Visual validation of the mechanical stabilizing effects of positive end-expiratory pressure at the alveolar level

Inspiratory respiratory mechanics estimation by using expiratory data for reverse-triggered breathing cycles

BACKGROUND AND OBJECTIVE

Model-based lung mechanics monitoring can provide clinically useful information for guiding mechanical ventilator treatment in intensive care. However, many methods of measuring lung mechanics are not appropriate for both fully and partially sedated patients, and are unable provide lung mechanics metrics in real-time. This study proposes a novel method of using lung mechanics identified during passive expiration to estimate inspiratory lung mechanics for spontaneously breathing patients.

METHODS

Relationships between inspiratory and expiratory modeled lung mechanics were identified from clinical data from 4 fully sedated patients. The validity of these relationships were assessed using data from a further 4 spontaneously breathing patients.

RESULTS

For the fully sedated patients, a linear relationship was identified between inspiratory and expiratory elastance, with slope 1.04 and intercept 1.66. The r value of this correlation was 0.94. No cohort-wide relationship was determined for airway resistance. Expiratory elastance measurements in spontaneously breathing patients were able to produce reasonable estimates of inspiratory elastance after adjusting for the identified difference between them.

CONCLUSIONS

This study shows that when conventional methods fail, typically ignored expiratory data may be able to provide clinicians with the information needed about patient condition to guide MV therapy.

Impacts in the Respiratory Mechanics of the Ventilator Hyperinsuflation in the Flow Bias Concept: a Narrative Review

AbstractPatients who require invasive ventilatory support are subject to the deleterious effects of this, mainly ventilator-associated pneumonia (VAP). The physiotherapist, a member of the multiprofessional team, assists the patient with the purpose of promoting the recovery and preservation of the functionality, being able to minimize / avoid secondary complications. This study aims to identify the repercussions of mechanical ventilation hyperinflation (MVH) in the flow bias concept in respiratory mechanics. This study is a narrative review. MVH is an important resource commonly used in clinical practice that involves the manipulation of mechanical ventilator configurations to provide larger pulmonary volumes, and the generated airflow gradient may play a relevant role in mucus transport, with the concept of flow bias the main factor responsible for its direction. For the mobilization of the mucus towards the cephalic direction to occur, there must be a predominant expiratory flow, guaranteeing the peak ratio of expiratory flow / inspiratory flow peak (EFP / IFP) greater than 1.11. Maintenance of mechanical ventilation assures the patient to maintain the positive end expiratory pressure (PEEP) and the oxygen inspired fraction, avoiding the deleterious effects of the mechanical ventilator disconnection. MVH is able to improve lung compliance without, however, increasing airway resistance. MVH in the cephalic flow bias concept is effective for the mucus mobilization in the central direction, being able to improve pulmonary compliance and peripheral oxygen saturation.Keywords: Respiration, Artificial. Intensive Care Units. Physical Therapy Department, Hospital.ResumoOs pacientes internados que necessitam de suporte ventilatório invasivo estão sujeitos aos efeitos deletérios deste, principalmente a pneumonia associada à ventilação mecânica (PAV). O fisioterapeuta, integrante da equipe multiprofissional, assiste o paciente com a finalidade de promover a recuperação e preservação da funcionalidade, podendo minimizar/evitar complicações secundárias. Este estudo consiste em identificar as repercussões da hiperinsuflação com ventilador mecânico (HVM) no conceito flow bias na mecânica respiratória. O presente estudo trata-se de uma revisão narrativa. A HVM é um importante recurso comumente utilizado na prática clínica que envolve a manipulação das configurações do ventilador mecânico para fornecer maiores volumes pulmonares, e o gradiente de fluxo de ar gerado pode desempenhar um papel relevante no transporte do muco, sendo o conceito de flow bias cefálico o principal fator responsável pelo direcionamento deste. Para que a mobilização do muco em direção cefálica ocorra, deve existir um fluxo expiratório predominante, garantindo a razão pico de fluxo expiratório/pico de fluxo inspiratório (PFE/PFI) maior do que 1,11. A manutenção da assistência ventilatória mecânica assegura ao paciente a manutenção da pressão positiva ao final da expiração (PEEP) e a fração inspirada de oxigênio (FiO2), evitando os efeitos deletérios da desconexão do ventilador mecânico. A HVM é capaz de melhorar a complacência pulmonar sem, no entanto, aumentar a resistência das vias aéreas. A HVM no conceito flow bias cefálico é eficaz para a mobilização do muco em direção central, sendo capaz de melhorar a complacência pulmonar e saturação periférica de oxigênio (SpO2).Palavras-chave: Respiração Artificial. Unidades de Terapia Intensiva. Serviço Hospitalar de Fisioterapia.

A comparison of the effects of manual hyperinflation and ventilator hyperinflation on restoring end-expiratory lung volume after endotracheal suctioning: A pilot physiologic study

PURPOSE

Endotracheal suctioning (ES) of mechanically ventilated patients decreases end-expiratory lung volume (EELV). Manual hyperinflation (MHI) and ventilator hyperinflation (VHI) may restore EELV post-ES but it remains unknown which method is most effective. The primary aim was to compare the efficacy of MHI and VHI in restoring EELV post-ES.

MATERIALS AND METHODS

ES was performed on mechanically ventilated intensive care patients, followed by MHI or VHI, in a randomised crossover design. The washout period between interventions was 1 h. End-expiratory lung impedance (EELI), measured by electrical impedance tomography, was recorded at baseline, during ES, during hyperinflation and 1, 5, 15 and 30 min post-hyperinflation.

RESULTS

Nine participants were studied. ES decreased EELI by 1672z (95% CI, 1204 to 2140) from baseline. From baseline, MHI increased EELI by 1154z (95% CI, 977 to 1330) while VHI increased EELI by 769z (95% CI, 457 to 1080). Five minutes post-VHI, EELI remained 528z (95% CI, 4 to 1053) above baseline. Fifteen minutes post-MHI, EELI remained 351z (95% CI, 111 to 592) above baseline. At subsequent time-points, EELI returned to baseline.

CONCLUSIONS

MHI and VHI effectively restore EELV above baseline post-ES and should be considered post suctioning.

Positive end-expiratory pressure during resuscitation at birth in very-low birth weight infants: A randomized-controlled pilot trial

BACKGROUND

There is limited evidence of the effect of positive end-expiratory pressure (PEEP) during resuscitation soon after birth. Premature neonates may experience respiratory distress from surfactant insufficiency and providing PEEP after the very first breath, may improve outcomes following appropriate resuscitation. The objective of this study was to evaluate the short term respiratory outcomes after positive pressure ventilation (PPV) with PEEP in preterm infants at birth.

METHODS

A prospective randomized-controlled, pilot trial was conducted. Premature neonates ≤ 32 weeks gestational age or birth weight < 1500 g were recruited. Subjects were allocated to either PEEP of 5 cm H₂O (PEEP-5) or no PEEP (PEEP-0) if PPV was administered. Pre-ductal, peripheral capillary oxygen saturation (SpO₂) and fraction of inspired oxygen concentration (FiO₂) were monitored at 1, 3, 5, 10, 15, and 20 min after birth. FiO₂ was adjusted to achieve targeted SpO₂ using the 2010 neonatal resuscitation protocol guidelines.

RESULTS

56% (14/25; PEEP-0) and 50% (13/26; PEEP-5) infants received PPV. Mean gestational age was 30 (PEEP-0) vs 31 (PEEP-5) weeks. The mean [SD] birthweight (g) of PEEP-0 was significantly lower than PEEP-5 (1050.4 [262.7] vs 1218.8 [236.8], p = 0.02). Pre-ductal SpO_2, FiO₂ delivered at each time point, and rates of pneumothorax, surfactant administration and oxygen dependency at 36 weeks postmenstrual age or death was similar.

CONCLUSION

Due to the small sample size and potential bias accrued through random allocation of higher birthweight infants to the PEEP-5 group, the results did not confirm differences in outcomes between the groups, despite evidence favoring postnatal ventilation with PEEP. A further randomized, controlled clinical trial with a larger sample size is warranted to determine the utility and safety of PEEP during the resuscitation of premature infants immediately after birth.

Differences in delivery of respiratory treatments by on-call physiotherapists in mechanically ventilated children: a randomised crossover trial

OBJECTIVES

To investigate differences, if any, in the delivery of respiratory treatments to mechanically ventilated children between non-respiratory on-call physiotherapists and specialist respiratory physiotherapists.

SETTING

Paediatric, tertiary care hospital in the United Kingdom.

PARTICIPANTS

93 children (aged between 3 days and 16 years), and 22 physiotherapists (10 specialist respiratory physiotherapists) were recruited to the study.

INTERVENTIONS

Recruited children received two physiotherapy treatments during a single day, one delivered by a non-respiratory physiotherapist, the other by a specialist respiratory physiotherapist in a randomised order. Selection, delivery and effects of techniques were recorded for each treatment.

OUTCOME MEASURES

Primary outcomes were selection and application of treatment components. Secondary outcomes included respiratory effects (in terms of changes in flow, volume and pressure) of selected treatment components.

RESULTS

Both non-respiratory on-call physiotherapists and specialist respiratory physiotherapists used combinations of saline instillation, manual lung inflations, chest wall vibrations and endotracheal suction during treatments. However specialist respiratory physiotherapists used combinations of chest wall vibrations with suction, and recruitment manoeuvres, significantly more frequently than non-respiratory on-call physiotherapists (92% vs 52%, and 87% vs 46% of treatments respectively, P<0.001). Chest wall vibrations delivered by non-respiratory on-call physiotherapists were 15% less effective at increasing peak expiratory flow.

CONCLUSION

Clinically important differences between non-respiratory and specialist respiratory physiotherapists' treatment outcomes may be related to differences in the selection and application of techniques. This suggests an important training need for non-respiratory on-call physiotherapists, particularly in the effective delivery of physiotherapy techniques.

TRIAL REGISTRATION

Clinicaltrials.gov NCT01999426.

Visualisation of time-varying respiratory system elastance in experimental ARDS animal models

BACKGROUND

Patients with acute respiratory distress syndrome (ARDS) risk lung collapse, severely altering the breath-to-breath respiratory mechanics. Model-based estimation of respiratory mechanics characterising patient-specific condition and response to treatment may be used to guide mechanical ventilation (MV). This study presents a model-based approach to monitor time-varying patient-ventilator interaction to guide positive end expiratory pressure (PEEP) selection.

METHODS

The single compartment lung model was extended to monitor dynamic time-varying respiratory system elastance, Edrs, within each breathing cycle. Two separate animal models were considered, each consisting of three fully sedated pure pietrain piglets (oleic acid ARDS and lavage ARDS). A staircase recruitment manoeuvre was performed on all six subjects after ARDS was induced. The Edrs was mapped across each breathing cycle for each subject.

RESULTS

Six time-varying, breath-specific Edrs maps were generated, one for each subject. Each Edrs map shows the subject-specific response to mechanical ventilation (MV), indicating the need for a model-based approach to guide MV. This method of visualisation provides high resolution insight into the time-varying respiratory mechanics to aid clinical decision making. Using the Edrs maps, minimal time-varying elastance was identified, which can be used to select optimal PEEP.

CONCLUSIONS

Real-time continuous monitoring of in-breath mechanics provides further insight into lung physiology. Therefore, there is potential for this new monitoring method to aid clinicians in guiding MV treatment. These are the first such maps generated and they thus show unique results in high resolution. The model is limited to a constant respiratory resistance throughout inspiration which may not be valid in some cases. However, trends match clinical expectation and the results highlight both the subject-specificity of the model, as well as significant inter-subject variability.

Monitoring of intratidal lung mechanics: a Graphical User Interface for a model-based decision support system for PEEP-titration in mechanical ventilation

In mechanical ventilation, a careful setting of the ventilation parameters in accordance with the current individual state of the lung is crucial to minimize ventilator induced lung injury. Positive end-expiratory pressure (PEEP) has to be set to prevent collapse of the alveoli, however at the same time overdistension should be avoided. Classic approaches of analyzing static respiratory system mechanics fail in particular if lung injury already prevails. A new approach of analyzing dynamic respiratory system mechanics to set PEEP uses the intratidal, volume-dependent compliance which is believed to stay relatively constant during one breath only if neither atelectasis nor overdistension occurs. To test the success of this dynamic approach systematically at bedside or in an animal study, automation of the computing steps is necessary. A decision support system for optimizing PEEP in form of a Graphical User Interface (GUI) was targeted. Respiratory system mechanics were analyzed using the gliding SLICE method. The resulting shapes of the intratidal compliance-volume curve were classified into one of six categories, each associated with a PEEP-suggestion. The GUI should include a graphical representation of the results as well as a quality check to judge the reliability of the suggestion. The implementation of a user-friendly GUI was successfully realized. The agreement between modelled and measured pressure data [expressed as root-mean-square (RMS)] tested during the implementation phase with real respiratory data from two patient studies was below 0.2 mbar for data taken in volume controlled mode and below 0.4 mbar for data taken in pressure controlled mode except for two cases with RMS < 0.6 mbar. Visual inspections showed, that good and medium quality data could be reliably identified. The new GUI allows visualization of intratidal compliance-volume curves on a breath-by-breath basis. The automatic categorisation of curve shape into one of six shape-categories provides the rational decision-making model for PEEP-titration.

Gas exchange in the respiratory distress syndromes

This article describes the gas exchange abnormalities occurring in the acute respiratory distress syndrome seen in adults and children and in the respiratory distress syndrome that occurs in neonates. Evidence is presented indicating that the major gas exchange abnormality accounting for the hypoxemia in both conditions is shunt, and that approximately 50% of patients also have lungs regions in which low ventilation-to-perfusion ratios contribute to the venous admixture. The various mechanisms by which hypercarbia may develop and by which positive end-expiratory pressure improves gas exchange are reviewed, as are the effects of vascular tone and airway narrowing. The mechanisms by which surfactant abnormalities occur in the two conditions are described, as are the histological findings that have been associated with shunt and low ventilation-to-perfusion.

Expiratory model-based method to monitor ARDS disease state

Introduction

Model-based methods can be used to characterise patient-specific condition and response to mechanical ventilation (MV) during treatment for acute respiratory distress syndrome (ARDS). Conventional metrics of respiratory mechanics are based on inspiration only, neglecting data from the expiration cycle. However, it is hypothesised that expiratory data can be used to determine an alternative metric, offering another means to track patient condition and guide positive end expiratory pressure (PEEP) selection.

Methods

Three fully sedated, oleic acid induced ARDS piglets underwent three experimental phases. Phase 1 was a healthy state recruitment manoeuvre. Phase 2 was a progression from a healthy state to an oleic acid induced ARDS state. Phase 3 was an ARDS state recruitment manoeuvre. The expiratory time-constant model parameter was determined for every breathing cycle for each subject. Trends were compared to estimates of lung elastance determined by means of an end-inspiratory pause method and an integral-based method. All experimental procedures, protocols and the use of data in this study were reviewed and approved by the Ethics Committee of the University of Liege Medical Faculty.

Results

The overall median absolute percentage fitting error for the expiratory time-constant model across all three phases was less than 10 %; for each subject, indicating the capability of the model to capture the mechanics of breathing during expiration. Provided the respiratory resistance was constant, the model was able to adequately identify trends and fundamental changes in respiratory mechanics.

Conclusion

Overall, this is a proof of concept study that shows the potential of continuous monitoring of respiratory mechanics in clinical practice. Respiratory system mechanics vary with disease state development and in response to MV settings. Therefore, titrating PEEP to minimal elastance theoretically results in optimal PEEP selection. Trends matched clinical expectation demonstrating robustness and potential for guiding MV therapy. However, further research is required to confirm the use of such real-time methods in actual ARDS patients, both sedated and spontaneously breathing.

Effect of body position on ventilation distribution in ventilated preterm infants

RATIONALE

Positioning is considered vital to the maintenance of good lung ventilation by optimizing oxygen transport and gas exchange in ventilated premature infants. Previous studies suggest that the prone position is advantageous; however, no data exist on regional ventilation distribution for this age group.

OBJECTIVES

To investigate the effect of body position on regional ventilation distribution in ventilated and nonventilated preterm infants using electrical impedance tomography.

DESIGN

Randomized crossover study design.

SETTING

Neonatal ICU.

PATIENTS

A total of 24 ventilated preterm infants were compared with six spontaneously breathing preterm infants.

INTERVENTIONS

Random assignment of the order of the positions supine, prone, and quarter prone.

MEASUREMENTS AND MAIN RESULTS

Ventilation distribution was measured with regional impedance amplitudes and global inhomogeneity indices using electrical impedance tomography. In the spontaneously breathing infants, regional impedance amplitudes were increased in the posterior compared with the anterior lung (p < 0.01) and in the right compared with the left lung (p = 0.03). No differences were found in the ventilated infants. Ventilation was more inhomogeneous in the ventilated compared with the healthy infants (p < 0.01). Assessment of temporal regional lung filling showed that the posterior lung filled earlier than the anterior lung in the spontaneously breathing infants (p < 0.02) whereas in the in the ventilated infants the right lung filled before the left lung (p < 0.01).

CONCLUSIONS

In contrast to previous studies showing that ventilation is distributed to the nondependent lung in infants and children, this study shows that gravity has little effect on regional ventilation distribution.

Analysis of different model-based approaches for estimating dFRC for real-time application

BACKGROUND

Acute Respiratory Distress Syndrome (ARDS) is characterized by inflammation, filling of the lung with fluid and the collapse of lung units. Mechanical ventilation (MV) is used to treat ARDS using positive end expiratory pressure (PEEP) to recruit and retain lung units, thus increasing pulmonary volume and dynamic functional residual capacity (dFRC) at the end of expiration. However, simple, non-invasive methods to estimate dFRC do not exist.

METHODS

Four model-based methods for estimating dFRC are compared based on their performance on two separate clinical data cohorts. The methods are derived from either stress-strain theory or a single compartment lung model, and use commonly controlled or measured parameters (lung compliance, plateau airway pressure, pressure-volume (PV) data). Population constants are determined for the stress-strain approach, which is implemented using data at both single and multiple PEEP levels. Estimated values are compared to clinically measured values to assess the reliability of each method for each cohort individually and combined.

RESULTS

The stress-strain multiple breath (at multiple PEEP levels) method produced an overall correlation coefficient R2 = 0.966. The stress-strain single breath method produced R2 = 0.530. The single compartment single breath method produced R2 = 0.415. A combined method at single and multiple PEEP levels produced R2 = 0.963.

CONCLUSIONS

The results suggest that model-based, single breath and non-invasive approaches to estimating dFRC may be viable in a clinical scenario, ensuring no interruption to MV. The models provide a means of estimating dFRC at any PEEP level. However, model limitations and large estimation errors limit the use of the methods at very low PEEP.

Mechanobiology in lung epithelial cells: measurements, perturbations, and responses

Epithelial cells of the lung are located at the interface between the environment and the organism and serve many important functions including barrier protection, fluid balance, clearance of particulate, initiation of immune responses, mucus and surfactant production, and repair following injury. Because of the complex structure of the lung and its cyclic deformation during the respiratory cycle, epithelial cells are exposed to continuously varying levels of mechanical stresses. While normal lung function is maintained under these conditions, changes in mechanical stresses can have profound effects on the function of epithelial cells and therefore the function of the organ. In this review, we will describe the types of stresses and strains in the lungs, how these are transmitted, and how these may vary in human disease or animal models. Many approaches have been developed to better understand how cells sense and respond to mechanical stresses, and we will discuss these approaches and how they have been used to study lung epithelial cells in culture. Understanding how cells sense and respond to changes in mechanical stresses will contribute to our understanding of the role of lung epithelial cells during normal function and development and how their function may change in diseases such as acute lung injury, asthma, emphysema, and fibrosis.

Model-based optimal PEEP in mechanically ventilated ARDS patients in the intensive care unit

BACKGROUND

The optimal level of positive end-expiratory pressure (PEEP) is still widely debated in treating acute respiratory distress syndrome (ARDS) patients. Current methods of selecting PEEP only provide a range of values and do not provide unique patient-specific solutions. Model-based methods offer a novel way of using non-invasive pressure-volume (PV) measurements to estimate patient recruitability. This paper examines the clinical viability of such models in pilot clinical trials to assist therapy, optimise patient-specific PEEP, assess the disease state and response over time.

METHODS

Ten patients with acute lung injury or ARDS underwent incremental PEEP recruitment manoeuvres. PV data was measured at increments of 5 cmH2O and fitted to the recruitment model. Inspiratory and expiratory breath holds were performed to measure airway resistance and auto-PEEP. Three model-based metrics are used to optimise PEEP based on opening pressures, closing pressures and net recruitment. ARDS status was assessed by model parameters capturing recruitment and compliance.

RESULTS

Median model fitting error across all patients for inflation and deflation was 2.8% and 1.02% respectively with all patients experiencing auto-PEEP. In all three metrics' cases, model-based optimal PEEP was higher than clinically selected PEEP. Two patients underwent multiple recruitment manoeuvres over time and model metrics reflected and tracked the state or their ARDS.

CONCLUSIONS

For ARDS patients, the model-based method presented in this paper provides a unique, non-invasive method to select optimal patient-specific PEEP. In addition, the model has the capability to assess disease state over time using these same models and methods.

Dynamic functional residual capacity can be estimated using a stress-strain approach

BACKGROUND

Acute Respiratory Distress Syndrome (ARDS) results in collapse of alveolar units and loss of lung volume at the end of expiration. Mechanical ventilation is used to treat patients with ARDS or Acute Lung Injury (ALI), with the end objective being to increase the dynamic functional residual capacity (dFRC), and thus increasing overall functional residual capacity (FRC). Simple methods to estimate dFRC at a given positive end expiratory pressure (PEEP) level in patients with ARDS/ALI currently does not exist. Current viable methods are time-consuming and relatively invasive.

METHODS

Previous studies have found a constant linear relationship between the global stress and strain in the lung independent of lung condition. This study utilizes the constant stress-strain ratio and an individual patient's volume responsiveness to PEEP to estimate dFRC at any level of PEEP. The estimation model identifies two global parameters to estimate a patient specific dFRC, β and mβ. The parameter β captures physiological parameters of FRC, lung and respiratory elastance and varies depending on the PEEP level used, and mβ is the gradient of β vs. PEEP.

RESULTS

dFRC was estimated at different PEEP values and compared to the measured dFRC using retrospective data from 12 different patients with different levels of lung injury. The median percentage error is 18% (IQR: 6.49) for PEEP=5 cmH₂O, 10% (IQR: 9.18) for PEEP=7 cmH₂O, 28% (IQR: 12.33) for PEEP=10 cmH₂O, 3% (IQR: 2.10) for PEEP=12 cmH₂O and 10% (IQR: 9.11) for PEEP=15 cmH₂O. The results were further validated using a cross-correlation (N=100,000). Linear regression between the estimated and measured dFRC with a median R² of 0.948 (IQR: 0.915, 0.968; 90% CI: 0.814, 0.984) over the N=100,000 cross-validation tests.

CONCLUSIONS

The results suggest that a model based approach to estimating dFRC may be viable in a clinical scenario without any interruption to ventilation and can thus provide an alternative to measuring dFRC by disconnecting the patient from the ventilator or by using advanced ventilators. The overall results provide a means of estimating dFRC at any PEEP levels. Although reasonable clinical accuracy is limited to the linear region of the static PV curve, the model can evaluate the impact of changes in PEEP or other mechanical ventilation settings.

Management of mechanical ventilation during laparoscopic surgery

Laparoscopy is widely used in the surgical treatment of a number of diseases. Its advantages are generally believed to lie on its minimal invasiveness, better cosmetic outcome and shorter length of hospital stay based on surgical expertise and state-of-the-art equipment. Thousands of laparoscopic surgical procedures performed safely prove that mechanical ventilation during anaesthesia for laparoscopy is well tolerated by a vast majority of patients. However, the effects of pneumoperitoneum are particularly relevant to patients with underlying lung disease as well as to the increasing number of patients with higher-than-normal body mass index. Moreover, many surgical procedures are significantly longer in duration when performed with laparoscopic techniques. Taken together, these factors impose special care for the management of mechanical ventilation during laparoscopic surgery. The purpose of the review is to summarise the consequences of pneumoperitoneum on the standard monitoring of mechanical ventilation during anaesthesia and to discuss the rationale of using a protective ventilation strategy during laparoscopic surgery. The consequences of chest wall derangement occurring during pneumoperitoneum on airway pressure and central venous pressure, together with the role of end-tidal-CO2 monitoring are emphasised. Ventilatory and non-ventilatory strategies to protect the lung are discussed.

Regional ventilation distribution in non-sedated spontaneously breathing newborns and adults is not different

BACKGROUND

In adults, ventilation is preferentially distributed towards the dependent lung. A reversal of the adult pattern has been observed in infants using radionuclide ventilation scanning. But these results have been obtained in infants and children with lung disease. In this study we investigate whether healthy infants have a similar reverse pattern of ventilation distribution.

STUDY DESIGN

Measurement of regional ventilation distribution in healthy newborn infants during non-REM sleep in comparison to adults.

METHODS

Twenty-four healthy newborns and 13 adults were investigated with electrical impedance tomography (EIT) in supine and prone position. Regional ventilation distribution was assessed with profiles of relative impedance change. The phase lag between dependent and non-dependent ventilation was calculated as a measure of asynchronous ventilation.

RESULTS

In newborns and adults the geometric center of ventilation was centrally located in the lung at 52.2 +/- 6.2% from anterior to posterior and at 50.5 +/- 14.7%, respectively. Using impedance profiles, ventilation was equally distributed to the dependent and non-dependent lung regions in newborns. Ventilation distribution in adults was similar. Phase lag characteristics of the impedance signal showed that infants had slower emptying of the dependent lung than adults.

CONCLUSION

The speculated reverse pattern of regional ventilation distribution in healthy infants compared to adults could not be demonstrated. Gravity had little effect on ventilation distribution in both infants and adults measured in supine and prone position.

A minimal model of lung mechanics and model-based markers for optimizing ventilator treatment in ARDS patients

A majority of patients admitted to the Intensive Care Unit (ICU) require some form of respiratory support. In the case of Acute Respiratory Distress Syndrome (ARDS), the patient often requires full intervention from a mechanical ventilator. ARDS is also associated with mortality rate as high as 70%. Despite many recent studies on ventilator treatment of the disease, there are no well established methods to determine the optimal Positive End-Expiratory Pressure (PEEP) or other critical ventilator settings for individual patients. A model of fundamental lung mechanics is developed based on capturing the recruitment status of lung units. The main objective of this research is to develop a minimal model that is clinically effective in determining PEEP. The model was identified for a variety of different ventilator settings using clinical data. The fitting error was between 0.1% and 4% over the inflation limb and between 0.3% and 13% over the deflation limb at different PEEP settings. The model produces good correlation with clinical data, and is clinically applicable due to the minimal number of patient specific parameters to identify. The ability to use this identified patient specific model to optimize ventilator management is demonstrated by its ability to predict the patient specific response of PEEP changes before clinically applying them. Predictions of recruited lung volume change with change in PEEP have a median absolute error of 1.87% (IQR: 0.93-4.80%; 90% CI: 0.16-11.98%) for inflation and a median of 5.76% (IQR: 2.71-10.50%; 90% CI: 0.43-17.04%) for deflation, across all data sets and PEEP values (N=34predictions). This minimal model thus provides a clinically useful and relatively simple platform for continuous patient specific monitoring of lung unit recruitment for a patient.

Positive end-expiratory pressure enhances development of a functional residual capacity in preterm rabbits ventilated from birth

The factors regulating lung aeration and the initiation of pulmonary gas exchange at birth are largely unknown, particularly in infants born very preterm. As hydrostatic pressure gradients may play a role, we have examined the effect of a positive end-expiratory pressure (PEEP) on the spatial and temporal pattern of lung aeration in preterm rabbit pups mechanically ventilated from birth using simultaneous phase-contrast X-ray imaging and plethysmography. Preterm rabbit pups were delivered by caesarean section at 28 days of gestational age, anesthetized, intubated, and placed within a water-filled plethysmograph (head out). Pups were imaged as they were mechanically ventilated from birth with a PEEP of either 0 cmH2O or 5 cmH2O. The peak inflation pressure was held constant at 35 cmH2O. Without PEEP, gas only entered into the distal airways during inflation. The distal airways collapsed during expiration, and, as a result, the functional residual capacity (FRC) did not increase above the lung's anatomic dead space volume (2.5 ± 0.8 ml/kg). In contrast, ventilation with 5-cmH2O PEEP gradually increased aeration of the distal airways, which did not collapse at end expiration. The FRC achieved in pups ventilated with PEEP (19.9 ± 3.2 ml/kg) was significantly greater than in pups ventilated without PEEP (−2.3 ± 3.5 ml/kg). PEEP greatly facilitates aeration of the distal airways and the accumulation of FRC and prevents distal airway collapse at end expiration in very preterm rabbit pups mechanically ventilated from birth.

Imaging lung aeration and lung liquid clearance at birth using phase contrast X-ray imaging

The transition to extra-uterine life at birth is critically dependent on airway liquid clearance to allow the entry of air and the onset of gaseous ventilation. We have used phase contrast X-ray imaging to identify factors that regulate lung aeration at birth in spontaneously breathing term and mechanically ventilated preterm rabbit pups. Phase contrast X-ray imaging exploits the difference in refractive index between air and water to enhance image contrast, enabling the smallest air-filled structures of the lung (alveoli; < 100 microm) to be resolved. Using this technique, the lungs become visible as they aerate, allowing the air-liquid interface to be observed as it moves distally during lung aeration. Spontaneously breathing term rabbit pups rapidly aerate their lungs, with most fully recruiting their functional residual capacity (FRC) within the first few breaths. The increase in FRC occurs mainly during individual breaths, demonstrating that airway liquid clearance and lung aeration is closely associated with inspiration. We suggest that transpulmonary pressures generated by inspiration provide a hydrostatic pressure gradient for the movement of water out of the airways and into the surrounding lung tissue after birth. In mechanically ventilated preterm pups, lung aeration is closely associated with lung inflation and a positive end-expiratory pressure is required to generate and maintain FRC after birth. In summary, phase contrast X-ray imaging can image the air-filled lung with high temporal and spatial resolution and is ideal for identifying factors that regulate lung aeration at birth in both spontaneously breathing term and mechanically ventilated preterm neonates.

The role of time and pressure on alveolar recruitment

Inappropriate mechanical ventilation in patients with acute respiratory distress syndrome can lead to ventilator-induced lung injury (VILI) and increase the morbidity and mortality. Reopening collapsed lung units may significantly reduce VILI, but the mechanisms governing lung recruitment are unclear. We thus investigated the dynamics of lung recruitment at the alveolar level. Rats (n = 6) were anesthetized and mechanically ventilated. The lungs were then lavaged with saline to simulate acute respiratory distress syndrome (ARDS). A left thoracotomy was performed, and an in vivo microscope was placed on the lung surface. The lung was recruited to three recruitment pressures (RP) of 20, 30, or 40 cmH(2)O for 40 s while subpleural alveoli were continuously filmed. Following measurement of microscopic alveolar recruitment, the lungs were excised, and macroscopic gross lung recruitment was digitally filmed. Recruitment was quantified by computer image analysis, and data were interpreted using a mathematical model. The majority of alveolar recruitment (78.3 +/- 7.4 and 84.6 +/- 5.1%) occurred in the first 2 s (T2) following application of RP 30 and 40, respectively. Only 51.9 +/- 5.4% of the microscopic field was recruited by T2 with RP 20. There was limited recruitment from T2 to T40 at all RPs. The majority of gross lung recruitment also occurred by T2 with gradual recruitment to T40. The data were accurately predicted by a mathematical model incorporating the effects of both pressure and time. Alveolar recruitment is determined by the magnitude of recruiting pressure and length of time pressure is applied, a concept supported by our mathematical model. Such a temporal dependence of alveolar recruitment needs to be considered when recruitment maneuvers for clinical application are designed.

Alveolar dynamics during respiration: are the pores of Kohn a pathway to recruitment?

The change in alveolar size and number during the full breathing cycle in mammals remains unanswered, yet these descriptors are fundamental for understanding alveolar-based diseases and for improving ventilator management. Genetic and environmental mouse models are used increasingly to evaluate the evolution of disease in the peripheral lung; however, little is known regarding alveolar structure and function in the fresh, intact lung. Therefore, we have developed an optical confocal process to evaluate alveolar dynamics in the fresh intact mouse lung and as an initial experiment, have evaluated mouse alveolar dynamics during a single respiratory cycle immediately after passive lung deflation. We observe that alveoli become smaller and more numerous at the end of inspiration, and propose that this is direct evidence for alveolar recruitment in the mouse lung. The findings reported support a new hypothesis that requires recruitable secondary (daughter) alveoli to inflate via primary (mother) alveoli rather than from a conducting airway.

The critically ill injured patient

Patients admitted to the ICU after severe trauma require frequent procedures in the operating room, particularly in cases where a damage control strategy is used. The ventilatory management of these patients in the operating room can be particularly challenging. These patients often have severely impaired respiratory mechanics because of acute lung injury and abdominal compartment syndrome. Consequently, the pressure and flow generation capabilities of standard anesthesia ventilators may be inadequate to support ventilation and gas exchange. This article presents the problems that may be encountered in patients who have severe abdominal and lung injuries, and the current management concepts used in caring for these patients in the critical care setting, to provide guidelines for the anesthetist faced with these patients in the operating room.

Comparison of the effectiveness of manual and ventilator hyperinflation at different levels of positive end-expiratory pressure in artificially ventilated and intubated intensive care patients

BACKGROUND

Manual hyperinflation (MHI) and ventilator hyperinflation (VHI) are two methods of recruitment maneuvers used in ventilated patients to improve lung compliance and secretion mobilization. The use of VHI may minimize the adverse effects of disconnection from the ventilator, but it is uncertain whether high levels of positive end-expiratory pressure (PEEP) would decrease the peak expiratory flow rate (PEFR) and consequently affect secretion clearance.

OBJECTIVES

The aim of this study was to compare the effectiveness of MHI and VHI in terms of clearing pulmonary secretions (sputum wet weight and PEFR), improving static respiratory system compliance and oxygenation (arterial oxygen tension/fraction of inspired oxygen), and altering mean arterial pressure, heart rate, and carbon dioxide output at different levels of PEEP.

METHODS

This was a randomized crossover study involving 14 general intensive care patients who were intubated and mechanically ventilated.

RESULTS

Sputum production was similar in both techniques and levels of PEEP. There were no differences in improvement in oxygenation and static respiratory system compliance between MHI and VHI. However, VHI increased Cst significantly at 30 minutes posttreatment (P = .012), and a significant difference was observed between levels 5 and 7.5 cmH(2)O (P = .02) of PEEP for MHI. MHI generated higher PEFR than VHI (P < .05). No adverse change in heart rate or mean arterial pressure was observed during either technique; however, VCO(2) was significantly different for techniques (P = .045) and over time (P = .05).

CONCLUSION

The VHI technique seems to promote greater improvements in respiratory mechanics with less metabolic disturbance compared with MHI. Other variables such as sputum production, hemodynamics, and oxygenation were affected similarly by both techniques.

Effect of positive end-expiratory pressure and tidal volume on lung injury induced by alveolar instability

INTRODUCTION

One potential mechanism of ventilator-induced lung injury (VILI) is due to shear stresses associated with alveolar instability (recruitment/derecruitment). It has been postulated that the optimal combination of tidal volume (Vt) and positive end-expiratory pressure (PEEP) stabilizes alveoli, thus diminishing recruitment/derecruitment and reducing VILI. In this study we directly visualized the effect of Vt and PEEP on alveolar mechanics and correlated alveolar stability with lung injury.

METHODS

In vivo microscopy was utilized in a surfactant deactivation porcine ARDS model to observe the effects of Vt and PEEP on alveolar mechanics. In phase I (n = 3), nine combinations of Vt and PEEP were evaluated to determine which combination resulted in the most and least alveolar instability. In phase II (n = 6), data from phase I were utilized to separate animals into two groups based on the combination of Vt and PEEP that caused the most alveolar stability (high Vt [15 cc/kg] plus low PEEP [5 cmH2O]) and least alveolar stability (low Vt [6 cc/kg] and plus PEEP [20 cmH2O]). The animals were ventilated for three hours following lung injury, with in vivo alveolar stability measured and VILI assessed by lung function, blood gases, morphometrically, and by changes in inflammatory mediators.

RESULTS

High Vt/low PEEP resulted in the most alveolar instability and lung injury, as indicated by lung function and morphometric analysis of lung tissue. Low Vt/high PEEP stabilized alveoli, improved oxygenation, and reduced lung injury. There were no significant differences between groups in plasma or bronchoalveolar lavage cytokines or proteases.

CONCLUSION

A ventilatory strategy employing high Vt and low PEEP causes alveolar instability, and to our knowledge this is the first study to confirm this finding by direct visualization. These studies demonstrate that low Vt and high PEEP work synergistically to stabilize alveoli, although increased PEEP is more effective at stabilizing alveoli than reduced Vt. In this animal model of ARDS, alveolar instability results in lung injury (VILI) with minimal changes in plasma and bronchoalveolar lavage cytokines and proteases. This suggests that the mechanism of lung injury in the high Vt/low PEEP group was mechanical, not inflammatory in nature.

A novel mechanical lung model of pulmonary diseases to assist with teaching and training

BACKGROUND

A design concept of low-cost, simple, fully mechanical model of a mechanically ventilated, passively breathing lung is developed. An example model is built to simulate a patient under mechanical ventilation with accurate volumes and compliances, while connected directly to a ventilator.

METHODS

The lung is modelled with multiple units, represented by rubber bellows, with adjustable weights placed on bellows to simulate compartments of different superimposed pressure and compliance, as well as different levels of lung disease, such as Acute Respiratory Distress Syndrome (ARDS). The model was directly connected to a ventilator and the resulting pressure volume curves recorded.

RESULTS

The model effectively captures the fundamental lung dynamics for a variety of conditions, and showed the effects of different ventilator settings. It was particularly effective at showing the impact of Positive End Expiratory Pressure (PEEP) therapy on lung recruitment to improve oxygenation, a particulary difficult dynamic to capture.

CONCLUSION

Application of PEEP therapy is difficult to teach and demonstrate clearly. Therefore, the model provide opportunity to train, teach, and aid further understanding of lung mechanics and the treatment of lung diseases in critical care, such as ARDS and asthma. Finally, the model's pure mechanical nature and accurate lung volumes mean that all results are both clearly visible and thus intuitively simple to grasp.

Lung recruitment during mechanical positive pressure ventilation in the PICU: what can be learned from the literature?

A literature review was conducted to assess the evidence for recruitment manoeuvres used in conventional mechanical positive pressure ventilation. A total of 61 studies on recruitment manoeuvres were identified: 13 experimental, 31 ICU, 6 PICU and 12 anaesthesia studies. Recruitment appears to be a continuous process during inspiration and expiration and is determined by peak inspiratory pressure (PIP) and positive end expiratory pressure (PEEP). Single or repeated recruitment manoeuvres may result in a statistically significant increase in oxygenation; however, this is short lasting and clinically irrelevant, especially in late ARDS and pneumonia. Temporary PIP elevation may be effective but only after PEEP loss (for example disconnection and tracheal suctioning). Continuous PEEP elevation and prone positioning can increase P(a)O2 significantly. Adverse haemodynamic or barotrauma effects are reported in various studies. No data exist on the effect of recruitment manoeuvres on mortality, morbidity, length of stay or duration of mechanical ventilation. Although recruitment manoeuvres can improve oxygenation, they can potentially increase lung injury, which eventually determines outcome. Based on the presently available literature, prone position and sufficient PEEP as part of a lung protective ventilation strategy seem to be the safest and most effective recruitment manoeuvres. As paediatric physiology is essentially different from adult, paediatric studies are needed to determine the role of recruitment manoeuvres in the PICU.

The effect of positive end-expiratory pressure level on peak expiratory flow during manual hyperinflation

Including positive end-expiratory pressure (PEEP) in the manual resuscitation bag (MRB) may render manual hyperinflation (MHI) ineffective as a secretion maneuver technique in mechanically ventilated patients. In this study we aimed to determine the effect of increased PEEP or decreased compliance on peak expiratory flow rate (PEF) during MHI. A blinded, randomized study was performed on a lung simulator by 10 physiotherapists experienced in MHI and intensive care practice. PEEP levels of 0-15 cm H(2)O, compliance levels of 0.05 and 0.02 L/cm H(2)O, and MRB type were randomized. The Mapleson-C MRB generated significantly higher PEF (P < 0.01, d = 2.72) when compared with the Laerdal MRB for all levels of PEEP. In normal compliance (0.05 L/cm H(2)O) there was a significant decrease in PEF (P < 0.01, d = 1.45) for a PEEP more than 10 cm H(2)O in the Mapleson-C circuit. The Laerdal MRB at PEEP levels of more than 10 cm H(2)O did not generate a PEF that is theoretically capable of producing two-phase gas-liquid flow and, consequently, mobilizing pulmonary secretions. If MHI is indicated as a result of mucous plugging, the Mapleson-C MRB may be the most effective method of secretion mobilization.

Dynamic alveolar mechanics and ventilator-induced lung injury

OBJECTIVES

To review the mechanism of dynamic alveolar mechanics (i.e., the dynamic change in alveolar size and shape during ventilation) in both the normal and acutely injured lung; to investigate the alteration in alveolar mechanics secondary to acute lung injury as a mechanism of ventilator-induced lung injury (VILI); and to examine the hypothesis that the reduced morbidity and mortality associated with protective strategies of mechanical ventilation is related to the normalization of alveolar mechanics.

DATA EXTRACTION AND SYNTHESIS

This review is based on original published articles and review papers dealing with the mechanism of lung volume change at the alveolar level and the role of altered alveolar mechanics as a mechanism of VILI. In addition, data from our laboratory directly visualizing dynamic alveolar mechanics is reviewed and related to the literature.

CONCLUSIONS

The mechanism of alveolar inflation in normal lungs is unclear. Nonetheless, normal alveoli are very stable and change size very little with ventilation. Acute lung injury causes marked destabilization of individual alveoli. Alveolar instability causes pulmonary damage and is believed to be a major component in the mechanism of VILI. Ventilator strategies that reduce alveolar instability may potentially reduce the morbidity and mortality associated with VILI.

Alveolar instability causes early ventilator-induced lung injury independent of neutrophils

Intratracheal instillation of Tween causes a heterogeneous surfactant deactivation in the lung, with areas of unstable alveoli directly adjacent to normal stable alveoli. We employed in vivo video microscopy to directly assess alveolar stability in normal and surfactant-deactivated lung and tested our hypothesis that alveolar instability causes a mechanical injury, initiating an inflammatory response that results in a secondary neutrophil-mediated proteolytic injury. Pigs were mechanically ventilated (VT 10 cc/kg, positive end-expiratory pressure [PEEP] 3 cm H2O), randomized to into three groups, and followed for 4 hours: Control group (n = 3) surgery only; Tween group (n = 4) subjected to intratracheal Tween (surfactant deactivator causing alveolar instability); and Tween + PEEP group (n = 4) subjected to Tween with increased PEEP (15 cm H2O) to stabilize alveoli. The magnitude of alveolar instability was quantified by computer image analysis. Surfactant-deactivated lungs developed significant histopathology only in lung areas with unstable alveoli without an increase in neutrophil-derived proteases. PEEP stabilized alveoli and significantly reduced histologic evidence of lung injury. Thus, in this model, alveolar instability can independently cause ventilator-induced lung injury. To our knowledge, this is the first study to directly confirm that unstable alveoli are subjected to ventilator-induced lung injury whereas stable alveoli are not.

Positive end-expiratory pressure after a recruitment maneuver prevents both alveolar collapse and recruitment/derecruitment

We tested the hypothesis that collapsed alveoli opened by a recruitment maneuver would be unstable or recollapse without adequate positive end-expiratory pressure (PEEP) after recruitment. Surfactant deactivation was induced in pigs by Tween instillation. An in vivo microscope was placed on a lung area with significant atelectasis and the following parameters measured: (1) the number of alveoli per field and (2) alveolar stability (i.e., the change in alveolar size from peak inspiration to end expiration). We previously demonstrated that unstable alveoli cause lung injury. A recruitment maneuver (peak pressure = 45 cm H2O, PEEP = 35 cm H2O for 1 minute) was applied and alveolar number and stability were measured. Pigs were then separated into two groups with standard ventilation plus (1) 5 PEEP or (2) 10 PEEP and alveolar number and stability were again measured. The recruitment maneuver opened a significant number of alveoli, which were stable during the recruitment maneuver. Although both 5 PEEP and 10 PEEP after recruitment demonstrated improved oxygenation, alveoli ventilated with 10 PEEP were stable, whereas alveoli ventilated with 5 PEEP showed significant instability. This suggests recruitment followed by inadequate PEEP permits unstable alveoli and may result in ventilator-induced lung injury despite improved oxygenation.

Ventilator-induced lung injury

The clinical relevance of experimental ventilator-induced lung injury has recently received a resounding illustration by the Acute Respiratory Distress Syndrome Network trial that showed a 22% reduction of mortality in patients with acute respiratory disease syndrome when lung mechanical stress was lessened by tidal volume reduction during mechanical ventilation. This clinical confirmation of the concept of ventilator-induced lung injury has also undisputedly substantiated the experimental observation that excessive tidal volume and/or end-inspiratory lung volume is the main determinant of ventilator-induced lung injury. More recently, attention has focused on the roles and implication in the pathogenesis of ventilator-induced lung injury of inflammatory cells and mediators that may be activated and released either in the alveolar space or in the systemic circulation because of the rupture of the alveolar-capillary barrier and on the cellular response to mechanical stress.

Reinterpreting the pressure-volume curve in patients with acute respiratory distress syndrome

New evidence requires a reinterpretation of the inflation pressure-volume curve and suggests that neither the lower nor the upper inflection point provides reliable information to determine safe ventilator settings in the acute respiratory distress syndrome. Recruitment probably continues throughout the inflation pressure-volume curve, and studies of the deflation pressure-volume curve, reinflations after partial deflation, or decremental positive end-expiratory pressure trials after a recruitment maneuver are probably needed to determine open-lung positive end-expiratory pressure.