Use of microspheres in measurement of regional blood flows during +GZ stress

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Abstract
The use of the radiolabeled microsphere technique for the study of the effects of +Gz acceleration [as in high performance fighter aircraft] on regional blood flow is examined. A theoretical analysis of the limits of this technique in a high acceleration environment is presented. Chronically implanted, electromagnetic aortic flow probes were used to determine the relationship between aortic blood flow velocity and +G2 acceleration in conscious adult miniature swine. Conscious straining adult miniature swine, with the assistance of an inflated anti-G [anti-gravity] suit, compensate well to accleration levels .ltoreq. +7 Gz. Exposure to +9 Gz often resulted in unstable cardiovascular states involving relative bradycardia, often progressing to asystole, declining aortic blood pressure and diminished cardiac outputs approaching 0. If aortic pressure and heart rate attain a relatively steady state during acceleration, and if heart level mean aortic pressure is .gtoreq. 100 Torr, the application of the micorsphere technique during +Gz acceleration is theoretically valid. This hypothesis was tested using the microsphere technique (9.0 .+-. 0.8 .mu.m diameter) in conscious miniature swine during exposure to +Gz acceleration. Within the defined limits the radiolabeled microsphere technique is as accurate for use during acceleration studies as it is for use in routine laboratory studies.