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

1 Dipartimento di Scienze Biomediche Sperimentali e Cliniche, Università degli Studi dell'Insubria, Varese; and 2 Dipartimento di Chirurgia, Ospedale San Raffaele, Milan, Italy Submitted 20 January 2005 ; accepted in final form 15 March 2005 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 (P lymph ) and adjacent interstitial space (P int ) was measured using micropuncture. During spontaneous breathing, end-expiratory P lymph and corresponding P int 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, P lymph and P int 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 ( P tm = P lymph – P int ) was in favor of lymph formation throughout the whole respiratory cycle ( P tm = –6.8 ± 1.2 mmHg) during spontaneous breathing but not during mechanical ventilation ( P tm = –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. interstitial fluid pressure; tissue fluid homeostasis Address for reprint requests and other correspondence: D. Negrini, Dipartimento di Scienze Biomediche Sperimentali e Cliniche, Università degli Studi dell'Insubria, Via J.H. Dunant 5, 21100 Varese, Italy (E-mail: daniela.negrini{at}uninsubria.it )