Lung

Abstract

The commonest indication for this investigation is to detect suspected pulmonary emboli by displaying regional defects within the lungs of the distribution of right ventricular output. The total pulmonary blood volume is slightly less than 0.5 1; under resting conditions at any instant about 10% is in capillaries. Capillary blood volume increases two- to three-fold on exercise. Regional distribution is demonstrated by labelling blood entering the right atrium with particles large enough to be trapped in any capillary they encounter, which in practice means the pulmonary bed. There are two implicit assumptions, namely that all particles are trapped in the lungs on their first transit from the right ventricle and secondly that intravenously administered particles are completely mixed with blood coming from all other regions as they pass through the right atrium, right ventricle and pulmonary outflow tract, so that site of injection does not affect distribution within the lungs. Both of these assumptions are valid under most circumstances. The particles employed must be physiologically inert and should not be so large that they are trapped proximal to the capillary bed. Any larger than 150 µm (micrometres) impact in arterioles and thus affect relatively large drainage regions. Particles smaller than 10 µm are likely to pass through the capillary bed, to be extracted by the reticulo-endothelial system, particularly in liver and spleen, instead of being trapped in lung. Any soluble tracer not bound to particles initially mixes with the total plasma volume. Its subsequent fate depends on its chemical form.