Individualized Positive End-expiratory Pressure and Regional Gas Exchange in Porcine Lung Injury.

BACKGROUND: In acute respiratory failure elevated intraabdominal pressure aggravates lung collapse, tidal recruitment, and ventilation inhomogeneity. Low positive end-expiratory pressure (PEEP) may promote lung collapse and intrapulmonary shunting, whereas high PEEP may increase dead space by inspiratory overdistension. The authors hypothesized that an electrical impedance tomography-guided PEEP approach minimizing tidal recruitment improves regional ventilation and perfusion matching when compared to a table-based low PEEP/no recruitment and an oxygenation-guided high PEEP/full recruitment strategy in a hybrid model of lung injury and elevated intraabdominal pressure. METHODS: In 15 pigs with oleic acid-induced lung injury intraabdominal pressure was increased by intraabdominal saline infusion. PEEP was set in randomized order: (1) guided by a PEEP/inspired oxygen fraction table, without recruitment maneuver; (2) minimizing tidal recruitment guided by electrical impedance tomography after a recruitment maneuver; and (3) maximizing oxygenation after a recruitment maneuver. Single photon emission computed tomography was used to analyze regional ventilation, perfusion, and aeration. Primary outcome measures were differences in PEEP levels and regional ventilation/perfusion matching. RESULTS: Resulting PEEP levels were different (mean ± SD) with (1) table PEEP: 11 ± 3 cm H2O; (2) minimal tidal recruitment PEEP: 22 ± 3 cm H2O; and (3) maximal oxygenation PEEP: 25 ± 4 cm H2O; P < 0.001. Table PEEP without recruitment maneuver caused highest lung collapse (28 ± 11% vs. 5 ± 5% vs. 4 ± 4%; P < 0.001), shunt perfusion (3.2 ± 0.8 l/min vs. 1.0 ± 0.8 l/min vs. 0.7 ± 0.6 l/min; P < 0.001) and dead space ventilation (2.9 ± 1.0 l/min vs. 1.5 ± 0.7 l/min vs. 1.7 ± 0.8 l/min; P < 0.001). Although resulting in different PEEP levels, minimal tidal recruitment and maximal oxygenation PEEP, both following a recruitment maneuver, had similar effects on regional ventilation/perfusion matching. CONCLUSIONS: When compared to table PEEP without a recruitment maneuver, both minimal tidal recruitment PEEP and maximal oxygenation PEEP following a recruitment maneuver decreased shunting and dead space ventilation, and the effects of minimal tidal recruitment PEEP and maximal oxygenation PEEP were comparable.

Tidal recruitment assessed by electrical impedance tomography and computed tomography in a porcine model of lung injury*.

OBJECTIVES: To determine the validity of electrical impedance tomography to detect and quantify the amount of tidal recruitment caused by different positive end-expiratory pressure levels in a porcine acute lung injury model. DESIGN: Randomized, controlled, prospective experimental study. SETTING: Academic research laboratory. SUBJECTS: Twelve anesthetized and mechanically ventilated pigs. INTERVENTIONS: Acute lung injury was induced by central venous oleic acid injection and abdominal hypertension in seven animals. Five healthy pigs served as control group. Animals were ventilated with positive end-expiratory pressure of 0, 5, 10, 15, 20, and 25 cm H2O, respectively, in a randomized order. MEASUREMENTS AND MAIN RESULTS: At any positive end-expiratory pressure level, electrical impedance tomography was obtained during a slow inflation of 12 mL/kg of body weight. Regional-ventilation-delay indices quantifying the time until a lung region reaches a certain amount of impedance change were calculated for lung quadrants and for every single electrical impedance tomography pixel, respectively. Pixel-wise calculated regional-ventilation-delay indices were plotted in a color-coded regional-ventilation-delay map. Regional-ventilation-delay inhomogeneity that quantifies heterogeneity of ventilation time courses was evaluated by calculating the scatter of all pixel-wise calculated regional-ventilation-delay indices. End-expiratory and end-inspiratory computed tomography scans were performed at each positive end-expiratory pressure level to quantify tidal recruitment of the lung. Tidal recruitment showed a moderate inter-individual (r = .54; p < .05) and intra-individual linear correlation (r = .46 up to r = .73 and p < .05, respectively) with regional-ventilation-delay obtained from lung quadrants. Regional-ventilation-delay inhomogeneity was excellently correlated with tidal recruitment intra- (r = .90 up to r = .99 and p < .05, respectively) and inter-individually (r = .90; p < .001). CONCLUSIONS: Regional-ventilation-delay can be noninvasively measured by electrical impedance tomography during a slow inflation of 12 mL/kg of body weight and visualized using ventilation delay maps. Our experimental data suggest that the impedance tomography-based analysis of regional-ventilation-delay inhomogeneity provides a good estimate of the amount of tidal recruitment and may be useful to individualize ventilatory settings.

Impedance tomography as a new monitoring technique.

PURPOSE OF REVIEW: Electrical impedance tomography (EIT) noninvasively creates images of the local ventilation and arguably lung perfusion distribution at bedside. Methodological and clinical aspects of EIT when used as a monitoring tool in the intensive care unit are reviewed and discussed. RECENT FINDINGS: Whereas former investigations addressed the issue of validating EIT to measure regional ventilation, current studies focus on clinical applications such as detection of pneumothorax. Furthermore, EIT has been used to quantify lung collapse and tidal recruitment in order to titrate positive end-expiratory pressure. Indicator-free EIT measurements might be sufficient for the continuous measurement of cardiac stroke volume, but assessment of regional lung perfusion presumably requires the use of a contrast agent such as hypertonic saline. SUMMARY: Growing evidence suggests that EIT may play an important role in individually optimizing ventilator settings in critically ill patients.

Assessment of regional lung recruitment and derecruitment during a PEEP trial based on electrical impedance tomography.

OBJECTIVE: To investigate whether electrical impedance tomography (EIT) is capable of monitoring regional lung recruitment and lung collapse during a positive end-expiratory pressure (PEEP) trial. DESIGN: Experimental animal study of acute lung injury. SUBJECT: Six pigs with saline-lavage-induced acute lung injury. INTERVENTIONS: An incremental and decremental PEEP trial at ten pressure levels was performed. Ventilatory, gas exchange, and hemodynamic parameters were automatically recorded. EIT and computed tomography (CT) scans of the same slice were simultaneously taken at each PEEP level. MEASUREMENTS AND RESULTS: A significant correlation between EIT and CT analyses of end-expiratory gas volumes (r=0.98 up to 0.99) and tidal volumes (r=0.55 up to r=0.88) could be demonstrated. Changes in global and regional tidal volumes and arterial oxygenation (PaO2/FiO2) demonstrated recruitment/derecruitment during the trial, but at different onsets. During the decremental trial, derecruitment first occurred in dependent lung areas. This was indicated by lowered regional tidal volumes measured in this area and by a decrease of PaO2/FiO2. At the same time, the global tidal volume still continued to increase, because the increase of ventilation of the non-dependent areas was higher than the loss in the dependent areas. This indicates that opposing regional changes might cancel each other out when combined in a global parameter. CONCLUSIONS: EIT is suitable for monitoring the dynamic effects of PEEP variations on the regional change of tidal volume. It is superior to global ventilation parameters in assessing the beginning of alveolar recruitment and lung collapse.

Regional lung ventilation and perfusion by electrical impedance tomography compared to single-photon emission computed tomography.

OBJECTIVE: Electrical impedance tomography (EIT) is a noninvasive imaging modality that allows real-time monitoring of regional lung ventilation ([Formula: see text]) in intensive care patients at bedside. However, for improved guidance of ventilation therapy it would be beneficial to obtain regional ventilation-to-perfusion ratio ([Formula: see text]) by EIT. APPROACH: In order to further explore the feasibility, we first evaluate a model-based approach, based on semi-negative matrix factorization and a gamma-variate model, to extract regional lung perfusion ([Formula: see text]) from EIT measurements. Subsequently, a combined validation of both [Formula: see text] and [Formula: see text] measured by EIT against single-photon emission computed tomography (SPECT) is performed on data acquired as part of a porcine animal trial. Four pigs were ventilated at two different levels of positive end-expiratory pressure (PEEP 0 and 15 cm H2O, respectively) in randomized order. Repeated injections of an EIT contrast agent (NaCl 10%) and simultaneous SPECT measurements of [Formula: see text] (81mKr gas) and [Formula: see text] (99mTc-labeled albumin) were performed. MAIN RESULTS: Both [Formula: see text] and [Formula: see text] from EIT and SPECT were compared by correlation analysis. Very strong (r 2 = 0.94 to 0.95) correlations were found for [Formula: see text] and [Formula: see text] in the dorsal-ventral direction at both PEEP levels. Moderate (r 2 = 0.36 to 0.46) and moderate to strong (r 2 = 0.61 to 0.82) correlations resulted for [Formula: see text] and [Formula: see text] in the right-left direction, respectively. SIGNIFICANCE: The results of combined validation indicate that monitoring of [Formula: see text] and [Formula: see text] by EIT is possible. However, care should be taken when trying to quantify [Formula: see text] by EIT, as imaging artefacts and model bias may void necessary spatial matching.

Gamma-variate modeling of indicator dilution curves in electrical impedance tomography.

Electrical impedance tomography (EIT) is a non-invasive imaging technique, that can be used to monitor regional lung ventilation (V̇) in intensive care units (ICU) at bedside. This work introduces a method to extract regional lung perfusion (Q̇) from EIT image streams in order to quantify regional gas exchange in the lungs. EIT data from a single porcine animal trial, recorded during injection of a contrast agent (NaCl 10%) into a central venous catheter (CVC), are used for evaluation. Using semi-negative matrix factorization (Semi-NMF) a set of source signals is extracted from the data. A subsequent non-linear fit of a gamma-variate model to the source signals results in model signals, describing contrast agent flow through the cardio-pulmonary system. A linear fit of the model signals to the EIT image stream then yields functional images ofQ̇. Additionally, a pulmonary transit function (PTF) and parameters, such as mean transit time (MTT), time to peak (TTP) and area under curve (AUC) are derived. In result, EIT was used to track changes of regional lung ventilation to perfusion ratio (V̇/Q̇) during changes of positive end-expiratory pressure (PEEP). Furthermore, correlations of MTT and AUC with cardiac output (CO) indicate that CO measurement by EIT might be possible.

The effect of pumpless extracorporeal CO2 removal on regional perfusion of the brain in experimental acute lung injury.

BACKGROUND: Lung-protective mechanical ventilation with low tidal volumes (V(T)) is often associated with hypercapnia (HC), which may be unacceptable in patients with brain injury. CO2 removal using a percutaneous extracorporeal lung assist (pECLA) enables normocapnia despite low V(T), but its effects on regional cerebral blood flow (rCBF) remain ambiguous. We hypothesized that reversal of HC by pECLA impairs rCBF in a porcine lung injury model. METHODS: Lung injury was induced in 9 anesthetized pigs by hydrochloric acid aspiration. rCBF and systemic hemodynamics were measured by colored microsphere technique and transpulmonary-thermodilution during a randomized sequence of 4 experimental situations: pECLA shunt-on (1) with HC and (2) without HC, pECLA shunt-off (3) with HC and (4) without HC. RESULTS: HC increased rCBF (P<0.05). CO2 removal with pECLA resulting in normocapnia, decreased rCBF to levels comparable to those without pECLA and normocapnia. HC resulted in increased cardiac output (+25.5%). Cardiac output was highest during HC with pECLA shunt (+44.9%). During pECLA with CO2 removal, cardiac output (+38.1%) decreased compared with pECLA without CO2 removal, but stayed higher than during normocapnia/no pECLA shunt (P<0.05). CONCLUSIONS: In this animal model, mechanical ventilation with low V(T) was associated with HC and increased rCBF. CO2 removal by pECLA restored normocapnia, reduced rCBF to levels of normocapnia, but required a higher systemic blood flow for the perfusion of the pECLA device. If these results could be transferred to patients, extracorporeal CO2 removal might be an option for treatment of combined lung and brain injury in condition of a sufficient cardiac flow reserve.

Experimental case report: development of a pneumothorax monitored by electrical impedance tomography.

Electrical impedance tomography (EIT) is a non-invasive, radiation-free functional imaging technique, which allows continuous bedside measurement of regional lung ventilation. Pneumothorax is an uncommon but nevertheless potentially dangerous incident that may arise unexpectedly. We report an incident of an accidental tension pneumothorax during an experimental ventilation study in a pig that was continuously monitored by EIT. The early sign of the occurring pneumothorax, prior to all clinical signs, was a fast increase of end-expiratory impedance in the ventral part of the right lung indicating that non-ventilated air entered this part, followed by a disappearance of ventilation in this region. At the same time the ventilation-related impedance changes of the left lung remained almost unchanged. The pneumothorax onset was localized using a newly introduced pneumothorax dynamics map directly derived from dynamic EIT data. We conclude that non-invasive EIT may be helpful as a tool to detect the development of a pneumothorax, which could be of particular interest during invasive procedures such as insertion of a central venous catheter.