Effect of different seated positions on lung volume and oxygenation in acute respiratory distress syndrome

Physiological and clinical effects of trunk inclination adjustment in patients with respiratory failure: a scoping review and narrative synthesis

BACKGROUND

Adjusting trunk inclination from a semi-recumbent position to a supine-flat position or vice versa in patients with respiratory failure significantly affects numerous aspects of respiratory physiology including respiratory mechanics, oxygenation, end-expiratory lung volume, and ventilatory efficiency. Despite these observed effects, the current clinical evidence regarding this positioning manoeuvre is limited. This study undertakes a scoping review of patients with respiratory failure undergoing mechanical ventilation to assess the effect of trunk inclination on physiological lung parameters.

METHODS

The PubMed, Cochrane, and Scopus databases were systematically searched from 2003 to 2023.

INTERVENTIONS

Changes in trunk inclination.

MEASUREMENTS

Four domains were evaluated in this study: 1) respiratory mechanics, 2) ventilation distribution, 3) oxygenation, and 4) ventilatory efficiency.

RESULTS

After searching the three databases and removing duplicates, 220 studies were screened. Of these, 37 were assessed in detail, and 13 were included in the final analysis, comprising 274 patients. All selected studies were experimental, and assessed respiratory mechanics, ventilation distribution, oxygenation, and ventilatory efficiency, primarily within 60 min post postural change.

CONCLUSION

In patients with acute respiratory failure, transitioning from a supine to a semi-recumbent position leads to decreased respiratory system compliance and increased airway driving pressure. Additionally, C-ARDS patients experienced an improvement in ventilatory efficiency, which resulted in lower PaCO2 levels. Improvements in oxygenation were observed in a few patients and only in those who exhibited an increase in EELV upon moving to a semi-recumbent position. Therefore, the trunk inclination angle must be accurately reported in patients with respiratory failure under mechanical ventilation.

Changes in Respiratory Mechanics With Trunk Inclination Differs Between Patients With ARDS With and Without Obesity

BACKGROUND

Studies investigating the effect of trunk inclination on respiratory mechanics in mechanically ventilated patients with ARDS have reported postural differences in partition respiratory mechanics. Compared with more upright positions, the supine-flat position provided lower lung and chest wall elastance, allowing reduced driving pressures and end-inspiratory transpulmonary pressure. However, the effect of trunk inclination on respiratory mechanics in patients with obesity and ARDS is uncertain.

RESEARCH QUESTION

Does the effect of change in posture on partition respiratory mechanics differ between patients with ARDS with and without obesity?

STUDY DESIGN AND METHODS

In this single-center study, patients with ARDS with and without obesity were randomized into two 15-minute steps in which trunk inclination was changed from semi-recumbent (40° head up) to supine-flat (0°), or vice versa. At the end of each step partition respiratory mechanics, airway opening pressure and arterial blood gases were measured. Paired t test was used to examine respiratory mechanics and blood gas variables in each group.

RESULTS

Forty consecutive patients were enrolled. Twenty were obese (BMI, 38.4 [34.5-42.3]), and 20 were non-obese (BMI, 26.6 [25.2-28.5]). In the patients with obesity, lung and chest wall elastance, driving pressure, inspiratory transpulmonary pressure, Paco2, and ventilatory ratio were lower supine than semi-recumbent (P < .001). Airways resistance was greater supine (P = .006). In the patients without obesity, only chest wall elastance was lower in supine vs semi-recumbent (P < .001).

INTERPRETATION

In mechanically ventilated patients with ARDS and obesity, supine posture provided lower lung and chest wall elastance, and better CO2 clearance, than the semi-recumbent posture.

CLINICAL TRIAL REGISTRATION

This study was registered with Australian New Zealand Clinical Trials Registry (ACTRN12623000794606).

Lung impedance changes during awake prone positioning in COVID-19. A non-randomized cross-over study

BACKGROUND

The effects of awake prone positioning (APP) on respiratory mechanics in patients with COVID-19 are not well characterized. The aim of this study was to investigate changes of global and regional lung volumes during APP compared with the supine position using electrical lung impedance tomography (EIT) in patients with hypoxemic respiratory failure due to COVID-19.

MATERIALS AND METHODS

This exploratory non-randomized cross-over study was conducted at two university hospitals in Sweden between January and May 2021. Patients admitted to the intensive care unit with confirmed COVID-19, an arterial cannula in place, a PaO2/FiO2 ratio <26.6 kPa (<200 mmHg) and high-flow nasal oxygen or non-invasive ventilation were eligible for inclusion. EIT-data were recorded at supine baseline, at 30 and 60 minutes after APP-initiation, and 30 minutes after supine repositioning. The primary outcomes were changes in global and regional tidal impedance variation (TIV), center of ventilation (CoV), global and regional delta end-expiratory lung-impedance (dEELI) and global inhomogeneity (GI) index at the end of APP compared with supine baseline. Data were reported as median (IQR).

RESULTS

All patients (n = 10) were male and age was 64 (47-73) years. There were no changes in global or regional TIV, CoV or GI-index during the intervention. dEELI increased from supine reference value 0 to 1.51 (0.32-3.62) 60 minutes after APP (median difference 1.51 (95% CI 0.19-5.16), p = 0.04) and returned to near baseline values after supine repositioning. Seven patients (70%) showed an increase >0.20 in dEELI during APP. The other EIT-variables did not change during APP compared with baseline.

CONCLUSION

Awake prone positioning was associated with a transient lung recruiting effect without changes in ventilation distribution measured with EIT in patients with hypoxemic respiratory failure due to COVID-19.

Effect of automated head-thorax elevation during chest compressions on lung ventilation: a model study

Our goal was to investigate the effects of head-thorax elevation (HUP) during chest compressions (CC) on lung ventilation. A prospective study was performed on seven human cadavers. Chest was automatically compressed-decompressed in flat position and during progressive HUP from 18 to 35°. Lung ventilation was measured with electrical impedance tomography. In each cadaver, 5 sequences were randomly performed: one without CC at positive end-expiratory pressure (PEEP) 0cmH2O, 3 s with CC at PEEP0, 5 or 10cmH2O and 1 with CC and an impedance threshold device at PEEP0cmH2O. The minimal-to-maximal change in impedance (VTEIT in arbitrary unit a.u.) and the minimal impedance in every breathing cycle (EELI) the) were compared between flat, 18°, and 35° in each sequence by a mixed-effects model. Values are expressed as median (1st-3rd quartiles). With CC, between flat, 18° and 35° VTEIT decreased at each level of PEEP. It was 12416a.u. (10,689; 14,442), 11,239 (7667; 13,292), and 6457 (4631; 9516), respectively, at PEEP0. The same was true with the impedance threshold device. EELI/VTEIT significantly decreased from - 0.30 (- 0.40; - 0.15) before to - 1.13 (- 1.70; - 0.61) after the CC (P = 0.009). With HUP lung ventilation decreased with CC as compared to flat position. CC are associated with decreased in EELI.

Effects of changes in trunk inclination on ventilatory efficiency in ARDS patients: quasi-experimental study

BACKGROUND

Trunk inclination from semirecumbent head-upright to supine-flat positioning reduces driving pressure and increases respiratory system compliance in patients with acute respiratory distress syndrome (ARDS). These effects are associated with an improved ventilatory ratio and reduction in the partial pressure of carbon dioxide (PaCO2). However, these physiological effects have not been completely studied, and their mechanisms have not yet been elucidated. Therefore, this study aimed to evaluate the effects of a change in trunk inclination from semirecumbent (45°) to supine-flat (10°) on physiological dead space and ventilation distribution in different lung regions.

RESULTS

Twenty-two ARDS patients on pressure-controlled ventilation underwent three 60-min steps in which trunk inclination was changed from 45° (baseline) to 10° (intervention) and back to 45° (control) in the last step. Tunk inclination from a semirecumbent (45°) to a supine-flat (10°) position resulted in a higher tidal volume [371 (± 76) vs. 433 (± 84) mL (P < 0.001)] and respiratory system compliance [34 (± 10) to 41 (± 12) mL/cmH2O (P < 0.001)]. The CO2 exhaled per minute improved from 191 mL/min (± 34) to 227 mL/min (± 38) (P < 0.001). Accordingly, Bohr's dead space ratio decreased from 0.49 (± 0.07) to 0.41 (± 0.06) (p < 0.001), and PaCO2 decreased from 43 (± 5) to 36 (± 4) mmHg (p < 0.001). In addition, the impedance ratio, which divides the ventilation activity of the ventral region by the dorsal region ventilation activity in tidal images, dropped from 1.27 (0.83-1.78) to 0.86 (0.51-1.33) (p < 0.001). These results, calculated from functional EIT images, indicated further ventilation activity in the dorsal lung regions. These effects rapidly reversed once the patient was repositioned at 45°.

CONCLUSIONS

A change in trunk inclination from a semirecumbent (45 degrees) to a supine-flat position (10 degrees) improved Bohr's dead space ratio and reduced PaCO2 in patients with ARDS. This effect is associated with an increase in tidal volume and respiratory system compliance, along with further favourable impedance ventilation distribution toward the dorsal lung regions. This study highlights the importance of considering trunk inclination as a modifiable determinant of physiological parameters. The angle of trunk inclination is essential information that must be reported in ARDS patients.

Endotracheal tube, by the venturi effect, reduces the efficacy of increasing inlet pressure in improving pendelluft

In mechanically ventilated severe acute respiratory distress syndrome patients, spontaneous inspiratory effort generates more negative pressure in the dorsal lung than in the ventral lung. The airflow caused by this pressure difference is called pendelluft, which is a possible mechanisms of patient self-inflicted lung injury. This study aimed to use computer simulation to understand how the endotracheal tube and insufficient ventilatory support contribute to pendelluft. We established two models. In the invasive model, an endotracheal tube was connected to the tracheobronchial tree with 34 outlets grouped into six locations: the right and left upper, lower, and middle lobes. In the non-invasive model, the upper airway, including the glottis, was connected to the tracheobronchial tree. To recreate the inspiratory effort of acute respiratory distress syndrome patients, the lower lobe pressure was set at -13 cmH2O, while the upper and middle lobe pressure was set at -6.4 cmH2O. The inlet pressure was set from 10 to 30 cmH2O to recreate ventilatory support. Using the finite volume method, the total flow rates through each model and toward each lobe were calculated. The invasive model had half the total flow rate of the non-invasive model (1.92 L/s versus 3.73 L/s under 10 cmH2O, respectively). More pendelluft (gas flow into the model from the outlets) was observed in the invasive model than in the non-invasive model. The inlet pressure increase from 10 to 30 cmH2O decreased pendelluft by 11% and 29% in the invasive and non-invasive models, respectively. In the invasive model, a faster jet flowed from the tip of the endotracheal tube toward the lower lobes, consequently entraining gas from the upper and middle lobes. Increasing ventilatory support intensifies the jet from the endotracheal tube, causing a venturi effect at the bifurcation in the tracheobronchial tree. Clinically acceptable ventilatory support cannot completely prevent pendelluft.

PEEP Titration Is Markedly Affected by Trunk Inclination in Mechanically Ventilated Patients with COVID-19 ARDS: A Physiologic, Cross-Over Study

BACKGROUND

Changing trunk inclination affects lung function in patients with ARDS. However, its impacts on PEEP titration remain unknown. The primary aim of this study was to assess, in mechanically ventilated patients with COVID-19 ARDS, the effects of trunk inclination on PEEP titration. The secondary aim was to compare respiratory mechanics and gas exchange in the semi-recumbent (40° head-of-the-bed) and supine-flat (0°) positions following PEEP titration.

METHODS

Twelve patients were positioned both at 40° and 0° trunk inclination (randomized order). The PEEP associated with the best compromise between overdistension and collapse guided by Electrical Impedance Tomography (PEEPEIT) was set. After 30 min of controlled mechanical ventilation, data regarding respiratory mechanics, gas exchange, and EIT parameters were collected. The same procedure was repeated for the other trunk inclination.

RESULTS

PEEPEIT was lower in the semi-recumbent than in the supine-flat position (8 ± 2 vs. 13 ± 2 cmH2O, p < 0.001). A semi-recumbent position with optimized PEEP resulted in higher PaO2:FiO2 (141 ± 46 vs. 196 ± 99, p = 0.02) and a lower global inhomogeneity index (46 ± 10 vs. 53 ± 11, p = 0.008). After 30 min of observation, a loss of aeration (measured by EIT) was observed only in the supine-flat position (-153 ± 162 vs. 27 ± 203 mL, p = 0.007).

CONCLUSIONS

A semi-recumbent position is associated with lower PEEPEIT and results in better oxygenation, less derecruitment, and more homogenous ventilation compared to the supine-flat position.

Verticalization Therapy for Acute Respiratory Distress Syndrome Patients Receiving Veno-Venous Extracorporeal Membrane Oxygenation

Persistent hypoxemia during veno-venous extracorporeal membrane oxygenation (VV-ECMO) for supporting acute respiratory distress syndrome (ARDS) patients is a clinical challenge for intensive care medical providers. Prone positioning is an effective strategy to treat persistent hypoxemia; however, placing a patient in a prone position is resource intensive with significant risks to the patient. We present a patient with severe ARDS receiving VV-ECMO who underwent verticalization therapy and subsequently recovered pulmonary function.

Metabolic and ventilatory changes during postural change from the supine position to the reclining position in bedridden older patients

The prevention of pneumonia in bedridden older patients is important, and its recurrence in these patients is a relevant issue. Patients who are bedridden and inactive, and have dysphagia are considered to be at risk for pneumonia. Efforts to reduce the bedridden state and low activity may be necessary to reduce the risk of developing pneumonia in bedridden older patients. This study aimed to clarify the effects of postural change from the supine position to the reclining position on metabolic and ventilatory parameters and on safety in bedridden older patients. Using a breath gas analyzer and other tools, we assessed the following 3 positions: lying on the back (supine), resting in the Fowler position (Fowler), and resting in an 80° recline wheelchair (80°). Measurements were oxygen uptake, carbon dioxide output, gas exchange ratio, tidal volume (VT), minute volume, respiratory rate, inspiratory time, expiratory time, total respiratory time, mean inspiratory flow, metabolic equivalents, end-expiratory oxygen, and end-expiratory carbon dioxide as well as various vital signs. The study analysis included 19 bedridden participants. The change in oxygen uptake driven by changing the posture from the supine position to the Fowler position was as small as 10.8 mL/minute. VT significantly increased from the supine position (398.4 ± 111.2 mL) to the Fowler position (426.9 ± 106.8 mL) (P  = .037) and then showed a decreasing trend in the 80° position (416.8 ± 92.5 mL). For bedridden older patients, sitting in a wheelchair is a very low-impact physical activity, similar to that in normal people. The VT of bedridden older patients was maximal in the Fowler position, and the ventilatory volume did not increase with an increasing reclining angle, unlike that in normal people. These findings suggest that appropriate reclining postures in clinical situations can promote an increase in the ventilatory rate in bedridden older patients.

Chest wall loading in the ICU: pushes, weights, and positions

Clinicians monitor mechanical ventilatory support using airway pressures—primarily the plateau and driving pressure, which are considered by many to determine the safety of the applied tidal volume. These airway pressures are influenced not only by the ventilator prescription, but also by the mechanical properties of the respiratory system, which consists of the series-coupled lung and chest wall. Actively limiting chest wall expansion through external compression of the rib cage or abdomen is seldom performed in the ICU. Recent literature describing the respiratory mechanics of patients with late-stage, unresolving, ARDS, however, has raised awareness of the potential diagnostic (and perhaps therapeutic) value of this unfamiliar and somewhat counterintuitive practice. In these patients, interventions that reduce resting lung volume, such as loading the chest wall through application of external weights or manual pressure, or placing the torso in a more horizontal position, have unexpectedly improved tidal compliance of the lung and integrated respiratory system by reducing previously undetected end-tidal hyperinflation. In this interpretive review, we first describe underappreciated lung and chest wall interactions that are clinically relevant to both normal individuals and to the acutely ill who receive ventilatory support. We then apply these physiologic principles, in addition to published clinical observation, to illustrate the utility of chest wall modification for the purposes of detecting end-tidal hyperinflation in everyday practice.

Paradoxical Positioning: Does "Head Up" Always Improve Mechanics and Lung Protection?

OBJECTIVES

Head-elevated body positioning, a default clinical practice, predictably increases end-expiratory transpulmonary pressure and aerated lung volume. In acute respiratory distress syndrome (ARDS), however, the net effect of such vertical inclination on tidal mechanics depends upon whether lung recruitment or overdistension predominates. We hypothesized that in moderate to severe ARDS, bed inclination toward vertical unloads the chest wall but adversely affects overall respiratory system compliance (C rs ).

DESIGN

Prospective physiologic study.

SETTING

Two medical ICUs in the United States.

PATIENTS

Seventeen patients with ARDS, predominantly moderate to severe.

INTERVENTION

Patients were ventilated passively by volume control. We measured airway pressures at baseline (noninclined) and following bed inclination toward vertical by an additional 15°. At baseline and following inclination, we manually loaded the chest wall to determine if C rs increased or paradoxically declined, suggestive of end-tidal overdistension.

MEASUREMENTS AND MAIN RESULTS

Inclination resulted in a higher plateau pressure (supineΔ: 2.8 ± 3.3 cm H 2 O [ p = 0.01]; proneΔ: 3.3 ± 2.5 cm H 2 O [ p = 0.004]), higher driving pressure (supineΔ: 2.9 ± 3.3 cm H 2 O [ p = 0.01]; proneΔ: 3.3 ± 2.8 cm H 2 O [ p = 0.007]), and lower C rs (supine Δ: 3.4 ± 3.7 mL/cm H 2 O [ p = 0.01]; proneΔ: 3.1 ± 3.2 mL/cm H 2 O [ p = 0.02]). Following inclination, manual loading of the chest wall restored C rs and driving pressure to baseline (preinclination) values.

CONCLUSIONS

In advanced ARDS, bed inclination toward vertical adversely affects C rs and therefore affects the numerical values for plateau and driving tidal pressures commonly targeted in lung protective strategies. These changes are fully reversed with manual loading of the chest wall, suggestive of end-tidal overdistension in the upright position. Body inclination should be considered a modifiable determinant of transpulmonary pressure and lung protection, directionally similar to tidal volume and positive end-expiratory pressure.

Considerations for Upright Particle Therapy Patient Positioning and Associated Image Guidance

Particle therapy is a rapidly growing field in cancer therapy. Worldwide, over 100 centers are in operation, and more are currently in construction phase. The interest in particle therapy is founded in the superior target dose conformity and healthy tissue sparing achievable through the particles’ inverse depth dose profile. This physical advantage is, however, opposed by increased complexity and cost of particle therapy facilities. Particle therapy, especially with heavier ions, requires large and costly equipment to accelerate the particles to the desired treatment energy and steer the beam to the patient. A significant portion of the cost for a treatment facility is attributed to the gantry, used to enable different beam angles around the patient for optimal healthy tissue sparing. Instead of a gantry, a rotating chair positioning system paired with a fixed horizontal beam line presents a suitable cost-efficient alternative. Chair systems have been used already at the advent of particle therapy, but were soon dismissed due to increased setup uncertainty associated with the upright position stemming from the lack of dedicated image guidance systems. Recently, treatment chairs gained renewed interest due to the improvement in beam delivery, commercial availability of vertical patient CT imaging and improved image guidance systems to mitigate the problem of anatomical motion in seated treatments. In this review, economical and clinical reasons for an upright patient positioning system are discussed. Existing designs targeted for particle therapy are reviewed, and conclusions are drawn on the design and construction of chair systems and associated image guidance. Finally, the different aspects from literature are channeled into recommendations for potential upright treatment layouts, both for retrofitting and new facilities.

Monitoring Lung Injury Severity and Ventilation Intensity during Mechanical Ventilation

Acute respiratory distress syndrome (ARDS) is a severe form of respiratory failure burden by high hospital mortality. No specific pharmacologic treatment is currently available and its ventilatory management is a key strategy to allow reparative and regenerative lung tissue processes. Unfortunately, a poor management of mechanical ventilation can induce ventilation induced lung injury (VILI) caused by physical and biological forces which are at play. Different parameters have been described over the years to assess lung injury severity and facilitate optimization of mechanical ventilation. Indices of lung injury severity include variables related to gas exchange abnormalities, ventilatory setting and respiratory mechanics, ventilation intensity, and the presence of lung hyperinflation versus derecruitment. Recently, specific indexes have been proposed to quantify the stress and the strain released over time using more comprehensive algorithms of calculation such as the mechanical power, and the interaction between driving pressure (DP) and respiratory rate (RR) in the novel DP multiplied by four plus RR [(4 × DP) + RR] index. These new parameters introduce the concept of ventilation intensity as contributing factor of VILI. Ventilation intensity should be taken into account to optimize protective mechanical ventilation strategies, with the aim to reduce intensity to the lowest level required to maintain gas exchange to reduce the potential for VILI. This is further gaining relevance in the current era of phenotyping and enrichment strategies in ARDS.

Effect of head-end of bed elevation on respiratory mechanics in mechanically ventilated patients with moderate-to-severe COVID-19 ARDS - A cohort study

Background

Head-end elevation (HEE) is known to improve oxygenation and respiratory mechanics. In ARDS, poor lung compliance limits positive pressure ventilation causing delivery of inadequate minute ventilation (MVe). We observed that, in moderate-to-severe COVID-19 ARDS, the respiratory system compliance (Crs) reduces upon elevating the head-end of the bed, and vice-versa, which can be utilized to improve ventilation and avoid respiratory acidosis.We hypothesized that increasing the degree of HEE reduces Crs.

Methods

We included 20 consecutive mechanically ventilated, moderate-to-severe COVID-19 ARDS patients in this pilot study (CTRI/2021/06/034,182). The Crs, Mve and Rinsp were recorded at 0°, 10°, 20° and 30° HEE. Repeated measures ANOVA was used to determine significant differences in measurements with increasing degrees and repeated measures correlation (rmcorr) for correlation.

Results

Repeated measures ANOVA showed a significant difference (p < 0.0001) between values of Crs, MVe and Rinsp. Rmcorr showed a strong negative correlation between increasing degrees and Crs and Mve (r-0.87 [95% CI -0.79 to -0.92, p < 0.0001 and r-0.77 [95% CI -0.64 to -0.85, p < 0.0001]) and a moderate negative correlation for Rinsp (r-0.67; 95% CI -0.79 to -0.50; p < 0.0001).

Conclusions

Increasing degree of HEE reduces compliance in moderate-to-severe COVID-19 ARDS. Reducing HEE may optimize ventilation and mitigate ventilator induced lung injury.

The Effect of Physical Therapy on Regional Lung Function in Critically Ill Patients

Early mobilization has become an important aspect of treatment in intensive care medicine, especially in patients with acute pulmonary dysfunction. As its effects on regional lung physiology have not been fully explored, we conceived a prospective observational study (Registration number: DRKS00023076) investigating regional lung function during a 15-min session of early mobilization physiotherapy with a 30-min follow-up period. The study was conducted on 20 spontaneously breathing adult patients with impaired pulmonary gas exchange receiving routine physical therapy during their intensive care unit stay. Electrical impedance tomography (EIT) was applied to continuously monitor ventilation distribution and changes in lung aeration during mobilization and physical therapy. Baseline data was recorded in the supine position, the subjects were then transferred into the seated and partly standing position for physical therapy. Afterward, patients were transferred back into the initial position and followed up with EIT for 30 min. EIT data were analyzed to assess changes in dorsal fraction of ventilation (%dorsal), end-expiratory lung impedance normalized to tidal variation (ΔEELI), center of ventilation (CoV) and global inhomogeneity index (GI index).Follow-up was completed in 19 patients. During exercise, patients exhibited a significant change in ventilation distribution in favor of dorsal lung regions, which did not persist during follow-up. An identical effect was shown by CoV. ΔEELI increased significantly during follow-up. In conclusion, mobilization led to more dorsal ventilation distribution, but this effect subsided after returning to initial position. End-expiratory lung impedance increased during follow-up indicating a slow increase in end-expiratory lung volume following physical therapy.

Clinical Practice: Should we Radically Alter our Sedation of Critical Care Patients, Especially Given the COVID-19 Pandemics?

The high number of patients infected with the SARS-CoV-2 virus requiring care for ARDS puts sedation in the critical care unit (CCU) to the edge. Depth of sedation has evolved over the last 40 years (no-sedation, deep sedation, daily emergence, minimal sedation, etc.). Most guidelines now recommend determining the depth of sedation and minimizing the use of benzodiazepines and opioids. The broader use of alpha-2 adrenergic agonists ('alpha-2 agonists') led to sedation regimens beginning at admission to the CCU that contrast with hypnotics+opioids ("conventional" sedation), with major consequences for cognition, ventilation and circulatory performance. The same doses of alpha-2 agonists used for 'cooperative' sedation (ataraxia, analgognosia) elicit no respiratory depression but modify the autonomic nervous system (cardiac parasympathetic activation, attenuation of excessive cardiac and vasomotor sympathetic activity). Alpha-2 agonists should be selected only in patients who benefit from their effects ('personalized' indications, as opposed to a 'one size fits all' approach). Then, titration to effect is required, especially in the setting of systemic hypotension and/or hypovolemia. Since no general guidelines exist for the use of alpha-2 agonists for CCU sedation, our clinical experience is summarized for the benefit of physicians in clinical situations in which a recommendation might never exist (refractory delirium tremens; unstable, hypovolemic, hypotensive patients, etc.). Because the physiology of alpha-2 receptors and the pharmacology of alpha-2 agonists lead to personalized indications, some details are offered. Since interactions between conventional sedatives and alpha-2 agonists have received little attention, these interactions are addressed. Within the existing guidelines for CCU sedation, this article could facilitate the use of alpha-2 agonists as effective and safe sedation while awaiting large, multicentre trials and more evidence-based medicine.

Does endo-tracheal tube clamping prevent air leaks and maintain positive end-expiratory pressure during the switching of a ventilator in a patient in an intensive care unit? A bench study

Objectives

When patients with acute respiratory distress syndrome are moved out of an intensive care unit, the ventilator often requires changing. This procedure suppresses positive end expiratory pressure and promotes lung derecruitment. Clamping the endotracheal tube may prevent this from occurring. Whether or not such clamping maintains positive end-expiratory pressure has never been investigated. We designed a bench study to explore this further.

How the study was done

We used the Elysee 350 ventilator in ‘volume controlled’ mode with a positive end-expiratory pressure of 15 cmH2O, connected to an endotracheal tube with an 8 mm internal diameter inserted into a lung model with 40 ml/cmH2O compliance and 10 cmH2O/L/s resistance. We measured airway pressure and flow between the distal end of the endotracheal tube and the lung model. We tested a plastic, a metal, and an Extra Corporeal Membrane Oxygenation clamp, each with an oral/nasal, a nasal, and a reinforced endotracheal tube. We performed an end-expiratory hold then clamped the endotracheal tube and disconnected the ventilator. We measured the change in airway pressure and volume for 30 s following the disconnection of the ventilator.

Results

Airway pressure decreased thirty seconds after disconnection with all combinations of clamp and endotracheal tube. The largest fall in airway pressure (-17.486 cmH₂O/s at 5 s and -18.834 cmH₂O/s at 30 s) was observed with the plastic clamp combined with the reinforced endotracheal tube. The smallest decrease in airway pressure (0 cmH₂O/s at 5 s and -0.163 cmH₂O/s at 30 s) was observed using the Extra Corporeal Membrane Oxygenation clamp with the nasal endotracheal tube.

Conclusions

Only the Extra Corporeal Membrane Oxygenation clamp was efficient. Even with an Extra Corporeal Membrane Oxygenation clamp, it is important to limit the duration the ventilator is disconnected to a few seconds (ideally 5 s).

Continuous Negative Abdominal Pressure Reduces Ventilator-induced Lung Injury in a Porcine Model

BACKGROUND

In supine patients with acute respiratory distress syndrome, the lung typically partitions into regions of dorsal atelectasis and ventral aeration ("baby lung"). Positive airway pressure is often used to recruit atelectasis, but often overinflates ventral (already aerated) regions. A novel approach to selective recruitment of dorsal atelectasis is by "continuous negative abdominal pressure."

METHODS

A randomized laboratory study was performed in anesthetized pigs. Lung injury was induced by surfactant lavage followed by 1 h of injurious mechanical ventilation. Randomization (five pigs in each group) was to positive end-expiratory pressure (PEEP) alone or PEEP with continuous negative abdominal pressure (-5 cm H2O via a plexiglass chamber enclosing hindlimbs, pelvis, and abdomen), followed by 4 h of injurious ventilation (high tidal volume, 20 ml/kg; low expiratory transpulmonary pressure, -3 cm H2O). The level of PEEP at the start was ≈7 (vs. ≈3) cm H2O in the PEEP (vs. PEEP plus continuous negative abdominal pressure) groups. Esophageal pressure, hemodynamics, and electrical impedance tomography were recorded, and injury determined by lung wet/dry weight ratio and interleukin-6 expression.

RESULTS

All animals survived, but cardiac output was decreased in the PEEP group. Addition of continuous negative abdominal pressure to PEEP resulted in greater oxygenation (PaO2/fractional inspired oxygen 316 ± 134 vs. 80 ± 24 mmHg at 4 h, P = 0.005), compliance (14.2 ± 3.0 vs. 10.3 ± 2.2 ml/cm H2O, P = 0.049), and homogeneity of ventilation, with less pulmonary edema (≈10% less) and interleukin-6 expression (≈30% less).

CONCLUSIONS

Continuous negative abdominal pressure added to PEEP reduces ventilator-induced lung injury in a pig model compared with PEEP alone, despite targeting identical expiratory transpulmonary pressure.

Effects of patient positioning on respiratory mechanics in mechanically ventilated ICU patients

Changes in the body position of patients receiving mechanical ventilation in intensive care unit are frequent. Contrary to healthy humans, little data has explored the physiological impact of position on respiratory mechanics. The objective of present paper is to review the available data on the effect of changing body position on respiratory mechanics in ICU patients receiving mechanical ventilation. Supine position (lying flat) or lateral position do not seem beneficial for critically ill patients in terms of respiratory mechanics. The sitting position (with thorax angulation >30° from the horizontal plane) is associated with improvement of functional residual capacity (FRC), oxygenation and reduction of work of breathing. There is a critical angle of inclination in the seated position above which the increase in abdominal pressure contributes to increase chest wall elastance and offset the increase in FRC. The impact of prone position on respiratory mechanics is complex, but the increase in chest wall elastance is a central mechanism. To sum up, both sitting and prone positions provides beneficial impact on respiratory mechanics of mechanically ventilated patients as compared to supine position.

Building on the Shoulders of Giants: Is the use of Early Spontaneous Ventilation in the Setting of Severe Diffuse Acute Respiratory Distress Syndrome Actually Heretical?

Acute respiratory distress syndrome (ARDS) is not a failure of the neurological command of the ventilatory muscles or of the ventilatory muscles; it is an oxygenation defect. As positive pressure ventilation impedes the cardiac function, paralysis under general anaesthesia and controlled mandatory ventilation should be restricted to the interval needed to control the acute cardio-ventilatory distress observed upon admission into the critical care unit (CCU; "salvage therapy" during "shock state"). Current management of early severe diffuse ARDS rests on a prolonged interval of controlled mechanical ventilation with low driving pressure, paralysis (48 h, too often overextended), early proning and positive end-expiratory pressure (PEEP). Therefore, the time interval between arrival to the CCU and switching to spontaneous ventilation (SV) is not focused on normalizing the different factors involved in the pathophysiology of ARDS: fever, low cardiac output, systemic acidosis, peripheral shutdown (local acidosis), supine position, hypocapnia (generated by hyperpnea and tachypnea), sympathetic activation, inflammation and agitation. Then, the extended period of controlled mechanical ventilation with paralysis under general anaesthesia leads to CCU-acquired pathology, including low cardiac output, myoneuropathy, emergence delirium and nosocomial infection. The stabilization of the acute cardio-ventilatory distress should primarily itemize the pathophysiological conditions: fever control, improved micro-circulation and normalized local acidosis, 'upright' position, minimized hypercapnia, sympathetic de-activation (normalized sympathetic activity toward baseline levels resulting in improved micro-circulation with alpha-2 agonists administered immediately following optimized circulation and endotracheal intubation), lowered inflammation and 'cooperative' sedation without respiratory depression evoked by alpha-2 agonists. Normalised metabolic, circulatory and ventilatory demands will allow one to single out the oxygenation defect managed with high PEEP (diffuse recruitable ARDS) under early spontaneous ventilation (airway pressure release ventilation+SV or low-pressure support). Assuming an improved overall status, PaO₂/FiO₂≥150-200 allows for extubation and continuous non-invasive ventilation. Such fast-tracking may avoid most of the CCU-acquired pathologies. Evidence-based demonstration is required.

Hypothesis: Fever control, a niche for alpha-2 agonists in the setting of septic shock and severe acute respiratory distress syndrome?

During severe septic shock and/or severe acute respiratory distress syndrome (ARDS) patients present with a limited cardio-ventilatory reserve (low cardiac output and blood pressure, low mixed venous saturation, increased lactate, low PaO2/FiO2 ratio, etc.), especially when elderly patients or co-morbidities are considered. Rescue therapies (low dose steroids, adding vasopressin to noradrenaline, proning, almitrine, NO, extracorporeal membrane oxygenation, etc.) are complex. Fever, above 38.5-39.5°C, increases both the ventilatory (high respiratory drive: large tidal volume, high respiratory rate) and the metabolic (increased O2 consumption) demands, further limiting the cardio-ventilatory reserve. Some data (case reports, uncontrolled trial, small randomized prospective trials) suggest that control of elevated body temperature ("fever control") leading to normothermia (35.5-37°C) will lower both the ventilatory and metabolic demands: fever control should simplify critical care management when limited cardio-ventilatory reserve is at stake. Usually fever control is generated by a combination of general anesthesia ("analgo-sedation", light total intravenous anesthesia), antipyretics and cooling. However general anesthesia suppresses spontaneous ventilation, making the management more complex. At variance, alpha-2 agonists (clonidine, dexmedetomidine) administered immediately following tracheal intubation and controlled mandatory ventilation, with prior optimization of volemia and atrio-ventricular conduction, will reduce metabolic demand and facilitate normothermia. Furthermore, after a rigorous control of systemic acidosis, alpha-2 agonists will allow for accelerated emergence without delirium, early spontaneous ventilation, improved cardiac output and micro-circulation, lowered vasopressor requirements and inflammation. Rigorous prospective randomized trials are needed in subsets of patients with a high fever and spiraling toward refractory septic shock and/or presenting with severe ARDS.

[Acute respiratory distress syndrome : Basic principles and treatment]

Even after many years of intensive research acute respiratory distress syndrome (ARDS) is still associated with a high mortality. Epidemiologically, ARDS represents a central challenge for modern intensive care treatment. The multifactorial etiology of ARDS complicates the clear identification and evaluation of new therapeutic interventions. Lung protective mechanical ventilation and adjuvant therapies, such as the prone position and targeted extracorporeal lung support are of particular importance in the treatment of ARDS, depending on the severity of the disease. In order to guarantee an individualized and needs-adapted treatment, ARDS patients benefit from treatment in specialized centers.

Continuous negative abdominal pressure: mechanism of action and comparison with prone position

We recently reported that continuous negative abdominal pressure (CNAP) could recruit dorsal atelectasis in experimental lung injury and that oxygenation improved at different transpulmonary pressure values compared with increases in airway pressure (Yoshida T, Engelberts D, Otulakowski G, Katira BH, Post M, Ferguson ND, Brochard L, Amato MBP, Kavanagh BP. Am J Respir Crit Care Med 197: 534-537, 2018). The mechanism of recruitment with CNAP is uncertain, and its impact compared with a commonly proposed alternative approach to recruitment, prone positioning, is not known. We hypothesized that CNAP recruits lung by decreasing the vertical pleural pressure (P_pl) gradient (i.e., difference between dependent and nondependent P_pl), thought to be one mechanism of action of prone positioning. An established porcine model of lung injury (surfactant depletion followed by ventilator-induced lung injury) was used. CNAP was applied using a plexiglass chamber that completely enclosed the abdomen at a constant negative pressure (-5 cmH₂O). Lungs were recruited to maximal positive end-expiratory pressure (PEEP; 25 cmH₂O) and deflated in steps of PEEP (2 cmH₂O, 10 min each). CNAP lowered the P_pl in dependent but not in nondependent lung, and therefore, in contrast to PEEP, it narrowed the vertical P_pl gradient. CNAP increased respiratory system compliance and oxygenation and appeared to selectively displace the posterior diaphragm caudad (computerized tomography images). Compared with prone position without CNAP, CNAP in the supine position was associated with higher arterial partial pressure of oxygen and compliance, as well as greater homogeneity of ventilation. The mechanism of action of CNAP appears to be via selective narrowing of the vertical gradient of P_pl. CNAP appears to offer physiological advantages over prone positioning. NEW & NOTEWORTHY Continuous negative abdominal pressure reduces the vertical gradient in (dependent vs. nondependent) pleural pressure and increases oxygenation and lung compliance; it is more effective than prone positioning at comparable levels of positive end-expiratory pressure.

Reproducibility of patient setup in the seated treatment position: A novel treatment chair design

Radiotherapy in a seated position may be indicated for patients who are unable to lie on the treatment couch for the duration of treatment, in scenarios where a seated treatment position provides superior anatomical positioning and dose distributions, or for a low‐cost system designed using a fixed treatment beam and rotating seated patient. In this study, we report a novel treatment chair that was constructed to allow for three‐dimensional imaging and treatment delivery while ensuring robust immobilization, providing reproducibility equivalent to that in the traditional supine position. Five patients undergoing radiation treatment for head‐and‐neck cancers were enrolled and were setup in the chair, with immobilization devices created, and then imaged with orthogonal X‐rays in a scenario that mimicked radiation treatments (without treatment delivery). Six subregions of the acquired images were rigidly registered to evaluate intra‐ and interfraction displacement and chair construction. Displacements under conditions of simulated image guidance were acquired by first registering one subregion; the residual displacement of other subregions was then measured. Additionally, we administered a patient questionnaire to gain patient feedback and assess comparison to the supine position. Average inter‐ and intrafraction displacements of all subregions in the seated position were less than 2 and 3 mm, respectively. When image guidance was simulated, L‐R and A‐P interfraction displacements were reduced by an average of 1 mm, providing setup of comparable quality to supine setups. The enrolled patients, who had no indication for a seated treatment position, reported no preference in the seated or the supine position. The novel chair design provides acceptable inter‐ and intrafraction displacement, with reproducibility equivalent to that reported for patients in the supine position. Patient feedback will be incorporated in the refinement of the chair, facilitating treatment of head‐and‐neck cancer in patients who are unable to lie for the duration of treatment or for use in an economical fixed‐beam setup.

[Short version S2e guidelines: "Positioning therapy and early mobilization for prophylaxis or therapy of pulmonary function disorders"]

The German Society of Anesthesiology and Intensive Care Medicine (DGAI) commissioned a revision of the S2 guidelines on "positioning therapy for prophylaxis or therapy of pulmonary function disorders" from 2008. Because of the increasing clinical and scientific relevance the guidelines were extended to include the issue of "early mobilization" and the following main topics are therefore included: use of positioning therapy and early mobilization for prophylaxis and therapy of pulmonary function disorders, undesired effects and complications of positioning therapy and early mobilization as well as practical aspects of the use of positioning therapy and early mobilization. These guidelines are the result of a systematic literature search and the subsequent critical evaluation of the evidence with scientific methods. The methodological approach for the process of development of the guidelines followed the requirements of evidence-based medicine, as defined as the standard by the Association of the Scientific Medical Societies in Germany. Recently published articles after 2005 were examined with respect to positioning therapy and the recently accepted aspect of early mobilization incorporates all literature published up to June 2014.

End-Expiratory Lung Volume in Patients with Acute Respiratory Distress Syndrome: A Time Course Analysis

PURPOSE

Lung injury can be caused by ventilation and non-physiological lung stress (transpulmonary pressure) and strain [inflated volume over functional residual capacity ratio (FRC)]. FRC is severely decreased in patients with acute respiratory distress syndrome (ARDS). End-expiratory lung volume (EELV) is FRC plus lung volume increased by the applied positive end-expiratory pressure (PEEP). Measurement using the modified nitrogen multiple breath washout technique may help titrating PEEP during ARDS and allow determining dynamic lung strain (tidal volume over EELV) in patients ventilated with PEEP. In this observational study, we measured EELV for up to seven consecutive days in patients with ARDS at different PEEP levels.

RESULTS

Thirty sedated patients with ARDS (10 mild, 14 moderate, 6 severe) underwent decremental PEEP testing (20, 15, 10, 5 cm H2O) for up to 7 days after inclusion. At all PEEP levels examined, over a period of 7 days the measured absolute EELVs showed no significant change over time [PEEP 20 cm H2O 2464 ml at day 1 vs. 2144 ml at day 7 (p = 0.78), PEEP 15 cm H2O 2226 ml vs. 1990 ml (p = 0.36), PEEP 10 1835 ml vs. 1858 ml (p = 0.76) and PEEP 5 cm H2O 1487 ml vs. 1612 ml (p = 0.37)]. In relation to the predicted body weight (pbw), no significant change in EELV/kg pbw over time could be detected either at any PEEP level or over time [PEEP 20 36 ml/kg pbw at day 1 vs. 33 ml/kg pbw at day 7 (p = 0.66); PEEP 15 33 vs. 29 ml/kg pbw (p = 0.32); PEEP 10 27 vs. 27 ml/kg pbw (p = 0.70) and PEEP 5 22 vs. 24 ml/kg pbw (p = 0.70)]. Oxygenation significantly improved over time from PaO2/FiO2 of 169 mmHg at day 1 to 199 mmHg at day 7 (p < 0.01).

CONCLUSIONS

EELV did not change significantly for up to 7 days in patients with ARDS. By contrast, PaO2/FiO2 improved significantly. Bedside measurement of EELV may be a novel approach to individualise lung-protective ventilation on the basis of calculation of dynamic strain as the ratio of VT to EELV.

Lung volume assessment in acute respiratory distress syndrome

PURPOSE OF REVIEW

Measurements of lung volumes allow evaluating the pathophysiogical severity of acute respiratory distress syndrome (ARDS) in terms of the degree of reduction in aerated lung volume, calculating strain, quantifying recruitment and/or hyperinflation, and gas volume distribution. We summarize the current techniques for lung volume assessment selected according to their possible usage in the ICU and discuss the recent findings obtained with implementation of these techniques in patients with ARDS.

RECENT FINDINGS

Computed tomography technique remains irreplaceable in terms of quantitative aeration of different lung regions, but the commonly used cut-offs for classification may be questioned with recent findings on nonpathological lungs. Monitoring end expiratory lung volume using nitrogen washout technique enhanced our understanding on lung volume change during positioning, pleural effusion drainage, intra-abdominal hypertension, and recruitment maneuver. Recent studies supported that tidal volume could not surrogate tidal strain, which needs measurement of functional residual capacity and which is correlated with pro-inflammatory lung response.

SUMMARY

Although lung volume measurements are still limited to research area of ARDS, recent progress in technology provides clinicians more opportunities to evaluate lung volumes noninvasively at the bedside and may facilitate individualization of ventilator settings based on the specific physiological understandings of a given patient.

S2e guideline: positioning and early mobilisation in prophylaxis or therapy of pulmonary disorders : Revision 2015: S2e guideline of the German Society of Anaesthesiology and Intensive Care Medicine (DGAI)

The German Society of Anesthesiology and Intensive Care Medicine (DGAI) commissioneda revision of the S2 guidelines on "positioning therapy for prophylaxis or therapy of pulmonary function disorders" from 2008. Because of the increasing clinical and scientificrelevance the guidelines were extended to include the issue of "early mobilization"and the following main topics are therefore included: use of positioning therapy and earlymobilization for prophylaxis and therapy of pulmonary function disorders, undesired effects and complications of positioning therapy and early mobilization as well as practical aspects of the use of positioning therapy and early mobilization. These guidelines are the result of a systematic literature search and the subsequent critical evaluation of the evidence with scientific methods. The methodological approach for the process of development of the guidelines followed the requirements of evidence-based medicine, as defined as the standard by the Association of the Scientific Medical Societies in Germany. Recently published articles after 2005 were examined with respect to positioning therapy and the recently accepted aspect of early mobilization incorporates all literature published up to June 2014.

Functional residual capacity in beagle dogs with and without acute respiratory distress syndrome

BACKGROUND

Traditionally, the choice of tidal volume for mechanical ventilation was based on body weight (BW) and usually, predicted BW was used to correct actual BW inter-individual variations in obesity and muscle weight. The method of selecting tidal volume depended on the fact that normal lung volumes, especially functional residual capacity (FRC), were mainly determined by height (indirectly by predicted BW), sex and age in healthy persons. However, FRCs in patients with acute respiratory distress syndrome (ARDS) might not abide by the same rule and be significantly different from each other in patients with the same height and sex. We hypothesized that FRC was determined by body length (surrogate for predicted BW) and age in healthy male beagle dogs but not in lung injured ones.

METHODS

A total of 24 dogs were recruited and ARDS model was induced by intravenous injection of oleic acid. FRC was measured by chest computer tomography. Blood gas analysis, extra vascular lung water and respiratory system mechanics were tested at baseline and post-lung injury. Age, body length and actual BW were also recorded before experiments.

RESULTS

After lung injury, FRC decreased sharply from baseline (414±84) to (214±70) mL. For healthy lungs, FRC could be estimated by the following formula: FRC =21.86 × age (months) + 20.55 × body length (cm) - 1,337.98 (P<0.05), while for injured lungs, the formula of multiple linear regression was invalid (P=0.305).

CONCLUSIONS

FRC was linearly related to body length in healthy dogs but not in lung injured ones. The traditional view of setting tidal volume based on predicted BW should be challenged cautiously.

The effect of patient migration in bed on torso elevation

BACKGROUND

Elevating the hospital head of bed (HOB) to at least 30° is recommended practice to reduce the risk of ventilator-associated pneumonia (VAP) in mechanically ventilated patients. However, this common practice prescribes the position of the bed and not of the patient, which could be significantly different.

OBJECTIVE

The aim of this research was to determine the relationship between patient migration in bed and anatomic torso angle.

METHODS

Ten healthy participants were positioned in a hospital bed that was raised from flat to 30° and 45° HOB elevations. Prior to bed movement, participants were aligned to different locations along the length of the bed to represent different amounts of migration. A motion capture system was used to measure torso angle and migration toward the foot of the bed. The relationship between torso angle and migration was estimated by linear regression.

RESULTS

Patient migration resulted in lower torso angles for both 30° and 45° HOB articulations. A migration of 10 cm resulted in a loss of 9.1° and 13.0° of torso angle for HOB articulations of 30° and 45°, respectively (for 30° articulations: (Equation is included in full-text article.)= -0.91, R = .96; for 45° articulations: (Equation is included in full-text article.)= -1.30, R = .98).

DISCUSSION

Migration toward the foot of the bed flattens the torso. To maintain a torso angle that is likely to protect against VAP, healthcare providers need to manage both HOB angle and migration. Protocols and equipment that minimize patient migration will help support effective clinical practice. Future research on patient migration, as it relates to VAP or other outcomes, should measure patient torso angle to allow accurate translation of the results to care practice.

Role of Bed Design and Head-of-Bed Articulation on Patient Migration

The ramifications of patient migration toward the foot of the bed in intensive care units are not well understood. Migration may cause shear and friction between the patient and the mattress, reduce elevation of the patient's torso, and require frequent repositioning of the patient. This study assesses how bed design impacts both the amount of migration that patients undergo during head section articulation to 30° and 45° and the extent of torso compression following the articulation.

Seated and semi-recumbent positioning of the ventilated intensive care patient - effect on gas exchange, respiratory mechanics and hemodynamics

OBJECTIVES

To compare the effect of semi-recumbent and sitting positions on gas exchange, respiratory mechanics and hemodynamics in patients weaning from mechanical ventilation.

BACKGROUND

Upright positions are encouraged during rehabilitation of the critically ill but there effects have not been well described.

METHODS

A prospective, randomized, cross-over trial was conducted. Subjects were passively mobilized from supine into a seated position (out of bed) and from supine to a semi-recumbent position (>45° backrest elevation in bed). Arterial blood gas (PaO2/FiO2, PaO2, SaO2, PaCO2 and A-a gradient), respiratory mechanics (VE,VT, RR, Cdyn, RR/VT) and hemodynamic measurements (HR, MABP) were collected in supine and at 5 min and 30 min after re-positioning.

RESULTS

Thirty-four intubated and ventilated subjects were enrolled. The angle of backrest inclination in sitting (67 ± 5°) was greater than gained with semi-recumbent positioning (50 ± 5°, p < 0.001). There were no clinically important changes in arterial blood gas, respiratory mechanic or hemodynamic values due to either position.

CONCLUSIONS

Neither position resulted in significant changes in respiratory and hemodynamic parameters. Both positions can be applied safely in patients being weaned from ventilation.