IS FLOW IN SUBPLEURAL REGION TYPICAL OF THE REST OF THE LUNG - A STUDY USING LASER-DOPPLER FLOWMETRY

The microcirculation in the subpleural region of the lung is thought to be physiologically typical of the entire vasculature. To investigate this issue, an in situ blood-perfused dog lung lobe (500 ml/min) was prepared and the blood flow in the subpleural region (Qs) was monitored with laser-Doppler flowmetry (LDF). The flow rates into and out of the lobe were monitored with in-line flow probes, and the arterial and venous pressures were recorded from side ports in the cannulas. The LDF signal measures flow in arbitrary units over a region < 2 mm deep and 1 mm2. The LDF signal was independent of site of measurement and was linearly proportional to total flow rate (r2 > 0.9), suggesting that during baseline conditions Qs behaves similarly to, although not necessarily the same as, blood flow in the rest of the lung. However, if the vasculature is constricted by serotonin (arterial constriction) or by histamine (venous constriction), Qs decreases significantly relative to total flow. In fact, in some cases Qs approached zero during vasoconstriction, despite the fact that total flow was maintained constant and the pulmonary arterial pressure became elevated. Reduction in Qs most likely reflects a redistribution from the subpleural to the central regions of the lung. The results of this study suggest that LDF is a useful tool for monitoring flow in the subpleural region of the lung. Regardless of whether Qs during baseline is the same as or different from flow in the central regions, the results strongly suggest that regional differences in flow rate between the subpleural region and the central region are related to the state of vascular resistance, and therefore caution must be taken in interpreting data derived from the subpleural region.