PubMed · 8994746
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W B Fye. 1997. Thomas Young.. https://pubmed.ncbi.nlm.nih.gov/8994746/
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When accurate, non-urea-based methods of measuring recirculation are used, recirculation is usually absent in arteriovenous (AV) accesses. When urea-based methods are used to measure recirculation in AV accesses, falsely elevated recirculation rates are common. These errors are due to AV and venovenous disequilibrium (peripheral vein method), delayed systemic sampling (two-needle methods), and errors in urea measurement (all methods). The literature suggests that recirculation in central venovenous (CV) catheters is approximately 5%. The methods used for these determinations have all been urea based. However, there are few theoretical problems in using urea-based measurements for measuring recirculation in this setting, making it more likely that these values are accurate. When hemodialysis via CV and AV accesses are compared, equilibrated Kt/V values differ significantly for the same single-pool Kt/V when 15-second postdialysis blood urea nitrogen values are used for modeling, but differ minimally when 2-minute postdialysis samples are used. The impact of transient retrograde blood flow in the superior vena cava on recirculation and whether dialysis efficiency is influenced by the exact site of CV catheter placement (superior vena cava v right atrium) is uncertain.
An experimental system has been developed to verify the possibility of detecting flow activity using a technique called ultrasound Doppler vector tomography. A tomography algorithm is used to reconstruct blood flow vector fields using data from computer-controlled ultrasound continuous-wave Doppler scanning equipment. The result is a picture in which the brightness variations represent the reconstructed values of the curl of the velocity field (inverted delta x v). Continuous ultrasound is transmitted into a region with flow activity and the Doppler-shifted signals are received. To obtain measurement data suited for fan beam tomography, the scanning is performed in a plane from points encircling the region. Reconstructions have been achieved using measurement data from two different flow phantoms. A comparison between the experimental results and simulations shows good conformity.
Since the circulatory and pulmonary systems are both driven by pressure and share space in the thorax, it is inevitable that they interact. These mechanical interactions, whilst relatively few in number, are protean in their manifestations. The circulatory system of the critically ill is often particularly susceptible to interference from respiration. Compensatory reserve is limited, ventilatory effort increased, and many critical care respiratory interventions place strain on the circulation, not seen in health. This review will examine the basic physiological mechanisms through which the pulmonary and circulatory systems interact. These mechanisms will then be applied to a variety of weaning, positive end-expiratory pressure (PEEP), and cardiopulmonary resuscitation techniques. It is hoped that this will provide the tools to understand clinical observations which would otherwise appear inexplicable.