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.

The central nervous system during lung injury and mechanical ventilation: a narrative review.

Mechanical ventilation induces a number of systemic responses for which the brain plays an essential role. During the last decade, substantial evidence has emerged showing that the brain modifies pulmonary responses to physical and biological stimuli by various mechanisms, including the modulation of neuroinflammatory reflexes and the onset of abnormal breathing patterns. Afferent signals and circulating factors from injured peripheral tissues, including the lung, can induce neuronal reprogramming, potentially contributing to neurocognitive dysfunction and psychological alterations seen in critically ill patients. These impairments are ubiquitous in the presence of positive pressure ventilation. This narrative review summarises current evidence of lung-brain crosstalk in patients receiving mechanical ventilation and describes the clinical implications of this crosstalk. Further, it proposes directions for future research ranging from identifying mechanisms of multiorgan failure to mitigating long-term sequelae after critical illness.

Why COVID-19 Silent Hypoxemia Is Baffling to Physicians.

Patients with coronavirus disease (COVID-19) are described as exhibiting oxygen levels incompatible with life without dyspnea. The pairing-dubbed happy hypoxia but more precisely termed silent hypoxemia-is especially bewildering to physicians and is considered as defying basic biology. This combination has attracted extensive coverage in media but has not been discussed in medical journals. It is possible that coronavirus has an idiosyncratic action on receptors involved in chemosensitivity to oxygen, but well-established pathophysiological mechanisms can account for most, if not all, cases of silent hypoxemia. These mechanisms include the way dyspnea and the respiratory centers respond to low levels of oxygen, the way the prevailing carbon dioxide tension (PaCO2) blunts the brain's response to hypoxia, effects of disease and age on control of breathing, inaccuracy of pulse oximetry at low oxygen saturations, and temperature-induced shifts in the oxygen dissociation curve. Without knowledge of these mechanisms, physicians caring for patients with hypoxemia free of dyspnea are operating in the dark, placing vulnerable patients with COVID-19 at considerable risk. In conclusion, features of COVID-19 that physicians find baffling become less strange when viewed in light of long-established principles of respiratory physiology; an understanding of these mechanisms will enhance patient care if the much-anticipated second wave emerges.

Misconceptions of pathophysiology of happy hypoxemia and implications for management of COVID-19.

In the article "The pathophysiology of 'happy' hypoxemia in COVID-19," Dhont et al. (Respir Res 21:198, 2020) discuss pathophysiological mechanisms that may be responsible for the absence of dyspnea in patients with COVID-19 who exhibit severe hypoxemia. The authors review well-known mechanisms that contribute to development of hypoxemia in patients with pneumonia, but are less clear as to why patients should be free of respiratory discomfort despite arterial oxygen levels commonly regarded as life threatening. The authors propose a number of therapeutic measures for patients with COVID-19 and happy hypoxemia; we believe readers should be alerted to problems with the authors' interpretations and recommendations.

Long-Term Outcome after Prolonged Mechanical Ventilation. A Long-Term Acute-Care Hospital Study.

Rationale: Patients managed at a long-term acute-care hospital (LTACH) for weaning from prolonged mechanical ventilation are at risk for profound muscle weakness and disability. Objectives: To investigate effects of prolonged ventilation on survival, muscle function, and its impact on quality of life at 6 and 12 months after LTACH discharge. Methods: This was a prospective, longitudinal study conducted in 315 patients being weaned from prolonged ventilation at an LTACH. Measurements and Main Results: At discharge, 53.7% of patients were detached from the ventilator and 1-year survival was 66.9%. On enrollment, maximum inspiratory pressure (Pimax) was 41.3 (95% confidence interval, 39.4-43.2) cm H2O (53.1% predicted), whereas handgrip strength was 16.4 (95% confidence interval, 14.4-18.7) kPa (21.5% predicted). At discharge, Pimax did not change, whereas handgrip strength increased by 34.8% (P < 0.001). Between discharge and 6 months, handgrip strength increased 6.2 times more than did Pimax. Between discharge and 6 months, Katz activities-of-daily-living summary score improved by 64.4%; improvement in Katz summary score was related to improvement in handgrip strength (r = -0.51; P < 0.001). By 12 months, physical summary score and mental summary score of 36-item Short-Form Survey returned to preillness values. When asked, 84.7% of survivors indicated willingness to undergo mechanical ventilation again. Conclusions: Among patients receiving prolonged mechanical ventilation at an LTACH, 53.7% were detached from the ventilator at discharge and 1-year survival was 66.9%. Respiratory strength was well maintained, whereas peripheral strength was severely impaired throughout hospitalization. Six months after discharge, improvement in muscle function enabled patients to perform daily activities, and 84.7% indicated willingness to undergo mechanical ventilation again.

New device for nonvolitional evaluation of quadriceps force in ventilated patients.

INTRODUCTION: In mechanically ventilated patients, nonvolitional assessment of quadriceps weakness using femoral-nerve stimulation (twitch force) while the leg rests on a right-angle trapezoid or dangles from the bed edge is impractical. Accordingly, we developed a knee-support apparatus for use in ventilated patients. METHODS: Ninety subjects (12 ventilated patients, 28 ambulatory patients, and 50 healthy subjects) were enrolled. Twitches with leg-dangling, trapezoid, and knee-support setups were compared. RESULTS: Knee-support twitches were similar to trapezoid twitches but smaller than leg-dangling twitches (P < 0.0001). Inter- and intraoperator agreement was high for knee-support twitches at 1 week and 1 month. In ventilated patients, knee-support twitches were smaller than in healthy subjects and ambulatory patients (P < 0.004). DISCUSSION: The new knee-support apparatus allows accurate recording of quadriceps twitches. The ease of use in ventilated patients and excellent inter- and intraoperator agreement suggest that this technique is suitable for cross-sectional and longitudinal studies in critically ill patients. Muscle Nerve 57: 784-791, 2018.

Pulse oximetry.

Pulse oximetry is universally used for monitoring patients in the critical care setting. This article updates the review on pulse oximetry that was published in 1999 in Critical Care. A summary of the recently developed multiwavelength pulse oximeters and their ability in detecting dyshemoglobins is provided. The impact of the latest signal processing techniques and reflectance technology on improving the performance of pulse oximeters during motion artifact and low perfusion conditions is critically examined. Finally, data regarding the effect of pulse oximetry on patient outcome are discussed.

The application of esophageal pressure measurement in patients with respiratory failure.

This report summarizes current physiological and technical knowledge on esophageal pressure (Pes) measurements in patients receiving mechanical ventilation. The respiratory changes in Pes are representative of changes in pleural pressure. The difference between airway pressure (Paw) and Pes is a valid estimate of transpulmonary pressure. Pes helps determine what fraction of Paw is applied to overcome lung and chest wall elastance. Pes is usually measured via a catheter with an air-filled thin-walled latex balloon inserted nasally or orally. To validate Pes measurement, a dynamic occlusion test measures the ratio of change in Pes to change in Paw during inspiratory efforts against a closed airway. A ratio close to unity indicates that the system provides a valid measurement. Provided transpulmonary pressure is the lung-distending pressure, and that chest wall elastance may vary among individuals, a physiologically based ventilator strategy should take the transpulmonary pressure into account. For monitoring purposes, clinicians rely mostly on Paw and flow waveforms. However, these measurements may mask profound patient-ventilator asynchrony and do not allow respiratory muscle effort assessment. Pes also permits the measurement of transmural vascular pressures during both passive and active breathing. Pes measurements have enhanced our understanding of the pathophysiology of acute lung injury, patient-ventilator interaction, and weaning failure. The use of Pes for positive end-expiratory pressure titration may help improve oxygenation and compliance. Pes measurements make it feasible to individualize the level of muscle effort during mechanical ventilation and weaning. The time is now right to apply the knowledge obtained with Pes to improve the management of critically ill and ventilator-dependent patients.

Effect of Atypical Sleep EEG Patterns on Weaning From Prolonged Mechanical Ventilation.

BACKGROUND: Approximately one-third of acute ICU patients display atypical sleep patterns that cannot be interpreted by using standard EEG criteria for sleep. Atypical sleep patterns have been associated with poor weaning outcomes in acute ICUs. RESEARCH QUESTION: Do patients being weaned from prolonged mechanical ventilation experience atypical sleep EEG patterns, and are these patterns linked with weaning outcomes? STUDY DESIGN AND METHODS: EEG power spectral analysis during wakefulness and overnight polysomnogram were performed on alert, nondelirious patients at a long-term acute care facility. RESULTS: Forty-four patients had been ventilated for a median duration of 38 days at the time of the polysomnogram study. Eleven patients (25%) exhibited atypical sleep EEG. During wakefulness, relative EEG power spectral analysis revealed higher relative delta power in patients with atypical sleep than in patients with usual sleep (53% vs 41%; P < .001) and a higher slow-to-fast power ratio during wakefulness: 4.39 vs 2.17 (P < .001). Patients with atypical sleep displayed more subsyndromal delirium (36% vs 6%; P = .027) and less rapid eye movement sleep (4% vs 11% total sleep time; P < .02). Weaning failure was more common in the atypical sleep group than in the usual sleep group: 91% vs 45% (P = .013). INTERPRETATION: This study provides the first evidence that patients in a long-term acute care facility being weaned from prolonged ventilation exhibit atypical sleep EEG patterns that are associated with weaning failure. Patients with atypical sleep EEG patterns had higher rates of subsyndromal delirium and slowing of the wakeful EEG, suggesting that these two findings represent a biological signal for brain dysfunction.

Effect of pressure support vs unassisted breathing through a tracheostomy collar on weaning duration in patients requiring prolonged mechanical ventilation: a randomized trial.

IMPORTANCE: Patients requiring prolonged mechanical ventilation (>21 days) are commonly weaned at long-term acute care hospitals (LTACHs). The most effective method of weaning such patients has not been investigated. OBJECTIVE: To compare weaning duration with pressure support vs unassisted breathing through a tracheostomy collar in patients transferred to an LTACH for weaning from prolonged ventilation. DESIGN, SETTING, AND PARTICIPANTS: Between 2000 and 2010, a randomized study was conducted in tracheotomized patients transferred to a single LTACH for weaning from prolonged ventilation. Of 500 patients who underwent a 5-day screening procedure, 316 did not tolerate the procedure and were randomly assigned to receive weaning with pressure support (n = 155) or a tracheostomy collar (n = 161). Survival at 6- and 12-month time points was also determined. MAIN OUTCOME MEASURE: Primary outcome was weaning duration. Secondary outcome was survival at 6 and 12 months after enrollment. RESULTS: Of 316 patients, 4 were withdrawn and not included in analysis. Of 152 patients in the pressure-support group, 68 (44.7%) were weaned; 22 (14.5%) died. Of 160 patients in the tracheostomy collar group, 85 (53.1%) were weaned; 16 (10.0%) died. Median weaning time was shorter with tracheostomy collar use (15 days; interquartile range [IQR], 8-25) than with pressure support (19 days; IQR, 12-31), P = .004. The hazard ratio (HR) for successful weaning rate was higher with tracheostomy collar use than with pressure support (HR, 1.43; 95% CI, 1.03-1.98; P = .033) after adjusting for baseline clinical covariates. Use of the tracheostomy collar achieved faster weaning than did pressure support among patients who did not tolerate the screening procedure between 12 and 120 hours (HR, 3.33; 95% CI, 1.44-7.70; P = .005), whereas weaning time was equivalent with the 2 methods in patients who did not tolerate the screening procedure within 0 to 12 hours. Mortality was equivalent in the pressure-support and tracheostomy collar groups at 6 months (55.92% vs 51.25%; 4.67% difference, 95% CI, -6.4% to 15.7%) and at 12 months (66.45% vs 60.00%; 6.45% difference, 95% CI, -4.2% to 17.1%). CONCLUSION AND RELEVANCE: Among patients requiring prolonged mechanical ventilation and treated at a single long-term care facility, unassisted breathing through a tracheostomy, compared with pressure support, resulted in shorter median weaning time, although weaning mode had no effect on survival at 6 and 12 months. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT01541462.

Nurses and ventilators.

In the previous issue of Critical Care, Rose and colleagues report the results of a survey on the frequency with which ICU nurses are involved in decision-making in ventilator management. About 63 to 88% of the decisions were made by nurses in collaboration with physicians, and as much as 68% of ventilator adjustments were performed by nurses independent of physicians. Nurse involvement in decision-making was twice as likely in ICUs that use weaning protocols. The ICU nurse performs many roles, the most important being the continuous observation of a patient. The diversion of a nurse's attention from constant vigilance by performing tasks of no benefit, such as the use of weaning protocols, would be a most unfortunate turn of events.

Clinical review: Respiratory monitoring in the ICU - a consensus of 16.

Monitoring plays an important role in the current management of patients with acute respiratory failure but sometimes lacks definition regarding which 'signals' and 'derived variables' should be prioritized as well as specifics related to timing (continuous versus intermittent) and modality (static versus dynamic). Many new techniques of respiratory monitoring have been made available for clinical use recently, but their place is not always well defined. Appropriate use of available monitoring techniques and correct interpretation of the data provided can help improve our understanding of the disease processes involved and the effects of clinical interventions. In this consensus paper, we provide an overview of the important parameters that can and should be monitored in the critically ill patient with respiratory failure and discuss how the data provided can impact on clinical management.

Narrative review: ventilator-induced respiratory muscle weakness.

Clinicians have long been aware that substantial lung injury results when mechanical ventilation imposes too much stress on the pulmonary parenchyma. Evidence is accruing that substantial injury may also result when the ventilator imposes too little stress on the respiratory muscles. Through adjustment of ventilator settings and administration of pharmacotherapy, the respiratory muscles may be rendered almost (or completely) inactive. Research in animals has shown that diaphragmatic inactivity produces severe injury and atrophy of muscle fibers. Human data have recently revealed that 18 to 69 hours of complete diaphragmatic inactivity associated with mechanical ventilation decreased the cross-sectional areas of diaphragmatic fibers by half or more. The atrophic injury seems to result from increased oxidative stress leading to activation of protein-degradation pathways. Scientific understanding of ventilator-induced respiratory muscle injury has not reached the stage where meaningful controlled trials can be done, and thus, it is not possible to give concrete recommendations for patient management. In the meantime, clinicians are advised to select ventilator settings that avoid both excessive patient effort and excessive respiratory muscle rest. The contour of the airway pressure waveform on a ventilator screen provides the most practical indication of patient effort, and clinicians are advised to pay close attention to the waveform as they titrate ventilator settings. Research on ventilator-induced respiratory muscle injury is in its infancy and portends to be an exciting area to follow.