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.

Background: Real-time bedside information on regional ventilation and perfusion during mechanical ventilation (MV) may help to elucidate the physiological and pathophysiological effects of MV settings in healthy and injured lungs. We aimed to study the effects of positive end-expiratory pressure (PEEP) and tidal volume (V T ) on the distributions of regional ventilation and perfusion by electrical impedance tomography (EIT) in healthy and injured lungs. Methods: One-hit acute lung injury model was established in 6 piglets by repeated lung lavages (injured group). Four ventilated piglets served as the control group. A randomized sequence of any possible combination of three V T (7, 10, and 15 ml/kg) and four levels of PEEP (5, 8, 10, and 12 cmH 2 O) was performed in all animals. Ventilation and perfusion distributions were computed by EIT within three regionsof-interest (ROIs): nondependent, middle, dependent. A mixed design with one between-subjects factor (group: intervention or control), and two within-subjects factors (PEEP and V T ) was used, with a three-way mixed analysis of variance (ANOVA). Results: Two-way interactions between PEEP and group, and V T and group, were observed for the dependent ROI (p = 0.035 and 0.012, respectively), indicating that the increase in the dependent ROI ventilation was greater at higher PEEP and V T in the injured group than in the control group. A two-way interaction between PEEP and V T was observed for perfusion distribution in each ROI: nondependent (p = 0.030), middle (p = 0.006), and dependent (p = 0.001); no interaction was observed between injured and control groups. Conclusions: Large PEEP and V T levels were associated with greater pulmonary ventilation of the dependent lung region in experimental lung injury, whereas they affected pulmonary perfusion of all lung regions both in the control and in the experimental lung injury groups.

BackgroundTidal recruitment/derecruitment (R/D) of collapsed regions in lung injury has been presumed to cause respiratory oscillations in the partial pressure of arterial oxygen (PaO2). These phenomena have not yet been studied simultaneously. We examined the relationship between R/D and PaO2 oscillations by contemporaneous measurement of lung-density changes and PaO2.MethodsFive anaesthetised pigs were studied after surfactant depletion via a saline-lavage model of R/D. The animals were ventilated with a mean fraction of inspired O2 (FiO2) of 0.7 and a tidal volume of 10 ml kg−1. Protocolised changes in pressure- and volume-controlled modes, inspiratory:expiratory ratio (I:E), and three types of breath-hold manoeuvres were undertaken. Lung collapse and PaO2 were recorded using dynamic computed tomography (dCT) and a rapid PaO2 sensor.ResultsDuring tidal ventilation, the expiratory lung collapse increased when I:E <1 [mean (standard deviation) lung collapse=15.7 (8.7)%; P<0.05], but the amplitude of respiratory PaO2 oscillations [2.2 (0.8) kPa] did not change during the respiratory cycle. The expected relationship between respiratory PaO2 oscillation amplitude and R/D was therefore not clear. Lung collapse increased during breath-hold manoeuvres at end-expiration and end-inspiration (14% vs 0.9–2.1%; P<0.0001). The mean change in PaO2 from beginning to end of breath-hold manoeuvres was significantly different with each type of breath-hold manoeuvre (P<0.0001).ConclusionsThis study in a porcine model of collapse-prone lungs did not demonstrate the expected association between PaO2 oscillation amplitude and the degree of recruitment/derecruitment. The results suggest that changes in pulmonary ventilation are not the sole determinant of changes in PaO2 during mechanical ventilation in lung injury.

Background: Bedside lung volume measurement could personalise ventilation and reduce driving pressure in patients with acute respiratory distress syndrome (ARDS). We investigated a modified gas-dilution method, the inspired sinewave technique (IST), to measure the effective lung volume (ELV) in pigs with uninjured lungs and in an ARDS model. Methods: Anaesthetised mechanically ventilated pigs were studied before and after surfactant depletion by saline lavage. Changes in PEEP were used to change ELV. Paired measurements of absolute ELV were taken with IST (ELV IST) and compared with gold-standard measures (sulphur hexafluoride wash in/washout [ELV SF6 ] and computed tomography (CT) [ELV CT ]). Measured volumes were used to calculate changes in ELV (DELV) between PEEP levels for each method (DELV IST , DELV SF6 , and DELV CT). Results: The coefficient of variation was <5% for repeated ELV IST measurements (n¼13 pigs). There was a strong linear relationship between ELV IST and ELV SF6 in uninjured lungs (r 2 ¼0.97), and with both ELV SF6 and ELV CT in the ARDS model (r 2 ¼0.87 and 0.92, respectively). ELV IST had a mean bias of e12 to 13% (95% limits¼±17 e 25%) compared with ELV SF6 and ELV CT. DELV IST was concordant with DELV SF6 and DELV CT in 98e100% of measurements, and had a mean bias of e73 to e77 ml (95% limits¼±128 e 186 ml) compared with DELV SF6 and e1 ml (95% limits ±333 ml) compared with DELV CT. Conclusions: IST provides a repeatable measure of absolute ELV and shows minimal bias when tracking PEEP-induced changes in lung volume compared with CT in a saline-lavage model of ARDS.

Background Cardiac output monitoring can support the management of high-risk surgical patients, but the pulmonary artery catheterisation required by the current ‘gold standard’—bolus thermodilution —has the potential to cause life-threatening complications. We present a novel noninvasive and fully automated method that uses the inspired sinewave technique to continuously monitor cardiac output . Methods Over successive breaths the inspired nitrous oxide (N 2 O) concentration was forced to oscillate sinusoidally with a fixed mean (4%), amplitude (3%), and period (60 s). was determined in a single-compartment tidal ventilation lung model that used the resulting amplitude/phase of the expired N 2 O sinewave. The agreement and trending ability of were compared with during pharmacologically induced haemodynamic changes, before and after repeated lung lavages, in eight anaesthetised pigs. Results Before lung lavage, changes in and from baseline had a mean bias of –0.52 L min −1 (95% confidence interval [CI], –0.41 to –0.63). The concordance between and was 92.5% as assessed by four-quadrant analysis, and polar plot analysis revealed a mean angular bias of 5.98° (95% CI, –24.4°–36.3°). After lung lavage, concordance was slightly reduced (89.4%), and the mean angular bias widened to 21.8° (–4.2°, 47.6°). Impaired trending ability correlated with shunt fraction ( r =0.79, P <0.05). Conclusions The inspired sinewave technique provides continuous and noninvasive monitoring of cardiac output, with a ‘marginal–good’ trending ability compared with cardiac output based on thermodilution. However, the trending ability can be reduced with increasing shunt fraction, such as in acute lung injury.