Abstract
The present study describes the magnitude and spatial distribution of lung strain in healthy anesthetized, mechanically ventilated dogs with and without positive end-expiratory pressure (PEEP). Total lung strain (LSTOTAL) has a dynamic (LSDYNAMIC) and a static (LSSTATIC) component. Due to lung heterogeneity, global lung strain may not accurately represent regional total tissue lung strain (TSTOTAL), which may also be described by a regional dynamic (TSDYNAMIC) and static (TSSTATIC) component. Six healthy anesthetized beagles (12.4 ± 1.4 kg body weight) were placed in dorsal recumbency and ventilated with a tidal volume of 15 ml/kg, respiratory rate of 15 bpm, and zero end-expiratory pressure (ZEEP). Respiratory system mechanics and full thoracic end-expiratory and end-inspiratory CT scan images were obtained at ZEEP. Thereafter, a PEEP of 5 cmH2O was set and respiratory system mechanics measurements and end-expiratory and end-inspiratory images were repeated. Computed lung volumes from CT scans were used to evaluate the global LSTOTAL, LSDYNAMIC, and LSSTATIC during PEEP. During ZEEP, LSSTATIC was assumed zero; therefore, LSTOTAL was the same as LSDYNAMIC. Image segmentation was applied to CT images to obtain maps of regional TSTOTAL, TSDYNAMIC, and TSSTATIC during PEEP, and TSDYNAMIC during ZEEP. Compliance increased (p = 0.013) and driving pressure decreased (p = 0.043) during PEEP. PEEP increased the end-expiratory lung volume (p < 0.001) and significantly reduced global LSDYNAMIC (33.4 ± 6.4% during ZEEP, 24.0 ± 4.6% during PEEP, p = 0.032). LSSTATIC by PEEP was larger than the reduction in LSDYNAMIC; therefore, LSTOTAL at PEEP was larger than LSDYNAMIC at ZEEP (p = 0.005). There was marked topographic heterogeneity of regional strains. PEEP induced a significant reduction in TSDYNAMIC in all lung regions (p < 0.05). Similar to global findings, PEEP-induced TSSTATIC was larger than the reduction in TSDYNAMIC; therefore, PEEP-induced TSTOTAL was larger than TSDYNAMIC at ZEEP. In conclusion, PEEP reduced both global and regional estimates of dynamic strain, but induced a large static strain. Given that lung injury has been mostly associated with tidal deformation, limiting dynamic strain may be an important clinical target in healthy and diseased lungs, but this requires further study.
Highlights
General anesthesia is frequently associated with respiratory muscle relaxation, with cranial diaphragmatic displacement that results in reduced lung aeration [1, 2]
The results indicate that [1] positive end-expiratory pressure (PEEP) induces a significant global and regional increase in normoaerated tissue and a decrease in poorly aerated tissue compared with zero end-expiratory pressure (ZEEP), especially in dorsal quadrants
(2) TSDYNAMIC is heterogeneous, with the lowest levels developing in the apicoventral regions and the largest in the basodorsal regions, both with and without PEEP. [3] PEEP induces a significant reduction in TSDYNAMIC in most regions; TSTOTAL is higher during PEEP since the reduction in TSDYNAMIC is smaller than the increase in TSSTATIC. [4] TSSTATIC induced by PEEP follows a similar topographic distribution as that described for the TSDYNAMIC
Summary
General anesthesia is frequently associated with respiratory muscle relaxation, with cranial diaphragmatic displacement that results in reduced lung aeration [1, 2]. The deformation of the lung due to the applied tidal ventilation has been referred to as lung strain, which is defined as the change in lung volume relative to a reference volume (i.e., tidal volume/functional residual capacity) and can be estimated from CT images [7, 8]. Total lung strain (LSTOTAL) has both a dynamic (LSDYNAMIC) and a static component (LSSTATIC). Dynamic lung strain is defined as the ratio between the cyclic change in lung volume (VT) and the initial lung volume (functional residual capacity—FRC, or end-expiratory lung volume—EELV) [16]. The definition of LSDYNAMIC evidences that VT, and the reference volume of the lung (FRC or EELV), and the interaction between these two variables, are important in the generation of this type of global strain. At comparable global LSTOTAL, larger LSDYNAMIC has been shown to be more injurious compared with LSSTATIC [8]
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