Why do all mammals, except for sloths and manatees, have exactly seven cervical vertebrae? In other vertebrates and other regions, the vertebral number varies considerably. We investigated whether natural selection constrains the number of cervical vertebrae in humans. To this end, we determined the incidence of cervical ribs and other homeotic vertebral changes in radiographs of deceased human fetuses and infants, and analyzed several existing datasets on the incidence in infants and adults. Our data show that homeotic transformations that change the number of cervical vertebrae are extremely common in humans, but are strongly selected against: almost all individuals die before reproduction. Selection is most probably indirect, caused by a strong coupling of such changes with major congenital abnormalities. Changes in the number of thoracic vertebrae appear to be subject to weaker selection, in good correspondence with the weaker evolutionary constraint on these numbers. Our analysis highlights the role of prenatal selection in the conservation of our common body plan.
Heparin is the most widely used anticoagulant in continuous renal replacement procedures but little is known about the balance between filter coagulation and patient hemorrhage during treatment. Filter survival and hemorrhagic complications during 240 filter periods in 78 critically ill patients, treated with continuous arteriovenous hemofiltration and hemodiafiltration, were studied for this article. The crude incidence of filter coagulation was 17.7 +/- 2.5 (mean +/- SE) per 1000 h at an activated partial thromboplastin time (APTT) of 15 to 35 s, as determined in systemic blood samples. The incidence of filter coagulation gradually decreased to 9.0 +/- 2.7 per 1000 h at an APTT of 45 to 55 s (P = 0.031). The crude incidence of patient hemorrhage was 2.9 +/- 1.0 per 1000 h at an APTT of 15 to 35 s and increased almost threefold to 7.4 +/- 2.4 per 1000 h when the APTT was 45 to 55 s (P = 0.009). There was no difference in filter survival between treatment with hemofiltration only and hemodiafiltration. Mean survival of acrylonitrile filters (33.8 +/- 4.3) was significantly lower compared with the survival of polyamide filters (104.1 +/- 14.4 h, P = 0.003). After adjustment for the type of the filter, mean arterial blood pressure, and platelet count, the risk for filter coagulation decreased 25% (relative risk, 0.77; 95% confidence interval [CI], 0.62 to 0.96) for every 10-s increase in APTT. At the same time, the risk for patient hemorrhage increased 50% (relative risk, 1.57; 95% CI, 1.43 to 1.72). The occurrence of filter coagulation or hemorrhages were not correlated with the administered dose of heparin. Concurrent use of coumarin derivatives had a positive effect on filter survival, without increasing the overall incidence of hemorrhages. It was concluded that the systemic APTT is a good predictor of the risk for filter coagulation and patient hemorrhage. Safety and efficacy of heparin therapy seems optimal at an APTT between 35 and 45 s.
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