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factors for severe hypoglycaemia in the study were a longer duration of diabetes and a history of previous
hypoglycaemia. Another worrying feature from the DCCT research group was that no warning symptoms
were experienced in 36 per cent of severe hypoglycaemic episodes, which occurred while patients were
awake. While loss of hypoglycaemic awareness is associated with strict diabetic control, it is also a
ICAO Preliminary Unedited Version — March 2010 III-4-20
complication acquired with increasing duration of diabetes, which may underline the emergence of age
and duration of diabetes as risk factors for severe hypoglycaemia.
Type 2 Diabetes
Type 2 diabetics can be managed on diet, diet and sulphonylureas, diet and biguanides or diet and a
combination of sulphonylurea and biguanide and more recently incretin based therapy (the dipeptidyl
peptidase 4 [DPP-4] inhibitors sitagliptin and vildagliptin: the glucagon-like peptide 1 [GLP-1] mimetic
exenatide (Barnett and Grice 2009)). The alpha-glycosidase inhibitors, which have recently been
introduced, may potentiate the hypoglycaemic effect of a sulphonylurea. Increasingly the glitazones,
which enhance the sensitivity of the insulin receptor, are being used as monotherapy or in combination
with the agents above. Incretin based therapy has the advantage that it increases insulin secretion from the
beta cells and decreases the secretion of glucagon from the alpha cell. Their mechanism of action is
glucose dependant and thus hypoglycaemia is uncommon. Thus, in assessing the risk of hypoglycaemia,
it is vitally important that the precise therapeutic regime of the diabetic is detailed.
Severe hypoglycaemia associated with some sulphonylureas is well documented, but the frequency of
mild hypoglycaemia not requiring urgent hospital admission is more difficult to assess, because
symptoms are often brief and many patients treated with oral agents have poor knowledge of the
symptoms of hypoglycaemia. Despite these difficulties, trials have recorded an incidence of symptomatic
hypoglycaemia ranging from 1.9 to seven per cent per annum. A study by Jennings et al(1989) found a
prevalence of symptomatic hypoglycaemia of the order of 20 per cent when using direct questioning of
the patients and the relatives. When assessing risk, it is important to know which agent the patient is
taking, since the risk of sulphonylurea induced hypoglycaemia appears to be greater for some agents than
others. Taking the incidence of hypoglycaemia among patients treated with chlorpropamide as 100, the
standardised incidence ratios are 111 for glibenclamide, 46 for glipizide and 21 for tolbutamide (Berger et
al, 1986). There is no mathematical formula, neither simple nor complex, which predicts with certainty
hypoglycaemia in sulphonylurea treated patients. The risk factors for sulphonylurea induced
hypoglycaemia are primarily:
a) age over 60
b) impaired renal function
c) poor nutrition and, often forgotten,
d) multi-drug therapy.
Second generation sulphonylureas, however, have a lower rate of hypoglycaemia and Heller (2007)
confirmed this in a study of drivers.
Since the withdrawal of phenformin in the early 1970s, due to the incidence of metabolic acidosis, the
only biguanide in use in the United Kingdom is metformin. Its mechanism of action does not involve the
stimulation of insulin secretion and it does not cause hypoglycaemia. A rare, but serious side-effect of
metformin is metabolic acidosis. The incidence has been recorded as 0.04 cases per 1000 patient years,
with a mortality of 0.024 per thousand patient years (Berger, 1985). The mortality risk from metformin
induced lactic acidosis has been estimated to be not significantly different from that of sulphonylurea
induced hypoglycaemia (Berger, 1986). The risk of metabolic acidosis may be almost eliminated by not
exceeding 2.5 g per day and excluding patients with any renal or hepatic insufficiency.
ICAO Preliminary Unedited Version — March 2010 III-4-21
In summary, therefore, the sulphonylureas carry a risk of hypoglycaemia which lies outside the usually
accepted one per cent per annum. It is likely, however, that a highly selected pilot group with Type 2
diabetes will lie at the lower end of the range of hypoglycaemia i.e. 2% per annum, although this remains
outside the normally accepted risk of incapacitation(see Part I, Chapter 3 – Flight crew incapacitation).
On the other hand, the biguanide, metformin does not cause hypoglycaemia and it carries an extremely
low risk of metabolic acidosis which is acceptable in appropriately selected pilots (see below).
Aeromedical considerations
Diet control
Hypoglycaemia is not the issue in the risk assessment in this group of pilots. The main area of concern is
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Manual of Civil Aviation Medicine 1(133)
