Mechanical Ventilation Redistributes Blood to Poorly Ventilated Areas in Experimental Lung Injury*

Abstract

Objectives: Determine the intra-tidal regional gas and blood volume distributions at different levels of atelectasis in experimental lung injury. Test the hypotheses that pulmonary aeration and blood volume matching is reduced during inspiration in the setting of minimal tidal recruitment/derecruitment and that this mismatching is an important determinant of hypoxemia. Design: Preclinical study. Setting: Research laboratory. Subjects: Seven anesthetized pigs 28.7 kg (sd, 2.1 kg). Interventions: All animals received a saline-lavage surfactant depletion lung injury model. Positive end-expiratory pressure was varied between 0 and 20 cm H2O to induce different levels of atelectasis. Measurements and Main Results: Dynamic dual-energy CT images of a juxtadiaphragmatic slice were obtained, gas and blood volume fractions within three gravitational regions calculated and normalized to lung tissue mass (normalized gas volume and normalized blood volume, respectively). Ventilatory conditions were grouped based upon the fractional atelectatic mass in expiration (< 20%, 20–40%, and ≥ 40%). Tidal recruitment/derecruitment with fractional atelectatic mass in expiration greater than or equal to 40% was less than 7% of lung mass. In this group, inspiration-related increase in normalized gas volume was greater in the nondependent (818 µL/g [95% CI, 729–908 µL/g]) than the dependent region (149 µL/g [120–178 µL/g]). Normalized blood volume decreased in inspiration in the nondependent region (29 µL/g [12–46 µL/g]) and increased in the dependent region (39 µL/g [30–48 µL/g]). Inspiration-related changes in normalized gas volume and normalized blood volume were negatively correlated in fractional atelectatic mass in expiration greater than or equal to 40% and 20–40% groups (r 2 = 0.56 and 0.40), but not in fractional atelectatic mass in expiration less than 20% group (r 2 = 0.01). Both the increase in normalized blood volume in the dependent region and fractional atelectatic mass in expiration negatively correlated with Pao 2/Fio 2 ratio (ρ = –0.77 and –0.93, respectively). Conclusions: In experimental atelectasis with minimal tidal recruitment/derecruitment, mechanical inspiratory breaths redistributed blood volume away from well-ventilated areas, worsening Pao 2/Fio 2.

Real-time effects of PEEP and tidal volume on regional ventilation and perfusion in experimental lung injury

“…Our findings on regional ventilation may be interpreted as predominantly gravity-related, since the ventilation of the more dependent region exhibited a greater increase in its percentage with increasing V T and PEEP in the injured group than in the control one. On the other hand, our findings on regional perfusion, where all regions were altered and the presence or not of lung injury did not have an effect, suggest that V T and PEEP interacted all over the lung parenchyma on the redistribution of regional perfusion to the dependent zones [20,21], also in agreement with pulmonary blood volume tidal redistribution [22].…”

Bedside monitoring of lung volume available for gas exchange

“…This approach could have contributed to the observed V PaO2 variability due to potentially overlooked changes in oxygen uptake, especially at higher lung volumes (end-inspiratory breath holds) in the saline lavage lung injury model. Here, greater positive airway pressure could have redistributed pulmonary perfusion [ 44 ] and reduced cardiac output during the breath-holding, hence reducing pulmonary oxygen uptake, possibly leading to an underestimation of lung volume. A greater accuracy would be expected with appropriate data collection timing, such as simultaneous measurement of oxygen uptake and V PaO2 .…”

Effects of impaired microvascular flow regulation on metabolism‐perfusion matching and organ function

“…Mechanical ventilation itself can cause lung injury 216 and was shown to worsen ventilation-perfusion matching and oxygenation in an animal model of lung injury by decreasing blood flow to poorly ventilated areas during inspiration. 217 Flow-controlled ventilation has been proposed to attenuate lung injury and improves homogeneity of aeration in dependent portions of the lung. 218 In summary, local regulation of pulmonary blood flow is crucial for maintaining oxygen saturation in critically ill patients.…”

“…When supplemental oxygen is not sufficient to raise arterial oxygen saturation, patients are placed on positive pressure ventilation (PPV). Mechanical ventilation itself can cause lung injury 216 and was shown to worsen ventilation‐perfusion matching and oxygenation in an animal model of lung injury by decreasing blood flow to poorly ventilated areas during inspiration 217 . Flow‐controlled ventilation has been proposed to attenuate lung injury and improves homogeneity of aeration in dependent portions of the lung 218 …”