Influence of respiratory rate and end-expiratory pressure variation on cyclic alveolar recruitment in an experimental lung injury model

Intra-tidal PaO2 oscillations associated with mechanical ventilation: a pilot study to identify discrete morphologies in a porcine model

BACKGROUND

Within-breath oscillations in arterial oxygen tension (PaO2) can be detected using fast responding intra-arterial oxygen sensors in animal models. These PaO2 signals, which rise in inspiration and fall in expiration, may represent cyclical recruitment/derecruitment and, therefore, a potential clinical monitor to allow titration of ventilator settings in lung injury. However, in hypovolaemia models, these oscillations have the potential to become inverted, such that they decline, rather than rise, in inspiration. This inversion suggests multiple aetiologies may underlie these oscillations. A correct interpretation of the various PaO2 oscillation morphologies is essential to translate this signal into a monitoring tool for clinical practice. We present a pilot study to demonstrate the feasibility of a new analysis method to identify these morphologies.

METHODS

Seven domestic pigs (average weight 31.1 kg) were studied under general anaesthesia with muscle relaxation and mechanical ventilation. Three underwent saline-lavage lung injury and four were uninjured. Variations in PEEP, tidal volume and presence/absence of lung injury were used to induce different morphologies of PaO2 oscillation. Functional principal component analysis and k-means clustering were employed to separate PaO2 oscillations into distinct morphologies, and the cardiorespiratory physiology associated with these PaO2 morphologies was compared.

RESULTS

PaO2 oscillations from 73 ventilatory conditions were included. Five functional principal components were sufficient to explain ≥ 95% of the variance of the recorded PaO2 signals. From these, five unique morphologies of PaO2 oscillation were identified, ranging from those which increased in inspiration and decreased in expiration, through to those which decreased in inspiration and increased in expiration. This progression was associated with the estimates of the first functional principal component (P < 0.001, R2 = 0.88). Intermediate morphologies demonstrated waveforms with two peaks and troughs per breath. The progression towards inverted oscillations was associated with increased pulse pressure variation (P = 0.03).

CONCLUSIONS

Functional principal component analysis and k-means clustering are appropriate to identify unique morphologies of PaO2 waveform associated with distinct cardiorespiratory physiology. We demonstrated novel intermediate morphologies of PaO2 waveform, which may represent a development of zone 2 physiologies within the lung. Future studies of PaO2 oscillations and modelling should aim to understand the aetiologies of these morphologies.

Influences of assisted breathing and mechanical ventilator settings on tidal volume and alveolar pressures in acute respiratory distress syndrome: a bench study

OBJECTIVE

To evaluate the influences of respiratory muscle efforts and respiratory rate setting in the ventilator on tidal volume and alveolar distending pressures at end inspiration and expiration in volume-controlled ventilation and pressure-controlled ventilation modes in acute respiratory distress syndrome.

METHODS

An active test lung (ASL 5000™) connected to five intensive care unit ventilators was used in a model of acute respiratory distress syndrome. Respiratory muscle efforts (muscle pressure) were configured in three different ways: no effort (muscle pressure: 0cmH2O); inspiratory efforts only (muscle pressure:-5cmH2O, neural inspiratory time of 0.6s); and both inspiratory and expiratory muscle efforts (muscle pressure:-5/+5cmH2O). Volume-controlled and pressure-controlled ventilation modes were set to deliver a target tidal volume of 420mL and positive end-expiratory pressure of 10cmH2O. The tidal volume delivered to the lungs, alveolar pressures at the end of inspiration, and alveolar pressures at end expiration were evaluated.

RESULTS

When triggered by the simulated patient, the median tidal volume was 27mL lower than the set tidal volume (range-63 to +79mL), and there was variation in alveolar pressures with a median of 25.4cmH2O (range 20.5 to 30cmH2O). In the simulated scenarios with both spontaneous inspiratory and expiratory muscle efforts and with a mandatory respiratory rate lower than the simulated patient's efforts, the median tidal volume was higher than controlled breathing.

CONCLUSION

Adjusting respiratory muscle effort and pulmonary ventilator respiratory rate to a value above the patient's respiratory rate in assisted/controlled modes generated large variations in tidal volume and pulmonary pressures, while the controlled mode showed no variations in these outcomes.

Dexmedetomidine reduces ventilator-induced lung injury via ERK1/2 pathway activation

Mechanical ventilation (MV) can contribute to ventilator-induced lung injury (VILI); dexmedetomidine (Dex) treatment attenuates MV-related pulmonary inflammation, but the mechanisms remain unclear. Therefore, the present study aimed to explore the protective effect and the possible molecular mechanisms of Dex in a VILI rodent model. Adult male Sprague-Dawley rats were randomly assigned to one of seven groups (n=24 rats/group). Rats were euthanized after 4 h of continuous MV, and pathological changes, lung wet/dry (W/D) weight ratio, the levels of inflammatory cytokines (IL-1β, TNF-α and IL-6) in the bronchoalveolar lavage fluid (BALF), and the expression levels of Bcl-2 homologous antagonist/killer (Bak), Bcl-2, pro-caspase-3, cleaved caspase-3 and the phosphorylation of ERK1/2 in the lung tissues were measured. Propidium iodide uptake and TUNEL staining were used to detect epithelial cell death. The Dex pretreatment group exhibited fewer pathological changes, lower W/D ratios and lower expression levels of inflammatory cytokines in BALF compared with the VILI group. Dex significantly attenuated the ratio of Bak/Bcl-2, cleaved caspase-3 expression levels and epithelial cell death, and increased the expression of phosphorylated ERK1/2. The protective effects of Dex could be partially reversed by PD98059, which is a mitogen-activated protein kinase (upstream of ERK1/2) inhibitor. Overall, dexmedetomidine was found to reduce the inflammatory response and epithelial cell death caused by VILI, via the activation of the ERK1/2 signaling pathway.

PO2 oscillations induce lung injury and inflammation

BACKGROUND

Mechanical ventilation can lead to ventilator-induced lung injury (VILI). In addition to the well-known mechanical forces of volutrauma, barotrauma, and atelectrauma, non-mechanical mechanisms have recently been discussed as contributing to the pathogenesis of VILI. One such mechanism is oscillations in partial pressure of oxygen (PO2) which originate in lung tissue in the presence of within-breath recruitment and derecruitment of alveoli. The purpose of this study was to investigate this mechanism's possible independent effects on lung tissue and inflammation in a porcine model.

METHODS

To separately study the impact of PO2 oscillations on the lungs, an in vivo model was set up that allowed for generating mixed-venous PO2 oscillations by the use of veno-venous extracorporeal membrane oxygenation (vvECMO) in a state of minimal mechanical stress. While applying the identical minimal-invasive ventilator settings, 16 healthy female piglets (weight 50 ± 4 kg) were either exposed for 6 h to a constant mixed-venous hemoglobin saturation (SmvO2) of 65% (which equals a PmvO2 of 41 Torr) (control group), or an oscillating SmvO2 (intervention group) of 40-90% (which equals PmvO2 oscillations of 30-68 Torr)-while systemic normoxia in both groups was maintained. The primary endpoint of histologic lung damage was assessed by ex vivo histologic lung injury scoring (LIS), the secondary endpoint of pulmonary inflammation by qRT-PCR of lung tissue. Cytokine concentration of plasma was carried out by ELISA. A bioinformatic microarray analysis of lung samples was performed to generate hypotheses about underlying pathomechanisms.

RESULTS

The LIS showed significantly more severe damage of lung tissue after exposure to PO2 oscillations compared to controls (0.53 [0.51; 0.58] vs. 0.27 [0.23; 0.28]; P = 0.0025). Likewise, a higher expression of TNF-α (P = 0.0127), IL-1β (P = 0.0013), IL-6 (P = 0.0007), and iNOS (P = 0.0013) in lung tissue was determined after exposure to PO2 oscillations. Cytokines in plasma showed a similar trend between the groups, however, without significant differences. Results of the microarray analysis suggest that inflammatory (IL-6) and oxidative stress (NO/ROS) signaling pathways are involved in the pathology linked to PO2 oscillations.

CONCLUSIONS

Artificial mixed-venous PO2 oscillations induced lung damage and pulmonary inflammation in healthy animals during lung protective ventilation. These findings suggest that PO2 oscillations represent an independent mechanism of VILI.

Oleic Acid-Injection in Pigs As a Model for Acute Respiratory Distress Syndrome

The acute respiratory distress syndrome is a relevant intensive care disease with an incidence ranging between 2.2% and 19% of intensive care unit patients. Despite treatment advances over the last decades, ARDS patients still suffer mortality rates between 35 and 40%. There is still a need for further research to improve the outcome of patients suffering from ARDS. One problem is that no single animal model can mimic the complex pathomechanism of the acute respiratory distress syndrome, but several models exist to study different parts of it. Oleic acid injection (OAI)-induced lung injury is a well-established model for studying ventilation strategies, lung mechanics and ventilation/perfusion distribution in animals. OAI leads to severely impaired gas exchange, deterioration of lung mechanics and disruption of the alveolo-capillary barrier. The disadvantage of this model is the controversial mechanistic relevance of this model and the necessity for central venous access, which is challenging especially in smaller animal models. In summary, OAI-induced lung injury leads to reproducible results in small and large animals and hence represents a well-suited model for studying ARDS. Nevertheless, further research is necessary to find a model that mimics all parts of ARDS and lacks the problems associated with the different models existing today.

Detection of inspiratory recruitment of atelectasis by automated lung sound analysis as compared to four-dimensional computed tomography in a porcine lung injury model

Background

Cyclic recruitment and de-recruitment of atelectasis (c-R/D) is a contributor to ventilator-induced lung injury (VILI). Bedside detection of this dynamic process could improve ventilator management. This study investigated the potential of automated lung sound analysis to detect c-R/D as compared to four-dimensional computed tomography (4DCT).

Methods

In ten piglets (25 ± 2 kg), acoustic measurements from 34 thoracic piezoelectric sensors (Meditron ASA, Norway) were performed, time synchronized to 4DCT scans, at positive end-expiratory pressures of 0, 5, 10, and 15 cmH₂O during mechanical ventilation, before and after induction of c-R/D by surfactant washout. 4DCT was post-processed for within-breath variation in atelectatic volume (Δ atelectasis) as a measure of c-R/D. Sound waveforms were evaluated for: 1) dynamic crackle energy (dCE): filtered crackle sounds (600–700 Hz); 2) fast Fourier transform area (FFT area): spectral content above 500 Hz in frequency and above −70 dB in amplitude in proportion to the total amount of sound above −70 dB amplitude; and 3) dynamic spectral coherence (dSC): variation in acoustical homogeneity over time. Parameters were analyzed for global, nondependent, central, and dependent lung areas.

Results

In healthy lungs, negligible values of Δ atelectasis, dCE, and FFT area occurred. In lavage lung injury, the novel dCE parameter showed the best correlation to Δ atelectasis in dependent lung areas (R² = 0.88) where c-R/D took place. dCE was superior to FFT area analysis for each lung region examined. The analysis of dSC could predict the lung regions where c-R/D originated.

Conclusions

c-R/D is associated with the occurrence of fine crackle sounds as demonstrated by dCE analysis. Standardized computer-assisted analysis of dCE and dSC seems to be a promising method for depicting c-R/D.

Electronic supplementary material

The online version of this article (10.1186/s13054-018-1964-6) contains supplementary material, which is available to authorized users.

Respiratory oscillations in alveolar oxygen tension measured in arterial blood

Arterial oxygen partial pressure can increase during inspiration and decrease during expiration in the presence of a variable shunt fraction, such as with cyclical atelectasis, but it is generally presumed to remain constant within a respiratory cycle in the healthy lung. We measured arterial oxygen partial pressure continuously with a fast intra-vascular sensor in the carotid artery of anaesthetized, mechanically ventilated pigs, without lung injury. Here we demonstrate that arterial oxygen partial pressure shows respiratory oscillations in the uninjured pig lung, in the absence of cyclical atelectasis (as determined with dynamic computed tomography), with oscillation amplitudes that exceeded 50 mmHg, depending on the conditions of mechanical ventilation. These arterial oxygen partial pressure respiratory oscillations can be modelled from a single alveolar compartment and a constant oxygen uptake, without the requirement for an increased shunt fraction during expiration. Our results are likely to contribute to the interpretation of arterial oxygen respiratory oscillations observed during mechanical ventilation in the acute respiratory distress syndrome.

Cyclic PaO2 oscillations assessed in the renal microcirculation: correlation with tidal volume in a porcine model of lung lavage

Background

Oscillations of the arterial partial pressure of oxygen induced by varying shunt fractions occur during cyclic alveolar recruitment within the injured lung. Recently, these were proposed as a pathomechanism that may be relevant for remote organ injury following acute respiratory distress syndrome. This study examines the transmission of oxygen oscillations to the renal tissue and their tidal volume dependency.

Methods

Lung injury was induced by repetitive bronchoalveolar lavage in eight anaesthetized pigs. Cyclic alveolar recruitment was provoked by high tidal volume ventilation. Oscillations of the arterial partial pressure of oxygen were measured in real-time in the macrocirculation by multi-frequency phase fluorimetry and in the renal microcirculation by combined white-light spectrometry and laser-Doppler flowmetry during tidal volume down-titration.

Results

Significant respiratory-dependent oxygen oscillations were detected in the macrocirculation and transmitted to the renal microcirculation in a substantial extent. The amplitudes of these oscillations significantly correlate to the applied tidal volume and are minimized during down-titration.

Conclusions

In a porcine model oscillations of the arterial partial pressure of oxygen are induced by cyclic alveolar recruitment and transmitted to the renal microcirculation in a tidal volume-dependent fashion. They might play a role in organ crosstalk and remote organ damage following lung injury.

Endexpiratory lung volume measurement correlates with the ventilation/perfusion mismatch in lung injured pigs

Background

In acute respiratory respiratory distress syndrome (ARDS) a sustained mismatch of alveolar ventilation and perfusion (V_A/Q) impairs the pulmonary gas exchange. Measurement of endexpiratory lung volume (EELV) by multiple breath-nitrogen washout/washin is a non-invasive, bedside technology to assess pulmonary function in mechanically ventilated patients. The present study examines the association between EELV changes and V_A/Q distribution and the possibility to predict V_A/Q normalization by means of EELV in a porcine model.

Methods

After approval of the state and institutional animal care committee 12 anesthetized pigs were randomized to ARDS either by bronchoalveolar lavage (n = 6) or oleic acid injection (n = 6). EELV, V_A/Q ratios by multiple inert gas elimination and ventilation distribution by electrical impedance tomography were assessed at healthy state and at five different positive endexpiratory pressure (PEEP) steps in ARDS (0, 20, 15, 10, 5 cmH₂O; each maintained for 30 min).

Results

V_A/Q, EELV and tidal volume distribution all displayed the PEEP-induced recruitment in ARDS. We found a close correlation between V_A/Q < 0.1 (representing shunt and low V_A/Q units) and changes in EELV (spearman correlation coefficient −0.79). Logistic regression reveals the potential to predict V_A/Q normalization (V_A/Q < 0.1 less than 5%) from changes in EELV with an area under the curve of 0.89 with a 95%-CI of 0.81–0.96 in the receiver operating characteristic. Different lung injury models and recruitment characteristics did not influence these findings.

Conclusion

In a porcine ARDS model EELV measurement depicts PEEP-induced lung recruitment and is strongly associated with normalization of the V_A/Q distribution in a model-independent fashion. Determination of EELV could be an intriguing addition in the context of lung protection strategies.

Systemic PaO2 Oscillations Cause Mild Brain Injury in a Pig Model

OBJECTIVE

Systemic PaO2 oscillations occur during cyclic recruitment and derecruitment of atelectasis in acute respiratory failure and might harm brain tissue integrity.

DESIGN

Controlled animal study.

SETTING

University research laboratory.

SUBJECTS

Adult anesthetized pigs.

INTERVENTIONS

Pigs were randomized to a control group (anesthesia and extracorporeal circulation for 20 hr with constant PaO2, n = 10) or an oscillation group (anesthesia and extracorporeal circulation for 20 hr with artificial PaO2 oscillations [3 cycles min⁻¹], n = 10). Five additional animals served as native group (n = 5).

MEASUREMENTS AND MAIN RESULTS

Outcome following exposure to artificial PaO2 oscillations compared with constant PaO2 levels was measured using 1) immunohistochemistry, 2) real-time polymerase chain reaction for inflammatory markers, 3) receptor autoradiography, and 4) transcriptome analysis in the hippocampus. Our study shows that PaO2 oscillations are transmitted to brain tissue as detected by novel ultrarapid oxygen sensing technology. PaO2 oscillations cause significant decrease in NISSL-stained neurons (p < 0.05) and induce inflammation (p < 0.05) in the hippocampus and a shift of the balance of hippocampal neurotransmitter receptor densities toward inhibition (p < 0.05). A pathway analysis suggests that cerebral immune and acute-phase response may play a role in mediating PaO2 oscillation-induced brain injury.

CONCLUSIONS

Artificial PaO2 oscillations cause mild brain injury mediated by inflammatory pathways. Although artificial PaO2 oscillations and endogenous PaO2 oscillations in lung-diseased patients have different origins, it is likely that they share the same noxious effect on the brain. Therefore, PaO2 oscillations might represent a newly detected pathway potentially contributing to the crosstalk between acute lung and remote brain injury.

Experimental investigation of the effect of polymer matrices on polymer fibre optic oxygen sensors and their time response characteristics using a vacuum testing chamber and a liquid flow apparatus

Very fast sensors that are able to track rapid changes in oxygen partial pressure (PO₂) in the gas and liquid phases are increasingly required in scientific research - particularly in the life sciences. Recent interest in monitoring very fast changes in the PO₂ of arterial blood in some respiratory failure conditions is one such example. Previous attempts to design fast intravascular electrochemical oxygen sensors for use in physiology and medicine have failed to meet the criteria that are now required in modern investigations. However, miniature photonic devices are capable of meeting this need. In this article, we present an inexpensive polymer type fibre-optic, oxygen sensor that is two orders of magnitude faster than conventional electrochemical oxygen sensors. It is constructed with biologically inert polymer materials and is both sufficiently small and robust for direct insertion in to a human artery. The sensors were tested and evaluated in both a gas testing chamber and in a flowing liquid test system. The results showed a very fast T₉₀ response time, typically circa 20 ms when tested in the gas phase, and circa 100 ms in flowing liquid.

High PEEP levels are associated with overdistension and tidal recruitment/derecruitment in ARDS patients

BACKGROUND

Positive end-expiratory pressure (PEEP) improves gas exchange and respiratory mechanics, and it may decrease tissue injury and inflammation. The mechanisms of this protective effect are not fully elucidated. Our aim was to determine the intrinsic effects of moderate and higher levels of PEEP on tidal recruitment/derecruitment, hyperinflation, and lung mechanics, in patients with acute respiratory distress syndrome (ARDS).

METHODS

Nine patients with ARDS of mainly pulmonary origin were ventilated sequential and randomly using two levels of PEEP: 9 and 15 cmH2 O, and studied with dynamic computed tomography at a fix transversal lung region. Tidal recruitment/derecruitment and hyperinflation were determined as non-aerated tissue and hyperinflated tissue variation between inspiration and expiration, expressed as percentage of total weight. We also assessed the maximal amount of non-aerated and hyperinflated tissue weight.

RESULTS

PEEP 15 cmH2 O was associated with decrease in non-aerated tissue in all the patients (P < 0.01). However, PEEP 15 cmH2 O did not decrease tidal recruitment/derecruitment compared to PEEP 9 cmH2 O (P = 1). In addition, PEEP 15 cmH2 O markedly increased maximal hyperinflation (P < 0.01) and tidal hyperinflation (P < 0.05). Lung compliance decreased with PEEP 15 cmH2 O (P < 0.001).

CONCLUSION

In this series of patients with ARDS of mainly pulmonary origin, application of high levels of PEEP did not decrease tidal recruitment/derecruitment, but instead consistently increased tidal and maximal hyperinflation.

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 30 min, 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.

Inhalation therapy with the synthetic TIP-like peptide AP318 attenuates pulmonary inflammation in a porcine sepsis model

BACKGROUND

The lectin-like domain of TNF-α can be mimicked by synthetic TIP peptides and represents an innovative pharmacologic option to treat edematous respiratory failure. TIP inhalation was shown to reduce pulmonary edema and improve gas exchange. In addition to its edema resolution effect, TIP peptides may exert some anti-inflammatory properties. The present study therefore investigates the influence of the inhaled TIP peptide AP318 on intrapulmonary inflammatory response in a porcine model of systemic sepsis.

METHODS

In a randomized-blinded setting lung injury was induced in 18 pigs by lipopolysaccharide-infusion and a second hit with a short period of ventilator-induced lung stress, followed by a six-hour observation period. The animals received either two inhalations with the peptide (AP318, 2×1 mg kg(-1)) or vehicle. Post-mortem pulmonary expression of inflammatory and mechanotransduction markers were determined by real-time polymerase chain reaction (IL-1β, IL-6, TNF-α, COX-2, iNOS, amphiregulin, and tenascin-c). Furthermore, regional histopathological lung injury, edema formation and systemic inflammation were quantified.

RESULTS

Despite similar systemic response to lipopolysaccharide infusion in both groups, pulmonary inflammation (IL-6, TNF-α, COX-2, tenascin-c) was significantly mitigated by AP318. Furthermore, a Western blot analysis shows a significantly lower of COX-2 protein level. The present sepsis model caused minor lung edema formation and moderate gas exchange impairment. Six hours after onset pathologic scoring showed no improvement, while gas exchange parameters and pulmonary edema formation were similar in the two groups.

CONCLUSION

In summary, AP318 significantly attenuated intrapulmonary inflammatory response even without the presence or resolution of severe pulmonary edema in a porcine model of systemic sepsis-associated lung injury. These findings suggest an anti-inflammatory mechanism of the lectin-like domain beyond mere edema reabsorption in endotoxemic lung injury in vivo.

Intra-breath arterial oxygen oscillations detected by a fast oxygen sensor in an animal model of acute respiratory distress syndrome

Background

There is considerable interest in oxygen partial pressure (Po₂) monitoring in physiology, and in tracking Po₂ changes dynamically when it varies rapidly. For example, arterial Po₂ (PaO2) can vary within the respiratory cycle in cyclical atelectasis (CA), where PaO2 is thought to increase and decrease during inspiration and expiration, respectively. A sensor that detects these PaO2 oscillations could become a useful diagnostic tool of CA during acute respiratory distress syndrome (ARDS).

Methods

We developed a fibreoptic Po₂ sensor (<200 µm diameter), suitable for human use, that has a fast response time, and can measure Po₂ continuously in blood. By altering the inspired fraction of oxygen (FIO2) from 21 to 100% in four healthy animal models, we determined the linearity of the sensor's signal over a wide range of PaO2 values in vivo. We also hypothesized that the sensor could measure rapid intra-breath PaO2 oscillations in a large animal model of ARDS.

Results

In the healthy animal models, PaO2 responses to changes in FIO2 were in agreement with conventional intermittent blood-gas analysis (n=39) for a wide range of PaO2 values, from 10 to 73 kPa. In the animal lavage model of CA, the sensor detected PaO2 oscillations, also at clinically relevant PaO2 levels close to 9 kPa.

Conclusions

We conclude that these fibreoptic PaO2 sensors have the potential to become a diagnostic tool for CA in ARDS.

Low tidal volume pressure support versus controlled ventilation in early experimental sepsis in pigs

Background

In moderate acute respiratory distress syndrome (ARDS) several studies support the usage of assisted spontaneous breathing modes. Only limited data, however, focus on the application in systemic sepsis and developing lung injury. The present study examines the effects of immediate initiation of pressure support ventilation (PSV) in a model of sepsis-induced ARDS.

Methods

18 anesthetized pigs received a two-staged continuous lipopolysaccharide infusion to induce lung injury. The animals were randomly assigned to PSV or volume controlled (VCV) lung protective ventilation (tidal volume each 6 ml kg^-1, n = 2x9) over six hours. Gas exchange parameters, hemodynamics, systemic inflammation, and ventilation distribution by multiple inert gas elimination and electrical impedance tomography were assessed. The post mortem analysis included histopathological scoring, wet to dry ratio, and alveolar protein content.

Results

Within six hours both groups developed a mild to moderate ARDS with comparable systemic inflammatory response and without signs of improving gas exchange parameters during PSV. The PSV group showed signs of more homogenous ventilation distribution by electrical impedance tomography, but only slightly less hyperinflated lung compartments by multiple inert gas elimination. Post mortem and histopathological assessment yielded no significant intergroup differences.

Conclusions

In a porcine model of sepsis-induced mild ARDS immediate PSV was not superior to VCV. This contrasts with several experimental studies from non-septic mild to moderate ARDS. The present study therefore assumes that not only severity, but also etiology of lung injury considerably influences the response to early initiation of PSV.

Ventilation/perfusion ratios measured by multiple inert gas elimination during experimental cardiopulmonary resuscitation

BACKGROUND

During cardiopulmonary resuscitation (CPR) the ventilation/perfusion distribution (VA /Q) within the lung is difficult to assess. This experimental study examines the capability of multiple inert gas elimination (MIGET) to determine VA /Q under CPR conditions in a pig model.

METHODS

Twenty-one anaesthetised pigs were randomised to three fractions of inspired oxygen (1.0, 0.7 or 0.21). VA/ Q by micropore membrane inlet mass spectrometry-derived MIGET was determined at baseline and during CPR following induction of ventricular fibrillation. Haemodynamics, blood gases, ventilation distribution by electrical impedance tomography and return of spontaneous circulation were assessed. Intergroup differences were analysed by non-parametric testing.

RESULTS

MIGET measurements were feasible in all animals with an excellent correlation of measured and predicted arterial oxygen partial pressure (R(2)  = 0.96, n = 21 for baseline; R(2)  = 0.82, n = 21 for CPR). CPR induces a significant shift from normal VA /Q ratios to the high VA /Q range. Electrical impedance tomography indicates a dorsal to ventral shift of the ventilation distribution. Diverging pulmonary shunt fractions induced by the three inspired oxygen levels considerably increased during CPR and were traceable by MIGET, while 100% oxygen most negatively influenced the VA /Q. Return of spontaneous circulation were achieved in 52% of the animals.

CONCLUSIONS

VA /Q assessment by MIGET is feasible during CPR and provides a novel tool for experimental purposes. Changes in VA /Q caused by different oxygen fractions are traceable during CPR. Beyond pulmonary perfusion deficits, these data imply an influence of the inspired oxygen level on VA /Q. Higher oxygen levels significantly increase shunt fractions and impair the normal VA /Q ratio.

TIP peptide inhalation in experimental acute lung injury: effect of repetitive dosage and different synthetic variants

Background

Inhalation of TIP peptides that mimic the lectin-like domain of TNF-α is a novel approach to attenuate pulmonary oedema on the threshold to clinical application. A placebo-controlled porcine model of acute respiratory distress syndrome (ARDS) demonstrated a reduced thermodilution-derived extravascular lung water index (EVLWI) and improved gas exchange through TIP peptide inhalation within three hours. Based on these findings, the present study compares a single versus a repetitive inhalation of a TIP peptide (TIP-A) and two alternate peptide versions (TIP-A, TIP-B).

Methods

Following animal care committee approval ARDS was induced by bronchoalveolar lavage followed by injurious ventilation in 21 anaesthetized pigs. A randomised-blinded three-group setting compared the single-dosed peptide variants TIP-A and TIP-B as well as single versus repetitive inhalation of TIP-A (n = 7 per group). Over two three-hour intervals parameters of gas exchange, transpulmonary thermodilution, calculated alveolar fluid clearance, and ventilation/perfusion-distribution were assessed. Post-mortem measurements included pulmonary wet/dry ratio and haemorrhage/congestion scoring.

Results

The repetitive TIP-A inhalation led to a significantly lower wet/dry ratio than a single dose and a small but significantly lower EVLWI. However, EVLWI changes over time and the derived alveolar fluid clearance did not differ significantly. The comparison of TIP-A and B showed no relevant differences. Gas exchange and ventilation/perfusion-distribution significantly improved in all groups without intergroup differences. No differences were found in haemorrhage/congestion scoring.

Conclusions

In comparison to a single application the repetitive inhalation of a TIP peptide in three-hour intervals may lead to a small additional reduction the lung water content. Two alternate TIP peptide versions showed interchangeable characteristics.

Year in review 2012: Critical Care--Respirology

Acute respiratory failure is a dominant feature of critical illness. In this review, we discuss 17 studies published last year in Critical Care. The discussion focuses on articles on several topics: respiratory monitoring, acute respiratory distress syndrome, noninvasive ventilation, airway management, secretion management and weaning.

A fibre optic oxygen sensor that detects rapid PO2 changes under simulated conditions of cyclical atelectasis in vitro

•

Real time detection of cyclical atelectasis is fundamental for individualised mechanical-ventilation therapy in ARDS.

•

Intra-arterial oxygen sensors could be used to detect the breath-by-breath oscillations in PO₂ during cyclical atelectasis.

•

The fidelity with which oxygen sensors can detect these arterial PO₂ oscillations depends on the sensors’ speed of response.

•

We present a system for testing fast-response fibre optic oxygen sensors under simulated conditions of cyclical atelectasis.

•

We show that a prototype fibre optic oxygen sensor, compatible with clinical use, can detect rapid PO₂ changes in vitro.

Two challenges in the management of Acute Respiratory Distress Syndrome are the difficulty in diagnosing cyclical atelectasis, and in individualising mechanical ventilation therapy in real-time. Commercial optical oxygen sensors can detect PaO2 oscillations associated with cyclical atelectasis, but are not accurate at saturation levels below 90%, and contain a toxic fluorophore. We present a computer-controlled test rig, together with an in-house constructed ultra-rapid sensor to test the limitations of these sensors when exposed to rapidly changing PO2 in blood in vitro. We tested the sensors’ responses to simulated respiratory rates between 10 and 60 breaths per minute. Our sensor was able to detect the whole amplitude of the imposed PO2 oscillations, even at the highest respiratory rate. We also examined our sensor's resistance to clot formation by continuous in vivo deployment in non-heparinised flowing animal blood for 24 h, after which no adsorption of organic material on the sensor's surface was detectable by scanning electron microscopy.

TIP peptide inhalation in oleic acid-induced experimental lung injury: a post-hoc comparison

BACKGROUND

The lectin-like domain of TNF-α mimicked by an inhaled TIP peptide represents a novel approach to attenuate a pulmonary edema in respiratory failure, which is on the threshold to clinical application. In extension to a previously published study, which reported an improved pulmonary function following TIP peptide inhalation in a porcine model of lavage-induced lung injury, a post-hoc comparison to additional experiments was conducted. This analysis addresses the hypothesis that oleic acid injection-induced capillary leakage and alveolar necrosis blunts the previously reported beneficial effects of TIP peptide inhalation in a porcine model.

FINDINGS

Following animal care committee approval lung injury was induced by oleic acid injection in six pigs with a setting strictly according to a previously published protocol that was used for lung-lavaged pigs. Ventilation/perfusion-distribution by multiple inert gas elimination, parameters of gas exchange and pulmonary edema were assessed as surrogates of the pulmonary function. A significantly improved ventilation/perfusion-distribution following TIP inhalation was recognized only in the bronchoalveolar lavage model but not following oleic acid injection. The time course after oleic acid injection yielded no comparable impact of the TIP peptide on gas exchange and edema formation.

CONCLUSIONS

Reported beneficial effects of the TIP peptide on gas exchange and pulmonary edema were not reproducible in the oleic acid injection model. This analysis assumes that sustained alveolar epithelial necrosis as induced by oleic acid injection may inhibit the TIP-induced edema resolution. Regarding the on-going clinical development of the TIP peptide this approach should hardly be effective in states of severe alveolar epithelial damage.

Correlation of thermodilution-derived extravascular lung water and ventilation/perfusion-compartments in a porcine model

PURPOSE

This study examines the correlation between the transpulmonary thermodilution derived extravascular lung water content (EVLW) and the ventilation/perfusion-distribution ([Formula: see text]) measured by multiple inert gas elimination (MIGET) in a porcine model.

METHODS

[Formula: see text] measured by micropore membrane inlet mass spectrometry-MIGET (MMIMS-MIGET) and EVLW were simultaneously measured in twelve pigs in the heathy state, with impaired gas exchange from repetitive lung lavage and after 3 h of ventilation. The relationship between [Formula: see text] compartments and EVLW was analysed by linear correlation and regression.

RESULTS

Considerable increases in EVLW and [Formula: see text] mismatching were induced through the lavage procedure. Significant correlations between the EVLW and the [Formula: see text] fractions representing pulmonary shunt and low [Formula: see text] were found. Perfusion to the normal [Formula: see text] regions was inversely correlated to the EVLW.

CONCLUSIONS

Increased EVLW is associated with increased low [Formula: see text] and shunt, but not equal to pulmonary shunt alone. Beneath true shunt EVLW can also be associated with low [Formula: see text] regions.

Multi frequency phase fluorimetry (MFPF) for oxygen partial pressure measurement: ex vivo validation by polarographic clark-type electrode

Background

Measurement of partial pressure of oxygen (P_O2) at high temporal resolution remains a technological challenge. This study introduces a novel P_O2 sensing technology based on Multi-Frequency Phase Fluorimetry (MFPF). The aim was to validate MFPF against polarographic Clark-type electrode (CTE) P_O2 measurements.

Methodology/Principal Findings

MFPF technology was first investigated in N = 8 anaesthetised pigs at F_IO2 of 0.21, 0.4, 0.6, 0.8 and 1.0. At each F_IO2 level, blood samples were withdrawn and P_O2 was measured in vitro with MFPF using two FOXY-AL300 probes immediately followed by CTE measurement. Secondly, MFPF-P_O2 readings were compared to CTE in an artificial circulatory setup (human packed red blood cells, haematocrit of 30%). The impacts of temperature (20, 30, 40°C) and blood flow (0.8, 1.6, 2.4, 3.2, 4.0 L min⁻¹) on MFPF-P_O2 measurements were assessed. MFPF response time in the gas- and blood-phase was determined. Porcine MFPF-P_O2 ranged from 63 to 749 mmHg; the corresponding CTE samples from 43 to 712 mmHg. Linear regression: CTE = 15.59+1.18*MFPF (R² = 0.93; P<0.0001). Bland Altman analysis: mean_diff 69.2 mmHg, range_diff -50.1/215.6 mmHg, 1.96-SD limits -56.3/194.8 mmHg. In artificial circulatory setup, MFPF-P_O2 ranged from 20 to 567 mmHg and CTE samples from 11 to 575 mmHg. Linear regression: CTE = −8.73+1.05*MFPF (R² = 0.99; P<0.0001). Bland-Altman analysis: mean_diff 6.6 mmHg, range_diff -9.7/20.5 mmHg, 1.96-SD limits -12.7/25.8 mmHg. Differences between MFPF and CTE-P_O2 due to variations of temperature were less than 6 mmHg (range 0–140 mmHg) and less than 35 mmHg (range 140–750 mmHg); differences due to variations in blood flow were less than 15 mmHg (all P-values>0.05). MFPF response-time (monoexponential) was 1.48±0.26 s for the gas-phase and 1.51±0.20 s for the blood-phase.

Conclusions/Significance

MFPF-derived P_O2 readings were reproducible and showed excellent correlation and good agreement with Clark-type electrode-based P_O2 measurements. There was no relevant impact of temperature and blood flow upon MFPF-P_O2 measurements. The response time of the MFPF FOXY-AL300 probe was adequate for real-time sensing in the blood phase.

PaO2 oscillations caused by cyclic alveolar recruitment can be monitored in pig buccal mucosa microcirculation

BACKGROUND

Cyclic alveolar recruitment and derecruitment play a role in the pathomechanism of acute lung injury and may lead to arterial partial pressure of oxygen (PaO(2) ) oscillations within the respiratory cycle. It remains unknown, however, if these PaO(2) oscillations are transmitted to the microcirculation. The present study investigates if PaO(2) oscillations can be detected in the pig buccal mucosa microcirculation.

METHODS

Respiratory failure was induced by surfactant depletion in seven pigs. PaO(2) oscillations caused by cyclic recruitment and derecruitment were measured in the thoracic aorta by fast fluorescence quenching of oxygen technology. Haemoglobin oxygen saturation, haemoglobin amount and blood flow in the buccal mucosa microcirculation were determined by combined fast white light spectrometry and laser Doppler flowmetry additionally to systolic arterial pressure. Measurements were performed during baseline conditions and during cyclic recruitment and derecruitment.

RESULTS

Measurements remained stable during baseline. Respiratory-dependent oscillations occurred in the systemic circulation [PaO(2) oscillations 92 (69-172) mmHg; systolic arterial pressure oscillations 33 (13-35) %] and were related to the respiratory rate (5.0 ± 0.2/min) as confirmed by Fourier analysis. Synchronised oscillations were detected to the pig buccal mucosa microcirculation [haemoglobin oxygen saturation oscillations 3.4 (2.7-4.9) %; haemoglobin amount oscillations 8.5 (2.3-13.3) %; blood flow oscillations 66 (18-87) %]. The delay between PaO(2) -\ and microcirculatory oxygen oscillations was 7.2 ± 2.8 s.

CONCLUSION

The present study suggests that PaO(2) oscillations caused by cyclic recruitment and derecruitment were transmitted to the buccal mucosa microcirculation. This non-invasive approach of measuring oxygen waves as a surrogate parameter of cyclic recruitment and derecruitment could be used to monitor PaO(2) oscillations at the bedside.