Dynamic hyperinflation and auto-positive end-expiratory pressure: lessons learned over 30 years

Effects of Similar Mechanical Power Resulting From Different Combinations of Respiratory Variables on Lung Damage in Experimental Acute Respiratory Distress Syndrome

OBJECTIVES

Mechanical power is a crucial concept in understanding ventilator-induced lung injury (VILI). We adopted the null hypothesis that under the same mechanical power, resulting from combinations of different static and dynamic variables-some with high stress per cycle and others without-would inflict similar degrees of damage on lung epithelial and endothelial cells as well as on the extracellular matrix in experimental acute respiratory distress syndrome (ARDS). To test this hypothesis, we varied tidal volume (Vt), which correlates with the stretching force per cycle, while adjusting respiratory rate (RR) to yield similar mechanical power values for identical durations across all experimental groups.

DESIGN

Animal study.

SETTING

Laboratory investigation.

SUBJECTS

Thirty male Wistar rats (333 ± 26 g).

INTERVENTIONS

Twenty-four hours after intratracheal administration of Escherichia coli lipopolysaccharide, animals were anesthetized and mechanically ventilated (positive end-expiratory pressure = 3 cm H2O) with combination of Vt and RR sufficient to induce similar mechanical power (n = 8/group): Vt = 6 mL/kg, RR = 140 breaths/minute (low Vt-high RR [LVT-HRR]); Vt = 12 mL/kg, RR = 70 breaths/minute (high Vt-low RR [HVT-LRR]); and Vt = 18 mL/kg, RR = 50 breaths/minute (very-high Vt-very-low RR [VHVT-VLRR]). All groups were ventilated for 80 minutes. A control group, not subjected to mechanical ventilation (MV), was used for molecular biology analyses.

MEASUREMENTS AND MAIN RESULTS

After 80 minutes of MV, lung overdistension, alveolar/interstitial edema, fractional area of E-cadherin, and biomarkers of lung inflammation (interleukin-6), lung stretch (amphiregulin), damage to epithelial (surfactant protein B) and endothelial cells (vascular cell adhesion molecule 1 and angiopoietin-2), and extracellular matrix (versican and syndecan) were higher in group VHVT-VLRR than LVT-HRR. Plateau pressure and driving pressure increased progressively from LVT-HRR to HVT-LRR and VHVT-VLRR.

CONCLUSIONS

In the current experimental model of ARDS, mechanical power alone is insufficient to account for VILI. Instead, the manner in which its components are applied determines the extent of injury at a given mechanical power value.

Bedside Assessment of the Respiratory System During Invasive Mechanical Ventilation

Assessing the respiratory system of a patient receiving mechanical ventilation is complex. We provide an overview of an approach at the bedside underpinned by physiology. We discuss the importance of distinguishing between extensive and intensive ventilatory variables. We outline methods to evaluate both passive patients and those making spontaneous respiratory efforts during assisted ventilation. We believe a comprehensive assessment can influence setting mechanical ventilatory support to achieve lung and diaphragm protective ventilation.

A self-regulated expiratory flow device for mechanical ventilation: a bench study

INTRODUCTION

Unregulated expiratory flow may contribute to ventilator-induced lung injury. The amount of energy dissipated into the lungs with tidal mechanical ventilation may be used to quantify potentially injurious ventilation. Previously reported devices for variable expiratory flow regulation (FLEX) require, either computer-controlled feedback, or an initial expiratory flow trigger. In this bench study we present a novel passive expiratory flow regulation device.

METHODS

The device was tested using a commercially available mechanical ventilator with a range of settings (tidal volume 420 ml and 630 ml, max. inspiratory flow rate 30 L/min and 50 L/min, respiratory rate 10 min-1, positive end-expiratory pressure 5 cmH2O), and a test lung with six different combinations of compliance and resistance settings. The effectiveness of the device was evaluated for reduction in peak expiratory flow, expiratory time, mean airway pressure, and the reduction of tidal dissipated energy (measured as the area within the airway pressure-volume loop).

RESULTS

Maximal and minimal reduction in peak expiratory flow was from 97.18 ± 0.41 L/min to 25.82 ± 0.07 L/min (p < 0.001), and from 44.11 ± 0.42 L/min to 26.30 ± 0.06 L/min, respectively. Maximal prolongation in expiratory time was recorded from 1.53 ± 0.06 s to 3.64 ± 0.21 s (p < 0.001). As a result of the extended expiration, the maximal decrease in I:E ratio was from 1:1.15 ± 0.03 to 1:2.45 ± 0.01 (p < 0.001). The greatest increase in mean airway pressure was from 10.04 ± 0.03 cmH2O to 17.33 ± 0.03 cmH2O. Dissipated energy was significantly reduced with the device under all test conditions (p < 0.001). The greatest reduction in dissipated energy was from 1.74 ± 0.00 J to 0.84 ± 0.00 J per breath. The least reduction in dissipated energy was from 0.30 ± 0.00 J to 0.16 ± 0.00 J per breath. The greatest and least percentage reduction in dissipated energy was 68% and 33%, respectively.

CONCLUSIONS

The device bench tested in this study demonstrated a significant reduction in peak expiratory flow rate and dissipated energy, compared to ventilation with unregulated expiratory flow. Application of the device warrants further experimental and clinical evaluation.

The ventilator of the future: key principles and unmet needs

Persistent shortcomings of invasive positive pressure ventilation make it less than an ideal intervention. Over the course of more than seven decades, clinical experience and scientific investigation have helped define its range of hazards and limitations. Apart from compromised airway clearance and lower airway contamination imposed by endotracheal intubation, the primary hazards inherent to positive pressure ventilation may be considered in three broad categories: hemodynamic impairment, potential for ventilation-induced lung injury, and impairment of the respiratory muscle pump. To optimize care delivery, it is crucial for monitoring and machine outputs to integrate information with the potential to impact the underlying requirements of the patient and/or responses of the cardiopulmonary system to ventilatory interventions. Trending analysis, timely interventions, and closer communication with the caregiver would limit adverse clinical trajectories. Judging from the rapid progress of recent years, we are encouraged to think that insights from physiologic research and emerging technological capability may eventually address important aspects of current deficiencies.

Heart-Lungs interactions: the basics and clinical implications

Heart-lungs interactions are related to the interplay between the cardiovascular and the respiratory system. They result from the respiratory-induced changes in intrathoracic pressure, which are transmitted to the cardiac cavities and to the changes in alveolar pressure, which may impact the lung microvessels. In spontaneously breathing patients, consequences of heart-lungs interactions are during inspiration an increase in right ventricular preload and afterload, a decrease in left ventricular preload and an increase in left ventricular afterload. In mechanically ventilated patients, consequences of heart-lungs interactions are during mechanical insufflation a decrease in right ventricular preload, an increase in right ventricular afterload, an increase in left ventricular preload and a decrease in left ventricular afterload. Physiologically and during normal breathing, heart-lungs interactions do not lead to significant hemodynamic consequences. Nevertheless, in some clinical settings such as acute exacerbation of chronic obstructive pulmonary disease, acute left heart failure or acute respiratory distress syndrome, heart-lungs interactions may lead to significant hemodynamic consequences. These are linked to complex pathophysiological mechanisms, including a marked inspiratory negativity of intrathoracic pressure, a marked inspiratory increase in transpulmonary pressure and an increase in intra-abdominal pressure. The most recent application of heart-lungs interactions is the prediction of fluid responsiveness in mechanically ventilated patients. The first test to be developed using heart-lungs interactions was the respiratory variation of pulse pressure. Subsequently, many other dynamic fluid responsiveness tests using heart-lungs interactions have been developed, such as the respiratory variations of pulse contour-based stroke volume or the respiratory variations of the inferior or superior vena cava diameters. All these tests share the same limitations, the most frequent being low tidal volume ventilation, persistent spontaneous breathing activity and cardiac arrhythmia. Nevertheless, when their main limitations are properly addressed, all these tests can help intensivists in the decision-making process regarding fluid administration and fluid removal in critically ill patients.

Modelling lung diffusion-perfusion limitation in mechanically ventilated SARS-CoV-2 patients

This is the first study to describe the daytime evolution of respiratory parameters in mechanically ventilated COVID-19 patients. The data base refers to patients hospitalised in the intensive care unit (ICU) at Arequipa Hospital (Peru, 2335 m) in 2021. In both survivors (S) and non-survivors (NS) patients, a remarkable decrease in respiratory compliance was observed, revealing a proportional decrease in inflatable alveolar units. The S and NS patients were all hyperventilated and their SatO2 was maintained at >90%. However, while S remained normocapnic, NS developed progressive hypercapnia. We compared the efficiency of O2 uptake and CO2 removal in the air blood barrier relying on a model allowing to partition between diffusion and perfusion limitations to gas exchange. The decrease in O2 uptake was interpreted as diffusion limitation, while the impairment in CO2 removal was modelled by progressive perfusion limitation. The latter correlated with the increase in positive end-expiratory pressure (PEEP) and plateau pressure (Pplat), leading to capillary compression, increased blood velocity, and considerable shortening of the air-blood contact time.

Early and late effects of volatile sedation with sevoflurane on respiratory mechanics of critically ill COPD patients

BACKGROUND

The objective was to compare sevoflurane, a volatile sedation agent with potential bronchodilatory properties, with propofol on respiratory mechanics in critically ill patients with COPD exacerbation.

METHODS

Prospective study in an ICU enrolling critically ill intubated patients with severe COPD exacerbation and comparing propofol and sevoflurane after 1:1 randomisation. Respiratory system mechanics (airway resistance, PEEPi, trapped volume, ventilatory ratio and respiratory system compliance), gas exchange, vitals, safety and outcome were measured at inclusion and then until H48. Total airway resistance change from baseline to H48 in both sevoflurane and propofol groups was the main endpoint.

RESULTS

Sixteen patients were enrolled and were sedated for 126 h(61-228) in the propofol group and 207 h(171-216) in the sevoflurane group. At baseline, airway resistance was 21.6cmH2O/l/s(19.8-21.6) in the propofol group and 20.4cmH2O/l/s(18.6-26.4) in the sevoflurane group, (p = 0.73); trapped volume was 260 ml(176-290) in the propofol group and 73 ml(35-126) in the sevoflurane group, p = 0.02. Intrinsic PEEP was 1.5cmH2O(1-3) in both groups after external PEEP optimization. There was neither early (H4) or late (H48) significant difference in airway resistance and respiratory mechanics parameters between the two groups.

CONCLUSIONS

In critically ill patients intubated with COPD exacerbation, there was no significant difference in respiratory mechanics between sevoflurane and propofol from inclusion to H4 and H48.

Guide to Lung-Protective Ventilation in Cardiac Patients

The incidence of acute respiratory insufficiency has continued to increase among patients admitted to modern-day cardiovascular intensive care units. Positive pressure ventilation (PPV) remains the mainstay of treatment for these patients. Alterations in intrathoracic pressure during PPV has distinct effects on both the right and left ventricles, affecting cardiovascular performance. Lung-protective ventilation (LPV) minimizes the risk of further lung injury through ventilator-induced lung injury and, hence, an understanding of LPV and its cardiopulmonary interactions is beneficial for cardiologists.

Expiratory flow limitation during mechanical ventilation: real-time detection and physiological subtypes

BACKGROUND

Tidal expiratory flow limitation (EFLT) complicates the delivery of mechanical ventilation but is only diagnosed by performing specific manoeuvres. Instantaneous analysis of expiratory resistance (Rex) can be an alternative way to detect EFLT without changing ventilatory settings. This study aimed to determine the agreement of EFLT detection by Rex analysis and the PEEP reduction manoeuvre using contingency table and agreement coefficient. The patterns of Rex were explored.

METHODS

Medical patients ≥ 15-year-old receiving mechanical ventilation underwent a PEEP reduction manoeuvre from 5 cmH2O to zero for EFLT detection. Waveforms were recorded and analyzed off-line. The instantaneous Rex was calculated and was plotted against the volume axis, overlapped by the flow-volume loop for inspection. Lung mechanics, characteristics of the patients, and clinical outcomes were collected. The result of the Rex method was validated using a separate independent dataset.

RESULTS

339 patients initially enrolled and underwent a PEEP reduction. The prevalence of EFLT was 16.5%. EFLT patients had higher adjusted hospital mortality than non-EFLT cases. The Rex method showed 20% prevalence of EFLT and the result was 90.3% in agreement with PEEP reduction manoeuvre. In the validation dataset, the Rex method had resulted in 91.4% agreement. Three patterns of Rex were identified: no EFLT, early EFLT, associated with airway disease, and late EFLT, associated with non-airway diseases, including obesity. In early EFLT, external PEEP was less likely to eliminate EFLT.

CONCLUSIONS

The Rex method shows an excellent agreement with the PEEP reduction manoeuvre and allows real-time detection of EFLT. Two subtypes of EFLT are identified by Rex analysis.

TRIAL REGISTRATION

Clinical trial registered with www.thaiclinicaltrials.org (TCTR20190318003). The registration date was on 18 March 2019, and the first subject enrollment was performed on 26 March 2019.

Setting positive end-expiratory pressure in the severely obstructive patient

PURPOSE OF REVIEW

The response to positive end-expiratory pressure (PEEP) in patients with chronic obstructive pulmonary disease (COPD) requiring mechanical ventilation depends on the underlying pathophysiology. This review focuses on the pathophysiology of COPD, especially intrinsic PEEP (PEEPi) and its consequences, and the benefits of applying external PEEP during assisted ventilation when PEEPi is present.

RECENT FINDINGS

The presence of expiratory airflow limitation and increased airway resistance promotes the development of dynamic hyperinflation in patients with COPD during acute respiratory failure. Dynamic hyperinflation and the associated development of PEEPi increases work of breathing and contributes to ineffective triggering of the ventilator. In the presence of airflow limitation, application of external PEEP during patient-triggered ventilation has been shown to reduce inspiratory effort, facilitate ventilatory triggering and enhance patient-ventilator interaction. To minimize the risk of hyperinflation, it is advisable to limit the level of external PEEP during assisted ventilation after optimization of ventilator settings to about 70% of the level of PEEPi (measured during passive ventilation).

SUMMARY

In patients with COPD and dynamic hyperinflation receiving assisted mechanical ventilation, the application of low levels of external PEEP can minimize work of breathing, facilitate ventilator triggering and improve patient-ventilator interaction.

Incomplete Exhalation during Resuscitation-Theoretical Review and Examples from Ventilation of Newborn Term Infants

BACKGROUND

Newborn resuscitation guidelines recommend positive pressure ventilation (PPV) for newborns who do not establish effective spontaneous breathing after birth. T-piece resuscitator systems are commonly used in high-resource settings and can additionally provide positive end-expiratory pressure (PEEP). Short expiratory time, high resistance, rapid dynamic changes in lung compliance and large tidal volumes increase the possibility of incomplete exhalation. Previous publications indicate that this may occur during newborn resuscitation. Our aim was to study examples of incomplete exhalations in term newborn resuscitation and discuss these against the theoretical background.

METHODS

Examples of flow and pressure data from respiratory function monitors (RFM) were selected from 129 term newborns who received PPV using a T-piece resuscitator. RFM data were not presented to the user during resuscitation.

RESULTS

Examples of incomplete exhalation with higher-than-set PEEP-levels were present in the recordings with visual correlation to factors affecting time needed to complete exhalation.

CONCLUSIONS

Incomplete exhalation and the relationship to expiratory time constants have been well described theoretically. We documented examples of incomplete exhalations with increased PEEP-levels during resuscitation of term newborns. We conclude that RFM data from resuscitations can be reviewed for this purpose and that incomplete exhalations should be further explored, as the clinical benefit or risk of harm are not known.

The Effects of Endotracheal Tube Size During Bronchoscopy in Simulated Models of Intubated Patients

OBJECTIVE

The aim is to use a simulation lung model to assess the possibility of performing bronchoscopy through endotracheal tubes (ETT) less than 8.0-mm while appropriately ventilating patients with normal and ARDS lungs in the setting of SARS-CoV-2.

METHODS

Five SHERIDAN® ETTs were used to ventilate SimMan® 3G under respiratory compliance levels representing normal and severe ARDS lungs. Baseline measurements of peak pressure, plateau pressure, and auto-positive end expiratory pressure (auto-PEEP) were recorded at four different inspiratory times (Ti). Three different-sized disposable bronchoscopes were inserted, and all measurements were repeated.

RESULTS

Normal lung model: Slim bronchoscopes in 6.0-mm ETTs resulted in plateau pressures <30 cm H₂ O, and increasing Ti to minimize peak pressure resulted in low auto-PEEP. Regular bronchoscopes in 7.0-mm ETTs had similar results. Large bronchoscopes in 7.5-mm ETTs generated plateau pressures ranging from 28 to 35 cm H₂ O with modest auto-PEEP. Severe ARDS lung model: Slim bronchoscopes in 6.0-mm ETTs resulted in plateau pressures of 46 and an auto-PEEP of 5 cm H₂ O. Regular bronchoscopes in 7.0-mm ETTs generated similar results. Large bronchoscopes in 8.0-mm ETTs displayed plateau pressures of 44 and an auto-PEEP of 2 cm H₂ O.

CONCLUSION

To mitigate risk of laryngeal injury, larger ETTs during bronchoscopy should be avoided. Our data show bronchoscopy with any ETT causes auto-PEEP and high plateau pressures, especially in lungs with poor compliance; however, ETT less than 7.5 mm can be used when considering several factors. Our data also suggest similar studies in patients with varying degrees of ARDS would be informative.

LEVEL OF EVIDENCE

NA Laryngoscope, 133:147-153, 2023.

A novel method to calculate compliance and airway resistance in ventilated patients

BACKGROUND

The respiratory system's static compliance (C_rs) and airway resistance (R_rs) are measured during an end-inspiratory hold on volume-controlled ventilation (static method). A numerical algorithm is presented to calculate C_rs and R_rs during volume-controlled ventilation on a breath-by-breath basis not requiring an end-inspiratory hold (dynamic method).

METHODS

The dynamic method combines a numerical solution of the equation of motion of the respiratory system with frequency analysis of airway signals. The method was validated experimentally with a one-liter test lung using 300 mL and 400 mL tidal volumes. It also was validated clinically using airway signals sampled at 32.25 Hz stored in a historical database as 131.1-s-long epochs. There were 15 patients in the database having epochs on volume-controlled ventilation with breaths displaying end-inspiratory holds. This allowed for the reliable calculation of paired C_rs and R_rs values using both static and dynamic methods. Epoch mean values for C_rs and R_rs were assessed by both methods and compared in aggregate form and individually for each patient in the study with Pearson's R² and Bland-Altman analysis. Figures are shown as median[IQR].

RESULTS

Experimental method differences in 880 simulated breaths were 0.3[0.2,0.4] mL·cmH₂O^-1 for C_rs and 0[- 0.2,0.2] cmH₂O·s· L^-1 for R_rs. Clinical testing included 78,371 breaths found in 3174 epochs meeting criteria with 24[21,30] breaths per epoch. For the aggregate data, Pearson's R² were 0.99 and 0.94 for C_rs and R_rs, respectively. Bias ± 95% limits of agreement (LOA) were 0.2 ± 1.6 mL·cmH₂O^-1 for C_rs and - 0.2 ± 1.5 cmH₂O·s· L^-1 for R_rs. Bias ± LOA median values for individual patients were 0.6[- 0.2, 1.4] ± 0.9[0.8, 1.2] mL·cmH₂O^-1 for C_rs and - 0.1[- 0.3, 0.2] ± 0.8[0.5, 1.2] cmH₂O·s· L^-1 for R_rs.

DISCUSSION

Experimental and clinical testing produced equivalent paired measurements of C_rs and R_rs by the dynamic and static methods under the conditions tested.

CONCLUSIONS

These findings support to the possibility of using the dynamic method in continuously monitoring respiratory system mechanics in patients on ventilatory support with volume-controlled ventilation.

A structured narrative review of clinical and experimental studies of the use of different positive end-expiratory pressure levels during thoracic surgery

OBJECTIVES

This study aimed to present a review on the general effects of different positive end-expiratory pressure (PEEP) levels during thoracic surgery by qualitatively categorizing the effects into detrimental, beneficial, and inconclusive.

DATA SOURCE

Literature search of Pubmed, CNKI, and Wanfang was made to find relative articles about PEEP levels during thoracic surgery. We used the following keywords as one-lung ventilation, PEEP, and thoracic surgery.

RESULTS

We divide the non-individualized PEEP value into five grades, that is, less than 5, 5, 5-10, 10, and more than 10 cmH₂ O, among which 5 cmH₂ O is the most commonly used in clinic at present to maintain alveolar dilatation and reduce the shunt fraction and the occurrence of atelectasis, whereas individualized PEEP, adjusted by test titration or imaging method to adapt to patients' personal characteristics, can effectively ameliorate intraoperative oxygenation and obtain optimal pulmonary compliance and better indexes relating to respiratory mechanics.

CONCLUSIONS

Available data suggest that PEEP might play an important role in one-lung ventilation, the understanding of which will help in exploring a simple and economical method to set the appropriate PEEP level.

Myths and Misconceptions of Airway Pressure Release Ventilation: Getting Past the Noise and on to the Signal

In the pursuit of science, competitive ideas and debate are necessary means to attain knowledge and expose our ignorance. To quote Murray Gell-Mann (1969 Nobel Prize laureate in Physics): "Scientific orthodoxy kills truth". In mechanical ventilation, the goal is to provide the best approach to support patients with respiratory failure until the underlying disease resolves, while minimizing iatrogenic damage. This compromise characterizes the philosophy behind the concept of "lung protective" ventilation. Unfortunately, inadequacies of the current conceptual model-that focuses exclusively on a nominal value of low tidal volume and promotes shrinking of the "baby lung" - is reflected in the high mortality rate of patients with moderate and severe acute respiratory distress syndrome. These data call for exploration and investigation of competitive models evaluated thoroughly through a scientific process. Airway Pressure Release Ventilation (APRV) is one of the most studied yet controversial modes of mechanical ventilation that shows promise in experimental and clinical data. Over the last 3 decades APRV has evolved from a rescue strategy to a preemptive lung injury prevention approach with potential to stabilize the lung and restore alveolar homogeneity. However, several obstacles have so far impeded the evaluation of APRV's clinical efficacy in large, randomized trials. For instance, there is no universally accepted standardized method of setting APRV and thus, it is not established whether its effects on clinical outcomes are due to the ventilator mode per se or the method applied. In addition, one distinctive issue that hinders proper scientific evaluation of APRV is the ubiquitous presence of myths and misconceptions repeatedly presented in the literature. In this review we discuss some of these misleading notions and present data to advance scientific discourse around the uses and misuses of APRV in the current literature.

Lung Mechanics Over the Century: From Bench to Bedside and Back to Bench

Lung physiology research advanced significantly over the last 100 years. Respiratory mechanics applied to animal models of lung disease extended the knowledge of the workings of respiratory system. In human research, a better understanding of respiratory mechanics has contributed to development of mechanical ventilators. In this review, we explore the use of respiratory mechanics in basic science to investigate asthma and chronic obstructive pulmonary disease (COPD). We also discuss the use of lung mechanics in clinical care and its role on the development of modern mechanical ventilators. Additionally, we analyse some bench-developed technologies that are not in widespread use in the present but can become part of the clinical arsenal in the future. Finally, we explore some of the difficult questions that intensive care doctors still face when managing respiratory failure. Bringing back these questions to bench can help to solve them. Interaction between basic and translational science and human subject investigation can be very rewarding, as in the conceptualization of “Lung Protective Ventilation” principles. We expect this interaction to expand further generating new treatments and managing strategies for patients with respiratory disease.

Respiratory-Cardiovascular Interactions During Mechanical Ventilation: Physiology and Clinical Implications

Positive-pressure inspiration and positive end-expiratory pressure (PEEP) increase pleural, alveolar, lung transmural, and intra-abdominal pressure, which decrease right and left ventricular (RV; LV) preload and LV afterload and increase RV afterload. The magnitude and clinical significance of the resulting changes in ventricular function are determined by the delivered tidal volume, the total level of PEEP, the compliance of the lungs and chest wall, intravascular volume, baseline RV and LV function, and intra-abdominal pressure. In mechanically ventilated patients, the most important, adverse consequences of respiratory-cardiovascular interactions are a PEEP-induced reduction in cardiac output, systemic oxygen delivery, and blood pressure; RV dysfunction in patients with ARDS; and acute hemodynamic collapse in patients with pulmonary hypertension. On the other hand, the hemodynamic changes produced by respiratory-cardiovascular interactions can be beneficial when used to assess volume responsiveness in hypotensive patients and by reducing dyspnea and improving hypoxemia in patients with cardiogenic pulmonary edema. Thus, a thorough understanding of the physiological principles underlying respiratory-cardiovascular interactions is essential if critical care practitioners are to anticipate, recognize, manage, and utilize their hemodynamic effects. © 2022 American Physiological Society. Compr Physiol 12:1-24, 2022.

Effects of time-controlled adaptive ventilation on cardiorespiratory parameters and inflammatory response in experimental emphysema

The time-controlled adaptive ventilation (TCAV) method attenuates lung damage in acute respiratory distress syndrome. However, so far, no study has evaluated the impact of the TCAV method on ventilator-induced lung injury (VILI) and cardiac function in emphysema. We hypothesized that the use of the TCAV method to achieve an expiratory flow termination/expiratory peak flow (E_FT/E_PF) of 25% could reduce VILI and improve right ventricular function in elastase-induced lung emphysema in rats. Five weeks after the last intratracheal instillation of elastase, animals were anesthetized and mechanically ventilated for 1 h using TCAV adjusted to either E_FT/E_PF 25% or E_FT/E_PF 75%, the latter often applied in ARDS. Pressure-controlled ventilation (PCV) groups with positive end-expiratory pressure levels similar to positive end-release pressure in TCAV with E_FT/E_PF 25% and E_FT/E_PF 75% were also analyzed. Echocardiography and lung ultrasonography were monitored. Lung morphometry, alveolar heterogeneity, and biological markers related to inflammation (interleukin [IL]-6, CINC-1), alveolar pulmonary stretch (amphiregulin), lung matrix damage (metalloproteinase [MMP]-9) were assessed. E_FT/E_PF 25% reduced respiratory system peak pressure, mean linear intercept, B lines at lung ultrasonography, and increased pulmonary acceleration time/pulmonary ejection time ratio compared with E_FT/E_PF 75%. The volume fraction of mononuclear cells, neutrophils, and expression of IL-6, CINC-1, amphiregulin, and MMP-9 were lower with E_FT/E_PF 25% than with E_FT/E_PF 75%. In conclusion, TCAV with E_FT/E_PF 25%, compared with E_FT/E_PF 75%, led to less lung inflammation, hyperinflation, and pulmonary arterial hypertension, which may be a promising strategy for patients with emphysema.

Three bedside techniques to quantify dynamic pulmonary hyperinflation in mechanically ventilated patients with chronic obstructive pulmonary disease

BACKGROUND

Dynamic pulmonary hyperinflation may develop in patients with chronic obstructive pulmonary disease (COPD) due to dynamic airway collapse and/or increased airway resistance, increasing the risk of volutrauma and hemodynamic compromise. The reference standard to quantify dynamic pulmonary hyperinflation is the measurement of the volume at end-inspiration (Vei). As this is cumbersome, the aim of this study was to evaluate if methods that are easier to perform at the bedside can accurately reflect Vei.

METHODS

Vei was assessed in COPD patients under controlled protective mechanical ventilation (7 ± mL/kg) on zero end-expiratory pressure, using three techniques in a fixed order: (1) reference standard (Vei_reference): passive exhalation to atmosphere from end-inspiration in a calibrated glass burette; (2) ventilator maneuver (Vei_maneuver): measuring the expired volume during a passive exhalation of 45s using the ventilator flow sensor; (3) formula (Vei_formula): (Vt × P_plateau)/(P_plateau - PEEP_i), with Vt tidal volume, P_plateau is plateau pressure after an end-inspiratory occlusion, and PEEP_i is intrinsic positive end-expiratory pressure after an end-expiratory occlusion. A convenience sample of 17 patients was recruited.

RESULTS

Vei_reference was 1030 ± 380 mL and had no significant correlation with P_plateau (r² = 0.06; P = 0.3710) or PEEP_i (r² = 0.11; P = 0.2156), and was inversely related with P_drive (calculated as P_plateau -PEEP_i) (r² = 0.49; P = 0.0024). A low bias but rather wide limits of agreement and fairly good correlations were found when comparing Vei_maneuver and Vei_formula to Vei_reference. Vei remained stable during the study period (low bias 15 mL with high agreement (95% limits of agreement from - 100 to 130 mL) and high correlation (r² = 0.98; P < 0.0001) between both measurements of Vei_reference).

CONCLUSIONS

In patients with COPD, airway pressures are not a valid representation of Vei. The three techniques to quantify Vei show low bias, but wide limits of agreement.

Discordance Between Respiratory Drive and Sedation Depth in Critically Ill Patients Receiving Mechanical Ventilation

OBJECTIVES

In mechanically ventilated patients, deep sedation is often assumed to induce "respirolysis," that is, lyse spontaneous respiratory effort, whereas light sedation is often assumed to preserve spontaneous effort. This study was conducted to determine validity of these common assumptions, evaluating the association of respiratory drive with sedation depth and ventilator-free days in acute respiratory failure.

DESIGN

Prospective cohort study.

SETTING

Patients were enrolled during 2 month-long periods in 2016-2017 from five ICUs representing medical, surgical, and cardiac specialties at a U.S. academic hospital.

PATIENTS

Eligible patients were critically ill adults receiving invasive ventilation initiated no more than 36 hours before enrollment. Patients with neuromuscular disease compromising respiratory function or expiratory flow limitation were excluded.

INTERVENTIONS

Respiratory drive was measured via P0.1, the change in airway pressure during a 0.1-second airway occlusion at initiation of patient inspiratory effort, every 12 ± 3 hours for 3 days. Sedation depth was evaluated via the Richmond Agitation-Sedation Scale. Analyses evaluated the association of P0.1 with Richmond Agitation-Sedation Scale (primary outcome) and ventilator-free days.

MEASUREMENTS AND MAIN RESULTS

Fifty-six patients undergoing 197 bedside evaluations across five ICUs were included. P0.1 ranged between 0 and 13.3 cm H2O (median [interquartile range], 0.1 cm H2O [0.0-1.3 cm H2O]). P0.1 was not significantly correlated with the Richmond Agitation-Sedation Scale (RSpearman, 0.02; 95% CI, -0.12 to 0.16; p = 0.80). Considering P0.1 terciles (range less than 0.2, 0.2-1.0, and greater than 1.0 cm H2O), patients in the middle tercile had significantly more ventilator-free days than the lowest tercile (incidence rate ratio, 0.78; 95% CI, 0.65-0.93; p < 0.01) or highest tercile (incidence rate ratio, 0.58; 95% CI, 0.48-0.70; p < 0.01).

CONCLUSIONS

Sedation depth is not a reliable marker of respiratory drive during critical illness. Respiratory drive can be low, moderate, or high across the range of routinely targeted sedation depth.

A review of therapies for the overlap syndrome of obstructive sleep apnea and chronic obstructive pulmonary disease

Obstructive sleep apnea (OSA) and chronic obstructive pulmonary disease (COPD) are common chronic diseases. These two noncommunicable diseases (NCDs) are prevalent among approximately 10% of the general population. Approximately 1% of the population is affected by the co-existence of both conditions, known as the overlap syndrome (OS). OS patients suffer from greater degrees of nocturnal oxygen desaturation and cardiovascular consequences than those with either condition in isolation. Besides OS, patients with COPD may suffer from a spectrum of sleep-related breathing disorders, including hypoventilation and central sleep apnea. The article provides an overview of the pathogenesis, associated risk factors, prevalence, and management of sleep-related breathing disorders in COPD. It examines respiratory changes during sleep caused by COPD and OSA. It elaborates upon the factors that link the two conditions together to lead to OS. It also discusses the clinical evaluation and diagnosis of these patients. Subsequently, it reviews the pathophysiological basis and the current evidence for three potential therapies: positive airway pressure therapy [including continuous positive airway pressure (CPAP) and bilevel positive airway pressure], oxygen therapy, and pharmacological therapy. It also proposes a phenotypic approach toward the diagnosis and treatment of OS and the entire spectrum of sleep-related breathing disorders in COPD. It concludes with the current evidence gaps and future areas of research in the management of OS.

Respiratory Mechanics

Today's management of the ventilated patient still relies on the measurement of old parameters such as airway pressures and flow. Graphical presentations reveal the intricacies of patient-ventilator interactions in times of supporting the patient on the ventilator instead of fully ventilating the heavily sedated patient. This opens a new pathway for several bedside technologies based on basic physiologic knowledge; however, it may increase the complexity of measurements. The spread of the COVID-19 infection has confronted the anesthesiologist and intensivist with one of the most severe pulmonary pathologies of the last decades. Optimizing the patient at the bedside is an old and newly required skill for all physicians in the intensive care unit, supported by mobile technologies such as lung ultrasound and electrical impedance tomography. This review summarizes old knowledge and presents a brief insight into extended monitoring options.

Degree of convexity calculated from expiratory flow-volume curves for identifying airway obstruction in nonsedated and nonparalyzed ventilated patients

The predictive performance of applying the degree of convexity in expiratory flow-volume (EFV) curves to detect airway obstruction in ventilated patients has yet to be investigated. We enrolled 33 nonsedated and nonparalyzed mechanically ventilated patients and found that the degree of convexity had a significant negative correlation with FEV₁% predicted. The mean degree of convexity in EFV curves in the chronic obstructive pulmonary disease (COPD) group (n = 18) was significantly higher than that in the non-COPD group (n = 15; 26.37 % ± 11.94 % vs. 17.24 % ± 10.98 %, p = 0.030) at a tidal volume of 12 mL/kg IBW. A degree of convexity in the EFV curve > 16.75 at a tidal volume of 12 mL/kg IBW effectively differentiated COPD from non-COPD (AUC = 0.700, sensitivity = 77.8 %, specificity = 53.3 %, p = 0.051). The degree of convexity calculated from EFV curves may help physicians to identify ventilated patients with airway obstruction.

How to ventilate obstructive and asthmatic patients

Exacerbations are part of the natural history of chronic obstructive pulmonary disease and asthma. Severe exacerbations can cause acute respiratory failure, which may ultimately require mechanical ventilation. This review summarizes practical ventilator strategies for the management of patients with obstructive airway disease. Such strategies include non-invasive mechanical ventilation to prevent intubation, invasive mechanical ventilation, from the time of intubation to weaning, and strategies intended to prevent post-extubation acute respiratory failure. The role of tracheostomy, the long-term prognosis, and potential future adjunctive strategies are also discussed. Finally, the physiological background that underlies these strategies is detailed.

Go with the flow-clinical importance of flow curves during mechanical ventilation: A narrative review

Most clinicians pay attention to tidal volume and airway pressures and their curves during mechanical ventilation. On the other hand, inspiratory–expiratory flow curves also provide a plethora of information, but much less attention is paid to them. Flow curves chronologically show the velocity and direction of inspiration and expiration and are influenced by the respiratory mechanics, the patient’s effort, and the mode of ventilation and its settings. When the ventilator setting does not synchronize with the patient’s respiratory pattern, the patient can easily have worsening breathing effort, patient–ventilator asynchrony, which can lead to prolonged ventilator support or lung injury. The information provided by the flow curves during mechanical ventilation, such as respiratory mechanics, the patient’s effort, and patient–ventilator interactions, are very helpful when adjusting the ventilator setting. If clinicians can monitor and assess the flow curves information appropriately, it can be a useful diagnostic and therapeutic tool at the bedside. There may be association between inspiratory effort and flow, and this may further guide us, especially in the weaning process and when patients are not synchronizing with the ventilator. In this review, we try to gather information about “flow” that is scattered around in the literature and textbooks in one place. We will summarize the different flow waveforms utilized in commonly used ventilator modes with their advantages and disadvantages, information gained by the flow curves (i.e., flow-time, flow-volume, and flow-pressure), how to detect and manage asynchronies, and some ideas for future uses. Flow waveforms shapes and patterns are very beneficial for the management of patients undergoing mechanical ventilatory support. Attention to those waveforms can potentially improve patient outcomes. Clinicians should be familiar with this information and how to act upon them.

Titration of extra-PEEP against intrinsic-PEEP in severe asthma by electrical impedance tomography: A case report and literature review

RATIONALE

The use of extra-positive end-expiratory pressure (PEEP) at a level of 80% intrinsic-PEEP (iPEEP) to improve ventilation in severe asthma patients with control ventilation remains controversial. Electrical impedance tomography (EIT) may provide regional information for determining the optimal extra-PEEP to overcome gas trapping and distribution. Moreover, the experience of using EIT to determine extra-PEEP in severe asthma patients with controlled ventilation is limited.

PATIENTS CONCERNS

A severe asthma patient had 12-cmH2O iPEEP using the end-expiratory airway occlusion method at Zero positive end-expiratory pressures (ZEEP). How to titrate the extra-PEEP to against iPEEP at bedside?

DIAGNOSES AND INTERVENTIONS

An incremental PEEP titration was performed in the severe asthma patient with mechanical ventilation. An occult pendelluft phenomenon of the ventral and dorsal regions was found during the early and late expiration periods when the extra-PEEP was set to <6 cmH2O. If the extra-PEEP was elevated from 4 to 6 cmH2O, a decrease in the end-expiratory lung impedance (EELI) and a disappearance of the pendelluft phenomenon were observed during the PEEP titration. Moreover, there was broad disagreement as to the "best" extra-PEEP settings according to the various EIT parameters. The regional ventilation delay had the lowest extra-PEEP value (10 cmH2O), whereas the value was 12 cmH2O for the lung collapse/overdistension index and 14 cmH2O for global inhomogeneity.

OUTCOMES

The extra-PEEP was set at 6 cmH2O, and the severe whistling sound was improved. The patient's condition further became better under the integrated therapy.

LESSONS

A broad literature review shows that this was the 3rd case of using EIT to titrate an extra-PEEP to against PEEPi. Importantly, the visualization of occult pendelluft and possible air release during incremental PEEP titration was documented for the first time during incremental PEEP titration in patients with severe asthma. Examining the presence of the occult pendelluft phenomenon and changes in the EELI by EIT might be an alternative means for determining an individual's extra-PEEP.

The use of extracorporeal carbon dioxide removal in acute chronic obstructive pulmonary disease exacerbation: a narrative review

Chronic obstructive pulmonary disease (COPD) exacerbation induces hypercapnic respiratory acidosis. Extracorporeal carbon dioxide removal (ECCO2R) aims to eliminate blood carbon dioxide (CO2) in order to reduce adverse effects from hypercapnia and the related acidosis. Hypercapnia has deleterious extra-pulmonary consequences in increasing intracranial pressure and inducing and/or worsening right heart failure. During COPD exacerbation, the use of ECCO2R may improve the efficacy of non-invasive ventilation (NIV) in terms of CO2 removal, decrease respiratory rate and reduce dynamic hyperinflation and intrinsic positive end expiratory pressure, which all contribute to increasing dead space. Moreover, ECCO2R may prevent NIV failure while facilitating the weaning of intubated patients from mechanical ventilation. In this review of the literature, the authors will present the current knowledge on the pathophysiology related to COPD, the principles of the ECCO2R technique and its role in acute and severe decompensation of COPD. However, despite technical advances, there are only case series in the literature and few prospective studies to clearly establish the role of ECCO2R in acute and severe COPD decompensation.

Dynamic hyperinflation and intrinsic positive end-expiratory pressure in ARDS patients

Background

In ARDS patients, changes in respiratory mechanical properties and ventilatory settings can cause incomplete lung deflation at end-expiration. Both can promote dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEP). The aim of this study was to investigate, in a large population of ARDS patients, the presence of intrinsic PEEP, possible associated factors (patients’ characteristics and ventilator settings), and the effects of two different external PEEP levels on the intrinsic PEEP.

Methods

We made a secondary analysis of published data. Patients were ventilated with a tidal volume of 6–8 mL/kg of predicted body weight, sedated, and paralyzed. After a recruitment maneuver, a PEEP trial was run at 5 and 15 cmH₂O, and partitioned mechanics measurements were collected after 20 min of stabilization. Lung computed tomography scans were taken at 5 and 45 cmH₂O. Patients were classified into two groups according to whether or not they had intrinsic PEEP at the end of an expiratory pause.

Results

We enrolled 217 sedated, paralyzed patients: 87 (40%) had intrinsic PEEP with a median of 1.1 [1.0–2.3] cmH₂O at 5 cmH₂O of PEEP. The intrinsic PEEP significantly decreased with higher PEEP (1.1 [1.0–2.3] vs 0.6 [0.0–1.0] cmH₂O; p < 0.001). The applied tidal volume was significantly lower (480 [430–540] vs 520 [445–600] mL at 5 cmH₂O of PEEP; 480 [430–540] vs 510 [430–590] mL at 15 cmH₂O) in patients with intrinsic PEEP, while the respiratory rate was significantly higher (18 [15–20] vs 15 [13–19] bpm at 5 cmH₂O of PEEP; 18 [15–20] vs 15 [13–19] bpm at 15 cmH₂O). At both PEEP levels, the total airway resistance and compliance of the respiratory system were not different in patients with and without intrinsic PEEP. The total lung gas volume and lung recruitability were also not different between patients with and without intrinsic PEEP (respectively 961 [701–1535] vs 973 [659–1433] mL and 15 [0–32] % vs 22 [0–36] %).

Conclusions

In sedated, paralyzed ARDS patients without a known obstructive disease, the amount of intrinsic PEEP during lung-protective ventilation is negligible and does not influence respiratory mechanical properties.

Clinical Guideline for Treating Acute Respiratory Insufficiency with Invasive Ventilation and Extracorporeal Membrane Oxygenation: Evidence-Based Recommendations for Choosing Modes and Setting Parameters of Mechanical Ventilation

For patients with acute respiratory insufficiency, mechanical ("invasive") ventilation is a fundamental therapeutic measure to ensure sufficient gas exchange. Despite decades of strong research efforts, central questions on mechanical ventilation therapy are still answered incompletely. Therefore, many different ventilation modes and settings have been used in daily clinical practice without scientifically sound bases. At the same time, implementation of the few evidence-based therapeutic concepts (e.g., "lung protective ventilation") into clinical practice is still insufficient. The aim of our guideline project "Mechanical ventilation and extracorporeal gas exchange in acute respiratory insufficiency" was to develop an evidence-based decision aid for treating patients with and on mechanical ventilation. It covers the whole pathway of invasively ventilated patients (including indications of mechanical ventilation, ventilator settings, additional and rescue therapies, and liberation from mechanical ventilation). To assess the quality of scientific evidence and subsequently derive recommendations, we applied the Grading of Recommendations, Assessment, Development and Evaluation method. For the first time, using this globally accepted methodological standard, our guideline contains recommendations on mechanical ventilation therapy not only for acute respiratory distress syndrome patients but also for all types of acute respiratory insufficiency. This review presents the two main chapters of the guideline on choosing the mode of mechanical ventilation and setting its parameters. The guideline group aimed that - by thorough implementation of the recommendations - critical care teams may further improve the quality of care for patients suffering from acute respiratory insufficiency. By identifying relevant gaps of scientific evidence, the guideline group intended to support the development of important research projects.

Invasive Mechanical Ventilation

Invasive mechanical ventilation is a potentially lifesaving intervention for acutely ill patients. The goal of this review is to provide a concise, clinically focused overview of basic invasive mechanical ventilation for the many clinicians who care for mechanically ventilated patients. Attention is given to how common ventilator modes differ in delivering a mechanical breath, evaluation of respiratory system mechanics, how to approach acute changes in airway pressure, and the diagnosis of auto-positive end-expiratory pressure. Waveform interpretation is emphasized throughout the review.

Power to mechanical power to minimize ventilator-induced lung injury?

Mechanical ventilation is a life-supportive therapy, but can also promote damage to pulmonary structures, such as epithelial and endothelial cells and the extracellular matrix, in a process referred to as ventilator-induced lung injury (VILI). Recently, the degree of VILI has been related to the amount of energy transferred from the mechanical ventilator to the respiratory system within a given timeframe, the so-called mechanical power. During controlled mechanical ventilation, mechanical power is composed of parameters set by the clinician at the bedside—such as tidal volume (V_T), airway pressure (Paw), inspiratory airflow (V′), respiratory rate (RR), and positive end-expiratory pressure (PEEP) level—plus several patient-dependent variables, such as peak, plateau, and driving pressures. Different mathematical equations are available to calculate mechanical power, from pressure-volume (PV) curves to more complex formulas which consider both dynamic (kinetic) and static (potential) components; simpler methods mainly consider the dynamic component. Experimental studies have reported that, even at low levels of mechanical power, increasing V_T causes lung damage. Mechanical power should be normalized to the amount of ventilated pulmonary surface; the ratio of mechanical power to the alveolar area exposed to energy delivery is called “intensity.” Recognizing that mechanical power may reflect a conjunction of parameters which may predispose to VILI is an important step toward optimizing mechanical ventilation in critically ill patients. However, further studies are needed to clarify how mechanical power should be taken into account when choosing ventilator settings.

Effects of breathing exercises using home-based positive pressure in the expiratory phase in patients with COPD

BACKGROUND

Patients with chronic obstructive pulmonary disease (COPD) commonly have higher intrinsic positive end-expiratory pressure (PEEPi). A breathing exercise programme strategy employing an appropriate PEEP may improve their pulmonary functional capacity, exercise tolerance and health-related quality of life. Breathing with an expiratory resistive load, which is a method of modulating spontaneous breathing against PEEPi, has not been fully studied in patients with COPD. The objective of this study was to investigate the role of changing spontaneous breathing in home-based conditions and regulating spontaneous breathing with breathing exercises in patients with COPD.

METHODS

This was a prospective randomised trial including 64 patients with a diagnosis of stage III or IV COPD. Patients were randomised into two groups: standard treatment and standard treatment combined with breathing exercise rehabilitation. The effects of the treatments on the COPD assessment test (CAT) score, 6-minute walk test (6MWT) results and pulmonary function were compared at 0, 6, 12 and 18 months within and between the two groups.

RESULTS

All outcomes showed no significant differences between the two groups at the beginning of the study, while the 6MWT and CAT scores exhibited clinically and statistically significant improvements (p<0.001) by the end of the study. At month 18, the change in the predicted percentage of forced expiratory volume in 1 s (FEV₁%pred) differed between the two groups (p<0.05). In addition, there were statistically significant differences in the 6MWT results, CAT scores and FEV₁%pred values between the baseline and month 18 (p<0.0001) in the intervention group.

CONCLUSIONS

Improvements in 6MWT results, pulmonary function and CAT scores are associated with a successful response to breathing against PEEPi in patients with COPD.

TRIAL REGISTRATION

This trial was registered at research registry.com (identifier research registry 4816).

Endotoxin-Induced Emphysema Exacerbation: A Novel Model of Chronic Obstructive Pulmonary Disease Exacerbations Causing Cardiopulmonary Impairment and Diaphragm Dysfunction

Chronic obstructive pulmonary disease (COPD) is a progressive disorder of the lung parenchyma which also involves extrapulmonary manifestations, such as cardiovascular impairment, diaphragm dysfunction, and frequent exacerbations. The development of animal models is important to elucidate the pathophysiology of COPD exacerbations and enable analysis of possible therapeutic approaches. We aimed to characterize a model of acute emphysema exacerbation and evaluate its consequences on the lung, heart, and diaphragm. Twenty-four Wistar rats were randomly assigned into one of two groups: control (C) or emphysema (ELA). In ELA group, animals received four intratracheal instillations of pancreatic porcine elastase (PPE) at 1-week intervals. The C group received saline under the same protocol. Five weeks after the last instillation, C and ELA animals received saline (SAL) or E. coli lipopolysaccharide (LPS) (200 μg in 200 μl) intratracheally. Twenty-four hours after saline or endotoxin administration, arterial blood gases, lung inflammation and morphometry, collagen fiber content, and lung mechanics were analyzed. Echocardiography, diaphragm ultrasonography (US), and computed tomography (CT) of the chest were done. ELA-LPS animals, compared to ELA-SAL, exhibited decreased arterial oxygenation; increases in alveolar collapse (p < 0.0001), relative neutrophil counts (p = 0.007), levels of cytokine-induced neutrophil chemoattractant-1, interleukin (IL)-1β, tumor necrosis factor-α, IL-6, and vascular endothelial growth factor in lung tissue, collagen fiber deposition in alveolar septa, airways, and pulmonary vessel walls, and dynamic lung elastance (p < 0.0001); reduced pulmonary acceleration time/ejection time ratio, (an indirect index of pulmonary arterial hypertension); decreased diaphragm thickening fraction and excursion; and areas of emphysema associated with heterogeneous alveolar opacities on chest CT. In conclusion, we developed a model of endotoxin-induced emphysema exacerbation that affected not only the lungs but also the heart and diaphragm, thus resembling several features of human disease. This model of emphysema should allow preclinical testing of novel therapies with potential for translation into clinical practice.

Alternative Modes of Mechanical Ventilation

Modern mechanical ventilators are more complex than those first developed in the 1950s. Newer ventilation modes can be difficult to understand and implement clinically, although they provide more treatment options than traditional modes. These newer modes, which can be considered alternative or nontraditional, generally are classified as either volume controlled or pressure controlled. Dual-control modes incorporate qualities of pressure-controlled and volume-controlled modes. Some ventilation modes provide variable ventilatory support depending on patient effort and may be classified as closed-loop ventilation modes. Alternative modes of ventilation are tools for lung protection, alveolar recruitment, and ventilator liberation. Understanding the function and application of these alternative modes prior to implementation is essential and is most beneficial for the patient.

Circulatory Collapse due to Hyperinflation in a Patient with Tracheobronchomalacia: A Case Report and Brief Review

We present a case of repeated cardiac arrests derived from dynamic hyperinflation in a patient with severe tracheobronchomalacia. Mechanical ventilation led to auto-PEEP with hemodynamic impairment and pulseless electric activity. Adjusted ventilation settings, deep sedation, and muscle paralysis followed by acute stenting of the affected collapsing airways restored ventilation and prevented recurrent circulatory collapse. We briefly review the pathophysiology and treatment options in patients with dynamic hyperinflation.

The basics of respiratory mechanics: ventilator-derived parameters

Mechanical ventilation is a life-support system used to maintain adequate lung function in patients who are critically ill or undergoing general anesthesia. The benefits and harms of mechanical ventilation depend not only on the operator's setting of the machine (input), but also on their interpretation of ventilator-derived parameters (outputs), which should guide ventilator strategies. Once the inputs-tidal volume (V_T), positive end-expiratory pressure (PEEP), respiratory rate (RR), and inspiratory airflow (V')-have been adjusted, the following outputs should be measured: intrinsic PEEP, peak (Ppeak) and plateau (Pplat) pressures, driving pressure (ΔP), transpulmonary pressure (P_L), mechanical energy, mechanical power, and intensity. During assisted mechanical ventilation, in addition to these parameters, the pressure generated 100 ms after onset of inspiratory effort (P_0.1) and the pressure-time product per minute (PTP/min) should also be evaluated. The aforementioned parameters should be seen as a set of outputs, all of which need to be strictly monitored at bedside in order to develop a personalized, case-by-case approach to mechanical ventilation. Additionally, more clinical research to evaluate the safe thresholds of each parameter in injured and uninjured lungs is required.

Improved lung recruitment and oxygenation during mandatory ventilation with a new expiratory ventilation assistance device: A controlled interventional trial in healthy pigs

BACKGROUND

In contrast to conventional mandatory ventilation, a new ventilation mode, expiratory ventilation assistance (EVA), linearises the expiratory tracheal pressure decline.

OBJECTIVE

We hypothesised that due to a recruiting effect, linearised expiration oxygenates better than volume controlled ventilation (VCV). We compared the EVA with VCV mode with regard to gas exchange, ventilation volumes and pressures and lung aeration in a model of peri-operative mandatory ventilation in healthy pigs.

DESIGN

Controlled interventional trial.

SETTING

Animal operating facility at a university medical centre.

ANIMALS

A total of 16 German Landrace hybrid pigs.

INTERVENTION

The lungs of anaesthetised pigs were ventilated with the EVA mode (n=9) or VCV (control, n=7) for 5 h with positive end-expiratory pressure of 5 cmH2O and tidal volume of 8 ml kg. The respiratory rate was adjusted for a target end-tidal CO2 of 4.7 to 6 kPa.

MAIN OUTCOME MEASURES

Tracheal pressure, minute volume and arterial blood gases were recorded repeatedly. Computed thoracic tomography was performed to quantify the percentages of normally and poorly aerated lung tissue.

RESULTS

Two animals in the EVA group were excluded due to unstable ventilation (n=1) or unstable FiO2 delivery (n=1). Mean tracheal pressure and PaO2 were higher in the EVA group compared with control (mean tracheal pressure: 11.6 ± 0.4 versus 9.0 ± 0.3 cmH2O, P < 0.001 and PaO2: 19.2 ± 0.7 versus 17.5 ± 0.4 kPa, P = 0.002) with comparable peak inspiratory tracheal pressure (18.3 ± 0.9 versus 18.0 ± 1.2 cmH2O, P > 0.99). Minute volume was lower in the EVA group compared with control (5.5 ± 0.2 versus 7.0 ± 1.0 l min, P = 0.02) with normoventilation in both groups (PaCO2 5.4 ± 0.3 versus 5.5 ± 0.3 kPa, P > 0.99). In the EVA group, the percentage of normally aerated lung tissue was higher (81.0 ± 3.6 versus 75.8 ± 3.0%, P = 0.017) and of poorly aerated lung tissue lower (9.5 ± 3.3 versus 15.7 ± 3.5%, P = 0.002) compared with control.

CONCLUSION

EVA ventilation improves lung aeration via elevated mean tracheal pressure and consequently improves arterial oxygenation at unaltered positive end-expiratory pressure (PEEP) and peak inspiratory pressure (PIP). These findings suggest the EVA mode is a new approach for protective lung ventilation.

Positive Pressure Ventilation in the Cardiac Intensive Care Unit

Contemporary cardiac intensive care units (CICUs) provide care for an aging and increasingly complex patient population. The medical complexity of this population is partly driven by an increased proportion of patients with respiratory failure needing noninvasive or invasive positive pressure ventilation (PPV). PPV often plays an important role in the management of patients with cardiogenic pulmonary edema, cardiogenic shock, or cardiac arrest, and those undergoing mechanical circulatory support. Noninvasive PPV, when appropriately applied to selected patients, may reduce the need for invasive mechanical PPV and improve survival. Invasive PPV can be lifesaving, but has both favorable and unfavorable interactions with left and right ventricular physiology and carries a risk of complications that influence CICU mortality. Effective implementation of PPV requires an understanding of the underlying cardiac and pulmonary pathophysiology. Cardiologists who practice in the CICU should be proficient with the indications, appropriate selection, potential cardiopulmonary interactions, and complications of PPV.

Physiology-guided management of hemodynamics in acute respiratory distress syndrome

Skillfully implemented mechanical ventilation (MV) may prove of immense benefit in restoring physiologic homeostasis. However, since hemodynamic instability is a primary factor influencing mortality in acute respiratory distress syndrome (ARDS), clinicians should be vigilant regarding the potentially deleterious effects of MV on right ventricular (RV) function and pulmonary vascular mechanics (PVM). During both spontaneous and positive pressure MV (PPMV), tidal changes in pleural pressure (P_PL), transpulmonary pressure (P_TP, the difference between alveolar pressure and P_PL), and lung volume influence key components of hemodynamics: preload, afterload, heart rate, and myocardial contractility. Acute cor pulmonale (ACP), which occurs in 20-25% of ARDS cases, emerges from negative effects of lung pathology and inappropriate changes in P_PL and P_TP on the pulmonary microcirculation during PPMV. Functional, minimally invasive hemodynamic monitoring for tracking cardiac performance and output adequacy is integral to effective care. In this review we describe a physiology-based approach to the management of hemodynamics in the setting of ARDS: avoiding excessive cardiac demand, regulating fluid balance, optimizing heart rate, and keeping focus on the pulmonary circuit as cornerstones of effective hemodynamic management for patients in all forms of respiratory failure.

Biologic Impact of Mechanical Power at High and Low Tidal Volumes in Experimental Mild Acute Respiratory Distress Syndrome

Background

The authors hypothesized that low tidal volume (VT) would minimize ventilator-induced lung injury regardless of the degree of mechanical power. The authors investigated the impact of power, obtained by different combinations of VT and respiratory rate (RR), on ventilator-induced lung injury in experimental mild acute respiratory distress syndrome (ARDS).

Methods

Forty Wistar rats received Escherichia coli lipopolysaccharide intratracheally. After 24 h, 32 rats were randomly assigned to be mechanically ventilated (2 h) with a combination of different VT (6 ml/kg and 11 ml/kg) and RR that resulted in low and high power. Power was calculated as energy (ΔP,L2/E,L) × RR (ΔP,L = transpulmonary driving pressure; E,L = lung elastance), and was threefold higher in high than in low power groups. Eight rats were not mechanically ventilated and used for molecular biology analysis.

Results

Diffuse alveolar damage score, which represents the severity of edema, atelectasis, and overdistension, was increased in high VT compared to low VT, in both low (low VT: 11 [9 to 14], high VT: 18 [15 to 20]) and high (low VT: 19 [16 to 25], high VT: 29 [27 to 30]) power groups. At high VT, interleukin-6 and amphiregulin expressions were higher in high-power than in low-power groups. At high power, amphiregulin and club cell protein 16 expressions were higher in high VT than in low VT. Mechanical energy and power correlated well with diffuse alveolar damage score and interleukin-6, amphiregulin, and club cell protein 16 expression.

Conclusions

In experimental mild ARDS, even at low VT, high mechanical power promoted ventilator-induced lung injury. To minimize ventilator-induced lung injury, low VT should be combined with low power.

Expiratory Flow Limitation During Mechanical Ventilation

Expiratory flow limitation (EFL) is present when the flow cannot rise despite an increase in the expiratory driving pressure. The mechanisms of EFL are debated but are believed to be related to the collapsibility of small airways. In patients who are mechanically ventilated, EFL can exist during tidal ventilation, representing an extreme situation in which lung volume cannot decrease, regardless of the expiratory driving forces. It is a key factor for the generation of auto- or intrinsic positive end-expiratory pressure (PEEP) and requires specific management such as positioning and adjustment of external PEEP. EFL can be responsible for causing dyspnea and patient-ventilator dyssynchrony, and it is influenced by the fluid status of the patient. EFL frequently affects patients with COPD, obesity, and heart failure, as well as patients with ARDS, especially at low PEEP. EFL is, however, most often unrecognized in the clinical setting despite being associated with complications of mechanical ventilation and poor outcomes such as postoperative pulmonary complications, extubation failure, and possibly airway injury in ARDS. Therefore, prompt recognition might help the management of patients being mechanically ventilated who have EFL and could potentially influence outcome. EFL can be suspected by using different means, and this review summarizes the methods to specifically detect EFL during mechanical ventilation.

Extracorporeal carbon dioxide removal in acute exacerbations of chronic obstructive pulmonary disease

Extracorporeal carbon dioxide removal (ECCO2R) has been proposed as an adjunctive intervention to avoid worsening respiratory acidosis, thereby preventing or shortening the duration of invasive mechanical ventilation (IMV) in patients with exacerbation of chronic obstructive pulmonary disease (COPD). This review will present a comprehensive summary of the pathophysiological rationale and clinical evidence of ECCO2R in patients suffering from severe COPD exacerbations.

Esophageal pressure: research or clinical tool?

Esophageal manometry has traditionally been utilized for respiratory physiology research, but clinicians have recently found numerous applications within the intensive care unit. Esophageal pressure (P_Es) is a surrogate for pleural pressures (P_Pl), and the difference between airway pressure (P_AO) and P_Es provides a good estimate for the pressure across the lung also known as the transpulmonary pressure (P_L). Differentiating the effects of mechanical ventilation and spontaneous breathing on the respiratory system, chest wall, and across the lung allows for improved personalization in clinical decision making. Measuring P_L in acute respiratory distress syndrome (ARDS) may help set positive end expiratory pressure (PEEP) to prevent derecruitment and atelectrauma, while assuring peak pressures do not cause over distension during tidal breathing and recruitment maneuvers. Monitoring P_Es allows improved insight into patient-ventilator interactions and may help in decisions to adjust sedation and paralytics to correct dyssynchrony. Intrinsic PEEP (auto-PEEP) may be monitored using esophageal manometry, which may also improve patient comfort and synchrony with the ventilator. Finally, during weaning, P_Es may be used to better predict weaning success and allow for rapid intervention during failure. Improved consistency in definition and terminology and further outcomes research is needed to encourage more widespread adoption; however, with clear clinical benefit and increased ease of use, it appears time to reintroduce basic physiology into personalized ventilator management in the intensive care unit.

Positive end-expiratory pressure attenuates hemodynamic effects induced by an overload of inspiratory muscles in patients with COPD

BACKGROUND

Inspiratory muscle training (IMT) using a Threshold^® device is commonly used to improve the strength and endurance of inspiratory muscles. However, the effect of IMT, alone or with positive end-expiratory pressure (PEEP), on hemodynamic parameters in patients with chronic obstructive pulmonary disease (COPD) remains unknown.

OBJECTIVE

To assess the effects of an overload of inspiratory muscles using IMT fixed at 30% of the maximal inspiratory pressure (MIP), and IMT associated with 5 cmH₂O of PEEP (IMT + PEEP), on the echocardiographic parameters in healthy subjects and patients with COPD.

METHODS

Twenty patients with COPD (forced expiratory volume in 1 second 53.19±24.71 pred%) and 15 age-matched healthy volunteers were evaluated using spirometry, MIP, the COPD assessment test (CAT), and the modified Medical Research Council (mMRC) dyspnea scale. The E- (fast-filling phase) and A- (atrial contraction phase) waves were evaluated at the tricuspid and mitral valves during inspiration and expiration in the following sequence: at basal conditions, using IMT, and using IMT + PEEP.

RESULTS

Patients with COPD had reduced MIPs versus the control group. Ten patients had CAT scores <10 and 12 patients had mMRC scores <2. E-wave values at the mitral valve were significantly decreased with IMT during the inspiratory phase in both groups. These effects were normalized with IMT + PEEP. During the expiratory phase, use of IMT + PEEP normalized the reduction in E-wave values in the COPD group. During inspiration at the tricuspid valve, reduction in E-wave values during IMT was normalized by IMT + PEEP in COPD group. During the expiratory phase, the value of the E-waves was significantly reduced with overload of the inspiratory muscles in both groups, and these effects were normalized with IMT + PEEP. A-waves did not change under any conditions.

CONCLUSION

Acute hemodynamic effects induced by overloading of the inspiratory muscles were attenuated and/or reversed by the addition of PEEP in COPD patients.