Body posture and gravity as determinants of lung perfusion and ventilation

Sammanfattning: Normal lung gas exchange depends on an intimate match between regional ventilation and regional blood flow in the lungs. The beneficial effect of the prone posture in acute respiratory failure is well documented but the mechanisms involved are not completely understood. The overall aim of this thesis was to gain further knowledge about the influence of gravity and body posture on the distributions of ventilation (V) and lung blood flow (Q) in healthy humans. In the studies included in this thesis, regional lung blood flow was marked with intravenous injection of macroaggregates of albumin labelled with 113mIn or 99mTc, while ventilation was marked with Technegas (99mTc). The radiotracers remain fixed in the lungs after administration. The distribution of the radiotracers, thus representing the distribution of V and Q at the time of administration, was mapped with quantitative single photon emission computed tomography (SPECT). In Study I, we compared regional lung blood flow and ventilation in supine and prone anesthetized and mechanically ventilated healthy humans, with and without PEEP (10 cm H2O). Half of the subjects were studied supine, the other half prone. All subjects were studied twice; once with, and once without PEEP. We found that in supine subjects, PEEP caused similar redistributions of both perfusion and ventilation towards dependent lung regions with little changes in V/Q ratios. In prone subjects on the other hand, the addition of PEEP caused a much greater redistribution of perfusion compared to ventilation towards dependent lung regions, leading to an increased V/Q mismatch. With PEEP, the vertical ventilation-toperfusion gradient was similar between supine and prone postures. However, without PEEP the gradient was less in prone than in supine posture. These results lead us to the conclusion that PEEP should be titrated differently according to the actual body posture. In Study II, the influence of gravity on regional lung blood flow in the upright and headdown posture was evaluated. For each subject, one radiotracer was administered while standing upright and the other in the head-down posture using a tilt table. A shift from upright to the head-down posture resulted in a clear redistribution of blood flow from basal to apical regions of the lung. The results further demonstrated that lung structure, and not gravity, is the major determinant of regional lung blood flow in the upright and head-down posture. In Study III and IV, the human centrifuge at Karolinska Institutet was used to determine the distribution of regional ventilation in supine and prone humans exposed to hypergravity. In Study III, a technique to map regional ventilation during exposure to hypergravity was developed, using Technegas and SPECT. The results demonstrated a significant redistribution of ventilation from dependent to non-dependent lung regions in both supine and prone subjects exposed to three times normal gravity. We also found that the hypergravity-induced arterial desaturation was less pronounced in the prone posture.