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Pulmonary perfusion is more uniform in the prone than in the supine position: scintigraphy in healthy humans
Critical Carevolume 1, Article number: 102 (1999)
The authors have examined an area of lung function that is often overlooked: studying ventilation changes with positive pressure ventilation or with the prone position. However, the trial used young healthy volunteers and it may not be directly applicable to older, injured lungs that are often the problem in ICU. Follow-up studies in different patient groups may reveal perfusion differences between the healthy and injured subjects.
Much has been made of the prone position for patients with severely impaired gas exchange in the intensive care unit (ICU). However, despite the apparent benefit of this change in position, the underlying mechanisms are still not completely clear. Much radiological evidence [computed tomography (CT)] shows beneficial changes to the ventilation of a collapsed lung with the prone position. This study examines the theory that regional differences in lung perfusion, resulting in improved ventilation-perfusion ratios in the prone position, contribute to the clinical improvement seen in such patients.
The study aimed to examine lung perfusion by scintigraphy in healthy human volunteers in both supine and prone positions, during normal respiration and with 10 cm H2O of continuous positive airway pressure (CPAP). The investigation aimed to examine whether the dominant dorsal lung perfusion in the supine position changes to a dominant ventral perfusion in the prone position. The investigators also looked at whether CPAP (by producing lung distension) altered positional variations in lung perfusion.
Ten healthy volunteers were used in the study (after approval by local ethics and radiation protection committees). Three of the subjects had their lung perfusion examined following either normal respiration or CPAP. The other seven subjects were able to act as their own controls for normal respiration and CPAP. Lung perfusion was determined using technetium-99m labelled macroaggregates of albumin, delivered intravenously. These macroaggregates are rapidly trapped in the pulmonary capillaries in proportion to blood flow. The radioactivity was measured using tomographic gamma camera examination [single-photon emission computed tomography (SPECT)]. Activity profiles for the right lung for each individual were measured during normal breathing and after 10 min of 10 cm H2O of CPAP.
During normal breathing, the only differences in perfusion were seen in the diaphragmatic sections of the lung. There was more uniform lung perfusion in diaphragmatic sections of lung in the prone position. With CPAP, perfusion was greater in dependent areas while the volunteers were supine. In the supine position there was a more pronounced effect of CPAP on gravitational blood flow dependence, especially in the diaphragmatic regions of the lung.
The main findings of the study are:
lung perfusion was more uniformly distributed in the prone position
CPAP enhanced perfusion differences due to gravity, with a more pronounced effect in the supine than in the prone position.
The use of SPECT is now established as a technique for tomographic investigation of radiotracer distribution. The subtraction technique used in this study (before and after injection of labelled albumin) allowed the volunteers to act as their own controls. This had the advantage of eliminating any anatomical variations. The right lung was used to avoid any interference with the heart. The authors concluded that the combination of gravity and other factors, such as vascular anatomy, results in a greater vertical gradient of perfusion in the supine position than the prone position. With CPAP there was a more marked gravity dependence on blood flow in the supine position than in the prone position. For these reasons, ventilation-perfusion matching during positive pressure ventilation is probably less favourable in the supine position than in the prone position.
Nyren S, Mure M, Jacobsson H, Larsson SA, Lindahl SGE: Pulmonary perfusion is more uniform in the prone than in the supine position: scintigraphy in healthy humans. J Appl Physiol. 1999, 86: 1135-1141.