Salvador Benito

A COMPARISON OF FOUR METHODS OF WEANING PATIENTS FROM MECHANICAL VENTILATION

BACKGROUND: Weaning patients from mechanical ventilation is an important problem in intensive care units. Weaning is usually conducted in an empirical manner, and a standardized approach has not been developed. METHODS: We carried out a prospective, randomized, multicenter study involving 546 patients who had received mechanical ventilation for a mean (+/- SD) of 7.5 +/- 6.1 days and who were considered by their physicians to be ready for weaning. One hundred thirty patients had respiratory distress during a two-hour trial of spontaneous breathing. These patients were randomly assigned to undergo one of four weaning techniques: intermittent mandatory ventilation, in which the ventilator rate was initially set at a mean (+/- SD) of 10.0 +/- 2.2 breaths per minute and then decreased, if possible, at least twice a day, usually by 2 to 4 breaths per minute (29 patients); pressure-support ventilation, in which pressure support was initially set at 18.0 +/- 6.1 cm of water and then reduced, if possible, by 2 to 4 cm of water at least twice a day (37 patients); intermittent trials of spontaneous breathing, conducted two or more times a day if possible (33 patients); or a once-daily trail of spontaneous breathing (31 patients). Standardized protocols were followed for each technique. RESULTS: The median duration of weaning was 5 days for intermittent mandatory ventilation (first quartile, 3 days; third quartile, 11 days), 4 days for pressure-support ventilation (2 and 12 days, respectively), 3 days for intermittent (multiple) trials of spontaneous breathing (2 and 6 days, respectively), and 3 days for a once-daily trial of spontaneous breathing (1 and 6 days, respectively). After adjustment for other covariates, the rate of successful weaning was higher with a once-daily trial of spontaneous breathing than with intermittent mandatory ventilation (rate ratio, 2.83; 95 percent confidence interval, 1.36 to 5.89; P < 0.006) or pressure-support ventilation (rate ratio, 2.05; 95 percent confidence interval, 1.04 to 4.04; P < 0.04). There was no significant difference in the rate of success between once-daily trials and multiple trials of spontaneous breathing. CONCLUSIONS: A once-daily trial of spontaneous breathing led to extubation about three times more quickly than intermittent mandatory ventilation and about twice as quickly as pressure-support ventilation. Multiple daily trials of spontaneous breathing were equally successful.

Gentamicin volume of distribution in critically ill septic patients

Gentamicin intrapatient pharmacokinetics variations were studied in 40 critically ill medical patients, suffering gram-negative sepsis. These patients were studied in two phases throughout gentamicin treatment: firstly, on the second day of treatment, when aggressive fluid therapy was required, and secondly, five days later, when patients had achieved a more stable clinical condition. Pharmacokinetic parameters were determined using least squares linear regression analysis assuming a one-compartment model using the Sawchuk-Zaske method. The apparent volume of distribution (Vd) in the first phase of the study was 0.43±0.12 L/kg, while on the seventh day of treatment it was 0.29±0.17 L/kg (p<0.001). Statistically significant differences were also observed for peak serum concentration (p<0.001), total dosage recommended (p<0.001) and half-life (p<0.05), whilst differences were not found for trough levels. From the analysis of the results obtained, we recommend increasing the initial dosage and monitoring plasma levels within the first days of therapy in critically ill patients treated with gentamicin, since important variations in aminoglycoside Vd related to disease, fluid balance and renal function, commonly occur in these patients.

Early prediction of successful weaning during pressure support ventilation in chronic obstructive pulmonary disease patients

ObjectiveThe aim of this study was to examine variables for early prediction of successful weaning in chronic obstructive pulmonary disease (COPD) patients during pressure support ventilation weaning. DesignThirteen COPD patients were pro-spectively studied to compare the respiratory pattern (inspiratory time, expiratory time, total breath cycle duration, tidal volume, respiratory rate, minute ventilation), the respiratory drive (airway occlusion pressure at 0.1 sec, tidal vol-ume/inspiratory time), and blood gases after 30 mins of pressure support weaning. SettingThe study was performed in the 20-bed General Critical Care Unit of the Rome “La Sapienza” University Hospital. PatientsWe evaluated 13 consecutive COPD patients fulfilling the standard weaning criteria (including clinical status, blood gases, forced vital capacity, maximum inspiratory pressure, and spontaneous respiratory rate after a 30-min T-piece trial) in which we compared respiratory pattern, respiratory drive, and blood gases after 30 mins of pressure support weaning. Measurements and Main ResultsAfter 30 mins of pressure support ventilation weaning (pressure support level 20cm H2O), we measured respiratory pattern (airway pressure and airflow tracing), airway occlusion pressure at 0.1 sec (occluding the inspiratory line during expiration with a rubber balloon), tidal volume/in-spiratory time, maximal inspiratory pressure, and blood gases. According to the result of the weaning trial, the patients were divided into two groups (not weaned and weaned), and the statistical difference between the evaluated variables was analyzed in weaned and not weaned groups.We did not observe a significant difference in breathing pattern data and arterial blood gases between weaned and not weaned patients. By contrast, airway occlusion pressure at 0.1 sec and maximum inspiratory pressure measured after 30 mins of weaning trial appeared significantly (p <.001) different in patients in whom the weaning trial succeeded or failed. Considering maximum inspiratory pressure, we could not separate weaned from not weaned patients, while all patients showing values of airway occlusion pressure at 0.1 sec <4.5 cm H2O were easily weaned. ConclusionsThis study confirms that conventional weaning criteria are often inadequate in predicting successful weaning of COPD patients, while airway occlusion pressure at 0.1 sec during the first phase of pressure support ventilation weaning can represent a good weaning predictor. (Crit Care Med 1992; 20:366–371)

P0.1/PIMax: An index for assessing respiratory capacity in acute respiratory failure

We studied airway occlusion pressure (P 0.1) and maximal inspiratory pressure (PIMax) in 10 healthy volunteers (Group A), 10 early postsurgical cardiac patients on spontaneous breathing (Group B), 10 patients mechanically ventilated for ARF (Group C), 10 patients weaning from mechanical ventilation after ARF (Group D) and 10 patients extubated after post-ARF (Group E). We calculated the index P 0.1/PIMax in an attempt to link the ventilatory demands and muscle ventilatory reserve. We found that the sensitivity and specificity in diagnosing the need for either full (C), partial (D) or no ventilatory support (A, B, E) by means of the P 0.1 were C=(50%, 95%), D=(70%, 72%) and A+B+E=(83%, 90%) respectively. When the index P 0.1/PIMax was used they were C=(90%, 100%), D=(80%, 87%) and A+B+E=(86%, 90%). We conclude that the index P 0.1/PIMax increases the reliability of P 0.1 alone to correctly classify the patients that will need either full, partial or no ventilatory support in ARF.

Variability analysis of the respiratory volume based on non-linear prediction methods

This work proposed and studied a method of automatically classifying respiratory volume signals as high or low variability by means of non-linear analysis of the respiratory volume. The analysis used volume signals generated by the respiratory system to construct a model of its dynamics and to estimate the quality of the predictions made with the model. Different methods of prediction evaluation, prediction horizons and embedding dimensions were also analysed. Assessment of the method was made using a database that contained 40 respiratory volume signals classified using clinical criteria into two classes: low or high variability. The results obtained using the method of surrogate data provided evidence of non-linear determinism in the respiratory volume signals. A discriminant analysis carried out using non-linear prediction variables classified the respiratory volume signals with an accuracy of 95%.

Decelerating flow ventilation effects in acute respiratory failure

PURPOSE The purpose of this article is to analyze the effect of a pressure-regulated volume-controlled ventilation mode on lung mechanics and gas exchange in patients with acute respiratory failure. MATERIALS AND METHODS We ventilated 10 patients with two pressure-limited modes: pressure-controlled ventilation (PC) and pressure-regulated volume-controlled ventilation (PRVC) in random order, for 1 hour each. Patients were stabilized on volume-controlled ventilation (VC) for 30 minutes before, between, and at the end of PC and PRVC to reach baseline conditions. At the end of every VC period and at 30 and 60 minutes of PC and PRVC, respiratory mechanics, gasometrics, and hemodynamic parameters were collected. RESULTS We found no significant differences between the three VC periods. Comparing VC with the two pressure-limited ventilation modes, peak pressure decreased from 29.4+/-9.1 cm H2O (VC) to 25.9+/-8.4 (PC 60 minutes) and 26.1+/-8.2 (PRVC 60 minutes), and PaCO2 decreased significantly from 38.6+/-3.1 mm Hg (VC) to 36.7+/-2.8 (PC 60 minutes) and 36.8+/-2.9 (PRVC 60 minutes). CONCLUSIONS Pressure-limited ventilation allows mechanical ventilation for the same tidal volume as VC but results in a lower peak inspiratory pressure and a slightly lower PaCO2. The mechanism responsible for this gas exchange effect is unknown but is probably related to a better air distribution of the decelerated flow. The clinical relevance of this phenomenon remains to be established.

Symbolic Dynamic Analysis of Relations Between Cardiac and Breathing Cycles in Patients on Weaning Trials

Traditional time-domain techniques of data analysis are often not sufficient to characterize the complex dynamics of the cardiorespiratory interdependencies during the weaning trials. In this paper, the interactions between the heart rate (HR) and the breathing rate (BR) were studied using joint symbolic dynamic analysis. A total of 133 patients on weaning trials from mechanical ventilation were analyzed: 94 patients with successful weaning (group S) and 39 patients that failed to maintain spontaneous breathing (group F). The word distribution matrix enabled a coarse-grained quantitative assessment of short-term nonlinear analysis of the cardiorespiratory interactions. The histogram of the occurrence probability of the cardiorespiratory words presented a higher homogeneity in group F than in group S, measured with a higher number of forbidden words in group S as well as a higher number of words whose probability of occurrence is higher than a probability threshold in group S. The discriminant analysis revealed the best results when applying symbolic dynamic variables. Therefore, we hypothesize that joint symbolic dynamic analysis provides enhanced information about different interactions between HR and BR, when comparing patients with successful weaning and patients that failed to maintain spontaneous breathing in the weaning procedure.

Inspiratory effort and occlusion pressure in triggered mechanical ventilation

We have studied eleven patients ventilated in the assisted mode during recovery from acute respiratory failure. We have measured the effort required to trigger the pressure demand valve for 3 different ventilators, and have measured the occulusion pressure as an index of neuromuscular inspiratory drive. We found a delay in the opening of the demand valve, as previously described by other authors. We also found a close correlation between the effort required to open the demand valve and the occlusion pressure. We conclude that the inspiratory effort required to open the demand valve, in the assist mode, is greater than the preset trigger level and that it is well correlated with the neuromuscular inspiratory drive. This inspiratory effort against the closed demand valve, allows the measurement of the occlusion pressure.

Intrinsic PEEP: a cause of inspiratory muscle ineffectivity.

Intrinsic PEEP has been described as an inspiratory threshold load to the inspiratory muscles that could increase the work of breathing. We report a patient who presented ineffective inspiratory efforts to trigger the ventilator; when we applied PEEP in similar amounts of measured intrinsic PEEP, his inspiratory efforts became able to trigger the ventilator, suggesting that the inspiratory load was alleviated.

Pulmonary compliance measurement in acute respiratory failure

Many automatic devices to measure effective compliance can now be incorporated in mechanical ventilators. We compared measurements from these devices against those obtained from a pressure-volume curve made with a super-syringe. The methods produced closely correlating results (r = 0.854; p < .001) in 20 adult patients with acute respiratory failure. Both techniques provide useful information about the elastic properties of the lung and can be used to adjust mechanical ventilation.