The effects of pulmonary blood flow on respiratory mechanics have been studied mostly in children undergoing surgery for congenital heart diseases. Most studies reported an increase of respiratory system elastance and resistance with increased pulmonary blood flow. Recently only respiratory mechanics were measured in experiments on healthy rats by the end-inflation occlusion technique, in which the pulmonary blood flow was changed by acute blood volume expansion (BVE). Modelling the respiratory system as composed by two compartments, the technique allows the measurement of static and dynamic elastances, of the ohmic resistance of the airway, and of the additional resistance due to pendelluft and stress-relaxation. Furthermore, the measurements were not subjected to the possible confounding effects of chronic modifications of lung parenchyma and/or vessels. Pulmonary blood flow increment was confirmed to have a detrimental effect on respiratory mechanics, causing an increased work of breathing secondary to respiratory system elastances and resistances increments. Resistive pressure dissipation due to the visco-elastic behaviour of the lung parenchyma (stress-relaxation) and lung mechanical inhomogeneity (pendelluft) increased also, as well as respiratory system hysteresis. The literature allows to conclude that pulmonary blood flow significantly influences respiratory mechanics and the related work of breathing. The subject is of both theoretical and practical interest, since BVE is a frequent medical intervention, and the associated increase of pulmonary blood flow may happen in different physiopathological situations. It is suggested that blood volume expansions need to be cautiously performed, most of all in the treatment of unstable patients with mechanical respiratory failure.
Keywords: Blood volume expansion, end-inflation occlusion method, respiratory mechanics, respiratory system hysteresis, work of breathing
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