Articles: biological-models.
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We describe a model for evaluating techniques of infant ventilation during resuscitation. The utility of the model is illustrated by testing performance of emergency medical technicians in mouth to mouth ventilation of a model 4 kg infant. Ventilation was generally adequate with mean (+/- S. ⋯ Assuming normal metabolic rate and respiratory dead space, alveolar gas composition resulting from the simulated resuscitations would be PACO2 = 31 +/- 20 and PAO2 = 110 +/- 19 mmHg. Nine of ten rescuers would have achieved satisfactory PACO2 less than or equal to 50 and PAO2 greater than or equal to 100. However, the rescuers' exhaled oxygen concentration is not adequate to correct hypoxemia if associated with hypoventilation or a wide alveolar to arterial oxygen gradient.
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Patellofemoral loads are calculated by means of a mathematical model based upon an elliptical approximation of the articulating profile. Differences in the sagittal curvature of morphological and arthroplastic gliding surfaces can be approximated by a differing eccentricity of the ellipse. The joint is balanced in a static situation by two tension forces. ⋯ The model also takes into account the patellar ligament turning around the proximal tibia at wide flexion angles. Besides the design of the prosthesis, patellofemoral loads depend on the height of implantation of the femoral component. It must be borne in mind that forces will vary in magnitude at different flexion angles; the patient's activities after the operation must also be considered.
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The mechanism by which optic nerve sheath fenestration relieves papilledema associated with increased intracranial pressure has not been clearly defined. A model was constructed to determine if the induction of fluid flow along the nerve sheath and through the fenestration could account for a reduction in pressure around the optic nerve, as might be expected according to Bernoulli's equation of fluid dynamics. The model states that as the velocity of a fluid increases, the pressure it exerts decreases. ⋯ The reduced intrasheath pressures persisted even when the intracranial pressure was again elevated to pathologic levels as long as active fluid flow continued along the nerve sheaths. Thus, induction of cerebrospinal fluid flow along the optic nerve sheath by fenestration appears to locally reduce the pressure around the optic nerve, in spite of persistently elevated intracranial pressure. This is in accordance with what would be predicted by Bernoulli's equation.
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A logistic regression analysis of eighteen variables in eighty-three lower limb amputations was performed in order to predict stump failure. Five variables were identified as having a significant effect on the logistic model: Age had an inverse relation to failure rate (p less than 0.005). This effect was mediated through a subgroup of 23 patients who had had a vascular operation (p less than 0.02), as this group had a higher failure rate and were younger than those without previous vascular surgery. ⋯ In addition, it was confirmed that the higher the skin perfusion pressure (p less than 0.05) and the amputation level, (p less than 0.05) the better the healing. A model including "skin perfusion pressure," "previous vascular surgery," "amputation level" and "surgical experience" had a good predictive capability with a misclassification rate of 0.08-0.11. Therefore it is suggested that a logistic model including these variables could be a helpful tool to predict the risk of stump failure.
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Pain is a common experience of the ICU patient, with a diverse clinical manifestation. To manage pain we must understand its anatomic pathways and physiology. This article reviews the development of our understanding of the theory of pain from Descartes to the gate theory of Melzack and Wall. We will review the anatomy of the pathways of pain and the interrelationship of "A" and "C" fibers and the unique nature of the opiate receptor.