, a predominantly cassava-eating zone. In the Zambian part of this region we found2 a prevalence-rate of 1.1 % for new diabetes. This was in marked contrast to the rest of Zambia where maize is the staple food and diabetes is uncommon. Over 80% of the men and 60% of the women had evidence of malnutrition. A dietary survey done at the same time3 showed that the average total calorie intake was 1439 per day, and 1340 of that was from cassava. BX’e were unable to X-ray the paients for evidence of pancreatic calcification. At that time none of the authorities on diabetes

5. Rosenbloom AL, Giordano BP. Am J Dis Child 1977; 131: 881. 1 McGlashan ND. Geographical evidence on medical hypotheses. Trop Geogr Med 1967, 19: 333-44. 2. Davidson JC, McGlashan ND, Nightingale EA, Upadhya JM. The prevalance of diabetes mellitus in the Kalene Hill area of Zambia Med Proc

2: 179-87.

(a) Normal fall in fetal tCP02 after contractions starting about 50 after the onset of contractions. (b) Almost simultaneous and abrupt fall in fetal tcP02 and in "flow" due to reduced blood-flow caused by pressure on electrode.

s

1969, 15: 426-32. 3. McGlashan ND. A diet survey in the Kalene Hill

Part of continuous record of fetal beat-to-beat heart-rate (FHR), uterine contractions, fetal tcPO2, and "flow" 90 and 58 min before delivery.

area.

Med J Zambia 1969;