Tracheal Occlusion Pressure Research Articles

Are diaphragmatic indices assessed by ultrasound good predictors of weaning outcome in mechanically ventilated respiratory patients?

Background Prediction of weaning outcome is still challenging. Among the reported causes of weaning failure is the diaphragmatic dysfunction. Diaphragmatic ultrasound indices were endorsed for predicting weaning outcome but with conflicting results. Therefore, this study aimed to evaluate ultrasonographic diaphragmatic indices for predicting weaning outcome in respiratory intensive care unit (RICU) patients. Patients and methods The study enrolled 40 RICU patients who were considered eligible for spontaneous breathing trial (SBT). Ultrasound of the right hemidiaphragm was performed immediately after the start of SBT. Assessment of the diaphragmatic excursion using M-mode in semisitting position was done. Moreover, the diaphragmatic thickness at the end of inspiration (Tdi) and expiration (Tdex), with the patient in supine position, was assessed. Diaphragmatic thickness fraction was then calculated. During SBT, patients’ respiratory rate, heart rate, rapid shallow breathing index (RSBI), and minute ventilation were recorded. Tracheal airway occlusion pressure (P0.1) and negative inspiratory force were measured using the ventilator software. Results The diaphragmatic indices did not appear to have a significant role in predicting successful weaning with area under the curve less than 0.56 for all of them. Moreover, no statistically significant difference in Tdi, Tdexe, thickness fraction, and diaphragmatic excursion was observed between the success and failure weaning groups. However, the RSBI performed as the best parameter in predicting weaning success, with area under the curve of 0.831. A cutoff value of 78 had 91% sensitivity and 84% specificity. Conclusion Ultrasonographic diaphragmatic indices are not satisfactory tools to predict weaning outcome in RICU compared with RSBI and other reported traditional indices.

Comparison of Maximal Inspiratory Pressure, Tracheal Airway Occlusion Pressure, and Its Ratio in the Prediction of Weaning Outcome: Impact of the Use of a Digital Vacuometer and the Unidirectional Valve

To investigate the predictive value of the maximal inspiratory pressure obtained by a digital vacuometer using a unidirectional valve (P(ImaxUV)) as to weaning outcome, and to compare its performance with the respiratory drive using airway occlusion pressure at 0.1 second (P(0.1)), and P(0.1)/P(ImaxUV). Patients on mechanical ventilation for > 24 hours who fulfilled the weaning criteria were prospectively enrolled. Measurements of P(ImaxUV) and P(0.1) were accomplished with a digital vacuometer with a unidirectional valve that allows only exhalation. Measured values were electronically recorded and stored on the digital vacuometer measurement device. Cutoff points for the used parameters were: absolute values of P(ImaxUV) > 30 cm H(2)O, P(0.1) < 2.3 cm H(2)O, and P(0.1)/P(ImaxUV) < 0.10. Receiver operating characteristic curves were calculated to compare the predictive values of the indexes. One hundred three subjects completed the test. The areas under the receiver operating characteristic curve were 0.79 ± 0.04, 0.65 ± 0.05, and 0.74 ± 0.04 for P(ImaxUV), P(0.1), and P(0.1)/P(ImaxUV), respectively. The area under the receiver operating characteristic curve for P(ImaxUV) was higher than for P(0.1) and P(0.1)/P(ImaxUV), but statistical significance was only found against P(0.1) (P = .007). Every studied index had only a modest performance regarding prediction of weaning outcome. Of note, P(ImaxUV) values obtained by digital technology using a unidirectional valve performed better than historically reported using a conventional techniques, surpassing P(0.1) and P(0.1)/P(ImaxUV) in this regard.

Evaluation of maximal inspiratory pressure, tracheal airway occlusion pressure, and its ratio in the weaning outcome

Purpose The objective of this study is to evaluate the predictive performance of maximal inspiratory pressure (P imax), airway occlusion pressure (P 0.1), and its ratio (P 0.1/P imax) in the weaning outcome. Materials and Methods Seventy patients on mechanical ventilation for more than 24 hours, who fulfilled weaning criteria, were prospectively evaluated. P imax less than −25 cm H 2O, P 0.1 less than 4.2 cm H 2O, and P 0.1/P imax less than 0.14 were evaluated in all patients before spontaneous breathing trials. The receiver operating characteristic (ROC) curve was calculated to evaluate the predictive performance of each index. Results P imax presented the area under the ROC curves smaller than those for P 0.1 and P 0.1/P imax (0.52 × 0.76 and 0.52 × 0.78; P = .004 and P = .0006, respectively), being the criteria of worst performance. P 0.1/P imax presented excellent predictive performance in weaned patients, with sensitivity of 98.08, but with the area under the ROC curves only slightly larger than those for P 0.1 (0.78 × 0.76, respectively; P = .69). Conclusion In our study, P 0.1 and P 0.1/P imax ratio were moderately accurate, whereas P imax was less accurate in predicting the weaning outcome.

A new integrative weaning index of discontinuation from mechanical ventilation

IntroductionIndexes predicting weaning outcome are frequently inaccurate. We developed a new integrative weaning index aimed at improving the accuracy of the traditional indexes.MethodsThree hundred and thirty-one patients mechanically-ventilated for more than 24 hours were evaluated. Initially, the threshold values of each index that best discriminate between a successful and an unsuccessful weaning outcome were determined in 115 patients. In the second phase, the predictive performance of these values was tested prospectively in the other 216 patients. Frequency/tidal volume ratio (f/Vt ratio), tidal volume (Vt), tracheal airway occlusion pressure 0.1 s (P 0.1), the product of P 0.1 and f/Vt (P 0.1 × f/Vt), respiratory rate (f), static compliance of the respiratory system (Cst,rs), ratio of arterial oxygen tension to fraction of inspired oxygen (PaO2/FiO2 ratio) and the new integrative weaning index IWI (Cst,rs × arterial oxygen saturation/f/Vt ratio) were evaluated in all patients. The readiness for weaning and the decision to return to mechanical ventilation was made by the physician in charge, based on the signs of poor tolerance. The receiver operating characteristic (ROC) curves were calculated in order to evaluate the predictive performance of each index. The Bayes' theorem was used to assess the probability of each test of predicting weaning.ResultsIn the prospective-validation set, successful weaning was observed in 183 patients (84.7%) and weaning failure in 33 (15.27%). IWI presented the highest accuracy, with the area under the ROC curves larger than that under the curves for the f/Vt ratio (0.96 × 0.85 respectively; P = 0.003), and also larger than that under the curves for the other indexes. IWI presented a higher probability of successful weaning when the test was positive (0.99) and a lower probability when the test was negative (0.14). Measurement of Cst,rs during the weaning process was considered one of the study limitations.ConclusionsIWI was the best predictive performance index of weaning outcome and can be used in the intensive care unit setting.Trial Registrationcontrolled-trials.com ISRCTN92117906

Modeling of the expiratory flow pattern of spontaneously breathing cats.

A mathematical model was developed describing the entire expiratory flow pattern during spontaneous, tidal breathing in the absence of expiratory muscle activity. It provides estimates for the time constants of the respiratory system (tau RS(model)) and of the decay of continuing inspiratory muscle activity in early expiration (tau mus(model)). In ten anesthetized, tracheostomized cats flow, tracheal pressure and diaphragmatic EMG were measured during normal expirations and expirations with four different added resistances. No significant differences were found between tau RS(model) (0.21-0.49 sec) obtained by fitting the model to the flow data and tau RS obtained from the straight part of the expiratory flow-volume curve. tau mus(model) (0.050-0.052 sec) was comparable to similar time constants obtained from the integrated diaphragmatic EMG or from end-inspiratory, tracheal occlusion pressure. Fitted peak flow and time to peak tidal expiratory flow were not significantly different from those measured. In conclusion, for spontaneously breathing, anesthetized cats our model provides a close fit of the expiratory flow and parameter estimates were comparable with independently measured values.

Changes in breathing pattern and respiratory muscle performance parameters during difficult weaning.

This study examined, using noninvasive means, the changes in breathing pattern and inspiratory muscle pressure-time indices during difficult progressive withdrawal of pressure-support ventilation. A prospective analysis of the temporal evolution of several respiratory variables in difficult-to-wean patients. A university hospital intensive care unit. A heterogeneous group of 17 patients receiving prolonged mechanical ventilation. Daily measurements of breathing pattern and respiratory muscle performance parameters in difficult-to-wean patients. We examined breathing pattern variables, rapid shallow breathing (respiratory rate/tidal volume), tracheal occlusion pressure, maximal inspiratory pressure (P(I)max), and the tension-time index of the inspiratory muscles (TTmus = P(I)/P(I)max x Ti/Ttot) (where Ti/Ttot is inspiratory fraction of the cycle). All measurements were repeated at 24-hr intervals throughout the difficult weaning period. The patients were extubated on satisfying ten of 12 classical weaning criteria. Eleven patients were successfully weaned from mechanical ventilation while six patients were not. Weaning failure was associated with the following: a) longer periods of mechanical ventilation before weaning; b) high values of tracheal occlusion pressure, respiratory rate, minute ventilation, and effective impedance maintained throughout the difficult weaning period; and c) persistent high PaCO2 and intrinsic positive end-expiratory pressure values. As the weaning failure patients' inspiratory muscles confronted an increasing inspiratory load, values of the tension-time index of the inspiratory muscles entered or remained in the fatigue zone. In contrast, weaning success patients normalized their breathing pattern and decreased their tracheal occlusion pressure, effective impedance, and tension-time index values. Breathing pattern alterations and respiratory muscle performance impairments lead to ventilator dependency after prolonged mechanical ventilation. The measurement of variables such as the noninvasive tracheal occlusion pressure, inspiratory power of breathing, and tension-time index of the inspiratory muscles facilitate the management of difficult-to-wean patients.

Breathing Pattern and Occlusion Pressure Waveform in Humans Anesthetized With Halothane or Sevoflurane

To examine the ventilatory effects of sevoflurane, breathing pattern, airway occlusion pressure waveform, and the mechanical variables of the respiratory system were determined in seven subjects anesthetized with sevoflurane and in an additional seven subjects anesthetized with halothane. All patients breathed 1 MAC of anesthetic using oxygen as the carrier gas, and the measurements were performed in the absence of surgical stimulation. The durations of inspiration and expiration were significantly longer during sevoflurane than during halothane administration. Tidal volumes were larger in the sevoflurane group than in the halothane group. Occlusion pressure waveforms were also markedly different between the two groups. Occlusion pressure during the initial 300-400 ms tended to be less in the sevoflurane-anesthetized than in the halothane-anesthetized subjects. There was no evidence of an active Hering-Breuer reflex with either anesthetic. Mechanical variables of the respiratory system were essentially identical between the two anesthetics. We conclude that (a) the ventilatory effects of halothane and sevoflurane are different, (b) the difference in the respiratory timing and depth of breathing originates from the action of the anesthetics on the central respiratory neural network, and (c) the different shape of the tracheal occlusion pressure may be largely due to the different effects of halothane and sevoflurane on the muscles of the rib cage.

Aminophylline enhances ventilation in phrenicotomized rats

The aim was to test the effects of aminophylline (AP) on breathing of phrenicotomized rats. Fifty seven male rats of the Wistar strain were anaesthetized with 1.3 g.kg-1 urethane i.p. They were phrenicotomized, divided into 6 groups and given 5, 10, 20, 40 and 80 mg.kg-1 AP i.v. or a corresponding volume of saline. Ventilation, tracheal occlusion pressure and arterial blood gases were measured. In all animals phrenicotomy resulted in hypoventilation with corresponding hypoxaemia (from control 11.9 +/- 1.0 to 10.4 +/- 0.8 kPa) and hypercapnia (from control 4.6 +/- 0.5 to 5.5 +/- 0.6 kPa). In the control group (with saline) 4 h later the PaO2 was 8.6 +/- 1.1 kPa and PaCO2 7.2 +/- 0.6 kPa. After AP 1 h after phrenicotomy the minute ventilation increased in a dose-dependent manner by 1-66%. 4 h after phrenicotomy the minute ventilation of rats with 20 and 40 mg.kg-1 of AP was significantly higher than that of the control group. AP prevented hypoventilation when injected into phrenicotomized rats. The results give no unequivocal basis from which to decide the proportion of central and peripheral effects of AP.

Vagal influences on respiratory mechanics, pressures, and control in rats

In eight spontaneously breathing anesthetized rats airflow, volume, and tracheal pressure were measured. The passive and active mechanical properties of the respiratory system, the shape of the tracheal occlusion pressure wave (P°tr), the decay of inspiratory muscle pressure during expiration, and parameters related to the control of breathing were computed both before and after bilateral cervical vagotomy. Pre- and post-vagotomy values of passive elastance, resistance, and time constant were similar. Active mechanics disclosed an increase of elastance and a decrease in resistance and in the time constant after vagotomy. The time couse of P°tr showed a downward concavity and was not modified by vagotomy in the range of control inspiratory times, whereas the shape of inspiratory muscle pressure decay during expiration was changed. The present data help to explain why after vagotomy late load-compensatory mechanisms are less effective.

Tracheal occlusion pressure: a simple index to monitor respiratory muscle fatigue during acute respiratory failure in patients with chronic obstructive pulmonary disease.

To assess respiratory muscle fatigue in acute respiratory failure in patients with chronic obstructive pulmonary disease and evaluate its influence on weaning patients from mechanical ventilation. We studied the time course of tracheal occlusion pressure (P0.1) and high-to-low ratio of the diaphragmatic electromyogram in 16 patients in acute respiratory failure with chronic obstructive pulmonary disease. All patients were intubated and studied during a 15-minute weaning period from ventilation. Minute ventilation (VE), arterial blood gases, P0.1 and high-to-low ratio of the diaphragm were measured every day from the onset to the end of acute failure (before extubation) at 5 and 15 minutes into the weaning period. The diaphragmatic electromyogram was recorded with an esophageal electrode and the high-to-low ratio of the electrical signal analyzed to assess diaphragmatic fatigue. In all patients P0.1 was markedly increased (7.1 +/- 2.4 cm H2O, mean +/- SE) on the first day of acute failure and did not change during weaning. In 11 patients, P0.1 had decreased to 4.7 +/- 1.8 cm H2O (P less than or equal to 0.002) before extubation (which was done after 5 to 9 days). In these patients, the high-to-low ratio of the diaphragm decreased rapidly-during the first minutes of weaning on the first day of acute failure and remained low throughout weaning, whereas before extubation no decrease in high-to-low ratio was seen during weaning. In 5 patients, P0.1 did not change significantly from the onset of acute failure and the high-to-low ratio remained low before extubation. These 5 patients had to be reintubated within 2 to 6 days. In both groups of patients, VE did not change significantly from the first to last day of acute failure (10.3 +/- 3 compared with 10.7 +/- 2.1 min-1), whereas blood gases during room air breathing improved significantly from the first to last day of acute failure, respectively, in each group (arterial oxygen pressure [PaO2], 33.5 +/- 1.5 compared with 44 +/- 9 mm Hg (P less than or equal to 0.05) and PaO2 56 +/- 2.3 compared with 49 +/- 2 mm Hg (P less than 0.005). Extubation should not be done in patients with respiratory muscle fatigue despite improvement in arterial blood gases and clinical status; and P0.1 provides a valid and simple index to assess the likelihood of respiratory muscle fatigue.

Response of abdominal muscle to graded mechanical loads.

Abdominal muscle electromyograms were monitored in response to graded mechanical loads added to either inspiration or expiration in anesthetized cats. The surgical preparation, the apparatus, and the levels of loads applied (resistive, tracheal occlusion, and continuous positive pressure) were matched with those employed in our previous study on medullary expiratory neurons [Baker et al, 1979]. Although expiratory neuron firing was significantly increased by each of three graded levels of expiratory resistive loads, abdominal muscles were activated in only about 50% of the animals exposed to the highest-level resistive load. The smaller resistive loads failed to elicit any discernible abdominal muscle activity. These findings suggest that the lower motor neurons have a higher threshold for activation than the medullary neurons. Recruitment of medullary expiratory neurons and integration of synaptic input at the spinal level must play important roles in the response to expiratory loading. Abdominal muscles did not respond to mechanical loading during inspiration. Bilateral cervical vagotomy eliminated the abdominal muscle responses to expiratory loads.

Effect of intermittent mandatory ventilation on respiratory drive and timing.

Seven patients receiving chronic ventilatory support were studied to better define the effects of intermittent mandatory ventilation (IMV) on the control and timing of spontaneous breathing between mandatory breaths. Each of these patients could sustain adequate spontaneous ventilation, as reflected by stable end-tidal carbon dioxide concentration (FETCO2), and arterial oxygen saturation (SO2) during periods of unassisted ventilation of sufficient duration to allow study. Inspiratory time (TI), respiratory cycle duration (Ttot), tidal volume (VT), and tracheal occlusion pressure (P0.1) were measured as IMV rate was progressively reduced. Respiratory timing was unaltered by decreasing IMV frequency; however, VT increased progressively. The P0.1 and mean inspiratory flow rate (VT/TI) also increased with each decrease in IMV rate, whereas FETCO2 and arterial SO2 remained constant. Thus, in these stable but ventilator-dependent patients, IMV did not alter respiratory timing or chemical stimuli, but it did alter respiratory drive as measured by VT/TI and P0.1.

Active impedance of respiratory system in anesthetized cats.

We have assessed the validity of the method of Siafakas et al. (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 51: 109-121, 1981) for determining active elastance (E'rs) and flow resistance (R'rs) of the respiratory system. In six cats anesthetized with pentobarbital sodium we have measured flow, volume, and tracheal occlusion pressure during spontaneous breathing. This allowed us to compute E'rs and R'rs. From these data and the occlusion pressure wave we predicted the time course of volume during inspirations with added linear flow resistances (delta R). These were compared to the actual loaded inspirograms. The agreement was generally good, except for small predictable discrepancies with the highest delta R values, which could be attributed to decompression of thoracic gas. These results indicate that the approach of Siakafas et al. to determine E'rs and R'rs is valid. In addition, we have quantified the "terminal inhibition" of inspiratory activity, which occurs toward the end of unoccluded breaths (both loaded and unloaded).

Temperature and CO2 effect on phrenic activity and tracheal occlusion pressure.

The present investigation was undertaken to study the interaction of CO2 and body temperature on phrenic activity (moving average) and tracheal occlusion pressure. Studies were performed on spontaneously ventilated cats anesthetized with pentobarbital sodium at different body temperatures (32-41 degrees C) while breathing room air, 2 and 4% CO2 in 50% O2. At any given chemical drive, increased body temperature caused a similar increase in rate of phrenic activity and tracheal occlusion pressure, while their peak values remained virtually unchanged. At any given body temperature, increased chemical drive caused an increase in both rate of rise and peak values of phrenic activity and tracheal occlusion pressure. These results confirm previous findings that body temperature affects the rate of rise of the central inspiratory activity (CIA), but not the inspiratory "off-switch" threshold, while CO2 increases both the rate of rise of CIA and off-switch threshold. In addition the results indicate that tracheal occlusion pressure provides a similar index of CIA as "integrated" phrenic activity.