End-inspiratory pause prolongation in acute respiratory distress syndrome patients: effects on gas exchange and mechanics

Effect of individualized end-inspiratory pause guided by driving pressure on respiratory mechanics during prone spinal surgery: a randomized controlled trial

Purpose

The prone position is commonly used in spinal surgery, but it can lead to decreased lung compliance and increased airway pressure. This study aimed to evaluate the effect of individualized end-inspiratory pause guided by driving pressure on respiratory mechanics in patients undergoing prone spinal surgery.

Methods

A randomized controlled trial was conducted from August to October 2023. Patients scheduled for elective prone spinal surgery were randomly assigned to either a study group, receiving individualized end-inspiratory pause, or a control group, receiving a fixed end-inspiratory pause (10% of total inspiratory time). Mechanical ventilation parameters, including tidal volume, plateau pressure, driving pressure, and peak pressure, were recorded at different time points. Arterial blood gases were collected at baseline and at specified intervals.

Results

Data from 36 subjects (18 in each group) were included in the final analysis. The study group exhibited a significant increase in respiratory system compliance (P < 0.05) and improved intraoperative oxygenation (P < 0.05). In addition, the individualized end-inspiratory pause significantly decreased plateau pressure (P < 0.05) and driving pressure (P < 0.05) compared to the control group.

Conclusion

The individualized end-inspiratory pause guided by driving pressure effectively optimized pulmonary compliance and improved oxygenation during prone spinal surgery. These findings suggest that this ventilation strategy may enhance respiratory mechanics and reduce the risk of postoperative pulmonary complications.

Effect of end-inspiratory pause duration on respiratory system compliance calculation in mechanically ventilated dogs with healthy lungs

OBJECTIVE

To compare respiratory system compliance (CRS), expressed per kilogram of bodyweight (CRSBW), calculated without end-inspiratory pause (EIP) and after three EIP times (0.2, 0.5 and 1 seconds) with that after 3 second EIP (considered the reference EIP for static CRS) and to determine the EIP times that provided CRSBW values in acceptable agreement with static CRSBW during controlled mechanical ventilation (CMV) in anaesthetized dogs.

STUDY DESIGN

Prospective, randomized, nonblinded, crossover clinical study.

ANIMALS

A group of 24 client-owned dogs with healthy lungs undergoing surgery in lateral recumbency.

METHODS

During CMV in dogs undergoing general anaesthesia, five EIPs [0 (no EIP), 0.2, 0.5, 1 and 3 seconds] were consecutively applied in random order. Tidal volume (Vt) was set at 10 mL kg-1 and positive end-expiratory pressure (PEEP) was not applied. Respiratory rate and inspiratory time were established according to each EIP time, setting EIP between 0 and 50% of the inspiratory time. The CRSBW was calculated as [expired Vt/(plateau pressure - PEEP)]/bodyweight and recorded every 15 seconds for 2 minutes after a 5 minute equilibration period with each EIP. One-way anova for repeated measures and the Bland-Altman analysis were used to compare CRSBW and evaluate agreement between EIP times, respectively.

RESULTS

The CRSBW was significantly greater as the EIP time increased up to 1 second (p < 0.05). In the Bland-Altman analysis, none of the tested EIPs (0, 0.2, 0.5 and 1 seconds) provided 95% confidence intervals for limits of agreement within the maximum allowed difference considered for acceptable agreement with 3 second EIP.

CONCLUSIONS

and clinical relevance An EIP ≤ to 1 second does not provide a CRSBW value in acceptable agreement with static CRSBW in healthy dogs. Besides, the application of an EIP ≤ to 0.5 seconds underestimates the static CRSBW to an increasing extent as the EIP time decreases.

Effect of end-inspiratory pause on airway and physiological dead space in anesthetized horses

OBJECTIVE

To evaluate the impact of a 30% end-inspiratory pause (EIP) on alveolar tidal volume (V_Talv), airway (V_Daw) and physiological (V_Dphys) dead spaces in mechanically ventilated horses using volumetric capnography, and to evaluate the effect of EIP on carbon dioxide (CO₂) elimination per breath (Vco₂br^-1), PaCO_2, and the ratio of PaO₂-to-fractional inspired oxygen (PaO₂:FiO₂).

STUDY DESIGN

Prospective research study.

ANIMALS

A group of eight healthy research horses undergoing laparotomy.

METHODS

Anesthetized horses were mechanically ventilated as follows: 6 breaths minute^-1, tidal volume (V_T) 13 mL kg^-1, inspiratory-to-expiratory time ratio 1:2, positive end-expiratory pressure 5 cmH₂O and EIP 0%. Vco₂br^-1 and expired tidal volume (V_TE) of 10 consecutive breaths were recorded 30 minutes after induction, after adding 30% EIP and upon EIP removal to construct volumetric capnograms. A stabilization period of 15 minutes was allowed between phases. Data were analyzed using a mixed-effect linear model. Significance was set at p < 0.05.

RESULTS

The EIP decreased V_Daw from 6.6 (6.1-6.7) to 5.5 (5.3-6.1) mL kg^-1 (p < 0.001) and increased V_Talv from 7.7 ± 0.7 to 8.6 ± 0.6 mL kg^-1 (p = 0.002) without changing the V_TE. The V_Dphys to V_TE ratio decreased from 51.0% to 45.5% (p < 0.001) with EIP. The EIP also increased PaO₂:FiO₂ from 393.3 ± 160.7 to 450.5 ± 182.5 mmHg (52.5 ± 21.4 to 60.0 ± 24.3 kPa; p < 0.001) and Vco₂br^-1 from 0.49 (0.45-0.50) to 0.59 (0.45-0.61) mL kg^-1 (p = 0.008) without reducing PaCO₂.

CONCLUSIONS AND CLINICAL RELEVANCE

The EIP improved oxygenation and reduced V_Daw and V_Dphys, without reductions in PaCO₂. Future studies should evaluate the impact of different EIP in healthy and pathological equine populations under anesthesia.

Lung morphology impacts the association between ventilatory variables and mortality in patients with acute respiratory distress syndrome

BACKGROUND

Acute respiratory distress syndrome (ARDS) patients with different lung morphology have distinct pulmonary mechanical dysfunction and outcomes. Whether lung morphology impacts the association between ventilatory variables and mortality remains unclear. Moreover, the impact of a novel combined ventilator variable [(4×DP) + RR] on morality in ARDS patients needs external validation.

METHODS

We obtained data from the Chinese Database in Intensive Care (CDIC), which included adult ARDS patients who received invasive mechanical ventilation for at least 24 h. Patients were further classified into two groups based on lung morphology (focal and non-focal). Ventilatory variables were collected longitudinally within the first four days of ventilation. The primary outcome was 28-day mortality. Extended Cox regression models were employed to explore the interaction between lung morphology and longitudinal ventilatory variables on mortality.

FINDINGS

We included 396 ARDS patients with different lung morphology (64.1% non-focal). The overall 28-day mortality was 34.4%. Patients with non-focal lung morphology have more severe and persistent pulmonary mechanical dysfunction and higher mortality than those with focal lung morphology. Time-varying driving pressure (DP) was more significantly associated with 28-day mortality in patients with non-focal lung morphology compared to focal lung morphology patients (P for interaction = 0.0039). The impact of DP on mortality was more significant than that of respiratory rate (RR) only in patients with non-focal lung morphology. The hazard ratio (HR) of mortality for [(4×DP) + RR] was significant in patients with non-focal lung morphology (HR 1.036, 95% CI 1.027-1.045), not in patients with focal lung morphology (HR 1.019, 95% CI 0.999-1.039).

INTERPRETATION

The association between ventilator variables and mortality varied among patients with different lung morphology. [(4×DP) + RR] was only associated with mortality in patients with non-focal lung morphology. Further validation is needed.

The role of acute hypercapnia on mortality and short-term physiology in patients mechanically ventilated for ARDS: a systematic review and meta-analysis

PURPOSE

Hypercapnia is frequent during mechanical ventilation for acute respiratory distress syndrome (ARDS), but its effects on morbidity and mortality are still controversial. We conducted a systematic review and meta-analysis to explore clinical consequences of acute hypercapnia in adult patients ventilated for ARDS.

METHODS

We searched Medline, Embase, and the Cochrane Library via the OVID platform for studies published from 1946 to 2021. "Permissive hypercapnia" defined hypercapnia in studies where the group with hypercapnia was ventilated with a protective ventilation (PV) strategy (lower VT targeting 6 ml/kg predicted body weight) while the group without hypercapnia was managed with a non-protective ventilation (NPV); "imposed hypercapnia" defined hypercapnia in studies where hypercapnic and non-hypercapnic patients were managed with a similar ventilation strategy.

RESULTS

Twenty-nine studies (10,101 patients) were included. Permissive hypercapnia, imposed hypercapnia under PV, and imposed hypercapnia under NPV were reported in 8, 21 and 1 study, respectively. Studies testing permissive hypercapnia reported lower mortality in hypercapnic patients receiving PV as compared to non-hypercapnic patients receiving NPV: OR = 0.26, 95% CI [0.07-0.89]. By contrast, studies reporting imposed hypercapnia under PV reported increased mortality in hypercapnic patients receiving PV as compared to non-hypercapnic patients also receiving PV: OR = 1.54, 95% CI [1.15-2.07]. There was a significant interaction between the mechanism of hypercapnia and the effect on mortality.

CONCLUSIONS

Clinical effects of hypercapnia are conflicting depending on its mechanism. Permissive hypercapnia was associated with improved mortality contrary to imposed hypercapnia under PV, suggesting a major role of PV strategy on the outcome.

Removal of a catheter mount and heat-and-moisture exchanger improves hypercapnia in patients with acute respiratory distress syndrome: A retrospective observational study

ABSTRACT

To avoid ventilator-associated lung injury in acute respiratory distress syndrome (ARDS) treatment, respiratory management should be performed at a low tidal volume of 6 to 8 mL/kg and plateau pressure of ≤30 cmH2O. However, such lung-protective ventilation often results in hypercapnia, which is a risk factor for poor outcomes. The purpose of this study was to retrospectively evaluate the effectiveness and safety of the removal of a catheter mount (CM) and using heated humidifiers (HH) instead of a heat-and-moisture exchanger (HME) for reducing the mechanical dead space created by the CM and HME, which may improve hypercapnia in patients with ARDS.This retrospective observational study included adult patients with ARDS, who developed hypercapnia (PaCO2 > 45 mm Hg) during mechanical ventilation, with target tidal volumes between 6 and 8 mL/kg and a plateau pressure of ≤30 cmH2O, and underwent stepwise removal of CM and HME (replaced with HH). The PaCO2 values were measured at 3 points: ventilator circuit with CM and HME (CM + HME) use, with HME (HME), and with HH (HH), and the overall number of accidental extubations was evaluated. Ventilator values (tidal volume, respiratory rate, minutes volume) were evaluated at the same points.A total of 21 patients with mild-to-moderate ARDS who were treated under deep sedation were included. The values of PaCO2 at HME (52.7 ± 7.4 mm Hg, P < .0001) and HH (46.3 ± 6.8 mm Hg, P < .0001) were significantly lower than those at CM + HME (55.9 ± 7.9 mm Hg). Measured ventilator values were similar at CM + HME, HME, and HH. There were no cases of reintubation due to accidental extubation after the removal of CM.The removal of CM and HME reduced PaCO2 values without changing the ventilator settings in deeply sedated patients with mild-to-moderate ARDS on lung-protective ventilation. Caution should be exercised, as the removal of a CM may result in circuit disconnection or accidental extubation. Nevertheless, this intervention may improve hypercapnia and promote lung-protective ventilation.

Progress of mechanical power in the intensive care unit

Mechanical power of ventilation, currently defined as the energy delivered from the ventilator to the respiratory system over a period of time, has been recognized as a promising indicator to evaluate ventilator-induced lung injury and predict the prognosis of ventilated critically ill patients. Mechanical power can be accurately measured by the geometric method, while simplified equations allow an easy estimation of mechanical power at the bedside. There may exist a safety threshold of mechanical power above which lung injury is inevitable, and the assessment of mechanical power might be helpful to determine whether the extracorporeal respiratory support is needed in patients with acute respiratory distress syndrome. It should be noted that relatively low mechanical power does not exclude the possibility of lung injury. Lung size and inhomogeneity should also be taken into consideration. Problems regarding the safety limits of mechanical power and contribution of each component to lung injury have not been determined yet. Whether mechanical power-directed lung-protective ventilation strategy could improve clinical outcomes also needs further investigation. Therefore, this review discusses the algorithms, clinical relevance, optimization, and future directions of mechanical power in critically ill patients.

Management of hypercapnia in critically ill mechanically ventilated patients-A narrative review of literature

The use of lower tidal volume ventilation was shown to improve survival in mechanically ventilated patients with acute lung injury. In some patients this strategy may cause hypercapnic acidosis. A significant body of recent clinical data suggest that hypercapnic acidosis is associated with adverse clinical outcomes including increased hospital mortality. We aimed to review the available treatment options that may be used to manage acute hypercapnic acidosis that may be seen with low tidal volume ventilation. The databases of MEDLINE and EMBASE were searched. Studies including animals or tissues were excluded. We also searched bibliographic references of relevant studies, irrespective of study design with the intention of finding relevant studies to be included in this review. The possible options to treat hypercapnia included optimising the use of low tidal volume mechanical ventilation to enhance carbon dioxide elimination. These include techniques to reduce dead space ventilation, and physiological dead space, use of buffers, airway pressure release ventilation and prone positon ventilation. In patients where hypercapnic acidosis could not be managed with lung protective mechanical ventilation, extracorporeal techniques may be used. Newer, minimally invasive low volume venovenous extracorporeal devices are currently being investigated for managing hypercapnia associated with low and ultra-low volume mechanical ventilation.

Individualized mechanical ventilation in a shared ventilator setting: limits, safety and technical details

The COVID-19 pandemic has resulted in an increased need for ventilators. The potential to ventilate more than one patient with a single ventilator, a so-called split ventilator setup, provides an emergency solution. Our hypothesis is that ventilation can be individualized by adding a flow restrictor to limit tidal volumes, add PEEP, titrate FiO₂ and monitor ventilation. This way we could enhance optimization of patient safety and clinical applicability. We performed bench testing to test our hypothesis and identify limitations. We performed a bench testing in two test lungs: (1) determine lung compliance (2) determine volume, plateau pressure and PEEP, (3) illustrate individualization of airway pressures and tidal volume with a flow restrictor, (4a) illustrate that PEEP can be applied and individualized (4b) create and measure intrinsic PEEP (4c and d) determine PEEP as a function of flow restriction, (5) individualization of FiO₂. The lung compliance varied between 13 and 27 mL/cmH₂O. Set ventilator settings could be applied and measured. Extrinsic PEEP can be applied except for settings with a large expiratory time. Volume and pressure regulation is possible between 70 and 39% flow restrictor valve closure. Flow restriction in the tested circuit had no effect on the other circuit or on intrinsic PEEP. FiO₂ could be modulated individually between 0.21 and 0.8 by gradually adjusting the additional flow, and minimal affecting FiO₂ in the other circuit. Tidal volumes, PEEP and FiO₂ can be individualized and monitored in a bench testing of a split ventilator. In vivo research is needed to further explore the clinical limitations and outcomes, making implementation possible as a last resort ventilation strategy.

A simple method of mechanical power calculation: using mean airway pressure to replace plateau pressure

The reference method for mechanical power (MP) calculation proposed by Gattinoni et al. is based on plateau pressure (P_plat) which needs an inspiratory hold. This study aims to introduce and validate a simple surrogate for MP calculation without any intervention in ventilated patients with or without acute respiratory distress syndrome (ARDS). The introduced equation is as:[Formula: see text]where P_mean is mean airway pressure, VE is minute ventilation, PEEP is positive end-expiratory pressure, and T_e/T_i is expiratory-to-inspiratory ratio. 50 patients with ARDS and 50 post-operative patients without ARDS were enrolled. P_mean-derived MP and reference MP were obtained at the inspiratory plateau time (T_plat) of 0 and 0.5 s (s). When T_plat was adjusted from 0 to 0.5 s, higher P_mean [non-ARDS cases: 9.3 (8.8-9.9) cmH₂O versus 8.2 (7.9-8.8) cmH₂O, P < 0.001; ARDS cases: 14 (13-16) cmH₂O versus 13 (11-14) cmH₂O, P < 0.001] and shorter T_e/T_i [non-ARDS cases: 1.4 (1.2-1.7) versus 2.4 (2.0-3.0), P < 0.001; ARDS cases: 1.3 (1.2-1.5) versus 2.5 (2.3-2.9), P < 0.001] were found. At both T_plat levels, the P_mean-derived MP correlated well with the reference MP both in patients with or without ARDS (non-ARDS: slopes = 1.05, 0.94, R² = 0.95, 0.93, bias + 0.76, + 0.51; ARDS: slopes = 1.03, 0.95, R² = 0.96, 0.96, bias + 0.97, + 0.78. P < 0.0001 for all). In patients with or without ARDS, P_mean-derived MP allows rapid and dynamic estimation of mechanical power without any intervention at the bedside.

Lung aeration in experimental malaria-associated acute respiratory distress syndrome by SPECT/CT analysis

Malaria-associated acute respiratory distress syndrome (ARDS) is an inflammatory disease causing alveolar-pulmonary barrier lesion and increased vascular permeability characterized by severe hypoxemia. Computed tomography (CT), among other imaging techniques, allows the morphological and quantitative identification of lung lesions during ARDS. This study aims to identify the onset of malaria-associated ARDS development in an experimental model by imaging diagnosis. Our results demonstrated that ARDS-developing mice presented decreased gaseous exchange and pulmonary insufficiency, as shown by the SPECT/CT technique. The pulmonary aeration disturbance in ARDS-developing mice on the 5th day post infection was characterized by aerated tissues decrease and nonaerated tissue accumulation, demonstrating increased vascular permeability and pleural effusion. The SPECT/CT technique allowed the early diagnosis in the experimental model, as well as the identification of the pulmonary aeration. Notwithstanding, despite the fact that this study contributes to better understand lung lesions during malaria-associated ARDS, further imaging studies are needed.

Dead space in acute respiratory distress syndrome

Dead space is the portion of each tidal volume that does not take part in gas exchange and represents a good global index of the efficiency of the lung function. Dead space is not routinely measured in critical care practice, because the difficulties in in interpreting capnograms and the different methods of calculations. Different dead space indices can provide useful information in acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) patients, where changes in microvasculature are the main determinants for the increase in dead space and consequently a worsening of the outcome. Lung recruitment is a dynamic process that combines recruitment manoeuvres (RMs) with positive end expiratory pressure (PEEP) and low Vt to recruit collapsed alveoli. Dead space guided recruitment allows avoiding regional overdistension or reduction in cardiac output in critical care patients with ALI or ARDS. Different patterns of ventilation affect also CO₂ elimination; in fact, end-inspiratory pause prolongation reduces dead space, increasing respiratory system compliance; plateau pressure and consequently driving pressure increase accordingly. Dead space measurement is a reliable method that provides important clinical and prognostic information. Different capnographic indices can be useful to evaluate therapeutic interventions or setting mechanical ventilation.

Should we use driving pressure to set tidal volume?

PURPOSE OF REVIEW

Ventilator-induced lung injury (VILI) can occur despite use of tidal volume (VT) limited to 6 ml/kg of predicted body weight, especially in patients with a smaller aerated compartment (i.e. the baby lung) in which, indeed, tidal ventilation takes place. Because respiratory system static compliance (CRS) is mostly affected by the volume of the baby lung, the ratio VT/CRS (i.e. the driving pressure, ΔP) may potentially help tailoring interventions on VT setting.

RECENT FINDINGS

Driving pressure is the ventilatory variable most strongly associated with changes in survival and has been shown to be the key mediator of the effects of mechanical ventilation on outcome in the acute respiratory distress syndrome. Observational data suggest an increased risk of death for patients with ΔP more than 14 cmH2O, but a well tolerated threshold for this parameter has yet to be identified. Prone position along with simple ventilatory adjustments to facilitate CO2 clearance may help reduce ΔP in isocapnic conditions. The safety and feasibility of low-flow extracorporeal CO2 removal in enhancing further reduction in VT and ΔP are currently being investigated.

SUMMARY

Driving pressure is a bedside available parameter that may help identify patients prone to develop VILI and at increased risk of death. No study had prospectively evaluated whether interventions on ΔP may provide a relevant clinical benefit, but it appears physiologically sound to try titrating VT to minimize ΔP, especially when it is higher than 14 cmH2O and when it has minimal costs in terms of CO2 clearance.

Fifty Years of Research in ARDS. Vt Selection in Acute Respiratory Distress Syndrome

Mechanical ventilation (MV) is critical in the management of many patients with acute respiratory distress syndrome (ARDS). However, MV can also cause ventilator-induced lung injury (VILI). The selection of an appropriate Vt is an essential part of a lung-protective MV strategy. Since the publication of a large randomized clinical trial demonstrating the benefit of lower Vts, the use of Vts of 6 ml/kg predicted body weight (based on sex and height) has been recommended in clinical practice guidelines. However, the predicted body weight approach is imperfect in patients with ARDS because the amount of aerated lung varies considerably due to differences in inflammation, consolidation, flooding, and atelectasis. Better approaches to setting Vt may include limits on end-inspiratory transpulmonary pressure, lung strain, and driving pressure. The limits of lowering Vt have not yet been established, and some patients may benefit from Vts that are lower than those in current use. However, lowering Vts may result in respiratory acidosis. Tactics to reduce respiratory acidosis include reductions in ventilation circuit dead space, increases in respiratory rate, higher positive end-expiratory pressures in patients who recruit lung in response to positive end-expiratory pressure, recruitment maneuvers, and prone positioning. Mechanical adjuncts such as extracorporeal carbon dioxide removal may be useful to normalize pH and carbon dioxide levels, but further studies will be necessary to demonstrate benefit with this technology.