Effects of left atrial and airway pressures on airway blood flow

The blood flow contributions of the pulmonary and systemic circulations to airway blood flow from the trachea down to the 5th generation airways when either 15 cmH2O positive end expiratory pressure (PEEP) or 20 mmHg left atrial pressure was applied were assessed in anesthetized dogs by injecting 15-microns radiolabeled microspheres into the right and left heart, respectively. After the microsphere injections, the animals were killed, and the tracheal cartilage, tracheal muscle-mucosa, main bronchi and the 2nd generation bronchi down to the 5th generation airways were excised and collected for radioactive counting. The results of this study showed that under normal conditions, tracheal blood flow was primarily systemic (> 95% of total tracheal blood flow) averaging 15-26 ml.min-1 x (100 g)-1, while both the pulmonary [10 ml.min-1 x (100 g)-1] and systemic circulations [15 ml.min-1 x (100 g)-1] contributed substantially to main bronchi blood flow. The systemic blood flow contribution to the lower airway generations (2nd to the 5th generation airways) stayed relatively constant, averaging 13-31 ml.min.(100 g)-1 while the pulmonary blood flow contribution increased significantly in these airway segments to a maximum of over 100 ml.min-1 x (100 g)-1 at the 5th generation. At 15 cmH2O PEEP, the systemic contribution to all of the airway segments (except the 5th generation, P < 0.09) and the pulmonary component to the main bronchi, significantly decreased (P < 0.05). In contrast, elevated left atrial pressure had little effect on the pulmonary and systemic contributions to airway blood flow. The results of this study suggest that elevated PEEP has a greater effect than elevated left atrial pressure on the systemic blood flow contribution to the airways, while the pulmonary blood flow contribution to the airways is affected very little by either of these perturbations.