Dorsal Lung Regions Research Articles

Gas Exchange in the Prone Posture.

The prone posture is known to have numerous effects on gas exchange, both under normal conditions and in patients with ARDS. Clinical studies have consistently demonstrated improvements in oxygenation, and a multi-center randomized trial found that, when implemented within 48 h of moderate-to-severe ARDS, placing subjects in the prone posture decreased mortality. Improvements in gas exchange occur via several mechanisms: alterations in the distribution of alveolar ventilation, redistribution of blood flow, improved matching of local ventilation and perfusion, and reduction in regions of low ventilation/perfusion ratios. Ventilation heterogeneity is reduced in the prone posture due to more uniform alveolar size secondary to a more uniform vertical pleural pressure gradient. The prone posture results in more uniform pulmonary blood flow when compared with the supine posture, due to an anatomical bias for greater blood flow to dorsal lung regions. Because both ventilation and perfusion heterogeneity decrease in the prone posture, gas exchange improves. Other benefits include a more uniform distribution of alveolar stress, relief of left-lower-lobe lung compression by the heart, enhanced secretion clearance, and favorable right-ventricular and systemic hemodynamics.

Prone Positioning Improves Ventilation Homogeneity in Children With Acute Respiratory Distress Syndrome.

To determine the effect of prone positioning on ventilation distribution in children with acute respiratory distress syndrome. Prospective observational study. Paediatric Intensive Care at Red Cross War Memorial Children's Hospital, Cape Town, South Africa. Mechanically ventilated children with acute respiratory distress syndrome. Electrical impedance tomography measures were taken in the supine position, after which the child was turned into the prone position, and subsequent electrical impedance tomography measurements were taken. Thoracic electrical impedance tomography measures were taken at baseline and after 5, 20, and 60 minutes in the prone position. The proportion of ventilation, regional filling characteristics, and global inhomogeneity index were calculated for the ventral and dorsal lung regions. Arterial blood gas measurements were taken before and after the intervention. A responder was defined as having an improvement of more than 10% in the oxygenation index after 60 minutes in prone position. Twelve children (nine male, 65%) were studied. Four children were responders, three were nonresponders, and five showed no change to prone positioning. Ventilation in ventral and dorsal lung regions was no different in the supine or prone positions between response groups. The proportion of ventilation in the dorsal lung increased from 49% to 57% in responders, while it became more equal between ventral and dorsal lung regions in the prone position in nonresponders. Responders showed greater improvements in ventilation homogeneity with R improving from 0.86 ± 0.24 to 0.98 ± 0.02 in the ventral lung and 0.91 ± 0.15 to 0.99 ± 0.01 in the dorsal lung region with time in the prone position. The response to prone position was variable in children with acute respiratory distress syndrome. Prone positioning improves homogeneity of ventilation and may result in recruitment of the dorsal lung regions.

Regional distribution of ventilation in horses in dorsal recumbency during spontaneous and mechanical ventilation assessed by electrical impedance tomography: a case series

ObjectiveTo evaluate the regional distribution of ventilation in horses during spontaneous breathing and controlled mechanical ventilation (CMV) using electrical impedance tomography (EIT). Study designProspective, experimental case series. AnimalsFour anaesthetized experimental horses. MethodsHorses were anaesthetized with isoflurane in an oxygen-air mixture and medetomidine continuous rate infusion, placed in dorsal recumbency with an EIT belt around the thorax, and allowed to breathe spontaneously until PaCO2 reached 13.3 kPa (100 mmHg), when volume CMV was started. For each horse, the EIT signal was recorded for at least 2 minutes immediately before (T1), and at 30 (n = 3) or 60 (n = 1) minutes after the start of CMV (T2). The centre of ventilation (CoV), dependent silent spaces (DSS) (likely to represent atelectatic lung areas), non-dependent silent spaces (NSS) (likely to represent lung areas with low ventilation) and total ventilated area (TVA) were evaluated. Cardiac output (CO) was measured and venous admixture and oxygen delivery (DO2) were calculated at T1 and T2. Data are presented as median and range. ResultsAfter the initiation of CMV, the CoV moved ventrally towards the non-dependent lung by 10% [from 57.4% (49.6–60.2%) to 48.3% (41.9–54.4%)]. DSS increased [from 4.1% (0.2–13.9%) to 18.7% (7.5–27.5%)], while NSS [21.7% (9.4–29.2%) to 9.9% (1.0–20.7%)] and TVA [920 (699–1051) to 837 (662–961) pixels] decreased. CO, venous admixture and DO2 also decreased. Conclusions and clinical relevanceIn spontaneously breathing anaesthetized horses in dorsal recumbency, ventilation was essentially centred within the dependent dorsal lung regions and moved towards non-dependent ventral regions as soon as CMV was started. This shows a major lack of ventilation in the dependent lung, which may be indicative of atelectasis.

Perioperative assessment of regional ventilation during changing body positions and ventilation conditions by electrical impedance tomography

BackgroundLung-protective ventilation is claimed to be beneficial not only in critically ill patients, but also in pulmonary healthy patients undergoing general anaesthesia. We report the use of electrical impedance tomography for assessing regional changes in ventilation, during both spontaneous breathing and mechanical ventilation, in patients undergoing robot-assisted radical prostatectomy. MethodsWe performed electrical impedance tomography measurements in 39 patients before induction of anaesthesia in the sitting (M1) and supine position (M2), after the start of mechanical ventilation (M3), during capnoperitoneum and Trendelenburg positioning (M4), and finally, in the supine position after release of capnoperitoneum (M5). To quantify regional changes in lung ventilation, we calculated the centre of ventilation and ‘silent spaces’ in the ventral and dorsal lung regions that did not show major impedance changes. ResultsCompared with the awake supine position [2.3% (2.3)], anaesthesia and mechanical ventilation induced a significant increase in silent spaces in the dorsal dependent lung [9.2% (6.3); P <0.05]. Capnoperitoneum and the Trendelenburg position led to a significant increase in such spaces [11.5% (8.9)]. Silent space in the ventral lung remained constant throughout anaesthesia. ConclusionElectrical impedance tomography was able to identify and quantify on a breath-by-breath basis circumscribed areas, so-called silent spaces, within healthy lungs that received little or no ventilation during general anaesthesia, capnoperitoneum, and different body positions. As these silent spaces are suggestive of atelectasis on the one hand and overdistension on the other, they might become useful to guide individualized protective ventilation strategies to mitigate the side-effects of anaesthesia and surgery on the lungs.

The "baby lung" became an adult.

The baby lung was originally defined as the fraction of lung parenchyma that, in acute respiratory distress syndrome (ARDS), still maintains normal inflation. Its size obviously depends on ARDS severity and relates to the compliance of the respiratory system. CO2 clearance and blood oxygenation primarily occur within the baby lung. While the specific compliance suggests the intrinsic mechanical characteristics to be nearly normal, evidence from positron emission tomography suggests that at least a part of the well-aerated baby lung is inflamed. The baby lung is more a functional concept than an anatomical one; in fact, in the prone position, the baby lung "shifts" from the ventral lung regions toward the dorsal lung regions while usually increasing its size. This change is associated with better gas exchange, more homogeneously distributed trans-pulmonary forces, and a survival advantage. Positive end expiratory pressure also increases the baby lung size, both allowing better inflation of already open units and adding new pulmonary units. Viewed as surrogates of stress and strain, tidal volume and plateau pressures are better tailored to baby lung size than to ideal body weight. Although less information is available for the baby lung during spontaneous breathing efforts, the general principles regulating the safety of ventilation are also applicable under these conditions.

A Comprehensive Review of Prone Position in ARDS.

Prone position (PP) has been used since the 1970s to treat severe hypoxemia in patients with ARDS because of its effectiveness at improving gas exchange. Compared with the supine position (SP), placing patients in PP effects a more even tidal volume distribution, in part, by reversing the vertical pleural pressure gradient, which becomes more negative in the dorsal regions. PP also improves resting lung volume in the dorsocaudal regions by reducing the superimposed pressure of both the heart and the abdomen. In contrast, pulmonary perfusion remains preferentially distributed to the dorsal lung regions, thus improving overall alveolar ventilation/perfusion relationships. Moreover, the larger tissue mass suspended from a wider dorsal chest wall effects a more homogeneous distribution of pleural pressures throughout the lung that reduces abnormal strain and stress development. This is believed to ameliorate the severity or development of ventilator-induced lung injury and may partly explain why PP reduces mortality in severe ARDS. Over 40 years of clinical trials have consistently reported improved oxygenation in approximately 70% of subjects with ARDS. Early initiation of PP is more likely to improve oxygenation than initiation during the subacute phase. Maximal oxygenation improvement occurs over a wide time frame ranging from several hours to several days. Meta-analyses of randomized controlled trials suggest that PP provides a survival advantage only in patients with relatively severe ARDS (PaO2 /FIO2 < 150 mm Hg). Moreover, survival is enhanced when patients are managed with a smaller tidal volume (≤ 8 mL/kg), higher PEEP (10-13 cm H2O), and longer duration of PP sessions (> 10-12 h/session). Combining adjunctive therapies (high PEEP, recruitment maneuvers, and inhaled vasodilators) with PP has an additive effect in improving oxygenation and may be particularly helpful in stabilizing gas exchange in very severe ARDS.

Auto-recruitment of dorsal lung regions in horses after anaesthesia

Collapse of dependent lung regions and high intrapulmonary shunt fractions are very common in anaesthetized horses. We have observed that horses show breath-holding after recovering from anaesthesia, the purpose of which has not been previously evaluated.

Pulmonary effects of expiratory-assisted small-lumen ventilation during upper airway obstruction in pigs.

Novel devices for small-lumen ventilation may enable effective inspiration and expiratory ventilation assistance despite airway obstruction. In this study, we investigated a porcine model of complete upper airway obstruction. After ethical approval, we randomly assigned 13 anaesthetised pigs either to small-lumen ventilation following airway obstruction (n=8) for 30min, or to volume-controlled ventilation (sham setting, n=5). Small-lumen ventilation enabled adequate gas exchange over 30 min. One animal died as a result of a tension pneumothorax in this setting. Redistribution of ventilation from dorsal to central compartments and significant impairment of the distribution of ventilation/perfusion occurred. Histopathology demonstrated considerable lung injury, predominantly through differences in the dorsal dependent lung regions. Small-lumen ventilation maintained adequate gas exchange in a porcine airway obstruction model. The use of this technique for 30 min by inexperienced clinicians was associated with considerable end-expiratory collapse leading to lung injury, and may also carry the risk of severe injury.

Regional lung tissue changes with mechanical ventilation and fluid load

ABSTRACTPurpose: To investigate the regional gravity-dependent impact of mechanical ventilation and fluid overload on lung extracellular matrix (ECM) in healthy lungs. Materials and Methods: The glycosaminoglycans (GAGs) composition of the ventral and dorsal lung parenchyma was determined in anesthetized supine healthy rats mechanically ventilated for 4 hours in air: (a) at low (∼7.5 mL/kg) or high (∼ 23 mL /kg) tidal volume (VT) and 0 cmH2O positive end-expiratory pressure (PEEP); (b) at low or high VT at 5 cmH2O PEEP and (c) with or without 7 mL /(kg·h) intravenous saline infusion. Results: Mechanical ventilation degraded lung ECM, with alveolar septa thinning and structural GAGs disorganization. Low VT ventilation was associated with significant tissue structure changes in both ventral and dorsal lung regions, while high VT mainly affected the dependent ones. PEEP decreased ECM injury mainly in the ventral lung regions, although it did not prevent matrix fragmentation and washout at high VT. Intravascular fluid load increased lung damage prevalently in the ventral lung regions. Conclusion: Mechanical ventilation and fluid load may cause additive injuries in healthy lungs, mainly in ventral regions.

Assessment of respiratory system compliance with electrical impedance tomography using a positive end-expiratory pressure wave maneuver during pressure support ventilation: a pilot clinical study.

IntroductionAssessment of respiratory system compliance (Crs) can be used for individual optimization of positive end-expiratory pressure (PEEP). However, in patients with spontaneous breathing activity, the conventional methods for Crs measurement are inaccurate because of the variable muscular pressure of the patient. We hypothesized that a PEEP wave maneuver, analyzed with electrical impedance tomography (EIT), might be suitable for global and regional assessment of Crs during assisted spontaneous breathing.MethodsAfter approval of the local ethics committee, we performed a pilot clinical study in 18 mechanically ventilated patients (61 ± 16 years (mean ± standard deviation)) who were suitable for weaning with pressure support ventilation (PSV). For the PEEP wave, PEEP was elevated by 1 cmH2O after every fifth breath during PSV. This was repeated five times, until a total PEEP increase of 5 cmH2O was reached. Subsequently, PEEP was reduced in steps of 1 cmH2O in the same manner until the original PEEP level was reached. Crs was calculated using EIT from the global, ventral and dorsal lung regions of interest. For reference measurements, all patients were also examined during controlled mechanical ventilation (CMV) with a low-flow pressure-volume maneuver. Global and regional Crs(low-flow) was calculated as the slope of the pressure-volume loop between the pressure that corresponded to the selected PEEP and PEEP +5 cmH2O. For additional reference, Crs during CMV (Crs(CMV)) was calculated as expired tidal volume divided by the difference between airway plateau pressure and PEEP.ResultsRespiratory system compliance calculated from the PEEP wave (Crs(PEEP wave)) correlated closely with both reference measurements (r = 0.79 for Crs(low-flow) and r = 0.71 for Crs(CMV)). No significant difference was observed between the mean Crs(PEEP wave) and the mean Crs(low-flow). However, a significant bias of +17.1 ml/cmH2O was observed between Crs(PEEP wave) and Crs(CMV).ConclusionAnalyzing a PEEP wave maneuver with EIT allows calculation of global and regional Crs during assisted spontaneous breathing. In mechanically ventilated patients with spontaneous breathing activity, this method might be used for assessment of the global and regional mechanical properties of the respiratory system.Electronic supplementary materialThe online version of this article (doi:10.1186/s13054-014-0679-6) contains supplementary material, which is available to authorized users.

Inhibition of Constitutive Nitric Oxide Synthase Does Not Influence Ventilation-Perfusion Matching in Normal Prone Adult Sheep With Mechanical Ventilation.

Local formation of nitric oxide in the lung induces vasodilation in proportion to ventilation and is a putative mechanism behind ventilation-perfusion matching. We hypothesized that regional ventilation-perfusion matching occurs in part due to local constitutive nitric oxide formation. Ventilation and perfusion were analyzed in lung regions (≈1.5 cm) before and after inhibition of constitutive nitric oxide synthase with N-nitro-L-arginine methyl ester (L-NAME) (25 mg/kg) in 7 prone sheep ventilated with 10 cm H2O positive end-expiratory pressure. Ventilation and perfusion were measured by the use of aerosolized fluorescent and infused radiolabeled microspheres, respectively. The animals were exsanguinated while deeply anesthetized; then, lungs were excised, dried at total lung capacity, and divided into cube units. The spatial location for each cube was tracked and fluorescence and radioactivity per unit weight determined. After administration of L-NAME, pulmonary artery pressure increased from a mean of 16.6-23.6 mm Hg, P = .007 but PaO2, PaCO2, and SD log(V/Q) did not change. Distribution of ventilation was not influenced by L-NAME, but a small redistribution of perfusion from ventral to dorsal lung regions was observed. Perfusion to regions with the highest ventilation (fifth quintile of the ventilation distribution) remained unchanged after L-NAME. We found minimal or no influence of constitutive nitric oxide synthase inhibition by L-NAME on the distributions of ventilation and perfusion, and ventilation-perfusion in prone, anesthetized, ventilated, and healthy adult sheep with normal gas exchange.

Effect of High-Flow Nasal Cannula and Body Position on End-Expiratory Lung Volume: A Cohort Study Using Electrical Impedance Tomography

Electrical impedance tomography measures changes in lung impedance, which are mainly related to changes in lung volume. We used electrical impedance tomography to investigate the effects of high-flow nasal cannula (HFNC) and body position on global and regional end-expiratory lung impedance variation (ΔEELI). Prospective study with 20 healthy adults. Two periods were defined: the first in supine position and the second in prone position. Each period was divided into 3 phases. In the first and the third phases the subjects were breathing ambient air, and in the second HFNC was implemented. Four regions of interest were defined: 2 ventral and 2 dorsal. For each respiratory cycle, global and regional ΔEELI were measured by electrical impedance tomography and were expressed as a function of the tidal variation of the first stable respiratory cycle (units). HFNC increased global EELI by 1.26 units (95% CI 1.20-1.31, P < .001) in supine position, and by 0.87 units (95% CI 0.82-0.91, P < .001) in prone position. The distribution of ΔEELI was homogeneous in prone position, with no difference between ventral and dorsal lung regions (-0.01 units, 95% CI -0.01 to 0, P = .18), while in supine position a significant difference was found (0.22 units, 95% CI 0.21-0.23, P < .001) with increased EELI in ventral areas. HFNC increased global EELI in our population, regardless of body position, suggesting an increase in functional residual capacity. Prone positioning was related to a more homogeneous distribution of ΔEELI, while in supine position ΔEELI was higher in the ventral lung regions.

Dynamics of regional lung aeration determined by electrical impedance tomography in patients with acute respiratory distress syndrome

Background: Lung tissue of patients with acute respiratory distress syndrome (ARDS) is heterogeneously damaged and prone to develop atelectasis. During inflation, atelectatic regions may exhibit alveolar recruitment accompanied by prolonged filling with air in contrast to regions with already open alveoli with a fast increase in regional aeration. During deflation, derecruitment of injured regions is possible with ongoing loss in regional aeration. The aim of our study was to assess the dynamics of regional lung aeration in mechanically ventilated patients with ARDS and its dependency on positive end-expiratory pressure (PEEP) using electrical impedance tomography (EIT). Methods: Twelve lung healthy and twenty ARDS patients were examined by EIT during sustained step increases in airway pressure from 0, 8 and 15 cm H2O to 35 cm H2O and during subsequent step decrease to the corresponding PEEP. Regional EIT waveforms in the ventral and dorsal lung regions were fitted to bi-exponential equations. Regional fast and slow respiratory time constants and the sizes of the fast and slow compartments were subsequently calculated. Results: ARDS patients exhibited significantly lower fast and slow time constants than the lung healthy patients in ventral and dorsal regions. The time constants were significantly affected by PEEP and differed between the regions. The size of the fast compartment was significantly lower in ARDS patients than in patients with healthy lung under all studied conditions. Conclusion: These results show that regional lung mechanics can be assessed by EIT. They reflect the lower respiratory system compliance of injured lungs and imply more pronounced regional recruitment and derecruitment in ARDS patients.

Dynamics of regional lung aeration determined by electrical impedance tomography in patients with acute respiratory distress syndrome

BackgroundLung tissue of patients with acute respiratory distress syndrome (ARDS) is heterogeneously damaged and prone to develop atelectasis. During inflation, atelectatic regions may exhibit alveolar recruitment accompanied by prolonged filling with air in contrast to regions with already open alveoli with a fast increase in regional aeration. During deflation, derecruitment of injured regions is possible with ongoing loss in regional aeration. The aim of our study was to assess the dynamics of regional lung aeration in mechanically ventilated patients with ARDS and its dependency on positive end-expiratory pressure (PEEP) using electrical impedance tomography (EIT).MethodsTwelve lung healthy and twenty ARDS patients were examined by EIT during sustained step increases in airway pressure from 0, 8 and 15 cm H2O to 35 cm H2O and during subsequent step decrease to the corresponding PEEP. Regional EIT waveforms in the ventral and dorsal lung regions were fitted to bi-exponential equations. Regional fast and slow respiratory time constants and the sizes of the fast and slow compartments were subsequently calculated.ResultsARDS patients exhibited significantly lower fast and slow time constants than the lung healthy patients in ventral and dorsal regions. The time constants were significantly affected by PEEP and differed between the regions. The size of the fast compartment was significantly lower in ARDS patients than in patients with healthy lung under all studied conditions.ConclusionThese results show that regional lung mechanics can be assessed by EIT. They reflect the lower respiratory system compliance of injured lungs and imply more pronounced regional recruitment and derecruitment in ARDS patients.

Effect of Nasal Continuous and Biphasic Positive Airway Pressure on Lung Volume in Preterm Infants

To monitor regional changes in end-expiratory lung volume (EELV), tidal volumes, and their ventilation distribution during different levels of nasal continuous positive airway pressure (nCPAP) and nasal biphasic positive airway pressure (BiPAP) in stable preterm infants. By using electrical impedance tomography and respiratory inductive plethysmography, we measured changes in EELV and tidal volumes in 22 preterm infants (gestational age 29.7±1.5 weeks) during 3 nCPAP levels (2, 4, and 6 cmH2O) and unsynchronized BiPAP (nCPAP=6 cmH2O; pressure amplitude=3 cmH2O; frequency=50/min; inspiration time=0.5 seconds) at 10-minute intervals. We assessed the distribution of these volumes in ventral and dorsal chest regions by using electrical impedance tomography. EELV increased with increasing nCPAP with no difference between the ventral and dorsal lung regions. Tidal volume also increased, and a decrease in phase angle and respiratory rate was noted by respiratory induction plethysmography. At the regional level, electrical impedance tomography data showed a more dorsally oriented ventilation distribution. BiPAP resulted in a small increase in EELV but without changes in tidal volume or its regional distribution. Increasing nCPAP in the range of 2 to 6 cmH2O results in a homogeneous increase in EELV and an increase in tidal volume in preterm infants with a more physiologic ventilation distribution. Unsynchronized BiPAP does not improve tidal volume compared with nCPAP.

Comparative effects of proportional assist and variable pressure support ventilation on lung function and damage in experimental lung injury*

To investigate the effects of proportional assist ventilation, variable pressure support, and conventional pressure support ventilation on lung function and damage in experimental acute lung injury. : Randomized experimental study. University hospital research facility. : Twenty-four juvenile pigs. Pigs were anesthetized, intubated, and mechanically ventilated. Acute lung injury was induced by saline lung lavage. After resuming of spontaneous breathing, animals were randomly assigned to 6 hrs of assisted ventilation with pressure support ventilation, proportional assist ventilation, or variable pressure support (n = 8 per group). Mean tidal volume was kept at ≈6 mL/kg in all modes. Lung functional parameters, distribution of ventilation by electrical impedance tomography, and breathing patterns were analyzed. Histological lung damage and pulmonary inflammatory response were determined postmortem. Variable -pressure support and proportional assist ventilation improved oxygenation and venous admixture compared with pressure support ventilation. Proportional assist ventilation led to higher esophageal pressure time product than variable pressure support and pressure support ventilation, and redistributed ventilation from central to dorsal lung regions compared to pressure support ventilation. Variable pressure support and proportional assist ventilation yielded higher tidal volume variability than pressure support ventilation. Such pattern was deterministic (self-organized) during proportional assist ventilation and stochastic (random) during variable pressure support. Subject-ventilator synchrony as well as pulmonary inflammatory response and damage did not differ among groups. In a lung lavage model of acute lung injury, both variable pressure support and proportional assist ventilation increased the variability of tidal volume and improved oxygenation and venous admixture, without influencing subject-ventilator synchrony or affecting lung injury compared with pressure support ventilation. However, variable pressure support yielded less inspiratory effort than proportional assist ventilation at comparable mean tidal volumes of 6 mL/kg.

Regional respiratory time constants during lung recruitment in high-frequency oscillatory ventilated preterm infants

PurposeTo assess the regional respiratory time constants of lung volume changes during stepwise lung recruitment before and after surfactant treatment in high-frequency oscillatory ventilated preterm infants.MethodsA stepwise oxygenation-guided recruitment procedure was performed before and after surfactant treatment in high-frequency oscillatory ventilated preterm infants. Electrical impedance tomography was used to continuously record changes in lung volume during the recruitment maneuver. Time constants were determined for all incremental and decremental pressure steps, using one-phase exponential decay curve fitting. Data were analyzed for the whole cross section of the chest and the ventral and dorsal lung regions separately.ResultsBefore surfactant treatment, the time constants of the incremental pressure steps were significantly longer (median 27.3 s) than those in the decremental steps (16.1 s). Regional analysis showed only small differences between the ventral and dorsal lung regions. Following surfactant treatment, the time constants during decremental pressure steps almost tripled to 44.3 s. Furthermore, the time constants became significantly (p < 0.01) longer in the dorsal (61.2 s) than into the ventral (40.3 s) lung region.ConclusionsLung volume stabilization during stepwise oxygenation-guided lung recruitment in high-frequency oscillatory ventilated preterm infants with respiratory distress syndrome is usually completed within 5 min and is dependent on the position of ventilation on the pressure volume curve, the surfactant status, and the region of interest of the lung.

Regional Respiratory Time Constants During Lung Recruitment in High Frequency Oscillatory Ventilated Preterm Infants

Background and aims: To assess the regional respiratory time constants of lung volume changes during oxygenation-guided lung recruitment before- and after surfactant treatment in high-frequency oscillatory ventilated preterm infants with respiratory distress syndrome. Methods: Using electrical impedance tomography, changes in lung volume were continuously recorded during a stepwise recruitment procedure before and after surfactant treatment in 22 preterm infants. Time constants were determined for all incremental and decremental pressure steps, using one phase exponential decay curve fitting. Data were analyzed for the whole cross-section of the chest and the ventral and dorsal lung regions separately. Results: Before surfactant treatment, the time constants of the incremental pressure steps were significantly longer (median 27.3 s) compared to the decremental steps (16.1 s). Regional analysis showed only small differences between the ventral and dorsal lung regions. Following surfactant treatment, the time constants during decremental pressure steps almost tripled to 44.3 s. Furthermore, the time constants became significantly longer in the dorsal (61.2 s) than in to the ventral (40.3 s) lung region. Conclusions: Lung volume stabilization during stepwise oxygenation-guided lung recruitment in high-frequency oscillatory ventilated preterm infants with respiratory distress syndrome is usually completed within 5 minutes and is dependent on the position of ventilation on the pressure volume curve, the surfactant status and the region of interest of the lung

Prone Positioning

Prone Positioning

Positive End-expiratory Pressure Redistributes Regional Blood Flow and Ventilation Differently in Supine and Prone Humans

Animal studies have demonstrated an interaction between posture and the effect of positive end-expiratory pressure (PEEP) on regional ventilation and lung blood flow. The aim of this study was to explore this interaction in humans. Regional lung blood flow and ventilation were compared between mechanical ventilation with and without PEEP in the supine and prone postures. Six normal subjects were studied in each posture. Regional lung blood flow was marked with In-labeled macroaggregates and ventilation with Technegas (Tc). Radiotracer distributions were mapped using quantitative single-photon emission computed tomography. In supine subjects, PEEP caused a similar redistribution of both ventilation and blood flow toward dependent (dorsal) lung regions, resulting in little change in the V/Q correlation. In contrast, in prone subjects, the redistribution toward dependent (ventral) regions was much greater for blood flow than for ventilation, causing increased V/Q mismatch. Without PEEP, the vertical ventilation-to-perfusion gradient was less in prone postures than in supine, but with PEEP, the gradient was similar. During mechanical ventilation of healthy volunteers, the addition of PEEP, 10 cm H2O, causes redistribution of both lung blood flow and ventilation, and the effect is different between the supine and prone postures. Our results suggest that the addition of PEEP in prone might be less beneficial than in supine and that optimal use of the prone posture requires reevaluation of the applied PEEP.