Heterogeneous capillary recruitment among adjoining alveoli

Abstract

. Heterogeneous capillary recruitment among adjoining alveoli. J Appl Physiol 95: 469-476, 2003; 10.1152/ japplphysiol.01115.2002.-Pulmonary capillaries recruit when microvascular pressure is raised. The details of the relationship between recruitment and pressure, however, are controversial. There are data supporting 1) gradual homogeneous recruitment, 2) sudden and complete recruitment, and 3) heterogeneous recruitment. The present study was designed to determine whether alveolar capillary networks recruit in a variety of ways or whether one model predominates. In isolated, pump-perfused canine lung lobes, fields of six neighboring alveoli were recorded with video microscopy as pulmonary venous pressure was raised from 0 to 40 mmHg in 5-mmHg increments. The largest group of alveoli (42%) recruited gradually. Another group (33%) recruited suddenly (sheet flow). Half of the neighborhoods had at least one alveolus that paradoxically derecruited when pressure was increased, even though neighboring alveoli continued to recruit capillaries. At pulmonary venous pressures of 40 mmHg, 86% of the alveolar-capillary networks were not fully recruited. We conclude that the pattern of recruitment among neighboring alveoli is complex, is not homogeneous, and may not reach full recruitment, even under extreme pressures. pulmonary microcirculation; isolated perfused lung lobes; video microscopy; dogs PULMONARY CAPILLARIES ARE recruited with increasing microvascular pressure. Different investigators, however, have found different patterns of recruitment. With the introduction of the zone model, West et al. (31) showed that pulmonary blood flow increased down a vertical gradient as pulmonary arterial and venous pressures increased relative to alveolar pressure. Glazier and colleagues (6) studied red blood cell distributions in rapidly frozen lungs and showed that there was gradual, homogeneous recruitment as capillary pressure increased, especially in zone 2. Fung and Sobin (5) developed the sheet flow theory, which suggested that the entire capillary bed opened suddenly and completely as capillary transmural pressure exceeded alveolar pressure. This hypothesis was supported experimentally by Sobin et al. (19), who studied latex casts of the capillary bed. Godbey et al. (9) showed in excised lungs that sheet flow tended to occur at low airway pressure when alveoli were not highly distended, whereas gradual recruitment tended to occur in more distended alveoli. Although the zone and sheet flow models predict homogeneous recruitment, Warrell et al. (28), studying rapidly frozen lungs under zone 2 conditions, found an uneven distribution of capillary red blood cells within areas likely to have been supplied by a common arteriole. The scatter in the data of Godbey et al. as capillaries recruited suggests confirmation of the Warrell data, i.e., that capillary recruitment could be heterogeneous. West et al. (32), using an elegant computer model, deduced that recruitment in a network of resistors could be heterogeneous. Nevertheless...

Lung Circulation

“…In all positions, vertical gradients of perfusion were observed, although multiple stepwise linear regression analysis revealed that both gravity and geometry of the vascular tree influenced regional blood flow. Further complexity is added by heterogeneity in blood flow even between neighboring groups of alveoli, as shown by Baumgartner et al (39). Consideration of these factors has led to the proposal of a refined model of lung zones (217,219) whereby all three zones can exist within the same horizontal (isogravimetric) plane, but the numbers of different zones within each plane shifts with increasing hydrostatic pressure down the lung, from predominantly zones 1 and 2 at the apex to all zone 3 conditions in the dependent lung regions.…”

A perpetual switching system in pulmonary capillaries

“…All animals were handled according to guidelines established by the National Institutes of Health and Institute of Laboratory Animal Resources. The protocols were based on techniques previously developed in our laboratory (3,4,6,36,37,38,39,40,41,42,43) and extended by others who studied the pulmonary microcirculation in vivo (2,5,20,21,22,29,30,33,34).…”

Ventilation and Perfusion at the Alveolar Level: Insights From Lung Intravital Microscopy

“…In addition to the detection of edema formation, IVM has also been used extensively to characterize the processes of capillary recruitment and derecruitment (not to be confused with alveolar R/D discussed above). IVM studies in mechanically ventilated rabbits or dogs or isolated rat lungs have demonstrated the considerable potential for capillary recruitment in the pulmonary vasculature at high perfusion rates or pressures (Wagner, 1988;Hanson et al, 1989;Kuhnle et al, 1995;Baumgartner et al, 2003), respectively. Importantly, this topic has recently gained considerable translational attention since several clinical studies of pulmonary hemodynamics at rest and during exercise in healthy resting supine subjects suggest that most of the pulmonary microvasculature is already recruited at baseline.…”