Reduced breathing variability as a predictor of unsuccessful patient separation from mechanical ventilation

Evolution of pattern of breathing during a spontaneous breathing trial predicts successful extubation

PURPOSE

Rapid shallow breathing may occur at any time during spontaneous breathing trials (SBT), questioning the utility of a single determination of the rapid shallow breathing index (RSBI). We hypothesize that change in RSBI during SBT may more accurately predict successful extubation than a single determination.

METHODS

Prospective observational study. Seventy-two subjects were extubated. At 24 h, 63/72 remained extubated (Extubation Success), and 9 were re-intubated (Extubation Failure). Respiratory rate (RR), tidal volume (VT) and RSBI were measured every 30 min during 2-h T-piece SBT. Change in respiratory parameters was assessed as percent change from baseline.

RESULTS

Initial RSBI was similar in Extubation Success and Extubation Failure groups (77.0 +/- 4.8, 77.0 +/- 4.8, p = ns). Nevertheless, RSBI tended to remain unchanged or decreased in the Extubation Success group; in contrast RSBI tended to increase in the Extubation Failure group because of either increased RR and/or decreased VT (p < 0.001 for mean percent change RSBI over time), indicating worsening of the respiratory pattern. Quantitatively, only 7/63 subjects of the Extubation Success group demonstrated increased RSBI >or=20% at any time during the SBT. In contrast, in the Extubation Failure group, RSBI increased in all subjects during the SBT, and eight of nine subjects demonstrated an increase greater than 20%. Thus, with a 2-h SBT the optimal threshold was a 20% increase (sensitivity = 89%, specificity = 89%). Similar results were obtained at 30 min (threshold = 5% increase). Percent change of RSBI predicted successful extubation even when initial values were >or=105.

CONCLUSION

Percent change of RSBI during an SBT is a better predictor of successful extubation than a single determination of RSBI.

Biosignal analysis techniques for weaning outcome assessment

Discontinuation of mechanical ventilation in critically ill patients is a challenging task and involves a careful weighting of the benefits of early extubation and the risks of premature spontaneous breathing trial. Recently, apart from studying different physiological variables by means of descriptive statistical tests, breathing pattern variability analysis has been performed for the assessment of weaning readiness. A limited number of clinical studies implementing different weaning protocols in heterogeneous groups of patients and using a variable set of signal processing techniques have appeared in the critical care literature, with varying results. The purpose of this review article is 3-fold: (1) to describe the different signal processing techniques being implemented for the assessment of weaning readiness, (2) to provide insight into the pathophysiological mechanisms that may govern breath-to-breath variability/complexity in health and disease, and (3) to present results from the critical care literature derived from the application of biosignal analysis tools for the identification of possible weaning indices.

Work of breathing during successful spontaneous breathing trial

OBJECTIVE

The aim of this study was to evaluate the work of breathing (WOB) behavior during a 120-minute successful spontaneous breathing trial (SBT) with T-tube trial, and its predictive value for extubation outcome.

DESIGN

A prospective cohort study.

SETTING

2 medical-surgical intensive care units.

PATIENTS

Fifty-one consecutive patients mechanically ventilated for more than 48 hours after a successful SBT were extubated based on the institutional protocol and followed for the occurrence of postextubation respiratory distress during 48 hours.

MEASUREMENTS AND MAIN RESULTS

All cases were serially monitored during 120 minutes of SBT using the respiratory monitoring system Ventrak 1500 (Medical Novametrix Systems, Wallingford, CT). Successful extubation occurred in 38 (74.5%) of 51 of the sample. Respiratory and hemodynamic parameters, APACHE II score, sex, days on mechanical ventilation, and cause of respiratory failure were unable to predict extubation outcome. The WOB significantly increased during SBT in extubation failure patients (WOB at 1st minute 0.24 +/- 0.06 J/L vs WOB at 120th minute = 0.39 +/- 0.07 J/L; P < .01) when compared to successfully extubated patients (WOB at 1st minute 0.21 +/- 0.08 J/L vs WOB at 120th minute = 0.24 +/- 0.11 J/L; P = .12). The WOB variation was able to predict extubation outcome only after the 90th minute of SBT (extubation failure = 0.35 +/- 0.08 J/L vs extubation success = 0.22 +/- 0.11 J/L; P = .01).

CONCLUSION

An increase in the WOB could predict extubation failure during a T-tube trial of 120 minutes.

Clinical review: liberation from mechanical ventilation

Mechanical ventilation is the defining event of intensive care unit (ICU) management. Although it is a life saving intervention in patients with acute respiratory failure and other disease entities, a major goal of critical care clinicians should be to liberate patients from mechanical ventilation as early as possible to avoid the multitude of complications and risks associated with prolonged unnecessary mechanical ventilation, including ventilator induced lung injury, ventilator associated pneumonia, increased length of ICU and hospital stay, and increased cost of care delivery. This review highlights the recent developments in assessing and testing for readiness of liberation from mechanical ventilation, the etiology of weaning failure, the value of weaning protocols, and a simple practical approach for liberation from mechanical ventilation.

Source of human ventilatory chaos: lessons from switching controlled mechanical ventilation to inspiratory pressure support in critically ill patients

Ventilatory flow measured at the airway opening in humans exhibits a complex dynamics that has the features of chaos. Currently available data point to a neural origin of this feature, but the role of respiratory mechanics has not been specifically assessed. In this aim, we studied 17 critically ill mechanically ventilated patients during a switch form an entirely machine-controlled assistance mode (assist-controlled ventilation ACV) to a patient-driven mode (inspiratory pressure support IPS). Breath-by-breath respiratory variability was assessed with the coefficient of variation of tidal volume, total cycle time, inspiratory time, expiratory time, mean inspiratory flow, duty cycle. The detection of chaos was performed with the noise titration technique. When present, chaos was characterized with numerical indexes (correlation dimension, irregularity; largest Lyapunov exponent, sensitivity to initial conditions). Expectedly, the coefficients of variations of the respiratory variables were higher during IPS than during ACV. During ACV, noise titration failed to detect nonlinearities in 12 patients who did not exhibit signs of spontaneous respiratory activity. This indicates that the mechanical properties of the respiratory system were not sufficient to produce ventilatory chaos in the presence of a nonlinear command (ventilator clock). A positive noise limit was found in the remaining 5 cases, but these patients exhibited signs of active expiratory control (highly variable expiratory time, respiratory frequency higher than the set frequency). A positive noise limit was also observed in 16/17 patients during IPS (p<0.001). These observations suggest that ventilatory chaos predominantly has a neural origin (intrinsic to the respiratory central pattern generators, resulting from their perturbation by respiratory afferents, or both), with little contribution of respiratory mechanics, if any.

Monitoring the mechanically ventilated patient

In the intensive care setting, monitored data relevant to the output, efficiency, and reserve of the respiratory system alert the clinician to sudden untoward events, aid in diagnosis, help guide management decisions, aid in determining prognosis, and enable the assessment of therapeutic response. This review addresses those aspects of monitoring we find of most value in the care of patients receiving ventilatory support. We concentrate on those modalities and variables that are routinely available or easily calculated from data readily collected at the bedside.

Liberation from mechanical ventilation: what monitoring matters?

Over the past 2 decades, the art of "weaning" from mechanical ventilation has been informed by increasing published basic science and outcomes studies. Although monitoring technologies can provide vast amounts of information before, during, and after liberation from mechanical ventilation, little data exists on how to maximally harness even routinely monitored, basic physiologic parameters. Overdependence on technology and derived variables, without data to demonstrate benefit, may even inhibit the patient's progress if it is used inappropriately. We review the scientific evidence for best using routinely available physiologic data and a few more sophisticated and invasive monitoring technologies during weaning. We also suggest future study designs that would better inform the process of liberation from the ventilator and endotracheal extubation.

Daily variation in endotoxin levels is associated with increased organ failure in critically ill patients

High blood levels of endotoxin on admission to the intensive care unit are predictive of adverse outcomes, including organ failure and death. However, the significance of changes in endotoxin levels over time has not been evaluated. We examined whether dynamic daily changes in endotoxin levels resulted in the development of greater organ dysfunction over time in critically ill patients. The study was a retrospective analysis of data from the longitudinal phase of a prospective observational multicenter cohort study of endotoxin levels in patients admitted to the intensive care unit. We analyzed 345 patients. Daily variation in endotoxin levels was assessed by calculating the number of inflections in the curve generated by plotting endotoxin levels against time. The degree of organ dysfunction over time was analyzed using a calculation of the total area under the curve generated by plotting the Multi Organ Dysfunction Score against time. From 1,301 endotoxin activity assay results, patients with dynamic daily variation in endotoxin levels as measured by a greater number of inflections had a greater degree of total organ dysfunction as measured by Multi Organ Dysfunction Score against time (P < 0.05). The arithmetic mean standard deviation of endotoxin activity assay results increased stepwise in the zero, one, and two inflection groups supporting the association between inflections and variability. Endotoxin activity assay variability was found to be independent of infection status (P = 0.52). Daily dynamic variation in endotoxin levels is a marker of increased severity of illness as measured by burden of total organ dysfunction over time. Further studies are warranted to assess the role of daily variation in endotoxin levels in the pathogenesis and potential therapy of organ failure in the critically ill.

Biologically variable ventilation improves gas exchange and respiratory mechanics in a model of severe bronchospasm*

OBJECTIVE

Mechanical ventilation can be lifesaving for status asthmaticus, but how best to accomplish mechanical ventilation is unclear. Biologically variable ventilation (mechanical ventilation that emulates healthy variation) and conventional control mode ventilation (monotonously regular) were compared in an animal model of bronchospasm to determine which approach yields better gas exchange and respiratory mechanics.

DESIGN

A randomized prospective trial of biologically variable ventilation vs. control mode ventilation in swine.

SETTING

University research laboratory.

SUBJECTS

Eighteen farm-raised pigs.

INTERVENTIONS

Methacholine was administered as a nebulized aerosol to initiate bronchospasm, defined as doubling of peak inspiratory pressure and respiratory system resistance, and then randomized (n = 9 each group) to either continue control mode ventilation or switch to biologically variable ventilation at the same minute ventilation. Over the next 4 hrs, hemodynamics, blood gases, respiratory mechanics, and carbon dioxide expirograms were recorded hourly. At end-experiment, tracheobronchial lavage was undertaken to determine interleukin-6 and -10 concentrations.

MEASUREMENTS AND MAIN RESULTS

Measurements of physiologic variables and inflammatory cytokines showed that biologically variable ventilation significantly improved gas exchange, with greater arterial oxygen tensions (p = .006; group x time interaction), lower arterial carbon dioxide tensions (p = .0003; group effect), lower peak inspiratory pressures (p = .0001; group x time), greater static compliance (p = .0001; group x time), greater dynamic compliance (p = .0001; group x time), and lower total respiratory system resistance (p = .028; group x time), compared with conventional ventilation. The appearance of inflammatory cytokines in bronchoalveolar lavage fluid (interleukin-6 and -10) was not affected by mode of ventilation.

CONCLUSIONS

In this experimental model, biologically variable ventilation was superior to control mode ventilation in terms of gas exchange and respiratory mechanics during severe bronchospasm.