Effect of body posture on regional ventilation and perfusion at normal and increased gravity

Abstract: The prone position is increasingly used to improve arterial oxygenation in intensive care patients with severe acute respiratory failure. This beneficial response is well documented but our understanding of the responsible mechanisms remains incomplete, as does our knowledge of the effect of posture on lung function in healthy individuals. The overall aim of the present thesis is to further our understanding of the effect of posture and gravity on regional lung blood flow and ventilation in normal subjects breathing spontaneously. The current work included the development of a new quantitative SPECT (single photon emission computed tomography) method using 113mIn labeled macroaggregates and Technegas (99mTc) to simultaneously estimate the distributions of blood flow and ventilation for the whole lung in humans. Both radiotracers remain fixed in the lung tissue after administration. In the first study of the thesis the multiple inert-gas elimination technique (MIGET) and whole lung respiratory gas exchange were used as physiological evaluations of the SPECT method. We validated the new SPECT method through a good agreement between measured and SPECT-predicted exchange of MIGET and respiratory gases in healthy volunteers. In the second and third study, prone and supine healthy volunteers were exposed to hypergravity using the human centrifuge at Karolinska Institutet. Five times normal gravity (5 G) caused arterial desaturation in both postures, but the effect was significantly less when prone. We also marked regional lung blood flow in both postures at 3 G. Contrary to our hypothesis, we found that hypergravity caused a paradoxical redistribution of blood flow from dependent to non-dependent lung regions. In the final two studies, SPECT was used to explore the effect of posture on regional blood flow and ventilation at normal gravity. Images were obtained in both the prone and supine postures after radiotracer administration in either posture. Regardless of posture at administration, we found greater blood flow and ventilation to regions dependent at the time of imaging. The results suggest that the shift in lung tissue with a change in posture has a major effect on the imaged distributions. Images obtained in one position, after radiotracer administration in sitting upright, prone and supine postures, were used to compare gradients of blood flow and ventilation per alveolus between postures. In supine and prone postures, blood flow and ventilation were evenly distributed between cranial and caudal regions, while when upright both blood flow and ventilation were greater in caudal regions. Posture had little effect on the ventral-to-dorsal distribution of blood flow, while ventilation of dorsal regions was greater in the horizontal postures compared to the upright posture. We conclude that: The SPECT method can be used for simultaneous measurements of regional blood flow and ventilation. During hypergravity, lung function is better preserved when prone compared to when supine and redistribution of blood flow is not the primary cause of arterial desaturation at 3 G. Gravity redistributes regional blood flow and ventilation when upright, while the influence is less in the supine and prone postures. Moreover, a change in posture shifts lung tissue, which constitutes the main effect on imaged blood flow and ventilation distributions.