Bronchial temperature as a key to the interior pulmonary capillary bed of anaesthetized dogs

The terminal airways are separated from the surrounding pulmonary capillaries by a tissue layer of a few micrometers in thickness only. Therefore, it should be possible to gain information about in vivo transcapillary heat transport of the interior pulmonary vascular bed by recording the terminal bronchial temperature. For this purpose, we studied temperature-time curves in the pulmonary artery, bronchial system and the aorta of six anaesthetized dogs permanently instrumented for measuring pulmonary blood flow. Thermistors recorded temperature changes at the three locations after injection of 5 ml cold solution into the right atrium. From the observed temperature-time curves mean transit times between the three recording sites were calculated for various pulmonary blood flows (integral of delta Ttdt/integral of delta Tdt) (range 1.1-3.5 1/ min). We found that the temperature-time curves of the bronchial system resemble typical "dilution" curves and are interspaced between those in the pulmonary artery and those in the aorta. Regardless of pulmonary blood flow, mean transit times from the pulmonary artery to the distal bronchial system and from there to the aorta were about equal. We conclude that transcapillary heat transfer generates bronchial temperature-time curves which permit an estimation of the relation of precapillary to postcapillary mean transit times in the interior of the lung.