Reverse triggering ? a novel or previously missed phenomenon?

BACKGROUND: Reverse triggering (RT) was described in 2013 as a form of patient-ventilator asynchrony, where patient's respiratory effort follows mechanical insufflation. Diagnosis requires esophageal pressure (Pes) or diaphragmatic electrical activity (EAdi), but RT can also be diagnosed using standard ventilator waveforms. HYPOTHESIS: We wondered (1) how frequently RT would be present but undetected in the figures from literature, especially before 2013; (2) whether it would be more prevalent in the era of small tidal volumes after 2000. METHODS: We searched PubMed, EMBASE, and the Cochrane Central Register of Controlled Trials, from 1950 to 2017, with key words related to asynchrony to identify papers with figures including ventilator waveforms expected to display RT if present. Experts labelled waveforms. 'Definite' RT was identified when Pes or EAdi were in the tracing, and 'possible' RT when only flow and pressure waveforms were present. Expert assessment was compared to the author's descriptions of waveforms. RESULTS: We found 65 appropriate papers published from 1977 to now, containing 181 ventilator waveforms. 21 cases of 'possible' RT and 25 cases of 'definite' RT were identified by the experts. 18.8% of waveforms prior to 2013 had evidence of RT. Most cases were published after 2000 (1 before vs. 45 after, p = 0.03). 54% of RT cases were attributed to different phenomena. A few cases of identified RT were already described prior to 2013 using different terminology (earliest in 1997). While RT cases attributed to different phenomena decreased after 2013, 60% of 'possible' RT remained missed. CONCLUSION: RT has been present in the literature as early as 1997, but most cases were found after the introduction of low tidal volume ventilation in 2000. Following 2013, the number of undetected cases decreased, but RT are still commonly missed. Reverse Triggering, A Missed Phenomenon in the Literature. Critical Care Canada Forum 2019 Abstracts. Can J Anesth/J Can Anesth 67 (Suppl 1), 1-162 (2020). https://doi-org.myaccess.library.utoronto.ca/ https://doi.org/10.1007/s12630-019-01552-z .

Electrical Impedance Tomography to Monitor Hypoxemic Respiratory Failure.

Hypoxemic respiratory failure is one of the leading causes of mortality in intensive care. Frequent assessment of individual physiological characteristics and delivery of personalized mechanical ventilation (MV) settings is a constant challenge for clinicians caring for these patients. Electrical impedance tomography (EIT) is a radiation-free bedside monitoring device that is able to assess regional lung ventilation and changes in aeration. With real-time tomographic functional images of the lungs obtained through a thoracic belt, clinicians can visualize and estimate the distribution of ventilation at different ventilation settings or following procedures such as prone positioning. Several studies have evaluated the performance of EIT to monitor the effects of different MV settings in patients with acute respiratory distress syndrome, allowing more personalized MV. For instance, EIT could help clinicians find the positive end-expiratory pressure that represents a compromise between recruitment and overdistension and assess the effect of prone positioning on ventilation distribution. The clinical impact of the personalization of MV remains to be explored. Despite inherent limitations such as limited spatial resolution, EIT also offers a unique noninvasive bedside assessment of regional ventilation changes in the ICU. This technology offers the possibility of a continuous, operator-free diagnosis and real-time detection of common problems during MV. This review provides an overview of the functioning of EIT, its main indices, and its performance in monitoring patients with acute respiratory failure. Future perspectives for use in intensive care are also addressed.

Lung Recruitment Assessed by Electrical Impedance Tomography (RECRUIT): A Multicenter Study of COVID-19 Acute Respiratory Distress Syndrome.

Rationale: Defining lung recruitability is needed for safe positive end-expiratory pressure (PEEP) selection in mechanically ventilated patients. However, there is no simple bedside method including both assessment of recruitability and risks of overdistension as well as personalized PEEP titration. Objectives: To describe the range of recruitability using electrical impedance tomography (EIT), effects of PEEP on recruitability, respiratory mechanics and gas exchange, and a method to select optimal EIT-based PEEP. Methods: This is the analysis of patients with coronavirus disease (COVID-19) from an ongoing multicenter prospective physiological study including patients with moderate-severe acute respiratory distress syndrome of different causes. EIT, ventilator data, hemodynamics, and arterial blood gases were obtained during PEEP titration maneuvers. EIT-based optimal PEEP was defined as the crossing point of the overdistension and collapse curves during a decremental PEEP trial. Recruitability was defined as the amount of modifiable collapse when increasing PEEP from 6 to 24 cm H2O (ΔCollapse24-6). Patients were classified as low, medium, or high recruiters on the basis of tertiles of ΔCollapse24-6. Measurements and Main Results: In 108 patients with COVID-19, recruitability varied from 0.3% to 66.9% and was unrelated to acute respiratory distress syndrome severity. Median EIT-based PEEP differed between groups: 10 versus 13.5 versus 15.5 cm H2O for low versus medium versus high recruitability (P < 0.05). This approach assigned a different PEEP level from the highest compliance approach in 81% of patients. The protocol was well tolerated; in four patients, the PEEP level did not reach 24 cm H2O because of hemodynamic instability. Conclusions: Recruitability varies widely among patients with COVID-19. EIT allows personalizing PEEP setting as a compromise between recruitability and overdistension. Clinical trial registered with www.clinicaltrials.gov (NCT04460859).

A novel capnogram analysis to guide ventilation during cardiopulmonary resuscitation: clinical and experimental observations.

BACKGROUND: Cardiopulmonary resuscitation (CPR) decreases lung volume below the functional residual capacity and can generate intrathoracic airway closure. Conversely, large insufflations can induce thoracic distension and jeopardize circulation. The capnogram (CO2 signal) obtained during continuous chest compressions can reflect intrathoracic airway closure, and we hypothesized here that it can also indicate thoracic distension. OBJECTIVES: To test whether a specific capnogram may identify thoracic distension during CPR and to assess the impact of thoracic distension on gas exchange and hemodynamics. METHODS: (1) In out-of-hospital cardiac arrest patients, we identified on capnograms three patterns: intrathoracic airway closure, thoracic distension or regular pattern. An algorithm was designed to identify them automatically. (2) To link CO2 patterns with ventilation, we conducted three experiments: (i) reproducing the CO2 patterns in human cadavers, (ii) assessing the influence of tidal volume and respiratory mechanics on thoracic distension using a mechanical lung model and (iii) exploring the impact of thoracic distension patterns on different circulation parameters during CPR on a pig model. MEASUREMENTS AND MAIN RESULTS: (1) Clinical data: 202 capnograms were collected. Intrathoracic airway closure was present in 35%, thoracic distension in 22% and regular pattern in 43%. (2) Experiments: (i) Higher insufflated volumes reproduced thoracic distension CO2 patterns in 5 cadavers. (ii) In the mechanical lung model, thoracic distension patterns were associated with higher volumes and longer time constants. (iii) In six pigs during CPR with various tidal volumes, a CO2 pattern of thoracic distension, but not tidal volume per se, was associated with a significant decrease in blood pressure and cerebral perfusion. CONCLUSIONS: During CPR, capnograms reflecting intrathoracic airway closure, thoracic distension or regular pattern can be identified. In the animal experiment, a thoracic distension pattern on the capnogram is associated with a negative impact of ventilation on blood pressure and cerebral perfusion during CPR, not predicted by tidal volume per se.

The PROMIZING trial enrollment algorithm for early identification of patients ready for unassisted breathing.

BACKGROUND: Liberating patients from mechanical ventilation (MV) requires a systematic approach. In the context of a clinical trial, we developed a simple algorithm to identify patients who tolerate assisted ventilation but still require ongoing MV to be randomized. We report on the use of this algorithm to screen potential trial participants for enrollment and subsequent randomization in the Proportional Assist Ventilation for Minimizing the Duration of MV (PROMIZING) study. METHODS: The algorithm included five steps: enrollment criteria, pressure support ventilation (PSV) tolerance trial, weaning criteria, continuous positive airway pressure (CPAP) tolerance trial (0 cmH2O during 2 min) and spontaneous breathing trial (SBT): on fraction of inspired oxygen (FiO2) 40% for 30-120 min. Patients who failed the weaning criteria, CPAP Zero trial, or SBT were randomized. We describe the characteristics of patients who were initially enrolled, but passed all steps in the algorithm and consequently were not randomized. RESULTS: Among the 374 enrolled patients, 93 (25%) patients passed all five steps. At time of enrollment, most patients were on PSV (87%) with a mean (± standard deviation) FiO2 of 34 (± 6) %, PSV of 8.7 (± 2.9) cmH2O, and positive end-expiratory pressure of 6.1 (± 1.6) cmH2O. Minute ventilation was 9.0 (± 3.1) L/min with a respiratory rate of 17.4 (± 4.4) breaths/min. Patients were liberated from MV with a median [interquartile range] delay between initial screening and extubation of 5 [1-49] hours. Only 7 (8%) patients required reintubation. CONCLUSION: The trial algorithm permitted identification of 93 (25%) patients who were ready to extubate, while their clinicians predicted a duration of ventilation higher than 24 h.

High Flow Nasal Cannula compared to Continuous Positive Airway Pressure: a bench and physiological study.

High-flow nasal cannula (HFNC) is extensively used for acute respiratory failure. However, questions remain regarding its physiological effects. We explored 1) whether HFNC produced similar effects to continuous positive airway pressure (CPAP); 2) possible explanations of respiratory rate changes; 3) the effects of mouth opening. Two studies were conducted: a bench study using a manikin's head with lungs connected to a breathing simulator while delivering HFNC flow rates from 0 to 60L/min; a physiological cross-over study in 10 healthy volunteers receiving HFNC (20 to 60L/min) with the mouth open or closed and CPAP 4cmH2O delivered through face-mask. Nasopharyngeal and esophageal pressures were measured; tidal volume and flow were estimated using calibrated electrical impedance tomography. In the bench study, nasopharyngeal pressure at end-expiration reached 4cmH2O with HFNC at 60L/min, while tidal volume decreased with increasing flow. In volunteers with HFNC at 60L/min, nasopharyngeal pressure reached 6.8cmH2O with mouth closed and 0.8cmH2O with mouth open; p<0.001. When increasing HFNC flow, respiratory rate decreased by lengthening expiratory time, tidal volume did not change, and effort decreased (pressure-time product of the respiratory muscles); at 40L/min, effort was equivalent between CPAP and HFNC40L/min and became lower at 60L/min (p=0.045). During HFNC with mouth closed, and not during CPAP, resistance to breathing was increased, mostly during expiration. In conclusion, mouth closure during HFNC induces a positive nasopharyngeal pressure proportional to flow rate and an increase in expiratory resistance that might explain the prolonged expiration and reduction in respiratory rate and effort, and contribute to physiological benefits.

An Introduction to the Clinical Application and Interpretation of Electrical Impedance Tomography.

Electrical impedance tomography is no longer a new technology, but its clinical use at the bedside is still in its primary stage. Global research has drastically increased since its commercial availability, and this has slowly begun to make its way into routine clinical bedside use in some areas of the world. This paper will provide the bedside clinician an introduction to the technology, how it is used, and the most common applications found in the literature.

Cuff Leak Test and Airway Obstruction in Mechanically Ventilated Intensive Care Unit Patients: A Pilot Randomized Controlled Clinical Trial.

Rationale: Laryngeal edema is a known complication of endotracheal intubation that may cause airway obstruction upon extubation. The only test available to predict this complication is the cuff leak test (CLT). Objectives: Given the uncertainty of the CLT's clinical utility, we conducted the COMIC (Cuff Leak Test and Airway Obstruction in Mechanically Ventilated ICU Patients) pilot study to examine the feasibility of undertaking a larger trial. Methods: COMIC is a multicentered, parallel-group randomized trial performed in Canada, Saudi Arabia, and Poland. We enrolled mechanically ventilated adults admitted to the intensive care unit who were deemed ready for extubation. Those allocated to the intervention arm had the results of their CLT communicated to the healthcare team, who then decided to proceed with extubation or not. In those randomized to the control arm, the CLT results were not communicated to the healthcare team and patients were extubated, regardless of the CLT result. The primary outcomes focused on feasibility. Results: One hundred patients (56 in the intervention and 44 in the control arm) were enrolled. All feasibility criteria were met, including 1) recruitment rate of 7.6 patients/month, 2) consent rate of 88.3% (95% confidence interval [CI], 82.1-94.5%), and 3) protocol adherence of 98% (95% CI, 95-100%). There were two episodes of clinically significant stridor in the intervention group and four patients who required reintubation in each group. Conclusions: The results of the COMIC pilot trial support the feasibility of a larger trial to determine the effect of the CLT on reintubation and clinically important stridor.Clinical trial registered with www.clinicaltrials.gov (NCT03372707).

AARC Clinical Practice Guideline: Management of Adult Patients With Oxygen in the Acute Care Setting.

Providing supplemental oxygen to hospitalized adults is a frequent practice and can be administered via a variety of devices. Oxygen therapy has evolved over the years, and clinicians should follow evidence-based practices to provide maximum benefit and avoid harm. This systematic review and subsequent clinical practice guidelines were developed to answer questions about oxygenation targets, monitoring, early initiation of high-flow oxygen (HFO), benefits of HFO compared to conventional oxygen therapy, and humidification of supplemental oxygen. Using a modification of the RAND/UCLA Appropriateness Method, 7 recommendations were developed to guide the delivery of supplemental oxygen to hospitalized adults: (1) aim for [Formula: see text] range of 94-98% for most hospitalized patients (88-92% for those with COPD), (2) the same [Formula: see text] range of 94-98% for critically ill patients, (3) promote early initiation of HFO, (4) consider HFO to avoid escalation to noninvasive ventilation, (5) consider HFO immediately postextubation to avoid re-intubation, (6) either HFO or conventional oxygen therapy may be used with patients who are immunocompromised, and (7) consider humidification for supplemental oxygen when flows > 4 L/min are used.

One-Year Readmission Following Undifferentiated Acute Hypercapnic Respiratory Failure.

Patients with acute hypercapnic respiratory failure (AHRF) often require hospitalization and respiratory support. Early identification of patients at risk of readmission would be helpful. We evaluated 1-y readmission and mortality rates of patients admitted for undifferentiated AHRF and identified the impact of initial severity on clinically important outcomes. We retrospectively analyzed patients who presented with AHRF to the emergency department of St Michael's Hospital in 2017. We collected data about patients' characteristics, hospital admission, readmission and mortality one year after the index admission. We analyzed predictors of readmission and mortality and conducted a survival analysis comparing patients who did and did not receive ventilatory support. A cohort of 212 patients with AHRF who survived their hospital admission were analyzed. At one year, 150 patients (70.8%) were readmitted and 19 (9%) had died. Main diagnoses included chronic obstructive pulmonary disease (60%), congestive heart failure (36%), asthma (22%) and obesity (19%), and these categories of patients had similar 1 y readmission rates. One third had more than one coexisting chronic illness. Although comorbidities were more frequent in readmitted patients, only a history of previous hospital admissions remained associated with 1 y readmission and mortality in multivariate analysis. Need for ventilatory support at admission was not associated with higher 1 y probability of readmission or death. Undifferentiated AHRF is the presentation of multiple chronic illnesses. Patients who survive one episode of AHRF and with previous history of admission have the highest risk of readmission and death regardless of whether they receive ventilatory support during index admission.

Noninvasive Respiratory Support.

Despite its life-saving nature, invasive mechanical ventilation does not come without risk, and the avoidance of invasive mechanical ventilation is the primary goal of noninvasive respiratory support. Noninvasive respiratory support in the form of continuous or bi-level positive airway pressure were considered the only viable options to accomplish this for many years. Innovation and research have led to high-flow nasal cannula being added to the list of specialized therapies clinically shown to reduce escalation of care and intubation rates in patients presenting with acute respiratory failure. The amount of research being performed in this clinical space is impressive, to say the least, and it is rapidly evolving. It is the responsibility of the clinicians trained to use these therapies in the management of respiratory failure to understand the currently available evidence, benefits, and risks associated with the type of noninvasive respiratory support being used to treat our patients.

Reverse Triggering Dyssynchrony 24 h after Initiation of Mechanical Ventilation.

BACKGROUND: Reverse triggering is a delayed asynchronous contraction of the diaphragm triggered by passive insufflation by the ventilator in sedated mechanically ventilated patients. The incidence of reverse triggering is unknown. This study aimed at determining the incidence of reverse triggering in critically ill patients under controlled ventilation. METHODS: In this ancillary study, patients were continuously monitored with a catheter measuring the electrical activity of the diaphragm. A method for automatic detection of reverse triggering using electrical activity of the diaphragm was developed in a derivation sample and validated in a subsequent sample. The authors assessed the predictive value of the software. In 39 recently intubated patients under assist-control ventilation, a 1-h recording obtained 24 h after intubation was used to determine the primary outcome of the study. The authors also compared patients' demographics, sedation depth, ventilation settings, and time to transition to assisted ventilation or extubation according to the median rate of reverse triggering. RESULTS: The positive and negative predictive value of the software for detecting reverse triggering were 0.74 (95% CI, 0.67 to 0.81) and 0.97 (95% CI, 0.96 to 0.98). Using a threshold of 1 µV of electrical activity to define diaphragm activation, median reverse triggering rate was 8% (range, 0.1 to 75), with 44% (17 of 39) of patients having greater than or equal to 10% of breaths with reverse triggering. Using a threshold of 3 µV, 26% (10 of 39) of patients had greater than or equal to 10% reverse triggering. Patients with more reverse triggering were more likely to progress to an assisted mode or extubation within the following 24 h (12 of 39 [68%]) vs. 7 of 20 [35%]; P = 0.039). CONCLUSIONS: Reverse triggering detection based on electrical activity of the diaphragm suggests that this asynchrony is highly prevalent at 24 h after intubation under assist-control ventilation. Reverse triggering seems to occur during the transition phase between deep sedation and the onset of patient triggering.

Automated detection and quantification of reverse triggering effort under mechanical ventilation.

BACKGROUND: Reverse triggering (RT) is a dyssynchrony defined by a respiratory muscle contraction following a passive mechanical insufflation. It is potentially harmful for the lung and the diaphragm, but its detection is challenging. Magnitude of effort generated by RT is currently unknown. Our objective was to validate supervised methods for automatic detection of RT using only airway pressure (Paw) and flow. A secondary objective was to describe the magnitude of the efforts generated during RT. METHODS: We developed algorithms for detection of RT using Paw and flow waveforms. Experts having Paw, flow and esophageal pressure (Pes) assessed automatic detection accuracy by comparison against visual assessment. Muscular pressure (Pmus) was measured from Pes during RT, triggered breaths and ineffective efforts. RESULTS: Tracings from 20 hypoxemic patients were used (mean age 65 ± 12 years, 65% male, ICU survival 75%). RT was present in 24% of the breaths ranging from 0 (patients paralyzed or in pressure support ventilation) to 93.3%. Automatic detection accuracy was 95.5%: sensitivity 83.1%, specificity 99.4%, positive predictive value 97.6%, negative predictive value 95.0% and kappa index of 0.87. Pmus of RT ranged from 1.3 to 36.8 cmH20, with a median of 8.7 cmH20. RT with breath stacking had the highest levels of Pmus, and RTs with no breath stacking were of similar magnitude than pressure support breaths. CONCLUSION: An automated detection tool using airway pressure and flow can diagnose reverse triggering with excellent accuracy. RT generates a median Pmus of 9 cmH2O with important variability between and within patients. TRIAL REGISTRATION: BEARDS, NCT03447288.

The cuff leak test in critically ill patients: An international survey of intensivists.

BACKGROUND: The cuff leak test (CLT) is used to assess laryngeal edema prior to extubation. There is limited evidence for its diagnostic accuracy and conflicting guidelines surrounding its use in critically ill patients who do not have risk factors for laryngeal edema. The primary study aim was to describe intensivists' beliefs, attitudes, and practice regarding the use of the CLT. METHODS: A 13-item survey was developed, pilot-tested, and subjected to clinical sensibility testing. The survey was distributed electronically through MetaClinician®. Descriptive statistics and multivariable regression analysis were performed to examine associations between participant demographics and survey responses. RESULTS: 1184 practicing intensivists from 17 countries in North and South America, Europe, Oceania, and Asia participated. The majority (59%) of respondents reported rarely or never perform the CLT prior to extubating patients not at high risk of laryngeal edema, which correlated with 54% of respondents reporting they believed a failed CLT did not predict reintubation. Intensivists from the Middle East were 2.4 times more likely to request a CLT compared to those from North America. Intensivists with base training in medicine or emergency medicine were more likely to request a CLT prior to extubation compared to those with base training in anesthesiology. CONCLUSION: Use of the CLT prior to extubating patients not at high risk of laryngeal edema in the intensive care unit is highly variable. Practice appears to be influenced by country of practice and base specialty training.

Lung Volume Measurement and Ventilation Distribution During Invasive Mechanical Ventilation.

Lung volume measurement performed during invasive mechanical ventilation can be used to determine functional residual capacity, changes in end-expiratory lung volume with the application of PEEP, and lung strain. However, many bedside measurements provide useful information without the use of specialized equipment. Ventilation distribution through the lung has traditionally been assessed with computed tomography, but more recently electrical impedance tomography has brought the ability to monitor this at the bedside, and without exposure to radiation. This review will describe techniques to measure lung volumes in the ICU and the relationship between lung strain, stress, and other measurements. This review will also discuss monitoring ventilation distribution at the bedside and the clinical assessment of regional compliance that this technology provides.

Gastric insufflation during cardiopulmonary resuscitation: A study in human cadavers.

INTRODUCTION: Bag-valve-mask ventilation is the first-line ventilation method during cardiopulmonary resuscitation (CPR). Risks include excessive volume delivery and gastric insufflation, the latter increasing the risk of pneumonia. The efficacy of ventilation can also be reduced by airway closure. We hypothesized that continuous chest compression (CC) could limit the risk of gastric insufflation compared to the recommended 30:2 interrupted CC strategy. This experimental study was performed in human "Thiel" cadavers to assess the respective impact of discontinuous vs. continuous chest compressions on gastric insufflation and ventilation during CPR. METHODS: The 30:2 interrupted CC technique was compared to continuous CC in 5 non-intubated cadavers over a 6 min-period. Flow and Airway Pressure were measured at the mask. A percutaneous gastrostomy allowed measuring the cumulative gastric insufflated volume. Two additional cadavers were equipped with esophageal and gastric catheters instead of the gastrostomy. RESULTS: For the 7 cadavers studied (4 women) median age of death was 79 [74-84] years. After 6 min of CPR, the cumulative gastric insufflation measured in 5 cadavers was markedly reduced during continuous CC compared to the interrupted CC strategy: (1.0 [0.8-4.1] vs. 5.9 [4.0-5.6] L; p < 0.05) while expired minute ventilation was slightly higher during continuous than interrupted CC (1.9 [1.4-2.8] vs. 1.6 [1.1-2.7] L/min; P < 0.05). In 2 additional cadavers, the progressive rise in baseline gastric pressure was lower during continuous CC than interrupted CC (1 and 2 cmH2O vs. 12 and 5.8 cmH2O). CONCLUSION: Continuous CC significantly reduces the volume of gas insufflated in the stomach compared to the recommended 30:2 interrupted CC strategy. Ventilation actually delivered to the lung is also slightly increased by the strategy.