Pulmonary scintigraphy in canine lobar and sublobar airway obstruction

The intrapulmonary distribution of the inhalation-to-perfusion ratio (I/P) was studied after placement of an obstruction in a sublobar bronchus (SLO) or in a lobar bronchus (LO) in 17 anesthetized prone (sternal recumbent) dogs. Placement of an SLO or LO did not induce any significant changes in the standard ventilatory and hemodynamic parameters measured. With the use of 99mTc aerosol inhalation combined with 99mTc perfusion lung scintigraphy, parameters quantifying the inhalation (I) to perfusion (P) mismatching at regional as well as at intraregional levels were calculated. The SLO increased the relative ventilation to the lung containing the obstruction, induced a shift of blood from the obstructed segment to the rest of the same lung, and increased the ratio between the mean I and the mean P (regional mismatching factor) in both the obstructed segment and the lung containing the obstruction. The LO diverted air and blood away from the obstructed lobe to the contralateral lung, but blood to a lesser extent. The LO decreased the regional mismatching factor in the obstructed lobe but also in the lung containing the obstruction. It also increased significantly the intraregional I to P mismatching in the obstructed lobe as well as in the rest of the same lung. After withdrawal of LO and reinsufflation of the collapsed lobe, blood continued to leave this lobe, while the withdrawal of SLO allowed recovery of the initial perfusion. This suggests that both hypoxic vasoconstriction and mechanical factors such as vascular distortion are involved in the blood shifts observed, and require some delay to be relieved. It is concluded that in dogs compensating mechanisms such as collateral ventilation are very effective. In case of SLO, I to the lung involved is increased. In case of LO, vasoconstriction in response to alveolar hypoxia could not fully compensate for the decreased I to the obstructed lobe.