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

¹Dipartimento di Scienze Biomediche Sperimentali e Cliniche, Università degli Studi dell'Insubria, Varese; and ²Dipartimento 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

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