Observation of ventilation-induced Spo(2) oscillations in pigs: first step to noninvasive detection of cyclic recruitment of atelectasis?

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.

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.

Hints for cyclical recruitment of atelectasis during ongoing mechanical ventilation in lavage and oleic acid lung injury detected by SpO₂ oscillations and electrical impedance tomography

PURPOSE OF THE STUDY

Detection of cyclical recruitment of atelectasis after induction of lavage (LAV) or oleic acid injury (OAI) in mechanically ventilated pigs. Primary hypothesis is that oxygen oscillations within the respiratory cycle can be detected by SpO₂ recordings (direct hint). SpO₂ oscillations reflect shunt oscillations that can only be explained by cyclical recruitment of atelectasis. Secondary hypothesis is that electrical impedance tomography (EIT) depicts specific regional changes of lung aeration and of pulmonary mechanical properties (indirect hint).

MATERIALS AND METHODS

Three groups (each n = 7) of mechanically ventilated pigs were investigated applying above mentioned methods before and repeatedly after induction of lung injury: (1) sham treated animals (SHAM), (2) LAV, and (3) OAI.

RESULTS

Early oxygen oscillations occurred in the LAV group (mean calculated amplitude: 73.8 mmHg reflecting shunt oscillation of 11.2% in mean). In the OAI group oxygen oscillations occurred hours after induction of lung injury (mean calculated amplitude: 57.1 mmHg reflecting shunt oscillations of 8.4% in mean). The SHAM group had no relevant oxygen oscillations (<30 mmHg, shunt oscillations < 1.5%). Synchronously to oxygen oscillations, EIT depicted (1) a decrease of ventilation in dorsal areas, (2) an increase in ventral areas, (3) a decrease of especially dependent expiratory impedance, 3) an increase in late inspiratory flow especially in the dependant areas, (4) an increase in the speed of peak expiratory flow (PEF), and (5) a decrease of dorsal late expiratory flow.

CONCLUSIONS

SpO2 and EIT recordings detect events that are interpreted as cyclical recruitment of atelectasis.

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

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

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

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

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

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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.

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.

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

Introduction

Cyclic alveolar recruitment/derecruitment (R/D) is an important mechanism of ventilator-associated lung injury. In experimental models this process can be measured with high temporal resolution by detection of respiratory-dependent oscillations of the paO₂ (ΔpaO₂). A previous study showed that end-expiratory collapse can be prevented by an increased respiratory rate in saline-lavaged rabbits. The current study compares the effects of increased positive end-expiratory pressure (PEEP) versus an individually titrated respiratory rate (RR_ind) on intra-tidal amplitude of Δ paO₂ and on average paO₂ in saline-lavaged pigs.

Methods

Acute lung injury was induced by bronchoalveolar lavage in 16 anaesthetized pigs. R/D was induced and measured by a fast-responding intra-aortic probe measuring paO₂. Ventilatory interventions (RR_ind (n = 8) versus extrinsic PEEP (n = 8)) were applied for 30 minutes to reduce Δ paO₂. Haemodynamics, spirometry and Δ paO₂ were monitored and the Ventilation/Perfusion distributions were assessed by multiple inert gas elimination. The main endpoints average and Δ paO₂ following the interventions were analysed by Mann-Whitney-U-Test and Bonferroni's correction. The secondary parameters were tested in an explorative manner.

Results

Both interventions reduced Δ paO₂. In the RR_ind group, ΔpaO₂ was significantly smaller (P < 0.001). The average paO₂ continuously decreased following RR_ind and was significantly higher in the PEEP group (P < 0.001). A sustained difference of the ventilation/perfusion distribution and shunt fractions confirms these findings. The RR_ind application required less vasopressor administration.

Conclusions

Different recruitment kinetics were found compared to previous small animal models and these differences were primarily determined by kinetics of end-expiratory collapse. In this porcine model, respiratory rate and increased PEEP were both effective in reducing the amplitude of paO₂ oscillations. In contrast to a recent study in a small animal model, however, increased respiratory rate did not maintain end-expiratory recruitment and ultimately resulted in reduced average paO₂ and increased shunt fraction.

Assessing gas exchange in acute lung injury/acute respiratory distress syndrome: diagnostic techniques and prognostic relevance

PURPOSE OF REVIEW

To provide the most recent insights on the assessment of gas exchange in acute lung injury.

RECENT FINDINGS

Central venous blood may be used as a surrogate of arterial blood to assess carbon dioxide tension and acid-base status. In contrast arterial oxygenation cannot be estimated with confidence from venous blood. However, the use of venous blood associated with pulse oximetry may provide the SvO2 which is useful for monitoring and targeting the resuscitation therapy. Impaired CO2 clearance and increased dead space have been confirmed as useful prognostic indices of structural lung damage and mortality in acute respiratory failure. A simplified technique based on multiple inert gas technique has been described to assess ventilation-perfusion mismatch while a new analysis of pulse oximetry has been suggested to detect lung opening and closing. Finally, new insight has been provided on the relationship between lung anatomy, as detected by computed tomography, oxygenation and CO2 clearance.

SUMMARY

Although oxygenation assessment is of primary importance during respiratory lung injury, dead space and CO2 retention are more strictly associated with outcome. The association of central venous blood analysis and pulse oximetry may provide more information than arterial blood alone.

Novel technologies to detect atelectotrauma in the injured lung

Cyclical recruitment and derecruitment of lung parenchyma (R/D) remains a serious problem in ALI/ARDS patients, defined as atelectotrauma. Detection of cyclical R/D to titrate the optimal respiratory settings is of high clinical importance. Image-based technologies that are capable of detecting changes of lung ventilation within a respiratory cycle include dynamic computed tomography (dCT), synchrotron radiation computed tomography (SRCT), and electrical impedance tomography (EIT). Time-dependent intra-arterial oxygen tension monitoring represents an alternative approach to detect cyclical R/D, as cyclical R/D can result in oscillations of PaO₂ within a respiratory cycle. Continuous, ultrafast, on-line in vivo measurement of PaO₂ can be provided by an indwelling PaO₂ probe. In addition, monitoring of fast changes in SaO₂ by pulse oximetry technology at the bedside could also be used to detect those fast changes in oxygenation.