Transmural pressure in rat initial subpleural lymphatics during spontaneous or mechanical ventilation

Abstract

The role played by the mechanical tissue stress in supporting lymph formation and propulsion in thoracic tissues was studied in deeply anesthetized rats ( n = 13) during spontaneous breathing or mechanical ventilation. After arterial and venous catheterization and insertion of an intratracheal cannula, fluorescent dextrans were injected intrapleurally to serve as lymphatic markers. After 2 h, the fluorescent intercostal lymphatics were identified, and the hydraulic pressure in lymphatic vessels (Plymph) and adjacent interstitial space (Pint) was measured using micropuncture. During spontaneous breathing, end-expiratory Plymph and corresponding Pint were −2.5 ± 1.1 (SE) and 3.1 ± 0.7 mmHg ( P < 0.01), which dropped to −21.1 ± 1.3 and −12.2 ± 1.3 mmHg, respectively, at end inspiration. During mechanical ventilation with air at zero end-expiratory alveolar pressure, Plymph and Pint were essentially unchanged at end expiration, but, at variance with spontaneous breathing, they increased at end inspiration to 28.1 ± 7.9 and 28.2 ± 6.3 mmHg, respectively. The hydraulic transmural pressure gradient (ΔPtm = Plymph − Pint) was in favor of lymph formation throughout the whole respiratory cycle (ΔPtm = −6.8 ± 1.2 mmHg) during spontaneous breathing but not during mechanical ventilation (ΔPtm = −1.1 ± 1.8 mmHg). Therefore, data suggest that local tissue stress associated with the active contraction of respiratory muscles is required to support an efficient lymphatic drainage from the thoracic tissues.