Changes In End-expiratory Lung Volume Research Articles

End-Expiratory Volume and Oxygenation: Targeting PEEP in ARDS Patients.

Changes in end-expiratory lung volume (∆EELV) in response to changes in PEEP (∆PEEP) have not been reported in mechanically ventilated patients with ARDS. The purpose of this study was to determine the utility of measurements of ∆EELV in determining optimal PEEP in ARDS patients. Nine patients with ARDS were prospectively recruited. ∆EELV was measured using magnetometers during serial decremental PEEP trials. Changes in PaO2 (∆PaO2) were simultaneously measured. Static respiratory system compliance (CRS), ∆PaO2/∆PEEP, and ∆EELV/∆PEEP were calculated at each level of PEEP. For the group, ∆EELV decreased by 1.09 ± 0.13 L (mean ± SD) as PEEP was reduced from 20 to 0 cm H2O with the greatest changes in ∆EELV occurring over the mid range of the decremental PEEP curve. Optimal values for CRS, ∆EELV/∆PEEP, and ∆PaO2/∆PEEP could be identified for each patient and occurred at PEEP levels ranging from 10 to 17.5 cm H2O. There was a significant correlation (r = 0.712, p = 0.047) between ∆PaO2/∆PEEP and ∆EELV/∆PEEP. ∆EELV can be measured from a decremental PEEP curve. Since ∆EELV is highly correlated with ∆PaO2, measures of ∆PaO2/∆PEEP may provide a surrogate for measures of ∆EELV/∆PEEP. Combining measures of ∆EELV/∆PEEP with measures of CRS may provide a novel means of determining optimal PEEP in patients with ARDS.

Influence of different electrode belt positions on electrical impedance tomography imaging of regional ventilation: a prospective observational study.

BackgroundElectrical impedance tomography (EIT) is a non-invasive bedside tool which allows an individualized ventilator strategy by monitoring tidal ventilation and lung aeration. EIT can be performed at different cranio-caudal thoracic levels, but data are missing about the optimal belt position. The main goal of this prospective observational study was to evaluate the impact of different electrode layers on tidal impedance variation in relation to global volume changes in order to propose a proper belt position for EIT measurements.MethodsEIT measurements were performed in 15 mechanically ventilated intensive care patients with the electrode belt at different thoracic layers (L1-L7). All respiratory and hemodynamic parameters were recorded. Blood gas analyses were obtained once at the beginning of EIT examination. Off-line tidal impedance variation/tidal volume (TV/VT) ratio was calculated, and specific patterns of impedance distribution due to automatic and user-defined adjustment of the colour scale for EIT images were identified.ResultsTV/VT ratio is the highest at L1. It decreases in caudal direction. At L5, the decrease of TV/VT ratio is significant. We could identify patterns of diaphragmatic interference with ventilation-related impedance changes, which owing to the automatically adjusted colour scales are not obvious in the regularly displayed EIT images.ConclusionsThe clinical usability and plausibility of EIT measurements depend on proper belt position, proper impedance visualisation, correct analysis and data interpretation. When EIT is used to estimate global parameters like VT or changes in end-expiratory lung volume, the best electrode plane is between the 4th and 5th intercostal space. The specific colour coding occasionally suppresses user-relevant information, and manual rescaling of images is necessary to visualise this information.

Dynamics of end expiratory lung volume after changing positive end-expiratory pressure in acute respiratory distress syndrome patients.

IntroductionLung recruitment maneuvers followed by an individually titrated positive end-expiratory pressure (PEEP) are the key components of the open lung ventilation strategy in acute respiratory distress syndrome (ARDS). The staircase recruitment maneuver is a step-by-step increase in PEEP followed by a decremental PEEP trial. The duration of each step is usually 2 minutes without physiologic rationale.MethodsIn this prospective study, we measured the dynamic end-expiratory lung volume changes (ΔEELV) during an increase and decrease in PEEP to determine the optimal duration for each step. PEEP was progressively increased from 5 to 40 cmH2O and then decreased from 40 to 5 cmH2O in steps of 5 cmH2O every 2.5 minutes. The dynamic of ΔEELV was measured by direct spirometry as the difference between inspiratory and expiratory tidal volumes over 2.5 minutes following each increase and decrease in PEEP. ΔEELV was separated between the expected increased volume, calculated as the product of the respiratory system compliance by the change in PEEP, and the additional volume.ResultsTwenty-six early onset moderate or severe ARDS patients were included. Data are expressed as median [25th-75th quartiles]. During the increase in PEEP, the expected increased volume was achieved within 2[2-2] breaths. During the decrease in PEEP, the expected decreased volume was achieved within 1 [1–1] breath, and 95 % of the additional decreased volume was achieved within 8 [2–15] breaths. Completion of volume changes in 99 % of both increase and decrease in PEEP events required 29 breaths.ConclusionsIn early ARDS, most of the ΔEELV occurs within the first minute, and change is completed within 2 minutes, following an increase or decrease in PEEP.

Lung volume distribution in prolonged mechanical ventilation patients from assist control mode to spontaneous trial mode of automatic tubing compensation in electrical impedance tomography

Electrical impedance tomography (EIT) is a non-invasive and portable lung imaging technique for dynamic evaluation of lung volume distribution. Previous studies had proved the EIT can provide information on regional distribution of ventilation and changes in end-expiratory lung volume (△EELV). The effect of EIT application in ventilation distribution of prolonged mechanical ventilation (PMV) patients underwent weaning program is unknown.

Effects of Sigh on Regional Lung Strain and Ventilation Heterogeneity in Acute Respiratory Failure Patients Undergoing Assisted Mechanical Ventilation.

In acute respiratory failure patients undergoing pressure support ventilation, a short cyclic recruitment maneuver (Sigh) might induce reaeration of collapsed lung regions, possibly decreasing regional lung strain and improving the homogeneity of ventilation distribution. We aimed to describe the regional effects of different Sigh rates on reaeration, strain, and ventilation heterogeneity, as measured by thoracic electrical impedance tomography. Prospective, randomized, cross-over study. General ICU of a single university-affiliated hospital. We enrolled 20 critically ill patients intubated and mechanically ventilated with PaO2/FIO2 up to 300 mm Hg and positive end-expiratory pressure at least 5 cm H2O (15 with acute respiratory distress syndrome), undergoing pressure support ventilation as per clinical decision. Sigh was added to pressure support ventilation as a 35 cm H2O continuous positive airway pressure period lasting 3-4 seconds at different rates (no-Sigh vs 0.5, 1, and 2 Sigh(s)/min). All study phases were randomly performed and lasted 20 minutes. In the last minutes of each phase, we measured arterial blood gases, changes in end-expiratory lung volume of nondependent and dependent regions, tidal volume reaching nondependent and dependent lung (Vtnondep and Vtdep), dynamic intratidal ventilation heterogeneity, defined as the average ratio of Vt reaching nondependent/Vt reaching dependent lung regions along inspiration (VtHit). With Sigh, oxygenation improved (p < 0.001 vs no-Sigh), end-expiratory lung volume of nondependent and dependent regions increased (p < 0.01 vs no-Sigh), Vtnondep showed a trend to reduction, and Vtdep significantly decreased (p = 0.11 and p < 0.01 vs no-Sigh, respectively). VtHit decreased only when Sigh was delivered at 0.5/min (p < 0.05 vs no-Sigh), while it did not vary during the other two phases. Sigh decreases regional lung strain and intratidal ventilation heterogeneity. Our study generates the hypothesis that in ventilated acute respiratory failure patients, Sigh may enhance regional lung protection.

The effect of prolonged lateral positioning during routine care on regional lung volume changes in preterm infants.

During routine nursing care, preterm infants are often placed in lateral position for several hours, but the effect of this procedure on regional lung volume and ventilation is unknown. In our study we examined this effect during 3 hrs of lateral positioning in stable preterm infants. Preterm infants on non-invasive respiratory support were eligible for the study. Infants were placed in supine position and subsequently transferred to right or left lateral position, according to their individual routine nursing schedule. Changes in end-expiratory lung volume (EELV), tidal volume (VT ) and ventilation distribution were recorded using electrical impedance tomography (EIT), starting 10 min before and up to 180 min after the positional change. Additionally, oxygen requirement, transcutaneous oxygen saturation and respiratory rate were recorded. 15 infants were included (GA 28.9 ± 2.0 wk, BW 1167 ± 290 g). EELV increased significantly after changing to lateral position, stabilizing at a median value of 40.8 (IQR 29.0-99.3) AU/kg at 30 min. This increase could almost be exclusively attributed to the non-dependent lung regions. Tidal volume, oxygenation, and respiratory rate remained stable. Changing to the right, but not the left, lateral position resulted in a rapid but transient shift in ventilation to the dependent lung regions. After 180 min there were no differences in ventilation distribution between lateral and supine positioning. This study shows that lateral position up to 3 hours, as part of normal nursing care of preterm infants, has no adverse effects on lung volumes and its regional distribution.

Determination of Lung Volume and Hemodynamic Changes During High-Frequency Ventilation Recruitment in Preterm Neonates With Respiratory Distress Syndrome.

To evaluate the changes in end-expiratory lung volume during an oxygenation-guided stepwise recruitment procedure in elective high-frequency ventilation. We hypothesized that high continuous distending pressure impedes pulmonary blood flow as evidenced by reduced lung volume measurements using respiratory inductive plethysmography. Changes in oxygenation, ventilation, and peripheral perfusion were evaluated as secondary outcomes. A prospective, single center, observational, nonrandomized study. The study was conducted in a neonatal ICU in Italy. High-frequency ventilated preterm infants with respiratory distress syndrome. During the recruitment procedure, end-expiratory lung volume measured by respiratory inductive plethysmography, oxygen saturation, perfusion index, regional cerebral and perirenal tissue oxygenation, heart rate, transcutaneous PCO2, and tidal volume were simultaneously recorded at each airway pressure step. In 12 preterm newborns (gestational age, 27.4 ± 0.2 wk; birth weight, 979 ± 198 g), high-frequency ventilation was initiated at a continuous distending pressure of 10 cm H2O and incrementally increased by 1-2 cm H2O every 2-5 minutes until FIO2 was less than or equal to 0.25. End-expiratory lung volume progressively increased during the initial recruitment, but decreased at the maximum airway pressure in nine patients, indicative of a reduction in pulmonary perfusion. At the end of recruitment, tidal volume was significantly higher (p = 0.002) and oxygenation was significantly improved (p = 0.002); however, mean perfusion index, postductal saturation, and mean renal tissue oxygenation values were significantly reduced (p < 0.05) compared with baseline. Mean cerebral tissue oxygenation and mean transcutaneous PCO2 values were reduced but failed to reach significance. High distending lung pressures increased oxygenation but decreased peripheral perfusion with no adverse cerebral side effects. Coupled with the reduction in respiratory inductive plethysmography-derived lung volume, high continuous distending pressure had adverse cardiopulmonary effects. Incorporation of lung volume and hemodynamic and oxygenation variables may guide optimum lung volume determination during high-frequency ventilation recruitment procedure while preventing adverse effects on the pulmonary circulation.

The Effect of Caffeine on Diaphragmatic Activity and Tidal Volume in Preterm Infants

To determine the effect of caffeine on diaphragmatic activity, tidal volume (Vt), and end-expiratory lung volume (EELV) in preterm infants. Using transcutaneous electromyography of the diaphragm (dEMG), we measured diaphragmatic activity from 30 minutes before (baseline) to 3 hours after administration of an intravenous caffeine-base loading dose in 30 spontaneously breathing preterm infants (mean gestational age, 29.1 ± 1.3 weeks), most of whom were on noninvasive respiratory support. Diaphragmatic activity was expressed as the percentage change in dEMG amplitude, area under the curve, respiratory rate, and inspiratory and expiratory times. Using respiratory inductive plethysmography, we measured changes in Vt and EELV from baseline. These outcome variables were calculated at 8 fixed time points after caffeine administration (5, 15, 30, 60, 90, 120, 150, and 180 minutes) and compared with baseline. Caffeine administration resulted in rapid (within 5 minutes) increases in dEMG amplitude (median, 43%; IQR, 24%-63%; P < .001) and area under the curve (median, 28%; IQR, 14%-48%; P < .001). Vt also increased by a median of 30% (IQR, 7%-48%), and this change was significantly correlated with the change in dEMG amplitude (r = 0.67; P < .001). These effects were relatively stable until 120 minutes after caffeine administration. Caffeine did not consistently impact EELV, respiratory rate, or inspiratory and expiratory times. Caffeine treatment results in a rapid and sustained increase in diaphragmatic activity and Vt in preterm infants.

Transpulmonary pressure and lung elastance can be estimated by a PEEP-step manoeuvre.

Transpulmonary pressure is a key factor for protective ventilation. This requires measurements of oesophageal pressure that is rarely used clinically. A simple method may be found, if it could be shown that tidal and positive end-expiratory pressure (PEEP) inflation of the lungs with the same volume increases transpulmonary pressure equally. The aim of the present study was to compare tidal and PEEP inflation of the respiratory system. A total of 12 patients with acute respiratory failure were subjected to PEEP trials of 0-4-8-12-16 cmH2O. Changes in end-expiratory lung volume (ΔEELV) following a PEEP step were determined from cumulative differences in inspiratory-expiratory tidal volumes. Oesophageal pressure was measured with a balloon catheter. Following a PEEP increase from 0 to 16 cmH2O end-expiratory oesophageal pressure did not increase (0.5 ± 4.0 cmH2O). Average increase in EELV following a PEEP step of 4 cmH2O was 230 ± 132 ml. The increase in EELV was related to the change in PEEP divided by lung elastance (El) derived from oesophageal pressure as ΔPEEP/El. There was a good correlation between transpulmonary pressure by oesophageal pressure and transpulmonary pressure based on El determined as ΔPEEP/ΔEELV, r(2) = 0.80, y = 0.96x, mean bias -0.4 ± 3.0 cmH2 O with limits of agreement from 5.4 to -6.2 cmH2O (2 standard deviations). PEEP inflation of the respiratory system is extremely slow, and allows the chest wall complex, especially the abdomen, to yield and adapt to intrusion of the diaphragm. As a consequence a change in transpulmonary pressure is equal to the change in PEEP and transpulmonary pressure can be determined without oesophageal pressure measurements.

Cross-Sectional Changes in Lung Volume Measured by Electrical Impedance Tomography Are Representative for the Whole Lung in Ventilated Preterm Infants

Electrical impedance tomography measures lung volume in a cross-sectional slice of the lung. Whether these cross-sectional volume changes are representative of the whole lung has only been investigated in adults, showing conflicting results. This study aimed to compare cross-sectional and whole lung volume changes using electrical impedance tomography and respiratory inductive plethysmography. A prospective, single-center, observational, nonrandomized study. The study was conducted in a neonatal ICU in the Netherlands. High-frequency ventilated preterm infants with respiratory distress syndrome. Cross-sectional and whole lung volume changes were continuously and simultaneously measured by, respectively, electrical impedance tomography and respiratory inductive plethysmography during a stepwise recruitment procedure. End-expiratory lung volume changes were assessed by mapping the inflation and deflation limbs using both the pressure/impedance and pressure/inductance pairs and characterized by calculating the inflection points. In addition, oscillatory tidal volume changes were assessed at each pressure step. Twenty-three infants were included in the study. Of these, eight infants had to be excluded because the quality of the registration was insufficient for analysis (two electrical impedance tomography and six respiratory inductive plethysmography). In the remaining 15 infants (gestational age 28.0 ± 2.6 wk; birth weight 1,027 ± 514 g), end-expiratory lung volume changes measured by electrical impedance tomography were significantly correlated to respiratory inductive plethysmography measurements in 12 patients (mean r = 0.93 ± 0.05). This was also true for the upper inflection point on the inflation (r = 0.91, p < 0.01) and deflation limb (r = 0.83, p < 0.01). In 13 patients, impedance and inductance data also correlated significantly on oscillatory tidal volume/pressure relationships (mean r = 0.81 ± 0.18). This study shows that cross-sectional lung volume changes measured by electrical impedance tomography are representative for the whole lung and that this concept also applies to newborn infants.

Reading Pulmonary Vascular Pressure Tracings. How to Handle the Problems of Zero Leveling and Respiratory Swings

The accuracy of pulmonary vascular pressure measurements is of great diagnostic and prognostic relevance. However, there is variability of zero leveling procedures, and the current recommendation of end-expiratory reading may not always be adequate. A review of physiological and anatomical data, supported by recent imaging, leads to the practical recommendation of zero leveling at the cross-section of three transthoracic planes, which are, respectively midchest frontal, transverse through the fourth intercostal space, and midsagittal. As for the inevitable respiratory pressure swings, end-expiratory reading at functional residual capacity allows for minimal influence of elastic lung recoil on pulmonary pressure reading. However, hyperventilation is associated with changes in end-expiratory lung volume and increased intrathoracic pressure, eventually exacerbated by expiratory muscle contraction and dynamic hyperinflation, all increasing pulmonary vascular pressures. This problem is amplified in patients with obstructed airways. With the exception of dynamic hyperinflation states, it is reasonable to assume that negative inspiratory and positive expiratory intrathoracic pressures cancel each other out, so averaging pulmonary vascular pressures over several respiratory cycles is most often preferable. This recommendation may be generalized for the purpose of consistency and makes sense, as pulmonary blood flow measurements are not corrected for phasic inspiratory and expiratory changes in clinical practice.

Head-of-bed elevation improves end-expiratory lung volumes in mechanically ventilated subjects: a prospective observational study.

Head-of-bed elevation (HOBE) has been shown to assist in reducing respiratory complications associated with mechanical ventilation; however, there is minimal research describing changes in end-expiratory lung volume. This study aims to investigate changes in end-expiratory lung volume in a supine position and 2 levels of HOBE. Twenty postoperative cardiac surgery subjects were examined using electrical impedance tomography. End-expiratory lung impedance (EELI) was recorded as a surrogate measurement of end-expiratory lung volume in a supine position and at 20° and then 30°. Significant increases in end-expiratory lung volume were seen at both 20° and 30° HOBE in all lung regions, except the anterior, with the largest changes from baseline (supine) seen at 30°. From baseline to 30° HOBE, global EELI increased by 1,327 impedance units (95% CI 1,080-1,573, P < .001). EELI increased by 1,007 units (95% CI 880-1,134, P < .001) in the left lung region and by 320 impedance units (95% CI 188-451, P < .001) in the right lung. Posterior increases of 1,544 impedance units (95% CI 1,405-1,682, P < .001) were also seen. EELI decreased anteriorly, with the largest decreases occurring at 30° (-335 impedance units, 95% CI -486 to -183, P < .001). HOBE significantly increases global and regional end-expiratory lung volume; therefore, unless contraindicated, all mechanically ventilated patients should be positioned with HOBE.

B-15 Thematic Poster - Advances in Epidemiology

BACKGROUND: The effect of weight loss on lung function at rest in men and women is well documented. Weight loss also improves breathing mechanics during exercise in obese men. However, the effect of weight loss on breathing mechanics during exercise is unknown in obese women, who have smaller respiratory reserves than obese men. PURPOSE: To investigate the effects of weight loss on breathing mechanics during submaximal exercise in obese women. We hypothesized that weight loss would improve breathing mechanics during submaximal exercise via an increase in end-expiratory lung volume (EELV). METHODS: In 29 obese women (age: 33 ± 8 y, weight: 96.8 ± 14.0 kg, BMI: 36.2 ± 3.5 kg/m2, percent body fat: 45.6 ± 4.5%; Mean ± SD), body composition, fat distribution (MRI), pulmonary function, and breathing mechanics during exercise were studied before and after a 12-week diet and resistance exercise weight loss program. Data were analyzed by paired t-tests. Relationships among variables were investigated by Pearson’s correlations. RESULTS: Subjects lost 7.5 ± 3.1 kg of body weight (P < 0.001). Total lung capacity, functional residual capacity (FRC), and expiratory reserve volume were significantly increased (4.8 ± 0.7 to 4.9 ± 0.8 L, 2.0 ± 0.4 to 2.2 ± 0.5 L, 0.8 ± 0.3 to 1.1 ± 0.5 L respectively; P < 0.05) while inspiratory capacity was decreased (2.8 ± 0.5 to 2.7 ± 0.5 L; P < 0.05) after weight loss. Changes in FRC were moderately associated with changes in body weight, BMI, abdominal fat and total chest fat + abdominal fat (r = -0.58 to -0.65; P < 0.05). EELV increased at rest (42.7 ± 7.5 to 45.7 ± 7.6 %TLC; P = 0.004) and during constant load exercise (40.4 ± 5.1 to 42.5 ± 5.7 %TLC; P = 0.02). There was no significant association between the increase in EELV with weight loss and the magnitude of expiratory flow limitation. There were no meaningful correlations between the changes in EELV during constant load exercise and changes in body composition or pulmonary function. CONCLUSIONS: Moderate weight loss improves breathing mechanics not only at rest but also during submaximal exercise in obese women, which is clinically important since submaximal exercise is a major component in the treatment and prevention of obesity. FUNDING SOURCES: NIH R01 HL096782-01A2, King Charitable Foundation Trust, Texas Health Presbyterian Hospital Dallas

Effect of PEEP and Tidal Volume on Ventilation Distribution and End-Expiratory Lung Volume: A Prospective Experimental Animal and Pilot Clinical Study

IntroductionLung-protective ventilation aims at using low tidal volumes (VT) at optimum positive end-expiratory pressures (PEEP). Optimum PEEP should recruit atelectatic lung regions and avoid tidal recruitment and end-inspiratory overinflation. We examined the effect of VT and PEEP on ventilation distribution, regional respiratory system compliance (CRS), and end-expiratory lung volume (EELV) in an animal model of acute lung injury (ALI) and patients with ARDS by using electrical impedance tomography (EIT) with the aim to assess tidal recruitment and overinflation.MethodsEIT examinations were performed in 10 anaesthetized pigs with normal lungs ventilated at 5 and 10 ml/kg body weight VT and 5 cmH2O PEEP. After ALI induction, 10 ml/kg VT and 10 cmH2O PEEP were applied. Afterwards, PEEP was set according to the pressure-volume curve. Animals were randomized to either low or high VT ventilation changed after 30 minutes in a crossover design. Ventilation distribution, regional CRS and changes in EELV were analyzed. The same measures were determined in five ARDS patients examined during low and high VT ventilation (6 and 10 (8) ml/kg) at three PEEP levels.ResultsIn healthy animals, high compared to low VT increased CRS and ventilation in dependent lung regions implying tidal recruitment. ALI reduced CRS and EELV in all regions without changing ventilation distribution. Pressure-volume curve-derived PEEP of 21±4 cmH2O (mean±SD) resulted in comparable increase in CRS in dependent and decrease in non-dependent regions at both VT. This implied that tidal recruitment was avoided but end-inspiratory overinflation was present irrespective of VT. In patients, regional CRS differences between low and high VT revealed high degree of tidal recruitment and low overinflation at 3±1 cmH2O PEEP. Tidal recruitment decreased at 10±1 cmH2O and was further reduced at 15±2 cmH2O PEEP.ConclusionsTidal recruitment and end-inspiratory overinflation can be assessed by EIT-based analysis of regional CRS.

Comparison of Static End-expiratory and Effective Lung Volumes for Gas Exchange in Healthy and Surfactant-depleted Lungs

Effective lung volume (ELV) for gas exchange is a new measure that could be used as a real-time guide during controlled mechanical ventilation. The authors established the relationships of ELV to static end-expiratory lung volume (EELV) with varying levels of positive end-expiratory pressure (PEEP) in healthy and surfactant-depleted rabbit lungs. Nine rabbits were anesthetized and ventilated with a modified volume-controlled mode where periods of five consecutive alterations in inspiratory/expiratory ratio (1:2-1.5:1) were imposed to measure ELV from the corresponding carbon dioxide elimination traces. EELV and the lung clearance index were concomitantly determined by helium wash-out technique. Airway and tissue mechanics were assessed by using low-frequency forced oscillations. Measurements were collected at PEEP 0, 3, 6, and 9 cm H2O levels under control condition and after surfactant depletion by whole-lung lavage. ELV was greater than EELV at all PEEP levels before lavage, whereas there was no evidence for a difference in the lung volume indices after surfactant depletion at PEEP 6 or 9 cm H2O. Increasing PEEP level caused significant parallel increases in both ELV and EELV levels, decreases in ventilation heterogeneity, and improvement in airway and tissue mechanics under control condition and after surfactant depletion. ELV and EELV exhibited strong and statistically significant correlations before (r=0.84) and after lavage (r=0.87). The parallel changes in ELV and EELV with PEEP in healthy and surfactant-depleted lungs support the clinical value of ELV measurement as a bedside tool to estimate dynamic changes in EELV in children and infants.

Evaluation of Recruited Lung Volume at Inspiratory Plateau Pressure With PEEP Using Bedside Digital Chest X-ray in Patients With Acute Lung Injury/ARDS

We wanted to assess whether there was a significant relationship between recruited lung volume (V(rec)) and change in density on digital processed chest x-ray measured at 2 different levels of inspiratory plateau pressure corresponding to 2 PEEP levels in patients with acute lung injury or ARDS. In 14 subjects, PEEP 5 cm H2O and 15 cm H2O were prospectively applied in a random order for 10 min. At the end of each period, chest x-ray was taken using a digital portable device, and a pressure-volume curve of the respiratory system was performed. We also assessed P(aO2), and the static and the dynamic (C(dyn,rs)) compliance of the respiratory system. Change in end-expiratory lung volume between tidal breath and relaxation volume of the respiratory system was determined. Radiological attenuation was measured on chest x-rays in 4 regions of interest in the right lung, and in 3 regions of interest in the left lung, drawn in posterior intercostal spaces from top to bottom, by using dedicated software. The ratio of lung density in each region between PEEP 15 and PEEP 5 (rP15/P5) and their arithmetic mean (μP15/P5) were computed. V(rec) was determined from the pressure-volume curves. The median value of rP15/P5 in the 98 lung levels was 0.91 (0.80-1.01), which was significantly different from 1 (P < .001). The values of rP15/P5 were not significantly different between the lung levels. The median values of V(rec) and μP15/P5 were 288 (173-402) mL and 0.90 (0.80-0.97), respectively. There was a significant negative correlation between V(rec) and μP15/P5 (R = -0.77, P = .01). The reduction in μP15/P5 tended to correlate with the increase in C(dyn,rs) (R = -0.49, P = .077) or in P(aO2) (R = -0.53, P = .05) between PEEP 15 cm H2O and PEEP 5 cm H2O. Digital chest x-ray done at the bedside in acute lung injury/ARDS subjects was able to detect a reduction in density between PEEP 5 cm H2O and PEEP 15 cm H2O, which correlated with V(rec).

A Method for Determining Optimal Mean Airway Pressure in High-Frequency Oscillatory Ventilation

"Optimal" mean airway pressure (MAP) during high-frequency oscillatory ventilation (HFOV) can be defined as the pressure that allows for maximal alveolar recruitment while minimizing alveolar overdistension. Choosing a MAP near or just below the point of maximal curvature (PMC) of the volume-pressure characteristics of the lung can serve as a guide to avoid overdistention during HFOV, while simultaneously preventing derecruitment. The purpose of this study was to assess whether optimal MAP at the PMC can be determined by using measures of PaO(2) in patients with acute respiratory distress syndrome (ARDS) undergoing HFOV. We prospectively studied seven patients with ARDS who underwent HFOV after failed conventional ventilation. In addition, 11 healthy subjects were studied to validate measurements of changes in end-expiratory lung volume (∆EELV) using magnetometers. Using this validated method, plots of ∆EELV and MAP were constructed during decremental changes in MAP following a recruitment maneuver in seven ventilated patients with ARDS. The PMC was defined as the point where the slope of the ∆EELV versus MAP curve acutely changed. The MAP at the PMC was compared to that determined from plots of PaO(2) versus MAP. In the healthy cohort, measurements of ∆EELV obtained by magnetometry approximated the line of identity when compared to those obtained by spirometry. The MAP determined using either the ∆EELV or PaO(2) techniques were identical in all seven HFOV ventilated patients. Additionally, there was a significant correlation between the MAP associated changes in PaO2 and the MAP associated changes in ∆EELV (p<0.001). The finding that MAP at the PMC is the same whether determined by measures of ∆EELV or PaO(2) suggest that bedside measures PaO(2) may provide an acceptable surrogate for measures of EELV when determining "optimal" MAP during HFOV.

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.

146 Lung Volume in Very Preterm Infants with Early Respiratory Distress Syndrome (RDS) Receiving Nasal Continuous Positive Airway Pressure (CPAP)

Background and aimsAlthough CPAP is extensively used for early RDS in very preterm infants from birth, the influence of CPAP on lung behaviour in early illness remains unclear. This study...

Respiratory Muscle Fatigue following Exercise in Patients with Interstitial Lung Disease

Background: It is not known whether respiratory muscle fatigue occurs as a consequence of exercise in patients with interstitial lung disease (ILD) and, if so, to what extent it is related to changes in dynamic lung volumes. Objectives: To assess the development of respiratory muscle fatigue in patients with ILD and relate it to the respiratory pattern during exercise. Methods: Sixteen ILD patients (11 women) performed incremental, symptom-limited cycle ergometry with inspiratory capacity manoeuvres used to measure changes in end-expiratory lung volume (EELV). Twitch transdia-phragmatic pressure (TwPdi) and twitch gastric pressure (TwT₁₀Pga), in response to magnetic stimulation, were used to assess the development of fatigue. Results: TwPdi did not differ significantly before and after exercise (21.8 ± 8 vs. 20.2 ± 8 cm H₂O; p = 0.10), while TwT₁₀Pga fell from 28.6 ± 18 to 25.2 ± 14 cm H₂O (p = 0.02). EELV fell from 2.18 ± 0.65 to 1.91 ± 0.59 liters following exercise (p = 0.04). The fall in TwT10Pga correlated with peak oxygen uptake at peak of exercise (r = -0.52, p = 0.041), increase in heart rate (r = 0.53, p = 0.032) and with the decrease of EELV during exercise (r = 0.57, p = 0.021). Abdominal muscle fatiguers (n = 9, 56%), defined as having a ≥10% fall in TwT10Pga, had a fall in EELV of 22 ± 22% compared to 0.7 ± 8% in non-fatiguers (p = 0.016). Conclusion: Abdominal muscle fatigue develops during exercise in some ILD patients in association with increased expiratory muscle activity manifested by reduced EELV.