Annals of biomedical engineering
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New techniques for determining the hematocrit (Hct) and oxygen saturation (SO2) of whole blood from backscattered light measurements are described. First, theoretical and experimental results are presented which show that the empirical linear relationship between SO2 and the infrared-red backscattered light intensity ratio on which previous instruments have been based is an inadequate description primarily because it does not account for the strong effects of Hct and transducer geometry. Then it is shown that the ratio of backscattered intensities from two appropriately positioned infrared sources can be plotted against the infrared-red intensity ratio to produce a family of calibration curves from which SO2 and Hct can be independently determined. Finally, a practical implementation of an oximetry system which employs a microelectronic catheter-tip optical sensor and a microprocessor-based signal processor is proposed.
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Cardiac output is the volume of blood ejected by the heart per unit time. It is a useful measurement in that it can be used to evaluate overall cardiac status in both critically ill patients and patients with suspected cardiovascular disease. An ideal cardiac output measurement system would have automated continuous output capability, be minimally invasive, accurate, fast, small, low cost and clinically adaptable. ⋯ Included are the Fick method, indicator dilution techniques, velocity measurements and transthoracic impedance and combined Doppler ultrasound as noninvasive techniques. In addition, several experimental methods are described along with their desirable features and possible constraints. These include intravascular heating/recording, thermistor tracking of cardiac output, ejection fraction measurements and magnetic susceptibility plethysmography.
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Autoregulation of cerebral blood flow is heterogeneous in several ways: regional, segmental, and temporal. We have found regional heterogeneity of the autoregulatory response during both acute reductions and increases in systemic arterial pressure. Changes in blood flow are less in brain stem than in cerebrum during decreases and increases in cerebral perfusion pressure. ⋯ Sudden increases in arterial pressure produce transient increases in blood flow, which are not observed under steady-state conditions. In addition, the blood-brain barrier is more susceptible to hypertensive disruption after rapid, compared to step-wise, increases in arterial pressure. Thus, when investigating cerebral vascular autoregulation, regional, segmental, and temporal differences in the autoregulatory response must be taken into consideration.
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A computerized system that uses feedback of end-tidal CO2 fraction (FETCO2) to adjust minute volume of a ventilator has been developed and tested. The effectiveness and robustness of the controller were evaluated in five anesthetized dogs. The controller responded to step-changes in the set-point for FETCO2 by adjusting minute volume so that the FETCO2 settled to the new set-point in less than 60 sec with less than 20% overshoot. ⋯ A disturbance to the controlled system was produced by releasing an occlusion of a branch of the pulmonary artery. The controller always responded to this disturbance in a stable manner, returning the FETCO2 to its desired value within 30 sec. Accurate control of arterial partial pressure of CO2(PaCO2) will require modifications enabling the system to determine the relationship between FETCO2 and PaCO2.
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Comparative Study
Acute and chronic implantation of coiled wire intraneural electrodes during cyclical electrical stimulation.
The posterior tibial nerves of 18 rabbits were intraneurally implanted with coiled wire electrodes for up to 9 weeks to evaluate their usefulness for neuromuscular electrical stimulation. In one group an electrode was implanted and removed in one leg while the other leg was chronically implanted. A second group was chronically implanted without electrical stimulation in one leg and implanted with cyclical electrical stimulation applied through the electrode in the other leg. ⋯ One cat was implanted in both the posterior tibial and peroneal nerves of each leg for a 4-year period. Threshold current showed very little change during the implantation period. The nerves showed minimal focal demyelination at the electrode site and the muscles showed normal fibers.